Agriculture Reference
In-Depth Information
Table 14. Volatiles and Fixed carbon content of Fuels and blends tested
(% of combustible matter)
Volatiles
(%)
Fixed carbon
(%)
V/FC
Normalised V/FC
with coal
Coal
38.2
61.8
0.62
1
50/50 Coal-Pulp Blend
52.5
47.5
1.1
1.77
60/40 Coal-Pulp Blend
48.5
51.5
0.94
1.52
70/30 Coal-Pulp Blend
45.3
54.7
0.83
1.34
However, higher amount of CaO does not necessarily mean a higher sulphur capture
efficiency as Fe
2
O
3
can have a catalytic effect on the reaction of CaO and SO
2
[Yeh at al.
1987]. Zhang et al. (1991) concluded by a comparative study on six different facilities and
nine different coals that it is not possible to determine a general relationship between de-
sulphurisation efficiency and Ca/S ratio. Moreover, microstructure of calcium also affects its
sulphur capture efficiency. Larger specific surface area CaO has more capability to capture
SO
2
as compared to that having smaller specific surface area [Duan et al. 2010]. It is possible
that CaO present in different fuels may have different pore structures and thus different
sulphur capture efficiency and may be a possible reason for variations and data scatter in
sulphur emissions during tests with different fuels.
Effect of excess air on SO
2
emissions is plotted in Figure 8 for 50/50 coal-pulp blends.
The figures show that the emissions decrease with increase in excess air. This shows that
sulphur retention is increased at higher excess air levels. During 50/50 coal-pulp blend
combustion SO
2
emissions decreased from around 900 to 600 mg/Nm
3
, a reduction of around
33%, when excess air is increased from 8.5% to 10.8% O
2
in the flue gas.
Increase in excess air levels increases fluidizing velocity. So an intuitive conclusion is
that sulphur emissions decrease with increase in fluidizing velocity. However, not only that
fluidizing velocity is influenced by operating temperature, it also influences other variables
such as residence time and mixing. Higher fluidization velocity reduces residence time of gas
in the bed and freeboard but increases intensity of mixing and attrition. Therefore, it is
difficult to determine the effect of fluidization velocity on sulphur capture. However, Oka and
Anthony, (2003) stated that normally, higher SO
2
emissions should be expected at increased
fluidization velocity. Similar results were witnessed by Valk et al. (1989) who obtained lower
de-sulphurisation efficiency with higher fluidization velocity.However, Zhang et al. (1989)
observed that fluidization velocity does not significantly affect sulphur retention in fluidized
beds. As explained earlier, SO
2
emissions observed to be decreased with increase in fluidizing
velocity.
Emissions of SO
2
are also affected by changes in bed temperature. With coal-pulp blends
effect of bed temperature on SO
2
emissions is pronounced and the emissions observed to be
higher at higher bed temperatures. The emissions increased from around 600 to 1200
mg/Nm
3
, 1000 to 1500 mg/Nm
3
and 1800 to 2400 mg/Nm
3
with increase in bed temperature
from 780 to 885
°
C, 810 to 860
°
C and 825 to 925 °C during firing of 50/50, 60/40 and 70/30
coal-pulp blends, respectively. Increased emissions of SO
2
with temperature may be due to
decomposition of calcium sulphate (CaSO
4
), present in the fuel or formed by forward reaction
(7) in the early stages at lower temperatures, into CaO and SO
2
by the following reverse
reaction [Duan et al. 2010].
