Abstract: The dominating form of mercury is elemental Hg in high temperature coal combustion. With the decrease of flue gas temperatures the shift from Hg to HgCl2 takes place and a portion of HgCl2 is adsorbed by ash residue that is then collected by dedust equipment to decrease the mercury emission into air. In terms of mercury conversion in coal combustion, the mechanism is described via combining the thermodynamics model with adsorption model. The factors such as Cl concentration in coal and the decrease rate of flue gas temperatures and the residence time of flue gas were researched with numerical simulation in this paper. The results show that the mercury adsorption rate is over 40% when the Cl concentration in coal exceeds 0.08×10－6and the residence time of flue gas exceeds 6 seconds. The model is tested with the data from a 580MJ/h beachscale entrainedflow reactor with 150℃ of flue gas temperature , which testifies the model credibility.