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

Juan chen et al.
flame during coal combustion, which substantially increased the limestone particle
temperature to a level overweighing the inhibitory effect of CO2 partial pressure. As can
be seen in fig. 2, adding 18% CaCO3 to coal combusted in DTF, increased the limestone
particle temperature from 1183 K to 1223 K in O2/CO2 and 1194 K in air.
Unreacted CaCO3, %
20
15
10
5
0
Air
Oxy-fuel (a)
18% 5% 1%
Mass percentage of CaCO3 in coal
Unreacted CaCO3, %
20
15
10
5
0
Air+SO2
Oxy-fuel+SO2 (b)
18% 5% 1%
Mass percentage of CaCO3 in coal
Fig 3Variation of the fractions of unreacted limestone with the addition of limestone
The mass ratio of limestone to ash in coal and the concentration of SO2 in flue gas were
also confirmed playing noticeable role on the extent of limestone calcination. As
demonstrated in fig. 3, in either pure gas, air versus 27% O2/73% CO2 at a reactor
length of 1200 mm in panel (a), with the decrease in the addition percentage of
limestone, i.e. increase in the mass ratio of coal ash/sulfur to limestone, the fraction of
unreacted CaCO3 was decreased drastically. If the reactions (1) and (2) shown above
are the single routes for limestone calcination and sulfation, the presence of coal ash and
sulfur in the furnace should not affect the extent of the right shift of reaction (1),
because they do not participant in this reaction. The SO2 emitted from coal-bound
sulphur is also insignificant, as the sulphur in coal only accounts for 0.9 wt%. It is thus
referrable that ash (2.24% content) plays the main role on the calcination extent of
limestone. This clearly indicates the interaction between limestone and coal ash,
according to the following reaction (16) and (17), respectively.
CaO+ Al O ⋅ SiO →CaO⋅
Al O ⋅ SiO
(16)
2
3
2
CaCO 3 + Al2O3
⋅ SiO2
→ CaO ⋅ Al2O3
⋅ SiO2
+ CO2
(17)
The Ca-Al-Si formed could be shed away from limestone surface, which in turn ensured
the exposure of fresh surface of limestone to undergo decomposition and react with SO2.
The direction reaction (17) should occur slowly in O2/CO2 with high CO2 partial
pressure when compared with the interaction (16) between calcium oxide (derived from
limestone calcination) and coal ash which is predominant in air (Zhang et al, 2002), as
indicated by the constantly higher unreacted CaCO3 fraction over the mass ratio of
limestone to coal in O2/CO2 than in air. Moreover, as suggested by the results for adding
1000 ppmV SO2 in fig. 3(b), it is clear that the un-reacted limestone fraction was
reduced quickly with the addition of SO2 compared to that without SO2 addition in
panel (a). The limestone calcination extent in O2/CO2 was even increased to the similar
level with that obtained in air. This is a clear reflection of the importance of the direct
sulfation reaction (3) at high SO2 concentration in an oxy-fuel boiler.
2
3
2
6