Gas-phase reactions of mercury and halogens in combustion environments

Regulations in the U.S. on the control of mercury emissions from industrial and utility boilers have spurred an interest in better understanding the behavior of mercury in combustion flue gas. Pilot- and full-scale studies have shown that the addition of halogen compounds such as chlorine and bromine to combustion systems influences the chemistry of mercury and its removal in air pollution control devices. The chemistry of mercury and chlorine in combustion systems such as incinerators and coal-fired boilers has been probed in many experimental and theoretical studies. However, the chemistry of mercury and bromine and of mixed bromine - chlorine species, has not been studied in much detail. We have assembled a set of elementary, gas phase (homogeneous) reactions and associated thermochemical parameters for calculation of reverse rate constants from microscopic reversibility (thermochemical consistent kinetics). These are used to model combustion flue gas that contains the appropriate mercury and halogen compounds for behavior of mercury in typical coal combustion flue gases. Modeling results suggest that addition of bromine enhances mercury oxidation in flue gas via heterogeneous reactions.