Agriculture Reference
In-Depth Information
Table 13. H/N, O/N and O/H ratios of the fuels tested on DAF basis [Akram, 2012]
H/N
O/N
O/H
Thoresby Singles Coal
1.1
4.2
3.8
Sugar Beet Pulp
12.3
74.5
6.1
50/50 Coal-Pulp Blend
3.33
10.16
3.05
60/40 Coal-Pulp Blend
3.16
8.5
2.69
70/30 Coal-Pulp Blend
2.82
6.75
2.4
The effect of temperature on NO
x
emissions for different fuels is observed to be different.
For coal only firing the emissions found to be slightly increased with increase in bed
temperature. Emissions of NO
x
increase with increase in bed temperature for coal due to
decreased influence of char at higher temperatures. At higher bed temperatures char is
consumed quicker and NO
x
reduction by char is lower and emissions increase. Moreover, at
higher bed temperature combustion rate is higher and production rate of radicals increases
[Kilpinen and Hupa, 1991] which can result in increased conversion of nitrogen to NO.
Combustion temperature has the largest effect on NO
x
and N
2
O emissions during fluidized
bed combustion [Amand and Leckner, 1988; Oka and Anthony 2003]. With an increase in
bed temperature concentration of NO
x
increases while that of N
2
O decreases. Braun et al,
(1990 and 1991) also observed the effect of bed temperature on NO
x
emissions and found that
when bed temperature was increased concentration of NO
x
increased from 0 to 150 mg/m
3
while those of N
2
O decreased from 250 to 100 mg/m
3
for a low volatile coal.
During co-firing coal-pulp blends NO
x
emissions found to be decreased with increase in
bed temperature. Bed temperature increases with decrease in excess air. This reduced air flow
results in lower oxygen availability to convert NH
3
to NO by reaction (1) and thus the
emissions are reduced. Although the data is scattered, the effect of temperature on NO
x
emissions is more pronounced at higher pulp proportions in the blend possibly due to increase
in moisture content of the blend with increase in pulp proportion. During 50/50 blend
combustion the emissions increased from 1256 to 1488 mg/Nm
3
(corrected to 6% O
2
) with
decrease in bed temperature from 865 to 790
°
C. Similarly, during 60/40 blend combustion
the emissions increased from 1173 to 1454 mg/Nm
3
(corrected to 6% O
2
) with decrease in
bed temperature from 910 to 770
°
C and during 70/30 blend combustion the emissions
increased from 1169 to 1530 mg/Nm
3
(corrected to 6% O
2
) with decrease in bed temperature
from 925 to 798
°
C. At lower bed temperature the emissions for all the three blends are not
very much different but at higher bed temperatures the emissions show more variation with
changes in bed temperature with 50/50 blend the most affected. Thus, the effect of bed
temperature is more pronounced at higher bed temperatures compared to that at lower bed
temperatures. This may be due to the production of more O and OH radicals, at higher
temperatures and higher moisture contents, which can convert NO to N
2
by reactions (3) and
(4).
Bed solids especially char play a key role in the destruction of NO
x
and N
2
O and
formation of molecular nitrogen. With lower volatile content fuels, char concentration in the
bed is increased creating favourable conditions for destruction of NO
x
and formation of
molecular nitrogen [Oka and Anthony, 2003]. There is conflicting information in the
literature on the effect of volatiles on NO
x
emissions. Oka and Anthony, (2003) described that
