Agriculture Reference
In-Depth Information
nitrogen is mostly present in aromatic structures. Moreover, biomass has higher hydrogen and
lower nitrogen content than coal. That is why N
2
O is hardly detected during fluidized bed
combustion of biomass [Knobig et al. 1997] as it is converted to N
2
and OH by reacting with
H by reaction (2).
Effect of fuel characteristics and operating conditions on NO
x
emissions is discussed
hereunder.
The ratio of O to N is a describing parameter for HCN to NH
3
ratio. High O/N ratio
corresponds to low HCN/NH
3
ratio [Hamalainen and Aho, 1994]. Fuels with higher O/N
ratios form more OH radicals and other oxygen containing radicals which are responsible for
the conversion of HCN to NH
3
[Aho et al. 1993]. Hydrogen is also possibly required to
convert HCN to NH
3
[Winter et al. 1999].
Ratios of H/N, O/N and O/H for all the fuels tested are given in Table 13. The table
shows that O/N ratio for 50/50 coal-pulp blend is higher than 60/40 and 70/30 blends which
can results in the formation of more NH
3
by mechanisms described above. The NH
3
can
reduce NO to N
2
by reaction (5) thus emissions of NO for 50/50 blend should be lower as
compared to the other two. However, the emissions for all the three blends are in the same
range, see Table 11. The table also shows that O/H ratio for the 50/50 blend is the highest
than the other two.Comparatively lower amount of H in the 50/50 blend may result in lower
NH
3
production by reaction (3) and thus reduction of NO to N
2
by reaction (5) is reduced.
Also, higher fuel moisture content can produce more O and OH radicals which can result in
lower NO emissions by reaction (4). As 50/50 blend has the highest moisture content, it
should have the lowest NO emissions of the three blends. Moreover, nitrogen content of the
50/50 blend is lower than the other two due to lower coal proportion, thus it should give
lower NO emissions. The influence of all of these parameters alongside moisture variation
may have balanced and the resultant NO
x
emissions don‟t show significant variations.
Therefore, due to the influence of other parameters it is not possible to discuss the results in
relation to these elemental ratios.
Increase in moisture content results in reduction in NO
x
emissions due to reduced fuel
nitrogen as well as lower bed temperatures. With increase in moisture content of fuel more O
and OH radicals are produced. With increase in radicals more NCO is produced which
converts NO to N
2
O by reaction (4) and thus NO
x
emissions are reduced. However, NO
x
emissions for different coal-pulp blends are observed to be in the same range (1200 - 1600
mg/Nm
3
) indicating that the effect of moisture on NO
x
emissions is not noticeable or is
balanced by the influence of other operating parameters. Moreover, when two fuels are
blended in different proportions, it is not only the moisture content but other characteristics
also change and thus it is not possible to distinguish the effect of moisture from other
influencing parameters.
Concentration of NO
x
decreases for different moisture contents and excess air values by
reactions of NO with fuel carbon and CO on the surface of chars. However, in a study of NO
x
emissions from co-firing saw dust (moisture content 51 - 63%) and Polish and Russian coals
(moisture content 9 - 13% and 9 - 11%, respectively), Savolainen (2003) found that high
moisture content in fuel tends to delay ignition and overall NO
x
emissions could be higher
than with pure coal. The net effect of delayed ignition to increase NO
x
and lower nitrogen
content, lower temperatures and reactions on the char surface to decrease NO
x
is probably
balanced and no considerable effect of moisture on NO
x
emissions is observed.
