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CONTROL OF NOx IN THERMAL POWER

PLANTS

submitted byPATI JAYA CHANDRA113MN0480

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WHY?

NO 2 is an air pollutant.

NO 2 reacts in the atmosphere to form ozone (O3) and acid rain.

Tropospheric ozone has been and continues to be a significant air

pollution problem in the United States and is the primary constituent

of smog.

Standard for NO2 is 0.053 parts per million (ppm) in the atmosphere.

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WHERE DOES NOx COME FROM?

Auto mobiles & other mobile sources

Thermal Power Plants Other sources

50% 20% 30%

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CONTROL OF NOx:• NOx control can be achieved by:

· Catalytic Combustion

· Modification of operating conditions

· Tail-end control equipment

Selective Catalytic Reduction

Selective Non - Catalytic Reduction

Electron Beam Radiation

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Catalytic Combustion

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Catalytic Combustion

• A catalyst is used to react fuel with air at a lower temperature than normal

combustion at which generation of significant amounts of NOx does not

occur.

• Emissions under 1 ppm NOx have been reported.

• This technology has a relatively high capital and operation and maintenance

cost because there is both a substantial initial investment and a replacement

cost for the catalyst

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Modification Of Operating Conditions

Off-stoichiometric combustion (staged combustion)

· 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.

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Staged Combustion

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Modification Of Operating ConditionsSteam/water injection:

• To reduce combustion temperature, steam or water can be mixed with the air

flow.

• This lowers combustion temperature to below 1,400F to limit NOx generation to

about 40 ppm.

• This can cause the concentration of CO and unburned hydrocarbons emitted from

a turbine to be increased.(drawback)

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Selective Catalytic Reduction• 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

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Selective Catalytic Reduction

• Selective catalytic reduction catalyst is best at around 300 to 400C

• Typical efficiencies are around 80 %.

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Selective Non-catalytic Reduction (SNR)• At higher temperatures (900-1000C), 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.

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Electron Beam Radiation• This treatment process is under development, and is not widely used.

• 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.

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References:1. RAO M.N. & RAO H, Air pollution, Tata McGraw Hill.

2. Mahajan S.P., pollution control in process industries, Tata McGraw Hill.

3. https://www3.epa.gov/ttncatc1/dir1/fnoxdoc.pdf