For the development of a dry solid sorbent having quite fast CO2 sorption kinetics in an intermediate temperature range of 245-300 ℃ to be applicable to a riser-type fluidized bed carbonator, samples of Al2O3-supported MgCO3 (1.2mmol/g) promoted with different molar amounts of Na2CO3 (1.2, 1.8mmol/g) and/or NaNO3 (0.6mmol/g) were prepared by incipient wetness pore volume impregnation. For a reference, an unsupported bulk phase sorbent of NaNO3-Na2Mg(CO3)2 was also prepared. From the sorption reaction using a gas mixture containing CO2 by 2.5-10% at 1 bar for the sorbents after their activation to MgO, Al2O3-supported sorbents were featured by their rapid carbonation kinetics in contrast to the unsupported sorbent showing a quite slow carbonation behavior. The addition of Na2CO3 to the MgCO3/Al2O3 sorbent made MgO species more reactive for the carbonation, bringing about a markedly enhanced kinetic rate and conversion, as compared with the unpromoted MgCO3/Al2O3 sorbent having a small negligible reactivity. The addition of NaNO3 to MgCO3/Al2O3 or to Na2CO3-MgCO3/Al2O3 induced the same promotional effects, but to a lesser magnitude, as observed for the Na2CO3 addition. It was also characteristic for all these MgCO3-based sorbents that initial carbonation conversions with time appeared as sigmoid curves. For the Al2O3-supported sorbent comprised of NaNO3, Na2CO3, and MgCO3 by 0.6, 1.8, and 1.2mmols, respectively, per gram sorbent, showing the best kinetic performance, a kinetic equation capable of reflecting such sigmoid conversion behavior was established, and its applicability to a riser carbonator was examined throughout a simple model calculation based on the kinetics obtained.