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The sulfation model of Simons and Rawlins (1980) is extended to include the effect of product deposits. The model includes: 1. the plugging of the smallest pores and the subsequent loss in the internal surface area, 2. the diffusion of the SO, through the product deposits, and 3. the loss of intraparticle diffusion due to the complete plugging of the largest pores. It is shown that the plugging of the smallest pores is generally rate-controlling. G. A. Simons, A. R. GarmanPhysical Sciences Inc.Andover, MA 01810 SCOPECurrent sulfation models (Hartman and Coughlin, 1976, 1978; Bhatia and Perlmutter, 1981a,b;Christman and Edgar, 1983;Bardakci, 1984;Marsh and Ulrichson, 1982; Ramachandran and Smith, 1977) describe the intraparticle diffusion of SO, through porous CaO, the intrinsic kinetics of the CaO + SO, reaction, the buildup of the product layer (CaSO,), the subsequent particle deactivation due to the relatively slow diffusion of SO, through the CaSO, to the unreacted CaO, and the ultimate termination of the reaction due to the plugging of the entire local porosity with the product deposits. These models are analyzed, and the results suggest an alternative deactivation mechanism: the plugging of the smallest pores and the associated loss in internal surface area. A pore-plugging model is developed that is compatible with a treelike description of pore branching (Simons, 1982). Product deposits systematically fill pores of all sizes, with the result that the entire local porosity is eventually plugged. The model is validated with both SO, and H, S sorption data without invoking any adjustable parameters, such as the product layer diffusion coefficient. The model is readily available for parametric studies of the effect of sulfur concentration, particle size, porosity, internal surface area, and temperature on the calcium utilization achieved within specified time scales appropriate to various combustor or gasification devices. CONCLUSIONS AND SIGNIFICANCEThe sorption of H,S and SO, by a porous sorbent material (CaO) is described. The present model is distinct from previous models in its treatment of the role of the deposit layer, CaS for H, S sorption and CaSO, for SO, sorption. The development of this product layer induces the deactivation of the sorbent particle. Previous models attribute this deactivation to diffusion through the product layer, whereas the present model attributes this deactivation to the plugging of the smallest pores and the subsequent loss of internal surface area. The plugging of small pores is validated by comCorrespondence concerning this paper should be addressed to G. A. Simons.parison of the present theory to data on BET surface evolution with sulfur sorption. The time-dependent sorbent model is validated by extensive comparisons with SO, and H,S sorption data. The larger late time utilization of CaO by H, S and the faster late time utilization rate are explained solely on the basis of the smaller molar volume of the product, CaS. The plugging of small pores simply occurs at higher ...

The intrinsic rate constant for the rate of reaction of SO, with porous CaO in an excess of 0, to form CaSO, has been determined from SO, sorption data on a wide range of particle diameters (1 pm-1 mm), SO, partial pressures (60 Pa to 5 kPa), and temperatures (973-1,478 K). An intrinsic SO, reaction order of unity has been validated through current fixed-bed experiments in the 60 to 300 Pa SO, partial pressure range, while fluidized-bed data were used to validate the reaction order from 500 Pa to 5 kPa SO, partial pressure. The absolute value of the rate constant and the activation energy was derived from data on small (1 pm) particles, which approach the limit of kinetic control. While analysis of this data base does minimize the role of intraparticle diffusion through the porous sorbent particle, the filling of the small pores by the product deposits and the associated loss of internal surface area is critical to the data reduction. It is believed that the intrinsic rate constant is known to the accuracy to which the reactivity and the BET surface area may be measured.

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