Agriculture Reference
In-Depth Information
Table 10. Results of Coal and Pressed Pulp tests
50/50
60/40
70/30
100/0
Thermal input (kW)
11.8 - 14.2
13.3 - 19.7
12.7 - 14.6
8.7 - 18.2
O
2
in flue gas (%)
5 - 11.5
7 - 12.1
8.9 - 12.8
9.7 - 14.5
Efficiency at Bed Temperature (BT),(%)
58 - 83
59 - 83
67 - 93
72.5 - 92.3
Efficiency at Freeboard temperature
(FBT), (%)
71 - 97
69 - 94
76 - 100
80.5 - 99
Fluidizing velocity (%)
1.6 - 2.6
1.9 - 2.4
1.8 - 2.3
1.9 - 3.1
The variation of bed temperature with increasing oxygen level and hence excess air is
presented in Figure 2 for coal and all the three blends of coal and pressed pulp at virtually
constant net thermal input of approximately 14 k W. The data represented in the Figure is
smoothed over the range of conditions tested. Generally as the proportion of the pressed pulp
is increased the cooling effect of the additional moisture is also increased. Air supplied to
practical combustion systems is well above stoichiometric. Thus, the combustion or adiabatic
flame temperature is significantly reduced as some of the heat of the combustion process is
used for heating up of the excess air. Consequently, when firing with high moisture fuel, such
as by-products of sugar industry, a fixed bed temperature can be maintained at a lower excess
air (oxygen) level since “additional” air is not required to cool the bed. This reduction in the
necessary air flow to maintain a fixed bed temperature can be seen more clearly in Figure 3
which plots, for a constant thermal input, the measured air flow to the fluidized bed against
the proportion by mass of the pressed pulp in the blended fuel at particular fixed bed
temperatures.
A blend of 70/30 coal-pulp has around 27% lower CV and 350% higher moisture than
coal, so the excess air required to get a fixed bed temperature should have been drastically
reduced when firing 70/30 coal-pulp blend as compared to coal alone. However, the data
shows that there is little reduction in excess air (only 1 - 2%) over the conditions tested when
fuel is switched from coal to 70/30 coal-pulp blend. Thus, the effect of fuel moisture on the
reduction of excess air when firing 70/30 coal-pulp blend is not significant. This may be due
to the dominance of coal in the blend which is contributing around 88% of the total dry matter
and is contributing around 96% towards the total energy input from the blend. Thus blends of
small proportions of pulp with coal can be co-fired with little effect on the performance of the
combustor. This is in line with the findings of Howe and Divilio (1993) who assessed pilot
and commercial experience on fluidized bed combustion and found that co-firing with 10 -
20% (weight basis) of secondary fuel is possible with minimal impact on performance and
design.
However, when 60/40 coal-pulp blend is combusted reduction in excess air is significant
although the change in moisture of fuel in this case is only 25% and change in energy value is
12%, as compared to 70/30 blend. The excess air requirement to get a bed temperature of 800
°C is reduced by around 10% as compared to 70/30 blend and around 11% as compared to
coal. Thus the effect of moisture on the reduction of excess air is more for 60/40 blend (32%
moisture) as compared to 70/30 blend (25% moisture). Similarly, excess air requirement for
getting a bed temperature of 900 °C is reduced by more than 11.5% as compared to 70/30
blend and by 13.3% as compared to coal. In the case of 60/40 blend dry matter contribution of
coal is 83% and its energy contribution is 93%. Neither the energy contribution of coal nor
