Study of modified calcium hydroxides for enhancing SO2 removal during sorbent injection in pulverized coal boilers

Several modified calcium hydroxides were prepared by lime hydration using NaCl, KCl, calcium lignosulfonate, ethanol-water solutions and combinations of them, to obtain sorbents with enhanced capacity to remove SO2. The sorbents were characterized in a drop tube reactor in similar conditions to those existing during sorbent injection in pulverized coal boilers. The greatest increases in SO2 retention were obtained with calcium lignosulfonate as additive or using ethanol-water solutions in the CaO hydration process. The alkali promoters (NaCl and KCl) acted mainly on the product layer diffusivity. Calcium lignosulfonate modified the pore size distribution of the calcined hydrate and decreased the mean particle size of the modified calcium hydroxide by up to five times. The use of ethanol-water solutions during the hydration process decreased the mean particle size of the modified hydrate and improved the porous structure of the calcined hydrate, mainly by increasing the porosity. To combine the favourable effects of these additives, several sorbents were prepared by combinations of them. The best sulfur retentions were obtained with sorbents modified by a combination of calcium lignosulfonate and hydration with ethanol-water solutions. The effect of the combination of additives on the sorbent properties was not the same as that of their action separately, and their effect on sulfur retention was not cumulative. (C) 1997 Elsevier Science Ltd.