Agriculture Reference
In-Depth Information
combustion of high volatile, high nitrogen content coal generally gives lower NO
x
emissions.
On the other hand Braun et al., (1990 and 1991) observed that during the combustion of high
volatile coal more NO
x
is formed. However, Moritomi, et al. (1991) found that NO
x
and N
2
O
emissions are not affected by the volatile content of the coal in a bubbling fluidized bed.
Emissions for coal-pulp blends do not show a noticeable effect of volatiles on NO
x
emissions. The emissions for all the coal-pulp blends are almost in the same range despite
change in pulp proportion in the blends. However, it should be noted here that the ratio of
volatiles to fixed carbon for the blends are not very much different due to very low dry mass
contribution of pulp in the blends, see Table 14. Thus small variation of volatiles does not
show significant changes in NO
x
emissions.
Also nitrogen in different fuels is present in different forms. The type of nitrogen
compound produced during combustion depends upon nitrogen species present in the fuel and
thus effects NO
x
emissions. During fluidized bed combustion more of the nitrogen present in
biomass fuels is released as NO
x
as compared to coal [Leckner and Karlsson, 1993]. Thus
NO
x
emissions should be higher when co-firing biomass with coal as compared to those
obtained when firing coal alone. However, due to lower nitrogen content of biomass than
coal, lower loading and higher reactivity of biomass char [Tillman, 1991] opposite trend is
observed.
It is observed that the emissions of NO
x
increase with increase in excess air. Increase in
excess air increases oxygen level which creates favourable conditions for the oxidation of
volatile nitrogen compounds. At lower excess air NO is reduced due to the presence of fuel
rich conditions. During combustion of 50/50 coal-pulp blend the emissions increased from
1256 to 1488 mg/Nm
3
(corrected to 6% O
2
) with increase in excess air from 9.3% to 11.5%
O
2
in the flue gas. Similarly, during 60/40 coal-pulp blend combustion the emissions
increased from 1173 to 1454 mg/Nm
3
(corrected to 6% O
2
) with increase in excess air from
8.2% to 12.1% O
2
in the flue gas and during 70/30 coal-pulp blend combustion the emissions
increased from 1169 to 1530 mg/Nm
3
(corrected to 6% O
2
) with increase in excess air from
7.4% to 12% O
2
in the flue gas.
The effect of excess air is more pronounced at higher temperatures [Mahmoudi et al.
2010]. Increase in NO emissions with increase in excess air due to fuel NO
x
formation
mechanism was also observed by Permchart and Kouprianov, (2004).
Oka and Anthony (2003) found that an increase in excess air simultaneously affects the
formation and destruction of nitrogen oxides. During combustion typically 20 - 40% of fuel
bound nitrogen is converted to NO
x
[Hoy and Gill, 1987]. Conversion of fuel nitrogen
increases with increase in equivalence ratio and mixing [Pohl and Sarofim, 1976]. Fuel NO
x
are not very much sensitive to temperature but are sensitive to mixing. During pulverized coal
combustion at 2480K, Pershing and Wendt (1977) however, found that 75% of the total NO
x
produced came from fuel NO
x
mechanism. As fluidized bed provides excellent mixing of fuel
and air, it can be assumed that all the NO
x
formed are from fuel nitrogen [Mahmoudi et al.
2010].
The effect of excess air on NO
x
emissions is also influenced by the presence of Ca in the
fuel. Catalytic effect of CaO leads to significant NO
x
emission differences from combustion
in fluidized beds using silica bed [Amand and Leckner, 1988]. Presence of CaO may result in
increased NO
x
formation if excess oxygen is present [Leckner and Amand, 1987]. It is
observed, during coal-pulp blends, that NO
x
emissions tend to increase with increase in
