Page 240 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

216 7 Combustion Process and Air Emission Formation

NO equilibrium concentration drops sharply as the temperature goes down, and
becomes negligible below 600 °C.

7.7.1.5 NOx Formation in Char Combustion

Char is formed during combustion of solid fuels, especially coal. This char residue
forms N 2 O under appropriate conditions. The N 2 O formed from char nitrogen
varies, depending on fuel property and devolatilization conditions. However, the
formation mechanism of N 2 O from char nitrogen is not yet fully clarified and
different mechanisms have been proposed in literature.
Much of the NO formation in coal combustion is by char–nitrogen interaction
[48]. Nitrogen atoms can be adsorbed by char, which is porous anyway. The
char–nitrogen is oxidized to produce NO, and it can be described with a simplified
reaction formula as
1
ð N þ CÞ þ O 2 ! NO þ C s ð7:77Þ
2
where (N + C) represents nitrogen atom bounded with carbon by chemisorption on
char. NO reduction may take place by the following two reactions

2NO þ 2CO ! N 2 þ 2CO 2 ð7:78Þ
1
k f6
2NO þ C s ! O þ Cð Þ þ N 2
2 ð7:79Þ
4 2
k f6 ¼ 0:21 exp 1:31 10 =T kmol m s atm
where (O + C) is the oxygen bounded with char carbon by chemisorption, and it
reacts with CO to produce CO 2
k f7
ð
CO þ O þ CÞ ! CO 2 þ C s
ð7:80Þ
3
2

k f7 ¼ 7:4 10 4 exp 9:56 10 T kmol m s atm
With the increasing temperature, the oxygen adsorbed onto the surface of carbon
may also produce CO directly,
k f8
ð O þ CÞ ! CO
ð7:81Þ
4
2

k f8 ¼ 1:5 10 2 exp 2:01 10 T kmol m s atm
Then, the overall rate of NO reduction can be determined as follows [48].
ð
k f6 P NO k f7 P CO þ k f8 Þ
r NO ¼ ð7:82Þ
k f6 P NO þ k f7 P CO þ k f8