Agriculture Reference
In-Depth Information
Softening temperature of the ash is thought to be the agglomeration occurrence
temperature in fluidized beds.Softening temperature of coal is above 1060 °C. However,
during the test agglomeration occurred at a temperature of 874 °C. This shows the influence
of alkalis present in the fuel and silica in the bed to form a composition which tends to reduce
agglomeration temperature considerably.
The average content (by mass) of sodium and potassium in the outer layer of the particles
removed from the bed after the testas measured by SEM analysis were 2.1% and 7.8%
respectively. The high levels of potassium are associated with the formation of potassium
silicate eutectics which can melt at temperatures as low as 760°C. The resultant molten,
sticky silicate deposits on sand and ash particles appears to be the likely cause of
agglomeration of the bed. The substantial “pick up” of potassium is not unexpected since
raffinate typically contains very high levels (13.0%) of this metal, see Table 19. During
combustion potassium transforms from organic to inorganic forms of low melting point K
2
O-
SiO
2
[Lin et al. 1999].
Emissions data given in Table 20 is taken as an average over a period of at least 10
minutes. For coal and raffinate co-firing, the emissions data is averaged over a period for
which coal flow was the same as for “coal only” analysis in the above Table. It can be
observed from the table that concentration of O
2
decreased while that of CO
2
increased after
the introduction of raffinate indicating the combustion of raffinate. Considerable increase in
CO emissions was observed due to incomplete combustion. The presence of water in raffinate
(almost 50% of raffinate flow) could have contributed to the conversion of CO to CO
2
by
water gas shift reaction (8), given below [Twigg, 1989], as can be evidenced by the
considerable increase in the concentration of H
2
. The endothermic nature of the water gas
shift reaction, alongside water, may have contributed towards bed temperature decrease in the
early stages of raffinate introduction as can be observed in Figure 9.
Table 19. Alkali Metal Contents of Raffinate, Vinasse and Sugar Beet
Pulp on dry basis[Akram, 2012]
Sodium (Na) %
Potassium (K) %
Calcium (Ca) %
Raffinate
3.6
13.0
0.76
Vinasse
2.5
7.0
0.05
Pressed Pulp
0.1
0.75
1.2
Table 20. Flue gas analysis of Coal and Raffinate Co-Firing test
Coal only
Co
a
l + Raffinate
Measured
Corrected to 6% O
2
Measured
Corrected to 6% O
2
O
2
(%)
14.3
6
11.4
6
CO
2
(%)
3.8
8.5
5.4
8.4
CO (ppm)
160
358
2845
4445
NO (ppm)
386
864
386.5
604
NO
2
(ppm)
1
2
0.6
1
NO
x
(ppm)
387
866
387.1
605
SO
2
(ppm)
379.1
849
244.6
382
H
2
(ppm)
8.6
19
611
954
