Agriculture Reference
In-Depth Information
70
60
50
40
30
20
10
0
Coal
70/30
60/40
50/50
Blneding ratio (Coal/Pulp)
Figure 6. Sulphur conversion for coal and coal-PP blends.
The lower conversion with increased proportion of pressed pulp may be possibly due to
the increase in moisture which absorbs SO
2
. With increasing pulp proportion, moisture
content of the blend increases and thus more OH radical are available to convert produced
SO
2
to H
2
SO
4
by the reactions mentioned in Section 4.2. Also retention of sulphur by biomass
ash could be another possibility. The figure 5 also shows that the emissions increase with
increasing bed temperature. The effect of bed temperature on SO
2
emissions seems to be
more pronounced at higher pulp proportions in the blends.
The maximum SO
2
capture by CaO is achieved at Ca/S molar ratio between 2.5 and 4
[Nowak, 2003]. As the Ca/S ratio for all the fuels and fuel blends tests is very much lower
than this optimum requirement, see Tables 12 and 16, that is why the conversion of sulphur to
SO
2
is relatively higher (14 - 60% for coal-pulp blends).
Sulphur forms gaseous compounds SO
2
, and SO
3
, and alkali sulphates during the
combustion process. About 40% - 90% of total sulphur in the biomass fuel is bound in the
ash and the remainder is carried in the flue gas as SO
2
and to a minor extent as SO
3
. Thus SO
2
emissions are lower when blending biomass with coal [Savolainen 2003]. Higher proportion
of biomass during co-firing can result in lower SO
2
emissions due to sulphur retention by
biomass ash.
Emissions of CO normalised to 6% O
2
for coal and pressed pulp blends against O
2
concentration in the flue gas (excess air) are plotted in Figure 7. The figure shows that CO
emissions are the highest for 70/30 blend and decrease with increase in pressed pulp
proportion in the blend possibly due to delayed evolution of volatiles as a result of increased
moisture content with increasing pulp proportion in the blend. The figure also shows that the
emissions decrease with increase in excess air.
