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HALDIA INSTITUTE OF TECHNOLOGY Kushagra Abhishek 11/CH/25 HIT LOGO

Nitrogen Oxides

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reduction of nox

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Page 1: Nitrogen Oxides

HALDIA INSTITUTE OF TECHNOLOGY

Kushagra Abhishek 11/CH/25

HIT LOGO

Page 2: Nitrogen Oxides

SELECTIVE CATALYTIC REDUCTION OF NOX

CONTENTS Page no.

Preface 3

Acknowledgement 4

Abstract 5

Introduction 6

What is NOX 6

Contribution of NOx to atmosphere by various sources

6

Why should be control NOx 7

Types of NOX formation 10

What is selective catalytic reduction of NOX

14

How does ammonia reduce Nitrogen Oxide?

14

SCR catalysts 18

How catalysts works in SCR technology 18

Limitation of SCR of NOx 19

CONCLUSION 20

References 21

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PrefacePresence of nox in air us very harmful and dangerous to all of living organism of biosphere.These toxic gases are emitted by human activity like automobiles, industry boilers, furnace kiln operation and many other.Exhaust coming out of these is very toxic so they are treated before releasing it into environment.There are various methods involves in treatment of this flue gas In this report one of the most efficient method have been put in detail. This method is very simple, clean and reliable method with highest efficiency. This is nothing but “SELECTIVE CATALYTIC REDUCTION OF NOx” .

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AcknowledgementI would like to thank Mr. Biswajeet Mondal sir , Astt. Prof. at HIT Haldia for giving me a very challenging and interesting project on “selective catalytic reduction of NOx” . His proper guidance and instruction throughout the project work keeps me energetic and enthusiastic. I would like to thank him again for giving me a wonderful opportunity to have a firsthand report and presentation experience. I would also like to thank Dr. R.N. Jana sir and DR. Avijit Ghosh sir (both Astt. Prof). for their valuable assistance throughout the project As and when required.

At last but not the least I would also like to thank my friends on presentation and report preparation related issues.

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Abstract

The SCR technology for reducing nitrogen oxides (NOx) finds applications worldwide as an after treatment system for power plants and waste furnaces. Ammonia (NH3) could also be used directly as a reagent, but the solution of urea in water is by far the best reagent since it is a non-toxic product and there are no restrictions for its transport on rail, road or ships. Furthermore, urea is a product largely used in agriculture and in industry and urea of various quality grades is readily available. An oxidation catalyst may be used to improve the efficiency of the SCR by converting NO into NO2 and by oxidising CO and hydrocarbons. Accurate dosing of the urea solution and appropriate strategies during transient modes prevent an NH3 slip. The SCR technology, by converting directly NOx to N2 outside the engine, allows the retaining of the engine calibrations, which correspond to the best compromise between fuel consumption and the formation of pollutants during the combustion process

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INTRODUCTION

What is NOX?Nitrogen oxides (NOx) are a very interesting and important family of air polluting chemical Compounds. NOx is a generic term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere as air pollution can be significant. NOx gases are formed whenever combustion occurs in the presence of nitrogen – as in an air-breathing engine; they also are produced naturally by lightning. In atmospheric chemistry, the term means the total concentration of NO and NO2. NOx gases react to form smog and acid rain as well as being central to the formation of tropospheric ozone.

Contribution of NOx to atmosphere by various sources is depicted below in fig:1

Fig:1

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WHY SHOULD WE CONTROL NOx?NOx represent a family of seven compounds. Actually, EPA(Environmental Protection Agency) regulates only 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 air pollutant by itself, but also reacts in the atmosphere to form ozone (O3) and acid rain. It is important to note that the ozone that we want to minimize is tropospheric ozone; that is, ozone in the ambient air that we breathe.Actually Stratospheric ozone protects us and the troposphere from ionizing radiation coming from the sun. EPA has established National Ambient Air Quality Standards (NAAQS) for NO2 and Tropospheric ozone. The NAAQS define levels of air quality that are necessary, with a Reasonable margin of safety, to protect public health (primary standard) and public welfare (Secondary standard) from any known or anticipated adverse effects of pollution. The primary and secondary standard for NO2 is 0.053 parts per million (ppm) (100 micrograms per cubic meter), annual arithmetic mean concentration.

Fig:2

The NO in air, then reacts with free radicals in the atmosphere, which arealso created by the UV acting on volatile organic compounds (VOC). The free radicals then recycle NO to NO2. In this way, each molecule of NO can produce ozone multiple times.40 This will continue until the VOC are reduced to short

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chains of carbon compounds that cease to be photo reactive (a reaction caused by light). A VOC molecule can usually do this about 5 times.In addition to the NO2 and Ozone NAAQS concerns, NOx and sulfur oxides (SOx) in the atmosphere are captured by moisture to form acid rain. Acid rain, along with cloud and dry deposition, severely affects certain ecosystems and directly affects some segments of our economy. All of these facts indicate an obvious need to reduce NOx emissions. However, to successfully do so, we must understand the generation and control of the NOx family of air pollutants. picture depicted in fig:1 is indicating the overall situation as mentioned preceding lines.

WHAT IS A NITROGEN OXIDE?

Diatomic molecular nitrogen (N2) is a relatively inert gas that makes up about 80% of the air we breathe. However, the chemical element nitrogen (N), as a single atom, can be reactive and have ionization levels (referred to as valence states) from plus one to plus five. Thus nitrogen can form several different oxides. Using the Niels Bohr model of the atom, valence state relates to the number of electrons which are either deficient (positive valence) or surplus (negative valence) in the ion when compared with the neutral molecule. The family of NOx compoundsand their properties are listed in Table 1.

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Oxygen ions are always at valence minus 2. Depending upon the number of oxygen ions (alwaysbalanced by the valence state of nitrogen), NOx can react to either deplete or enhance ozoneconcentrations. The nitrogen ion in these oxides really does a dance in which it has (at differenttimes) various numbers of oxygen ions as partners. Nitrogen changes its number of partnerswhen it changes its ionization energy level. This happens whenever NOx:

(1) is hit with a photon of ionizing radiation (UV or a shorter wavelength light) (2) is hit with enough photons that together transfer enough energy to change its ionization level (3) is catalyzed (4) is stimulated sufficiently by thermal (IR) energy (5) reacts with a chemically oxidizing or reducing radical (an ionized fragment of a molecule); (6) reacts with a chemically oxidizing or reducing ion (an atom with unbalanced electrical charge).When any of these oxides dissolve in water and decompose, they form nitric acid (HNO3) ornitrous acid (HNO2). Nitric acid forms nitrate salts when it is neutralized. Nitrous acid formsnitrite salts. Thus, NOx and its derivatives exist and react either as gases in the air, as acids indroplets of water, or as a salt. These gases, acid gases and salts together contribute to pollutioneffects that have been observed and attributed to acid rain.Nitrous oxide (N2O), NO, and NO2 are the most abundant nitrogen oxides in the air. N2O (alsoknown as laughing gas) is produced abundantly by biogenic sources such as plants and yeasts. Itis only mildly reactive, and is an analgesic N2O is an ozone depleting substance which reacts with o3 in both the troposphere and stratosphere.

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Except for NO from soils, lightning and natural fires, NO is largely anthropogenic (i.e., generated by human activity). Biogenic sources are generally thought to account for less than 10% of total NO emissions.

WHERE DOES NOx COME FROM?Automobiles and other mobile sources contribute about half of the NOx that is emitted. Electricpower plant boilers produce about 40% of the NOx emissions from stationary sources.Additionally, substantial emissions are also added by such anthropogenic sources as industrialboilers, incinerators, gas turbines, reciprocating spark ignition and Diesel engines in stationarysources, iron and steel mills, cement manufacture, glass manufacture, petroleum refineries, andnitric acid manufacture. Biogenic or natural sources of nitrogen oxides include lightning, forestfires, grass fires, trees, bushes, grasses, and yeasts.1 These various sources produce differingamounts of each oxide. The anthropogenic sources are approximately shown as:

This shows a graphic portrayal of the emissions of our two greatest sources of NOx. If wecould reduce the NOx emissions from just these two leading categories, we might be able to livewith the rest. However, don’t expect either of these categories to become zero in the foreseeablefuture. We cannot expect the car, truck, bus, and airplane to disappear. The zero-emission car isstill on the drawing board and not on the production line. Also, social customs will have tochange before consumption of electricity can be reduced.

In all combustion there are three opportunities for Types of NOx formation. They are:1. Thermal NOx - The concentration of “thermal NOx” is controlled by the nitrogen andoxygen molar concentrations and the temperature of combustion. Combustion at temperatureswell below 1,300_C (2,370_F) forms much smaller concentrations of thermal NOx.2. Fuel NOx - Fuels that contain nitrogen (e.g., coal) create “fuel NOx” that results fromoxidation of the already-ionized nitrogen contained in the fuel.3. Prompt NOx - Prompt NOx is formed from molecular nitrogen in the air combining with fuelin fuel-rich conditions which exist, to some extent, in all combustion. This nitrogen then oxidizesalong with the fuel and becomes NOx during combustion, just like fuel NOx. The abundance of

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prompt NOx is disputed by the various writers of articles and reports - probably because theyeach are either considering fuels intrinsically containing very large or very small amounts ofnitrogen, or are considering burners that are intended to either have or not have fuel-rich regionsin the flame.

Table 2 lists principles or methods that are used to reduce NOx. Basically there are six:

Method 1. Reducing Temperature -- Reducing combustion temperature means avoiding the

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stoichiometric ratio (the exact ratio of chemicals that enter into reaction). Essentially, thistechnique dilutes calories with an excess of fuel, air, flue gas, or steam. Combustion controls usedifferent forms of this technique and are different for fuels with high and low nitrogen content.

Control of NOx from combustion of high nitrogen content fuels (e.g., coal) can be understood bythe net stoichiometric ratio. Control of the NOx from combustion of low nitrogen fuels (such asgas and oil) can be seen as lean versus rich fuel/air ratios. Either way, this technique avoids theideal stoichiometric ratio because this is the ratio that produces higher temperatures that generatehigher concentrations of thermal NOx.Combustion temperature may be reduced by: (1) using fuel rich mixtures to limit the amount ofoxygen available; (2) using fuel lean mixtures to limit temperature by diluting energy input;(3) injecting cooled oxygen-depleted flue gas into the combustion air to dilute energy;(4) injecting cooled flue gas with added fuel; or (5) injecting water or steam. Low-NOx burnersare based partially on this principle.8,9,10 The basic technique is to reduce the temperature ofcombustion products with an excess of fuel, air, flue gas, or steam. This method keeps the vastmajority of nitrogen from becoming ionized (i.e., getting a non-zero valence).

Method 2. Reducing Residence Time -- Reducing residence time at high combustiontemperatures can be done by ignition or injection timing with internal combustion engines. It canalso be done in boilers by restricting the flame to a short region in which the combustion airbecomes flue gas. This is immediately followed by injection of fuel, steam, more combustionair, or recirculating flue gas. This short residence time at peak temperature keeps the vastmajority of nitrogen from becoming ionized. This bears no relationship to total residence timeof a flue gas in a boiler.

Method 3. Chemical Reduction of NOx – This technique provides a chemically reducing (i.e.,reversal of oxidization) substance to remove oxygen from nitrogen oxides. Examples includeSelective Catalytic Reduction (SCR) which uses ammonia, Selective Non-Catalytic Reduction(SNCR) which use ammonia or urea, and Fuel Reburning (FR). Non-thermal plasma, anemerging technology, when used with a reducing agent, chemically reduces NOx. All of thesetechnologies attempt to chemically reduce the valence level of nitrogen to zero after the valencehas become higher.11 Some low-NOx burners also are based partially on this principle.

Method 4. Oxidation of NOx -- This technique intentionally raises the valence of the nitrogenion to allow water to absorb it (i.e., it is based on the greater solubility of NOx at higher valence).This is accomplished either by using a catalyst, injecting hydrogen peroxide, creating ozone

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within the air flow, or injecting ozone into the air flow. Non-thermal plasma, when used withouta reducing agent, can be used to oxidize NOx. A scrubber must be added to the process toabsorb N2O5 emissions to the atmosphere. Any resultant nitric acid can be either neutralized bythe scrubber liquid and then sold (usually as a calcium or ammonia salt), or collected as nitricacid to sell to customers.12, 49

Method 5. Removal of nitrogen from combustion -- This is accomplished by removingnitrogen as a reactant either by: (1) using oxygen instead of air in the combustion process; or(2) using ultra-low nitrogen content fuel to form less fuel NOx. Eliminating nitrogen by usingoxygen tends to produce a rather intense flame that must be subsequently and suitably diluted.Although Method 2 can lower the temperature quickly to avoid forming excessive NOx, it cannot eliminate nitrogen oxides totally if air is the quench medium. Hot flue gas heats the air that is used to quench it and this heating generates some thermal NOx. This method also includesreducing the net excess air used in the combustion process because air is 80% nitrogen. Usingultra-low-nitrogen content fuels with oxygen can nearly eliminate fuel and prompt NOx.13

Method 6. Sorption, both adsorption and absorption -- Treatment of flue gas by injection ofsorbents (such as ammonia, powdered limestone, aluminum oxide, or carbon) can remove NOxand other pollutants (principally sulfur). There have been successful efforts to make sorptionproducts a marketable commodity. This kind of treatment has been applied in the combustionchamber, flue, and baghouse. The use of carbon as an adsorbent has not led to a marketableproduct, but it is sometimes used to limit NOx emissions in spite of this. The sorption method isoften referred to as using a dry sorbent, but slurries also have been used. This method uses eitheradsorption or absorption followed by filtration and/or electrostatic precipitation to remove thesorbent.

Method 7. Combinations of these methods -- Many of these methods can be combined toachieve a lower NOx concentration than can be achieved alone by any one method. For example,a fuel-rich cyclone burner (Method 1) can be followed by fuel reburn (Method 3) and over-fireair (Method 1). This has produced as much as a 70% reduction in NOx.55 Other controltechnologies that are intended to primarily reduce concentrations of sulfur also strongly affect the nitrogen oxide concentration. For example, the SOx-NOx-ROx-Box (SNRB) technology uses a limestone sorbent in the flue gas from the boiler to absorb sulfur. This is followed by ammoniainjection and SCR using catalyst fibers in the baghouse filter bags. The sulfur is recovered fromthe sorbent and the sorbent regenerated by a Claus process. This has demonstrated removal of upto 90% of the NOx along with 80% of the SOx.39, 42 EBARA of Japan reported that an electronbeam reactor with added ammonia removed 80% of the SO2 and 60% of the NOx for a utility

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boiler in China.54 FLS Milo and Sons reported at the same symposium that 95% of the SO2 and70%-90% of the NOx were removed in several demonstrations of their SNAP technology, whichis based upon an aluminum oxide adsorber with Claus regeneration.

Selective catalytic reduction technology for reducing content of NOx in air/flue gas. Q.)What is scr?Ans.) SCR uses a catalyst to react injected ammonia to chemically reduce NOx. It can achieve up to a 94% reduction in NOx and is one of the most effective NOx abatement techniques. However, this technology has a high initial cost. In addition, catalysts have a finite life in flue gas and some ammonia “slips through” without being reacted. SCR has historically used precious metal catalysts, but can now also use base-metal and zeolite catalysts. The base-metal and operate at much different temperatures then the precious metal catalysts

Selective catalytic reduction (SCR) -Involves using beds containing ammonia or urea to reduce nitrogen oxides to molecular nitrogen and water. Two or three catalysts (usually tungsten and vanadium) are arranged in honeycomb shapes in the beds so air can flow through. NOx reduction efficiencies ranging from 75 to 90% are possible when the amount of catalyst is sufficient, the catalyst is in good condition, the ammonia reagent flow is sufficient, and the ammonia is adequately distributed across the gas stream.

How does ammonia reduce Nitrogen Oxide?Injection of ammonia in the flue gas in presence of a catalyst which causes chemical reactions that convert the NOx to free nitrogen and water vapor.

–NO4 NO + 4 NH3 + O2 ----------- 4 N2 + 6 H2O

–NO24 NO2 + 8 NH3 + 2 O2 ----------- 6 N2 + 12 H2OTemperatures for ideal de-nitrification must take place between 392 and 1022 degree Fahrenheit. Temperatures below 400 can cause the formation of ammonia SCR

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salts. Temperatures above 850 can cause a reverse reaction where ammonia is converted back into NOx. There are various operation modes of a combined cyclepower plant, which inherently require varying amounts of ammonia to be injected into the flue gas stream as part of the SCR system. In SCR systems, ammonia or a compound of ammonia is used as the reducing agent and is injected into the flue gas stream, passing over a catalyst. NOx emission reductions over 80-90% are achieved. Temperatures for ideal de-nitrification must take place between 392F and 1022F. Temperatures below 400F can cause the formation of ammonia salts. Temperatures above 850F can cause a reverse reaction where ammoniais converted back into NOx.

several secondary reactions during reduction of NOx by NH3 are:

2SO2 + O2 → 2SO3

2NH3 + SO3 + H2O → (NH4)2SO4

NH3 + SO3 + H2O → NH4HSO4

Note: The reaction for urea instead of either anhydrous or

aqueous ammonia is:

4NO + 2(NH2)2CO + O2 → 4N2 + 4H2O + 2CO2

SCR CATALYSTS:

SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components are usually

either oxides of base metals (such vanadium, molybdenum and tungsten), 

zeolites, or various precious metals. Each catalyst component has advantages and disadvantages.

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Base metal catalysts, such as the vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly seen in industrial and utility boiler applications. Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filter with forced regeneration. They also have a high catalyzing potential to oxidize SO2 into SO3, which can be extremely damaging due to its acidic properties.

Zeolite catalysts have the potential to operate at substantially higher temperature than base metal catalysts; they can withstand prolonged operation at temperatures of 900 K and transient conditions of up to 1120 K. Zeolites also have a lower potential for potentially damaging SO2 oxidation.Iron- and copper-exchanged zeolite urea SCRs have been developed with approximately equal performance to that of vanadium-urea SCRs if the fraction of the NO2 is 20% to 50% of the total NOx. The two most common designs of SCR catalyst geometry used today are honeycomb and plate. The honeycomb form usually is an extruded ceramic applied homogeneously throughout the ceramic carrier or coated on the substrate. Like the various types of catalysts, their configuration also has advantages and disadvantages. Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but plate configurations are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily. A third type is corrugated, comprising only about 10% of the market in power plant applications.

Reductants:

Several reductants are currently used in SCR applications including anhydrous ammonia, aqueous ammonia or urea. All those three reductants are widely available in large quantities.

Pure anhydrous ammonia is extremely toxic and difficult to safely store, but needs no further conversion to operate within an SCR. It is typically

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favoured by large industrial SCR operators. Aqueous ammonia must be hydrolyzed in order to be used, but it is substantially safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion to ammonia through thermal decomposition in order to be used as an effective reductant.

SCR technology working stepsin boiler is shown below in Fig:3

Fig:3

How catalysts works in SCR technologySCR

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In SCR method there is a bed of catalyst arranged in Plate configuration or Honey comb configuration.

There are two beds of catalyst for ex: in V &Ti or Pt/Pd or Rh type catalyst arrangement, when flue gas coming from exhaust of silencer of automobiles then there is ammonia injection in the path of flue gas. As we know flue gas are composed of harmful nox gases then reduction reaction takes place. Here catalyst bed attracts or bind towards itself N2 . Again at second bed of catalysts hydrocarbon oxidise co. This co reacts with O2 to get less harmful Co2 gas and water vapour. At last there is a control system to regulate and keep air fuel ratio at optimum level. This situation is depicted as shown below in Fig:4

Fig:4

Limitation of SCR of NOx:

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SCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many SCRs are given a finite life due to known amounts of contaminants in the untreated gas. The large majority of catalyst on the market is of porous construction. A clay planting pot is a good example of what SCR catalyst feels like. This porosity is what gives the catalyst the high surface area essential for reduction of NOx. However, the pores are easily plugged by a variety of compounds present in combustion/flue gas. Some examples of plugging contaminates are: fine particulate, ammonia sulfur compounds, ammonium bisulfate (ABS) and silicon compounds. Many of these contaminants can be removed while the unit is on line, for example by soot blowers. The unit can also be cleaned during a turnaround or by raising the exhaust temperature. Of more concern to SCR performance is poisons, which will destroy the chemistry of the catalyst and render the SCR ineffective at NOx reduction or cause unwanted oxidation of ammonia (forming more NOx). Some of these poisons include: halogens, alkaline metals, arsenic, phosphorus, antimony, copper.

Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.

Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or catalyst has degraded (see above).

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Temperature is one of the largest limitations of SCR. Gas turbines, cars, and diesel engines all have a period during a start-up where exhaust temperatures are too cool for NOx reduction to occur.

CONCLUSION:

Selective catalytic reduction of NOx is very simple, clean and reliable technique for reducing NOx content in flue gas.

Although its initial installation cost is high but its efficiency in reducing nox is upto 95%,largest by any other available technology. This technique enhances engine life of automobile, also less consumption of fuel.

Staging of the combustion is implicit in several pollution prevention techniques. Tandem application (or use of hybrid control technology) of NOx control techniques (first SNCR, then SCR in the duct, and then sorption before the ESP which is referred to as “polishing”) have been used to achieve an overall reduction of 90+% in NOx and 80% in SOx, even without using low- NOx burners to lower NOx generation

References:

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1. Nitrogen Oxides (NOx),Why and How They Are Controlled by Clean Air Technology Center (MD-12)Information Transfer and Program Integration DivisionOffice of Air Quality Planning and StandardsU.S. Environmental Protection Agency

2. Selective Catalytic Reduction (Final Report) by ACEA.

3. Effects of Nitrogen Oxides". Electric Power Research Institute, 1989

4) Gieshoff, J; M. Pfeifer, A. Schafer-Sindlinger, P. Spurk, G. Garr, T. Leprince (March 2001). "Advanced Urea Scr Catalysts for Automotive Applications" (PDF). Society of Automotive Engineers. Retrieved 2009-05-

4. "NOx Removal". Branch Environmental Corp. Archived from the original on 2007-10-08. Retrieved 2007-12-26.

5. http://en.wikipedia.org/wiki/Selective_catalytic_reduction

6. www.epa.gov/ttncatc1/dir1/fnoxdoc.pdf

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