Performance of a Novel Synthetic Ca-Based Solid Sorbent Suitable for Desulfurizing Flue Gases in a Fluidized Bed

The extent and mechanism of sulfation and carbonation of limestone, dolomite, and chalk, were compared with a novel, synthetic sorbent (85 wt % CaO and 15 wt % Ca12Al14O33), by means of experiments undertaken in a small, electrically heated fluidized bed. The sorbent particles were used either (i) untreated, sieved to two particle sizes and reacted with two different concentrations of SO2, or (ii) after being cycled 20 times between carbonation, in 15 vol % CO2 in N-2, and calcination, in pure N-2, at 750 degrees C. The uptake of untreated limestone and dolomite was generally low (<0.2 g(SO2)/g(sorbent)), confirming previous results, However, the untreated chalk and the synthetic sorbent were found to be substantially more reactive with SO2, and their final uptake was significantly higher (>0.5 g(SO2)/g(sorbent)) and essentially independent of the particle size. Here, comparisons are made on the basis of the sorbents in the calcined state. Hg-intrusion porosimetry, performed on calcined sorbents, revealed that the pore volume of limestone and dolomite was distributed almost entirely across small pores, which were easily plugged and hindered the diffusion of SO2 through the particle; this was confirmed by EDS analysis. Conversely, the pore size distribution of calcined chalk and fresh synthetic sorbent revealed the presence of pore volume also in the macroporous region; these bigger pores were not prematurely blocked by the newly formed CaSO4 and therefore allowed sulfation to proceed uniformly throughout the particle. It was also found that the uptake Of SO2 by limestone, dolomite, and chalk was substantially lower when the particles had been subjected to cycles of calcination and carbonation in CO2 prior to sulfation; this was attributed to the loss in pore volume in the small pores with cycles of carbonation and calcination, as confirmed by Hg-intrusion porosimetry. The uptake of the synthetic sorbent, on the other hand, was much closer to that achieved when the sorbent was used untreated, because its pore volume in large pores was not lost with cycling. The capacities for the uptake Of SO2, on a basis of unit mass of calcined sorbent, were comparable for the chalk and the synthetic sorbent. However, previous work has demonstrated the ability of the synthetic sorbent to retain its capacity for CO2 over many cycles of carbonation and calcination: much more so than natural sorbents such as chalk and limestone. Accordingly, the advantage of the synthetic sorbent is that it could be used to remove CO2 from flue gases and, at the end of its life, to remove SO2 on a once-through basis.