a study on the calcination and sulfation behavior of limestone during ...

Juan chen et al.
higher than the ash content, thus the gas-solid reaction with SO2 became the dominant
reaction, which compensated the affect of ash. As can be seen in fig. 5, the fractions of
Ca-Si/Al compounds were decreased with SO2 addition in both air and O2/CO2
conditions. The lower sulfation extent in O2/CO2 than in air was further confirmed.
Shaddix and Molina (2007) reported that O2 has a lower diffusion rate (about 20% less)
in CO2 layer than in N2. This phenomenon is explained by the lower SO2 diffusivity in
O2/CO2 than in air affecting the transport of SO2 to the surface of the limestone particle.
Fig. 6 depicts the changes to the fractions of
unreacted limestone and Ca-S with the
addition of 1000 ppmV SO2 to air and
O2/CO2 conditions. For the addition 18%
limestone to coal in O2/CO2 case, the
reduction on its unreacted fraction is nearly
equal to the increase in the percentage of
Ca-S, thus proving the sole significance of
the direction sulfation reaction (3) for a
high Ca/S molar ratio. For another two low
limestone addition percentages, in air
condition the unreacted fraction upon SO2
addition is not changed, but the Ca-S
fraction is still increased. For O2/CO2, the
reduction in the CaCO3 fraction is far lower
than the increase in the fraction of Ca-S,
particularly for the case of 1% limestone added to coal, indicative of Ca-S not only from
direct sulfation reaction (3), but also binding with sulfur from other approaches. Apart
from the increase in the S/Ca molar ratio, the interaction of coal ash with limestone via
reaction (16) and (17) may also be the cause for this phenomenon. The formed Ca-Al-Si
could be shed away from limestone surface, ensured the exposure of fresh surface of
limestone to undergo decomposition and react with SO2. Besides that, our previous
results (Zhang et al. 2002) confirmed that the added limestone reacted with
aluminosilicate in the ash and fixed into calcium aluminosilicate, rich in calcium, can
continue to react with gaseous SO2 to remove it.
5. CONCLUSIONS
18% 5% 1%
mass percentage of CaCO3 in coal
Mechanisms for calcination and sulfation of limestone added at the mass ratios of 18%,
5% and 1% to a brown coal during air and O2/CO2 mixture (27/73) in a lab-scale DTF
have been investigated. The major conclusions are drawn as follows: (1) The high CO2
partial pressure in an oxy-fuel furnace played a negative role on calcinations of
limestone. However, increase in the local temperature of limestone particles through the
radiative heat from coal flame favoured the decomposition of limestone to a level
similar with that achieved in air in a nominal gas residence time of approximately 4 s. (2)
The ash-forming metals in coal played an important role on the calcination and sulfation
of limestone during oxy-fuel combustion. The direct interaction between ash-forming
metals particularly aluminium and silicon and limestone was more favored in the CO2dominant
gas atmosphere than in air, which in turn promoted the decomposition of
limestone. The Ca-Al/Si formed could be partially shed away from limestone surface to
ensure the exposure of fresh surface to continue decomposition and sulfation. However,
Reduction/Increase in fraction, %
24
18
12
6
0
-6
-12
-18
-24
reduction in unreacted CaCO3(Air), %
increase in Ca-S(Air), %
reduction in unreacted CaCO3(O2/CO2), %
increase in Ca-S(O2/CO2), %
Fig.6 Changes to the fractions of unreacted
CaCO3 and CaSO4 upon SO2
addition to air and O2/CO2
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