TECHNOLOGY FOR AIR POLLUTION CONTROL – PART1

TECHNIQUES WITHOUT USING EMISSIONS CONTROL DEVICES Process Change

Wind, Geothermal, Hydroelectric, or Solar Unit instead of Fossil fired Unit.

Change in Fuel e.g. Use of Low Sulfur Fuel, instead of High Sulfur fuel.

Good Operating Practices Good Housekeeping Maintenance

Plant Shutdown

COMMONLY USED METHODS FOR AIR POLLUTION CONTROL

PARTICULATE

Cyclones Electrostatic Precipitators Fabric Filter Wet Scrubbers

GASES Adsorption Towers Thermal Incernation Catalytic Combustion

SOX CONTROL

GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS

Change to Low Sulfur Fuel Natural Gas

Liquefied Natural Gas

Low Sulfur Oil

Low Sulfur Coal

Use Desulfurized Coal and Oil Increase Effective Stack Height Build Tall Stacks

Redistribution of Stack Gas Velocity Profile

Modification of Plume Buoyancy

GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS (CONTD.) Use Flue Gas Desulfurization Systems

Use Alternative Energy Sources, such as Hydro-Power or Nuclear-Power

FLUE GAS DESULFURIZATION SO2 scrubbing, or Flue Gas Desulfurization processes can be

classified as: Throwaway or Regenerative, depending upon whether the recovered sulfur

is discarded or recycled. Wet or Dry, depending upon whether the scrubber is a liquid or a solid.

Flue Gas Desulfurization Processes The major flue gas desulfurization ( FGD ), processes are : Limestone Scrubbing Lime Scrubbing Dual Alkali Processes Lime Spray Drying Wellman-Lord Process

LIMESTONE SCRUBBING

LIMESTONE SCRUBBING Limestone slurry is sprayed on the incoming flue gas.

The sulfur dioxide gets absorbed The limestone and the

sulfur dioxide react as follows :

CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2

CaCO3 + 2HSO3-+ Ca+2 ----> 2CaSO3 + CO2 + H2O

LIME SCRUBBING

LIME SCRUBBING The equipment and the processes are similar to those in

limestone scrubbing Lime Scrubbing offers better utilization of the reagent. The operation is more flexible. The major disadvantage is the high cost of lime compared to limestone.

The reactions occurring during lime scrubbing are :

CaO + H2O -----> Ca(OH)2

SO2 + H2O <----> H2SO3

H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O

CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O

DUAL ALKALI SYSTEM Lime and Limestone scrubbing lead to deposits inside spray tower. The deposits can lead to plugging of the nozzles through which the

scrubbing slurry is sprayed. The Dual Alkali system uses two regents to remove the sulfur

dioxide. Sodium sulfite / Sodium hydroxide are used for the absorption of

sulfur dioxide inside the spray chamber. The resulting sodium salts are soluble in water,so no deposits are

formed. The spray water is treated with lime or limestone, along with make-

up sodium hydroxide or sodium carbonate. The sulfite / sulfate ions are precipitated, and the sodium hydroxide

is regenerated.

LIME – SPRAY DRYINGLime Slurry is sprayed into the chamber

The sulfur dioxide is absorbed by the slurry

The liquid-to-gas ratio is maintained such that the spray dries before it reaches the bottom of the chamber

The dry solids are carried out with the gas, and are collected in fabric filtration unit

This system needs lower maintenance, lower capital costs, and lower energy usage

WELLMAN – LORD PROCESSSchematic process flow diagram – SO2 scrubbing and recovery system

WELLMAN – LORD PROCESS This process consists of the following subprocesses:

Flue gas pre-treatment. Sulfur dioxide absorption by sodium sulfite Purge treatment Sodium sulfite regeneration. The concentrated sulfur dioxide stream is processed to a

marketable product.

The flue gas is pre - treated to remove the particulate. The sodium

sulfite neutralizes the sulfur dioxide :

Na2SO3 + SO2 + H2O -----> 2NaHSO3

WELLMAN – LORD PROCESS (CONTD.) Some of the Na2SO3 reacts with O2 and the SO3 present in the flue

gas to form Na2SO4 and NaHSO3.

Sodium sulfate does not help in the removal of sulfur dioxide, and is

removed. Part of the bisulfate stream is chilled to precipitate the

remaining bisulfate. The remaining bisulfate stream is evaporated

to release the sulfur dioxide, and regenerate the bisulfite.

NOX CONTROL

BACKGROUND ON NITROGEN OXIDES There are seven known oxides of nitrogen :

NO

NO2

NO3

N2O

N2O3

N2O4

N2O5

NO and NO2 are the most common of the seven oxides listed

above. NOx released from stationary sources is of two types

GENERAL METHODS FOR CONTROL OF NOX EMISSIONS NOx control can be achieved by:

Fuel Denitrogenation

Combustion Modification

Modification of operating conditions

Tail-end control equipment

Selective Catalytic Reduction

Selective Non - Catalytic Reduction

Electron Beam Radiation

Staged Combustion

FUEL DENITROGENATION

o One approach of fuel denitrogenation is to remove a large part of the nitrogen

contained in the fuels. Nitrogen is removed from liquid fuels by mixing the fuels

with hydrogen gas, heating the mixture and using a catalyst to cause nitrogen in

the fuel and gaseous hydrogen to unite. This produces ammonia and cleaner

fuel.

This technology can reduce the nitrogen contained in both naturally occurring

and synthetic fuels.

COMBUSTION MODIFICATION Combustion control uses one of the following strategies:

Reduce peak temperatures of the flame zone. The methods are : increase the rate of flame cooling decrease the adiabatic flame temperature by dilution

Reduce residence time in the flame zone. For this we change the shape of the flame zone

Reduce Oxygen concentration in the flame one. This can be accomplished by:

decreasing the excess air controlled mixing of fuel and air using a fuel rich primary flame zone

CATALYTIC COMBUSTION

CATALYTIC EMISSION CONTROL

MODIFICATION OF OPERATING CONDITIONS The operating conditions can be modified to achieve

significant reductions in the rate of thermal NOx

production. the various methods are: Low-excess firing

Off-stoichiometric combustion ( staged combustion )

Flue gas recirculation

Reduced air preheat

Reduced firing rates

Water Injection

TAIL-END CONTROL PROCESSES o Combustion modification and modification of operating

conditions provide significant reductions in NOx, but not enough to meet regulations.

For further reduction in emissions, tail-end control equipment is required.

Some of the control processes are:

Selective Catalytic Reduction

Selective Non-catalytic Reduction

Electron Beam Radiation

Staged Combustion

SELECTIVE CATALYTIC REDUCTION (SCR)

Schematic process flow diagram – NOX control

SELECTIVE CATALYTIC REDUCTION (SCR) In this process, the nitrogen oxides in the flue gases are reduced to

nitrogen

During this process, only the NOx species are reduced

NH3 is used as a reducing gas

The catalyst is a combination of titanium and vanadium oxides. The

reactions are given below :

4 NO + 4 NH3 + O2 -----> 4N2 + 6H2O

2NO2 + 4 NH3+ O2 -----> 3N2 + 6H2O

Selective catalytic reduction catalyst is best at around 300 too 400 oC.

Typical efficiencies are around 80 %

SELECTIVE NON-CATALYTIC REDUCTION (SNR) At higher temperatures (900-1000oC), NH3 will reduce NOX

to nitrogen without a catalyst.

At NH3 : NOX molar ratios 1:1 to 2:1, about 40-60%reduction is obtained.

SNR is cheaper than SCR in terms of operation cost and capital cost.

Tight temperature controls are needed. At lower temperatures, un-reacted ammonia is emitted. At higher temperatures ammonia is oxidized to NO.

ELECTRON BEAM RADIATION This treatment process is under development, and is not

widely used. Work is underway to determine the

feasibility of electron beam radiation for neutralizing

hazardous wastes and air toxics. Irradiation of flue gases containing NOx or SOx produce

nitrate and sulfate ions. The addition of water and ammonia produces NH4NO3, and

(NH4)2SO4 The solids are removed from the gas, and are sold as

fertilizers.

STAGED COMBUSTION

STAGED COMBUSTION PRINCIPLE

Initially, less air is supplied to bring about incomplete

combustion

Nitrogen is not oxidized. Carbon particles and CO are released.

In the second stage, more air is supplied to complete the

combustion of carbon and carbon monoxide.

30% to 50% reductions in NOx emissions are achieved.

CARBON MONOXIDE CONTROL

FORMATION OF CARBON MONOXIDE

Due to insufficient oxygen

Factors affecting Carbon monoxide formation:

Fuel-air ratio

Degree of mixing

Temperature

GENERAL METHODS FOR CONTROL OF CO EMISSIONS Control carbon monoxide formation.

Note : CO & NOx control strategies are in conflict.

Stationary Sources Proper Design Installation Operation Maintenance

Process Industries Burn in furnaces or waste heat boilers.

CARBON DIOXIDE CONTROL

SOURCES OF CARBON DIOXIDEHuman-Related Sources Combustion of fossil fuels: Coal, Oil, and Natural Gas in

power plants, automobiles, and industrial facilities Use of petroleum-based products Industrial processes: Iron and steel production, cement,

lime, and aluminum manufactures

Natural Sources Volcanic eruptions Ocean-atmosphere exchange Plant photosynthesis

SOURCES OF CO2 EMISSIONS IN THE U.S.

Source: USEPA(x-axis units are teragrams of CO2 equivalent)

CO2 EMISSIONS FROM FOSSIL FUEL COMBUSTION BY SECTOR AND FUEL TYPE

Source: USEPA(y-axis units are teragrams of CO2 equivalent)

GENERAL METHODS FOR CONTROL OF CO2 EMISSIONS Reducing energy consumption, increasing the efficiency

of energy conversion

Switching to less carbon intensive fuels

Increasing the use of renewable sources

Sequestering CO2 through biological, chemical, or physical processes

CONTROL OF MERCURY EMISSIONS

MERCURY EMISSIONS Mercury exists in trace amounts in

Fossil fuels such as Coal, Oil, and Natural Gas Vegetation Waste products

Mercury is released to the atmosphere through combustion or natural processes

It creates both human and environmental risks Fish consumption is the primary pathway for human and

wildlife exposure United states is the first country in the world to regulate

mercury emissions from coal-fired power plants (March 15, 2005).

Source: Seingeur, 2004 and Mason and Sheu, 2002.

Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels, March 29-30, 2004

Types of Sources

Worldwide Distribution of Emissions

CONTROL TECHNOLOGIES FOR MERCURY EMISSIONS Currently installed control devices for SO2, NOX, and particulates, in a

power plant, remove some of the mercury before releasing from the stack

Activated Carbon Injection: Particles of activated carbon are injected into the exit gas flow, downstream

of the boiler. The mercury attaches to the carbon particles and is removed in a particle control device

Thief process for the removal of mercury from flue gas:It is a process which extracts partially burned coal from a pulverized coal-

fired combustor using a suction pipe, or "thief," and injects the resulting sorbent into the flue gas to capture the mercury.

PARTICULATE MATTER CONTROL

Range: 20 to 40000 mg/m**3First step: Process controlSecond step: Use of collection device

INDUSTRIAL SOURCES OF PARTICULATE EMISSIONS Iron & Steel Mills, the blast furnaces, steel making furnaces.

Petroleum Refineries, the catalyst regenerators, air-blown asphalt stills, and sludge burners.

Portland cement industry

Asphalt batching plants

Production of sulfuric acid

Production of phosphoric acid

Soap and Synthetic detergent manufacturing

Glass & glass fiber industry

Instant coffee plants

EFFECTS OF PARTICULATE EMISSIONS Primary Effects • Reduction of visibility

• size distribution and refractive index of the particles • direct absorption of light by particles • direct light scattering by particles • 150 micro g / m3 concentration ~ average visibility of 5 miles

( satisfactory for air and ground transportation )

• Soiling of nuisance• increase cost of building maintenance, cleaning of furnishings,

and households • threshold limit is 200 - 250 micro g / m3 ( dust ) • levels of 400 - 500 micro g / m3 considered as nuisance

GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS Five Basic Types of Dust Collectors :

Gravity and Momentum collectors Settling chambers, louvers, baffle chambers

Centrifugal Collectors Cyclones

Mechanical centrifugal collectors

Fabric Filters Baghouses

Fabric collectors

GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS (CONTD.)

Electrostatic Precipitators Tubular Plate Wet Dry

Wet Collectors Spray towers Impingement scrubbers Wet cyclones Peaked towers Mobile bed scrubbers

PARTICULATE COLLECTION MECHANISM Gravity Settling

Centrifugal Impaction

Inertial Impaction

Direct Interception

Diffusion

Electrostatic Effects

Tubular Dust Collector Arrangement for an ESP

Overall Collection η

Ci inlet concentrationCo outlet concentration

Note: The smaller the particle, the lower is efficiency of removal.

PROBLEM A cyclone operates removes 75% of particulate matter

fed to it. The filter is then fed to an ESP which operates with 90% efficiency. What is the overall efficiency of this particulate system?

Step 1: Assuming an initial feed of 100%, determine the percentage of

the stream fed to the ESP. 100% - (100% x 0.75) = 25%

Step 2: Determine final composition after ESP. 25% - (25% x 0.9) = 2.5%

Step 3: Determine overall efficiency hoverall = (initial – final)/ initial

= 100% - 2.5%100% = 97.5%