Recent studies [ I ] suggest that a 50 percent reduction in emissions of SO2 and NO, may be necessary to avoid environmental damage by acid deposition. The US. Environ-18, 607-18 (1979). Robert H. Borgwardt is a Chemical Engineer with EPA's Office ofResearch and Development where he has responsibilities for in-house evaluation of processes for controlling sulfur emissions from coal combustion. In addition to hasic research, he was extensively involved in pilot scale development of the limestone wet scrubbing process. He received his B. S. degree from M.I.T. Nancy Roache 15 A

The directdisplacement reaction of limestone wlth H2S was studied over the temperature range 570-850 O C in a differential reactor. Measurements with particles ranging in size from 1.6 to 100 pm showed an activation energy of 42 kcal/mol and a rate constant of 0.66 (L cm)/(g-mol of H,S min) at 750 O C . The reaction is inhiblted by H, and CaCI,. Sintering of CaCO,, dissociation of H2S, and product layer diffusion limit the rate of Cas formation at temperatures above 750 OC. Limestone also reacts with elemental sulfur, but the actlvation energy and rate are significantly lower than those of the H,S reaction.

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Calcination of 1-µ CaC03 particles at maximum rate and moderate temperature (600-950 °C) yields CaO with an initial BET (Brunauer-Emmett-Teller) surface area of 79 ± 6 m2/g, corresponding to a grain radius of about 110 Á. The grain radius can be increased to any value up to at least 1800 A by controlled sintering preceding exposure to the reactive gas. At a given temperature, the grain size can be adjusted by (1) stagnant sintering, (2) sintering with inert sweep gas, or (3) sintering with a sweep gas containing controlled concentrations of C02. Techniques for measuring the reactivity as a function of grain size are demonstrated for the CaO-SC2 reaction. The introduction of grain size as an independent experimental variable should contribute to improved understanding of reactions in porous solids.