There exist two reaction modes

direct and indirect

for the gas-solid desulphurization reaction between limestone and SO2 at high CO2 concentration from oxy-fuel combustion.The effect of temperature and SO2 concentration on direct desulphurization was studied with TGA.Based on spherical particle gas diffusion theory

a new method for calculating reaction rate constant and reaction order of SO2 utilizing shrinking unreacted core （SUC）model was proposed.Meanwhile

the method for calculating diffusion coefficient of SO2 through products layer was improved.Furthermore

simulation was carried out using SUC model for the direct desulphurization by limestone at various temperature and SO2 concentration

and the agreement between simulation results and experiment data was good.At last

the effect of temperature

porosity and average pore radius on effectiveness diffusion coefficient are discussed qualitatively based on the model proposed in the paper

and it is found that the effect of temperature is significant but the effects of porosity and average pore radius are comparatively minor.