Low-cost MgO-based sorbents were prepared through modification of natural dolomite and were found to be capable of capturing CO, from coal gas at elevated temperatures and pressures (i.e., 3300 degrees < T < 450 degrees C and P > 20 bar). The effect of steam on the reactivity of the MgO sorbents was investigated experimentally. The results indicate that the presence of steam significantly improves the overall rate of the carbonation reaction through two different mechanisms. The presence of steam improves the intrinsic rate of the carbonation reaction by altering the carbonation reaction pathway through the formation of more reactive transient MgO center dot H2O/Mg(OH)(2)(center dot) intermediate compounds. The formation of the transient intermediate compounds with larger molar volume (compared with MgO) results in the expansion of the inner pores, which lowers diffusion resistance to the reactant gas (i.e., CO2) during carbonation.