Abstract: The control of the amount of alkali vapors in the flue gas at the gas turbine inlet is very important for the coalfired combined cycle power system. Based on the Gibbs free energy minimization principle of chemical equilibrium, two models of coalfired process and alkali migration were developed using Aspen Plus software to explore the migration mechanisms of alkali metals during coalfired process. Moreover, the effect of conditions on the limit level of alkali vapors were predicted. The model results show that in a wide reactor temperature range from 850K to 1650K, the concentration of alkali vapors increases with increasing the reactor temperature. Below 1400K NaCl vapor and KCl vapor are the major alkali metal species in the gaseous phase, and the concentration of Na2SO4 vapor is much higher than that of K2SO4 vapor. At below 1500K, concentrations of alkali vapors increase with the increase of chlorine in coal. At a constant temperature, concentrations of alkali vapors decrease with increasing the reactor pressure. Under atmospheric pressure, the concentration of alkali vapors is decreased to 2.4×10－8 by weight in 972K with 0.1% chlorine in coal, and when the chlorine in coal are 0.05% and 0.01%, the reactor temperature should be increased to 990K and 1025K to limit concentrations of alkali vapors lower than 2.4×10－8, respectively.