ELSEVIER PII: SO016-2361(96)00204-9 Fuel Vol. 76, No. 3, pp. 257-265, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/97 $17.00+0.00 Study of modified calcium hydroxides for enhancing SO2 removal dur=ng sorbent injection in pulverized coal boilers Juan Ad6nez, Vanessa Fierro, Francisco Garcia-Labiano and Jos6 Maria Palacios* Instituto de Carboquirnica (C.S.I.C.), P.O. Box 589, 50080 Zaragoza, Spain *lnstituto de Cat~lisis y Petroleoquirnica (C.S.I.C.), Cantoblanco, 28049 Madrid, Spain (Received 28 May 1996; revised 8 October 1996) Several modified calcium hydroxides were prepared by lime hydration using NaC1, KC1, 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 (NaCI and KC1) 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. © 1997 Elsevier Science Ltd. All rights reserved. (Keywords: sulfur dioxide removal; sorbent injection; calcium hydroxides) The injection of calcium-based sorbents in pulverized coal boilers is a subject of great interest for the control of SO2 emissions, especially when it is combined with NOx reduction by reburning with natural gas or coal, or by means of LIMB technology. The main disadvantage of this desulfurization process is the low sorbent utilization. Since the molar volume of calcium sulfate is significantly higher than that of calcium oxide, there is a loss of porosity and pore blockage as the sulfation reaction proceeds. The high temperatures and low residence times (< 1 s) of the sorbents in the reactive zone of the boiler are the factors determining this low sorbent utilization. Retention is low when using limestones as sorbents (~ 18%), and only a little higher with calcium hydro- xides (~ 24%). However, sulfur retention (SR) can be greatly improved by using modified calcium hydroxides. Previous studies have shown that a smaller particle size of the sorbent 1-3 and a higher specific surface area 4 improve SR. Porosity and pore volume distribution also play an important role in reaching a greater SR 5'6. The largest pores are regarded as more favourable in avoiding both pore blockage and pore plugging. Sorbent reactivity also increases when sintering is delayed 7 or when the sulfation rate is promoted by increasing ionic diffusion through the CaSO4 product layer 8. As a process controlled by diffusion into the pores and in the product layer 9-11, sorbent utilization can be improved decreasing the aerodynamic particle size and agglomeration of the particles after they are formed 12. The presence of certain inert compounds in sorbents can improve their utilization, thus increasing SR in the boiler. Impurities present in the CaO and the additives added to the hydration water could be responsible for the differences in reactivity of modified calcium hydroxides. The temperature and pressure during CaO hydration can also affect the characteristics of the modified calcium hydroxide 13. Hydration with alcoholic solutions yields sorbents with higher specific surface area and smaller particle size 4. The relation between the specific surface area of calcinates and sorbent reactivity is well known 3'8'14-16. Borgwardt and Bruce 9 found that the sorbent reactivity varies with the square of the specific surface area over the 2 1 range 2-63m g-. However, not only the specific Fuel 1997 Volume 76 Number 3 257