Agriculture Reference
In-Depth Information
4.
E
FFECT OF
O
PERATING
C
ONDITIONS AND
F
UEL
C
HARACTERISTICS ON
E
MISSIONS
In order to devise a suitable energy recovery system it is essential to consider emissions
from such a process. Emissions control strategies need to be employed to keep them under
environmental limits. Knowledge of emissions formation mechanisms is necessary to develop
emission mitigation strategies. So far in this chapter NO
x
, SO
2
and CO emissions data for
tests conducted with different coal and pulp blends at a wide range of operating conditions is
presented. Following paragraphs discuss the potential formation pathways and the effect of
fuel characteristics and operating conditions on the emissions.
4.1. NO
x
Emissions
The mechanisms of NO
x
formation and destruction during combustion and the effect of
different parameters on NO
x
emissions are described as follows.
Nitrogen in fuel is the greatest source of NO
x
and N
2
O during fluidized bed combustion.
The conversion of coal nitrogen to NO
x
and N
2
O ranges from 10% [Braun, 1990] to 30%
[Suzuki et al. 1991] depending upon the type of coal. Only a small percentage of NO
x
come
from atmospheric nitrogen (thermal and prompt) at typical fluidized bed temperature of 800 -
900 °C [Halgaard, 1991]. Fuel NO
x
formation takes place by the combustion of nitrogen
containing species released during volatilisation and oxidation of nitrogen retained in char.
Fuel NO
x
are very sensitive to stoichiometric conditions. Prompt NO
x
, contributes the
minimum to the total NO
x
, and results from radicals of hydrocarbons reacting with
atmospheric nitrogen. These reactions give NH
3
and HCN under fuel rich conditions. These
NH
3
and HCN are oxidized to NO in lean zone of the flame [Mahmoudi et al. 2010].
Table 12. Nitrogen and Sulphur contents of Coal and Coal-PP blends
50/50
60/40
70/30
100/0
Nitrogen, AR (%)
0.88
1
1.2
Nitrogen, Dry (%)
1.42
1.50
1.57
Nitrogen, DAF (%)
1.51
1.59
1.66
Sulphur, AR (%)
1.08
1.26
1.45
2.01
Sulphur, Dry (%)
1.74
1.85
1.93
Sulphur, DAF (%)
1.84
1.95
2.04
Ca, AR (%)
0.66
0.55
0.44
0.12
K, AR (%)
0.41
0.34
0.27
Na, AR (%)
0.16
0.17
0.18
Fe, AR (%)
0.45
0.54
0.63
Ca/S (Mass)
0.61
0.44
0.31
0.06
Ca/S (Molar)
0.49
0.35
0.25
0.05
Moisture (%)
38.3
31.76
25.22
