The initial kinetics of direct sulfation of Faxe Bryozo, a porous bryozoan limestone was studied in the temperature interval from 873 to 973 K in a pilot entrained flow re- actor with very short reaction times (between 0.1 and 0.6 s). The initial conversion rate of the limestone—for conversions less than 0.3%—was observed to be significantly promoted by higher SO2 concentrations and lower CO2 concentrations, whereas O2 showed negligible influence. A mathematical model for the sulfation of limestone involving chemical reaction at calcite grain surfaces and solid-state diffusion of car- bonate ions in calcite grains is established. The validity of the model is limited to the initial sulfation period, in which nucleation of the solid product calcium sulphate is not started. This theoretical reaction-diffusion model gives a good simulation of the initial kinetics of the direct sulfation of Faxe Bryozo. The intrinsic rate of the direct sulfation of the limestone is estimated to have an activation energy of about 25 kJ/mol and reaction orders of about 0.9 and 20.75 for SO2 and CO2, respectively. The diffu- sivity of carbonate ions in the surface layer of the calcite grain is estimated to be about three orders of magnitude higher than the diffusivity of carbonate ions in the inner lattice of calcite grain and have an activation energy of about 202 kJ/mol.
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