Encyclopedia of Environmental Science and Engineering, Volume I and II

1236 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

TABLE 15

How current NOx control methods fare with fossil fuels

NOx reduction

Method Gas (%) Oil (%) Coal (%)

Flue-gas circulation 60 20 Not effective

Reduced combustion-air preheat 50 40 Not competitive

Steam or water injection 60 40 Not competitive

Staged combustion 55 40 40

Low excess air 20 20 20

Reduced heat-release rate 20 20 20

Combined staging, low excess air, and

reduced heat release

50 35 40

Change to fuel with lower percent N Not effective 40 20

Source: EPA report No. 650/2-74-066.

Sliding

Air

Damper

Air

Measuring

Device

Adjustable

Spin Vanes

Inner Secondary Air

With Some Recirculation

To Base of Flame

Outer Secondary

Air Mixing

Pulverized

Coal and

Primary Air

A

B

C

D

High temperature - fuel rich

devolatization zone

Production of reducing species

NOx decomposition zone

Char oxidizing zone

FIGURE 19 B & W dual register, low NOx burner (1993). (See Color Plate III)).

combustion has been demonstrated for gas and oil combus-

tion by Sensenbaugh and Jonakin (1960). Low excess air

combustion for coal may result in furnace slagging, smoke,

flame instability, and efficiency loss due to carbon carry

over. This technique is always implemented concurrently

with other combustion modifications. Reduced heat release

reduces the flame temperature and thereby NOx emissions.

The fuel NOx can be reduced by changing to a fuel with

lower %N. On an equivalent heat generation basis, fuels in

order of decreasing NOx formation are coal, oil, gas. Duhl

(1976) pointed out that NOx found in the methanol flue gas

was lower than that produced by natural gas, and even lower

than that derived from oil combustion. Southern California

Edison successfully tested methanol in a 26-MW gas tur-

bine and got NOx emissions of 45–50 ppm against 260 ppm

for distillate fuel in an identical unit (Parkinson, 1981). The

effects of the modification of operating conditions to reduce

NOx emissions by fossil fuel combustion are shown in Table 15

(Ricci, 1977).

MODIFICATION OF DESIGN CONDITIONS

Low NOx Burner Low NOx burners are either the delayed

or minimum mixing of fuel and air method to increase the

locally fuel-rich zones and thereby decrease volatile nitro-

gen conversion. Also secondary air or tertiary air is added

to ensure the carbon burn-out. For instance, the split-flame

burners, Figure 19, of Babcock and Wilcox, reduce NOx

emissions by minimizing mixing between the coal and pri-

mary air. The secondary air stream is introduced from the

outer annulus to complete the carbon burn-out. According to

the manufacturer, the NOx emission is 0.45lb/10^6 BTU with-

out overfire air or flue gas recirculation (Parkinson, 1981).

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