Characteristics and reactivities of Ca(OH) 2/silica fume sorbents for low-temperature flue gas desulfurization

Abstract

Ca(OH) 2/silica fume sorbents were prepared with various Ca(OH) 2/silica fume weight ratios and slurrying times at 65°C and a water/solid ratio of 10/1. Dry sorbents prepared were characterized, and their reactivities toward SO 2 were measured in a differential fixed-bed reactor at the conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization (FGD) processes. The reaction between Ca(OH) 2 and silica fume in the slurry was very fast. The formation of calcium silicate hydrates, which were mainly C–S–H(I), resulted in sorbent particles with a highly porous structure that seemed compressible under high pressures. The sorbents were mesoporous, and their specific surface areas and pore volumes were much larger than those of Ca(OH) 2 alone. The utilization of Ca of sorbent increased with increasing silica fume content mainly due to the increase in the specific surface area of sorbent. The sorbent with 70 wt% Ca(OH) 2 had the maximum 1 h SO 2 capture. Sorbents with Ca(OH) 2 contents less than 100 wt% and greater than 21 wt% would have a SO 2 capture greater than that of Ca(OH) 2 alone. Both the 1 h utilization of Ca and SO 2 capture per unit specific surface area of sorbent decreased in general with increasing specific surface area. At the same Ca(OH) 2 content, the 1 h utilization of Ca or SO 2 capture of the Ca(OH) 2/silica fume sorbent was greater than that of the Ca(OH) 2/fly ash sorbent; however, the amount of SO 2 captured per unit surface area of the former sorbent was smaller than that of the latter sorbent. The results of this study are useful to the preparation of silica-enhanced sorbents for use in the dry and semidry FGD processes.