Agriculture Reference
In-Depth Information
the moisture content of the 60/40 blend is significantly changed as compared to 70/30 blend
relative to change in these properties for 70/30 blend as compared to coal. Therefore, the
significant changes in excess air requirement for 60/40 blend may be due to combined effect
of increased moisture and reduced thermal input contribution by coal. This also suggests that
there is a specific moisture content at which its effect on the bed cooling is significant.
For 50/50 blend increase in moisture is 21% and reduction in energy value is 14% as
compared to 60/40 blend which is not very much different from the relative difference
between 60/40 and 70/30 blends.In the case of 50/50 blend excess air is reduced by 5 - 9% as
compared to 60/40 blend and 19 - 20% as compared to coal, with higher values at lower bed
temperatures. The reduction is 9% at 800 °C and 5% at 900 °C as compared to 60/40 blend.
This may be due to the reason that at higher temperature, moisture is evaporated relatively
quickly and fuel combustion rate increases which results in higher bed temperature and thus
requirement of excess air for a fixed bed temperature is increased. At lower temperature
moisture stays longer in the bed which slows fuel burning rate which results in lower bed
temperature. In order to increase the bed temperature air flow needs to be reduced so that
higher reduction in excess air is observed at lower bed temperatures. Therefore, the reduction
in air flow increases relatively sharply when PP proportion is increased to higher levels so
that overall about 20% less air is required when the ratio of the pressed pulp is increased to
50%. The air spared by moisture can be used to burn more coal and thus can result in
improvement in throughput. The discussion above concludes that the optimum blending ratio
of Thoresby coal and pressed sugar beet pulp is 50/50 (wt/wt) as it is the maximum ratio
which was successfully fired and that it gives maximum reduction in excess air requirement
for a fixed bed temperature.
1000
950
900
850
800
Coal
70/30 Coal/Pulp
60/40 Coal/Pulp
50/50 Coal/Pulp
750
700
650
600
7
8
9
10
11
12
13
14
15
O
2
(%) in flue gas
Figure 2. Bed Temperatures with Different Blends of Coal and Pressed Pulp.
