For an oxy-fuel circulating fluidized bed combustion system, the limestone calcination is normally prevented due to excessive CO2 partial pressures and the limestone is subject to a direct sulfation reaction. The enhancement of the direct sulfation of limestone by Na2CO3 was investigated under high CO2 partial pressure in a thermogravimetric apparatus (TGA) and scanning electron microscope (SEM) analysis method. A commercial limestone with a mean size of 18.8 mu m was used. Experimental results indicate that the incorporation of Na+ ions in solid product CaSO4 lattice structures results in formation of more extrinsic point defects in the crystal lattices of CaSO4 and a significantly increased solid-state diffusivity/mobility in the solid product. So the direct sulfation of Na2CO3-doped limestone shows higher rate and higher degree of conversion in the later stage of sulfation, in comparison with the direct sulfation of original limestone. The reaction changes from diffusional control to chemical reaction control in the presence of Na2CO3 because of the effect of foreign ions on accelerating the solid-state diffusion. (C) 2009 Elsevier B.V. All rights reserved.