Theoretical Studies of Properties and Reactions Involving Mercury Species Present in Combustion Flue Gases†

The thermochemical properties of mercury species present in combustion flue gases were studied using quantum mechanical methods combined with effective core potentials (ECP) basis sets. At various levels of theory, the calculated geometries, vibrational frequencies of the species, and the reaction enthalpies were compared with experimental data in order to validate the quantum mechanical method and basis set combination. The results show that the QCISD/RCEP28DVZ combination provides the most accurate results and the B3PW91/RCEP28DVZ and B3LYP/ECP28MWB combinations also perform well. On the basis of the evaluation of theoretical methods and basis sets of quantum chemistry, theoretical exploration on the mercury reaction mechanism in flue gas was conducted on the level of atoms and molecules. The properties of stable minimums were validated by vibration frequencies analysis. The activation energies were calculated by thermal energy calibration (including zero point energy calibration). The reaction rate constants in the temperature range of 298−1500 K were calculated from the transition state theory.