The kinetics and pore structure of sorbents during the simultaneous calcination/sulfation of limestone in CFB

Abstract

The interaction of calcination and sulfation in the simultaneous calcination/sulfation of limestone sorbent under circulating fluidized bed boiler conditions was studied. A specially designed constant-temperature reactor which can stop the reaction at a given time was employed. When limestone entered the furnace of mixed gases of CO2, O2, SO2, etc., its weight went down first, then up, so there was a minimum weight point. The whole reaction period could be divided into two stages by this minimum weight point, named the weight-loss stage and the weight-growth stage, which were dominated by the calcination reaction and by the sulfation reaction, respectively. In the weight-loss stage, the sulfation reaction took place and CaSO4 formed simultaneously together with limestone calcination as long as SO2 was present. In the weight-growth stage, the sulfation ratio at 60 min in simultaneous calcination/sulfation is 30.7% higher than that in the sequential calcination then sulfation process. The weight loss rate of limestone calcined in the presence of SO2 was lower than that without SO2 present but the final weight was higher. The calcination of limestone was slowed by the presence of SO2; a probable mechanism was proposed, namely that the CaSO4 formed may fill or plug the pores in the CaO layer, and impede the transfer of CO2 and, therefore, retard the calcination reaction. This mechanism was supported by the observation that the effective diffusion coefficient of CO2 in CaO produced in the presence of SO2 was reduced. The impeding effect increased with increasing SO2 concentration (0-3000 ppm), while, when the particle size decreased from 0.4-0.45 mm to 0.2-0.25 mm, the calcination rate of limestone was higher, no matter whether there was SO2 present or not. The impeding effect was less pronounced at 880°C than at 850°C. The reason for this appears to be the fact that there was less CaSO4 formed at 880°C and, therefore, fewer pores of the particle were filled or plugged.

The combined effect of H2O and SO2 on the simultaneous calcination/sulfation reaction in CFBs

“…One more phenomenon to be noted in Figure b is that the samples under all four conditions were calcined completely at 300 s. This is different from the findings on Massicci limestone in the work of Wang et al, where they found that the samples which experienced 90 min of reaction still contained 3%–5% mass fraction of undecomposed CaCO 3 . The reason may be that the sulfation reactivity of the limestone used in the present work is lower than for Massicci limestone, as has been pointed out elsewhere …”

“…For a spherical particle, η can be calculated by: where R is the radius of the particle, m; k v is the reaction rate constant per unit volume; and D e is the diffusion coefficient of CO 2 in pores of CaO layer. The procedure to calculate k v and D e can be found elsewhere . The effectiveness factors at different times for calcination with 0.3% SO 2 are shown in Figure , compared with those without SO 2 .…”

“…After the furnace reached the set temperature and was purged by synthetic flue gas for 15 min, the sample (80 ± 2 mg) was loaded into a quartz boat, and its mass was continually measured by a mass monitor (accuracy 0.1 mg) and recorded by computer. Our previous work with this system showed that it has sufficient accuracy for this type of study.…”

“…The actual process proceeds as follows: after limestone reaches hot flue gases containing SO 2 , the calcination and sulfation reactions occur simultaneously, and the two reactions affect each other. We call this process the “simultaneous calcination/sulfation of limestone” and have carried out preliminary investigations on it . Some important phenomena were demonstrated in our earlier work, in particular, the decomposition rate of limestone particles was impeded by SO 2 , and some of the CaCO 3 did not decompose completely even after 90‐min reaction in the presence of 0.38% SO 2 .…”

“…To answer these questions while addressing the shortcomings of TGA, a specially designed experimental system was employed here. With this equipment it is feasible to measure the calcination and sulfation rates separately . The sample collected from one test in this system is about 50 mg, so it is easier to obtain enough samples for the pore structure measurement.…”