Agriculture Reference
In-Depth Information
3500
50/50 Blend
3000
60/40 Blend
2500
70/30 Blend
2000
1500
1000
500
0
6
8
10
12
14
O
2
(%) in flue gas
Figure 7. Carbon monoxide emissions from coal-PP blends.
There are more than 80 possible reaction mechanisms for N
2
O and NO
x
formation in
fluidized bed combustion. There are over 90 reactions of HCN which play a key role in the
formation of NO
x
and N
2
O from the nitrogen released during de-volatilisation [Halgaard,
1991; Gustavsson and Leckner, 1990; Johnsson et al. 1990; Johnsson, 1990]. Most of the NO,
N
2
O, NH
3
and HCN is formed during de-volatilisation stage although de-volatilisation step is
much shorter than char combustion. NO is also formed in significant amounts during char
combustion, however N
2
O, NH
3
and HCN are not considered important [Winter, et al. 1999].
When a fuel particle is heated up, NH
3
and HCN are released and oxidised in complex
reaction mechanisms with O and OH to form NO [Kilpinen and Hupa 1991]. NH
3
is oxidized
to NO by the following reaction [Kilpinen and Hupa, 1991].
NH
3
+ 5/4O
2
NO + 3/2H
2
O
(1)
N
2
O is not observed as it is thought to be destroyed in the flame mainly by the following
reaction and is converted to molecular nitrogen [Winter, 1997].
N
2
O + H
N
2
+ OH
(2)
Formation of HCN and NH
3
also depends upon heating rate. Bassilakis et al. (1993)
found that at lower heating rates volatile nitrogen product is NH
3
while at higher heating rates
the product is HCN. HCN is evolved primarily from the fuel while NH
3
evolves from the fuel
in small amounts as well as produced mainly from the conversion of HCN by reaction (3)
with H.
