AbstractThe direct sulfation of three high purity limestones, all having more than 95% CaCO3but differing in petrographic texture, was studied in a thermogravimetric analyzer under simulated PFBC (Pressurized Fluidized‐Bed Combustion) conditions, that is, in the presence of enough CO2to prevent decomposition of CaCO3. Experiments were carried out over a wide range of particle size (53‐350 μm) at two temperatures (750 and 850°C), and two concentrations (1500 and 6000 mL/m3(ppmv) SO2). The particle size and temperature had a strong influence on the variation of the conversion with time. In contrast, the effect of concentration was rather marginal over the whole duration of the process, and the analysis of the initial reaction rate data gave a reaction order of 0.4, with respect to SO2, for all solids. The measured reaction rates were compared with those obtained in a past study for the sulfation of the calcines of the three solids under similar conditions. The initial rate of the calcine‐SO2reaction was found to be higher than that of the limestone‐SO2reaction, but since the former drops sharply as the pores at the external surface of the particles are filled with solid product, higher conversions may be obtained in some cases during direct sulfation for large react