Effect of CaSO4 on the structure and use of Ca(OH)2/fly ash sorbents for SO2 removal

Sorbents for SO2 removal from flue gas were prepared by hydration of fly ash, calcium hydroxide and calcium sulfate to simulate the reuse of desulfurant sorbents after reactivation, at different relative amounts and slurrying times. They were characterized by determining the specific surface area and pore volume distribution, and tested in the dry desulfurization reaction at low temperature. The purpose was to investigate the influence of CaSO4 in the system fly ash/calcium hydroxide, following the physical properties of the sorbents as well as the sorbent utilization in the desulfurization reaction, The sorbents prepared with CaSO4 showed structural properties and chemical composition that were different from those of sorbents without CaSO4 (with in general less specific surface area and mesopore volume). Sorbent utilization was expressed in two ways: as utilization of calcium hydroxide (mol of captured SO2/g of Ca(OH)(2)) and as total calcium utilization (total SO2 mol in the sorbent/total calcium mol in the sorbent). In the desulfurization test, all the sorbents prepared with and without CaSO4, at different fly ash/calcium hydroxide/calcium sulfate ratios and slurrying times, showed a great increase in the calcium utilization compared with commercial calcium hydroxide. The utilization of calcium hydroxide showed lower values for sorbents with CaSO4 than for sorbents without CaSO4. For all the sorbents studied, with and without CaSO4, a near to constant ratio of 70% of total calcium utilization was found. These results show that despite the different chemical composition and structural properties, only the total calcium amount determines the sorbent capacity for desulfurization. (C) 2001 Elsevier Science B.V. All rights reserved.