Agriculture Reference
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excess air which may be due to combined effect of the presence of Ca contributed by pulp as
well as the presence of oxygen.
However, Duan et al. (2012) observed decrease in NO x emissions with increase in excess
air. According to them it may be due to lower availability of CO at higher excess air and thus
NO x reduction by CO by reaction (6) is comparatively lower.
2NO + 2CO 2CO 2 + N 2
(6)
The emissions of NO x depend upon nitrogen content of the fuel. As biomass fuels have
lower nitrogen content than coal, NO x emissions during co-firing are found to be lower for all
the pulp blends with coal as compared to coal alone. The emissions from the combustion of
coal-pulp blends are also lower (1200 - 1600 mg/Nm 3 , corrected to 6% O 2 ) than coal due to
lower nitrogen content of pulp (0.14%) but the difference is small due to very low dry mass
contribution of pulp (11.6% in 70/30 blend and 23.5% in 50/50 blend).
Similar observations were made by Kuprianov et al. (2006) who found that higher
nitrogen content of rice husk in its blend with bagasse increases the overall nitrogen content
of the blend when its proportion is increased. They found that emissions of NO x from
conventional fluidized beds burning rice husk are high (100 - 180 ppm at 20 - 100% excess
air) due to higher nitrogen content in the fuel. Chakritthakul and Kuprianov (2011) observed
optimum performance of swirling fluidized bed combustor firing eucalyptus bark and
rubberwood sawdust in a ratio of 3:1 (sawdust:bark) at an excess air level of 50 - 55%.
4.2. SO 2 Emissions
The results of emissions for different coal-pulp blends are interesting and vary
considerably with change in blending ratio. The emissions decrease with increase in pulp
proportion in the blend despite very little contribution of dry matter from pulp. This may be
due to increase in the moisture content of the blend with increase in pulp ratio. The effect of
different parameters on SO 2 emissions is described hereunder.
During coal-pulp blends combustion the emissions of SO 2 are decreased with increase in
pulp proportion in the blend. The emissions varied between 500 - 1000, 1000 - 1500 and
1600 - 2400 mg/Nm 3 (corrected to 6% O 2 ) for 50/50, 60/40 and 70/30 coal-pulp blends,
respectively. The efficiency of sulphur fixation in the ash depends upon the concentration of
alkaline earth metals, especially calcium, in the ash. Calcium contained in the coal ash can
capture a considerable amount of SO 2 [Raymant, 1989]. Ash from certain coal types can
capture above 90% of sulphur [Zhang, et al. 1991]. Generally, de-sulphurisation efficiency
increases with increases in the Ca/S ratio [Oka and Anthony, 2003]. Analysis shows that Ca/S
ratio of pulp is 143 times and of 50/50 coal-pulp blend is 10 times that of coal, see Table 15.
Ca/S ratio increases with increase in pulp proportion in the blends due to its higher Ca and
lower S content. This could be one of the possible reasons for lower SO 2 emissions with
increase in pulp proportion in the blends. De-sulphurisation efficiency increases with increase
in contact time between SO 2 and CaO. Thus higher bed height may result in decreased SO 2
emissions [Oka and Anthony, 2003]. The decrease in SO 2 emissions with increase in pulp
proportion in coal-pulp blends may also be due to increase in moisture content. Presence of
moisture can convert SO 2 to H 2 SO 4 .
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