Encyclopedia of Environmental Science and Engineering, Volume I and II

VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1235

COMBUSTION MODIFICATION

Modification of Operating Conditions The production of

thermal NOx is very temperature sensitive. Reducing the

flame temperature is effective in reducing thermal NOx pro-

duction. This can be achieved by using flue gas recirculation

by reduced air preheat, and by steam or water injection. In

flue gas recirculation, the recirculated gas must be returned

to the combustion zone. The greatest reduction in flame

temperature is achieved by mixing the gas directly with the

combustion air. The above methods are not as effective for

coal fired boilers since coal contains high fuel nitrogen. Both

thermal and fuel NOx can be reduced by staged combustion,

low excess air, reduced heat release rate, and a combina-

tion of these methods. In staged combustion, fuel is mixed

with sub-stoichiometric amounts of air and burned in the

first stage. In the second stage fuel burn-out is completed

by injecting secondary air into the stage. Formation of NO

is thereby limited in the first stage because of the low air

level. By using interstage cooling, temperatures can be held

down in the second stage where the excess air is injected.

Low excess air decreases the NOx emissions by reducing

oxygen availability. The effectiveness of low excess air

TABLE 14

Differences between combustion flue gas and nitric acid plant tailgas

Source Item Flue gas Tailgas

NOx concentration low high

Key component of NOx NO NO 2

Flow rate high low

Gas pollutant NOx + SO 2 NOx

TABLE 13

Kinetic parameters for some bimolecular reactions (Laider, 1965)

Logarithm of frequency factor, cc mole-1 sec-1

Reaction Activation energy,

kcal per mole

Observed Calculated by absolute

rate theory

Calculated by simple

collision theory

Reference

NO + O 3 → NO 2 + O 3 2.5 11.9 11.6 13.7 a

NO + O 3 → NO 3 + O 7.0 12.8 11.1 13.8 b

NO 2 + F 2 → NO 2 F + F 10.4 12.2 11.1 13.8 c

NO 2 + CO → NO + CO 2 31.6 13.1 12.8 13.6 d

2NO 2 → 2NO + O 2 26.6 12.3 12.7 13.6 e

NO + NO 2 Cl → NOCl + NO 2 6.9 11.9 11.9 13.9 f

2NOCl → 2NO + Cl 2 24.5 13.0 11.6 13.8 g

NO + Cl 2 → NOCl + Cl 20.3 12.6 12.1 14.0 h

F 2 + ClO 2 → FClO 2 + F 8.5 10.5 10.9 13.7 i

2ClO → Cl 2 + O 2 0 10.8 10.0 13.4 j

Upper Limit in Oxygen

Upper Limit in Air

Lower Limit

in Oxygen

100% Oxygen

100% Oxygen

100% Methane

100% Nitrogen 100% Nitrogen

100% Hydrogen

100% Hydrogen

Limit in Air

Stoichiometric

Mixture

a)

b)

Mixtures of Fuel and Air

100% Fuel

Fuel

Composition

10–90

20–80

30–70

40–60

50–50

60–40

70–30

80–20

90–10

FIGURE 18 Flammability diagram. (a) Hydrogen–oxygen 1 atm 20C. (b) Fuel–oxygen 1 atm 20C (Talmage, 1971).

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