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Air PollutionAir PollutionControl TechnologiesControl Technologies

CompendiumCompendium

Prepared byY.M. Fahmy, P. Fornasiero, S. Zinoviev and S. Miertus

UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION

INTERNATIONAL CENTRE FOR SCIENCE AND HIGH TECHNOLOGYTrieste, 2007

070239 - ICS.Compendium.55.indd 1070239 - ICS.Compendium.55.indd 1 19-06-2007 8:50:2619-06-2007 8:50:26


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PrefacePreface

This present Compendium has been developed as part of the programme of the International Centre for Science and High Technologies of the United Nations Industrial Development Organization (ICS-UNDIO). UNIDO is a specialized agency of the United Nations dedicated to promoting sustainable industrial development in developing and transition-economy countries. It harnesses theforces of government and the private sector to foster competitive industrial production and international industrial partnership and promote socially equitable and environmentally friendly industrial development. The International Centre for Science and High Technology is an institute within the framework of UNIDO, with headquarters in Trieste, Italy at the AREA Science Park. The Centre’s mandate is the transfer of know-how and technology in favour of developing countries and is based on the premise that competitive industrial technological capability cannot be built-up without adequate scientifi c knowledge and commitment to a sustainable development approach based on new and environmentally friendly technologies.

In the programme of the Area of Pure and Applied Chemistry one of the subprogrammes is dedicated to Catalysis and sustainable Chemistry. The activities with this subprogramme are focused on specifi c topics such as catalysis for exploitation renewable resources, catalysis for environmentally friendly processes and catalytic technologies for cleaner production.

The rapid implementation of various technologies focused primarily on increasing the economic effi ciency of industry has had an important environmental impact.

Industry, besides of transport - is the largest consumer of natural resources and one of the main contributors to overall pollution. The industrial sector generates pollutants such as organic substances, CO2, SOx and NOx emissions, hydrocarbons, volatile organic compounds (VOC) and persistent organic pollutants (POPs). Is compendium focuses on air pollution control technologies including catalysis for pollution reduction and prevention both for stationary and mobile applications.

Stanislav MiertusChief of Area

Pure and Applied Chemistry



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Contents

1 Pollutants and Pollution Sources 91.1 Main Sources of Air Pollution 91.2 Air Pollution Effects on Human Health 101.3 Criteria for Air Pollution Control Technologies Selection 111.4 References 12

2 Techniques for Air Pollution Measurements 132.1 Sulfur Dioxide 132.2 Carbon Monoxide 142.3 Nitrogen Oxides 142.4 Photochemical Oxidant 142.5 Hydrocarbons 142.6 Particulate Matter 152.7 Environmental Quality Standards 152.7.1 National Ambient Air Quality Standards (NAAQS) 162.8 References 17

3 NOx Abatement Technologies 193.1 NOx Sources 193.2 NOx Abatement and Control Strategies 203.2.1 Selective Catalytic Reduction of NOx 223.2.2 NOx Abatement System in Gas Turbines 253.2.3 Internal Combustion Reciprocating Engines 253.2.4 Air-fuel Ratio and Ignition Type 263.2.5 Infl uence of Fuel Quality on the Combustion’s Emissions 263.2.6 Fluidized Bed Combustion 283.2.7 Combined Technology Approaches 283.3 Selected Available Technologies and Supplier References 293.4 Conclusions 443.5 References 45

4 SOx Abatement Technologies 474.1 Control of SOx and H2SO4 Emission 484.2 Absorption Techniques 494.3 Adsorption Technique 514.4 Alternative Fuels 524.5 Implemented or Commercial Technologies 524.6 References 60

5 Volatile Organic Compounds (VOCs) 615.1 Introduction 615.2 VOC Destruction Technology 615.2.1 Thermal Oxidation 615.2.2 Catalytic Oxidation 625.2.3 Adsorption 645.2.4 Condensation, Refrigeration and Cryogenics 66


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5.2.5 Biological Oxidation 665.2.6 General Applicability of VOC Control Systems 675.3 Some Commercial Technologies 685.3.1 Oxidizer Types 685.3.2 Direct Fired Thermal Oxidizers (DFTOS) 695.3.3 Recuperative Oxidizers 695.3.4 Regenerative Thermal Oxidizers (RTOs) 705.4 Example of Commercial Technologies 715.5 Examples of Commercial Catalysts 735.5.1 Engelhard Catalysts 735.5.2 Photocatalytic Self-cleaning Ceramic 735.6 References 74

6 POPs and Chlorinated Organic Pollutants 756.1 Introduction 756.2 Unintentionally produced POPs and air pollution 766.3 Formation of dioxins 766.4 Chlorinated VOC and Other Halogenated Pollutants 786.5 Prevention of PCDD/F formation during incineration 796.6 End-of-pipe prevention and removal techniques 806.7 Chemical destruction of POPs and chlorinated pollutants in emissions 826.8 Treatment of macro streams of POPs and chlorinated pollutants 876.9 References 90

7 Technologies for Particulate Emission Control 917.1 Introduction 917.2 Particulate Formation 927.3 Particulate Emissions Control Systems 947.3.1 Gravity Settling Chambers 947.3.2 Centrifugal Separators 957.3.3 Particulate Wet Scrubbers 967.3.4 Electrostatic Precipitators 997.3.5 Filters 1007.4 Commercially Available Technologies 1017.5 References 102

8 Abatement of the Hydrocarbon and CO Emissions from Gas Turbine 1038.1 Gas Turbines 1038.2. Catalytic combustion 1048.3 Catalytic control of Hydrocarbon Emissions from Gas Turbine 1058.2 Catalytic control of CO Emissions from Gas Turbine 1068.3 Gas Turbine Aftertreatment Catalyst Deactivation 1078.4 References 107

9 Ozone Control Strategies 1099.1 Strategies for Reducing Ground-level Ozone 1109.2 Ozone Abatement in Jet Aircraft 1109.3 References 110

10 Air Pollution from Mobile Sources 11110.1 Automotive Emission Characteristics and Regulations 11110.2 Catalytic Converter and Three-way Catalysts 113


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10.2.1 Principle and Operation of TWCs 11510.3 Next Generation Technology for Emissions Control 11910.3.1 Hydrocarbon Adsorber Systems 11910.3.2 Electrically/Chemically Heated Catalyst Systems 12010.3.3 Close-coupled Catalyst (CCC) 12010.3.4 New Catalysts 12110.4 Diesel Engine Emissions 12110.4.1 Emission Formation in Diesel Engines 12210.4.2 Diesel Fuel 12310.4.3 World Diesel Emission Standards 12510.4.4 Diesel Emission Control Technologies 12610.4.5 Advanced Diesel Engine Technologies 12710.4.6 Diesel Oxidation Catalyst 12810.4.7 Lean NOx Catalyst 12810.4.8 Selective Catalytic Reduction 12910.4.9 NOx Adsorbers 13010.4.10 Diesel Particulate Filters 13010.5 Diesel Filter Regeneration 13110.6 Other Control Technologies 13310.7 Future Trends 13310.8 References 134

11 Appendices 13711.1 Web Directory 13711.2 Glossary 138



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AIR POLLUTION CONTROL TECHNOLOGIES

1 Pollutants and Pollution Sources1 Pollutants and Pollution Sources

Air pollution is a topic, which, among many others, has been at the forefront of social concern for the past several years. Most of this concern has been directed toward the health, distributional, regulatory and technological aspects of prevention or reduction of pollutants emissions in the atmosphere.

As the population grows, industry expands to make more and increasingly diverse products and the use of transport increases. Therefore the emissions of some pollutants is inevitably increasing. High pollution emission have already, on several occasions and in several places, led to ground level concentrations that were associated with dramatic rises in mortality and morbidity.

The term air pollution is the presence in the atmosphere of solid particles, liquid droplets or gaseous compounds, which are not normally, present or which are present in a concentration substantially greater than normal and harmful to living organisms and to the environment.

1.1 Main Sources of Air Pollution1.1 Main Sources of Air Pollution

Major sources of air pollution are:w Combustion processes;w Industry;w Vehicles;w Unsanitary disposal of household and municipal wastes;w Indiscriminate use of insecticides and pesticides in the agriculture;w Respiration of man and animals.

Common air pollutants and the relative main sources are presented in Table 1.1

Table 1.1: Common air pollutants and their sources

Pollutants Main SourcesSuspended particulate matter (SPM) Automobiles, power plants, boilers, industries requiring

crushing and grinding like cement factory.Sulfur oxides (SOx) Power plants, boilers, sulfuric acid manufacture, petroleum

refi ning.Lead Battery manufacturing, automobiles.Nitrogen oxides (NOx) Automobiles, power plants, nitric acid manufacturing.Carbon monoxide (CO) Automobiles.Hydrogen sulfi de (H2S) Pulp and paper, petroleum refi ning.Hydrocarbons (HC) Automobiles, petroleum refi ning.Ammonia (NH3) Fertilizer plants.


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POLLUTANTS AND POLLUTION SOURCES

An excellent summary (1) of the major atmosphere pollutants was published by US Department of Energy.

NOx emission per year: > 30 mmt (millions of metric tons) globally, 21 mmt in the USA, 95 % from vehicles and power sources.

Non-CH4 VOCs emission per year: 79 mmt in the USA, 87% from stationary and mobile engines.

CO emission per year: > 107 mmt globally, 79 mmt in the USA, 80% from mobile engines.

SO2 emissions per year: > 42 mmt globally, 22 mmt in the USA.

CH4 emissions per year: 160 mmt from natural sources (rice, animals); 370 mmt from human derived sources (cars, refi neries, coal mines, and fi lls); 27.2 mmt from US human sources, but only 0.12 mmt of CH4 from US industry; 500 adsorbed by the Earth.

N2O emissions per year: 11 mmt from natural sources; 3 mmt from human sources (fertilizer, fuel oil, cars); 10 mmt adsorbed by the Earth.

CO2 emission per year: 160000 mmt generated naturally, worldwide; 8000 mmt from human derived sources, global; 165000 mmt adsorbed by the Earth, the balance is a global increase of 3000 mmt per year; 1375 mmt CO2 from energy production in the USA.

In the year 2005, the dramatic dimension of the problem has been confi rmed by the US Department of Energy (2). The evidences clearly indicate that CO2, CO, VOCs, NOx, and SO2 represent major air pollutants. Notably, although the amount of released N2O is relatively small, there is a growing attention to this pollutant due to its long lifetime in the atmosphere (over 150 years) (3-6)

1.2 Air Pollution Effects on Human Health1.2 Air Pollution Effects on Human Health

Air pollutants can have the following effects on the human health:

• Immediate effects: sudden increase in air pollution has often been associated with immediate increase in the mortality and morbidity especially due to respiratory disease.

• Delayed effects: chronic bronchitis, lung cancer, dermatitis due to irritants and carcinogens in smoke and other pollutants.

Carbon monoxide (CO) is one of the most toxic gases emitted by automobiles. It interferes with absorption of oxygen by hemoglobin (red blood cells), impairs perception and thinking, slows refl exes, cause drowsiness, brings or angina and can cause unconsciousness and death; it affect growth in pregnant women and tissue development of young children.

Nitrogen oxides (NOx) can increase susceptibility to viral infections such as infl uenza, bronchitis and pneumonia.

Lead affects circulatory, reproductive, nervous and kidney systems, suspected of causing hyperactivity and lowered learning ability in children, hazardous even after exposure ends.


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Sulfur dioxide (SO2), has irritating capacity and have development of bronchial obstruction. It has also settled down that the lingering exposition increase the risk of development of sharp and chronic conjunctivitis.

Particles irritates mucous membranes and may initiate a variety of respiratory diseases, line particles may cause cancer. A strong correlation exists between suspended particulates and infant mortality in urban areas.

Ozone (O3) Ozone can irritate lung airways and cause infl ammation much like a sunburn. Other symptoms include wheezing, coughing, pain when taking a deep breath, and breathing diffi culties during exercise or outdoor activities. People with respiratory problems are most vulnerable, but even healthy people that are active outdoors can be affected when ozone levels are high. Repeated exposure to ozone pollution for several months may cause permanent lung damage. Anyone who spends time outdoors in the summer is at risk, particularly children and other people who are active outdoors.

Even at very low levels, ground-level ozone triggers a variety of health problems including aggravated asthma, reduced lung capacity, and increased susceptibility to respiratory illnesses like pneumonia and bronchitis.

1.3 Criteria for Air Pollution Control Technologies Selection1.3 Criteria for Air Pollution Control Technologies Selection

Beside the main human activities (big industrial plants) also the following small and medium Enterprises contribute to air pollution and shuold be subjected to air pollution control. (7-20):

1. Agricultural Chemical Applicators 2. Asphalt Applicators 3. Asphalt Manufacturers 4. Auto Body Shops 5. Bakeries 6. Distilleries 7. Dry Cleaners 8. Foundries 9. Furniture Manufacturers10. Furniture Repairs11. Gasoline Service Stations12. General Contractors13. Hospitals

14. Laboratories15. Lawnmower Repair Shops16. Lumber Mills17. Metal Finishers18. Newspapers19. Pest Control Operators20. Photo Finishing Laboratories21. Printing Shops22. Refrigerator/Air Conditioning Service

and Repair23. Tar Paving Applicators24. Textile Mills25. Wood Finishers

The number of technology options available for reducing environmental impact are highest early on in the life cycle and then decrease drastically. In contrast, costs associated with resolving an environmental problem typically increase exponentially as the process matures and the scale of equipment gets larger. There are technologies to control generic classes of air pollutant emissions such as CO, Volate Organic Compounds (VOCs), NOx or SOx. The specifi c choice of control technology, in most cases, is driven by the need to meet regulatory compliance at the lowest cost. The most cost-effective strategy for a given application, depends on a number of factors, including the nature of the pollutant, its concentration, the fl ow rate of the exhaust, and regulation. The decision is based on:

• Capability of treating a variety of wastes with varying constituents with minimal pretreatment or characterization;

• Secondary waste stream volumes that are signifi cantly smaller than the original waste stream volumes and which contain no toxic reaction byproducts;


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POLLUTANTS AND POLLUTION SOURCES

• Complete mineralization of organic contaminants;• Offgas and secondary waste composition;• Cost and Risk.

1.4 References1.4 References

1. Emissions of greenhouse gases in the United States: 1987-11992, DOE/EIA Report 0573, US Government printing offi ces, Washington, DC, October, 1994.

2. http://www.eia.doe.gov/oiaf/1605/ggrpt/summary/index.html

3. Thiemens, M.H.&Trogler, W.C. Science, 1991, 251, 932.

4. Armor J.N., Catal. Today, 38 (1997) 163.

5. http//www.epa.gov

6. Air Pollution Control Engineering, de Nevers N., second Edition, 2000, Mc Graw Hill, New York.

7. Bell M.L., Davis D.L., Gouveia N., Borja-Aburto V.H., Cifuentes L.A., Environmental Research 100 (2006) 431.

8. Chow J.C., Watson J.G., Shah J.J., Kiang C.S., Loh C., Lev-On M., Lents J.M., Molina M.J., Molina L.T., Journal of Air & Waste Management Association 54 (2004) 1226.

9. Diem J.E., Ricketts C.E., Dean J.R., Climate Research 30 (2006) 201.

10. Gregg J.W., Jones C.G., Dawson T.E., Nature 424 (2003) 183.

11. Herndon S.C., Jayne J.T., Zahniser M.S., Worsnop D.R., Knighton B., Alwine E., Lamb B.K., Zavala M., Nelson D.D., McManus J.B., Shorter J.H., Canagaratna M.R., Onasch T.B., Kolb C.E., Faraday Discuss. 130 (2005) 327.

12. Klumpp A., Ansel W., Klumpp G., Calatayud V., Garrec J.P., He S., Penuelas J., Ribas A., Ro-Poulsen H., Rasmussen S., Sanz M.J., Vergne P., Environ. Pollut. 139 (2006) 515.

13. Marshall J., Nature 437 (2005) 312.

14. Molina M.J., Molina L.T., Journal of the Air & Waste Management Association 54 (2004) 644.

15. Seigneur C., Aiche Journal 51 (2005) 356.

16. Pataki D.E., Alig R.J., Fung A.S., Golubiewski N.E., Kennedy C.A., McPherson E.G., Nowak D.J., Pouyat R.V., Lankao P.R., Global Change Biology 12 (2006) 2092.

17. Kaye J.P., Groffman P.M., Grimm N.B., Baker L.A., Pouyat R.V., Trends in Ecology & Evolution 21 (2006) 192.

18. Mitchell V.G., Diaper C., Water Sci. Technol. 52 (2005) 91.

19. Mitchell V.G., Diaper C., Environmental Modelling & Software 21 (2006) 129.

20. de Gouw J, Warneke C, Mass Spectrometry Reviews 26 (2007) 223.


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2 Techniques for Air Pollution Measurements2 Techniques for Air Pollution Measurements

To monitor air pollution, trace substance must be measured at levels of ppm (part per million) or ppb (part per billion) using high-sensitive analytical methods. The infl uence of coexisting substances. Complicates both qualitative and quantitative determination. Most of the analytical methods currently performed on gas components are chemical methods where ambient air passes through an absorbent by bubbling to adsorb the desired component; the component trapped by the adsorbent is quantitatively determined. In recent years, physical methods have been developing by which air samples directly introduced into an analysis unit are quantitatively determined.This chapter will briefl y discuss systems most generally used in continuous analyzers focusing on measurement methods for substances specifi ed in environmental quality standard relating to air pollution. For each major air pollutant, EPA, defi ned a reference method, which is the test method that is considered the standard against which other methods can be tested. There are equivalent methods, which have been checked against the reference method and found to give similar results. The equivalent methods are generally simplier, cheaper and easier to use than the reference methods and therefore are commonly used by national and regional ambient monitoring agency.

2.1 Sulfur Dioxide2.1 Sulfur Dioxide

Sulfur dioxide is a pollution generated by combustion of fuels containing sulfur, including coal and petroleum. At present, environmental concentration of sulfur dioxide in air tends to decrease by progress of techniques of desulfurization (Prevention).

The so colled West-Gaeke method is the EPA reference method for SO2 (1-3). It consist in bubbling a known volume of air trough a solution of sodium tetracloromercurate, which forms a complex with SO2. After several intermediate reactions, the solution is treated with pararosaniline to form the intensely colored methyl solfonic acid, whose concentration is determined in a colorimeter. The EPA’s regulations for locating ambient air monitors are in 40CFR58 App. E. The description of the analytical methods to use for measuring pollutants in ambient air are in 40CFR50 App. A-H.

Solution conductometry is specifi ed as measurements method concerning environmental standards. When ambient sample air passes through an absorbent (hydrogen peroxide solution acidifi ed by sulfuric acid), SO2 contained in the air is absorbed by the absorbent to form sulfuric acid according to the following reaction formula, resulting in the elevation of conductivity.

H2O2 + SO2 ⇒ H2SO4

Based on this change in conductivity, the concentration of SO2 in the sample air is continuous determined.

In addition, JIS (Japan International Standard) also specifi es the fl ame photometric detection (FPD) method, coulometry and ultraviolet fl uorometry for continuous analyzers.


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TECHNIQUES FOR AIR POLLUTION MEASUREMENTS

2.2 Carbon Monoxide2.2 Carbon Monoxide

Measuring methods specifi ed by environmental standards and by JIS include non-dispersive infrared absorption (NDIR) method (continuously determined the concentration of the CO in ambient sample air due to change in the absorption of infrared rays by using a selective detector) and hydrogen fl ame ionization detection (FID) method and rated potential electrolytic method.

2.3 Nitrogen Oxides2.3 Nitrogen Oxides

Environmental standards are specifi ed for NO2, which is produced by the oxidation of NO arising in the course of combustion.

The measuring method specifi ed in environmental standards is absorptiometry using Saltzmann reagent and chemiluminescence method.

Absorptiometry Using SaltzmannThis method allows to continuously measures both the concentration of NO and

NO2 in an ambient sample air by means of absorptiometry using an absorbent (Saltzmann reagent). The absorbent (a mixed solution of N-1-naphthylethylene diamine dihydrochlorate, sulfanilic acid and glacial acetic acid) selectively absorbs NO2 contained in the ambient sample air to produce a purplish red color. The concentration of NO2 in ambient sample air is determined by measuring this absorption. NO is determined in the same manner after the introduction of ambient sample air, which has already been analyzed on NO2, into oxidizing solution (potassium permanganate solution acidifi ed by sulfuric acid) to oxidize NO into NO2.

2.4 Photochemical Oxidant2.4 Photochemical Oxidant

An oxidant is a generic name of oxidizing substances including ozone and nitrogen dioxide. Environmental standards defi ne photochemical oxidants as substances releasing iodine from neutral potassium iodide solution (total oxidant), other than nitrogen dioxide. There are continuous analyzers for total oxidant based on absorptiometry and coulometry, and those for ozone based on chemiluminescence and ultraviolet absorption method.

Absorptiometry Using Neutral Potassium Iodide SolutionThis is the method of measuring the concentration of the total oxidant in ambient

sample air with the absorbance of iodine, which is released by contacting ambient sample air with neutral potassium iodide absorbent at a constant ratio of fl ow.

2.5 Hydrocarbons2.5 Hydrocarbons

The thought that hydrocarbons other than methane (present in general ambient air around 2 ppm) should be treated as “non-methane hydrocarbons”, namely, pollutants is currently accepted generally. No environmental standards are established for non-methane hydrocarbons, and guideline value is given. JIS specifi es hydrogen fl ame ionization detection (FID) method as continuous analyzer for hydrocarbons in ambient air.

Hydrogen Flame Ionization Detection (FID) MethodThis is the method for determining the concentration of hydrocarbons in ambient air

by measuring minute electric current due to ions, which arise on combustion of hydrocarbons in hydrogen fl ame, after isolating methane and non-methane hydrocarbons from ambient sample air by means of gas chromatography.


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2.6 Particulate Matter2.6 Particulate Matter

Atmospheric particulate matter is fi ne solid matter or liquid matter fl oating in atmospheric air. Of the particulate, environmental standards specify particulate having a diameter lower than 10 μm, which is considered to affect health. Particulate arising during industrial combustion process may have small particle size and high contents of heavy metals.

Measurement of Atmospheric Particulate Matter by the Light Scattering MethodThis is the method for determining the weight concentration of atmospheric

particulate matter fl oating in atmospheric air by measuring the intensity of scattered light which is known to be proportional to the weight concentration of particulate matter within the same particulate system on exposing particulate to light.

Measurement of Atmospheric Particulate Matter by β-ray Absorption MethodThis is the method of determining the atmospheric particulate matter by measuring

the absorption quantity of β-ray emitted by low energy β-ray source caused by atmospheric particulate matter collected on the fi lter.

Chemiluminescence SpectroscopyChemiluminescence, like atomic emission spectroscopy (AES), uses quantitative

measurements of the optical emission from excited chemical species to determine analyses concentration; however, unlike AES, chemiluminescence is usually emission from energized molecules instead of simply excited atoms. The bands of light determined by this technique emanate from molecular emissions and are therefore broader and more complex then bands originating from atomic spectra. Furthermore, chemiluminescence can take place in either the solution or gas phase, whereas AES is almost strictly as gas phase phenomenon.

2.7 Environmental Quality Standards2.7 Environmental Quality Standards

Environmental quality is certainly a worldwide concern. Air pollution knows no boundaries, and reducing it is of the utmost importance. Countries are established environmental regulations more and more restrictive. However, a completely unpolluted environment at no cost to everyone is an impossible dream. The emission standards philosophy is based on the defi nition of maximum possible (or practical) degree of emission control. Notably, the degree for a certain pollutant varies between classes of sources. Determining the emission standard for each class of pollutants and for each type of emitters, the pollution emission rate will be the lowest possible, leading to the cleanest possible air. A different approach is that of Air Quality Standards, based on a zero-damage philosophy. It is assumed that for each pollutant exist (and can be determined) a threshold below which no air pollution would occur. Dose response data, time of exposure, sinergic effects, bio-accumulation are some of many important parameters which are used to prepare Quality Standards. An example of Environmental Quality Standards is reported in Table 2.1.


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TECHNIQUES FOR AIR POLLUTION MEASUREMENTS

Table 2.1: Environmental quality standards in Japan

Substance SO2ppm

COppm

Suspended* particulate matter mg/m3

NO2ppm

Photochemical**` oxidantsppm

Conditions 0.1/h0.04/day

20/h10/day

02/h0.1/day

00.4-0.06/d 0.06/h

Measuring methods

Solution conductimetry

-UV fl uorescence method

Non dispersive infrared method (NDIR)

Piezoelectric balance methodBeta-ray absorption method

-Light scattering method

Absorptiometry(Saltzman reagent solution)

Chemiluminescence method

Absorptiometry(NGKI)Coulometry

-UV AbsorptiometryChemiluminescence method

* Suspended particulated matter shall mean airborn particles of 10 μm or less in diameter.** Photochemical oxidants are oxidizing substances such as ozone and peroxiacetyl nitrate produced by photochemical

reactions (only those capable of isolating iodine from neutral potassium iodide, excluding nitrogen dioxide).

2.7.1 National Ambient Air Quality Standards (NAAQS)2.7.1 National Ambient Air Quality Standards (NAAQS)

The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards for pollutants considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of “sensitive” populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings (3-18).

The EPA Offi ce of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards for six principal pollutants, which are called “criteria” pollutants. They are listed in Table 2.2. Units of measure for the standards are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3).

Table 2.2: National ambient air quality standards

POLLUTANT STANDARD VALUE * STANDARD TYPE

Carbon Monoxide (CO) 8-hour Average 1-hour Average

9 ppm35 ppm

(10 mg/m3)(40 mg/m3)

PrimaryPrimary

Nitrogen Dioxide (NO2) Annual Arithmetic Mean 0.053 ppm (100 µg/m3) Primary & Secondary

Ozone (O3) 1-hour Average 8-hour Average

0.12 ppm0.08 ppm

(235 µg/m3)(157 µg/m3)

Primary & SecondaryPrimary & Secondary

Lead (Pb) Quarterly Average 1.5 µg/m3 Primary & Secondary

Particulate (PM 10) Particles with diameters of 10 micrometers or less Annual Arithmetic Mean 50 µg/m3 Primary & Secondary 24-hour Average 150 µg/m3 Primary & Secondary

Particulate (PM 2.5) Particles with diameters of 2.5 micrometers or less Annual Arithmetic Mean 15 µg/m3 Primary & Secondary 24-hour Average 65 µg/m3 Primary & Secondary

Sulfur Dioxide (SO2) Annual Arithmetic Mean 0.030 ppm (80 µg/m3) Primary 24-hour Average 0.14 ppm (365 µg/m3) Primary 3-hour Average 0.50 ppm (1300 µg/m3) Secondary

* Parenthetical value is an approximately equivalent concentration.


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1. de Nevers N., Pollution Control Engineering, 2000, Mc Graw Hill, New York.

2. Cooper, H. B. B., Jr. and Rossano A.T. Jr., Source Testing for Air Pollution Control, Environmental Research and Applications, Inc., Wilton, CT, 1971.

3. http://www.epa.gov/epahome/lawregs.htm and http://www.epa.gov/air/criteria.html

4. Pollution: Causes, Effects and Control, R. M. Harrison Ed., The Royal Society of Chemistry, Cambridge, 1996.

5. Diem J.E., Ricketts C.E., Dean J.R., Climate Research 30 (2006) 201.

6. Pataki D.E., Alig R.J., Fung A.S., Golubiewski N.E., Kennedy C.A., McPherson E.G., Nowak D.J., Pouyat R.V., Lankao P.R., Global Change Biology 12 (2006) 2092.

7. Kaye J.P., Groffman P.M., Grimm N.B., Baker L.A., Pouyat R.V., Trends in Ecology & Evolution 21 (2006) 192.

8. Mitchell V.G., Diaper C., Water Science and Technology 52 (2005) 91.

9. Mitchell V.G., Diaper C., Environmental Modelling & Software 21 (2006) 129.

10. Herndon S.C., Jayne J.T., Zahniser M.S., Worsnop D.R., Knighton B., Alwine E., Lamb B.K., Zavala M., Nelson D.D., McManus J.B., Shorter J.H., Canagaratna M.R., Onasch T.B., Kolb C.E., Faraday Discuss. 130 (2005) 327.

11. M.L.Bell, D.L.Davis, N.Gouveia, V.H.Borja-Aburto, L.A.Cifuentes, Environmental Research 100 (2006) 431.

12. Chow J.C., Watson J.G., Shah J.J., Kiang C.S., Loh C., Lev-On M., Lents J.M., Molina M.J., Molina L.T., Journal of the Air & Waste Management Association 54 (2004) 1226.

13. Gregg J.W., Jones C.G., Dawson T.E., Nature 424 (2003) 183.

14. de Gouw J, Warneke C, Mass Spectrometry Reviews 26 (2007) 223.

15. Klumpp A., Ansel W., Klumpp G., Calatayud V., Garrec J.P., He S., Penuelas J., Ribas A., Ro-Poulsen H., Rasmussen S., MSanz.J., Vergne P., Environ Pollut. 139 (2006) 515.

16. Marshall J., Nature 437 (2005) 312.

17. Molina M.J., Molina L.T., Journal of the Air & Waste Management Association 54 (2004) 644.

18. Seigneur C., Aiche Journal 51 (2005) 356.



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3 NO3 NOx Abatement Technologies Abatement Technologies

NOx represent a family of seven compounds (Table 3.1). EPA regulates nitrogen dioxide (NO2) as a surrogate for this family of compounds because it is the most prevalent form of NOx in the atmosphere that is generated by anthropogenic (human) activities. NO2 is not only an important primary air pollutant, but since it reacts in the atmosphere, it is also a secondary pollutant. NO2 react with water in the atmosphere to form acid rain. Furthermore, NO2 can react with air in the presence of ultraviolet light (UV) to form ozone and nitric oxide (NO). Then, NO can react with free radicals, created by UV irradiation of VOC, forming PAN (1).

Table 3.1: Nitrogen Oxides (NOx)

Formula Name Properties

N2O Nitrous oxide Colorless, water soluble

NON2O2

Nitric oxideDinitrogen dioxide

Colorless gas, slightly water soluble

N2O3 Dinitrogen trioxide Black solid, water soluble, decomposes in water

NO2

N2O4

Nitrogen dioxideDinitrogen tetroxide

Red-brown gas, very water soluble, decomposes in water

N2O5 Dinitrogen pentoxide White solid, very water soluble, decomposes in water

3.1 NO3.1 NOx Sources Sources

Mobile sources contribute for about 50% of the total NOx emissions. Electric power plant boilers produce about 40% of the NOx emissions from stationary sources (2). Additionally, substantial emissions arise from industrial boilers, incinerators, gas turbines, reciprocating spark ignition and Diesel engines in stationary sources, iron and steel mills, cement manufacture, glass manufacture, petroleum refi neries, and nitric acid manufacture. Biogenic or natural sources of nitrogen oxides include lightning, forest fi res, grass fi res, trees, bushes, grasses, and yeasts.

Nitrogen oxides can be grouped as follows accordingly with their formation processes during the combustion.

Thermal NOx: The concentration of “thermal NOx” is controlled by the nitrogen and oxygen molar concentrations and the temperature of combustion. Combustion at temperatures well below 1300 °C forms relatively low concentrations of thermal NOx.

Fuel NOx: Fuels that contain nitrogen (e.g., coal) create “fuel NOx” that results from oxidation of the already-ionized nitrogen contained in the fuel.

Prompt NOx: Prompt NOx is formed from molecular nitrogen in the air combining with fuel in fuel-rich conditions, which exist, to some extent, in all combustion. This nitrogen then oxidizes along with the fuel and becomes NOx during combustion, just like fuel NOx.


20

NOX ABATEMENT TECHNOLOGIES

3.2 NO3.2 NOx x Abatement and Control Strategies Abatement and Control Strategies

NOx abatement and control technology is a relatively complex issue. We shall try to provide a structure of the main NOx pollution prevention and control technologies by fi rst giving the principles that are used. Then we shall describe the more successful pollution prevention and emission control technologies and strategies. The Destruction or Removal Effi ciency (DRE) that each successful technology is capable of achieving, is one of the key parameters for selecting the best available technology. Therefore effectiveness of pollution control / reduction of NO and NO2 is also expressed in terms of relative DRE; i.e., comparison of the NOx generated in the presence of a NOx reduction technology and in its absence.

In the case of combustion process, specifi c boiler types and combustion systems will be considered along with the specifi c NOx technologies that can be applied. Notably, many new combustion systems incorporate NOx prevention methods into their design and signifi cantly reduce NOx emissions compared to similar but older systems. As a result, the use of DRE (even a relative DRE) for NOx may be inappropriate. In fact, the comparison between actual NOx emissions from a new, well-designed system with those emitted by a similar older system, equipped with the same NOX controlled technology, may be the best way of evaluating how effectively a new combustion system minimizes NOx emissions.

The selection of the specifi c NOx abatement technology for a combustion system should be based on the analysis of the required system modifi cations. Specifi cally, technical and economic fusibility are primary restrictions. Therefore, in the process of identifi cation of the best applicable pollution prevention and emission control technologies, it must be fi rst consider the combustion system design.

The major types of combustion systems are shown in Table 3.2

Table 3.2: Common combustion systems (4)

Type of Combustion Unit Fuel

Dry bottom boilers - wall-fi red, front-fi red or Opposed-fi red pulverized coal, gas, or liquid

Dry bottom boilers - tangentially fi red pulverized coal, gas, or liquid

Wet bottom (slag tap) boilers - cyclone-type burners pulverized coal, gas, or liquid

Fluidized bed coal

Stokers with traveling grate crushed coal

Stokers with spreader grate crushed coal

Gas turbines gas and liquid

Internal combustion engines gas and liquid

There are two primary control techniques:

1. combustion modifi cations to suppress the formation of nitrogen oxides,

2. add-on controls (fl ue gas treatment) to reduce nitrogen oxides to molecular nitrogen.

Table 3.3 lists principles or methods that are used to reduce NOx;


21


Table 3.3: NOx Control methods - (P) pollution prevention and (A) add-on technology *

Method Technologies Abatement Type(P and A)

1. Reducing peak temperature Combustion OptimizationBurners Out Of Service (BOOS)Less Excess Air (LEA)Inject Water or SteamFlue Gas Recirculation (FGR)Natural Gas ReburningLow NOx Burners (LNB)Catalytic CombustionOver Fire Air (OFA)Air StagingReduced Air Preheat

PPPPPPPPPPP

2. Reducing residence time

at peak temperature

Inject AirInject FuelInject Steam

PPP

3. Chemical reduction of NOx Fuel Reburning (FR)Low NOx Burners (LNB)Selective Catalytic Reduction (SCR)Selective Non-Catalytic Reduction(SNCR)

PPAA

4. Oxidation of NOx withSubsequent absorption

Non-Thermal Plasma ReactorInject Oxidant

AA

5. Removal of nitrogen Oxygen Instead Of AirUltra-Low Nitrogen Fuel

PP

6. Using a sorbent Sorbent In Combustion ChambersSorbent In Ducts

AA

7. Combinations of theseMethods

All Commercial Products P and A

w Summary of NOx Control Technologies and Their Availability and Extent of Application, EPA 450/3-92-004.

Reducing Temperature (Method 1): This technique is based on the possibility to carry out the combustion at reduced / lower temperatures. This is achieved by avoiding the stoichiometric air to fuel ratio (the exact ratio of reactants that are needed for full combustion). This technique avoids the ideal stoichiometric ratio because this is the ratio that produces higher temperatures and therefore generates higher concentrations of thermal NOx. Combustion temperature may be reduced by: (a) using fuel rich mixtures to limit the amount of oxygen available; (b) using fuel lean mixtures to limit temperature by diluting energy input; (c) injecting cooled oxygen-depleted fl ue gas into the combustion air to dilute energy; (d) injecting cooled fl ue gas with added fuel; or (e) injecting water or steam. Low-NOx burners are based partially on these principles.

Reducing Residence Time (Method 2): Reducing residence time at high combustion temperatures can be done by ignition or injection timing with internal combustion engines. It can also be done in boilers by restricting the fl ame to a short region in which the combustion air becomes fl ue gas. This is immediately followed by injection of fuel, steam, more combustion air, or recirculating fl ue gas. This short residence time at peak temperature keeps the vast majority of nitrogen from becoming ionized. This bears no relationship to total residence time of a fl ue gas in a boiler.

Chemical Reduction of NOx (Method 3): This technique is based on the use of a reducing agent, which react with NOx leading to N2 formation. Examples include Selective Catalytic Reduction (SCR), which uses ammonia, Selective Non-Catalytic Reduction (SNCR)


22


that use ammonia or urea, and Fuel Reburning (FR). Non-thermal plasma, in the presence of a reducing agent, is an emerging and innovative technology.

Oxidation of NOx (Method 4): This technique is based on the possibility to fully oxidize NOx to favor water to absorb them (i.e., it is based on the greater solubility of NOx at higher N valence state). This is accomplished either by using a catalyst, injecting hydrogen peroxide, creating ozone within the air fl ow, or injecting ozone into the air fl ow. Notably, non-thermal plasma, in the absence of a reducing agent, can effectively oxidize NOx. A scrubber must be added to the process to absorb N2 O5, avoiding emissions to the atmosphere. Emissions of nitric acid can be controlled as well. In this case, nitric acid can be neutralized by basic liquids in the scrubber. Notably, nitric acid can be collected and then sold (also as a calcium or ammonia salt).

Removal of nitrogen from combustion (Method 5): Reduction of NOx formation can be achieved by removing nitrogen from the combustion either by: (a) using oxygen instead of air in the combustion process; or (b) using ultra-low nitrogen content fuel. Notably, the use of pure oxygen produces a rather intense fl ame that must be subsequently and suitably diluted. Using ultra-low-nitrogen content fuels with oxygen can nearly eliminate fuel and prompt NOx.

Sorption (adsorption and absorption) (Method 6): Treatment of fl ue gas by injection of sorbents (such as ammonia, powdered limestone, aluminum oxide, or carbon) can remove NOx and other pollutants (principally sulfur). There have been successful efforts to make sorption products a marketable commodity. This kind of treatment has been applied in the combustion chamber, fl ue, and baghouse. The use of carbon as adsorbent has not led to a marketable product, but it is sometimes used to limit NOx emissions in spite of this. The sorption method is often based ao the use of a dry sorbent, but slurries have been used. This method uses either adsorption or absorption followed by fi ltration and/or electrostatic precipitation to remove the sorbent.

Combinations of these methods (Method 7): Many of these methods can be combined to achieve a higher NOx reduction effi ciency. For example, a fuel-rich cyclone burner (Method 1) can be followed by fuel reburn (Method 3) and over-fi re air (Method 1). Other control technologies that are intended to primarily reduce concentrations of sulfur also strongly affect the nitrogen oxide concentration.

Existing NOx abatement technologies for stationary sources can be divided into two categories, external combustion applications (e.g., boilers, furnaces and process heaters) and internal combustion applications (e.g., stationary internal combustion engines and turbines) (5-9). These categories can be further subdivided into pollution prevention (which reduces NOx generation) and add-on control technologies, which reduces NOx emissions (fl ue gas treatment). Each NOx abatement technology has different implementations, development histories, and, therefore, commercial status. Selection of a suitable technology must occur after an evaluation of the technical and economic feasibility of each NOx technology to accurate be implemented on an existing system. The available options may be limited by inability to adjust the existing combustion system.

3.2.1 Selective Catalytic Reduction of NO3.2.1 Selective Catalytic Reduction of NOx x

Selective Catalytic Reduction of NOx represent a key external combustion application which deserve great attention. The active catalytic component and temperature ranges may be classifi ed as indicated:


23


Low temperature (175-250 oC): Platinum - based catalystsMedium temperature (300-450 oC): Vanadium - based catalystsHigh temperature (350-600 oC): Zeolite - based catalysts

A more complete list of catalyst manufactures, general catalyst composition, catalyst support structures, and temperatures of operation is shown in Table 3.4 and Table 3.5 shows the solution for some problem for SCR.

Table 3.4: SCR NOx catalyst technologies.*

Company Catalyst Description Operating Temperature (oC)

Babcock Hitachi V/Ti/Metal plate 240-415

Camet Precious metal/metal monolith 225-275

Cormetech V/Ti/Extruded monolith 200-450

Engelhard Precious metal/Ceramic monolith 175-320

Engelhard V/Ti/Ceramic monolith 300-440

Engelhard Zeolite/Ceramic monolith 440-590

Hitachi Zosen V/Ti/Extruded monolith 330-420

Hitachi Zosen V/Ti/Wire mesh 330-420

JHI V/Ti/Extruded monolith 200-400

JMI V/Ti/Metal monolith 340-425

HKI V/TI/Extruded monolith 300-400

MIHI V/Ti/Extruded monolith 200-400

Norton Zeolite/Extruded monolith 220-520

Steuler Zeolite/Extruded monolith 300-520

UBE V/Ti/Extruded monolith 250-400

* Ref: R.M. Heck, R.J. Farrauto, Catalytic air pollution control: commercial technologies, van Nostrand Reinhold, New York, p. 170, 1995.

Table 3.5: Representative problems of SCR and some of the possible solutions.

Problems Solution

Poisoning by SO3 Use of TiO2-V2O5 catalyst

Clogging by dust Use of monolithic honeycomb type

Abrasion by coal dust Hardening the part of gas entrance in catalyst

Catalytic Oxidation SO2 to SO3 by catalyst Decrease of V2O5 loadingand addition WO3

Leak of ammonia Decrease of NH3/NOx molar ratio

Pollution by aged catalyst Recovery of heavy metal and TiO2 from spent catalyst

The position of the SCR reactor in the process effl uent of a thermal power plant strongly infl uences the selection of the catalyst composition and its physical structure should be used. For a high-ash, high-sulphur exhaust, there are three possible positions for the SCR catalyst.


24


High dust: for high-dust/coal-fi red boiler applications, a V2O5/TiO2 or zeolite-containing extruded catalyst of low cell density operating at 350-400 oC and from 3,000 to 5,000 hr-1 space velocity is preferred. The SCR catalyst is located before any fl ue gas particulate or scrubbing operations, so it must have large channels or holes to avoid plugging and excessive backpressure.

Low dust: The SCR catalyst can be positioned downstream of an electrostatic precipitator to provide the catalyst with a low-dust environment. Extruded homogenous honeycombs composed of V2O5/TiO2, zeolite, or (in some cases) combinations of these materials are commonly used. A major problem in low-dust system SCR is covered of the catalyst by very fi ne dust particles, which have passed the electrostatic fi lter, thus making it unavailable for catalytic action.

Tail end location: This location provides the cleanest feed gas to the catalyst since it has passed through an electrostatic precipitator and a fl ue gas desulfurizer. Extruded homogenous zeolites or ceramic or metal substrates washcoated with V2O5/TiO2 or zeolites can be used.

Trends in NOx control technologyRecently natural gas or other hydrocarbons tent to be used as reducing agent in

stationary SCR rather than NH3 (10). For many new power plants, natural gas is commonly used as a fuel and is readily available as effective NOx reducing agent. NH3 is more expensive, requires special handling and storage, and requires a sophisticated metering system to avoid NH3 slip.

An ideal situation for elimination of NOx from combustion processes is to avoid producing it initially. An exciting new possibility is the use of catalysts for both liquid and gaseous fuels in the primary combustion process, replacing conventional burners. In catalytically supported thermal combustion, a honeycomb-supported catalyst initiates oxidation of a fuel such as natural gas (11-12). The heat generated at the catalyst surface brings the bulk gas temperature to a condition where it can burn homogenously. The key to the process concept is that the initial fuel-air mixture is below the fl ammability limit, and thus the adiabatic temperature rise produces a gas below that required for nitrogen fi xation (< 1500 oC) so that almost no NOx is formed (<10 ppm). Furthermore, the oxidation reactions are so effi cient that the unburned hydrocarbon and CO contents of the exhaust are exceedingly small (<5 ppm). There are substantial materials and process problems to be solved before this technology can be commercialized. The temperatures that the catalyst and substrate will experience are considerably higher than any other known catalytic processes. After the exothermic reaction of the fuel brings about a high-temperature exposure of about 1300 oC, the catalyst must continue to function at inlet temperatures of less than 400 oC during start-up periods. For fuels like natural gas, where Pd is the most active catalyst, this is a major problem. The substrate material, be it ceramic or metal, must have excellent thermal shock resistance so as not degrade or crack during the periodic high-temperature surge common in power generation. For gas-fi red turbine power plants, inlet temperatures currently do not exceed about 1300-1400 oC, so this could be an ideal solution for future NOx emission control (13-19).

It is important to make that most NOx exhaust streams contain high levels of water vapor. Water vapor has been established (20-21) as a known inhibitor in most catalytic NOx reduction processes. Thus, any study announcing new catalysts for NOx removal must consider and evaluate the materials performance in wet NOx atmospheres water vapor of > 10%. Unless there is a worldwide reduction of sulfur levels in fuels, NOx, streams will usually have ppm levels of some sulfur compound; thus NOx-removal catalysts must also be tested for their resistance to sulfur (H2S or SOx).


25


3.2.2 NO3.2.2 NOx x Abatement System in Gas Turbines Abatement System in Gas Turbines

Gas turbines use the Brayton Cycle with a burner to raise temperature of gas after compression and before expansion through the turbine. Turbines mainly use reducing peak temperature and reducing residence time (Methods 1 and 2) approaches to limit NOx emissions. Because addition of particles to air fl ow entering the turbine would accelerate erosion of turbine blades, sorbents (Method 6) could only be applied after the expansion in the turbine. NOx reduction (Method 3) has been used to treat exhaust gases.

Table 3.6: Effi ciency of NOx abatement technologies currently used for gas turbines

NOx Abatement Method Technique Effi ciency

1.Reduction peak temperature Natural Gas Reburning (NGR)Low NOx Burners (LNB)Inject Water or SteamReduced Air PreheatCatalytic Combustion

70-85%

2.Reduction residence timeat peak temperature

Air Staging of CombustionInject Steam

70-80%

3.Chemical reduction of NOx Selective Catalytic Reduction (SCR)Selective Non-Catalytic Reduction (SNCR)Fuel Reburning (FR)Low NOx Burners (LNB)

70-90%


Non-Thermal Plasma Reactor (NTPR) NO-data

5. Removal of nitrogen Ultra-Low Nitrogen Fuel NO-data

6. Use of a sorbent Sorbent in Ducts 60-90%

There has also been some success in reducing NOx concentrations when burning biomass fuels in a boiler.

Table 3.6 reports the effi ciency of the presents NOx abatement technologies used for gas turbines.

It is worth noting that many turbine operators claim signifi cant improvement of air pollution control as a consequence of “good combustion practices. They mostly refers to reduction of particulate emissions (often focusing the attention to the reduction of the visible fraction of PM). However, the NOx emission data are rarely reported. Cogeneration units use a gas turbine to generate electricity and provide preheated combustion air for a boiler. Gas turbine exhaust contains typically 10-15% oxygen and therefore it can be effectively used to provide combustion air for a low-pressure boiler. This latter system can be used to provide steam for another turbine, a process heater, a space heater, or a combination of these. Notably, sorbent particles can be introduced into the exhaust gas turbine fl ow in order to control NOx.

3.2.3 Internal Combustion Reciprocating Engines3.2.3 Internal Combustion Reciprocating Engines

Internal combustion engines use air-to-fuel ratio and ignition/injection timing to control maximum temperature and residence time. This can reduce the concentration of NOx that is generated by reducing peak temperature (Method 1). Valve timing adjustments can reduce residence time at peak temperature (Method 2) to control NOx formation. Chemical reduction of NOx (Method 3) is used in catalytic converters to reduce NOx to N2. Some stationary engines use both Method 3 and NOx oxidation (Method 4). A non-thermal plasma


26


reactor was developed for treatment of diesel exhaust. A plasma ignition system allows greater freedom in the air-fuel ratio and the ignition timing of spark ignition engines. See Table 3.7 for NOx technologies used for stationary internal combustion engines.

Table 3.7: Effi ciency of NOx technologies currently used for stationary internal combustion engines

NOx Abatement Method Technique Effi ciency

1.Reduction of peak temperature Air/fuel RatioTiming of Ignition/Type of IgnitionPre-Stratifi ed Combustion

20-97%

2.Reducing residence timeat peak temperature

Valve Timing NO-data

3.Chemical reduction of NOx Selective Catalytic Reduction (SCR)Non-Selective Catalytic Reduction (NCSR)80-90%

80-90%


Non-Thermal Plasma Reactor (NTPR) 80-95%

5. Removal of nitrogen Ultra-Low Nitrogen Fuel NO-data

6. Use of sorbent Sorbent In Exhaust DuctsAdsorber in fi xed Bed

60-90%

3.2.4 Air-fuel Ratio and Ignition Type3.2.4 Air-fuel Ratio and Ignition Type

For internal combustion reciprocating engines, retardation of injection or spark ignition, or an air-fuel ratio that departs from stoichiometric conditions will reduce peak temperature. Lower peak temperature limits the amount of NOx formation. This technique can achieve up to 50% control effi ciency (22-23). When a three-way catalyst is used for spark ignition engines, exhaust gas must have no more than 0.5% oxygen. This technique can reach up to 98% effi ciency. The use of plasma ignition (an alternating current or AC system) instead of a direct current (DC) spark ignition system can also allow a greater fuel-lean departure from the stoichiometric ratio. NOx emissions from internal combustion engines using plasma ignition have been reported to be reduced by up to 97% (24-26). Delaying injection of fuel in a compression ignition (diesel) engine can signifi cantly reduce NOx emissions. The amount of this reduction will depend upon the engine, valuing, and fuel. However excessive timing retard can cause combustion instability or misfi re (27) .

3.2.5 Infl uence of Fuel Quality on the Combustion’s Emissions 3.2.5 Infl uence of Fuel Quality on the Combustion’s Emissions

The selection of combustion system strongly depends on the available fuel, environmental constrain, availability of technical support and fi nancial criteria. As far as concern with fuel quality, it must be clearly stated that the presence of impurities in a fuel strongly infl uence combustion performances and exhaust emissions (SOX, NOx, heavy metals, VOC…). The concentration of fuel’s impurities depends on the type of sources, purifi cation process, and supplier. Even “natural gas” (methane) may contain propane, butane, and carbon monoxide, in various percentage.

Solid FuelsThe combustion of a solid fuel, such as coal, is achieved by fi rst allowing the

gasifi cation of the volatile fraction of the fuel in a primary burner. A fl ow of air ensure


27


the removal of the volatile fraction from char and its oxidation. The remaining solid needs further combustion air to burn. This process provides supplementary heat, part of which is used to volatilize additional fuel. The simple control of combustion temperature would limit the amount of air which react with the char fraction. Therefore, the volatile fraction must be oxidized in over-fi re-air or a secondary stage of burner with its air control system. The balance of combustion air must be carefully adjusted taking into consideration the composition of fuel, the boiler loading, and transient loads. Fast dynamic air adjustments are recommended.

Pulverized coal can be burned in a similar way to oil. The fl ame is usually well defi ned and, depending on particle size of fuel, char may remain in suspension in fl ue gas throughout burning. Normally, the volatile fraction burns in air together with the fi ne fuel particles. However, if the particles of the fuel are too coarse, char will continue burning on its trajectory after leaving the fl ame. At a certain point the combustion ends. The trade jargon for this is “ unburned carbon (UBC),” “carbon in the ash (CIA),” or “loss of ignition (LOI).” These terms refer to carbon in char that does not burn along the trajectory. UBC can be minimized by grinding particles fi ner and separate the various fractions of fuel particles so that larger ones are returned to the roller mill or grinder.

Particles will become fl y ash if they are small enough. UBC ranging from 0.5% to 5% is considered acceptable. Therefore, the dimensions of the solid fuel particle is an important parameter for the control of air pollution. Great attention must be dedicated to the control of air to fuel stoichiometry and to the combustion temperature to minimize unburned carbon in the ash. Notably, biomass represent an important solid fuel. However, in general, it cannot be pulverized to small particles, but can burn to ash in a short time.

Even if particulate emissions from solid fuel are a serious problem, it exists a set of technologies to control them (slag tap or ash pit, baghouse, and/or electrostatic precipitator).

Liquid FuelsLiquid fuels burn like the volatile fraction of solid fuel provided that the droplets are

small enough. Liquid fuels usually have less nitrogen content than solid fuels. Combustion of liquids and gases can be controlled much more readily than that of solid fuel. Optimised combustion of liquids can be performed without residual ash. The fuel-air ratio can be used to control combustion temperature and therfore to minimize NOx generation. The fl ame can be well defi ned and combustion is essentially completed within the fl ame.

Semi-solid FuelsSemi-solid fuels are residuals from refi neries. They are not clean burning fuels

like distillates and often are not even liquid at room temperature. Many impurities, typically found in crude oil, are concentrated in semi-solid residual fuel. These fuels can contain more nitrogen than coal, but usually contain less sulfur (28). Therefore, semi-solid fuels are intermediate between coal and oil.

Gas FuelNatural gas is desulfurized before it is sent in a pipeline. Therefore, natural gas

has almost no sulfur, essentially no impurities, and no ash. The only thing that varies is heat content per cubic meter. This variance is caused by natural gas producers supplementing natural gas with propane, liquefi ed petroleum gas (butane), carbon monoxide, or other gaseous fuel. As a result, air to fuel ratio must be carfully cheked to prevent changes in the stoichiometric ratio.


28


3.2.6 Fluidized Bed Combustion3.2.6 Fluidized Bed Combustion

Fluidized bed combustion occurs in a bed of crushed coal that has air fl owing upward through it to make coal particles behave like a fl uid. Boiler pipes can be either submerged in the bed or exposed to the hot gases after they leave the bed. The fl uidized bed is temperature controlled (Method 1). The bed is also a chemically reducing region in which available oxygen is consumed by carbon (Method 3) that reduces ionization of nitrogen. Excess of air is injected (Method 2) over the fl uidized bed to complete combustion of CO and other burnables.In fl uidized bed combustion it is possible to cold pulverized limestone (Method 6) to control NOx emission. Notably, sulfur oxides, eventuallly produced during the combustion can react with the limestone to form gypsum, a marketable product. Therefore, gypsum must be separated from the ash. As a result, NOx generation can be essentially limited to prompt NOx and fuel NOx .

3.2.7 Combined Technology Approaches3.2.7 Combined Technology Approaches

The choice of technologies depends upon the type of combustion system, type of boiler or other energy conversion device, and type of fuel used. Available technologies will be narrowed by consideration of turndown ratio, stability of combustion, availability or access to burners, air supply controls, fuel impurities, and cost among other factors. There are many examples and here are a few of them. Selective catalytic reduction of NOx to N2 can be followed by selective oxidation of sulfur dioxide to sulfur trioxide. Then sulfuric acid is formed followed by scrubbing sulfuric acid from the fl ue gas (29). LNB can be used in conjunction with SCR or SNCR to achieve a greater overall DRE than any of these can achieve alone. Water/steam injection can be used with SCR to achieve a DRE greater than SCR can achieve alone. Fuel reburning and SCR can be used together as well as separately, to get the maximum NOx reduction (30).


29


3.3 Selected Available Technologies and Supplier References3.3 Selected Available Technologies and Supplier References

No. 1Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Less excess air (LEA)Application: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers,

dry bottom tangentially fi red boilers, wet bottom (slag tap boilers), fl uidized bed combustion, stokers with traveling grates and stokers with spread grates.

Description: Reduces oxygen availability: Excess airfl ow for combustion has been correlated the with amount of NOx generated. A net excess airfl ow lower than 2% can strongly limit NOx content of fl ue gas. Although there are fuel-rich and fuel-lean zones in the combustion region, the overall net excess air is limited when using this approach.

Advantages: Easy modifi cation.Disadvantages: Low NOx reduction, reduction combustion effi ciency.Ref./Source: www.cleaver-brooks.com/Emissions1.html

No. 2Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Burners out of service (BOOS)Application: All fuels, multiple burners for BOOS, used for dry bottom wall-fi red, front-

fi red or opposed-fi red boilers.Description: Staged combustion: multiple-burner equipment can have part of an array of

burners with some “burners out of service” (not feeding fuel, but supplying air or fl ue gas). This allows the burners around them to supply fuel and air to air or fl ue gas fl owing from the BOOS. The result is a multi stage combustion process by stages with temperatures always lower than when all burners are in service. Therrefore, thermal NOx emission are lower. The degree of NOx reduction depends upon the spatial relationship betwen the BOOS and the other burners.

Advantages: Low cost, no capital cost for BOOS.Disadvantages: Higher air fl ow for CORef./Source: Methods for Reducing NOx Emissions, C. Latta, Roy F. Weston Inc., Plant

Engineering, September 1998.

No. 3Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Over fi re air (OFA)Application: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers,

dry bottom tangentially fi red boilers, wet bottom (slag tap boilers), fl uidized bed combustion, stokers with traveling grates and stokers with spread grates.

Description: The OFA completes the combustion. When the primary combustion is carried out with a fuel-rich mixture. The temperature i keept low by working under off-stoichiometric combustion. The unburned fuel is fully oxidised in the OFA.

Advantages: Low operating cost.Disadvantages: High capital cost.


30


No. 4Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Low NOx burner (LNB)Application: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers

and stokers with spread grates.Description: A LNB provides a stable fl ame that has several different zones. For example,

the fi rst zone can be primary combustion. The second zone can be Fuel Reburning (FR) with fuel added to chemically reduce NOx. The third zone can be the fi nal combustion in low excess air to limit the temperature. There are many variations on the LNB design. The LNB has produced up to 80% DRE. This can be one of the less expensive pollution prevention technologies with high DRE.

Advantages: Low operating cost, compatible FGR.Disadvantages: Moderately high capital cost.Ref./Source: Gas Reburning for High Effi ciency NOx Control Boiler Durability Assessment,

B. A. Folsom, T. Sommer, D. Engelhardt, and S. Freedman, 96-RP139.04, Air & Waste Management Assn. Regulatory Developments in NOx Controls for Utility Boilers, C. Harrison, 96-RP139.03, Air & Waste Management Assn.

Overview of NOx Emission Control for Utility Boilers, J. E. Staudt, Proceedings of the American Power Conference, 1993.

Select the Right NOx Control Technology, S. Wood, Chemical Engineering Progress, January 1994.

No. 5Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Flue gas recirculation (FGR)Application: All fuels, low nitrogen fuel, used for dry bottom wall-fi red, front-fi red or

opposed-fi red boilers, bottom tangentially fi red boilers, wet bottom (slag tap) boilers. Fluidized bed combustion and stokers with traveling grates.

Description: < 30% fl ue gas recirculated with air, decreasing temperature: decreasing recirculation of cooled fl ue gas reduces temperature by diluting the oxygen content of combustion air and by causing heat to be diluted in a greater mass of fl ue gas. Heat in the fl ue gas can be recovered by a heat exchanger. This reduction of temperature lowers the NOx concentration. If combustion temperature is held down to below 760° C, thermal NOx formation can be negligible.

Advantages: High NOx reduction, potential for low nitrogen fuels.Disadvantages: Moderately high capital cost. And operating cost, affects heat transfer and

system pressures.Ref./Source: Fossil Fuel Combustion, A Source Book, W. Bartok and A. Sarofi m, Wiley-

Interscience, John Wiley & Sons.

No. 6Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Water or steam injectionApplication: All fuels, low nitrogen fuel, used for dry bottom wall-fi red, front-fi red or

opposed-fi red boilers, bottom tangentially fi red boilers, wet bottom (slag tap) boilers and stokers with traveling grates.

Description: Injection of water or steam causes the stoichiometry of the mixture to be changed and adds steam modifi cs the reaction stoichiometry and reduces the working temperature. Both of these actions leads to lower thermal NOx emissions.


31


Advantages: Moderate capital cost, NOx reduction similar to FGR.Disadvantages: Effi ciency penalty.

No. 7Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Reduced air preheatApplication: All fuels, low nitrogen fuel, used for dry bottom wall-fi red, front-fi red or

opposed-fi red boilers, bottom tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates and stokers with spread grates

Description: Air is usually preheated to cool the fl ue gases, reduce the heat losses, and gain effi ciency. However, this can raise the temperature of combustion favoring NOx formation. By reducing air preheat, the combustion temperature is lowered and NOx formation is reduced. This can lower effi ciency, but can limit NOx generation.

Advantages: High NOx reduction potential.Disadvantages: Signifi cant effi ciency loss.

No. 8Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Fuel reburning (FR)Application: All fuels, pulverized solid, used for bottom tangentially fi red boilers, wet

bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates and stokers with spread grates.

Description: Recirculation of cooled fl ue gas with added fuel (this can be natural gas, pulverized coal, or even oil spray) causes heat dilution, similar to that abserved in the case of FGR, and a reduction of the primary combustion temperature. When the FR is added in a secondary combustion stage, the presence of fuel chemically reduces newly generated NOx to molecular nitrogen. The fuel is only partially consumed in the reduction NOx of and its burning is completed in a subseguent stage using either combustion air nozzles or over-fi re-air. This technique has been demonstrated to be effective with residence times from 0.2 seconds to 1.2 seconds and has achieved up to 76% DRE.

Advantages: Moderate cost, moderate NOx removal.Disadvantages: Longer residence times.Ref./Source: Gas Reburning for High Effi ciency NOx Control Boiler Durability Assessment,

B. A. Folsom, T. Sommer, D. Engelhardt, and S. Freedman, 96-RP139.04, Air & Waste Management Assn.

No. 9Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Combustion optimizationApplication: Gas liquid fuel, used for dry bottom wall-fi red, front-fi red or opposed-fi red

boilers, stokers with traveling grates and stokers with spread grates.Description: Combustion optimization refers to the active control of combustion. In a

natural gas fi red boiler, by decreasing combustion effi ciency from 100% to 99%, NOx generation dropped to a much more acceptable level. For coal-fi red boilers a 20% to 60% reduction in NOx has been experienced. These active combustion control measures seek to fi nd optimum combustion effi ciency and to control combustion (and hence emissions) at that effi ciency. Another approach uses a neural network computer program to fi nd the optimum


32


control point. Still another approach is to use software to optimize inputs for the defi ned output. One vendor decreases the amount of air that is pre-mixed with fuel from the stoichiometric ratio (ratio that produces the hottest fl ame) to lengthen the fl ame at the burner and reduce the rate of heat release per unit volume. This can work where the boiler tubes are far enough away from the burner. Carbon monoxide, unburned fuel, and partially burned fuel that result can then be subsequently oxidized in over-fi re-air at a lower temperature. Combustion must be optimized for the conditions that are encountered. 50% DRE has been reported.

Advantages: Minimal cost.Disadvantages: Longer residence times.Ref./Source: www.marathonmonitors.com www.ucicl.eng.uci.edu/research/active-control/ GNOCIS - 1999 Update on the Generic NOx Control Intelligent System,

G. Warriner, AJA.Sorge, M. Slatsky, J. Noblett, J. Stallings, EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999.

Obtaining Reduced NOx and Improved Effi ciency Using Advanced Empirical Optimization on a Boiler Operated in Load-Following Mode, P. Patterson, EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999.

No. 10Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Air stagingApplication: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers,

dry bottom tangentially fi red boilers, wet bottom (slag tap) boilers, stokers with traveling grates and stokers with spread grates.

Description: Combustion air is divided into two streams. The fi rst stream is mixed with fuel in a ratio that produces a reduced fl ame. The second stream is injected downstream of the fl ame and makes the net ratio slightly rich air in DRE up to 99% have been reported.

Advantages: Reduce peak combustion temperature.Disadvantages: Extend combustion to a longer residence time at lower temperatureRef./Source Maximum Achievable Control Technology for NOx Emissions from Thermal

Oxidation, P.Nutcher and D. Lewandowski, 94-WA74A.03, Air and Waste Management Association, Annual Meeting, Cincinnati, OH, June 19-24 1994.

No. 11Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Fuel stagingApplication: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers,

dry bottom tangentially fi red boilers, wet bottom (slag tap) boilers, stokers with traveling grates and stokers with spread grates.

Description: This Technologyis is based on the staging of combustion. Fuel is divided into two streams. The fi rst stream feeds primary combustion that operates in a reducing fuel to air ratio. The second stream is injected downstream of primary combustion, causing the net fuel to air ratio to be only slightly oxidizing. Excess fuel in primary combustion dilutes heat to reduce temperature. The second stream oxidizes the fuel while reducing the NOx to N2. This is reported to achieve a 50% DRE.

Advantages: Reduce peak combustion temperature.


33


Disadvantages: Extend combustion to a longer residence time at lower temperature.Ref./Source: Maximum Achievable Control Technology for NOx Emissions from Thermal

Oxidation, P. Nutcher and D. Lewandowski, 94-WA74A.03, Air and Waste Management Association, Annual Meeting, Cincinnati, OH, June 19-24, 1994.

No. 12Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Oxygen instead of airApplication: All fuels, used for wet bottom (slag tap) boilers.Description: Use oxygen to oxidize fuel: an example of this is a cyclone burner where the

fl ame is short and intense. This technique has reduced NOx by up to 20% in burners using conventional fuel. This technique also is usable with low-NOx burners to prevent the prompt NOx from being formed.

Advantages: Moderate to high cost, intense combustion.Disadvantages: Eliminates only prompt NOx, furnace alteration.Ref./Source: Reducing NOx Emissions, B. Chambers, Glass Industry, May 1993. Nitrogen Oxides Control Technology Fact Book, Leslie L. Sloss, Noyes Data

Corp., 1992.

No. 13Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Inject oxidantApplication: All fuels, used for dry bottom wall-fi red, front-fi red or opposed-fi red boilers,

stokers with traveling grates and stokers with spreader grates.Description: Chemical oxidant injected in fl ow: The oxidation of nitrogen to its higher

valence states makes NOx soluble in water. Gas absorbers can be effective used. Many oxidants have been injected into the airfl ow such as ozone, ion-ized oxygen, or hydrogen peroxide. Non-thermal plasma generating oxygen ions within the air fl ow have been also used. Water, hydrogen peroxide, or an alkaline fl uid can effectively absorb nitric acid.

Advantages: Moderate cost.Disadvantages: Nitric acid removal.

No. 14Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Catalytic combustionApplication: Gas liquid fuels.Description: Catalytic combustion at occurs lower temperature with respect to the

uncatalysed reach an the resulting reduction in the working temperature leads to a signifi cant NOx reduction. This technique is not used often because it is very load sensitive. However, where it is used, catalytic combustion can achieve less than 1-ppm concentration of NOx in the fl ue gas.

Advantages: Lowest possible NOx emissions.Disadvantages: Very high capital cost, high operating cost, catalyst deactivation.Ref./Source: Low-Emission Gas Turbines Using Catalytic Combustion, S. Vatcha, Energy

Conversion Management, Vol. 38 No. 10-13, pp. 1327-1334.


34


No. 15Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Ultra-low nitrogen fuelsApplication: All ultra-low nitrogen fuels, dry bottom wall-fi red, front-fi red or opposed-fi red

boilers, dry bottom tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates and stokers with spreader grates.

Description: Uses low-nitrogen fuel: These fuels can avoid NOx that results from nitrogen contained in conventional fuels. Up to 70% reduction in NOx emissions has been reported. Now there are ultra-low-nitrogen liquid fuel oils. These oils contain 15-20 times less nitrogen than standard fuel oil. This oil is now commercially available and competitively priced. Ultra-low-nitrogen oil is most frequently used in Southern California where the air pollution is particularly a problem. Natural gas can be considered a low-nitrogen fuel. Coke (the quenched char from coal) can also be an ultra-low-nitrogen fuel because nitrogen in the volatile fraction of the coal is removed in making coke.

Advantages: Eliminates fuel NOx, No capital cost.Disadvantages: Slight rise in operating cost.Ref./Source: www.cleaver-brooks.com/Emissions1.html

No. 16Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Non-thermal plasma reactor (NTPR)Application: Dry bottom wall-fi red, front-fi red or opposed-fi red boilers, dry bottom

tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates and stokers with spreader grates.

Description: Reducing agent ionized or oxidant created in fl ow: Using methane and hexane as reducing agents, non-thermal plasma has been shown to remove NOx in a laboratory setting with a reactor duct only 2 feet long. A transient high voltage that created non-thermal plasma ionized the reducing agents. The ionized reducing agents reacted with NOx and achieved a 94% DRE. There are indications that an even higher DRE can be achieved. A successful commercial vendor uses ammonia as a reducing agent to react with NOx in an electron beam generated plasma. Such a short reactor can meet available space requirements for virtually any plant. The non-thermal plasma reactor could also be used without reducing agent to generate ozone and use that ozone to raise the valence of nitrogen for subsequent absorption as nitric acid.

Advantages: Moderate cost, easy siting, high NOx removal.Disadvantages: Fouling possible, ozone emission possible.

No. 17Classifi cation of technology: Air Pollution (ADD-ON Control)Name of technology/technique: Selective catalytic reduction (SCR)Application: Dry bottom wall-fi red, front-fi red or opposed-fi red boilers, dry bottom

tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates and stokers with spreader grates.

Description: Catalyst located in the air fl ow, promotes reaction between ammonia and NOx: SCR uses a catalyst to react injected ammonia to chemically reduce NOx. It can achieve up to a 94% DRE and is one of the most effective NOx abatement techniques. However, this technology has a high initial


35


cost. In addition, catalysts have a fi nite life in fl ue gas and some ammonia “slips through” without being reacted. SCR can used precious metal based catalysts or base metal and zeolite based catalysts. However, the base metal and zeolite catalysts operate at higher temperatures then the precious metal catalysts.

Advantages: High NOx removalDisadvantages: Very high capital cost, high operating cost, catalyst siting, increased Pressure drop possible water wash required.Ref./Source: Selective Catalytic Reduction Control of NOx Emissions, SCR Committee of

Institute of Clean Air Companies, November 1997.

No. 18Classifi cation of technology: Air Pollution (ADD-ON Control)Name of technology/technique: Selective non-catalytic reduction (SNCR)Application: Dry bottom wall-fi red, front-fi red or opposed-fi red boilers, dry bottom

tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates, stokers with spreader grates.

Description: In SNCR ammonia or urea is injected within a boiler or in ducts in a region where temperature is between 900 oC and 1100 oC. This technology is based on temperature ionizing the ammonia or urea instead of using a catalyst or non-thermal plasma. It is necessary to control the working temperature windows to avoid ammonia slip or high NOx emissions. The temperature “window” is different for urea and ammonia. Reduction of the NOx by SNCR can have up to a 70% DRE.

Advantages: Using urea-low capital cost moderate NOx removal, Non-toxic chemical. Using ammonia: low operating cost, Moderate NOx removal.

Disadvantages: Temperature dependent NOx reduction, moderately high capital cost, area or ammonia storage, handling, injection system.

Ref./Source: Nitrogen Oxides Control Technology Fact Book, Leslie L. Sloss, Noyes Data Corp., 1992.

Selective Non-Catalytic Reduction for Controlling NOx Emissions, SNCR Committee of Institute of Clean Air Companies, October 1997

www.cleaver-brooks.com/Emissions1.html.

No. 19Classifi cation of technology: Air Pollution (ADD-ON Control)Name of technology/technique: Sorption-both! adsorption and absorptionApplication: Dry bottom wall-fi red, front-fi red or opposed-fi red boilers, dry bottom

tangentially fi red boilers, wet bottom (slag tap) boilers, fl uidized bed combustion, stokers with traveling grates, stokers with spreader grates. 1) Use sorbents in combustion, 2) duct to baghouse, 3) due to electrostatic precipitator.

Description: Use a chemical to absorb NOx or an adsorber to hold it: Several methods are used to inject and remove adsorbent or absorbent. One-method sprays dry powdered limestone into the fl ue gas. The limestone then reacts with both sulfuric acid and nitric acid. There is a spray dryer approach that consist of spraying slurry of powdered limestone and aqueous ammonia into the fl ue gas. The limestone preferentially reacts with the sulfur while the ammonia preferentially reacts with the NOx. In-duct injection of dry sorbents is another example of this technique and can reduce pollutants in three stages: (1) in the combustion chamber, (2) in the fl ue gas duct leading to the baghouse, and (3) in the fl ue gas duct leading to the electrostatic precipitator. The by


36


products formed by sorption are gypsum (calcium sulfate) that could be sold to make wallboard, and ammonium nitrate that can be sold to make either explosive or a fertilizer. Sorption is reported to have up to a 60% DRE. Another version uses carbon injected into the air fl ow to fi nish the capture of NOx. The carbon is captured in either the baghouse or the electrostatic precipitator (ESP) just like other sorbents. There are many absorbents and adsorbents available.

Advantages: Can control other pollutants as well as NOx. Moderate operating cost.Disadvantages: Cost of handling sorbent, space for the sorbent, storage and handling.Ref./Source: Nitrogen Oxides Control Technology Fact Book, Leslie L. Sloss, Noyes Data

Corp., 1992.

No. 20Classifi cation of technology: Air Pollution (Add-on Control)Name of technology/technique: Selective catalytic reductionApplication: Internal CombustionDescription: As with boilers, SCR can be used to obtain up to a 90% DRE of NOx. When

used with a LNB or steam/water injection, NOx can be reduced to 5-10 ppm. With compression ignition engines, zeolite catalysts achieve a DRE of 90+%, while base-metal catalysts can achieve a 80% to 90% DRE.

Advantages: 90% effi ciency can be achieved.Disadvantages: Operating cost.Ref./Source: Low-Emission Gas Turbines Using Catalytic Combustion, S. Vatcha, Energy

Conversion Management, Vol. 38 No. 10-13, pp. 1327-1334. Alternative Control Techniques Document -- NOx Emissions from Stationary

Reciprocating Internal Combustion Engines. EPA-453/R-93-032, July 1993.

No. 21Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Steam injectionApplication: Reduce the concentration of NOx discharged by gas turbine from

200ppm to 6ppm in accordance with the pollution prevention. Chemical; Petrochemical.

Description: Steam injection was provided in the combustion chamber of the gas turbine and the fl ue gas denigration system was installed in the exhaust gas boiler as a measure for reducing the amount of NOx discharge that was increasing in line with the installation of gas turbine equipment.1. Suppression of NOx generation by injecting steam into the gas turbine. Fuel/Steam = 1/1 ~ 1/1.2

2. Denigration by the ammonia catalytic reduction method.Catalyst : 4NO + 4NH3 + O2 → 4N2 + 6H2OCatalyst : NO + NO2 + 2NH3 → 2N2 + 3H2O

Implementing company: Ishihara Sangyo Co., Ltd., Yokkaichi Plant.

No. 22Classifi cation of technology: Air Pollution (Add – on Control)Name of technology/technique: Combined techniquesApplication:. Reduce NOx, SOx and soot in fl ue gas discharged from chimney stacks.Description: Flue gas is passed between multiple electrodes charged with high voltage

where soot particles in the fl ue gas are collected by charging the particles


37


negatively and attracting them to the positive electrode. The particles adhering to the electrode are removed mechanically by tapping with a hammer.1. Denigration (Dry ammonia catalytic reduction method) 4NO + 4NH3 + O2 → 4N2 + 6H2O

2. Desulphurization (magnesium hydroxide method) SO2 + Mg (OH)2 → MgSO3 + H2O 2MgSO3 + O2 → 2MgSO4

3. Soot ( Electric precipitator )Advantages: High Effi ciency Denigration (80%) Desulphurization (99%) Soot removed

(96%)Implementing company: Mitsubishi Chemical Co., Ltd.

No. 23Classifi cation of technology: Air Pollution (Add – on Control)Name of technology/technique: Removal of exhaust NOx by ammonia catalytic reduction

method (SCR)Application: Chemicals, petrochemical. As a result of changing the boiler fuel from heavy

oil to petroleum coke, the concentration of NOx in the exhaust gas increased to the nitrogen content in the fuel. Using the dry ammonia contact reduction method therefore carried out removal of NOx.

Description: Use a high performance honeycomb type catalyst.4NO + 4NH3 + O2 → 4N2 + 6H2O

Advantages: Removal of 85% of NOx generated in the boiler furnace in line with witching of its fuel.

Implementing company: Tosoh Company, Ltd. Yokkaichi Plant

No. 24Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Low NOx combustion technologyPreface: Low NOx combustion technologies include the following technologies.

Low NOx burner (aerodynamically controlled tope and split fl ame type) Combustion modifi cation such as air staging and fl ue gas mixingIFNR (In-Furnace NOx Reduction) combustion.

Application: Boilers for the utility thermal power plant and Boilers for industrial-owned thermal power plant.

Description: 1. Low NOx Burner Low NOx type burners control the strength of the fuel mixing with the

combustion air and realize fuel rich region locally in the process fi ring fuels. The split fl ame type burner is of off-stoichiometric fi ring type and is only adopted in fi ring fuel oils. DF (Dual Flow) type low NOx burner is of an aerodynamically mixing control type and is used for all fuels.

2. Combustion Modifi cation The air staging and the fl ue gas mixing technologies are adopted to reduce

NOx emissions in the combustion modifi cation. On staged fi ring, a part of combustion air is branched from the stream to the burner fi ring system, and introduced to the furnace through OAP’s (Overfi re Air Ports), distributed at the down stream of the burner region. Consequently, fuel rich fi ring can be realized by air staging and NOx formation can be therefore reduced. The fl ue gas mixing technology gives the decrease of the fl ame temperature. Lowering fl ame temperature, while fuel-NOx, originated by fuel-bound nitrogen, is insensitive to fl ame temperature, signifi cantly reduces thermal-


38


NOx originated by molecular nitrogen in the combustion air. This system needs a fan for recirculating the fl ue gas.

3. IFNR (in-furnace NOx reduction) NOx can be decomposed into molecular nitrogen through the reaction of CHi + NO-> HCN-> NHi-> N2

in the fi ring region. Here, CHi denotes the hydrocarbon species yielded in the process of fuel oxidation. Fuel rich fi ring will accelerate the reaction above, because the hydrocarbon species yield more under such conditions. IFNR technology adopts this concept of NOx decomposition. IFNR comprise of IAP’s (Interstage Air Pons) and OAP’s (Overfi re Air Ports). A part of combustion air also introduce to the furnace through IAP’s as well as that through OAP’s. IAP’s play a role accelerating both reactions for fuel oxidation and NOx decomposition, latter of which means the conversion from the intermediate nitrogen species (HCN, NHi) to N2.

Performance: The effi ciencies of NOx reduction are roughly shown below for each of low NOx technology.

Low NOx Burner: split fl ame oil atomizer: 20 - 30% depends on fuel nitrogen contentDF (dual fl ow) burner: 20 - 50% depends on the type of fuelCombustion Modifi cation: air staging: 20 - 50% depends on OAP capacityand the type of fuelfl ue gas mixing : 10 - 50%, depends on GM fan capacity and the type of fuelIFNR: IFNR normally used with other NOx reduction technologies, andfollowing NOx emissions have been already achieved.Gas fi ring: 15 - 20ppm (O2 5% corrected)Oil fi ring: 45 - 80ppm (O2 4% corrected) depends on fuel nitrogen contentCoal fi ring: 100- 200ppm (O2 6% corrected), depends on coal properties

Ref./Source: http://nett21.unep.or.jp/CTT_DATA/Contents4.html

No. 25Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: Combustion improvement systemDescription: Combustion air is mixed with portion of fl ue gas to slow combustion,

avoiding local rise in temperature and thereby reducing NOx generation. Part of the fuel is used as reducing agent to reduce NO generated in the combustion chamber, thereby reducing NOx generation.

Advantages: Established and widely employed technologyCapacity: NOx reduction effi ciency: Approx. 20 - 50% by combination of these

methodsApplication: Fossil power stations etc. (The Kansai Electric Power Co., Inc.)Source/Ref : http://nett21.unep.or.jp/CTT_DATA/Contents4.html

No. 26Classifi cation of technology: Air Pollution (Prevention Method)

Name of technology/technique: Low NOx burner for industrial furnaceDescription: Principle of Low NOx Burner. Slow combustion has been adapted as a heating effi ciency measure by

suppression of mixing of fuel and air during the initial stage of combustion. The direction of fuel injection is at an angle to the air stream axis, and the mixing of fuel and air in the primary combustion zone is limited. Therefore a local high temperature region cannot develop. Fuel, which is not burned in this zone gradually, mixes with air not used in the initial stages and


39


thus combustion is completed. It is therefore possible to obtain uniform combustion. The drop both in the maximum fl ame temperature and local oxygen partial pressure effectively suppresses thermal NOx and fuel NOx and thus the NOx value is drastically reduced.

Advantages: This burner is very simple, modifi cation of the burner refractory quarl is unnecessary and cost is very low. It is fully compatible with energy saving and with low NOx emission. It can be easily mounted on existing furnaces. NOx concentration decreased to a value less than half that obtained using a conventional burner.

Application: Applied to reheating furnaces, soaking pits, heavy forge furnaces and melting furnaces.

Implementing company: KOBE STEEL, LTD.Ref/Source: http://nett21.unep.or.jp/CTT_DATA/Contents4.html

No. 27Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: 3-Stage low NOx combustion systemDescription: High performance, super low NOx system, a combination of “ordinary

combustion type” burner and HITACHI ZOSEN’s patented 3-stage combustion type low NOx system.

Advantages: 1. In case of 3-stage combustion system, reducing combustion is limited only to the secondary combustion zone, so that reducing atmosphere space is very narrow. Furthermore, since measure are taken so that combustion gas with a strong reducing capability does not directly strike the water wall, adverse effects from such things as reducing corrosion and slagging on the heating surfaces inside the furnace due to reducing atmosphere, even in the case of super low NOx combustion, are extremely small compared to other NOx reduction systems such as OFA combustion system”, which expose a large part of the furnace inside to reducing atmosphere.

2. Normal oxidizing combustion is performed in the primary combustion zone (main combustion zone) where most of the fuel burns and combustion effi ciency is very high. The unburnt part from the secondary fuel in the secondary combustion zone (reducing combustion zone) is burnt in the tertiary combustion zone (combustion completion zone). Consequently, combustion effi ciency in a 3-stage type low NOx boiler is as high as in ordinary boilers without NOx reduction.

3. Due to the features mentioned above, the NOx value in a 3-stage combustion type low NOx boiler can reach to 100 ppm or less with ordinary bituminous coal. The NOx value decreases if the combustion gas remains longer in the secondary combustion zone (reducing combustion zone). The dimensionless residence time of combustion gas in the reducing zone based on the “specifi ed time”, the time the combustion gas stays in the reducing zone of a specifi ed boiler. The heavy line in the fi gure indicates the guaranteed NOx value (with standard design) with ordinary bituminous coal. From this, the dimensionless reducing time will be set at about 1.1 for a boiler with a guaranteed NOx of 100 ppm. As just explained, this low NOx

combustion system consists of three stages, that is, primary, secondary and tertiary stage, and the following is an explanation of each stage.

Primary Combustion Zone In the “normal combustion type” non-low NOx burner, normal oxidizing

combustion with an air ratio of 1.0 or over is performed. The amount of


40


fuel here is 65-75% of total boiler fuel and the fl ame is raised to a very high temperature and a high combustion effi ciency is obtained because the minimum combustion air needed for complete combustion is applied and also fl ame length is adjusted to be as short as possible. Consequently, the fl ame in this zone is a bright “golden color” and good combustion can be confi rmed at a glance. Generation of NOx in the primary combustion area is not controlled at all and the aim is only to reach 100% combustion effi ciency.

Secondary Combustion Zone 25-35% of the fuel for the boiler is blown in from the secondary fuel port.

This secondary fuel includes air for transport but any other air is strictly excluded to improve its performance as a denitration agent. The denitration reaction is as follows:1. Since the secondary fuel port is located just above the burner, secondary fuel is heated intensely by the burner fl ame and hydrogen (H2), hydrocarbon (CmHn), carbon monoxide (CO), charcoal (C), etc. are formed as very active reducing components.2. The atmosphere in the center of the secondary combustion zone has very high reaction ability because the air ratio is about 0.7-0.8 of the theoretical air ratio.3. The atmosphere temperature in the secondary combustion zone is a very ideal at about 1400 °C so that the chemical reaction of the reducing components and NOx from the primary combustion zone occurs intensely.4. The secondary fuel is injected in stripes in the horizontal cross section of the furnace and there are plenty of secondary fuel ports for complete mixing of the secondary fuel and the combustion gas from the primary combustion zone.5. In the course of the process mentioned above, most of NOx in the primary combustion gas is instantly reduced to inactive N2 by hydrogen, hydrocarbon, carbon monoxide, charcoal, etc. which are formed from the secondary fuel.6. Since combustion in the secondary combustion zone takes place with insuffi cient air, there is almost no new generation of NOx while a sizable amount of unburnt components remains from the secondary fuel. Consequently, the longer the gas remains in the secondary combustion zone, the lower NOx becomes.Tertiary Combustion zone30-40% of all the air used for combustion is injected from the OFA port and the unburnt components from the secondary combustion zone are burnt. The OFA port is positioned so that it is in the ideal gas temperature zone from the viewpoint of improved combustion of the unburnt components and for control of new generation of thermal NOx . OFA is injected in stripes on the horizontal cross section of the furnace, and there are a number of OFA ports so the OFA is completely mixed with the combustion gas from the secondary combustion zone. In the tertiary combustion zone, combustion effi ciency increases in accordance with the increase of the space between the OFA ports and the furnace exit. However, this increases the size and weight of the boiler, so the size of this space is determined from the total cost including facility costs and running cost of the boiler.

Implementing company: HITACHI ZOSEN CORPORATIONRef./Source: http://nett21.unep.or.jp/CTT_DATA/Contents4.html


41


No. 28Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: High-temperature combustion catalystApplication: The technology has not been commercialized, but following applications

are expected. Gas turbine, Boiler, Gas-heater, Byproduct gas combustor, Off-gas treatment, Deodorizer

Description: In order to reduce NOx emission, integrated combustion techniques such as lean-burn combustion and water injection have been developed. However, it is diffi cult to reduce NOx emission to ultra-low concentration, because thermal NOx is formed by thermo-chemical reaction between nitrogen and oxygen in air. The NOx formation reaction strongly depends on temperature, thermal NOx is extensively produced in high temperature zone over than 1500 deg. °C inside a combustion fl ame.Catalyst combustion is one kind of fl ame-less combustion in which fuel is oxidized perfectly without high temperature zone and which scarcely produces thermal NOx .Catalyst is made of Mn-substituted hexa-aluminate (Sr0.8La0.2MnAl11Ol9-a

Catalyst powder is extruded to a honeycomb shape and sintered at high temperature. Honeycomb shape with 300cells/inch2 is used for natural gas combustor. In gas-turbine combustor, three types of catalyst honeycomb are combined. At the top of the catalyst bed, conventional noble metal catalyst honeycombs, which have a high reaction activity under low temperature, is located. Two types of hexa-aluminate honeycombs are used in the middle and bottom parts. Fuel is mixed with air, heated by pre-burner to around 450 °C, and oxidized smoothly in the catalyst bed in which temperature rises continuously to around 1200 °C.It was known that catalyst combustion was effective for low NOx combustion, but catalyst having high temperature durability over than 1000 °C was not developed. Mn-hexaaluminate is the only one catalyst which can be stably used over than 1000 °C. 8000 hours of catalyst life at 1300 °C is confi rmed by atmospheric combustion test, and 1000 hours of stable operation is also confi rmed at high pressure combustion test. When large diameter honeycomb is used for a large-scale combustor, formation of cracks on the honeycomb caused by thermal stress is an important problem. Segmentation system, which is effective in reducing thermal stress, is developed, and optimum shapes of honeycomb segments are designed with FEM simulation model.

Effi ciency: A test device for the 150kW prototype catalytic combustion turbine was prepared. In the test, turbine pressure ratio, air fl ow rate and combustor inlet air temperature were 8.5, 1.8 kg/s and 350 °C, respectively, and both combustion effi ciency higher than 99% and NOx emissions less than 40 ppm were attained after 215 hours of continuous operation. In addition no visible cracks was observed in either the catalyst honeycomb or the catalyst holder after the test.

Implementing company: KOBE STEEL, LTD.Ref./Source: http://nett21.unep.or.jp/CTT_DATA/Contents4.html

No. 29Classifi cation of technology: Air Pollution (Prevention Method)Name of technology/technique: The SCR DENOX process-haldortopsoeApplications: The SCR process can be applied in various applications and is especially

dedicated to high NOx -removal effi ciencies.


42


Topsoe has applied its SCR DENOx process to applications such as:Coal fi red Boilers, Oil fi red Boilers, Lignite fi red Boilers, Pet Coke fi red Boilers, Gas fi red Boilers, Diesel Engines, Gas Engines, Gas Turbines, Waste Incinerators

Description: The SCR technology was developed in Japan in the 1970s and fi rst implemented at fossil fuel-based power plants in major Japan cities, where the level of photochemical smog had reached unacceptable levels. Since then, the technology has gradually spread to the rest of the word where it has found such wide spread use that it today is the most widely applied technique for reducing NOx in fl ue gases and exhaust gases. Selective catalytic reduction, SCR, is considered the most effi cient commercially proven method of removing nitrogen oxides (NOx ) from off-gases. In the SCR DeNOx process the nitrogen oxides (NO2 and NO) are reacted with ammonia (NH3) over a catalyst to form harmless and naturally occurring nitrogen (N2) and water vapor (H2O):4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O6 NO2 + 8 NH3 → 7 N2 + 12 H2OThe SCR process basically consists of a catalytic reactor and an ammonia storage and injection system. The reducing agent can be either liquid, water-free ammonia under pressure or an aqueous ammonia solution at atmospheric pressure. A solution of urea can be used as well. In case liquid ammonia is used, the ammonia is volatilized and subsequently diluted with air before being injected into the exhaust gas duct.Chemicals PlantsEach application has its own characteristics and different demands to the DeNOx catalyst and technology. For example, in the high-dust process for coal-fi red boilers, the SCR reactor is placed immediately after the boiler and upstream of the electrostatic precipitator. The catalyst is exposed to heavily dust-laden fl ue gas and must be resistant to erosion and possible poisons in the fl y ash. The SCR process can, in principle, be applied on any fl ue gas containing NOx and oxygen.Topsoe SCR DENOX Catalysts DNX Series.Topsoe’s DNX-series of catalysts comprises SCR DENOX catalysts, tailored to suit a comprehensive range of process requirements.The DNX catalysts feature: High NOx removal activity, Low pressure drop, Low SO2 oxidation rate and Excellent durabilityDNX catalysts are based on a corrugated, fi ber reinforced titanium dioxide (TiO2) carrier. The carrier is impregnated with the active components: vanadium pentoxide (V2O5) and tungsten trioxide (WO3). The catalyst is shaped to a monolithic structure with a large number of parallel channels.

Effi ciency: The unique catalyst design provides a highly porous structure with a large surface area and an ensuing large number of active sites. The high and well-defi ned porosity is the key to:- A high NOx removal level with minimum ammonia slip- A low activity towards SO2 oxidation, minimizing the risk of fouling downstream equipment

- A high poison resistance ensuring a long and stable service life- A substantially lower weight than for conventional plate or extruded catalysts, allowing a fast response to changes in operation

To ensure optimum performance in any type of operating environment, the DNX catalysts are available in a wide range of channel sizes and chemical formulations.


43


Ref./Source: www.haldortopsoe.com

No. 30Classifi cation of technology: Air PollutionName of technology/technique: SNOX process-haldortopsoe

The SNOX process is an innovative process, which removes sulphur and nitrogen oxides from fl ue gases, recovers the sulphur oxides as concentrated sulphuric acid and reduces the nitrogen oxides to free nitrogen. The process is based on catalytic reactions and does not consume water, absorbents or chemicals, except for ammonia for the reduction of nitrogen oxides. Further it does not generate secondary sources of pollution such as wastewater, slurries, or solids, and it does not release CO2 to the atmosphere. Process heat and the heat content of the fl ue gas down to a temperature of 100°C are recovered and utilized in the boiler to increase steam production. Contrary to other fl ue gas cleaning processes the operating costs decrease with increasing sulphur content in the fuel. Thus, with the SNOx process high-sulphur fuels can be utilized in an environmentally acceptable and economically attractive way.Furthermore, the SNOx process accepts fl ue gas with high content of SO3.Apart from being an effi cient and cost effective tool in the abatement of air pollution, the SNOx process meets future regulations and standards for solid waste management, water pollution, and resource conservation.

Applications: The process is applicable to fl ue gases from power stations, industrial and institutional boilers as well as industrial off-gases containing sulphur and nitrogen oxides.Particulates contained in the fl ue gas are removed in a high effi ciently fabric fi lter or electrostatic precipitator. The fl ue gas is heated in a rotary heat exchanger with fl ue gas from the SO2 converter. In the catalytic DENOX reactor the nitrogen oxides in the fl ue gas are reduced selectively with ammonia to free nitrogen. In the SO2 converter the sulphur dioxide is oxidized catalytically to sulphur trioxide, which is recovered in the WSA condenser as concentrated sulphuric acid. Ambient air preheated in the WSA condenser is used as combustion air for the boiler.The SNOx plant is fully automat zed and can be operated from the main control room of a power plant without requiring additional manpower. There is no hold-up of liquids or solids and response to variations in boiler load is very fast. The process is adaptable to new or retrofi t installations.All equipment, apart from the WSA condenser, is made of carbon steel or low alloy steel. The proprietary WSA condenser is in principle an air-cooled multi-tube falling-fi lm condenser with tubes of glass whereas other parts in contact with the acid are lined with acid proof bricks or coated with acid proof polymers. A SNOX plant has only few moving parts, resulting in low maintenance costs and high on-stream availability.95% of the sulphur contained in the fl ue gas is recovered as concentrated sulphuric acid of commercial grade.95% of the nitrogen oxides in the fl ue gas is reduced to free nitrogen.Fly ash and trace metals in the fl ue gas are almost quantitatively removed.No waste products or wastewater is produced.Apart from ammonia, no absorbents or auxiliary chemicals are used in the process.


44


The heat content of the fl ue gas is utilized down to 100 °C and, together with the heat of sulphuric acid formation, it is used for increased air preheat and steam production.The ammonia slip and most carbonaceous combustibles in the fl ue gas are oxidized completely.The volume of DENOX catalyst required for the denitrifi cation of the fl ue gas is substantially smaller than for an SCR process or an SCR process in combination with other fl ue gas desulphurization processes.The process is fully automated, the plant contains only few moving parts, and there is no circulation of slurries or solids.The operating costs decrease with increasing sulphur content in the fl ue gas.

Ref./Source: www.haldortopsoe.com

3.4 Conclusions3.4 Conclusions

The design of the boiler, internal combustion engine, or gas turbine has a major effect on the operation. NOx formation tends to increase with an increase in boiler capacity, because larger boilers tend to have more intense combustion with higher combustion temperatures and longer residence time for fl ue gases. The same appears to be true for engines and turbines.

Different fuels require different combustion, abatement and control techniques. For example different coals show a great variability in the content of volatile components. The nitrogen content of fuel is important, as are the content of sulfur, lead, mercury and other contaminants. Ultra-low nitrogen content fuels have been developed and are already cost competitive. Thus, we can achieve some control of NOx from the lowered concentration of nitrogen in the fuel without investing in changed burner design.

Tandem application (or use of hybrid control technology) of NOx control techniques (fi rst SNCR, then SCR in the duct, and then sorption before the ESP have been used to achieve an overall reduction of 90+% in NOx and 80% in SOx.

Combustion of natural gas and petroleum distillates can be controlled in the same way as pulverized coal. The major differences between coal and natural gas or oil are that these better ones: (I) generally are lower in sulfur and ash; (II) usually are lower in nitrogen; and (III) probably are lower in lead and mercury. Thus, gas and oil do not deactivate a catalyst used in Selective Catalytic Reduction (SCR) at the same rate that coal or semi-solid fuels do. The semi-solid petroleum products can actually have higher levels of sulfur, nitrogen and other impurities than coal. They do not have as much char or ash as coal, but have more than the lighter distillates.

Most of the NOx control systems are already available commercially available notably name of the considered technologies have proved to basses performances and a general applicably to all combustion system.

Please note that abatement and control of NOx from nitric acid manufacturing and “pickling”baths differs from abatement and control at combustion sources. Combustion sources all have NOx in a large fl ow of fl ue gas, while nitric acid manufacturing plants and pickling baths try to contain the NOx. Wet scrubbers (absorbers) can control NOx emissions from acid plants and pickling, and can use either alkali in water, water alone, or hydrogen peroxide as the liquid that captures the NOx. The wet scrubber operates by liquid fl owing downward by gravity through a packing medium, opposed by an upward fl ow of gas. Scrubbers operate on the interchange of substances between gas and liquid. This requires that the height of the absorber, type of packing, liquid fl ow, liquid properties, gas properties, and gas fl ow should collectively cause a scrubber to have the desired control effi ciency.


45



1. Nitrogen Oxides: Impacts on Public Health and the Environment, EPA 452/R-97-002.

2. Selective Catalytic Reduction Control of NOx Emissions, SCR Committee of Institute of Clean Air Companies, November 1997.

3. Nitrogen Oxides Control Technology Fact Book, Leslie L. Sloss, Noyes Data Corp., 1992.

4. NOx Control Technology Data, EPA 600/2-91-029.

5. Bauer F., Energy Engineering, 91 (1994) 17.

6. Alternate Control Techniques Document - NOx Emissions from Cement, Manufacturing EPA-453/R-94-004.

7. The Economic Feasibility of using Hydrogen Peroxide for the Ehanced Oxidation And Removal of Nitrogen Oxides from Coal-Fired Power Plant Flue Gases, Hatwood J., Cooper C., AWMA Journal, March 1998.

8. Simultaneous SO2, SO3, and NOx Removal by Commercial Application of the EBA Process, Hirano S., Aoki S., Izutsu M, and Yuki Y., EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999.

9. Update on SNAP Technology for Simultaneous SOx and NOx Removal, Felsvang K., Boscak V., Iversen S., Anderson P., EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999.

10. Iwamoto, M. and Hamada, H., Catal. Today, 10 (1991) 57.

11. Heck R.M. and Farrauto R.J., Catalytic air pollution control: commercial technologies, van Nostrand Reinhold, New York, 1995.

12. Zwinkels, M., Jaras, S., Menon, P., and Griffi n, T., Catal. Rev-Sci. Eng. 35(1993) 319.

13. Li, Y. and Armor, J.N., J.Catal., 150 (1994) 376.

14. Li, Y., Battavio, P.J. and Armor, J.N. (1993) J. Catal., 1993, 142, 561.

15. Alternative Control Techniques Document - NOx Emissions from Stationary Reciprocating Internal Combustion Engines, EPA 453/R-93-032.

16. Alternative Control Techniques Document - NOx Emissions from Stationary Reciprocating Internal Combustion Engines. EPA-453/R-93-032, July 1993.

17. Chess K., Yao S., Russell A. and Hsu H., Journal of the Air & Waste Management Association, 45 (1995) 627.

18. Anonimous, Oil & Gas Journal, 92 (1994) 134.

19. Manning M., Pipeline & Gas Journal, 222 (1995) 26.

20. Alternative Control Techniques Document - NOx Emissions from Stationary Reciprocating Internal Combustion Engines. EPA-453/R-93-032, July 1993.

21. Bartok, W., and Sarofi m, A.F., Fossil Fuel Combustion, John Wiley & Sons, New York, 1991

22. Kuehn S.E., Power Engineering, 98 (1994) 23-27.

23. Advanced Reburning for SIP Call NOx Control, EPRI-DOE-EPA Combined Utility Air Pollution Control Symposium: The MEGA Symposium, Atlanta, August 1999.

24. States’ Report on Nitrogen Oxides Reduction Technology Options for Application by the Ozone Transport Assessment Group, OTAG, 1996.

25. Alternative Control Techniques - Nitric Acid and Adipic Acid Manufacturing Plants EPA 450/3-91-026.



47


4 SO4 SOx Abatement Technologies Abatement Technologies

Sulfur oxides include sulfur dioxide (SO2), sulfur trioxide (SO3)SO2 is a colorless gas, which is moderately soluble in water and aqueous liquid.

The major source of sulfur dioxide is combustion of fossil fuels for generation of electric power. It accounts for 85% of the sulfur dioxide. Industrial processes such as nonferrous metal smelting contributes for about 8% to SO2 emissions and transportation for about 7 %. Once released to the atmosphere, SO2 reacts slowly to form H2SO3 and H2SO4, inorganic sulfate compounds, and organic sulfate compounds. Some of the SO2 is oxidized to SO3 at high temperature. Sulfur trioxide remains in the vapor state while the combustion gases are very hot. As the gases cool, most of the sulfur trioxide, which is extremely hygroscopic reaction with water to form sulfuric acid. Sulfuric acid vapor in moderate concentrations (2 to 8 ppm) is very benefi cial to electrostatic precipitators because it adsorb particle surfaces and creates a moderate resistively. High concentrations of H2SO4 can be detrimental to precipitator performances. High sulfuric acid levels can also cause signifi cant corrosion problems for precipitators, fabric fi lters, and other control devices. The temperature of fl ue gases should be kept well above the dew point for sulfuric acid to prevent condensation on ductwork surfaces and components in the air pollution control system. Sulfur oxides can also be released from chemical reactors plants that manufacture batteries and sulfuric acid plants. The sulfur in the fuel or waste being fi red enters the combustion process in a variety of chemical forms including but not limited to inorganic sulfates, organic sulfur compounds, and pyrite. A small fraction of the fuel or waste sulfur (usually less than fi ve percent) remains in the bottom ash leaving the combustion processes. The remaining 95+ percent is converted to sulfur dioxide, which remains in the gaseous form throughout the combustion system. A small fraction of the sulfur dioxide generated in the combustion zone is oxidized further to form sulfur trioxide. The reaction mechanisms that could contribute to the formation of this pollutant are not entirely known; however, they probably include the following:

• Free radical reaction of SO2 with atomic oxygen in the high temperature zones.• Catalytic oxidation of SO2 on the surfaces of particles entrained in the gas stream.• Thermal reactions between SO2 and other inorganic gases generated during

combustion.

The concentration of sulfur trioxide generated during combustion varies widely from unit to unit; however, sulfur trioxide concentrations are generally related directly to the concentration of sulfur in the fuel and the concentration of oxygen in the combustion zone. The sulfur trioxide concentrations are usually 0.5 to 2 percent of the sulfur dioxide concentration. Sulfur trioxide quickly converts to sulfuric acid upon cooling in the gas stream or atmosphere.


48

SOX ABATEMENT TECHNOLOGIES

4.1 Control of SO4.1 Control of SOx and H and H2SOSO4 Emission Emission

Air pollution control systems for SO2 removal are large and sophisticated. Sulfur dioxide is controlled by three different techniques: absorption, adsorption, and the use of low-sulfur fuels. The control systems used for SO2 are usually not designed to remove H2SO4. The sulfuric acid concentrations are usually below the levels where it is economically feasible or environmentally necessary to install control systems. Table 4.1 shows the list of desulfurization process and Table 4.2 shows the simplifi ed desulfurization process for developing countries.

Table 4.1: List of desulfurization processes

Wet Type Desulfurization process Absorbent By-product (treatment)Limestone (line) gypsum Limestone Gypsum (recovery) and

sludge (disposal)Indirect limestone (line) gypsum process (double alkali process)

Soda, ammonia and aluminum sulfate

Gypsum (recovery)

Valuable by-product recovery processes: soda absorption, magnesium absorption process

Caustic soda, magnesium and ammonia

(Recovery) Sodium sulfi te,sulfur/sulfuric acid and ammonia sulfate

Waste solution discharge process: soda absorption process and magnesium

Caustic soda and magnesium hydroxide

(Disposal)

Sodium sulfate and magnesium

Semi-dry type Spray drier process Slaked lime and hot water curing

Dry type (adsorption)

Lime process:

Intra-furnace fl ue blow-in process

Slaked (Disposal or lard reclamation) Calcium sulfi te and gypsum

Activated charcoal process, activated charcoal mobilized bred.

Activated charcoal Sulfuric acid

Sulfur and liquefi ed SO2)Electronic radiation process Ammonium sulfate

Fuel modifi cation

Coal preparation

Coal cleaning

Coal briquette including SOx absorbent

Table 4.2: Simplifi ed desulfurization process for developing countries*

1. Fuel modifi cationCoal preparationCoal cleaningCoal briquette including SOx absorbent2. Fluidized bed combustionCirculating fl uidized bed combustion (CFBC)Internally circulating fl uidized bed boiler (ICFB)

3. Simplifi ed fl ue gas desulfurization processSemi-dry typeSpray dryer typeSimplifi ed limestone-gypsum type.

*Advantages: Cost-effective for both initial and operating; easy to operate; 70-80% of SOx removal effi ciency


49


4.2 Absorption Techniques4.2 Absorption Techniques

Absorption processes use the solubility of sulfur dioxide in aqueous solutions to remove it from the gas stream. Once sulfur dioxide has dissolved in solution to form sulfurous acid (H2SO3), it reacts with oxidizers to form inorganic sulfi tes (SO3

2-) and sulfates (SO4

2-). This process prevents the dissolved sulfur dioxide from diffusing out of solution and being re-emitted. The most common type of sulfur dioxide absorber is the limestone-wet scrubber. (Figure 4.1). Limestone is the alkali most often used to react with the dissolved sulfur dioxide. Limestone slurry is sprayed into the sulfur dioxide-containing gas stream. The chemical reactions in the recirculating limestone slurry and reaction products must be carefully controlled in order to maintain the desired sulfur dioxide removal effi ciency and to prevent operating problems. Wet scrubbers used for sulfur dioxide control usually operate at liquid pH levels between 5 to 9 to maintain high effi ciency removal. Typical removal effi ciencies for sulfur dioxide in wet scrubbers range from 80 to 95%. Limestone gypsum process is used most commonly for treating a great volume of gas from utility boilers, etc. to cool fl ue gas by water spray. A prescrubber can be used to dust remove. Gas is then treated with limestone slurry in a subrequent scrubber. In this confi guration 85% to 95% of SO2 is removed (reaction 1 and 2). Calcium sulfi te generated by reaction is oxidized into gypsum by air bubbling (reaction 3).

1. CaO + SO2 = CaSO3

2. CaCO3 + SO2 = CaSO3 + CO2

3. CaSO3 + ½ O2 + 2 H2O = CaSO4 2H2O

Gypsum is separated from the solution with a thickener or centrifuge.

SlurryUnderflow

RecirculationTank

ClarifierClarifier

VacuumFilter

So

lid t

oD

isp

osa

l MixingTank

FeederFeeder

City Water

MistEliminator

Fan

AlkaliStorage

Tank(Powder)

FlueGas

Absorber

PumpPump

Pump

Pump

Pump

PurgeStream

Rec

ircu

lati

on

Slu

rry

Lim

e o

rL

imes

ton

e S

lurr

y

Figure 4.1: Scheme of a limestone wet scrubber system (adapted from ref 1)


50


Spray towers scrubbers have the best design for sulfur dioxide removal. They have a spray mechanism for introducing the slurry into the gas stream. Also, their simple, open design presents fewer opportunities for the alkaline slurry to plug parts. The residence time is also adequate for sulfur dioxide to be absorbed into the slurry droplets. Venturis scrubbers are not a good choice in this situation because the residence time is too short. In sulfur dioxide removal, there must be adequate contact between the pollutant gas and the alkaline slurry for at least a minimal length of time. While Venturi scrubbers can be used to remove both particulate and gaseous pollutants, they are primarily designed to maximize the collection of particles by creating a large difference in velocity between particles and liquid droplets in the gas stream. Venturi scrubbers also have large pressure drops, so they are more expensive to operate than spray towers. Impingement plate scrubbers are not good candidates in this situation because the alkaline slurry would plug the small holes in the impingement plates.

The wet scrubber (absorber) vessels do not effi ciently remove particulate matter smaller than approximately 5 micrometers. However, as in the case with low-effi ciency particulate wet scrubbers, the particulate removal effi ciency increases rapidly with particle size above 5 micrometers. Usually, a moderate-to-high effi ciency particulate control system is used upstream from the sulfur dioxide absorber to reduce the particulate matter emissions in the less than 3 micrometer size range. These upstream collectors also reduce the quantity of particulate matter that is captured in the absorber. The evaporation of water that occurs in wet scrubber vessels can keep gas temperatures relatively cold, in the range of 43 to 60 oC. These gas temperatures are well below the typical operating temperatures of other air pollution control systems used on sources that generate sulfur dioxide emissions. Another type of absorption system is called a spray atomizer dry scrubber, which belongs to a group of scrubbers called spray-dryer-type dry scrubbers (Figure 4.2). In this case, alkaline slurry is sprayed into the hot gas stream at a point upstream from the particulate control device. As the slurry droplets are evaporating, sulfur dioxide absorbs into the droplet and reacts with the dissolved and suspended alkaline material. Large spray dryer chambers are used to ensure that all of the slurry droplets evaporate to dryness prior to going to a high effi ciency particulate control system. The term “dry scrubber” refers to the condition of the dried particles approaching the particulate control system. Fabric fi lters or electrostatic precipitators are often used for high effi ciency particulate control. Spray-dryer-type absorption systems have effi ciencies that are similar to those for wet-scrubber-type absorption systems. These generate a waste stream that is dry and, therefore, easier to handle than the sludge generated in a wet scrubber. However, the equipment used to atomize the alkaline slurry is complicated and can require considerably more maintenance than the wet scrubber systems. Spray-dryer-type absorption systems operate at higher gas temperatures than wet scrubbers do and are less effective for the removal of other pollutants in the gas stream such as condensable particulate matter. The choice between a wet-scrubber absorption system and a spray-dryer absorption system depends primarily on site-specifi c costs. The options available for environmentally sound disposal of the waste products are also an important consideration in selecting the type of system for a specifi c application. Both types of systems are capable of providing high effi ciency sulfur dioxide removal.


51


Slaker

LimeSilo

Air PollutionControl SystemWaste Product

Truck Delivery ofCalcium Oxide

FabricFilter

Spray DryerAbsorber

FlueGasfromBoiler

Recycling System

(optional)

Lim

eS

lurr

y

SlurryPump

Calcium HydroxideSlurry Holding Tank

ScreenGrit to

Disposal

SlakingWater

DilutionWater

AtomizerFeed Tank

InducedDraft Fan

Slaker

LimeSiloLimeSilo

Air PollutionControl SystemWaste Product

Truck Delivery ofCalcium Oxide

FabricFilter

Spray DryerAbsorber

FlueGasfromBoiler

Recycling System

(optional)

Lim

eS

lurr

y

SlurryPump

Calcium HydroxideSlurry Holding Tank

ScreenGrit to

Disposal

SlakingWater

DilutionWater

AtomizerFeed Tank

InducedDraft Fan

Figure 4.2: Spray-dryer-type dry scrubber (adapted from ref 1)

4.3 Adsorption Technique4.3 Adsorption Technique

Sulfur dioxide can be collected by adsorption systems. In this type of control system, a dry alkaline powder is injected into the gas stream (Figure 4.3). Sulfur dioxide adsorbs to the surface of the alkaline particles and reacts to form compounds that cannot be re-emitted to the gas stream. Hydrated lime (calcium hydroxide) is the most commonly used alkali. However, a variety of alkalis can be used effectively. A dry-injection-type dry scrubber can be used on smaller systems as opposed to using the larger, more complicated spray-dryer-type dry scrubber. However, the dry injection system is slightly less effi cient, and requires more alkali per unit of sulfur dioxide (or other acid gas) collected. Accordingly, the waste disposal requirements and costs are higher for adsorption systems than absorption systems.

CombustionProcess

HeatExchanger

ParticulateControl Device

InducedDraft Fan

AlkaliStorage

Tank

Feeder

Blower

Solid ResidueRecycling System

(optional)

AcidGas

Note: flue gas recycle streamand heat not shown

Figure 4.3: A fl owchart for a dry-injection-type dry scrubber (adsorber) (adapted from ref 1)


52


4.4 Alternative Fuels4.4 Alternative Fuels

Other techniques used for limiting the emissions of sulfur dioxide are simply to switch to fuels that have less sulfur or to convert to synthetic (processed) fuels that have low sulfur levels. The sulfur dioxide emission rate is directly related to the sulfur levels in coal, oil, and synthetic fuels. However, not all boilers can use these types of fuels. Each type of boiler has a number of very specifi c and important fuel characteristic requirements and not all low sulfur fuels meet these fuel-burning characteristics.

4.5 Implemented or Commercial Technologies4.5 Implemented or Commercial Technologies

No. 1Classifi cation of technology: Air pollution controlName of technology/technique: Flue gas desulfurization (FGD)Application: Processing of SOx, NOx and dust discharged from Coal Burning Boilers.Description: Mitsubishi gas chemical to adapted FGD to reduce discharge of SOx, NOx and

dust. Flue gas desulfurization facilities, exhaust gas denitrizer and exhaust gas dust collector (bag fi lter) were installed in line with the installation of the private coal burning power generating plant.

1. Flue gas desulfurization facilities SO2 + Mg (OH)2 → MgSO3 + H2O

MgSO3 + ½ O2 → MgSO4

High desulfurization is realized compared with the lime gypsum method.Extended periods of continuous operation are possible.

2. Exhaust gas denitrizer. 4NO + 4 NH3 + O2 → 4N2 + 6H2O 2NO2 + 4 NH3 + O2 → 3N2 + 6H2O Dry type ammonia catalytic reduction method was adopted because it show

high level of reliability and safety, reaching some at the time high conversion effi ciency.

(3) Exhaust gas dust collector. The bag fi lter system was used since in the case of coal burning boilers, the

electric resistance of fl y ash in the exhaust gas is high and can cause the reverse electric dissociation leading to low dust-collecting effi ciency.

Advantages: Rate of removal: SO2 = 90%, NOx = 70%, Dust = 99%.Implementing company: Mitsubishi Gas Chemical Co., Ltd. Yokkaichi PlantRef./Source Sources. www.icett.or.jp/te

No. 2Classifi cation of technology: Air pollution controlName of technology/technique: Activated carbonApplication: Reduction of exhaust SO2.Description: Reduction of the amount of SO2 discharged, by installing the fl ue gas

desulfurization equipment in the factory, Nippon steel corporation installed a dry type fl ue gas desulfurization equipment with activated carbon. This system that does not require wastewater processing and is suitable for processing large quantities of exhaust gas.This process is based on adsorption/desorption reactions on activated carbon.

(I) Adsorption SO2 + H2O + ½ O2 → H2SO4

SO2 is adsorbed on the surface of the activated carbon and is oxidized to form sulfuric acid.


53


(II) H2SO4 + 1/2 C → SO2 + CO2 + H2O Desorption of SO2 occurs after heating to provote decomposition SO2

becomes one of the reagents for ammonium sulfate synthesis.Advantages: Amount of gas processed 900,000 mN3/h. Desulfurization effi ciency over

95%, Adsorption column 450,000 mN3/h x 2, Cross fl ow moving bed system, space velocity 800 h-1, Desorption column 15 T/H x 1, 350 oC Indirect heating moving bed type heating equipment COG combustion and combustion gas recirculating system. Byproducts: sulfuric acid.

Implementing company: Nippon Steel Corporation, Nagoya WorksRef./Sources: www.icett.or.jp

No. 3Classifi cation of technology: Air pollution controlName of technology/technique: Magnesium hydroxide method wet type flue gas

desulfurizationApplication: Coal burning boiler/Reduction of SOx in boiler exhausts gas.Description: The compact and low-pressure loss magnesium hydroxide method with

high desulfurization and dust removal performance was introduced as the fl ue gas desulfurization measures when installing the coal burning thermal power station.

Mechanism of desulfurization reaction 1. Exhaust gas containing SO2 comes into contact with the absorption

fl uid in the scrubber and, in addition to being humidifi ed and cooled until saturated, SO2 and dust are removed.

2. Air is blown into the bottom part of the scrubber, where magnesium sulfi te of low solubility is oxidized to the magnesium sulfate of high solubility.

SO2 + Mg(OH)2 → MgSO3 → MgSO4

3. Waste water from the desulfurization process is discharged after treatment and sludge separation pH of fl uid absorbed; 5.5 - 6.0, counter fl ow contact spray type with extremely low-pressure loss suitable for removing dust with grain diameter of 20~30 microns.

Advantages: 1. DeNOx: (Inlet) 240 - 260 ppm → (Outlet) 20 ppm or below. 2. Dust removal: (Inlet) 30 - 40 mg/Nm3 → (Outlet) 10 mg/Nm3 or below

Implementing company: Nippon Steel Corporation yard, Tokai Co-operative Power Company Inc.

Ref./Sources: www.icett.or.jp

No. 4Classifi cation of technology: Air Pollution ControlName of technology/technique: ScrubbersApplication: Paper/Pulp- Dust Removal and Desulfurization using ScrubbersDescription: Scrubbers are used with the objective of treating exhaust gas of a oil - fuel

boiler. The exhausted gasses have been previously treated in an electric dust-precipitator. the gasses enter the main scrubber unit in a tangential direction, rising as it creates a swirling motion. At the same time, liquid is sprayed out radially from the spray nozzles in the center. The net effect is that the swirling motion of the gas and the radial motion of the droplets collect dust. The main reaction taking place in the absorption process:

2NaOH + SO2 → Na2SO3 + H2O The side reaction occurs: Na2SO3 + SO2 + H2O → 2 NaHSO3


54


The main specifi cations for the dust-removal scrubber are as follows:1. Treated gas: 488,000 m/h, at 150 °C

2. Dust content at loading port: 0.35 g/Nm3

3. Dust content at discharge port: 0.12 g/Nm3

4. General dimensions Body diameter: 6,800 mm, Total height: 50,000 mmImplementing company: Oji Paper Co., Ltd., Kasugai PlantRef./Source: www.icett.or.jp

No. 5Classifi cation of technology: Air Pollution ControlName of technology/technique: AbsorptionApplication: Food industry, Reduction of SO2 and Offensive Odors from Process Exhaust

GasDescription: SO2 emissions could be reduced from 250ppm to below under 15ppm,

dust from 200ppm to below and the problem of bad smell was signifi cantly minimized. 60ppm. The reaction adopted for SO2 removal was:

SO2 + 2NaOH + NaOCl → Na2SO4 + NaCl + H2O. Deodorization, desulfurization and reduction of dust are carried out by

passing the exhausted gas through a sequence of a cooling tower, an alkali washing tower and oxidizing tower.

Implementing company: Shikishima Starch Co., Ltd., Suzuka FactoryRef/Source: www.icett.or.jp

No. 6Classifi cation of technology: Air Pollution ControlName of technology/technique: Alkali scrubber (absorption tower)Application: Chemical; Petrochemical industry/Reduction of SOx and dust in Process

Exhaust GasDescription: The tecnology has been designed to neutralize SOx and capture dust and

steam vapor which are generated in the dissolving process of titanium ore with sulfuric acid. The process use large amount of alkaline solution and sea water for the following reactions:

SO3 + 2NaOH → Na2SO4 + H2O SO2 + 2NaOH → Na2SO3 + H2O Na2SO3 + 1/2 O2 → Na2SO4

The amounts of discharged steam, SOx and dust were reduced by passing the exhaust gas through a mist catcher, a cooler and an alkali scrubber. In the scrubber, large amount of sea water are sprayed from the top of the column counter-currently to the gas, and NaOH solution is sprayed from nozzles located in several places at the middle of the column. Drain from the scrubber is discharged after pH adjustment and passing through a drain fi lter system.

Implementing company: Ishihara Sangyo Co., Ltd. Yokkaichi FactoryRef./Source: www.icett.or.jp

No. 7Classifi cation of technology: Air Pollution ControlName of technology/technique: Simplifi ed limestone/lime gypsum processDescription: This is the simplifi ed wet-type desulfurization system, realizing reduced

investment cost by simplifying the conventional limestone/lime gypsum process. Kawasaki open spray tower is adopted for the absorber with high performance spray nozzles and simple internal structures. Gypsum treatment


55


section is also simplifi ed by using continuous Gypsum Centrifuge. 1. Absorber Tower Section SO2 + H2O + (1/2)O2 → H2SO4

2. Absorber Sump Section CaCO3 + H2SO4 + H2O → CaSO4 2H2O + CO2 (g) or Ca(OH)2 + H2SO4 → CaSO4 2H2OAdvantages: Simplifi ed confi guration ensures low installation and operation costs more

over, the adopted equipment and the selected absorbents are rather inexpensive.

CaCO3 or Ca (OH)2 as absorbents. Process by-products (mixtures of gypsum and ash), can be utilized as

retardants for cement manufacturing. - DeSOx Performance 70 - 95% - By-Product Gypsum - Fuel Coal/Oil - Sulfur Content 0 - 4wt% in coal - Gas Volume 50 km3N/H This system has been adopted for simplifi ed fl ue gas desulfurization

systems in China.Ref./Source: Kawasaki Heavy Industries Ltd., www.khi.co.jp/index_.html

No. 8Classifi cation of technology: Air Pollution ControlName of technology/technique: Limestone-gypsum fl ue gas desulfurization systemApplication: Boiler for the utility thermal power plant, boiler for industry-owned thermal

power plant. This process has been adopted more than any other process in the world.

The reason is the facts. that limestone is a very cheap absorbent and this process gives high desulfurization effi ciency. Moreover, for the gypsum as by-product has a commercial value since it can be used production of plasterboard or as a retardant in the manufacture of cement.

Process Description: SO2 gas in the fl ue gas is absorbed by limestone and is oxidized by reacting with the air accordingly to the following reaction.

CaCO3 + SO2 + (1/2) O2 + 2H2O → CaSO4 2H2O + CO2

The by-product gypsum is recovered by centrifugation.Advantages: (I) High effi ciencies can be achieved. Desulfurization effi ciency: more than 96% Dust removal effi ciency: more than 90% COD in wastewater: less than 10 mg/l. (II) Limestone is less expensive than the other absorbents and gypsum has

a commercial value. (III) It is necessary to provide the countermeasures against scaling and

build-up of solid in slurry liquid. (IV) Spray tower is adopted as the Absorber, because its structure is simple

and has no scaling trouble. Treated gas fl ow rate: 2,400,000 m3N/h Inlet SOx concentration: 900 ppm Desulfurization effi ciency: 96% Dust removal effi ciency: 92% COD in waste water: 5 mg/lRef./Source: Ishikawajima-Harima Heavy Industries Co., Ltd., www.ihi.co.jp.


56


No. 9Classifi cation of technology: Air Pollution ControlName of technology/technique: Mitsui-ge type fl ue gas SO2 removal system Limestone or slaked lime is used as absorbent and gypsum is produced

as the by-product. Mitsui Mining delivered many systems for large-scale thermal power plants with coal-fi red boilers.

Description: In case of coal-fi red thermal power plants, the fl y ash and the acid gasses can give undesirable effects on the quality of gypsum produced and can reduce SO2 removal effi ciency. Therefore, treatment method has to be determined depending on the kind of coal used and the performance of EP.(a) Soot mixing method. This method has been developed as the most economical process to enable simultaneous dust and SO2 removal. Using small amount of Na catalyst, Mitsui Mining’s achieved stable performance of SO2 removal.

(b) Soot separating method This method has been developed for various kinds of overseas coals- fi red

thermal power plants. Pre-scrubber are installed in this confi guration to remove the impurities in fl ue gas. Stable performance of SO2 removal and high quality of gypsum are achieved.

(c) Oxidizing methods Oxidation inside of the absorber has been developed with patent of G.E.

(U.S.A.), and is able to recover high quality gypsum.Performance: I) SO2 removal effi ciency: More than 95% II) Dust removal effi ciency: Less than 15mg/m3N at outlet (in case of coal

fi red (Thermal power plants) III) Pressure loss: Less than 100 mmH2O as pressure loss of spray type

absorber IV) By product: High quality gypsum.Characteristics: I) This system uses the reliable spray type absorber with high performance.

In this absorber gas pressure loss is very low and the big capacity of gas treatment with one tower is completely performed. Moreover, this system has been improved for cutting cost and saving space, boost up fan for SO2

removal being excluded and served with IDF. II) Against the fl uctuations of gas volume, Mitsui Mining’s controlling system

can achieve the constantly stable performance. III) The by-product of gypsum is of good quality, which can be used, as

material of plaster board and cement.Ref./Source: MITSUI MINING CO., LTD

No. 10Classifi cation of technology: Air Pollution ControlName of technology/technique: Hitachi wet limestone-gypsum FGD systemDescription: Wet Limestone-Gypsum Flue Gas Desulphurization (FGD) System

removes SO2 (sulphur dioxide) contained in the fl ue gas in contact with limestone slurry droplets as an absorbent when the fl ue gas containing SO2 passes through the absorber. Limestone slurry absorbs SO2 then it is oxidized by air at the lower part of absorber to produce calcium sulphate which is extracted from the absorber as gypsum slurry and fi nally dewatered and reused in the form of gypsum powder.Chemical Reaction: CaCO3 + SO2 + 2H2O + (1/2)O2 → CaSO4 . 2H2O + CO2

Advantages: I) SO2 removal effi ciency higher than 90% can be achieved. II) By-product gypsum can be reused as material of cement or wall board.


57


III) Limestone, which is supplied stably, is used as an absorbent. IV) Single absorber tower, which has integrated function of prescrubbing,

absorption and oxidation, is adopted. Moreover it also removes particulate in the fl ue gas with high removal effi ciency.

Ref./Source: Babcock-Hitachi K.K.

No. 11Classifi cation of technology: Air Pollution ControlName of technology/technique: Blue sky 2000 processDescription: This plant absorbs effi ciently SO2 gas in the fl ue gas of oil or coal burning

boiler, glass melting furnace, kiln, etc. by calcium slurry with a little amount of HCOOH. Moreover the dust in the fl ue gas is spontaneously removed. The purity of gypsum as by-product is high and the gypsum can be sold for cement and wall-board.

Advantages: 1. SO2 absorption effi ciency: over than 95% 2. Dust conc. at absorber outlet: less than 0.03 g/m3N 3. Stoichiometric ratio of CaCO3: 1 - 3% excess 4. Gypsum purity: over than 90% 5. Pressure drop in absorber: less than 100 mmH2O 6. Purge liquid amount: less than 1/10 of conventional process This process has the following characteristics by the addition of HCOOH. 1. High SO2 removal ratio even in a low pH range of 4.2 - 5.2 2. Preclusion of scaling and plugging 3. High stoichiometric utilization of the absorbent 4. Low liquid to gas ratio (L/G) and consequently lower energy consumption

for washing fl uid circulation. 5. High SO2 absorption over the boiler load changes 6. Simple process, so small installation area 7. High Cl concentration in washing fl uid, so little amount of purge liquorPrinciple: The free HCOOH spontaneously dissolves the added calcium carbonate

intensively to form calcium formate, which is highly soluble in water. The washing fl uid then contains calcium formate in dissociated form and provides a large amount of calcium ions for SO2 absorption from the fl ue gas. Because of this, SO2 absorption ratio is enhanced very much. The details of each section are as follows.

1. Absorption The fl ue gas is contacted with the washing fl uid in a cocurrent/countercurrent

two-stage absorber. SO2 is absorbed from the fl ue gas and reacts to form bisulfi te ions.

SO2 + H2O → HSO3- + H+

Formate ions in solution react with the H+ formed during SO2 absorption. H+ + HCOO- → HCOOH Thus, an intensive SO2 absorption occurs in the pH range 4.2 - 5.2, the pH

range that insures the formation of bisulfate, the only water-soluble form of calcium and sulfur.

2. Oxidation At the absorber pH of 4.2 - 5.2, oxidation occurs easily without the need for

an acidifying step. Dissolved oxygen in the washing fl uid from boiler excess air and injected air in the absorber sump react to form sulfate.

2HSO3- + O2 → 2H+ + 2SO4

2-

Calcium ions present in solution combine to produce gypsum. Ca2+ + SO4

2- + 2H2O → CaSO4. 2H2O


58


3. Limestone addition Limestone slurry is added to the absorber sump to replace the calcium

lost with the gypsum. At the low pH, limestone dissociates to produce a washing fl uid with dissolved calcium several orders of magnitude higher than conventional process.

CaCO3 + H+ → Ca2+ + OH- + CO2(g)Ref./Source: TSUKISHIMA KIKAI CO., LTD.

No. 12Classifi cation of technology: Air Pollution ControlName of technology/technique: The fl ue gas treatment system of the municipal solid waste

incineration plantApplication: This device is designed mainly for the treatment of exhaust gas generated

by the incineration of municipal solid waste. However, it can also be used to process gases produced from various types of furnace.

Description: The exhaust gas generated from the incineration of municipal solid waste contains harmful substances that cause air pollution, such as dust, acid gases (SOx, HCl, NOx) and heavy metal particles (Pb, Zn, Cu, etc.). This device removes wide range of these harmful substances with high effi ciency. The exhaust gas from the municipal solid waste incinerator is cooled to a temperature of 180 to 200 °C by the gas cooler, and then enters the mixing chamber located at the inlet of the bag fi lter. Filtration agency and hydrated lime transported by air from those silos are injected into the mixing chamber. When mixed with the exhaust gas, the hydrated lime reacts with hydrogen chloride and sulphur oxides, forming solid matter, which is collected by the bag fi lter.

These chemical reactions are as follows: 2HCl + Ca(OH)2 → CaCl2 + 2H2O SOx + Ca(OH)2 → CaSOx + H2O The surface of the bag fi lter is also coated with fi ltration agency and hydrated

lime, and this coating effectively collects dust and heavy metal particles.Advantages: 1. The device uses dry-type processing which does not affect the exhaust

gas temperature directly. Therefore, no corrosion or dust adhesion trouble occurs in the equipment. 2. The fi ltering effect of the fi ltration agency and hydrated lime prevents

clogging of the fi lter cloth, providing stable plant operation without an increase in the fi ltering pressure loss.

3. The fi ltering effect of the fi ltration agency and hydrated lime removes dust and microscopic heavy metal particles effectively.

Processing capacity 1. Gas volume: no limitation 2. Gas temperature: 160 to 220 °CRef./Source KURIMOTO, LTD.

No. 13Classifi cation of technology: Air Pollution Control

Name of technology/technique: Mitsui-bf dry type desox/denox process/mitsui mining co., Ltd.

Description: Mitsui Mining successfully develops a dry-process, simultaneous desulfurization and denitrifi cation technology for commercial use on an industrial scale. This process is applicable for SOx, NOx, dust and toxic trace elements removal from boiler, furnace and chemical plant fl ue gases.


59


Applications: - Coal-fi red power station - Industrial boilers - Chemical plant- Petroleum refi neries (such as FCC) - Smelting works - Sintering furnaces- Incinerators used for trash, refuse, waste oil and sludge- Glass furnaces - Sulfuric acid plant tail gas

Description: The Mitsui-BF System is a three-part process: adsorption, desorption, and optional by-product recovery. The system is capable of simultaneous desulfurization and denitrifi cation. Alternatively, it can perform either DeSOx or optionally DeNOx functions.

Adsorption: Flue gas passes through a bed of activated coke moving downwards in the two-stage adsorber at a constant fl ow rate. The activated coke consists of carbon, and has a large, porous inner surface area. In the fi rst stage, sulfur oxides are largely removed from the gas by a process of adsorption into the activated coke, where oxidation occurs, yielding H2SO4 (sulfuric acid) maintained on the inner surfaces at a temperature of 100 to 200 °C. After desulfurization in the fi rst stage, the fl ue gas passes through the activated coke bed in the second stage. Here, it works as a catalyst in the decomposition of NOx to nitrogen (N2) and water (H2O) by the injection of ammonia gas (NH3) into the activated coke bed. This chemical reaction occurs at a temperature of 100 to 200 °C. The adsorber, with its moving activated coke bed, also functions as a particulate remover. Following the adsorption process, the particulate emissions will not exceed 30mg/Nm3 when inlet particulate concentrations are kept within 500 mg/Nm3. As the activated coke becomes loaded with sulfur oxides (SOx), its adsorption capacity deteriorates. The SOx-saturated activated coke is then conveyed by bucket elevators to the desorber, and regenerated at a temperature of 400 to 500°C. Simultaneously, ammonium sulfate ((NH4)2SO4) in the activated coke is decomposed to nitrogen (N2), sulfur dioxide (SO2) and water. After cooling, the regenerated activated coke passes over a vibrating screen to eliminate mechanically and/or chemically degraded material, and is then recycled back to the adsorber. This ensures that only activated coke meeting design-size parameters will be reused in the adsorber. Activated coke fi nes can be returned to the boiler as fuel. This desorption sequence optionally incorporates statistical process control, to optimize the adsorption capability for site-specifi c applications, as the adsorption capacity of the activated coke deteriorates in the process.BY-PRODUCT RECOVERYIn the process of regenerating the activated coke in the desorption unit, SO2-rich gas is released. This SO2-rich gas (20 to 25% volume in SO2 concentration) can be treated to form elemental sulfur (S) or sulfuric acid (H2SO4) by standard methods.Production of Elemental SulfurSO2-rich gas is reduced to H2S in a reduction column by using a carbonaceous reduction agent. The reduced gas is partially oxidized to SO2 by air injected at the upper portion of the reduction column and a mole ratio of H2S/SO2 is maintained at 2.0. Finally, the mixture of H2S and SO2 is converted to elemental sulfur in a Claus unit. The carbonyl sulfi de (COS), which is generated secondarily in the reduction column, is hydrolyzed in the Claus unit, and elemental sulfur is obtained.Production of Sulfuric AcidAfter dust and impurities are removed from the SO2-rich gas, the gas is oxidized in a converter to form SO3. The SO3 is then absorbed in an absorber to form 98% sulfuric acid.


60


ACTIVATED COKEMitsui Mining produces high-quality activated coke maximizing the effi ciency of the Mitsui-BF process.

Advantages: Dry processSOx removal effi ciency higher than 98%, NOx removal effi ciency higher than 80%- Flexibility Simultaneous DeSOx/DeNOx, or DeSOx-only, DeNOx-only operation

- Low power consumption - Minimum installation space required- No waste water generated - Gas reheating not required - High quality by-product

- Ease of operation - Low maintenance requirementsRef./Source: Mitsui Mining co., Ltd

No. 14Classifi cation of technology: Air Pollution ControlName of technology/technique: SOx reduction technology atmospheric fl uidized bed

combustion boiler system (AFBC: Atmospheric Fluidized Bed Combustion)Capacity: SOx removal effi ciency more than 90%Application: Coal-fi red power station etc.Description: When fl uidization medium (limestone, sand etc.) placed on the distribution

panel (perforated panel) is supplied with air from beneath, the medium fl oats in the air current within a specifi c range of the air speed, just as in a state of boiling. The layer of fl oating medium in this state is referred to as a fl uidized bed. When coarsely crushed coal is continuously supplied into the fl uidization medium, which is heated in the air-heating furnace to the ignition temperature of coal, the coal spontaneously starts burning. The furnace is turned off at this point, and while the coal keeps burning, coal supply volume is controlled so that the temperature of the fl uidized bed stays at 760 to 860 °C. For recovering heat, the fl uidized bed combustion boiler has heat exchanger tubes in its fl uidized bed and convection section. The use of limestone as fl uidization medium enables in-furnace desulfurization. Flue gas from the fl uidized bed combustion boiler includes various unburned components, which are collected by the mechanical precipitator and burned in the after-burner furnace (CBC) to improve combustion effi ciency.

Advantages: - In-furnace desulfurization/Fluidized bed combustion boiler can remove SOx in the furnace while burning coal, using limestone as the fl uidization medium.

- Use of various types of coal/Since the fl uidized bed combustion boiler ensures stable combustion at low temperature, as compared to the pulverized coal fi red boiler, various types of coal can be used.

Ref./Source: The Kansai Electric Power Co., Inc.


1. Air Pollution Control Engineering, N. de Nevers, second Edition, 2000, Mc Graw Hill, New York.

2. Pollution: Causes, Effects and Control, R. M. Harrison Ed., The Royal Society of Chemistry, Cambridge,

1996.


61


5 Volatile Organic Compounds (VOCs)5 Volatile Organic Compounds (VOCs)

5.1 Introduction5.1 Introduction

The majority of anthropogenic volatile Organic Compounds (VOCs) released into the atmosphere are from transportation sources and industrial processes utilizing solvents such as surface coating (paints), printing (inks), and petrochemical processing. Notably VOC compounds are not formed in industrial processes, they are lost.

VOCs are organic compounds that can volatilize and participate in photochemical reactions when the gas stream is released to the ambient air. Almost all of the organic compounds used as solvents and as chemical feedstock are VOCs. However, a few organic compounds, such as methane, are not considered to be VOCs.

5.2 VOC Destruction Technology5.2 VOC Destruction Technology

In order to reduce the quantities of VOCs that are lost as fugitive emissions, it is necessary to redesign the industrial processes, both form a chemical and an engineering point of view. End-of-pipe technology (thermal incinerators, catalytic incinerators, liquid and solids adsorbents, condensers, biodegradation….) must be applied when redesign of the process is inapplicable or insuffi cient (1).

5.2.1 Thermal Oxidation5.2.1 Thermal Oxidation

Thermal oxidation occurs by heating the polluted air to elevated temperatures (700 – 1000 °C). Thermal oxidation is a process whereby most of the VOCs are broken down and recombined with oxygen to produce water vapor and carbon dioxide. In a thermal oxidizer, the polluted air stream is heated to gas temperatures several hundred degrees Celsius above the autoignition temperature of the organic compounds that need to be oxidized. Due to these very high temperatures, thermal oxidizers have refractory-lined combustion chambers (also called fume incinerators) (see Figure 5.1), which increase their weight and size considerably. The effi ciency of oxidation and the design of most oxidizers is governed by the residence time (from a fraction of a second to more than two seconds), the combustion chamber temperature and the amount of turbulence the air stream sees. Thermal oxidizers usually provide VOC destruction effi ciencies that exceed 95% and often exceed 99%. One of the main limitations of thermal oxidizers is the large amount of fuel required to heat the gas stream to the temperature necessary for high-effi ciency VOC destruction. Termed regenerative thermal oxidizers (RTOs) use heat exchanger to recover some of the heat of the waste gas and to return it to the inlet gas stream. Therefore, these units, require less fuel to maintain the combustion chamber at the necessary temperature.


62

VOLATILE ORGANIC COMPOUNDS (VOCS)

Waste Gas Inlet100°C

Heat GasExchanger

350°C350°C

700-1000°C

Refractory-LinedCombustion Chamber

300°C

Figure 5.1: Thermal oxidizer with recuperative heat exchanger (adapted from ref 1)

Thermal oxidizers have the broadest applicability of all the VOC control devices. They can be used for almost any VOC compound. Thermal oxidizers can also be used for gas streams having VOC concentrations at the very low concentration range of less than 10 ppm up to the very high concentrations approaching 10,000 ppm.

Safety constraints impose to use thermal oxidizers for gas streams having VOC concentrations not exceeding approximately 25% of the lower explosive limit (LEL). This constrain has been introduced to be able to cope with possible short-term concentration spike that would exceed the LEL. The 25% LEL limit depends on the actual gas constituents and usually is in the 10,000 to 20,000 ppm range.

Thermal oxidizers handling VOC materials that contain chlorine, fl uorine, or bromine atoms generate HCl, Cl2, HF, and HBr as additional reaction products during oxidation. A gaseous absorber is used as part of the air pollution control system to collect these contaminants prior to gas stream release to the atmosphere.

5.2.2 Catalytic Oxidation5.2.2 Catalytic Oxidation

Due to the presence of a catalyst, oxidation reactions can be performed at substantially lower temperatures (250-550 °C) than thermal oxidizers (700-1000°C). Common types of catalysts include noble metals (i.e. platinum and palladium) and ceramic materials. VOC destruction by catalytic oxidizers usually exceeds 95% and often exceeds 99%. Due to the relatively low gas temperatures in the combustion chamber, there is no need for a refractory lining to protect the oxidizer shell. This minimizes the overall weight of catalytic oxidizers and provides an option for mounting the units on roofs close to the point of VOC generation. This placement can reduce the overall cost of the system by limiting the distance the VOC-laden stream must be transported in ductwork.

Catalytic oxidizers are also applicable to a wide range of VOC-laden streams; however, they cannot be used on sources that also generate small quantities of catalyst poisons. Catalyst poisons are compounds that react chemically in an irreversible manner with the catalyst. Common catalyst poisons include phosphorus, tin, and zinc. Another potential operating problem associated with catalytic oxidizers is their vulnerability to chemicals and/or particulate matter that masks or fouls the surface of the catalyst. (Masking is the


63

IR POLLUTION CONTROL TECHNOLOGIES

reversible reaction of a chemical with the catalyst and fouling is the coating of the catalyst with a deposited material.) If the conditions are potentially severe, catalytic units are not installed.

Clean Gas

Gas Inlet

Catalyst Bed

Tubular HeatExchanger

Burner(normally off)

Figure 5.2: Scheme of a catalytic oxidation system (adapted from ref 1)

As with thermal oxidizers, catalytic oxidizers should not exceed 25% of the LEL, a value that is often equivalent to a VOC concentration of 10,000 to 20,000 ppm.

The catalytic incineration method has become most popular because, in many case, it is more versatile and economic for the low concentrations of organic emissions (i.e., <5,000 vppm).

The actual operating temperature and amount of preheat varies, depending on the organic molecule, space velocity, composition of feed (i.e., contaminants water vapor, and so forth), and organic concentration. Typical examples of operating temperatures are given in Table 5.1. One-way of comparing thermal versus catalytic abatement is to look at the energy required (air preheat temperature) to obtain quantitative removal of a given hydrocarbon. The operating temperatures shown in Table 5.1 are well below the corresponding temperatures necessary to initiate thermal (noncatalytic) oxidation. The catalyst initiates reaction at lower temperatures. This demonstrates the major advantage of catalyzed processed, which is that they proceed faster than noncatalytic reactions, allowing lower temperatures for the same amount of conversion. This translates directly into improved economic for fuel use and less expensive reactor construction materials, since corrosion is greatly reduced. Selection of the catalytic material for various organic pollutants has been the subject of many studies. Because metal oxides, precious metals, and combinations are used both for hydrocarbons and chlorinated hydrocarbons. As a rule, precious metals (especially platinum and/or palladium dispersed on carriers) are preferred because of their, resistance to deactivation, and ability to be regenerated.


64


Table 5.1: Operating temperatures for catalytic abatement of organic compounds (1)

Name of constituent Chemical formulaTemperature rise

1,000ppm (°C)

Operating temperature

(°C)

Concentration before treatment

(ppm)

Styrene C6H6CHCH2 138 250 310

Acetaldehyde CH3CHO 35 350 240

Benzene C6H8 103 210 380

Toluene C6H5CH3 123 210 320

M-xylene C6H5(CH3) 143 210 270

Phenol C6H5OH 101 300 380

Formaldehyde HCHO 17 150 410

Acrolein CH2CHCHO 51 180 500

Acetic acid CH2COOH 26 350 590

Butyric acid C3H7COOH 66 20 370

Acetone CH3COCH3 57 350 410

Methyl ethyl keton CH3COC2H3 74 220 380

Methyl isobutyl keton CH3COC4H8 116 250 270

Ethyl acetate CH3 COOC2H5 68 350 350

Butyl acetate CH3COOC4H9 108 350 480

Methyl alcohol CH3OH 21 150 830

Ethyl alcohol C2H5OH 44 350 550

Isopropyl alchol C3H7OH 64 280 230

Butyl alcohol C4H9OH 84 260 330

Carbon monoxide CO 9 150 4,000

Methyl cellosolve HOCH2CH2OCH3 55 300 110

Ethyl cellosolve HOCH2CH2OC2H5 76 300 80

Buty cellosolve HOCH2CH2OC4H9 118 300 50

5.2.3 Adsorption5.2.3 Adsorption

Adsorption systems beds are generally used when the gas stream contains one to three volatile organic compounds, and it is economical to recover and reuse these compounds, or when a large number of organic compounds at low concentration, and it is necessary to pre-concentrate these organics prior to thermal or catalytic oxidation.

Figure 5.3 shows the diagram of a multi-bed adsorber system used for collection and recovery of organic solvent compounds. The VOC-laden gas is often cooled prior to entry into the adsorption system to improve the effectiveness of adsorption. As the gas stream passes through the bed, the organic compounds adsorb weakly onto the surfaces of the adsorbent (high surface area activated carbon, zeolite, or organic polymer). When the adsorbent is approaching saturation with organic vapor, a bed is isolated from the gas stream and desorbed. Low-pressure steam or hot N2 is often used to remove the weakly adsorbed organics. The concentrated stream from the desorption cycle is treated to recover the organic compounds. After desorption, the adsorption bed is returned to service, and another bed in the system is isolated and desorbed.


65


VOCVOCVOC

VOCVOCVOC

VOCVOCVOC

Steam and DesorbedSolvents Vapors

To

Atm

osp

her

e

ΔPΔPAA

TT

T

ParticulateFilter

Cooling Water

TT

LELLELSolvent-laden

Air

ΔP

SteamBed 1

Bed 2

Bed 3

Figure 5.3: Multi-bed adsorber system for solvent recovery

An adsorption system used for pre-concentration prior to solvent recovery is shown in Figure 5.4. The gas stream cointaining the VOC passes through a rotary wheel containing zeolite or carbon-based adsorbents. Approximately 75-90% of the wheel is in adsorption service while the remaining portion of the adsorbent passes through an area where the organics are desorbed into a very small, moderately hot gas stream. The concentrated organic vapors are then transported to a thermal or catalytic oxidizer for destruction. The preconcentration step substantially reduces the fuel requirements for the thermal or catalytic oxidizer.

Solvent-ladenExhaust Air

Fan

Pre-filter

PurifiedEchaust Air

Hot Air(Desorption Air)

Solvent-ladenDesorption Air(Concentrated)

RotaryWheel

Figure 5.4: Preconcentrator – type adsorption system (adapted from ref 1)

Adsorption systems are not recommended for gas streams that contain particulate matter and/or high moisture concentrations because the particulate matter and moisture compete with the gaseous pollutants for pore space on the adsorbent material.

The adsorption removal effi ciency usually exceeds 95% and is often in the 98% to 99% range for both solvent recovery and preconcentrator type systems. In both types of units, the removal effi ciency increases with reduced gas temperatures.


66


The suitability of an adsorption system for a particular situation should be considered on a case-by-case basis. However, as a general guideline, adsorption systems can be used for organic compounds having a molecular weight of more than 50 and less than approximately 200. In fact, the low molecular weight organics usually do not adsorb suffi ciently. The high molecular weight compounds adsorb so strongly that is it is diffi cult to remove these materials from the adsorbent during the desorption cycle. Adsorption systems can be used for a wide range of VOC concentrations from less than 10 ppm to approximately 10,000 ppm. The upper concentration limit is due to the potential explosion hazards when the total VOC concentration exceeds 25% of the LEV.

5.2.4 Condensation, Refrigeration and Cryogenics5.2.4 Condensation, Refrigeration and Cryogenics

Condensation, refrigeration, and cryogenic systems remove organic vapor by making them condense on cold surfaces. These cold conditions can be created by passing cold water through an indirect heat exchanger, by spraying cold liquid into an open chamber with the gas stream, by using a freon-based refrigerant to create very cold coils, or by injecting cryogenic gases such as liquid nitrogen into the gas stream. The concentration of VOCs is reduced to the level equivalent to the vapor pressures of the compounds at the operating temperature. Condensation and refrigeration systems are usually used on high concentration, low gas fl ow rate sources. Typical applications include gasoline loading terminals and chemical reaction vessels.

The removal effi ciencies attainable with this approach depend strongly on the outlet gas temperature. For cold-water-based condensation systems, the outlet gas temperature is usually in the 4 to 10°C range, and the VOC removal effi ciencies are in the 90 to 99% range depending on the vapor pressures of the specifi c compounds. For refrigerant and cryogenic systems, the removal effi ciencies can be considerably above 99% due to the extremely low vapor pressures of essentially all VOC compounds at the very low operating temperatures of -56°C to less than -130°C.

Condensation, refrigeration, and cryogenic systems are usually used on gas streams that contain only VOC compounds. High particulate concentrations are rare in the types of applications that can usually apply this type of VOC control system. However, if particulate matter is present, it could accumulate on heat exchange surfaces and reduce heat transfer effi ciency.

5.2.5 Biological Oxidation5.2.5 Biological Oxidation

VOCs can be removed by forcing them to absorb into an aqueous liquid or moist media inoculated with microorganisms that consume the dissolved and/or adsorbed organic compounds. The control systems usually consist of an irrigated packed bed that hosts the microorganisms (biofi lters). A presaturator is often placed ahead of the biological system to increase the gas stream relative humidity to more than 95%. The gas stream temperatures are maintained at less than approximately 40 °C to avoid harming the organisms and to prevent excessive moisture loss from the media.

Biological oxidation systems are used primarily for very low concentration VOC-laden streams. The VOC inlet concentrations are often less than 500 ppm and sometimes less than 100 ppm. The overall VOC destruction effi ciencies are often above 95%.

Biological oxidation systems are used for a wide variety of organic compounds; however, there are certain materials that are toxic to the organisms. In these cases, an alternative type of VOC control system is needed.


67


5.2.6 General Applicability of VOC Control Systems5.2.6 General Applicability of VOC Control Systems

Limiting the consideration of the VOC’s control systems to gas streams having total VOC concentrations less than approximately 25% of the LEL, it is possible to arbitrarily divide the control system applicability into two separate groups: low VOC concentration (less than 500 ppm) and high VOC concentration. It should be noted that there is no generally accepted distinction between low and high concentration. The low concentration group can be further divided into three main categories depending on the number of different VOC compounds in the gas stream and the value of recovering these compounds for re-use (Figure 5.5 ).

If there are a large number of separate VOC compounds, it is usually not economically feasible to recover and reuse the captured organics. In this case, thermal or catalytic oxidizers are used to oxidize the VOC compounds. Adsorbers can also be used as independent control systems or as preconcentrators for the oxidizers.

If there are less than or equal to 3 VOC compounds, it is usually possible to use either adsorbers or biological oxidation systems. However, it is necessary to confi rm that the compounds can be desorbed from regenerative-type adsorbers and that the specifi c organics are not toxic to the microorganisms in biological oxidation systems. Both thermal and catalytic oxidizers can also be used for these types of gas streams.

> 3 VOC CompoundsRecovery / Reuse

Not Feasible

? 3 VOC CompoundsRecovery / Reuse

Not Feasible


Feasible

Thermal OxidizersCatalytic Oxidizers

Adsorbers

AdsorbersBiological OxidizersThermal OxidizersCatalytic Oxidizers

Adsorbers

VOC Sources (< 500 ppm)


Not Feasible


Not Feasible


Feasible


Adsorbers

AdsorbersBiological OxidizersThermal OxidizersCatalytic Oxidizers

Adsorbers

VOC Sources (< 500 ppm)

Figure 5.5: Scheme of general applicability of VOC control systems for low concentration sources

If recovery and reuse are necessary, an adsorber system is generally used as the control technique. Due to the low VOC concentrations, the cost of organic compound recovery can be quite high.

The applicability of VOC control systems for high concentration systems also depends, in part, on the number of separate VOC compounds present in the gas stream and the economic incentives for recovery and reuse. Thermal oxidizers can be used in all cases in which recovery and reuse are not desired or economically feasible. Catalytic oxidizers can be used in these same situations if there are no gas stream components that would poison, mask, or foul the catalyst. Adsorbers can also be used for this service as long as there are environmentally acceptable means for disposal of the collected organics.

If recovery and reuse are desired, either adsorbers or condenser/refrigeration systems can be used. Usually, these systems are limited to gas streams containing at most three organic compounds due to the costs associated with separating the recovered material into individual components. However, if the process can reuse a multi-component organic stream, both adsorbers and condenser or refrigeration systems can be used without the costs of recovered material purifi cation and reprocessing.


68



Not Feasible


Not Feasible


Feasible


AdsorbersThermal OxidizersCatalytic Oxidizers

AdsorbersCondensers /Refrigeration

VOC Sources (> 500 ppm)


Not Feasible


Not Feasible


Feasible


AdsorbersThermal OxidizersCatalytic Oxidizers

AdsorbersCondensers /Refrigeration

VOC Sources (> 500 ppm)

Figure 5.6: Schema of general applicability of VOC control systems for high concentration sources

5.3 Some Commercial Technologies5.3 Some Commercial Technologies

Some of the major suppliers of VOC Catalytic systems are listed below

• Allied Signal• ARI• Degussa• Engelhard Corporation• Haldoe-topsoe• Hereaas• Johnson- Mattey• Nikki Universal• Nippon shojubai• Prototech/united catalyst• W.R.Grace

5.3.1 Oxidizer Types5.3.1 Oxidizer Types

The most reliable and acceptable means of destroying VOCs, HAPs, and odors available today is thermal oxidation. Oxidation, typically, is an energy intensive technology wherein a polluted air stream is heated to a high temperature setpoint that is predetermined by the nature of the pollutant. The simplest form of an oxidizer is a direct-fi red burner that elevates the air temperature from incoming levels to combustion levels. Because of the high cost of heating the process exhaust stream to the required oxidation temperature most thermal oxidizers incorporate some type of primary heat recovery. Primary heat recovery transfers energy from the hot clean gas stream exiting the oxidizer into the incoming polluted gas stream. This reduces the amount of additional energy required to achieve the oxidation temperatures. There are two widely used methods of recovering this thermal energy, recuperative and regenerative.

Sources: http://www.durrenvironmental.com


69


When a catalyst is used to enhance the operation of a thermal oxidizer, the system is generally referred to as a catalytic oxidizer.

Oxidizer Selection Criteria:

In order to select which type of oxidizer is most advantageous for a specifi c application, the following information must be known:

• Process exhaust fl ow rate• Process exhaust stream temperature• Pollutant concentration levels• Type of Pollutant• Particulate Emission levels• Required pollutant control effi ciency

In many cases the most advantageous type of oxidizer can be selected based on the following general guidelines. In other cases two or more oxidizer types may be practical and a detailed economic analysis based upon the specifi c costs of fuel and electricity is required to determine the best selection.

5.3.2 Direct Fired Thermal Oxidizers (DFTOS)5.3.2 Direct Fired Thermal Oxidizers (DFTOS)

The simplest Thermal Oxidizer is a Direct Fired unit (sometimes referred to as an After-Burner) that employs no heat recovery. In this system a fuel burner (mostly natural gas fi red) raises the temperature of the pollutant-laden air to a predetermined combustion temperature. In order to achieve the a high level of hydrocarbon destruction, the heated air is kept at the combustion chamber setpoint for a certain minimum time, called the residence or dwell time. In addition to temperature and dwell time, turbulence also plays an important role in making oxygen and hydrocarbon molecules to interact more vigorously. Since DFTOs employ no heat recovery, they are most often applied to very low volume air streams, usually with very high concentrations of VOCs, HAPs, and other pollutants. These oxidizers are characterized by specialized burners to insure mixing of combustion air and low volume, high concentration inert airstreams, which are often injected directly into the burner ports or directly in the fl ame cone via lances. This minimizes the risk of explosion and takes maximum advantage of the enriched fuel value of high VOC content exhaust streams.

5.3.3 Recuperative Oxidizers5.3.3 Recuperative Oxidizers

A Recuperative Oxidizer is a Direct Fired unit that employs integral primary heat recovery. To minimize the energy consumption of the oxidizer, the hot air exiting the combustion chamber is passed over an air-to-air heat exchanger. The heat recovered is used to preheat the incoming pollutant laden air. The primary heat exchangers are usually supplied as either a plate-type or a shell and tube type heat exchanger. These heat exchangers can be designed for various heat transfer effi ciencies, but the nominal maximum is 70%. Thus by the addition of a heat exchanger, the net heat load on the burner can be reduced by up to 70% of that required in a DFTO. The addition of the heat exchanger, because it is made of heat corrosion resistant alloy, substantially increases the cost of the oxidizer system. Also, the fan for moving the polluted gas through the oxidizer must be more powerful to overcome the additional pressure drop of the heat exchanger. In most cases, the savings in fuel will more than offset the additional up-front cost within the fi rst two years of operation, however, even with 70% heat recovery, recuperative oxidizers can be expensive to operate, especially


70


if the airfl ow is large and has dilute concentration levels, unless additional secondary heat recovery can be applied to the customer’s process.

5.3.4 Regenerative Thermal Oxidizers (RTOs)5.3.4 Regenerative Thermal Oxidizers (RTOs)

A Regenerative Oxidizer is also a Direct Fired oxidizer that employs integral primary heat recovery. However, the RTO operates is periodic, repetitive cycle rather than a steady state mode. Instead of conventional heat exchangers, which indirectly transfer heat from hot side to cold side across the exchanger walls, RTOs use a store and release mechanism. The nature of a RTOs heat recovery process requires it to have at least two beds of appropriate heat recovery media. In many applications, the additional step of purging a bed before reversing the fl ow through it from inlet to exhaust is necessary to maintain very high destruction effi ciencies. This purge step creates the requirement for an additional (or odd number) chamber making the RTO more complicated and more expensive than a recuperative oxidizer. RTO systems can utilize more than two beds (operating in parallel) in order to be capable of handling larger air volumes. The primary advantage of an RTO is lower operating costs due to high heat recovery and low fuel consumption. Depending on the mass of media included in an RTO, heat recoveries of up to 95% are common. Because of their capability for high heat recovery, RTOs are often operated in an “auto-thermal” or self-sustaining mode, where the heat content of the VOCs being oxidized is enough to sustain the combustion chamber temperature at setpoint, requiring no external fuel input.

Industries Served by Oxidizers:

• Automotive

• Surface Finishing/Coating

• Semiconductor

• Wood Panel Manufacturing

• Chemical Manufacturing

• Petrochemical

• Pharmaceutical

• Aerospace

• Glass Manufacturing

• Foundry

• Styrene

• Waste Water Treatment

• Tank Farms

• Printing & Flexography

• Wall Paper

• Flooring

• Solid Waste Treatment

• Fiber Manufacturing

• Pulp and Paper

• Rendering

• Corn Milling

Application Selection Chart

Choosing the right equipment for VOC control applications depends primarily on the exhaust air volume and the average concentration of VOCs. The chart displayed provides general guidelines for choosing equipment to fi t particular applications.


71


Sources: http://www.durrenvironmental.com

5.4 Example of Commercial Technologies5.4 Example of Commercial Technologies

Classifi cation of technology: Air Pollution (Add – on Control)Name of technology/technique: Catalytic incineratorApplied: Chemical industries, painting, printing, rubber and casting factoriesDescription: This equipment catches malodorous gases with the platinum catalyst and

decomposes them by oxidation to harmless and odorless carbonic acid gas and water. Compared with the direct combustion type, this equipment is capable of treating malodorous gases at lower temperature, running et lower cost. In case of low malodorous substances concentration, it is advisable to install Honeycomb Type Deodorization Equipment as pre-treatment equipment to save energy.

Exhaust gasses are pre-heated in a heat exchanger.Exhaust gasses are heated to the predesigned temperature by the auxiliary heater (generally to 300 °C ). Electricity, city gas, LPG, kerosene oil, etc. can be selected as heat source.When the polluted gas passes through the catalyst-bed, malodorous substances contained are decomposed by oxidation. Purifi ed gas is exhausted after passing through the heat exchanger where heat is exchanged between purifi ed gas and treating gas. In combination with a steam heater: In case electricity is used as heat source for the auxiliary heater, it is advisable to use a steam heater together with it to reduce electric consumption.


72


Advantages: Low running cost: Compared with the direct combustion system, this equipment is capable of treating malodorous substance at low temperature. Fuel costs can be reduced by 1/3.Low boiling organic solvents and wider ranges of malodorous substances can be purifi ed.

Installation example of the system

Ref./Source: Daikin Industries, Ltd.


73


5.5 Examples of Commercial Catalysts5.5 Examples of Commercial Catalysts

5.5.1 Engelhard Catalysts5.5.1 Engelhard Catalysts

Carbon monoxide, VOCs or HAPs control VOCat® PTA.This noble metals based catalyst family is designed to abate pollutants generated

during the process of purifi cation of terephthalic acid, a key raw material to produce polyethylenterephthalate (PET). The volatile organic compound (VOC) most diffi cult to abate during this process is methyl bromide. These catalysts were specifi cally, but not exclusively, designed.

VOCat® 350 HC and 360 PFC.These noble metals based two catalysts were specifi cally designed to selectively

abate both chlorinated and fl uorinated VOCs (including dioxins and furans) producing the relevant halogenated acids, easy to be scrubbed during a further step. Amount of generated chlorine and fl uorine, very nasty to be eliminated, is produced at lowest possible level.

VOCat® 310 ST S and ST HThese noble metals based, two catalysts provide effective VOC abatement in an

environment rich in sulfur.

VOCat® RCOThis is a noble metal-based catalyst family, designed to either retrofi t or build

up new regenerative oxidizers. It is useful to treat very large exhaust fl ows with a VOC total amount of more than 1 g/Nm3. Those catalysts substantially lower operating costs of regenerative oxidizers with a quick payback period (often less than 1 year).

Zeolite rotor concentratorsThis product family is aimed to optimize economics of either thermal or catalytic

oxidizers (also regenerative ones). It is economically convenient to treat exhaust fl ows larger than 20–30,000 Nm3/h and with a VOC total amount of less than 1–2 g/Nm3. These concentrators safely and effectively adsorb and desorb VOC in such a way that 85–95% of the original fl ow rate can directly proceed to the stack, whereas a much lower fl ow rate, rich of VOCs, is sent to the oxidizer. The latter can, therefore, work with much lower operative costs. The payback period is very quick. Special hydrophobic zeolites are coated onto a fi ber–ceramic honeycomb. The same technology–but with non-rotating panels–is used in VOC abatement for food service.

5.5.2 Photocatalytic Self-cleaning Ceramic5.5.2 Photocatalytic Self-cleaning Ceramic

The self-cleaning ceramic can degrade contaminants (e.g., oil) on the surface, deodorize some harmful gases, and kill bacteria or virus. It can be used for indoor or outdoor fi tment for kitchen, toilet, natatorium, operating rooms, and so on.

Solid superacid photocatalyst with high effi ciencyThe solid superacid photocatalyst possesses higher photocatalytic activity and

higher effi ciency for destroying organic contaminants at room temperature. It can be applied to the areas including environmental protection, reclamation of noble metal, preservation of fruits and so on.


74


Multi-functional photocatalytic air cleanerThe air cleaner has multi-functions such as destroying volatile organic compounds

(e.g., trichloroethylene, benzene, formaldehyde, etc.), killing bacteria, deodorization, and dedusting. It is favorable for reducing air pollution in home vehicles, hotel, offi ces, meeting rooms, and so on.

Source: http://www.nerc-cfc.com/production.html


1. Air Pollution Control Engineering, de Nevers N., second Edition, 2000, Mc Graw Hill, New York.

2. www.epin.ncsu.edu

3. Buscom.com

4. www.engelhard.com


75


6 POPs and Chlorinated Organic Pollutants6 POPs and Chlorinated Organic Pollutants


Persistent organic pollutants (POPs) are the group chemical substances of natural or anthropogenic origin, which are extremely toxic and very stable. These compounds resist photolytic, chemical and biological degradation. They are also characterized by low water solubility and high lipid solubility, resulting in bioaccumulation in fatty tissues of living organisms thereby posing a risk of adverse effects to human health and the environment. POPs are transported in the environment in low concentrations by fresh and marine waters and can transfer long distances in the atmosphere, resulting in widespread distribution across the globe. Thus, both humans and environmental organisms are exposed to POPs around the world, in many cases for extended period of time. Over the past several years, the risks posed by POPs have become of increased concern in many countries.

One of the fi rst dioxin public awareness raising events may be considered the accident at a chemical plant in Seveso, Italy on the 10th July 1976, when a toxic cloud containing 2,3,7,8-tetrachloro-dibenzo-p-dioxin, the most toxic POP and one of the most toxic man-made substances was released in the atmosphere. During the eighties the public concern on POP emissions grew, focusing mainly on waste incinerators, which are the main source of dioxins, but also on industrial organochlorine processes.

In the late eighties and in the nineties many local and international organizations began considering the POPs problem with more attention. In May 1995, the UNEP Governing Council adopted the Decision concerning Persistent Organic Pollutants. This process led in 2001 to the signature by the representatives of 92 countries of the Stockholm Convention on POPs, which included a list of 12 substances. The Stockholm Convention is now signed by over 150 countries and represents the main driving force in the international fi ght against POPs. The detailed information on the state of the art on POPs and Stockholm convention can be found on the websites of UNEP www.chem.unep.ch/pops and at www.pops.int.

The 12 POPs recognized by the Stockholm Convention on POPs as requiring the most urgent action are shortly described in this section. Some POP chemicals have been produced intentionally on the industrial scale to be used mainly in agriculture and electrical equipment. These are nine pesticides, namely chlordane, dieldrin, heptachlor, toxaphene, aldrin, endrin, DDT, mirex, hexachlorobenzene (HCB), and PCBs which are the group of substances known for their wide use as transformer and condenser heat exchange oils, insulation liquids, plasticizers, lubricants, etc. Polychlorinated dibenzo-para-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are other 2 POPs which group of substances on the list of POPs are the infamously known dioxins and furans, more precisely a group of 17 toxic congeners belonging to the classes of. These latter have been produced unintentionally, normally as by products of combustion processes and in some other sources, including chlorine industries. The threat by the PCDD/F to humans and the Environment is recognized as of major importance due to their extreme toxicity and very high persistence. It is mainly dioxins and furans that raise a special concern related to air contamination, as these


76

POPS AND CHLORINATED ORGANIC POLLUTANTS

substances, once occurred in the gaseous emissions of industrial plants then can transfer atmospherically over very long distances. Along with dioxins and furans also PCBs and HCB can be formed as byproducts in some processes and are also designated by the Convention as the so-called unintentionally produced (UP) POPs.

The provision is made in the Convention on POPs that all existing stockpiles of intentionally produced industrial POPs (pesticides and PCBs) have to be destroyed and their production be stopped. Regarding the UP POPs the Convention requires that their production be minimized and eliminated where possible. With this aim two basic approaches can be applied, namely prevention and end-of-pipe practices. This chapter will review some technical aspects on these practices related in particular to the presence of POPs in air emissions.

It is known that many other chlorinated and halogenated substances also manifest high or POP-like or toxicity. The 12 POPs by the Stockholm Convention are also comprised in the list of 16 chemicals subject to the Geneva Convention on Long-Range Transboundary Air Pollution (LRTAP). In addition to the 12 POPs, many other halogenated organic compounds are on the list of persistent toxic substances (PTS), ozone depleting substances (ODS), hazardous air pollutants, etc.

The technical approaches described here as applicable to the effi cient destruction of POPs, which are all chlorinated compounds, can be considered also useful for removal of other chlorinated/halogenated pollutants, in addition to what have been already presented in the previous chapter on VOC.

6.2 Unintentionally produced POPs and air pollution6.2 Unintentionally produced POPs and air pollution

Speaking about the air contamination by POPs and other chlorinated compounds the dioxin problem is by no doubt to be considered as priority. The issue of dioxin pollution owes attention mainly to waste incinerators, which have always been the main source of global dioxin contamination. Dioxins and furans present in the air and dust also constitute the second major way of dioxin intake after food.

The Stockholm Convention outlines a number of processes and devices recognized as the PCDD/F sources, namely: waste incinerators (municipal, medicinal, hazardous waste, and sewage sludge), some processes of pulp and paper, metallurgical, and organochlorine industries, cement production, fossil-fuel-fi red combustors, and forest fi res, etc. The Convention also requires that measures be taken regarding these sources so that the levels of UP POPs are reduced or eliminated, by using best available techniques (BAT) and best environmental practices (BEP). This means that the existing processes should be managed in the proper way or modifi ed or replaced so that PCDD/F formation is prevented by design or that proper post-treatment is applied in order to remove these contaminants from the emissions. Being the dioxin and furan compounds the more thermodynamically stable of the class, it can be guaranteed that given the proper practice of emission control to remove PCDD/F the other UP POPs present (HCB, PCB) and other chlorinated organics will be also effi ciently removed.

6.3 Formation of dioxins6.3 Formation of dioxins

There are a total of 17 individual compounds from the class of dioxins and furans (congeners) which are considered as dioxins and furans on the list of POPs. As it was mentioned, the 2,3,7,8-TCDD congener (see Fig. 6.1) is the most toxic from all the 17 and


77


its toxicity equivalent factor (TEF) was assigned one. The rule which defi nes the toxicity of a dioxin / furan congener is exactly the presence of the four chlorine atoms at all of positions 2,3,7,8 of the structure. If at least one chlorine at these positions is missing the congener is not toxic; if additional chlorines (up to eight possible) are present at other positions of the rings, such congeners are normally less toxic (TEF less than 1). Therefore the measure of dioxin contamination is usually presented in toxic equivalents (TEQ), i.e. the sum of concentrations of all 17 congeners (they usually occur together) where for each conger its component concentration is multiplied by its respective TEF. The TEQ value is often used in regulatory limits, and for air contamination levels the PCDD/F usually range in the order of nanograms per cubic meter TEQ.

Dioxins Furans

Figure 6.1. Toxic PCDD/PCDF congeners are shown which are characterized by the presence of chlorine atoms at positions 2,3,7,8 (lateral) and chlorine or hydrogen atoms at positions 1,4,6,9 (in brackets)

Chlorinated dioxins and furans are the most toxic products of incomplete combustion. The formation mechanisms for these compounds have not been yet completely elucidated. It is believed that there are at least three possible types of formation mechanisms. In principle, the dioxins are formed in any high temperature process given the presence of organic matter, oxygen and chlorine. The fi rst mechanism is the so-called ìpass-throughî mechanism whereby the dioxins originally present in the combusted material are released and exit unaltered. In the second mechanism, PCDD/F are formed from the precursors, which are the chlorinated aromatic compounds possessing the appropriate structure to yield, after structural rearrangement at elevated temperature, the structural skeleton of PCDD/F. Typical precursors are chlorophenols and PCBs. Dioxins and furans can also be formed in incinerators via the ìde-novoî synthesis, which is a catalytic process that takes place on the surface of fl y ash in the presence of organic matter, oxygen, chlorine and a transition metal, e.g. copper. PCDD/F formed by either of these mechanisms or their combination, normally are adsorbed on the fl y ash particles.

Usually the formation of dioxins from precursors occurs at cooling of the fl ue gases. PCDD/F formation appears to be more favored over the temperature range from 240 to 540°C. At temperatures well above 540°C PCDD/F are readily oxidized. However, even though high temperatures are involved in most incinerators, boilers and other combustion devices and PCDD/F are initially destroyed, the chlorinated precursors, which originated in the fuel and/or waste can volatilize and move with the gas stream through the combustion process until they reach the temperature range favorable for dioxin and furan formation (240 to 540°C).

There are several basic principles and approaches that should be applied to reduce dioxin and furan emissions in combustion processes:


78


Primary control or prevention techniques• Optimum burnout: temperatures >850°C, residence time >2 seconds, good turbulence• Quenching (rapid drop to low temperature)• Addition of inhibitors

End-of-pipe or removal methods• Dust separation (electrostatic precipitators, fabric fi lters)• Adsorption on carbon (fl ow injection, fi xed bed etc.)• Catalytic oxidation

The plot in Fig. 6.2 gives a comparison of effi ciencies of different approaches to reduce dioxins in the emission gases, from good combustion practices (prevention) to the end-of-pipe approaches, such as particulate fi ltration (electrostatic precipitators, fabric fi lters), adsorption on carbon, and catalytic oxidation approaches.

Figure 6.2. Effi ciency comparison of different post-treatment systems for PCDD/F removal in combustion plants.

6.4 Chlorinated VOC and Other Halogenated Pollutants6.4 Chlorinated VOC and Other Halogenated Pollutants

Combustion processes, which give rise to dioxins and UP POPs, can also produce a number of other chlorinated or halogenated hazards, especially when chlorinated waste is incinerated. Some hazardous chlorinated organics can be found in the emissions as the by-products of combustion and industrial processes or as undestroyed chemicals from waste disposal operations. Chlorinated chemicals from the VOC list include chlorides of methyl, vinyl, ethyl, di- and trichloroethanes, chloroform, trichloroethylene, mono- and dichlorobenzenes. Chlorinated PTS chemicals (see www.chem.unep.ch/pts) include polybrominated biphenyls (PBB) and diphenylethers, chlorinated naphthalenes and paraffi ns, pentachlorophenol (PCP), lindane, etc. Tens of chlorofl uorocarbons (CFCs) and other substances from the class of halogenated C1-C3 hydrocarbons belong to the class of ODS (http://www.epa.gov/ozone/


79


ods.html). There are many halogenated compounds on the EPA list of 188 hazardous air pollutants (http://www.epa.gov/ttn/atw/188polls.html).

The standard techniques for elimination of VOC and particulate in fl ue gases (see chapters 5 and 7) are usually suitable to get rid of the halogenated pollutants, as well. However, in the case the chlorinated and halogenated VOC are chemically destroyed, an additional problem connected with the formation of the halogen hydride arises. In some cases this just can be solved by connecting a caustic scrubber right after the treatment step. In other cases, this can result in harm produced to the system, e.g. catalyst poisoning, corrosion, etc. In addition, some halogenated chemicals are more stable and resistant to chemical treatment than normal VOC, which may require prolonged residence time of more severe conditions. In some processes special treatment, e.g. special catalysts, may be applied for chlorinated VOC removal which is different then for other VOC. On the contrary, if the emission control systems are designed to remove dioxins and POP, as a rule, the chlorinated VOC at the same concentrations can also be easily removed.

6.5 Prevention of PCDD/F formation during incineration6.5 Prevention of PCDD/F formation during incineration

The best way to control dioxin and furan emissions is preventing their formation by reducing or eliminating chlorine in the fuel and waste material being burned, e.g. in the case of municipal solid waste incinerators (MSWI) which contribute most of the PCDD/F air emissions. It can be argued whether it is at all worthwhile to incinerate municipal waste, the topic that has become an issue at the centre of attention of ecologists, industries, NGOs, governmental and international organizations. Even if a MSWI is provided with a system to capture dioxin emissions, one of its byproducts is the fl y ash. Fly ash contains very high dioxin levels and therefore has to be treated as a hazardous waste. Some ecologists and defenders of the classical MSW management say that there is no reason, neither economical nor ecological, to landfi ll a ton of a highly toxic fl y ash instead of landfi lling three tons of low toxic MSW. Unfortunately, the dioxin problem has been recognized in its dimension after a number of MSWI plants were already in operation. A strong public campaign against construction of new MSWI now exists worldwide. In developed countries substantial funds have been raised to modify these processes or to close them, but still many old incinerators remain in operation.

Should the chlorine containing materials be incinerated this must be carried out in the proper manner, in strict conformance with the technical requirements defi ned by the existing regulations. Since the dioxin formation is favored at cooling of the combustion gases, containing incomplete combustion products and chlorine atoms, as was previously mentioned, it is important to provide suffi cient oxygen and temperature so that that, before the effl uents are cooled, the organic matter is exhaustively defragmented and oxidized in order to suppress the ìpass-throughî and ìprecursorî routes of PCDD/F formation. The standard rules require that the incinerators be designed so that the combustion gases reside for at least 2 seconds in the zone of the furnace heated over 850°C and in the presence of at least 6% of oxygen.

Rapid quenching of combustion gases from temperatures of over 600°C to below 100°C is another practice which helps to reduce the formation of dioxins via ìde novoî and ìprecursorî mechanism. This way the temperature range of dioxin formation is passed very quickly, so only small quantities of dioxins are formed because their formation involves heterogeneous mass transfer processes that are relatively slow.


80


The particular attention should be paid to the best techniques and practices applied to the incineration of the hazardous and other POP waste, e.g. PCB and stockpiled chlorinated pesticides, since in this case the burnt material is the pure dioxin precursor and the risks of dioxins emissions are particularly high as their formation is intrinsically inevitable. Although this chapter is mainly dedicated to the techniques of reducing of UP POPs in emissions by the end-of-pipe approach, which presumes that these dangerous compounds have been formed, it has to be underlined that as in the case of MSW disposal practices, the alternative techniques of hazardous chlorinated waste treatment should be given priority where possible. A number of such modern techniques have been appearing recently and gaining popularity, also thanks to the support of environmental and international organizations. These emerging technologies are based on the processes other than incineration and high temperature oxidative treatment (e.g. reduction based processes and low temperature oxidation), so that the formation of PCDD/F is excluded by the nature of chemical process. UNIDO has been promoting the introduction of non-combustion techniques through demonstration projects with the support of GEF and of local governments in several countries. A number of such newly emerged non-combustion technologies are in commercial operation in Japan, Australia, Europe, Canada and USA and pilot and demonstration activities exist around the world. ICS-UNIDO has recently published a review of non-combustion technologies for destruction of stockpiled POPs (1).

In addition, incineration of hazardous waste can be conducted in a more effi cient manner by alterative design of the plant or combination with other physical treatment. Several techniques aim at homogenization of waste material in order to perform combustion in a homogeneous (non-fl ame) manner and thereby exclude low temperature zones in the combustion chamber and reduce the amount of partial combustion products. For example better mixing of solid waste in the combustion chamber can be achieved by using fl uidized bed incineration, and better heat temperature distribution throughout the reactor zone is achieved in the Isotherm power waste recovery process. A number of dense phase oxidation processes exist that also guaranty a homogeneous and fl ameless oxidation in the media, such as supercritical / subcritical water oxidation, wet air oxidation, and molten salt/metal/slug oxidation processes. Other technologies achieve better oxidation effi ciency by using more severe plasma treatment (a number of plasma arc oxidation and pyrolysis technologies exist) or by combining combustion with other treatment, such as infrared (IR), ultraviolet (UV), microwave (MW) irradiation. These assist mass transfer processes and defragment bigger organic molecules and simplify their oxidation.

6.6 End-of-pipe prevention and removal techniques6.6 End-of-pipe prevention and removal techniques

The prevention practices described above rarely allow reaching the required 0.1 ng-TEQ/m3 in the air emissions of incinerators. In addition to these, the end-of-pipe practices (additional modules) should be also applied in the processes based on combustion or high temperature oxidation of chlorinated substances. These modules are designed to capture, destroy, or prevent formation of dioxins in the post-combustion zone. This can be done by a number of approaches, usually by their combination. Most diffused modules include scrubbers, fi ltration with activated carbon, particulate capture systems (described in the following chapter), post-combustion chambers and afterburners, including catalytic oxidation systems. Some modern techniques for treatment of POPs in gaseous emissions use plasma or reductive environment. In this section we will briefl y describe the post-combustion modules which are installed in incinerators (Fig. 6.3) and in other potential POP production sources.


81


Figure 6.3. Organizational diagram of the rotary kiln type hazardous waste incineration plant. Reproduced from (2).

To widen the spectrum of approaches and methods that can be to destroy POPs and chlorinated VOC in the gaseous phase it is useful also to consider here the post-treatment systems employed in desorption processes, such as various technologies for soil remediation or for treatment of other highly contaminated matrices (PCB equipment, sludges, etc.) which are based on the evaporation of the contaminant as the fi rst step. As an example the soil remediation system based on vapor extraction could be considered, as presented in Fig. 6.4. Unlike incinerators, where the end-of-pipe systems are designed to treat micro amounts of pollutants (mainly PCDD/F), the post-treatment systems in desorption technologies usually have to address high concentrations of pollutants in the air or vapor steam (mainly original undestroyed contaminants which, in case if a chlorinated compound is desorbed thermally, would also contain some amount of PCDD/F).

Treatment of gaseous effl uents arising from the thermal desorption of polluted soils as well as treatment of some gaseous streams from chemical industries containing halogenated VOC is usually the same as applied for other VOC (post-combustion, catalytic oxidation, adsorption, etc.) (3). However, in the case of treatment of halogenated VOC vapors by oxidative techniques, post-treatment in caustic scrubber would be required in order to remove hydrochloric acid. In addition, if catalytic oxidation is applied to treat halogenated VOC, other catalyst formulations maybe used, especially for more resistant and more concentrated pollutants, as the oxidation catalysts based on platinum or chromium or magnesium oxides would be easily poisoned by acidic environment created by oxidation products.

Figure 6.4. Typical soil vapor extraction system. Reproduced from (4).


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The techniques and practices listed below in this and the following sections are applicable not only for POP destruction but also for a majority of VOC and organic pollutants. However, we have focused in a specifi c manner on the methodologies which are promising for destruction of polychlorinated pollutants and which are not listed in chapter 5, including some emerging ones. Before going to the techniques used to destroy POP and halogenated pollutants in effl uent gases (sections 6.6 and 6.7) letís consider some basic methods that are used either i) to intensify the main combustion step (post- or secondary combustion, in addition to the previously mentioned IR, UV, and MW intensifi cation methods in incineration), ii) to remove other hazards, e.g. chlorine and hydrochloric acid from combustion gases (scrubbers; can be also considered as preventive to PCDD/F formation), or iii) to remove contaminants physically without destruction (adsorption).

Secondary combustion chambers or afterburners are used in incinerators and are placed immediately after the furnace and are designed to destroy any undestroyed chlorinated waste if such is treated and to destroy the PCDD/F and incomplete combustion products formed during the fi rst step. To guarantee high temperature and effi cient oxidation, the post combustion chambers or afterburners are fuelled with methane or other auxiliary combustible and are equipped with air infl ow system. Additional liquid waste can be also added directly in the secondary combustion chamber.

Scrubbers are essential in any process where chlorinated waste undergo thermal high temperature treatment. Scrubbers remove hydrochloric acid or other halogen hydride and free chlorine from the combustion gases by reaction with alkali solution. Concentrated NaOH or KOH, CaOH solutions in water are usually used. This treatment is applied after the gases exit from the furnace or the secondary combustion chamber. The alkaline solution is introduced as a counter fl ow shower or the gases pass bubble through a vessel containing the solution. Several scrubbers can be placed consecutively and can be followed with the washing by pure water in a similar way. First scrubbers can be also designed in the way to provide excellent heat exchange and rapid quenching of the combustion gases which helps to order to reduce the dioxin formation rate. Scrubbing of incineration off-gases per se cannot effi ciently destroy halogenated chemicals; however, it eliminates hydrochloric acid and chlorine gases formed during incineration, therefore also reducing the probability of new POP formation in the post combustion zone.

Adsorption of PCDD/F from off-gases is one of the most effi cient treatment methods to remove these compounds. Such systems may have different design but are all based on the same principle of adsorption of dioxins on carbon, activated carbon, or coke. Other adsorbents can also be sometimes used, such as zeolites and synthetic polymers. Adsorption of the fl at aromatic dioxin and furan molecules on carbon surface is very strong and allows a very effi cient capture of these substances from the gaseous state as well as of other chlorinated aromatics. Filtering post-treatment systems are usually installed as the end modules of the process. Dioxins are not destroyed and remain in the adsorbed state, so the used fi lters have to be periodically replaced and treated as the hazardous waste, e.g. in the similar manner as the fl y ash.

6.7 Chemical destruction of POPs and chlorinated pollutants in 6.7 Chemical destruction of POPs and chlorinated pollutants in emissionsemissions

Modern incinerators and thermal desorption units can be also equipped with the catalytic oxidation systems or other type of post-oxidative treatment aimed to destroy any residual waste or newly formed dioxins coming after incineration or to destroy the pollutants


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coming from soil remediation by thermal desorption. Post-oxidation systems can be also installed in other processes that yield gaseous emissions containing chlorinated organics and POPs, such as organochlorine chemical production processes and non-combustion hazardous waste destruction technologies.

Catalytic oxidation or catalytic post-combustion systems (Fig. 6.5) are usually installed in incinerators as the fi nal step before the emissions go in the atmosphere. These systems are designed to treat small amounts of chlorinated chemicals, such as PCDD/F in the gaseous phase. In the case of bigger amounts of the treated pollutant in the emission, like in the case of chlorinated VOC treatment the catalytic oxidation module can be followed by scrubber and washing systems in order to remove hydrochloric acid.

Catalytic oxidizers and carbon fi lters are the essential components that allow effective reduction of the dioxin levels in production sources to the acceptable/required values. Unlike fi lters, the catalysts do not accumulate the pollutant and do not raise the issue of their disposal. The activated carbon fi lters should be more frequently replaced than catalysts. This increases the cost of operating incinerators, however even very stable catalysts require periodical replacement as these can be poisoned/deactivated by a number of substances that present in the emissions. Because of this, the catalysts may contribute more than the other post-treatment process modules to the overall maintenance and consumables costs of the plant. Many new catalysts and processes capable of treating streams containing chlorinated hydrocarbons have been commercialized in the last years.

Figure 6.5. Basic arrangement of the catalytic oxidation module. Reproduced from (5).

Particulate fi ltration and catalytic oxidation processes can be also coupled by introducing the catalyst in the membrane fi lter (catalytic fi ltration), as in the Gore process using TiO2/V2O5/WO3 catalyst and the polytetrafl uoroethylene fi ber (see Fig. 6.6) (6). This process has been successfully demonstrated/used in numerous combustion facilities, including waste incinerators, pyrometallurgical plants, cement kilns burning hazardous waste, etc.


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Figure 6.6. Remediaô Dioxin/Furan Catalytic Filter System. Reproduced from www.gore.com/remedia.

In general, most post-treatment catalytic systems employ catalysts based on TiO2 also using supported V2O5 and WO3. These catalysts also allow reduction of NOx and ammonia in the gas emissions. Operation temperatures can range from 160°C to 350°C, and different designs of the catalytic reactors are available. Noble metal catalysts can also be used. For example, the treatment of off-gases from the production of vinyl chloride is based on bimetallic Pt and Pd on γ-alumina catalysts. An activated alumina specially treated to resist chlorine anions is used to decompose the aromatic hydrocarbon solvents and some of the chlorinated hydrocarbons. (7)

Other post-treatment techniques based on oxidation applicable for moderate amounts of POPs and halogenated VOCs treatment in emissions include UV oxidation, biofi ltration and non-thermal plasma.

Photolytic (UV) oxidation technologies use UV light to ionize the organic molecules, by breaking molecular bonds and creating free radicals. UV or near UV light can be used (150-350 nm) which is suffi cient to destroy VOC and chlorinated VOC. The free radicals formed from oxygen and pollutant molecules then recombine and yield harmless oxidation products, such as carbon dioxide, water, and hydrogen chloride, in the case of chlorinated pollutants. Similar photo-catalytic technologies use UV light in the presence of titanium dioxide (TiO2) as a catalyst. The catalytic UV oxidation techniques allow carrying out the reactions at moderate or ambient temperatures.

Figure 6.7. Flow diagram of the PTI photolytic destruction process. Reproduced from US-EPA (8).


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A photolytic destruction system developed by Process Technologies, Inc. (PTI), feeds off-gas from an SVE system to a sorption chamber containing a fl uidized bed of adsorbent material (Fig. 6.7). The adsorption unit concentrates the contaminants from the air stream and is connected to a desorption column where the contaminants are realized and the more concentrated stream is then fed into the photolytic reactor. Short wavelength UV light (185-254 nm) is used, which effi ciently destroys halogenated hydrocarbons. A proprietary reagent is used (mainly calcium hydroxide), which reacts with the degradation products coming from the photolytic destruction step, forming solid and stable products. The technology was tested in various programmes by U.S. Navy and in the Superfund Innovative Technology Evaluation (SITE) Demonstration Program. Compounds treated in the system included several halogenated VOCs, such as 1,2-DCE, TCE, PCE, as well non-halogenated VOCs (3).

Adsorption-Integrated-Reaction (AIR) Process by KSE, Inc. is an example of photocatalytic technology which was demonstrated in full scale applications in groundwater strippers and soil vapor extraction processes at several air force bases and in the SITE Emerging Technology Program in 1995 (US) (3). In the photocatalytic processes, free radicals are formed when the infl uent stream contacts the catalyst (TiO2), which is activated by the UV light energy, but also in the gaseous phase by absorption of the UV light directly (photolytic pathway). The radicals then recombine to form water vapor, carbon dioxide, and, if chlorinated VOCs are treated, halogenated acids. More novel photocatalytic systems use coating glass fi bers with TiO2 and fi lling of the reaction vessel with UV-lit fi bers. Another approach is a solar-powered system in which the UV light from the sun activates the TiO2. DREs for photo-catalytic oxidation systems normally exceed 99% for chlorinated VOC, such as TCE, DCE, vinyl chloride, etc.

Biofi ltration processes can be used for treatment of remediation off-gas streams. The gas is passed through a bed of biologically active fi lter (Fig. 6.8). The organic content is adsorbed on the fi lter and degraded to carbon dioxide and water. Various media (organic or synthetic) can be used for these systems.

Figure 6.8. Basic arrangement of the biofi ltration process. Reproduced from (5).


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Until recently, biofi lters were not designed to treat chlorinated compounds. However, recent demonstrations have suggested that they can be used to remove these compounds, as well. The technique has been successfully applied to chlorinated VOCs such as PCE and TCE. While highly aerobic conditions achieve good removal of light to moderately halogenated organics, heavily halogenated species would require anaerobic or co-metabolic conditions (added methane, propane, or aromatics). However, only limited data exist on the use of biofi ltration for chlorinated pollutants. The method does not yet operates on the commercial scale for treatment of POP and chlorinated VOC.

Non-thermal plasma (see also chapter 3) is a relatively new technology which involves the destruction of organic compounds by ionization in the gas stream at temperatures from 30°C to 120°C and at atmospheric pressure. This method is particularly useful when treating pollutants at concentrations in the range of ppm. Specifi c plasma technologies include silent discharge plasma, gas-phase corona reactor, tunable hybrid plasma, and low-pressure surface wave plasma. The principle of non-thermal plasma treatment is basically the same as in the case of UV oxidation. Electrons generated in the electrical discharge then ionize the molecules of pollutants and oxygen in the gaseous phase producing radicals and ions, which then recombine yielding oxidation products. Recently, a number of hybrid non-thermal plasma techniques appeared, where plasma processing is combined with other methods such as wet processing, the use of adsorbents, and catalysis. For example, a photocatalyst (TiO2) can be introduced in order to intensify ionization of molecules. Wet processing during plasma treatment can be effi cient in incinerators, especially for POP destruction, in order to remove dust and hydrochloric acid.

Figure 6.9. Basic scheme of the non-thermal plasma set-up for treatment of pollutants in gaseous emissions. Reproduced from (4).

The principle components of a typical non-thermal plasma system (Fig. 6.9) are the reactor vessel and, if chlorinated compounds are treated, an acid scrubber. The reactor cell is connected to the power supply and control system for corona production. In a gas-phase corona reactor, contaminated infl uent gas is forced through a reaction chamber, where it is ionized by plasma. The reaction produces carbon dioxide and water, and in the case of halogenated hydrocarbons, halogen hydride.


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6.8 Treatment of macro streams of POPs and chlorinated pollutants6.8 Treatment of macro streams of POPs and chlorinated pollutants

In this group we will consider several destruction technologies which have been designed to process wastes containing POP as well as other hazardous chemicals, e.g. military waste and VOC. A detailed review of these and other non-combustion technologies suitable to process POP wastes was recently produced by ICS-UNIDO (1). Some of these technologies can treat pollutants in the gaseous phase, e.g. originally in the gaseous state, vaporised, or as aerosols. As a rule, such treatment is justifi ed to treat more concentrated pollutant streams than the previously described techniques for destruction of residual or micro amounts of POP in the emissions. Therefore, these methods can be suitable to treat chlorinated pollutants coming after thermal desorption processes used in remediation activities or from the industrial processes that produce gaseous streams of concentrated chlorinated pollutants, or other concentrated organic matter containing such. It is assumed that these technologies, originally designed to be stand alone, can be in principle coupled to the emission sources of different types and therefore can be also considered as POP pollution control methods.

There are not too many non-combustion chemical technologies that have been specifi cally designed to treat concentrated chlorinated chemicals in effl uents. The three non-combustion POP destruction processes that can treat gaseous streams directly are Gas-Phase Chemical Reduction (GPCR) and PLASCON, an ìin fl ightî plasma arc system. Plasma arc processes are sometimes assimilated to incinerators, but there are several substantial differences that concern the nature of processes, design, and effi ciency. As for the two above, CerOx process can also be considered theoretically applicable to gaseous emissions. In practice, none of these processes have been applied as an additional air pollution control module as these were designed for macro concentrated waste disposal. Some other processes, such as MCD (Mechanochemical Decomposition) by EDL in Australia and BCD (Base Catalysed Decomposition) by BCD Int. can also treat vaporised pollutants coming after desorption processes, however the treatment in these processes takes place in the dense phase, i.e. after condensation of the pollutants, therefore, such technologies can not be considered directly applicable to gaseous effl uents.

GPCR technology provided by Canadian Hallett Environmental was designed for treatment of stockpiled POPs by reduction of the chlorinated molecules in the gaseous phase with hydrogen. The technology operates at temperatures of 875°C and low (atmospheric) pressures. Emissions include mainly hydrogen chloride, methane and other low molecular weight hydrocarbons.

CxHyClxOz + H2 --> CH4 + H2O + HCl (thermal)

The process (see Fig. 6.10) can treat basically all types of chlorinated waste in different forms, namely bulk organic solids and liquids, high-strength PCB oils and mixed solid materials, aqueous waste, contaminated soils and sediments.

Since the GPCR process is based on the gas-phase reaction it is provided with a gas fed module, so POPs and other chlorinated organics present in the gaseous state can be treated directly in the continuous manner. This makes this technology a possible option to couple to a facility which produces POP emissions. However, the costs of this technology setup and operation may be considered too elevated to be applied for destruction of micro amounts of pollutants in the gaseous phase. This technology shows excellent destruction effi ciencies but can be suitable only for the oxygen free gaseous streams containing high concentration of chlorinated pollutants.


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Figure 6.10. Organizational diagram of the GPCR process. Reproduced from (9).

PLASCON, now marketed by DoloMatrix Int. Ltd., is an argon plasma arc process and is designed to process CFC and organic liquids, such as PCBs. PLASCON is an “in fl ight” plasma process which operates in the gaseous phase in the continuous regime (Fig. 6.11). The waste mixes directly with the argon plasma column. Argon is used as the plasma gas since it is inert and does not react with the torch components. The waste is fed directly into the plasma torch within the current of argon, where it is rapidly heated (one millisecond) up to 12.000°C and then passes into the fl ight tube where its pyrolysis occurs in nearly 20 ms at temperature of about 3000°C.

Figure 6.11. Basic scheme of the PLASCON plasma torch system. Reproduced from the vendor website (www.srlplasma.com.au) and (10).


89


In the beginning of the fl ight tube oxygen is also added (limited amount) to ensure that any carbon formed during pyrolysis is then converted to carbon monoxide. The high temperature generated by plasma causes compounds to dissociate into their elemental ions and atoms. Recombination occurs in a cooler area of the reaction chamber, followed by rapid (2 ms) alkaline quenching from 1500°C to less than 100°C. Such rapid quenching prevents the formation of dioxins and furans. The PLASCON process is applicable to the gaseous streams and was successfully applied for destruction of CFC in Australia. In principle, it can be applied as an add-on module to the processes that yield gaseous emissions containing high concentrations of POPs and chlorinated VOC. The process is highly effi cient, easy in operation and compact, however maybe costly for low concentrated pollutants due to high electricity and argon consumption.

CerOx, developed by CerOx Corporation, USA, is a mediated electrochemical oxidation process able to destroy hazardous organic waste derived from industrial processes and laboratory wastes, such as biogenic wastes, toxic chemicals, pesticides, herbicides, PCBs, dilute organics in water, etc. by converting them into CO2 and water.

The organic molecule to be destroyed is being put in contact with the aqueous solution of Ce4+ species, which is a strong oxidizing agent. Ce4+ then vigorously reacts with the organics oxidizing it to CO2 and water and being itself reduced to Ce3+. After the cerium anion is reduced to Ce 3+ by taking an electron from an organic compound, it is brought to the electrochemical cell where it is reoxidized back to Ce4+. The latter is again introduced in the reaction vessel thereby closing the process cycle. The two consecutive process modules treat organics in water and in the gas phase respectively (see Fig. 6.12). The second reactor module is designed to treat VOC and partial oxidation products in the emissions arising from the fi rst step. It uses a counter current fl ow of Ce4+ allowing the Ce4+ to oxidize organics.

Figure 6.12. Gas-phase CerOx treatment. Reproduced from the vendor website (www.cerox.com).


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The gas phase reactor can process small amounts of VOCs and can be sized to handle large quantities of compounds that transform into gas phase at lower than 95°C. In this connection, its application to treat chlorinated chemicals in emissions can be considered a possible option.


1. S. Zinoviev, P. Fornasiero, A. Lodolo, S. Miertus, Non-combustion technologies for POP destruction. Review and Evaluation, ICS-UNIDO Publications, 2007 (in press).

2. H.-U. Hartenstein, Performance of commonly used combustion technologies for POPs destruction, in STAP/GEF Technical Workshop on Emerging Innovative Technologies for the Destruction and Decontamination of Obsolete POPs, Washington, 1-3 October 2003.

3. Off Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice, U.S. EPA, March 2006, EPA-542-R-05-028.

4. Guide for Conducting Treatability Studies Under CERCLA, Soil Vapor Extraction, U.S. EPA, September 1991, EPA/540/2-91/019A.

5. FRTR Remediation Technologies Screening Matrix, available at www.frtr.gov.

6. M. Plinke, R.L. Sassa, W.P. Mortimer Jr., G.A. Brinckman, US 5620669, 1997.

7. Alternative technologies for the Destruction of Chemical agents and Munitions. National Research Council, Washington D.C.: National Academy of Sciences, 1993.

8. Technology Profi les Eleventh Edition. Volumes 1-3, U.S. EPA, September 2003, EPA 540/R-03/501.

9. GPCR presentation at South American regional workshop on the environmentally sound destruction of POPs and the decontamination of POP containing waste in the framework of Basel and Stockholm conventions, San Pablo, Brazil, 7-10 December 2004, available at crsbasilea.inti.gov.ar/evento11.htm.

10. T. Bridle, Scheduled Waste Management: The Australian Experience, in POPs Workshop on Emerging Innovative Technologies for the Destruction and Decontamination of Obsolete POPs, 1st-3rd October 2003.


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7 Technologies for Particulate Emission Control7 Technologies for Particulate Emission Control


The present chapter provides technologies available for particulate emission control for stationary applications: the use of gravity settling chambers, centrifugal separators, particulate wet scrubbers, electrostatic precipitators and fabric fi lters will be briefl y discussed (1,2).

Defi nitionsParticulate matter (PM) is a major class of air pollution. Black smoke, or soot,

is the most visible emission, but other, less visible pollutants are emitted. Combustion and non-combustion industrial processes, mining and construction activities, vehicles and incinerators are examples of potential sources of particulate. Beside these antropogenic emissions, natural sources (volcanos, forest fi res, wind storm, and so forth) are present, enhancing the negative impact on a global scale. An enormous number of parameters infl uence the particulate formation/ageing process, which leads to particulate matter in large variety of shapes and sizes, and therefore of physical and chemical properties. The concept of “aerodynamic diameter” has been developed to describe, in a simple and universal way, how a particle behaves in air. The aerodynamic diameter is the diameter of a spherical particle having a density of 1 gm/cm3 that has the same inertial properties (i.e. terminal settling velocity) in the gas as the particle of interest. The range of particle sizes of concern for air emission evaluation is quite broad. The Environmental Protection Agency (EPA) (http://www.epa.gov) has defi ned four terms for categorizing particles of different sizes (Table 7.1).

Table 7.1: Environmental protection agency terminology for particle size

EPA Description Particle sizeSupercoarse φ > 10 μmCoarse 2.5 μm< φ < 10 μmFine 0.1 μm< φ < 2.5 μmUltrafi ne < 0.1 μm

Using this division of particulates according to their size, the EPA has identifi ed various categories of particulate emission for the purposes of defi ning emission control: (I) Total Suspended Particulate Matter (TSPM); (II) PM10; (III) PM2.5; (IV) particles less than 1 micrometer; and (V) condensable particulate matter. As outlined below, these categories are based on the environmental impact of each.

Total Suspended Particulate Matter includes a broad range of particle sizes from 0.1 μm to about 30 μm in diameter, ie, fi ne, coarse, and supercoarse particles.

PM10 is defi ned as particulate matter with a diameter of 10 μm collected with 50% effi ciency by a PM10 sampling collection device. However, for convenience, the term PM10 is often used to include all particles having an aerodynamic diameter of less than or equal to 10 μm. PM10 is regulated as a specifi c type of "pollutant" because particles in this size range can penetrate into the lower respiratory tract.


92

TECHNOLOGIES FOR PARTICULATE EMISSION CONTROL

Analogously, PM2.5 is particulate matter with a diameter of 2.5 μm collected with 50% effi ciency by a PM2.5 sampling collection device. The EPA has chosen 2.5 μm as the partition between fi ne and coarse particulate matter. Relative to coarse and supercoarse particles, PM2.5 particles settle quite slowly in the atmosphere and are of particular concern from the point of view of human health due to their potentially long airborne retention time and the inability of the human respiratory system to defend itself against particles of this size.

Particles less than 1 micrometer in diameter is relatively hard to collect and therefore they can represent a signifi cant fraction of the particulate emissions from some types of industrial sources. Particles in the range of 0.2 to 0.5 μm are common in many types of combustion, waste incineration and metallurgical sources. Particulate can be much smaller than 0.1 μm. These sub-micrometer particles are composed of 20 to 50 molecules clustered together in a stable form. Particles with diameter between 0.01 and 0.1 μm tend to agglomerate rapidly to yield particles in a range greater than 0.1 μm.

Condensable particulate matter originates from condensing gases or vapors.

7.2 Particulate Formation7.2 Particulate Formation

In general, the mechanism of particle formation determines the size and composition of emitted particles. Not surprisingly, any such formation mechanism is in turn strictly connected to the source of particle emission. In the following paragraphs, the main sources of particulate emisions are summarised.

Physical attrition/mechanical dispersion occurs when two surfaces rub together and generates primarily moderate-to-large sized particles (5 μm < diameter <1000 μm). The composition and density of particulates thus formed are identical to those of the parent materials. Mining or construction activities are typical examples of processes where physical attrition is a relevant source of PM formation of this type. Figure 7.1 shows the grinding of a metal rod on a grinding wheel, which yields small particles that break off from both surfaces. In general, any part in movement contributes to particulate emission.

Housing

GrindingWhell

ParentMaterial

Particlesand Air

HoodAir Handling

Duct

To Fan

Figure 7.1: Particulate generated by physical attrition during the operation of a grinding wheel (adapted from ref 1)

Combustion process: When a fuel is injected into a hot combustion chamber, most of the organic compounds in the fuel are rapidly vaporized and oxidized in the gas stream. The fuel is quickly reduced to ash (incombustible material) and slow burning char (organic compounds) only. Eventually, most of the char will also burn leaving primarily the incombustible matter. Ash and char particles are primarily in the 1 to 100 µm range. This mechanism for particle formation can be termed combustion particle burnout. The combustion process of a fuel is rather complex and it is out of the scope of this brief


93


survey to analyze the mechanism of soot formation in detail. However, it is important to underline that condensation to clusters of polycyclic aromatic hydrocarbons present in the fuel, due to Van der Wall forces, and successive coagulation, can lead to primary soot particles. Soot is produced even in the combustion of methane (Figure 7.2). In fact, during the oxidation, a small portion of methane can decompose into methyl radicals which, under fuel rich conditions, form small molecules and reactive radicals of various sizes. Addition of hydrocarbon radicals to those species allows the growing of the radicalic hydrocarbons, which then can chemically condense to form chainlike aggregates (primary particles). The particles may be more or less liquid and they may contain small aromatic moieties, interconnected by fl exible hydrocarbon chains (3,4).

CH4

Methane fuel

• CH3Methylradical Acetylene

C CH H

• C3H3Propargylradical

• C6H5Phenylradical

Reactive smallmolecules and radicals

Pyrolise Chemicalcondensation

Desorption

Formation ofaromatic rings

Pyrens

Corones

Ovalenes

Graphitisation

Desorption

PAH(Polycycle Aromatic

Hydrocarbons)

Figure 7.2: Simplifi ed schema of particulate formation hypothesis of radicals during methane combustion

During heating of the particles in the fl ame, their chainlike, aliphatic structure is increasingly converted into aromatic rings and compounds, as a consequence of the extraordinary aromatic stability. Subsequently, the polyaromatic compounds may detach from the particles when they lay at the surface and all chemical bonds are saturated. As heating continues the particles form increasingly larger arrays of aromatic rings, becoming more and more graphitic and therefore denser. The particles tend to coagulate and pick up small radicals and acetylene for surface growth.

Homogeneous and heterogeneous nucleation occurs when the gas temperature decreases below the dew point and vapor phase materials condense to a particulate form (Figure 7.3). Homogeneous nuclei are composed of only one vapor phase compound, whereas heterogeneous nucleation is the condensation of material on the surfaces of existing particles, resulting in particulates composed of more than one compound. The vapor phase material which can nucleate in exhausted air can be grouped in two main categories: (I) organic compounds and (II) inorganic metals and metal compounds like mercury, lead, lead oxide, cadmium, cadmium oxide, cadmium chloride, and arsenic trioxide. Homogeneous and heterogeneous nucleation generally create particles that are very small, often between 0.1 and 1.0 μm.


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Figure 7.3: Municipal waste incinerator: homogeneous and heterogeneous nucleation of particulate (adapted from ref 1)

Droplet evaporation is caused by air pollution control systems that use recycled water from wet scrubbers (vide infra) to cool the gas streams. The recycled water, containing solid materials, is injected into the hot gas streams. The evaporation of the small water droplets leads to release of the suspended and dissolved solids as small particles in the size range 0.1 to 20 μm.

7.3 Particulate Emissions Control Systems7.3 Particulate Emissions Control Systems

Particulate emissions control systems apply forces to the particles in order to remove them from the gas stream. Depending on particle size, the collection mechanism can be based for instance on the effect of inertia, of Brownian diffusion, of gravity or of electrostatic attraction. The main particulate control techniques can be divided into the following fi ve categories.

7.3.1 Gravity Settling Chambers7.3.1 Gravity Settling Chambers

A gravity settler (Figure 7.4) is a long chamber through which the polluted gas is forced to pass slowly, allowing time for particles to settle by gravity to the bottom. These devices are used only for large particles (diameter greater than 75 μm). In fact, the very low terminal settling velocities of most particles encountered in the fi eld of air pollution limit the usefulness of gravity settling chambers. The cross section area (WH) is much larger than that of the duct approaching it, so that the linear gas velocity inside the chamber is lower


95


that in the inlet duct, thereby favoring settling of particulate. These devices must be cleaned manually at regular intervals, but their design and construction are simple. Through the last three decades, the stringent control requirements adopted have resulted in a sharp decline in the use of this type of collector. However, these devices still have some use in treating very dirty gases from metallurgical industries.

W

L

H

Inlet Baffles

Outlet Baffles

Gas FlowIN

Gas FlowOUT

Figure 7.4: Gravity settling chambers (adapted from ref 1)

7.3.2 Centrifugal Separators7.3.2 Centrifugal Separators

Centrifugal separators are mechanical collectors that use centrifugal force to drive the particle to the collection surface. In general, the gas stream is forced to spin in a cyclonic manner (Figure 7.5). The mass of the particles causes them to move toward the cyclone body wall and then settle into the hopper of the cyclone. The cleaned gas turns and exits the cyclone. There are two main types of mechanical collectors: (I) large-diameter cyclones, and (II) small-diameter multi-cyclones.

Large-diameter cyclones are usually 30-180 cm in diameter. They have a high effi ciency for particles of diameter higher then 20 μm, require low capital cost and they can operate at relatively high temperature. The fact that there are no moving parts reduce signifi cantly their maintenance cost.

Cleaned Gas

Dust

Dirty Gas

Inlet

Gas Outlet Tube

Cyclone body

Conical Section

Figure 7.5: Large-diameter cyclones (adapted from ref 1)


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In order to increase the spinning of the gas stream, small-diameter multi-cyclone tubes have been developed (Figure 7.6). Vanes located on the inlet of each of the tubes create an effi cient spinning movement of the gas stream. These type of cyclones usually have diameters of 8-30 cm. Due to the limited gas handling capacity of each tube, large numbers of tubes are mounted in parallel in a single collector. Small-diameter multi-cyclones are utilised to collect considerably smaller particles than large-diameter cyclones. In normal application, the separation effi ciency of small-diameter multi-cyclones is sharply decreased when particle diameter decreases below 5 μm; and these systems cannot be used for very large diameter material (above 30-50 μm), as large particles may plug the spinner vanes in the multi-cyclone tubes. However, it should be noted that specially designed cyclones with a different geometry with respect to that shown in Figure 7.5 have been developed to provide high-effi ciency particulate matter collection down to a particle size of 1 μm (5-8).

The presence of sticky and/or wet particulate matter in the gas stream is an important considereation in multi-cyclone systems. In fact, sticky and/or wet particulate matter can accumulate on the cyclone body wall or the inlet spinner vanes of conventional multi-cyclone collectors, reducing the performances of the system.

A strategy sometimes used with centrifugal separators is to use cyclones as a pre-collector of large-diameter embers generated in some combustion systems. Removal of embers is necessary to protect high-effi ciency particulate control systems located downstream from the mechanical collectors.

Dirty GasOutlet Tube

InletSpinnerVane

CollectingTube

Fly AshParticle

Traeted Gas


InletSpinnerVane

CollectingTube

Fly AshParticle


InletSpinnerVane

CollectingTube

Fly AshParticle

Traeted Gas

Figure 7.6: Small-diameter multi-cyclone (adapted from ref 1)

7.3.3 Particulate Wet Scrubbers7.3.3 Particulate Wet Scrubbers

The term scrubber was originally used to describe a device for collecting fi ne particles on liquid drops. Nowadays, this term has a more general meaning. In fact, even devices using a liquid to control sulfur dioxide emission, are called scrubbers.

The basic idea of scrubber is to make a liquid drop and the particle touch each other, so that the particle will adhere to the drop. Therefore it is necessary to properly introduce a large number of liquid drops (normally water) into the gas stream. The mixture of the gas and liquid containing the particulate can easily separated in a cheap cyclone, as depicted in Figure 7.7. Finally, the liquid can be separated from the solid, and recycled.


97


Scrubber

gas-liquidcontactor

Cyclone

gas-liquidseparator

Clean gasDirty gas

Dirty liquid

Mixed gas

and liquid

Clean liquid

Liquid recirculating pump

Collected solid

liquid-solidseparator

Figure 7.7: Schematic composition of a scrubber installation (adapted from ref 1)

Contact of the water drops with the gas to be cleaned may be achieved in a number of ways. In cross-fl ow scrubbers (Figure 7.8), multiple spray nozzles are located on the roof of the scrubber and dispense uniformly the liquid over the horizontally incoming gas. The liquid is collected at the bottom of the device. Effi ciency improves with decreasing drop size and increasing height of the scrubber. However care must be taken to ensure that the drops size is not too small, as this may result in a too slow vertical velocity. If this is slower than the horizontal gas velocity, the drops can pass out with the gas and they will therefore not be collected in the scrubber. This may represent a serious limitation for certain applications.

Gas in Gas out

Liquid out

Liquid in

Figure 7.8: Scheme of a cross-fl ow scrubber (adapted from ref 1)

In counter-fl ow scrubbers (Figure 7.9) the spray nozzles, are located in the roof of the scrubber. The liquid falls by gravity. The gas stream enter from the bottom of the scrubber and fl ows upwards. In this confi guration a potential drawback is blowing the drops out through top of the scrubber (ie, in the wrong direction).

Gas in

Gas out

Liquid out

Liquid in

Figure 7.9: Scheme of a counter-fl ow scrubber (adapted from ref 1)


98


In co-fl ow scrubbers (Figure 7.10), both the gas and the liquid enter at the left and exit at right. However, the liquid enters at right angle to the gas fl ow. The gas velocity is partially reduced by the liquid injection.

Gas in Gas out

Liquid out

Liquid in

Figure 7.10: Scheme of a co-fl ow scrubber (adapted from ref 1)

All three of these general methods have been incorporated into the many types particulate wet scrubbers are used in industrial processes: Venturis, impingement and sieve plates, spray towers, mechanically aided, condensation growth, packed beds, ejector, mobile bed, caternary grid, froth tower, oriented fi ber pad, wetted mist eliminators. As this list is quite extensive, for brevity only selected samples will be discussed here.

A typical co-fl ow Venturi scrubber is shown in Figure 7.11 (9). Particulate matter, which accelerates as it enters the throat, is driven into the slow moving, large water droplets that are introduced near the high velocity point at the inlet of the Venturi throat. The adjustable dampers in the unit illustrated are used to adjust the open cross-sectional area and thereby affect the speed of the particles entrained in the inlet gas stream. Venturi scrubbers include the following different design types: (i) fi xed throat, (ii) adjustable throat, (iii) collision (opposed-adjustable), (iv) single rod decks, and (v) multiple rod decks.

Gas Outlet

Gas InletSpray

Header

LiquidInlet Throat

MovableThroat

Dumper

Outlet

Figure 7.11: Co-fl ow venturi scrubber (adapted from ref 1)

Figure 7.12 shows an impingement plate scrubber. These scrubbers usually have one to three horizontal plates, each of which has a large number of small holes. The gas stream accelerating through the holes vapourises water droplets in the water layer above the plate. Particles impact with these water droplets.


99


Clean Gas

Dirty Gas

Mist Eliminator

Downcomer

Plate

Liquid Inlet

Figure 7.12: Impingement plate scrubber (adapted from ref 1)

Figure 7.13 report a typical spray tower scrubber. This is the simplest type of particulate wet scrubber in commercial service. Sets of spray nozzles located near the top of the scrubber vessel generate water droplets that impact with particles in the gas stream as the gas stream moves upwards.

Clean Gas

Dirty Gas

Liquid Sprays

Figure 7.13: Spray tower scrubber (adapted from ref 1)

7.3.4 Electrostatic Precipitators7.3.4 Electrostatic Precipitators

The basic idea an electrostatic precipitator is to give the particles an electrical charges and then to put them in an electrostatic fi led that drive them to the collecting wall. Figure 7.14 shows a simplifi ed scheme of an electrostatic precipitator with two plates. In this confi guration, the dirty gas passes between the plates, which are electrically grounded. Between the plates are rows of wires, held at a typical voltage of 40000 volts. The combination of charged wires and grounded plates produces at the same time the free electrons to charge the particles and the fi eld to drive them to the plates. On the plate the particle loses its charge, adheres to other preadsorbed particles forming a “cake”. This solid cake is removed by rapping the plates at regular intervals with a mechanical or electromagnetic rapper that strikes a blow on the edge of the plate. The composition of the particulate matter is very important because it infl uences the electrical conductivity within the dust layers on the collection plate (10-12).

There are three main styles of electrostatic precipitators: (i) negatively charged dry precipitators, (ii) negatively charged wetted-wall precipitators, and (iii) positively charged two-stage precipitators.


100


The negatively charged dry precipitators are the type most frequently used on large applications such as coal-fi red boilers, cement kilns, and kraft pulp mills. They are also widely used in air conditioning systems for buildings . In this case, charging and collecting are carried out in separate parts of the electrostatic precipitator, giving the name of two stage precipitators. Wetted-wall precipitators (sometimes called wet precipitators) are often used to collect mist and/or solid material that is moderately sticky. The positively charged two-stage precipitators are used only for the removal of mists.

Electrostatic precipitators can have very high effi ciencies due to the strong electrical forces applied to the small particles. These types of collectors can be used when the gas stream is not explosive and does not contain entrained droplets or other sticky material.

L2H

hDirty Gas

Clean Gas

Collected Duston Plates

Duct Removed fromPlates to Hoppers

Ground

Dust-collectionPlates

UpperWire

Support

LowerWire

Support

High-voltage Wires forCorona Discharge

Corona Dischargealong the Length

of a Wire

Figure 7.14: Scheme of an electrostatic precipitator with two plates and four wires (adapted from ref 1)

7.3.5 Filters7.3.5 Filters

Filters divide the gas fl ow into smaller parts where they can collect the particles. In general two categories of fi lter are used in air pollution control: surface fi lters and depth fi lters. Construction of fi lters with holes as small as many of the particle to collect is very diffi cult. Therefore, in general, the sizes of the holes of industrial fi lters are signifi cantly bigger than the particle diameter. Nevertheless, they act as effi cient surface fi lters (Figure 7.15a), as, when fi ne particles are caught on the side of a hole, they tend to bridge over the hole, reducing its dimensions. The cake of collected material becomes the “effective” fi lter, while the “original” fi lter act as a support. The growing cake have average pore size smaller than the diameter of the incoming particles. The particulate is collected on the front surface of the cake. For that reason this is called a surface fi lter.

Depth fi lters (Figure 7.15b) are the other important class of fi lter used in air pollution control. They collect particles throughout the entire fi lter body. Each layer has the same fi ltration effi ciency, however mass deposition decrease logarithmically from infl ow to outfl ow. The separation effi ciency depends on pore size of the fi lter (or the particulate diameter), on the depth of the fi lter, and on the gas velocity (13-14).


101


Filtered exhaust Filtered exhaust

Figure 7.15 a: Wall fl ow fi lter, surface fi ltration(adapted from ref 1)

Figure 7.15 b: Depth fi lter, depth fi ltration (adapted from ref 1)

7.4 Commercially Available Technologies7.4 Commercially Available Technologies

No.1Classifi cation of technology: Air Pollution ControlType of Industry: Chemical; PetrochemicalSubject: Reduction of Smoke Dust in Boiler Exhaust GasPurpose: To keep the concentration of the smoke dust discharged from coal boilers

under 0.03 g/Nm3 in accordance with the regulation of the local prefecture.Principle: The temperature of the exhaust gas is around 150°C.The bag fi lter chamber

is divided into 6 chambers with 450 fi lter cloths suspended in each chamber for a total of 2,700 pcs. Clogging of the fi lter cloth is effectively prevented: the dust adhered to the external surface of the fi lter cloth can be dropped into a hopper by means of periodical pulses of cleaning air jets from a rotating manifold mounted on the cell plate. The interval of these pulses can be arranged to keep the pressure difference on the fi lter cloth at about 100 mmAq.

Effect and Achievement: Operation is carried out with a bag fi lter outlet dust concentration of 0.025 g/Nm3 as compared with the inlet dust of 4 ~ 17g/Nm3.

The system can operate below 0.010g/Nm3 if there is no leakage in the cell plate packing. Although the bag fi lter will be bypassed when the boiler fuel is changed from coal to heavy oil, the regulation value of dust can be attained with desulfurization equipment installed downstream.

Future: Although the physical properties tests are conducted with samples for diagnosis of deterioration of the fi lter cloth, there is no appropriate guide on damage or deterioration, so there is a tendency towards over maintenance to avoid damage of the fi lter cloth during operation.

Implementing company: Japan Synthetic Rubber Co., Ltd. Yokkaichi Plant

No. 2Classifi cation of technology: Air Pollution ControlType of Industry: Petroleum refi ningSubject: Reduction of Dust Discharged from Catalyst RegeneratorPurpose: To reduce the dust in the exhaust gas discharged from the catalyst

regenerator unit of the fl uidized catalytic cracker from 0.30 g/Nm3 to 0.03 g/Nm3.

Details: Normally, the dust concentration is high in the exhaust gas discharged from the catalyst regenerator unit, since it contains scattered catalysts. The electric precipitator was therefore installed to collect the dust and to clean the exhaust gas.

Principle: The basic structure of the electric precipitator consists of 1) electric discharging poles, 2) dust collecting poles to collect the dust charged by corona current, ) striking devices to release the dust adhering to the pole


102


plates, 4) discharge device to process the dust collected and ) shielding of the dust collecting unit and high voltage insulating mechanism. The dust particles in the discharge gas are charged by corona discharge from the discharging pole and are attracted to the dust collecting pole by Coulomb force. The dust that has collected and grown on the dust collecting pole is separated from the pole by the impact of the striker and is removed after falling down by the gravity.

Future: It is necessary to adapt the optimum processing conditions to the maximise exhaust gas purifi cation, and in particular it must be 1) increase the corona discharge by maintaining a high load voltage, 2) maintain adequate gas temperature - higher than the dew point but not too high (normally 230-300°C ), 3) adjust strike time and 4) refrain from storing excessive dust in the hopper of the electric dust collector.

Implementing company: Showa Yokkaichi Sekiyu Co., Ltd. Yokkaichi Refi nery.


1. Air Pollution Control Engineering, N. de Nevers, second Edition, 2000, Mc Graw Hill, New York.

2. Pollution: Causes, Effects and Control, R. M. Harrison Ed., The Royal Society of Chemistry, Cambridge, 1996.

3. Gruenberger TM, Moghiman M, Bowen PJ, Syred N, Combustion Science and Technology, 2002, 174 (5-6): 67-86.

4. Barone AC, D’Alessio A, D’Anna A, Combustion and Flame, 2003, 132 (1-2): 181-187.

5. Cooke MJ, Ford NJ, Pragnell RJ, Journal of the Institute of Energy, 1991, 64 (461): 239-246.

6. Easom BH, Burlatsky SF, Altman RF, Chang R, Pollution Engineering, 1999, (7): 40-42.

7. Jaworek A, Balachandran W, Krupa A, Kulon J, Lackowski M, Environmental Science and Technology,40 (20): 6197-6207 OCT 15 2006

8. Lehner M, Hoffmann A, Chemical Engineering & Technology, 2004, 27 (7): 722-728.

9. Ananthanarayanan NV, Viswanathan S, Industrial & Engineering Chemistry Research, 1999, 38 (12): 4889-4900

10. Kim SH, Lee KW, Journal of Electrostatics, 1999, 48 (1): 3-25.

11. Zukeran A, Looy PC, Chakrabarti A, Berezin AA, Jayaram S, Cross JD, Ito T, Chang JS, IEEE Transactions on Industry Apllications, 1999, 35 (5): 1184-1191.

12. Prasad NVPRD, Lakshminarayana T, Narasimham JRK, Verman TM, Raju CSRK, IEEE Transactions on Industry Apllications, 1999, 35 (3): 561-567

13. Jung SC, Park JS, Yoon WS, Iternational Journal of Automotive Technology 8 (2): 165-177 APR 2007

14. Yamamoto K, Ochi F, Journal of the Energy Institute, 2006, 79 (4): 195-199

ReRere


103


8 Abatement of the Hydrocarbon and CO Emissions from 8 Abatement of the Hydrocarbon and CO Emissions from Gas TurbineGas Turbine

This short Chapter is specifi cally dedicated to the abatement of pollutant gasses from gas turbines, due to the importance of this application. Notably that NOX reduction is also discussed in Chapter 3.2.2, while the reduction of the emission of volatile organic compounds is discussed in Chapter 5 and the control of NOx, hydrocarbon and CO emission from vehicles is discussed in Chapter 10.

8.1 Gas Turbines8.1 Gas Turbines

Combustion turbines that burn fuel to generate heat are widely used for the co-generation of electricity and steam.(1) Gas turbines have usually a coaxial centrifugal air compressor. The preheated and compressed air at 200 - 500 °C meets the fuel stream in the combustion chamber. The exhausted gas have a temperature up to 1600-2000 °C, which is too high for the turbine. Therefore, part of the air is by-passed to cool the combus-tion gas before it goes to the turbine. Notably that nitrogen oxides are formed during the combustion process, trough the various mechanisms previously discussed in Chapter 3. We need to recall that the formation of thermal-NOx is signifi cant at temperatures above approxi-mately 1500 °C while prompt NOx is only signifi cant at close to stoichiometric conditions in fl ame combustors. Finally, the NOx formed as a reaction intermediate, is signifi cant at low temperatures in and is unstable at higher temperatures. The combustion of fuel containing N bound species most likely involves the thermal decomposition of the bound nitrogen species upstream of the combustion zone. Therefore, signifi cant amounts of low molecular weight molecules, such as NH3, NH2, NH, HCN, CN., can be fed to the gas turbine and partially remain in the exhaust.

The existing methods for NOx control in gas turbines includes prevention strategies to control the combustion and end–off pipe measures to clean up the exhaust gas. Primary measures include, beside catalytic combustion, water or steam injection, lean-premixed combustion and staged combustion. In this respect it mast be notice that since the 1980s, a common practice in operation of these gas turbines has been to use water or steam injection to lower the combustion fl ame temperature to reduce the NOx emissions. However, the quenching of the fl ame results in increased CO and hydrocarbon (HC) emissions. In fact, the use of water or steam injection typically can reduce the NOx emissions from 150 vppm to about 40 vppm, with a concurrent increase in CO emissions from 10 vppm to as high as 400 vppm and an increase in hydrocarbon emissions. Furthermore, a water purifi cation stage is need and water injection can lead to corrosion problem and / or can alter the combustion. However, water injection is still a viable technology, and in some case is used in combination with new burner developments and end-off pipe technology.

Lean-premixed combustion has been introduced lately and gives low emission levels, typically around 25 ppm thermal-NOx. The drawback of this approach is that a complicated and expensive control systems must added.


104

ABATEMENT OF THE HYDROCARBON AND CO EMISSIONS FROM GAS TURBINE

Staged combustors operate fi rst under fuel-rich conditions followed by a quick quench to fuel-lean. This system offers great potentiality and high effi ciency for fuels with large amount of bound nitrogen species, which can be effectively reduced in the fuel rich stage. Notably, however that the emissions of other pollutants than fuel-NOx are usually higher than for other prevention alternatives.

The catalytic combustion (2) is the most promising and challenging strategy for ultra-low emission gas turbines. It has been shown that it should be the lowest-cost and preferred technology for applications that require emission levels below 5 ppm (3). This application will be discussed in more detail in the following 8.2.

The most important end-off pipe technology which can be applied to gas turbine is the effective selective catalytic reduction (SCR). The major drawbacks of SCR are the high capital and operating costs, the large bulky catalytic reactor, the need of NOx/NH3 analytical equipment as well as an ammonia or urea storage and distribution system The selective non-catalytic reduction (SNCR) is far less effective in NOx reduction even though it is much cheaper and easy to control.

In any case a critical design features showed to be the design of the postcatalyst stage with homogeneous combustion to reach low HC and CO emissions.

8.2. Catalytic combustion 8.2. Catalytic combustion

The catalytic combustor (2) is basically a lean-premixed combustor, in which the combustion is stabilized by a catalyst. Here, the combustion can be ultra-lean, since the catalyst can operate below the limits of homogeneous fl ammability. Therefore, the thermal-NOx emission can be very low. Notably that the presence of a catalyst lowers signifi cantly the temperature of the combustion chamber and of the exhausted gasses (900-1300 °C), which can go directly to the gas turbine without need for a by-pass of cooling air, as shown in Figure 8.1.

Fig. 8.1 Schematic representation of a catalytic combustor and a fl ame combustor in an open-cycle gas turbine.

Typically, the inlet air temperature to the combustor is raised to approximately 200 °C for a low

pressure turbine and to 400 °C for a high pressure turbine at full load. Notably that at idle and partial load conditions, the gas turbines do not reach the same inlet temperature. Therefore, a suitable catalyst must be able to ignite the fuel already at signifi cantly low – ignition temperature. Alternatively to the design and use of highly active catalyst, pilot fl ames


105


can be used to heat the gas to the catalyst ignition temperature. However, this requires a careful designed, to control the amounts of thermal-NOx (4).

The main factors which infl uence the maximum outlet temperature of the combustor are the amount of cooling air, homogeneity or pattern factor of the fuel-air mixture temperature losses in the transition duct between combustor and turbine, turbine inlet temperatures and adiabatic temperature rise on combustion of the fuel. Finally, it must be noted that gas turbines are designed for a specifi c turbine inlet temperature, which is not the same as the combustor outlet temperature. For example, if the gas turbine is designed for an inlet temperature of 1250 °C, approximately 100 °C could be lost in cooling air and temperature losses in the transition duct. Further, a 5% inhomogeneity in the air-to-fuel mixing could give a temperature deviation of ≈ ± 50°C in adiabatic temperature rise. Therefore, the catalyst must be designed to work at 1400 °C and not at 1250 °C.

Combustor can be made without any large change in the gas turbine design.

8.3 Catalytic control of Hydrocarbon Emissions from Gas Turbine 8.3 Catalytic control of Hydrocarbon Emissions from Gas Turbine

While NOx emissions from gas turbine have been the subject of many regulations, only recently abatement of HC, CO and SOx have been considered. Notably that air quality regulations have for long time not even considered methane emissions. This is because methane is non toxic and is largely considered unreactive with regards to the atmospheric processes that cause photochemical air pollution. Thus, hydrocarbon reduction effi ciency usually concerns the destruction of nonmethane hydrocarbons (NMHC). To meet these requirements, new catalysts has been developed for simultaneous CO and HC removal (5-7).The catalytic oxidation activity of these catalysts, strongly depends on the type of hydrocarbon. For example, unsaturated or substituted hydrocarbons such as alkenes (e.g., propylene), aromatics (e.g., toluene), and aldehydes (e.g., formaldehyde) are much easier to oxidize (i.e., require lower temperature) than saturated paraffi ns such as propane. Among saturated compounds, those with lower carbon numbers are more diffi cult to convert than those with higher carbons numbers (e.g., propane is much more diffi cult to convert than hexane).

Hydrocarbons in the exhaust of combustion turbines are composed of “combustion-derived” and “fuel-derived” products. Fuel-derived hydrocarbons are uncombusted hydrocarbons from the fuel that reached the turbine exhaust, whereas combustion-derived hydrocarbons are compounds that have undergone a partial chemical change in the turbine combustor that falls short of complete conversion to carbon dioxide and water. Depending on the fuel source, the exhaust HC species and composition can vary greatly. It has been reported that, for aircraft turbine engines operating with jet A fuel, combustion-derived ethylene, acyetlene, propylene, and formaldehyde accounted for a major portion (22 to 43 percent) of the nonmethane hydrocarbons (8). Fuel-derived, C7-C9 hydrocarbons represent a large portion of the remaining NMHC in the exhaust. On the other hand, the NMHC in the exhaust of natural gas fi red turbines are usually reported to consist of fuel-derived ethane and propane, with trace levels of combustion-derived formaldehyde, benzene, and other substances. The requirements of a catalytic NMHC emissions abatement system can vary widely as a result of these distinctly different NMHC compositions for turbines using different fuels.

Except for C2-C5 paraffi ns (i.e., ethane, propane, butane, and pentane), the conversion rates of NMHC compounds are controlled by the rate of gas mass transfer to the catalyst surface when operating at temperatures greater than 250 °C. These reactive, often combustion-derived, hydrocarbons include alkenes, alkynes, aromatics, C+6 paraffi ns, and partially oxygenated hydrocarbons. This conversion effi ciency will depend primarily on their gas


106

ABATEMENT OF THE HYDROCARBON AND CO EMISSIONS FROM GAS TURBINE

phase diffusivities and, as in CO oxidation, on the geometric surface area of the honeycomb catalyst. The conversion rates of reactive hydrocarbons will always increase with increasing catalyst cell density, because the geometric surface area for reaction increasing.

However, the absolute conversion level for each species will depend on its diffusion rate in the exhaust gas. In general, larger, heavier molecules (like C8 and C9 molecules) will diffuse more slowly than smaller, lighter molecules such as ethylene.

The primary hydrocarbon types found in the exhaust of a natural gas fi red combustion turbine are light paraffi ns, and these are among the less reactive molecules for oxidation. Methane, ethane, propane, butane, and (to a lesser extent) pentane) require special catalysts, higher temperatures, or both before they can be destroyed using practical volumes of catalyst in a combustion turbine exhaust.

8.2 Catalytic control of CO Emissions from Gas Turbine8.2 Catalytic control of CO Emissions from Gas Turbine

The catalyst used for the abatement of CO emissions from gas turbine can range from 0.05 to 0.5 percent Pt catalyst dispersed on γ-Al2O3 on a ceramic honeycomb with 100 to 400 cpsi. Since this reaction is basically bulk mass transfer controlled above 150 °C, increasing the linear velocity, the reactor volume, and the number of cpsi of the honeycomb can increase the catalyst effi ciency. However, this is at the penalty of increased pressure drop. Increasing the space velocity diminishes the maximum conversion obtainable. The oxidation catalyst can be located anywhere downstream in the gas turbine exhaust as long as the temperature is above 100 °C (preferably above 250 °C) to minimize catalyst requirements (e.g., volume).

These catalytic systems are assembled in modules (e.g., 2*2 feet) and fabricated into walls of catalyst. Typical catalytic depths range from 3 to 6 inches. These catalyst walls have been fabricated up to 30 feet wide. Depending on the installation, space velocities range from 100, 000 to 300, 000 hr-1 and cell geometries from 100 to 400 cpsi. Pressure drops of 1 to 10 inches of water are typical. Either ceramic or metallic substrates can be utilized (9).

CO abatement systems for fuels containing sulphur require special catalyst formulation and process conditions that minimize the SO2 to SO3 reaction and reduce the poisoning effect of both SO2 and SO3 on the catalyst. Notably that SO2 strongly chemisorbs onto the Pt sites at temperatures below about 300 °C, resulting in inhibition of the CO oxidation reaction. Above 300-350 °C, the SO2 is converted to SO3, which can react with the γ-Al2O3 washcoat to form Al2(SO4)3. This lead to deactivation by pore blockage. Therefore catalyst must be designed in such a way that scavenger additives protect the active phase from SOx adsorption, limits the reaction between SO3 and the washcoat and includes additives that suppress the activity of Pt toward SO3 production at the reaction conditions.

Supported noble metal catalysts and in particular supported gold nanoparticles on transition metal oxides show interesting performances (10). Commercial Engelhard Technology Camet® CO Oxidation catalyst (11) has interesting performances for such type of applications. This catalyst, noble metal-based, coated onto a open cross fl ow channel (“herringbone” or “skew” type) corrugated metal foil, wound into monolithic structure, is a good available solution to abate CO from gas turbines exhausts. It can be combined to the SCR catalyst below for complete exhaust abatement service and provides the lowest possible pressure drop with very high surface area. It is washable and non-hazardous when spent. The same technology can be used in several other VOC abatement processes (10).


107


8.3 Gas Turbine Aftertreatment Catalyst Deactivation8.3 Gas Turbine Aftertreatment Catalyst Deactivation

There are three primary sources of catalyst poisons and contaminants in the exhausts of co-generation combustion turbines: fuel contaminants, boiler leaks, and turbine lubricants. Of the three, sulphur oxides from liquid fuels are the most common. Poisoning from boiler feed additives and turbine lubricants can be much more severe.

At temperatures below about 300 °C, SO2 chemisorbs on to the active metal sites and inhibits the oxidation of both CO and HC (12). It is for this reason that most processes operate above 300 °C . Notably that SO3 formed from the oxidation of SO2, can react with alumina leading to formation of Al2(SO4)3, which further deactivates the catalyst. Catalysts are therefore formulated to minimize SO3 production. New washcoats have been developed that adequately disperse the active catalytic

metals but are unreactive towards SO3.Catalyst contamination from sources such as turbine lubricant and boiler feed

water additives (e.g., Zn, P, Ca) is usually much more sever than deactivation by sulphur compounds in the turbine exhaust. This is because these materials irreversibly mask the catalyst surface. Therefore, catalyst regeneration remains the best option for maintaining good conversion in the dirtiest environments. Commercial experience led to the development of catalyst regeneration procedures that can restore most of the original catalyst activity, however this requires shat down of the process.


1. Walsh P. and Fletcher P., Gas Turbine Performance, Second Edition, Blackwell Publishing House, 2004.

2. Magnus Johansson E. et al. in Catalytic Combustion for Gas Turbine Applications, Chapter 6, Catalysis, Volume 14, The Royal Society of Chemistry, 1999.

3. Dalla Betta R.A., Catal. Today, 47 (1997) 369-375.

4. Sadamori H., Catal. Today, 47 (1999) 325-338.

5. Speronello, B., Chen, J., and Heck, R. 1992. A Family of Verstile Catalyst Technologies for NOx and CO Removal in CO-Generation. 92-109.06, 85th Annual Awma Meeting, June 21-26, 1992, Kansas City, Mo.

6. Chen, J., Speronello, B., and Heck, R. 1993. Catalytic Control of Unburned Hydrocarbon Emissions in Combustion Gas Turbine Exhaust. 93-mp 7.03, 8th Annual AWMA Meeting, june, 1993, Denver, Co.

7. Pereira, C., Gulian, F., Czarnecki, L., Rieck, J. 1990. Dual Function Catalyst for Clean Gas Application. 90-105.6, 83rd Annual AWMA Meeting, June 24-29, 1990, Pittsburgh, pa.

8. Spicer, C., Holdren, M., and Smith, D. 1990. Chemical Composition of Exhaust from Aircraft Engines. Gas Turbine and Aeroengine Congress and Exposition, June 11-14, 1990, Brussels, Belgium.

9. Pereira, C., Plumblee, K., and Evans, M. 1988. Camet Metal Monolith Catalyst System for Cogen Applications. 2nd International Symposium on Turbomachinery. Combined Cycle Technologies and Cogeneration. IGTI-Vol 3. Ed. G. Serovy and T. Fransson, Book No. 1002705.

10. Centi G., Ciambelli P., Perathoner S., Russo P., Cat. Today, 75 (2002) 31-15.

11. http:\\www.engelhard.com/etg

12. Heck R.M. and Farrauto R.J., Catalytic Air Pollution Control, Wiley-Interscience; 2 edition, New York (2002).



109


9 Ozone Control Strategies9 Ozone Control Strategies

Ozone, the triatomic form of oxygen (O3) is a gaseous atmospheric constituent. In the troposphere, it is created both naturally and by photochemical reactions involving volatile organic compounds resulting from human activities (photochemical smog). Ozone is a strong photochemical oxidant and therefore, in relatively high concentrations, it can be dangerous and harmful for human health and can damage the ecosystem, the agricultural activities and reduce the life of many materials (for instance it can increase the rate of oxidation of plastic materials). (1-2)

Notably that there is a general great interest towards understanding of the ozone reaction in the atmosphere and therefore to its control and regulations. In fact it has been established that chlorine and bromine containing compounds, such as chlorofl uorocarbons (CFCs), halons, a broad range of industrial chemicals used as propellants, refrigerants, fi re retardants, solvents, process agents, foaming agents and fumigants, can attach the tropospheric ozone layer allowing ultraviolet-B radiation to reach the ground, which could raise the incidence of skin cancer, cataracts, and other adverse effects on the human immune. In this respect, a series of international agreements on the reduction and eventual elimination of production and use of ozone depleting substances have been put in place. The well known Montreal Protocol became effective in 1989. Currently, more then 160 countries participate in the Protocol. Efforts are dedicated to the recovery of the ozone layer in about 50 years. In the United States, the U.S. Environmental Protection Agency (EPA) continues to establish regulations to phase out these chemicals. The Clean Air Act requires warning labels on all products containing CFCs or similar substances, prohibits nonessential ozone depleting products, and prohibits the release of refrigerants used in car and home air conditioning units.

Furthermore, in view of the harmful effects of photochemical pollution in the lower levels of the atmosphere, the European Council adopted in 1992 Directive 92/72/EEC on ozone air pollution (the Ozone Directive). The Directive came into force in March 1994. It established procedures for harmonized monitoring of ozone concentrations, for exchange of information, for communication with and alerting of the population regarding ozone and to optimise the action needed to reduce ozone formation.

More delicate problem are related those related to the abatement of ozone concentration in the urban atmosphere. In fact, motor vehicle exhaust and industrial emissions, gasoline vapours, and chemical solvents are the major sources of ozone precursors. Strong sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. Many polluted urban areas tend to have high levels of “bad” ozone, but even relatively clean areas can suffer of similar problems due to the effects of winds, which can carry VOCs and NOx emissions hundreds of miles away from their original sources. Ozone concentrations can signifi cantly vary from year to year, accordingly with weather conditions (number of hot / sunny days or periods of air stagnation). Therefore long-term ozone concentration predictions are extremely diffi cult. The Clean Air Act Amendments of 1990 require EPA, states, and cities to implement programs to further reduce emissions of ozone precursors from sources such as cars, fuels, industrial facilities, power plants, and consumer/commercial products. Power plants have partially reduced emissions, cleaner cars and fuels have been developed, gas


110

OZONE CONTROL STRATEGIES

stations are increasingly using special nozzles at the pumps to recapture gasoline vapours, and vehicle inspection programs are being improved to reduce emissions.

9.1 Strategies for Reducing Ground-level Ozone9.1 Strategies for Reducing Ground-level Ozone

Strategies for reducing ground-level ozone include are base on prevention and in particular on the reduction of NOx and VOC emissions from power plants and industrial combustion sources, on the introduction of low-emission cars and trucks, and on the use of “cleaner” fuels. (3) All this aspect are discussed separately in the other book chapters.

9.2 Ozone Abatement in Jet Aircraft9.2 Ozone Abatement in Jet Aircraft

Great effort (3-5) has been devoted to the development of ozone abatement technology suitable for application in Jet Aircraft. Adsorption and thermal and catalytic decompositions have been investigated so far. The amount of adsorbent (e.g., carbon) required for such applications would weight too much for practical use. High fuel penalty limits the use of thermal processes which require high temperatures. Catalytic decomposition occurs at much lower temperature and is more acceptable. Therefore, the majority of the aircrafts equipped for ozone abatement, use a catalyst to decompose ozone. (2)

Both precious and base metals were explored for this application. It must be noticed that, due to the ozone high oxidation capability, it has been observed a rapid deactivation of many catalytic materials after a short exposure. This caused considerable problems in early testing because many materials looked good on bench tests but failed in the aircraft after a relatively short time. (4). The most effective catalytic material is about 1 percent Pd on gama alumina supported on a high cell density ceramic or metallic monolith.


1. Pollution: Causes, Effects and Control, Harrison R. M. Ed., The Royal Society of Chemistry, Cambridge, 1996.

2. Heck R.M. and Farrauto R.J., Catalytic Air Pollution Control, Wiley-Interscience; 2 edition, New York (2002).

3. Couach O., Kirchner F., Jimenez R., Balin I., Perego S., van den Bergh H., Atmospheric Environment, 38 (2004) 1425–1436.

4. Heck R., Farrauto R., and Lee H., Cat.Today, 13 (1992) 43-85.

5. Galligan M.P. and Dettling J.C., United States Patent 5620672, 1997.


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10 Air Pollution from Mobile Sources10 Air Pollution from Mobile Sources

Air pollution generated from mobile sources is a problem of general interest. In the last sixty-years the world vehicle fl eet has increased from about 40 million vehicles to over 700 million; this fi gure is estimated to increase up to 920 million by the year 2010 (1). The environmental concern originated by mobile sources is due to the fact that the majority of engines employ combustion of fuels derived from crude oil as a source of energy. Burning of hydrocarbon ideally leads to formation of water and carbon dioxide, however, due to non-perfect combustion control and high temperatures reached in the combustion chamber, the exhaust contains signifi cant amounts of pollutants which need to be transformed into harmless compounds. This chapter is mainly focussed on the catalytic aspects of pollution abatement, even though the reader should consider that technological solutions such a electrically heated catalysts, reformulation of the fuel, etc., may heavily affect the converter performances (2).

10.1 Automotive Emission Characteristics and Regulations10.1 Automotive Emission Characteristics and Regulations

Engine exhausts consist of a complex mixture, the composition depending on a variety of factors such as: type of engine, driving conditions, vehicle speed, etc. The exhaust contains principally three types of pollutants, unburned or partially burned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), mostly NO (see Table 10.1) (3-8). In general, the emissions strongly depend on air-to-fuel (A/F) ratio, as exemplifi ed in Figure 10.1.

Po

lluta

nt

em

isio

ns

(a.u

.)

Air : Fuel ratio, by weight

10 14 18 22

Rich Lean

HCCO

NO Engine power

0.68 0.95 1.22 1.50 l

Figure 10.1: The effect of air-to-fuel ratio (w/w) on engine emissions (after ref. (8))

Tuning of the engine to rich feed gives the highest power output, which, however, occurs at expenses of high fuel consumption. Under lean-conditions lower combustion temperatures lead to lower NOx emissions, however, at very high A/F engine misfi re occurs,


112

AIR POLLUTION FROM MOBILE SOURCES

leading again to high HC emissions. Under any A/F conditions catalytic abatement of pollutants is needed to comply with the legislation limits. Only at stoichiometric conditions appropriate amounts of reducing and oxidising agents are present in the exhaust to carry out simultaneously the catalytic reactions of NOx reduction and CO and hydrocarbon oxidation.

Table 10.1: Exhaust emission of vehicles. (from ref 4)

Exhaust components and conditions a

Diesel engine Four stroke spark ignited-engine

Four-stroke lean-burn spark- ignited engine

Two-stroke spark-ignited engine

NOx 350 - 1000 ppm 100 - 4000 ppm ≈ 1200 ppm 100-200 ppm

HC 50 - 330 ppmC 500 - 5000 ppmC ≈ 1300 ppmC 20000 - 30000 ppmC

CO 300 - 1200 ppm 0.1 - 6 % ≈ 1300 ppm 1 - 3 %

O2 10-15 % 0.2-2 % 4 - 12 % 0.2 - 2 %

H2O 1.4-7 % 10-12 % 12 % 10 - 12 %

CO2 7 % 10-13.5 % 11 % 10 - 13 %

SOx 10 - 100 ppm b 15 - 60 ppm 20 ppm ≈ 20 ppm

PM 65 mg/m3

Temperatures rt - 650 °C rt - 1100 °C c rt - 850 °C rt - 1000 °C

λ (A/F) e ≈ 1.8 (26) ≈ 1 (14.7) ≈ 1.16 (17) ≈ 1 (14.7) d

a N2 is remainder. b For comparison: diesel fuels with 500 ppm of sulphur produce about 20 ppm of SO2.

c Close coupled catalyst. d Part of the fuel is employed for scavenging of the exhaust, which does not allow to defi ne a precise defi nition of the A/F. e λ defi ned as ratio of actual A/F to stoichiometric A/F, λ = 1 at stoichiometry (A/F = 14.7) .

In the past, exhaust emission standards (limits) have been set in most industrialised countries for passenger cars, light-duty trucks and heavy-duty trucks (gasoline and diesel engines). For the gasoline engines, nowadays, the requirements are to reduce emissions by about or even more than 95% in comparison to typical amount of pollutants emitted by late-sixties automobiles with uncontrolled emissions. As an example, under the European limits the uncontrolled CO emissions in the late sixties were around 60 g of CO km-1, which should drop, according to an EU proposal, down 1.0 g km-1 by year 2005. This is equivalent to a 98% reduction of the CO emissions. Similarly, a very restrictive situation is foreseen for HC emissions. Both lowering of the limits down to 0.10 g km-1 and the inclusion of the cold start in the vehicle test are important targets which need to be fulfi lled.

The Environmental Protection Agency (EPA) established a federal Test Procedure (FTP) simulating the average driving conditions in the U.S in which CO, HC, and NOx would be measured. The FTP cycle was conducted on an engine dynamometer and included measurements from the automobile during three conditions: (i) cold start, after the engine was idle for eight hours, (ii) hot start, and (iii) a combination of urban and highway, driving conditions. Separate bags would collect the emissions from all three modes, and a weighing factor was applied for calculating the total emissions.


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The need to control engine emissions according to Clean Act Air, recent amendments have set up more stringent requirements for automotive emissions (2). The catalysts will be required to last 100,000 miles for new automobiles after 1996. Furthermore, the amendments (contingent on tier 2 standards to be set by environmental protection agency) reduce nonmethane hydrocarbon (NMHC) emissions to a maximum of 0.125 g/mile by 2004 (down from 0.41 g/mile in 1991), carbon monoxide to 1.7 g/mile (down from 3.4 g in 1991), and nitrogen oxides to 0.2 g/mile (down from 1.0 g).

Engine manufactures have explored a wide variety of technologies to meet the requirements of the Clean Air Act. Catalysis has proven to be the most effective passive system. The major worldwide suppliers of automotive catalysts are Engelhard, Johnson Matthey, Degussa, and Allied Signal. As the automobile engine has become more sophisticated, the control devices and combustion modifi cations have proven to be very compatible with catalyst technology, to the point where today engineering design incorporates the emission control unit and strategy for each vehicle.

10.2 Catalytic Converter and Three-way Catalysts10.2 Catalytic Converter and Three-way Catalysts

Emission control catalysts eliminate dangerous engine pollutants from a wide range of fuels, including gasoline, diesel, natural gas, and alternate fuels. Catalysts transform pollutants into harmless gases within a catalytic converter before releasing them into the environment. Historically, four generations of de-pollution catalysts/converters can be distinguished. The fi rst generation converter typically contained a simple oxidation catalyst based on Pd/Pt supported on Al2O3. An additional air supply system was introduced to promote the oxidation. This arrangement was suffi cient to meet the initial standards. NOx control was generally achieved by using an Exhaust Gas Recirculation system. By recirculating part of the exhaust gases back into the combustion chamber, the combustion temperature was lowered, resulting in reduced NOx emissions. More stringent exhaust emissions standards, including tighter NOx control, had been set in the USA by 1977-1981, which led to an implementation of the control of the exhaust emissions by developing the so-called dual-way catalyst (second generation) and, subsequently, three-way catalyst (TWC, third and fourth generation). The former converter usually consisted of a primary bed containing a Rh/Pt-based catalyst for NOx removal, followed by a secondary bed containing an oxidation catalyst equipped with an additional air supply to promote the oxidation reactions. By tuning the engine to run close to stoichiometric conditions, reduction of NOx to N2 could be achieved in the fi rst bed and CO/HC oxidation to CO2 and H2O was carried out in the second bed. However, such devices could not withstand the progressively tightening demand for exhaust control in the USA in the early 80’s, mainly due to the imperfect A/F control resulting from the use of carburettors. Further, NOx can be easily reduced to NH3 under rich conditions, which can then be back converted over the oxidation catalyst, making NOx removal ineffi cient. The advent of oxygen sensors (the so-called λ sensor) based on zirconia type ceramics, led to the development of the modern TWC arrangement. The name is derived from the ability to simultaneously remove all of the three categories of pollutants, i.e. NOx, CO and HC, which are present in the exhausts. Of these, NOx removal/reduction is obviously most effi cient under net reducing conditions, in a defi ciency of O2. In contrast, both CO and HC are best converted under net oxidising conditions, in excess oxygen. However, if one could maintain the A/F ratio close the stoichiometry, then in principle all the three pollutants could be converted simultaneously. This is illustrated in Figure 10.2. In driving conditions, A/F is kept close to the stoichiometric point by a fuel injection device regulated by an electronic feedback system, which uses a signal from the λ sensor.


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Fig. 10.2: Effect of A/F ratio on the conversion effi ciency of three-way catalysts. (adapted from ref 4)

To obtain an effi cient control of the A/F ratio the amount of air is measured and the fuel injection is controlled by a computerised system which uses oxygen sensors (λ) located at the inlet of the catalytic converter. The signal from the λ sensor is used as a feed-back for the control loop. A second additional λ sensor is mounted at the outlet of the catalytic converter (Fig. 10.3) .

Figure 10.3: A picture of a modern TWC/engine/oxygen sensor (λ) control loop for the engine exhaust control (Courtesy of Mr. R. Crevatin, University of Trieste)


115


This confi guration constitutes the basis of the so called engine on-board diagnostics (EOBD). By comparing the oxygen concentration before and after the catalyst, A/F fl uctuations are detected. Extensive fl uctuations of A/F at the outlet signal system failure. This EOBD arrangement implicitly assumes that only a narrow A/F window at stoichiometric point is the fi ngerprint of an effective TWC type of system. The location of the catalytic converter is another critical point which determines the conversion effi ciency. TWCs typically feature a so-called light-off type conversion vs. temperature behaviour. This curves is characterised by conversion which steadily increases from 0 to 100 % conversion, the temperature of 50 % of conversion being indicated as the light-off temperatures. TWCs are characterised by a light-off temperature around 250-350°C (Fig. 10.4) (4). This means that an under-fl oor catalyst is heated above the light-off temperature within 90-120 s. Differently, when the catalyst is closely coupled to the engine (CCC close-coupled catalyst) the heating time typically drops down to 10-20 s. This dramatically affects vehicle emissions immediately after the start up of the engine. The ULEV (Californian ultra low emissions vehicle) limit (0.064 g of HC emitted/km) is typically surpassed within 40 s after the engine start-up (9). This requires for an almost instantaneous heating up of the converter to achieve the required >95-98% conversion. As a consequence of this location of the catalyst, temperatures up to 1100°C are easily met by a CCC.

NO

CO

Temperature / °C

% / n

oisrevn

oC

100

50

0

HC

200 300 400 500

Figure 10.4: Light off type catalyst (adapted from ref 4)

It must be realised that the latest US and European legislation (EURO phase V and US TIER II) limit for automotive emissions require application of the CCCs and EOBD technologies on the vehicles in order to meet the emissions standards. A high durability is also an important requirement for the present and future TWCs, for example a durability up to 130.000 miles of the catalyst will be demanded by US tier II regulations in 2004. It should be considered that if a signifi cant part of the vehicles fails the periodical exhaust emission controls, converter replacement becomes mandatory for a vehicle manufacturer. Accordingly, an extremely effi cient and robust catalyst is required for future vehicle applications.

10.2.1 Principle and Operation of TWCs10.2.1 Principle and Operation of TWCs

A typical design of a modern three-way catalytic converter is reported in Figure 10.5. Basically it is a stainless steel container which incorporates a honeycomb monolith made of cordierite (2MgO·2Al2O3·5SiO2) or metal (3, 4). The choice and geometrical characteristics of the honeycomb monolith play a key role in determining the effi ciency of the converter. In


116


fact, high conversion must be achieved in the converter and therefore the catalyst works under conditions where severe mass and heat transfer limitations apply. Typically, both metal and ceramic monoliths are employed nowadays. The major advantage of the metallic substrate is that the wall thickness is limited by the steel rolling mill’s capabilities, not strength. In a typical automotive 400 cell/inch2 application, the frontal fl ow area in a ceramic monolith is 69% open (31% closed), while the metallic version has 91% open area. This is due to the higher wall thickness of ceramic monoliths (0.007 inch (0.178mm)) compared to metallic ones (0.002 inch (0.050mm)). However, even in this fi eld there has been a strong improvement of the technology, cell densities as high as 900 cell/inch2 or even higher are now commonly available on the market for both types of monoliths. Traditionally, cordierite monoliths have been employed quite extensively, primarily due to their lower production cost. However, a major advantage of the metal monoliths resides in their high thermal conductivity and low heat capacity which allow very fast heating of the CCCs during the phase-in of the engine, minimising the light-off time.

Figure 10.5: Diagram of a typical car converter (1) monolith and (2) metallic honeycomb (adapted from ref 4)

The monolith is mounted in the container with a resilient matting material to ensure vibration resistance (10,11). The active catalysts is supported (washcoated) onto the monolith by dipping it into a slurry containing the catalyst precursors. The excess of the deposited material (washcoat) is then blown out with hot air and the honeycomb is calcined to obtained the fi nished catalyst. This is clearly a very simplifi ed and schematic description of the washcoating process as multiple layer technology, or multiple catalyst-bed converters are also employed (10,12). The exact method of deposition and catalyst composition are therefore highly proprietary and specifi c for every washcoating company. For example, the metallic honeycombs are non-porous, which makes adhesion of the washcoat diffi cult. Accordingly a FeCrAl based alloy is employed, which contains up to 5 wt% of aluminium; after an appropriate pre-treatment this element then acts as an anchoring centre for adhesion of the washcoat.

However, there are some common components, which represent the state-of-art of the washcoating composition:

• alumina, which is employed as a high surface area support;• CeO2-ZrO2 mixed oxides, principally added as oxygen storage promoters;• Noble metals (NM= Rh, Pt and Pd) as active phases;• Barium and/or lanthana oxides as stabilisers of alumina surface area.


117


Al2O3 The choice of Al2O3 as carrier is dictated by the necessity of increasing the surface area of the honeycomb monolith which is typically below 2-4 m2 l-1, where the volume is that of the honeycomb (13). This does not allow achievement of high NM dispersion. Alumina is chosen due to its high surface area and relatively good thermal stability under the hydrothermal conditions of the exhausts. In most of the studies γ-Al2O3 is employed due to its high surface area with respect to other transitional aluminas (14), however, also other high temperature aluminas such as δ- and θ-Al2O3 can be employed for high temperature applications such as in the CCCs because of their higher thermal stability compared to γ-Al2O3. Since temperatures above 1000°C can be met in the TWCs, stabilisation of transition aluminas is necessary to prevent their transformation to α-Al2O3, which typically features surface areas below 10 m2 g-1. A number of stabilising agents have been reported in the literature, lanthanum, barium, strontium, cerium and, more recently, zirconium oxides or salts being the most investigated (15-22). These additives are impregnated onto γ-Al2O3 or, sometimes, sol-gel techniques are employed to improve the stability of the surface area.

CeO2-ZrO2 mixed oxides The benefi cial effects of CeO2 containing formulations of the TWC performances has long been recognised (23). Many different promotional effects have been attributed to this component such as the ability to:

• promote the noble metal dispersion;• increase the thermal stability of the Al2O3 support;• promote the water gas shift (WGS) and steam reforming reactions;• favour catalytic activity at the interfacial metal-support sites;• promote CO removal trough oxidation employing a lattice oxygen;• store and release oxygen under respectively lean and rich conditions.

For a detailed discussion of these roles and their relative importance we refer the reader to earlier literature (4,24,25).

Among the different roles of CeO2 in TWCs, the OSC is certainly the most important one, at least from the technological point of view. This is due to the fact that unambiguous relationships between the TWC activity and OSC performances have been established (26). Starting from 1995, CeO2-ZrO2 mixed oxides have gradually replaced pure CeO2 as OSC materials in the TWCs (27), even though some low purity CeO2 materials may be employed for less demanding TWC technologies (28). The principal reason for the introduction of CeO2-ZrO2 mixed oxides in place of CeO2 is due to their higher thermal stability (27,29-32).

A remarkably property of the CeO2-ZrO2 mixed oxides compared to the CeO2 is their ability to easily remove bulk oxygen species under moderate temperature even in highly sintered samples (33). This was associated with the ability of ZrO2 to modify the oxygen sublattice in the CeO2-ZrO2 mixed oxides, generating defective structures and highly mobile oxygen atoms in the lattice which can be released even at moderate temperatures (34,35).

Noble metals Obviously NMs represent the key component of the TWC, as the catalytic activity occurs at the noble metal centre. However, we purposely discuss the aspects related to the NM at this point, since its interaction with the various components of the washcoat critically affects the activity of the supported NM.

In principle, the fi rst aspect to be considered is the choice of the NM and its loading in the washcoat. Rh, Pd an Pt have long been employed in the TWCs and there is a general agreement about the specifi city of Rh to promote NO dissociation, thus enhancing the NO removal (24,36-38), even if alternative mechanistic pathways for NO reduction have,


118


also, been proposed (39-41). Pt and Pd are considered as metal of choice to promote the oxidation reaction, even though Rh also has a good oxidation activity. In particular, except some initial use in 1975-76, Pd has extensively been added to TWC formulations starting from mid 90s due to its ability to promote HC oxidation (9,10). In fact, better A/F control (42) and modifi cation of the support provided high NOx conversion, comparable to the traditional Rh/Pt catalyst (43). The increase of the use of Pd in the TWC technology adversely affected Pd market price, which is now comparable to that of Pt. In fact, there is a large demand for Pd due to the fact that a straightforward way to increase the effi ciency of the TWCs at low temperatures, is that of increasing the NM loading, and particularly that of Pd, which for long was the cheapest NM among the three ones employed (Figure 10.6).

PGM Prices

0

500

1000

1500

2000

1997 1999 2001 2003

$ per oz

Rh Pt Pd

0

1

2

3

4

5

6

7

8

9

10

1987 1992 1997 2002

Million oz

Demand for PGMin Autocatalysts

Figure 10.6: Evolution of the cost and demand of noble metals

On the other hand, use of high NM loading may favour sintering at high temperatures, leading to deactivation of the TWCs, in addition to the fact that cost-effective TWCs are required by the market. In summary, the choice and loading of the NM is a compromise between the required effi ciency of the converter and the market price of the NM; ideally a car maker would prefer to have at the disposal a choice of TWCs with different NMs, which would allow the best choice to be made according to price fl uctuations of the NMs.

Generally speaking, sintering of NM, leading to decrease of the number of active sites, is a major pathway for the deactivation of the TWCs. In addition to sintering, poisoning of the catalyst may contribute to deactivation of the TWCs. The latter phenomenon is essentially related to the mileage travelled, quality of the fuel and the engine lubricating oil (13). However, there is a number of other phenomena which can contribute to deactivation of the TWCs: I) sintering of the OSC promoter leading to loss of OSC and, possibly, to encapsulation of the NM (44); II) sintering of Al2O3 and, more important, deactivation of Rh due to migration of Rh3+ into the alumina lattice (13). The comprehension of the relative importance of the different deactivation phenomena is diffi cult due to the variety of the reaction conditions,


119


TWC preparation methods, etc. For example, when NM were supported on CeO2-ZrO2 mixed oxides and aged at high temperatures under redox conditions, encapsulation of Pd and Rh within the pores of the support occurred, while it did not occur for Pt (45).

Albeit it received relatively scarce consideration (46), the issue of sulphur poisoning of TWC needs some consideration. As above discussion inclusion of CeO2-based promoters into the washcoat considerably enhances the conversion effi ciency of the TWCs. On the other hand, both CeO2 and ZrO2 are known to easily adsorb SOx species: sulphated ZrO2 is a well known solid acid catalyst (47). Investigation of reduced and oxidized CeO2 revealed that SO2 is adsorbed under various forms, both surface and bulk-type of sulphates being formed (48,49) and may even modify the microstrucutre of the CeO2-based oxide (50). Curiously, under oxidizing conditions bulk sulphates decomposed by 600°C whereas surface sulphates persisted up to 700°C (48). Use of reducing conditions favours elimination of sulphates as H2S which can be easily detected as rotten-egg odour (51), particularly in the presence of noble metal (52). The OSC of CeO2 is detrimentally affected by the presence of SO2, while addition of ZrO2 to CeO2 increases the resistance of CeO2 to sulphur poisoning, albeit more sulphur is adsorbed at the surface (46). This may be associated with the generally higher OSC effi ciency of the CeO2-ZrO2 mixed oxide compared to CeO2 and the possibility that ZrO2 acts as sulphur scavenger. Ni containing oxide are sometimes added to the washcoat in the USA as sulphur scavengers while their use in Europe is prohibited.

10.3 Next Generation Technology for Emissions Control10.3 Next Generation Technology for Emissions Control

As above discussed, TWCs represent a quite mature, highly effective technology for pollution abatement which, however, brings some inherent limitations which need further improvement and development. These aspects are essentially related to: I) low activity at low temperatures (start-up of the engine) and II) use of stoichiometric A/F. As for the fi rst aspect is concerned, it should be noticed that roughly 50-80 % of HC emissions during the test procedures are emitted before the TWC reaches the light-off temperature. When, in the last years, the emissions limits have been pushed down, it appeared clearly that minimization of warm-up HC emissions was a major problem in the automotive pollution abatement. This issue has been therefore addressed by introduction of the CCCs onto the market. This required development of TWCs featuring thermal stabilities well above 1000°C.

In reality, the issue of the start-up emissions can be addressed by different approaches.

10.3.1 Hydrocarbon Adsorber Systems10.3.1 Hydrocarbon Adsorber Systems

A fi rst possibility is that of collecting the HC emitted during the warm-up of the converter in a HC trap, typically composed of hydrophobic zeolite. In an optimal trap, HC are trapped at low temperatures and as the temperature is increased above 250-300°C trapped HC are released and converted on the TWCs (Fig. 10.7). A suitable trap must also feature very high thermal stability under hydrothermal conditions, which often is not the case for zeolite type of systems. We recall that temperatures as high as 850-900°C may be reached in the under-fl oor catalysts.


120


HC -adsorber

CatalystHC -

adsorberCatalyst

HC -adsorber

Catalyst

HC -adsorber

CatalystHC -

adsorberCatalyst

HC -adsorber

Catalyst

• Cold start

HC retained by adsorber

• Warmed-up

HC released from adsorber and converted by catalyst

Hydrocarbon adsorber (HC –adsorber)

Figure 10.7: Scheme of hydrocarbon adsorber systems

10.3.2 Electrically/Chemically Heated Catalyst Systems10.3.2 Electrically/Chemically Heated Catalyst Systems

While Hydrocarbon Adsorber Systems are still under investigation, alternative approaches have been indicated. It must be recognised that to minimize the emissions, the catalysts must be heated-up in a minimum time. This can be achieved, for example, by electrically (Fig. 10.8) or combustion/chemically heated catalyst. In the latter case hydrogen and oxygen, or CO-rich feed is fl owed over the catalysts (53). Oxidation of both CO and H2 is an easy reaction and occurs at the lowest temperatures over the TWCs (54) leading to rapid heating of the catalyst. However storage of H2 on the vehicle or use of rich A/F which generates high CO and H2 emissions brings complexity to the de-pollution system. HC are in fact emitted at rich A/F which requires for additional HC trap.

EHCMain

Catalyst

Elettrically heated catalyst (EHC)

Quick heat-up of the converter

Figure 10.8: Scheme of electrically heated catalyst systems

The substrate, onto which the catalyst is deposited, is generally made from metal so that, when an electric current is passed, it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds.

10.3.3 Close-coupled Catalyst (CCC)10.3.3 Close-coupled Catalyst (CCC)

Use of complex technology clearly pushes-up the costs while the simplest technology is desirable. Accordingly there has been a strong effort aimed at improving thermal stability of the washcoat (29). With availability of thermally stable washcoats, application of a start-up converter, i.e. converter that is closely coupled to the exhaust manifold (Fig. 10.9), became feasible. This converter allows extremely rapid heating of the catalyst, leading to enhanced conversions during the warm-up of the engine. Metallic converter can be easily shaped into the exhaust manifold and result very convenient for such an application also due to their low thermal capacity. In general, the composition of the close-coupled catalyst is related to that of the typical TWC in that NM metals and particularly Pd are employed to promote HC conversion. The OSC promoter may be omitted from these formulations since it promotes CO conversion, leading to local overheating because of this highly exothermic reaction. On the other hand, for the purpose of the OBD II technology, there is necessity to monitor the OSC effi ciency since the start-up of the engine. Accordingly ZrO2-rich doped CeO2 promoters that may feature very high thermal stabilities (29,55), are often added to this catalyst.


121


Start-upconverter

Maincatalyst

Start-up converters (close-coupled converters)

Heat up quicker New thermostable washcoats

Engine

Figure 10.9: Scheme of close-coupled catalyst

10.3.4 New Catalysts10.3.4 New Catalysts

An alternative approach is that of developing new catalysts showing high conversion effi ciency at low, nearly ambient, temperature (53). A large part of these investigation has been triggered by the observation by Haruta and co-workers that gold catalysts are able to effi ciently oxidize CO even at subambient temperature provided that nanodispersed Au particles are prepared on the support (56). Thus, light-off temperatures in the conversion of the exhausts as low as 100°C could be achieved by depositing small Au particles on reducible oxides such as CeO2 and TiO2 (57). However, the durability of gold catalysts under harsh conditions is still an issue, signifi cant deactivation of cobalt oxide promoted Au catalysts was observed already after 157 h of reaction at 500°C under simulated exhaust (58). There is in fact a fl ourishing activity in the fi eld of low temperatures catalysts (59-61), other noble metals, in addition to Au, being effective in low temperature oxidation reactions, provided that appropriate synthesis methodology is employed (59). To our knowledge, however, due to the nano-dispersed nature of these catalysts, the issue of thermal stability, even at moderately high temperatures has not been solved as yet. Supported metal nano-particles are, in fact, quite mobile on the surface, even at ambient temperature in the case of gold, which makes prevention of sintering phenomena diffi cult. We believe that thermal stabilisation of nano-dispersed metals may represent a new breakthrough point in the development of these environmental catalysts.

10.4 Diesel Engine Emissions10.4 Diesel Engine Emissions

The diesel engine, invented in the late 19th century by Dr. Rudolf Diesel, is the most energy effi cient power plant among all type of internal combustion engines known today. This high effi ciency translates to good fuel economy and low greenhouse gas emissions. Other diesel features that have not been matched by competing energy conversion machines include durability, reliability, and fuel safety. The downsides of diesels include noise, low specifi c power output, NOx and PM emissions, and high cost (61).

The diesel engine is a compression-ignition internal combustion heat engine, which can be operated, in both the four- and two-stroke cycle. The combustion process can be theoretically modeled by applying thermodynamic laws of mass and energy conservation to the processes in the engine cylinder. Basic design and performance parameters in diesel engines include compression ratio, swept volume, clearance volume, a number of scavenging characteristics in two-stroke engines, power output, indicated power, mechanical effi ciency, indicated mean effective pressure, brake mean effective pressure, specifi c fuel consumption, and more.


122


Driving behind a diesel bus or truck, whether on a highway or in the city, often is an unpleasant experience because of its emissions. The black smoke, or soot, is the most visible emission, but other, less visible pollutants are also present. The emissions from a diesel engine are composed of three phases: solids, liquids, and gases. The combined solids and liquids are called particulates, or total particulate matter (TPM), and are composed of dry carbon (soot), inorganic oxides (primarily as sulfates), and liquids. When diesel fuel is burned, a portion of the sulphur is oxidized to sulphate, which, upon reaction with the moisture in the exhaust, becomes H2SO4. The liquid are a combination of unburned diesel fuel and lubricating oils, called the soluble organic fractions (SOF) or volatile organic fractions (VOF), which from discrete erosols and/or are adsorbed within the dry carbon particles) (62). Gaseous hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur dioxide are the constituents of the third phase.

In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air. As the piston continues to move closer to top dead center, the mixture temperature reaches the fuel’s ignition point, causing instantaneous ignition of some pre-mixed quantity of fuel and air. The balance of fuel that had not participated in premixed combustion is consumed in the rate-controlled combustion phase, also known as diffusion combustion.

10.4.1 Emission Formation in Diesel Engines10.4.1 Emission Formation in Diesel Engines

Emissions formed during burning of the heterogeneous diesel air/fuel mixture depend on the conditions during combustion, during the expansion stroke, and especially prior to the exhaust valve opening. NOx emissions are formed manly during the premixed burning. PM, on the other hand, is generated in diesels primarily during the diffusion fl ame. The visible smoke emission can be classifi ed into black smoke, also known as hot or solid smoke, and white smoke also referred to as liquid smoke or fog.

Diesel particulate matter (DPM) is the most complex of diesel emissions. Diesel particulates, as defi ned by most emission standards, are sampled from diluted and cooled exhaust gases. This defi nition includes both solids, as well as liquid material, which condenses during the dilution process. The basic fractions of DPM are elemental carbon; heavy hydrocarbons derived from the fuel and lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates include small nuclei mode particles of diameters below 0.04 µm and their agglomerates of diameters up to 1µm.

The fl ame temperature, due to the high activation energy of the pyrolysis processes, is one of the key factors that infl uence soot formation. Soot formation occurs in fl ames above 1000 and 1300 °C. Soot is eventually emitted when burnout ceases at temperatures below 1000 °C. Depletion in local oxygen concentration strongly enhance soot production. Soot particle formation is also effected by the overall oxygen concentration, although this may be an indirect effect of the different fl ame temperatures associated with different oxygen concentrations. The high pressure used in some diesel can also promote soot formation. Improved atomisation caused by higher pressure lead to lower penetration of the droplets into the combustion chamber. Thus, the high concentration of fuel in the high temperature, low oxygen region establishes the conditions for soot formation. The presence of chemical impurities or additives of the fuel may affect particulate emissions. Halogens cause an increase of hydrocarbon radicals by scavenging H atoms. In addition, sulfur compounds present in the fuel are oxidized to SO2 during the combustion. A small fraction


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of SO2 can be oxidized to SO3 if the exhaust temperature exceed 450 °C. The presence of catalyst favors this oxidation process. Consequently, sulfuric acid and sulfate ions are formed. Fuel sulfate and calcium of the lubrication oil may react to form gypsum, which may strongly deactivate the particulate fi lter and/or hinder its regeneration. On the other hand, emulsifi cation with water may deplete soot formation. Metals such as nickel or manganese charge the soot particles electrostatically and reduce agglomeration. The smaller particles thus formed burn more easily. The characteristics of the fuel sach as viscosity, volatility, thermal stability, aromatic content and C/H ratio also affect soot occurrence. Hydrocarbons show the following smoking tendency:

aromatics > alkanes > alkenes > alkynes

Older type of diesel engine had relatively low injection pressure and operated at low excess of air. Due to ineffi cient fuel injectors, a signifi cant fraction of the fuel was stacked to the walls of the combustion chamber as large fuel drops. This resulted in big fl akes of soot. Modern diesel engine works with redesigned fuel and air injectors and a large excess of air. Therefore, an extremely effi cient mixing of air and fuel is guaranteed, resulting in nearly complete combustion and lower soot emissions.

10.4.2 Diesel Fuel10.4.2 Diesel Fuel

Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. The important properties, which are used to characterize diesel fuel, include cetane number (or cetane index), fuel volatility, density, viscosity, cold behavior, and sulfur content.

Fuel Properties and EmissionsThere is a clear correlation between some fuel properties and regulated

diesel emissions. Drawing general conclusions is, however, diffi cult due to such factors as intercorelation of different fuel properties, different engine technologies, or engine test cycles. In heavy-duty engines increasing the cetane number lowers HC, CO, and NOx emissions, while reducing fuel density lowers NOx and PM but increases HC and CO. Light-duty engines show different fuel sensitivity than the heavy-duty engines. Sulfur increases PM in both classes of engines. Sulfur is also known to interfere with several diesel emission control strategies.

Synthetic Diesel FuelSynthetic diesel fuels can be made from carbon containing feedstocks, such as

natural gas or coal, in a process developed by Fischer and Tropsch in the 1920’s. That process has been further developed by oil companies and is considered a viable option of natural gas utilization. Synthetic diesel fuels are characterized by excellent properties, such as very high cetane number and no sulfur content. They can be used in existing diesel engines without modifi cations or mixed with petrodiesel. Several studies found signifi cant reductions in all regulated diesel emissions, including NOx and PM, when using synthetic fuel.

BiodieselVegetable oil was used as a diesel fuel as early as 1900, when Rudolf Diesel

demonstrated that a diesel engine could run on peanut oil. However, its use as a fuel attracted little attention except in times of crisis such as during World War II and the energy shortages of the 1970s. Increasing environmental concerns, expensive overproduction in


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European agriculture and changes in government policies have resulted in expanded testing and usage of biodiesel. The name “biodiesel” was introduced in the United States in 1992 by the National SoyDiesel Development Board (now the National Biodiesel Board), which has pioneered the commercialization of biodiesel in the U.S. Biodiesel is defi ned as the mono alkyl esters of long chain fatty acids derived from renewable lipid sources. Biodiesel is typically produced through the reaction of a vegetable oil or animal fat with methanol in the presence of a catalyst to yield glycerin and biodiesel (chemically called methyl esters). Biodiesel has been registered with the US Environmental Protection Agency as a pure fuel or as a fuel additive and is a legal fuel for commerce. The compression-ignition engine has certain advantages over spark-ignition engines and role of biodiesel is to contribute to the longevity and cleanliness of diesel engines.

The use of biodiesel as alternative fuel present the following signifi cant advantages:

1. Biodiesel can be made from domestically produced, renewable materials.

2. Biodiesel is the only alternative fuel that runs in any conventional, unmodifi ed diesel engine. It can be stored anywhere that petroleum diesel fuel is stored.

3. Biodiesel can be used alone or mixed in any ratio with petroleum diesel fuel. The most common blend is a mix of 20% biodiesel with 80% petroleum diesel, or “B20.”

4. The life cycle production and use of biodiesel produces approximately 80% less carbon dioxide emissions. This is very important in view of the control of the green-house effect.

5. Combustion of biodiesel provides over a signifi cant reduction in total unburned hydrocarbons and an almost complete elimination of sulphur dioxide. Biodiesel further provides signifi cant reductions in particulates and carbon monoxide than petroleum diesel fuel. However, biodiesel provides a slight increase or decrease in nitrogen oxides depending on engine family and testing procedures.

6. Biodiesel is 11% oxygen by weight and contains no sulphur. The use of biodiesel can extend the life of diesel engines because it is more lubricating than petroleum diesel fuel, while fuel consumption, auto ignition, power output, and engine torque are relatively unaffected by biodiesel.

Biodiesel is safe to handle and transport because it is as biodegradable non toxic, and has a high fl ashpoint of about 150 °C compared to petroleum diesel fuel, which has a fl ash point of 50 °C.

Auto ignition, fuel consumption, power output, and engine torque are relatively unaffected by biodiesel. So basically, the engine just runs like normal (except for the smell).

Biodiesel can contribute to reduce dependence on petroleum, creating domestic manufacturing jobs.

Biodiesel is a proven fuel with over 30 million successful US road miles, and over 20 years of use in Europe.

Use of biodiesel in combination with the existing technologies for pollution abatement (EGR, catalysts, fi lters, etc.) further enhance the benefi ts to the environmental.

It should be underline that there are also some disadvantages of Biodiesel. The major one is its high production costs. The production of subsidized biofuels can be partially justifi ed if a high premium is put on the environmental cost of the alternatives. One additional concern is the environmental impacts of increased fertilizer and pesticide usage to increase oilseed production for use in manufacturing biodiesel. Finally, biodiesel


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has a higher viscosity than conventional diesel and therefore becomes less useful at lower temperatures. This limits its use in Canada, the northern U.S. and much of Europe. In these areas, biodiesel is marketed as an additive in a fi ve to ten percent blend with conventional diesel fuel.diesel engines.

Dimethyl EtherDimethyl ether (DME) can be made from a variety of fossil feedstock including

natural gas and coal as well as from renewable feedstock and waste. When used as a diesel fuel, DME offers NOx and PM emission benefi ts. Emissions of CO and HC which may increase with DME can be easily controlled by an oxidation catalyst. Energy effi ciency of DME is lower than that of diesel but higher than that of methanol/gasoline engines.

10.4.3 World Diesel Emission Standards10.4.3 World Diesel Emission Standards

Diesel emission are clearly more complex than those of gasoline engine and hence, their catalytic treatment is more complicated and requires new technology.

The popularity of diesel engines is derived primarily from their fuel effi ciency relative to the gasoline spark-ignited engine. Diesel operate very lean of stoichiometric, with air-to-fuel ratios greater than about 22. They have good fuel economy, producing less CO2. It is not uncommon for a diesel engine to have a life of 1 million miles, or about 10 times that of the gasoline engine. The diesel fuel and air mixture is highly composed, raising the gas temperature to the point of combustion. Its lean nature results in a cooler combustion with less gaseous NOx, CO, and HC emissions than its gasoline counterpart. The design of the combustion process, however, results in high particulate emission levels. In the past few years, engine manufactures have made great progress in redesigning the engine (fuel injectors, timing, and so on) to minimize particulate emissions (63). The units of measurement for particulates are grams per brake horsepower generated in an hour, or g/bhp-h. For example, in 1986, particulate emissions from a typical diesel truck engine were 0.6/bhp-h. By 1990, manufacturers had redesigned the engine to reduce the particulates to 0.25 g/bhp-h. In the U.S, emissions are measured over a standardized Federal Test Procedure (FTP), The FTP simulates U.S. truck driving conditions. The Clean Air Amendment of 1990 requires that by 1994 particulate emission be reduced to 0.1 g/bhp-h for truks and no grater than 0.07 g/bhp-h for buses. This is where catalytic aftertreatment can be used effectively to bring engines into compliance with the standards (64).

Diesel emission control is being addressed worldwide. In the U.S., Europe, and Japan, essentially all trucks and buses operate with diesel-fueled engines. In Europe, the favorable price of diesel fuel relative to gasoline has resulted in a large number of diesel fueled passenger cars. Trucks are used to ship goods; buses to transport groups of people (usually within urban areas), and passenger cars are utilized for individual or small groups in both urban and rural locations. Consequently, the allowable emissions standards differ for each application. Furthermore, the terrain in the U.S., Japan, and Europe differ suffi ciently that the test conditions must refl ect local driving habits. Each vehicle must specifi c emission standards as measured in standardized driving cycles that refl ect the duty cycle anticipated for the particular engine. Table 10.2 summarizes current and proposed emission standards worldwide. In Europe, passenger cars must meet standards refl ecting both urban (ECE) conditions and high-speed extra urban driving (EUDC). For European heavy-duty truck applications, a 13-mode steady-state test with various loads/torques and speed is utilized. This test puts emphasis on the high-temperature performance of the catalyst, refl ecting high load conditions. At such high temperatures, minimum generation of sulphate by catalytic oxidation of gaseous SO2 becomes the most demanding requirement.


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Table 10.2: Worldwide diesel emission standards

Country/year (vehicle) HC CO NOx Particulate HC + NOx

U.S/1991 (truck) 1.3 15.5 5.0 0.1

U.S/1994(truck) 1.3 15.5 5.0 0.1, 0.07 Bus

U.S/1998 (truck) 1.3 15.5 4.0 0.1

Calif./1998 (truck) 0.025 HCHO 0.05(ULEV) 2.5(ULEV)

Europe/1995(truck) 1.1 4.0 7.0 0.15

Europe/1996(car) 1.0 0.10(DI) 0.9(DI)

Europe/1999(car) 0.5 0.04 0.5

Japan/1998 (car) 0.4 0.08

U.S. truck: HD FTP Transient Cycle, g/bhp-h. Euro truck: R-49 13 Mode. Euro car: Cycle A, g/km. DI=Dir. Inj. Japan car: 10 Mode, g/km.

10.4.4 Diesel Emission Control Technologies10.4.4 Diesel Emission Control Technologies

This chapter discusses the general classifi cation of emission control options available for diesel engines today. Since nitrogen oxides and particulate emissions are usually the biggest concern, a brief overview of NOx and PM control strategies is presented.

Engine Design for Low EmissionsChanges in diesel engine design introduced over the period from the late 1980’s

to early 2000’s contributed to some 10-fold decrease in emissions without relying on exhaust gas aftertreatment. The most important of these engine technologies are advanced fuel injection systems, air intake improvements, combustion chamber modifi cations, and electronic engine control. Additionally, exhaust gas recirculation (EGR) was introduced on both light- and heavy-duty diesel engines to control NOx emissions.

Engine Design for NOx ControlNOx emissions from heavy-duty engines had to be reduced by about 70% to meet

emission standards of the 1990’s. Major technologies, which were employed, included injection timing retard and intake air-cooling. Implementing high injection pressures prevented the negative effect of timing retard on fuel economy.

Engine Design for PM ControlPM emissions from heavy-duty engines were reduced by over 90% to meet emission

standards of the 1990’s. Major PM reductions were realized through improvements in air management, combustion, oil consumption control, and fuel injection.

Exhaust Gas RecirculationExhaust gas recirculation (EGR) is an effective strategy to control NOx emissions

from diesel engines. The EGR reduces NOx through lowering the oxygen concentration in the combustion chamber, as well as through heat absorption. Several confi gurations have been proposed, including high- and low-pressure loop EGR, as well as hybrid systems. NOx emissions may be further reduced by cooled EGR, in which recirculated exhaust gas is cooled in EGR cooler using jacket water.

EGR effectively reduces NOx emissions while increasing PM emissions and fuel


127


consumption. EGR control systems, utilizing open or closed loop control layouts, have to provide precise EGR rates and proper A/F ratios in order to achieve their NOx reduction targets while minimizing PM and fuel economy penalty. Even more precise A/F ratio control is possible with variable geometry turbochargers and electrically driven chargers. EGR, especially in uncooled confi guration, in conjunction with high engine loads and aged lube oils may also cause premature engine wear.

Water in Diesel CombustionAddition of water to the diesel process decreases combustion temperatures and

lowers NOx emissions. The most common methods of introducing water are direct injection into the cylinder, a process commercialized in certain marine and stationary diesel engines, and water-in-fuel emulsions. Emulsifi ed fuels, due to increased mixing in the diesel diffusion fl ame, can be also effective in simultaneous reduction of PM and NOx emissions.

Ceramic In-Cylinder CoatingsZirconia based ceramic combustion chamber coatings originally developed for

adiabatic or low heat rejection engines have been shown to reduce diesel emissions. Reported results indicate that in-cylinder zirconia coatings are capable of reducing the carbonaceous fraction of diesel particulates without increasing NOx or other regulated emissions. Reductions in total PM emissions may be achieved by combining zirconia coatings with diesel oxidation catalysts. In-cylinder coatings are most effective in reducing emissions from older technology engines of relatively low thermal effi ciency.

10.4.5 Advanced Diesel Engine Technologies10.4.5 Advanced Diesel Engine Technologies

Emission challenges faced by the diesel engine require substantial reductions of NOx and PM emissions over the period of 2005-2010, to be achieved by a combination of engine, fuel, and exhaust aftertreatment technologies. The emerging engine technologies include optimization of the fuel injection, combustion, and air induction systems, new engine accessories and subsystem technologies, as well as entirely new combustion techniques such as the HCCI engine.

Advanced Technologies: Fuel Injection & CombustionDiesel fuel injection systems for meeting future emission standards require very

fl exible rate shaping capacity and capability for pilot- and post-injections with controllable parameters. Combustion systems for future engines, designed using computerized tools, provide optimized swirl conditions for effi cient air/fuel mixture preparation.

Advanced Technologies: Air InductionEmerging air induction technology options for meeting future emission standards

include improved air charging strategies, through the use of electric superchargers, charge air cooling, optimized intake manifolds and intake ports, and variable valve actuation.

Controls for Modern Diesel EnginesThe control system of a diesel engine is responsible for maintaining performance

at its optimum while at the same time keeping the engine from exceeding certain emission limits. The control system performs this function using three groups of components: sensors, processor, and actuators. Basic control system confi gurations are the open and the closed loop systems. A variation of the open loop system utilizing lookup tables, referred to as scheduled control, was common in diesel engines. Future control systems include model-based controls and neural networks.


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10.4.6 Diesel Oxidation Catalyst10.4.6 Diesel Oxidation Catalyst

Diesel oxidation catalysts promote chemical oxidation of CO and HC as well as the SOF portion of diesel particulates. They also oxidize sulfur dioxide, which is present in diesel exhaust from the combustion of sulfur containing fuels. The oxidation of SO2 leads to the generation of sulfate particulates and may signifi cantly increase total particulate emissions despite the decrease of the SOF fraction. Modern diesel oxidation catalysts are designed to be selective, i.e., to obtain a compromise between suffi ciently high HC and SOF activity and acceptably low SO2 activity.

Commercial Diesel Oxidation CatalystsThe catalysts currently used in the U.S. trucks and European passenger cars are

monolithic honeycomb structures of cordierite with 200-400 cpsi. The catalyzed washcoat is deposited onto the walls of each channel at a loading of about 2 g/in3 and is typically no more than about 40-80 microns in the thickest locations (corners or fi llets) on the monolith. The literature reports a Degussa catalyst composed of combinations of Pt 920-60 g/ft3), 3-10 percent V2O5, and balance TiO2, primarily for use in European passenger car applications (65-66). Johnson-Matthey is believed to be using a variation of Pt and vanadia for selected applications (73). The fi rst generation Engelhard catalyst is composed of about 50 percent catalytically active base metal oxide in combination with an equal amount of alumina with small amounts of Pt (0.5-2) g/ft3) for truck, bus, and selected passenger car applications in the U.S. and Europe (67). A joint venture between Nippon Shokubai and Degussa is promoting the use of a combination of Pd (40 g/ft3 ) supported on zirconia with various promoter oxides such as the rare earths for trucks (68). All of these technologies are in the early stages of commercialization, so precise details of the catalyst formulations are not available.

As general rule, the catalyst volume equals the displacement volume of the engine. Therefore, a 6.0 liter medium-duty truk engine has about 6.0 liters of catalyst. The catalyst is contained in a steel can, with a mounting material made of a ceramic wrapped around its outside diameter to ensure mechanical integrity and resistance to vibration (69). Space velocity vary between about 20,000 and 250,000/hr, depending on the duty cycle of the vehicle. Catalyst diameters are 7-*10 inches (17.78-25.40 cm), with lengths of 5-7 inches (12.7-17.18 cm) for trucks. They are smaller for passenger car applications.

Deactivation of Diesel CatalystThe causes for the deactivation of diesel catalysts are thermal degradation and

poisoning by lubrication oil additives, as well as by sulfur. Phosphorus is the most common oil-derived catalyst poison. Sulfur can be found uniformly distributed over the catalyst length and the washcoat depth, while phosphorus is selectively adsorbed at the catalyst inlet and in a thin, outer washcoat layer.

10.4.7 Lean NO10.4.7 Lean NOx Catalyst Catalyst

Two groups of catalyst systems are known for the reduction of NOx with hydrocarbons: a copper substituted zeolite ZSM5 catalyst, which is active at high temperatures, and a platinum/alumina catalyst, exhibiting low temperature activity. Both catalysts have narrow operating temperature windows, resulting in only a limited NOx reduction effi ciency, and exhibit other problems. Some lean NOx catalysts have been commercialized, primarily to provide small deNOx functionality in diesel oxidation catalysts. (4)


129


10.4.8 Selective Catalytic Reduction10.4.8 Selective Catalytic Reduction

Due to the limited success of hydrocarbons as effi cient reducing agent under lean conditions, the use of urea as an alternative reducing agent for NOx from heavy duty diesel vehicles has received attention. Selective catalytic reduction of NOx with NH3 in presence of excess of O2 is a well implemented technology for NOx abatement from stationary sources (70). Typically vanadia supported on TiO2, with different promoters (WO3 and MoO3) are employed in monolith type of catalysts. A sketch of an arrangement for the urea based NOx abatement technology is shown in Figure 10.10. Typically the urea solution is vaporised and injected into a pre-heated zone where hydrolysis occurs according to the reaction :

32222 2NHCOOHNHCONH +→+−−

Ammonia then reacts with NO and NO2 on the reduction catalyst via the following reactions:

OHNONHNO 2223 6444 +→++

OHNNHNO 2232 12786 +→+

This approach has proved to be quite successful and high NOx (up to 80%) could be achieved on Heavy Duty (HD) diesel engines under driving conditions, even after reasonably high mileages (200.000-300.000 km), the activity decreased to about 75-80% of the initial value after over 500.000 km (71,72).

Figure 10.10: A typical arrangement for abatement of NOx from a heavy futy diesel engine using urea as reducing agent (from ref 4)

A major problem of such system is that an extreme care must be exercised to develop a suitable urea injection strategy that avoids overloading of the system leading to ammonia slip (73). Typically ammonia slip should not exceed 10 ppm. While the effi ciency of urea-SCR technology is recognised (74) temperatures, the design of compact converter systems that require higher conversion effi ciency and last but not least the issue of generalised urea distribution when fuelling the vehicle.

Given the effi ciency of such systems, the application of the urea-SCR technology to Light-duty (LD) diesel engines vehicles was also investigated (75): while appreciable NOx can be achieved, it must be recognised that typically a ratio of engine displacement-to-


130


catalyst volume of 1:3 is typically employed for the urea-SCR systems that may represent a serious problem in compact LD vehicles. Clearly an important improvement of the catalytic performances is needed before such systems can be effectively considered for LD application.

10.4.9 NO10.4.9 NOx Adsorbers Adsorbers

NOx adsorbers (traps) are the newest control technology being developed for partial lean burn gasoline engines and for diesel engines (4). The adsorbers, which are incorporated into the catalyst washcoat, chemically bind nitrogen oxides during lean engine operation. After the adsorber capacity is saturated, the system is regenerated, and released NOx is catalytically reduced, during a period of rich engine operation (Figure 10.11) . Typically a Pt/BaO/Al2O3 catalyst is used to store NOx under oxidising conditions as adsorbed “nitrate” species, which are then released and reduced on a traditional TWC by temporarily running the engine under rich conditions.

Figure 10.11: Principle of operation of a NOx adsorber: NOx are stored under oxidizing conditions (1) and then reduced on a TWC when the A/F is temporarily switched to rich conditions

10.4.10 Diesel Particulate Filters10.4.10 Diesel Particulate Filters

Diesel particulate traps capture particle emissions through a combination of surface-type and deep-bed fi ltration mechanisms, such as diffusional deposition, inertial deposition, or fl ow-line interception. Collected particulates are removed from the fi lter, continuously or periodically, through thermal regeneration. Diesel fi lters are very effective in controlling the solid part of PM emission, but maybe ineffective in controlling non-solid particulate fractions. Filters have been commercialized for selected retrofi t applications and are on the verge of commercialization for highway light- and heavy-duty diesel engines.

Diesel fi lter materials should be characterized by high fi ltration effi ciencies, high maximum operating temperatures, low thermal expansion, resistance to thermal stress, and chemical resistance to metal oxides (ash) present in diesel particulates. A number of materials have been under development, including ceramic wall-fl ow monoliths, ceramic fi bers, or sintered metals.

Wall-Flow MonolithsWall-fl ow monoliths became the most popular diesel fi lter design. They are derived

from fl ow-through catalyst supports where channel ends are alternatively plugged to force the gas fl ow through porous walls acting as fi lters. Wall fl ow monoliths are made of specialized ceramic materials such as cordierite and silicon carbide. A number of mechanical and thermal properties have been defi ned to characterize and compare different monoliths. Filters of different sizes have been developed and are available as standard products.


131


Ceramic Fibers and CartridgesFilter cartridges for fi ltering of diesel particulates can be assembled from

high-temperature ceramic fi bers. Fiber fi lters capture particulates through depth fi ltration mechanisms. A number of cartridge designs have been developed, some of them incorporating electric heaters for regeneration.

Diesel fi lter systems are designed by combining fi lter materials with regeneration methods. The biggest challenge in the system design is to ensure adequate regeneration and durability. Filters must not be exposed to excessive thermal stress during regeneration to avoid fi lter media failure. Based on the principle of regeneration, fi lter systems are classifi ed into passive and active. Considering low exhaust temperatures in many diesel applications, both light- and heavy-duty, most future fi lter systems are likely to include some active strategies to support regeneration.

Catalyzed Diesel FiltersMost catalyzed diesel fi lters utilize monolithic wall-fl ow substrates coated with

a catalyst. The catalyst lowers the soot combustion temperature, allowing the fi lter to self-regenerate during periods of high exhaust gas temperature. A number of diesel fi lter catalysts have been developed, including both noble and base metal formulations. Catalyzed ceramic traps exhibit very good DPM fi ltration effi ciencies, but are characterized by relatively high exhaust gas pressure drop.

Traps with Fuel AdditivesFuel additives, also called fuel soluble catalysts, can be used in passive diesel

trap systems to lower the soot combustion temperature and to facilitate fi lter regeneration. The most popular additives include iron, cerium, copper, and platinum. Many laboratory experiments and fi eld tests have been conducted to evaluate the regeneration of various diesel fi lter media, e.g. the Nextel fi bers, using additives. Cerium additive was utilized in a commercial trap system for diesel cars designed by Peugeot.

10.5 Diesel Filter Regeneration10.5 Diesel Filter Regeneration

The regeneration of diesel fi lters is characterized by a dynamic equilibrium between the soot being captured and the soot being oxidized in the fi lter. Soot oxidation rates depend on the fi lter temperature, soot load in the fi lter, and a number of other factors. Continuously regenerating fi lters operate at a balance temperature, which can be determined through a laboratory measurement. To facilitate fi lter regeneration on diesel engines in real operation the exhaust gas temperature has to be increased or the soot ignition temperature has to be lowered using a catalyst.

Electrically Regenerated FiltersElectric regeneration of diesel fi lters can be performed in on-board and various

off-board confi gurations. On-board regeneration by means of an electric heater connected to the vehicle power source puts a signifi cant additional load on the vehicle electrical system. Partial fl ow layouts or regeneration with hot air are more energy effi cient. Filter systems have been also developed which must be connected to external power source or are removed from the vehicle for off-board regeneration.


132


Traps with Fuel BurnersDiesel fuel burners can be used to increase the exhaust gas temperature upstream

of a trap in order to facilitate fi lter regeneration. Fuel burner fi lters can be divided into single point systems and full fl ow systems. The full fl ow systems can be regenerated during regular vehicle operation but require complex control strategies to ensure a thermally balanced regeneration.

Filters rigeneration can be achieved thermally, by burning the soot deposits on the fi lter, using for example a dual fi lter systems such as depicted in Figure 10.12. However, such systems may be adopted only in the trucks where space requirements are less stringent compared to passenger cars. In addition there are problems arising from the high temperatures achieved during the regeneration step when the deposited soot is burned off. In fact, local overheating can easily occur leading to sintering with consequent permanent plugging of the fi lter.

Regeneration periodBurner on

Working periodBurner off

(1) (2)

Figure 2 10.12: Principle of fi lter operation (1) and fi lter re-generation (2) for a soot removal system, using fuel powered burners. (adapted from ref 4)

Microwave Regenerated FiltersDiesel soot, due to its microwave absorption properties, can be heated by

microwave irradiation for regeneration of diesel particulate fi lters. This method, when used with fi lter substrate materials that are transparent to microwaves, allows for selective heating of the particulates. In case the fi lter material does adsorb microwave power, microwave irradiation can be used to heat both the soot and the fi lter.

Figure 10.13: The working principle of the continuously regenerating particulate trap


133


Continuously regenerating particulate trapThe CRT is a trade name for a two-stage catalytic, passive fi lter confi guration

(Figure 10.13). The CRT system utilizes a ceramic wall-fl ow fi lter to trap particulates. The trapped particulate matter is continuously oxidized by nitrogen dioxide generated in an oxidation catalyst, which is placed upstream of the fi lter. The CRT requires ultra low sulfur fuel and a certain minimum NOx/PM ratio for proper operation.

10.6 Other Control Technologies10.6 Other Control Technologies

Non-thermal plasma technologies are being developed to reduce NOx emissions from gasoline and diesel exhaust. Since oxidation reactions dominate during plasma discharges in lean exhaust, the plasma alone is ineffective in reducing NOx. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NOx removal effi ciency. Non-thermal plasma reactors can be also designed as diesel particulate matter reducing devices. Plasma technologies still require a signifi cant improvement in their consumption of electrical energy and in other areas.

10.7 Future Trends10.7 Future Trends

Catalysts to enhance low temperature particulate removal to avoid channel plugging under extended idle operation. Improved light-off activity for hydrocarbons with minimum use of expensive precious metals will continue to be important for future HC+NOx standards. More active, selective, and durable catalysts will have to be developed to be compatible with improved exhaust system designs. Close coupling (i.e., manifold, mounting) of the catalyst to be the combustion chamber, hydrocarbon trapping, electrically heated monoliths, particulate traps, and so on all are under consideration by diesel engine manufactures to further improve the quality of the diesel exhaust.

Natural gas fueled lean-burn engines are growing in popularity for service vehicles such as buses, delivery trucks, and so on where refueling is handled daily at a central location (76). Popularity of natural gas fueled vehicles is derived primarily from their clean burning characteristics relative to diesel (77). Selected vehicles are currently equipped with Pt-and Pd-containing catalysts to reduce non-methane hydrocarbons and carbon monoxide, and nitric oxides for stoichiometric engines (78). Methane has been excluded from the standards in the U.S because of its uncreative disposition towards photochemical smog generation reactions. In the future, its reduction may become mandated because methane is a strong “greenhouse” gas. Current catalysts are not suffi ciently active to reduce the methane component of the emissions, so this remains a challenge for improved catalytic technology. The catalytic reduction of NOx from lean-burn engines may prove to be an even greater challenge since natural gas not proven to be highly effective as a reductant in real exhausts. The performance of the most promising zeolite-based catalysts thus far developed is strongly hindered by the water present in the exhaust (79). One of the most important technologies that will affect the design of diesel engine exhaust treatment systems in the near future is NOx reduction. The lean operation of the diesel engine gives rise to high fuel effi ciency, which, in turn, decreases CO2 emissions that contribute to the “greenhouse” effect. This advantage, however, may be offset by the inability of existing catalysts to reduce NOx to N2 in the high O2 content environment. Consequently, there is a strong driving force to be developing a four-way catalytic system that is capable of reducing NOx to N2 and oxidizing TPM, HC, and CO to CO2 and H2O.


134


10.8 References10.8 References 1. Guibet J.C. and Faure-Birchem E., “Fuels and engines. Technology. Energy. Environment.”, 1, Editions

Technip, Paris, 1999.

2. Farrauto R.J. and Heck R.M., Catal. Today 51 (1999) 351.

3. Degobert P., “Automobiles and Pollution” Society of Automotive Engineers, Inc., Warrendale, PA, USA, 1995.

4. a) J. Kašpar, P. Fornasiero, and N. Hickey, Catal. Today, 77 (2003) 419 and b) Kaspar, J., Graziani, M., and Fornasiero, P., “Handbook on the Physics and Chemistry of Rare Earths: The Role of Rare Earths in Catalysis.,” (Gschneidner Jr.K.A.and Eyring L., Eds.), Chap.184, pp.159-267, Amsterdam, Elsevier Science B.V., 2000.

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137


11 Appendices11 Appendices

11.1 Web Directory11.1 Web Directory

The web is getting a more and more important tool for fast and effi cient collection of information on any topic, including scientifi c and technological aspects. Nevertheless, the huge and increasingly amount of information can generate a confusion and inadequate selection of the priorities. Furthermore, the problem of absence of peer review process for most of the sources, can lead to superfi cial and sometimes wrong interpretations. We offer here a few suggestion on web site, which, in our opinion, can provide useful information regarding the topic of Air Pollution.

The web site of the U.S. Environmental Protection Agency (http://www.epa.gov/) has a detailed and rich information on any topics related to the protection of the environment.

The web site of the European Union offers many information on environment (http://europa.eu/pol/env/index_en.htm) and specifi cally also on air pollution (http://europa.eu/scadplus/leg/en/s15004.htm).

The web site of the World Bank (http://www.worldbank.org/) offers interesting documents on environmental protection strategies, including fi nancial aspects.

The web site of the Organisation for Economic Co-operation and Development (OECD) (http://www.oecd.org) is also useful to collect information on environment at http://www.oecd.org/topic/0,2686,en_2649_37465_1_1_1_1_37465,00.html.

The Department of the Environment and Water Resources of the Australian Government provides useful information on recent development in air pollution control at http://www.environment.gov.au/atmosphere/publications/index.html

The International Center for Environmental Technology Transfer has an useful website at http://www.icett.or.jp

The web site www.world.org address general problems of environmental pollution.

The web site www. cleanair.org provides information on air pollution worldwide..


138

APPENDICES

11.2 Glossary11.2 Glossary

AbsorptionThe transfer of molecules from the bulk of the gas to a liquid surface, followed by diffusion of these molecules to the bulk of the liquid.

ACFM The acronym for actual cubic feet per minute, which is the actual gas fl ow rate expressed in the American Engineering system of units. ACFM is a measure of the volume of gas that passes a given point during a one-minute period.

Acid Gas Scrubbing SystemA wet scrubbing system capable of achieving high acid gas removal effi ciency by introducing water, caustic, or hydrated lime into the gas stream.

Adsorbs The adhesion of a substance to the surface of a solid or liquid.

AdsorptionThe process whereby vapor phase compounds in the gas stream pass through a bed or layer of highly porous material (adsorbent). The vapor phase compounds diffuse to the surface of the adsorbent and are retained due to weak attractive forces.

AerosolsThe suspension of solid or liquid particles in the atmosphere.

Air PreheaterIndirect heat exchanger designed to transfer heat from combustion gas to the air stream added to the combustion zone.

Air Toxics (or Hazardous Air Pollutants)Pollutants that are known to cause or suspected of causing cancer or other serious health effects, such as developmental effects or birth defects.

Attainment AreaAn area that meets the air quality standard for a criteria pollutant (under NAAQS).

Autoignition TemperatureThe minimum temperature at which a substance ignites without application of a fl ame or spark.

BaghouseThis term is often used interchangeably with the term “fi ltration systems.” However, it is applicable only to pulse jet, cartridge, reverse air, and shaker-type fi ltration systems. The term “baghouse” does not have any clear meaning for HEPA fi ltration systems.

Bag BlindingConditions where the particles (dust) become embedded in the fabric fi lter over time and are not removed by the bag cleaning process.

Barometric PressureThe total pressure exerted by the atmosphere. This term is synonymous with “atmospheric pressure.”

Bottom AshIncombustible matter resulting from combustion that does not leave as fl y ash.

Burners Out Of Service (BOOS)An off-stoichiometric combustion modifi cation for control of NOx performed by operating alternate burners in the combustion zone as either fuel rich, air rich, or air only.

Carbon Bed AdsorberAn air pollution control system that is used to collect and concentrate organic compounds on an activated carbon adsorbent.

Catalytic OxidizerAn air pollution control device that uses a catalyst to accelerate the oxidation reaction at lower temperatures than possible in gas phase thermal oxidation.

Cascade ImpactorA sampling device used to determine the particle size distribution. Particles are separated and deposited on a series of stages that correspond to different aerodynamic diameters.

CatalystA substance, usually present in small amounts compared to the reactants, that speeds up the chemical reaction rate without being consumed in the process.


139


Clarifi erVessel where particulate matter of higher density than the surrounding liquid is separated and removed from the liquid by gravitational settling.

Collection Effi ciencyA ratio of pollutants entering a control device versus pollutants leaving the device expressed as a percent.

CombustionThe production of heat and light energy through a chemical process, usually oxidation. Products of complete combustion include water and carbon dioxide; while, incomplete combustion can yield partially oxidized organic compounds and carbon monoxide. Factors that promote complete combustion include the proper fuel-air ratio, temperature range, and adequate amount of time for the fuel and its by-products to complete the combustion reactions.

Combustion Particle BurnoutParticulate matter formed and released by the combustion of fossil fuels

Condensable Particulate MatterParticulate matter, contained almost entirely within the PM2.5 classifi cation that forms from condensing gases or vapors. It forms by chemical reactions as well as by physical phenomena.

CondenserA simple, relatively inexpensive device that normally uses water or air to cool and condense a vapor stream.

Cyclone (Large Diameter)A type of mechanical collector, usually ranging in size from one to six feet in diameter, that uses a spinning movement of the gas stream to collect particles ranging in size from one-sixteenth to more than 6 inches in diameter.

Cyclonic SeparatorA mechanical collector that uses centrifugal force to drive particles to the wall of the device.

DesorptionThe process of using low-pressure steam or hot nitrogen gas to remove compounds from an adsorbent bed.

Dew PointThe temperature at which the partial pressure of a substance (in vapor form) equals the equilibrium vapor pressure of the substance. At this temperature, a vapor begins to condense at a constant pressure.

DRE The acronym for destruction and removal effi ciency

Dry ScrubberAn air pollution control device used to remove an acid gas pollutant from a gas stream. The pollutant is collected on or in a solid or liquid material, which is injected into the gas stream. A dry scrubber produces a dry product that must be collected downstream from this control device.

Dust A mixture in air of irregular shaped mineral particulates in the size range from 1 to 200 μm formed by crushing, chipping, grinding or like operations or by natural disintegration of rock and soil.

Electrostatic PrecipitatorA type of air pollution control system that uses high voltage fi elds to electrically charge and collect particulate matter. The charged particles approach an electrically grounded collection plate and accumulate as a dust layer, which is partially removed by mechanical rapping (hammers) on a routine basis.

Emission Sampling TrainEquipment usually consisting of (1) a sampling nozzle and probe, (2) fi lter and impingers for collection of gaseous and/or particulate components, (3) fl ow meter and fl ow regulation devices, and (4) a vacuum pump for collecting a representative sample of a gas stream.

EPA The acronym for environmental protection agency.

ESP The acronym for electrostatic precipitator.

Evaporative Cooling TowerEquipment used to reduce the temperature of a gas stream. Fine droplets, injected into a vessel, are evaporated as they absorb heat from the gas stream.


140

APPENDICES

Fabric FilterA fi ltration device using one or more fi lter bags, sheets, or panels to remove particles from a gas stream.

Fan DriveThe way in which the motor shaft is linked to the fan wheel to transmit power and control speed.

Fine ParticlesEPA classifi cation of particles having aerodynamic diameters greater than 0.1 micrometer and less than or equal to 2.5 micrometers.

Flue Gas Desulfurization (FSD)The process by which sulfur is removed from combustion exhaust gas.

Fly AshUncombusted particulate matter in the combustion gases resulting from the burning of coal and other material.

FPD The acronym for fl ame photometric detection

Fuel NOxNitrogen oxides generated from the fuel or waste during combustion.

Fugitive EmissionsEmissions that escape from industrial processes and equipment.

Heavy Duty (HD) diesel enginesDiesel vehicles with an engine displacement greater than 8 litres and power outputs of greater than 150 kW. HD engines are found in heavy road transport, industrial and marine applications. Medium duty engines fi ll the gap in the middle and are found in medium size trucks, buses and light industrial equipment.

Hood A shaped inlet designed to capture contaminated air and conduct it into the exhaust duct system.

Hood Capture VelocityThe air velocity at any point in front of the hood or at the hood opening necessary to overcome opposing air currents and to capture the contaminated air at that point by pulling it into the hood.

Hood Static PressureThe static pressure in the duct immediately downstream from the hood.

Hopper A device for temporarily storing dust collected by an air pollution control device. Hoppers funnel solids into the solids handling system. There are two main types used on fi ltration systems: pyramidal and trough-type.

JIS The acronym for Japan International Standard.

Light-duty (LD) DieselDiesel vehicles with an engine displacement of less than 4 litres and power output of up to 100 kW, and are characterised by relatively high engine speeds. LD engines would normally be found in passenger vehicle and light commercial vehicle applications.

Low NOx BurnerAn off-stoichiometric combustion modifi cation for control of NOx where the mixing of fuel and air is controlled in a pattern that keeps the fl ame temperature low and dissipates the heat quickly.

Lower Explosive LimitThe lowest concentration at which a gas or vapor is fl ammable or explosive at ambient conditions.

Lowest Achievable Emission Rate (LAER)This represents the most stringent control technology achieved in practice regardless of cost. LAER is used to determine emission limits for the NSR program.


141


Maximum Achievable Control Technology (MACT)EPA standards mandated by the 1990 CAA for the control of toxic emissions from various industries.Minimum Transport VelocityThe minimum gas velocity that must be maintained to keep the contaminant from settling out of the gas fl ow stream and building up deposits in the ductwork.Mist EliminatorA component that passively removes most of the water droplets from a gas stream.

NAAQS National Ambient Air Quality Standards

Overfi re AirAn off-stoichiometric combustion modifi cation for control of NOx where the lower burners operate under fuel rich conditions and air injection nozzles located above these burners complete the combustion process.

Packed Bed Wet ScrubberA common type of gas absorber in which scrubbing liquid is dispersed over packed columns containing packing material. This design provides a large surface area for gas-liquid contact.

Photochemical ReactionA chemical change triggered by the radiant energy of the sun or other light source.

PM2.5 EPA defi nes PM2.5 as particulate matter with a diameter of 2.5 micrometers collected with 50% effi ciency by a PM2.5 sampling collection device. However, for convenience in this reference material, the term PM2.5 includes all particles having an aerodynamic diameter of less than or equal to 2.5 micrometers.

PM10 The U.S. EPA defi nes PM10 as particulate matter with a diameter of 10 micrometers collected with 50% effi ciency by a PM10 sampling collection device. However, for convenience in this reference material, the term PM10 includes all particles having an aerodynamic diameter of less than or equal to 10 micrometers.

Potential-to-EmitThe total emissions that a facility would release by operating at maximum load for 24 hours per day and 365 days per year.

PCB polychlorinated biphenyl.

POP persistent organic pollutant.

ppb part per billion.

ppm part per million.

ppm(V/V))The part per million concentrations that is determined by comparing the volume of one constituent with the total volume of the substance. Gas concentrations are always expressed in a ppm(v/v) format as opposed to the format often used for liquids. Throughout APTI courses, the term ppm when applied to gases means ppm(v/v).

ppm (W/W)The part per million concentrations that is determined by comparing the mass of one constituent with the total mass of the sample. Liquid concentrations are often expressed in a ppm (w/w) format as opposed to the ppm(v/v) format used for gases. Throughout APTI courses, the term ppm when applied to liquids means ppm(w/w). Note that the abbreviation “w/w” is used despite the fact that the ppm concentration is based on a ratio of masses.

Pulse Jet Fabric FilterA type of fi ltration system that uses a short duration pulse of compressed air injected on the “clean side” of the fi lter media to routinely clean the fi lter media. Pulse jets are one of the most common types of fi ltration systems. They are sometimes termed reverse jets (not to be confused with reverse air fi ltration systems).

PulverizerA device used to reduce a substance to a powder form usually by grinding.


142

APPENDICES

Scrubbing LiquidA liquid used to remove particulate or gaseous pollutants by absorption or chemical reaction through contact with the gas stream.of the treatment time (residence time). Space velocity has units of 1/seconds or 1/minutes.

SCWO for supercritical water oxidation.

Smoke A mixture in air of very fi ne particles formed by combustion or other chemical processes in the some range from 0.01-1 μm. The particles may be irregular in shape if formed of solid material or they may be spherical if formed by condensation.

Spray-Dryer-Type Dry ScrubberAn air pollution control device for removing acid gases where an alkaline slurry is introduced into the gas stream and the pollutants absorb into the droplets and react. The droplets dry in the gas stream leaving particulate matter, which is collected by a downstream particulate control device.

Terminal Settling VelocityThe velocity of a falling particle when the gravitational force downward is balanced by the air resistance (or drag) force upward.

Thermal NOxNitrogen oxides generated from atmospheric nitrogen during combustion.

Particulate MatterParticulate matter of all sizes is regulated as total fi lterable particulate matter. This category of air pollutants was the fi rst one that was subject to air pollution control regulations.

TurbidityThe degree of clearness of a liquid or the lack of visual clarity of a liquid. It is defi ned by the measure of light scattering due to the presence of particles suspended in the liquid. Turbidity is similar to opacity for particles suspended in the gas phase.

Ultrafi ne ParticlesEPA classifi cation of particles having aerodynamic diameters less than or equal to 0.1 micrometer.

Venturi ScrubberA type of wet scrubber that is usually highly effi cient but requires a large amount of energy to operate. (Wet scrubbers are air pollution control devices.) In venturi scrubbers, a scrubbing liquid is introduced into the gas stream, which then passes through a contracted area of the scrubber at a high velocity creating a high dispersion of fi ne droplets. These fi ne droplets capture the gaseous and particulate pollutants.

VOC Volatile organic conpounds.

Wet ScrubberA vessel used for removing pollutants from a gas stream by means of a liquid spray, liquid jet, or liquid layer.




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