Pergamon Chemical Enyineeriru d Science. Vol. 50, No. 13. pp. 2029 2040, 1995 Copyright ~C 1995 Elsevier Science Ltd Printed in Great Britain. All tights reserved 0~09-2509/95 $9.50 + 0.00 0009-2509(95)00043-7 ULTRAFAST CALCINATION AND SINTERING OF Ca(OH)2 POWDER: EXPERIMENTAL AND MODELING A. GHOSH-DASTIDAR, S. MAHULI, R. AGNIHOTRI and L.-S. FAN Department of Chemical Engineering,The Ohio State University, 121 Koffolt Laboratories, 140 West 19th Avenue, Columbus, OH 43210, U.S.A. (Received 26 October 1994; accepted in revised form 5 January 1995) Abstract--The kinetics of ultrafast calcination and sintering of Ca(OH)z powder is studied in the 900-1050°C temperature range in an entrained-flow reactor. Time-resolved kinetic data are obtained for 0-300 ms time scale with the residencetime of the solids being measured on-line.The reaction exhibits very high initial calcination rates with sharp attenuation and subsequent virtual "die-off" at higher residence times. The interplay of calcination kinetics and sintering kinetics and the net effect on the overall surface area evolution is clearly elucidated. A modifiedgrain model incorporating the sintering of the product CaO layer and the role of diffusion of evolved H20 is proposed to explain the overall phenomena. The model predictions are in good agreement with the observed experimental results. INTRODUCTION Injection of dry calcium-based sorbents [Ca(OH)2 or CaCO3] into the above-the-flame region of a coal- fired furnace is a potential retrofit technology for controlling SO2 emission from power plants. Current attention is focused on using hydroxide sorbent be- cause of its superior efficiency for SO2 capture (Cole et al., 1986; Bruce et al., 1989). When Ca(OH)2 is injected into the furnace, it decomposes or calcines to high-surface-area, high-porosity CaO: Ca(OH)2 ~ CaO + H20. The highly reactive CaO then reacts with SO2 in the presence of 02 to form solid CaSO4: CaO + SO2 + ½02 ~ CaSO4. Contrary to calcination, which acts as an activation step, the sulfation reaction is a deactivation phenom- enon which results in the build-up of the CaSO4 product layer and, hence, a loss in available surface area. Another mechanism by which active surface area is lost is thermal sintering. The grains coalesce to larger grains due to sintering, reducing the surface area and porosity of reactive CaO (Borgwardt, 1989). Both calcination and sintering phenomena are ex- tremely important in determining the effectiveness of sorbent in removing SO2 from combustion gases, and occur very rapidly under upper-furnace temperatures of 850--1200"C. Bortz and Flament (1985) reported 70% calcination of Ca(OH)2 particles within the first 25 ms of the reaction at 973 K. Mai and Edgar (1989) reported a comparatively slower decomposition rate, 35% calcination in lOOms at 1425 K for reagent-grade hydroxide powder (mass median diameter = 12.5/am). They suggested inadequate sorbent/gas mixing and slow particle heat-up to be the possible reasons for the low calcination rate in their studies. Studies on thermal sintering at such short time scales are, however, quite scarce. Nonetheless, those which are available report a very fast sintering rate leading to rapid reduction in surface area within 100-200 ms (Roman et al., 1985, Cole et al., 1986) of the reaction. For small sizes (less than 10/am) of sorbent par- ticles, intraparticle heat and mass transfer do not offer any significant resistance to calcination (Beruto and Searcy, 1974; Powell and Searcy, 1980). In Powell and Searcy's (1980) work, chemical reaction was suggested to be the rate controlling mechanism. They also showed that the calcination rate was proportional to the available surface area of the unreacted particle, which was later confirmed by Borgwardt (1985). Borgwardt (1989) found the maximum attainable sur- face area of CaO from the calcination of limestone and hydrated limes. At a temperature of 973 K, when the effect of sintering is minimal, surface area of the Ca(OH)2-derived nascent CaO was measured to be in the range of 70-80 m2/g for 2-10/am particles. How- ever, at higher temperatures, such a high surface area of the nascent CaO is readily lost, and studies with pre-calcines suggest that the rate of surface area re- duction is proportional to the square of the instan- taneous surface area (Milne et al., 1990; Cole et al., 1986; Silcox et al., 1989). In addition to observing a strong effect of temperature, Mai and Edgar (1989) also noticed a substantial enhancement of sintering in the presence of H20 and CO2 in the combustion gas. Some previous modeling efforts assumed Ca(OH)2 calcination to be instantaneous (Bortz et al., 1986, Simons and Garman, 1986). Silcox et al. (1989) modeled the simultaneous calcination and sintering of CaCO3 sorbent, and suggested that sintering of the product CaO surrounding the undecomposed sorbent core causes diffusional resistance on the outgoing CO2 gas which may limit the calcination rate at a later stage. They indicated that similar resistance to 2029