In situ sulfur removal from coal-fired boilers via calcium-based sorbents has been studied extensively for the past decade. Either a calcium carbonate or a calcium hyroxide is injected directly onto the postflame combustion zone, decomposes into CaO, and reacts with SO/sub 2/ via the global reaction. While the global reaction suggests that the oxygen reaction order is 0.5, it is well known that chemical reactions do not occur via three-body collisions but rather occur in a series of two-body reactions. Hence, the rate limiting step could be the adsorption of SO/sub 2/, while the O/sub 2/ adsorption is relatively fast. The effective O/sub 2/ reaction order would then be very small. To test this hypothesis, a series of tests was conducted by Physical Sciences, Inc. (PSI) in its entrained flow reactor at a temperature of 1173/sup 0/K and a pressure of 100 kPa. A well-characterized calcium hydroxide (Longview: 2.3 ..mu..m diameter, 75% porous, 15 m/sup 2//g internal surface area) was exposed to a gas mixture containing 3000 ppm SO/sub 2/ and 1-30% oxygen. The PSI entrained flow reactor is designed to simulate heterogeneous sorption and deposition processes, under well-characterized laminar flow conditions, in the postflame gases of combustion devices. Itmore » is presently being used to generate fundamental rate data on SO/sub 2/ sorption of calcium-based sorbents and for studies of coal mineral matter transformations and aerosol transport/deposition processes. A schematic of the reactor design is shown.« less