Encyclopedia of Environmental Science and Engineering, Volume I and II

1234 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

TABLE 10

Excess air at furnace outlet (Frying, 1996). At inlet (Stein, 1968)

Fuel Excess air, % Gas fuels air/gas (vol)

Solid Coal 10–40 Natural 9.8

fuels Coke 20–40 L.P. 29.5

Wood 25–50 Manufacturing gas 4.9

Bagasse 25–45 Coke oven 4.9

Liquid Oil 3–15 Blast furnaces 0.7

fuels Producer gas 1.0

Gaseous Natural gas 5–10 Sewage gas 6.3

fuels Refinery gas 8–15

Blast furnace gas 15–25

Coke over gas 5–10

TABLE 11

Typical complete combustion of bituminous coal

Gas Weight/100 lb coal Molar Basis Volume fraction

SO 2 3 3/64  0.047 0.015

CO 2 207 207/44  4.70 0.145

O 2 40 40/32  1.25 0.040

N 2 750 750/28  24.8 0.800

1000 lb  30.8 1,000

TABLE 12

Detonation limits

g/m^3 with air

C 2 H 2 26–844

NH 3 106–198

CoH 6 45–217

CO 145–863

C 2 H 4 36–373

H 2 3–62

CH 4 35–93

H 2 S 61–645

catalytic reduction, molecular sieve adsorption, extended

water-absorption, tailgas scrubbing with nitric acid, and a

there-stage absorber which combines gas chilling and urea

scrubbing (Ricci, 1977).

There are two types of catalytic reduction: selective

and nonselective catalytic reductions. Nonselective reduc-

tion uses natural gas (or H 2 ) as reductant to decompose the

NOx to nitrogen and oxygen. However, it has high capital

and operating costs, and results in lower plant efficiency and

higher CO emissions, nonselective catalytic reduction is not

an attractive method. Selective catalytic reduction involves

addition of a species, usually NH 3 , which selectively reduces

NOx in an oxygen-containing environment to N 2 and N 2 O.

The selective catalytic reduction may be used to control the

SO 2 and NOx emissions simultaneously.

Removal of entrained water and acid mist by using

a chilling method and mist eliminator is followed by gas

drying with a desiccant. Then the gas stream passes through

a molecular-sieve bed that catalytically converts NO to NO 2.

Finally, the sieve selectively absorbs the NO 2. The absorbed

NOx is then released by heating and returned to the absorber.

This method has high efficiency of NOx removal. However,

platinum catalyst may be lost and the method requires higher

energy.

The extended water-absorption process involves rout-

ing tailgas from the existing absorber tower to a second

unit for additional NOx absorption. Counter-current wash-

ing with water produces a weak acid that is pumped to the

first absorber. In order to increase the conversion of NO to

NCO 2 which is relatively easy to scrub out, high pressure

is required. For new plants, the pressure should surpass

150 psia.

There is no chemical consumption in the tailgas scrub-

bing with nitric acid process since nitric acid is recycled

internally. Low energy consumption is the other advan-

tage. However, its high liquid-gas ratio may lower tower

capacity.

About 80% of the initial NOx input is recovered as weak

nitric acid in the three-stage absorber which combines gas

chilling and the urea scrubbing process. The remainder is

urea and ammonium nitrate. They can serve to prepare liquid

fertilizers.

COMBUSTION FLUE GAS

There are two major categories of NOx control for stationary

combustion sources: combustion modification and flue gas

denitrification. Combustion modification involves change of

either operating or design conditions.

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