Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has been used to investigate the reaction of SO2, in He with CaO and CaCO3 particles at temperatures between 25 and 900°C. The reaction of SO2 with CaO occurs at 25°C, while the reaction of CaCO3 with SO2 is first evident at 550°C. The initial product of both of these reactions is CaSO3. The CaSO3 then further reacts with SO2 to form CaSO4, CaS2O3, and CaS at higher temperatures. At 900°C and above, the net production of SO4= and S2O3= is decreased because of the decomposition of SO3=. FT-IR/PAS depth-profiling experiments indicate the formation of a reaction-limiting product layer of SO3= and SO4= from the above reactions on the surface of the CaO and CaCO3 particles.

FTIR PAS has been used to characterize the reaction products on and near the surface of particulate CaCO3 and CaO during and after exposure to 1.5% SO2 in He. The reaction has been studied as a function of temperature (up to 900°C), time of exposure, concentration of SO2, presence of O2, precalcining, and addition of promoters. The PAS results confirm the initial reaction to be the formation of SO 3 = . The production of SO 4 = is found to occur via disproportionation and by further reaction of SO 3 = with SO2 (pseudodisproportionation). No evidence is found for gas phase oxidation, and the role of direct solid state oxidation in the formation of SO 4 = is of minimal importance. The mechanism of action of various additives in promoting or inhibiting sintering, oxidation, and/or disproportionation has also been investigated by following the course of the reaction by PAS as a function of temperature and time. Use of mirror velocity (modulation frequency) variation indicates that the limiting factor for the reaction is the formation of an impervious product layer on the surface of the particles. Implications of these results for the application of limestone injection in the control of SO x emission from fossil fuel plants are discussed.

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