Fundamentals of mercury transformations in coal combustion flue gas - A theoretical and modeling study

The emission of trace metals such as mercury and arsenic from fossil fuel combustion for electric power generation has become increasingly important because of the potential health risks associated with the presence of elevated concentrations of such species in the environment. Once emitted, the mercury compounds deposit in aquatic systems and transform into organic (methylated) mercury, which gets bioaccumulated and its concentration magnified in the aquatic food chain. Methyl mercury is a neurotoxin and the U.S. EPA estimates that each year nearly 300,000 children in the U.S. face an elevated risk of developing disabilities associated with exposure to elevated levels of mercury arising out of consumption of mercury contaminated fish. In the U.S., coal-fired electric utilities account for 48% of the approximately 110 tons of annual anthropogenic mercury emissions. A fundamental understanding of Hg transformations in combustion systems is required for developing Hg emission control technologies because the different forms of Hg have distinct removal characteristics. While elemental mercury (Hg0) is relatively transparent to capture in pollution control devices installed on power plants, the oxidized (Hg 2+) and particulate (Hg(p)) forms are easily captured in devices such as ESP (for particulate control) and FGD (for SO2 control). A comprehensive study of the chemical kinetic transformation pathways of various mercury species under postcombustion conditions of utility coal power plants was therefore conducted in this thesis. ^ A complete chemical kinetic mechanism was developed that consisted of gas phase Hg reactions with Cl species (Cl, Cl2, HCl, HOCl) coupled to heterogeneous reactions of Hg/Cl with unburned carbon in fly ash. The model also included an Hg oxidation mechanism on SCR catalysts (Ti/V) installed on power plants for the control of NO emissions, and an equilibrium analysis for predicting Hg retention in FGD systems. This model was used to predict mercury speciation and capture data from full-scale power plants for a broad range of conditions. Both measurements and predictions showed that power plants with SCR and FGD achieved the greatest reduction in air emissions of Hg and tests with high Cl and high levels of unburned carbon in fly ash correlated with higher extents of Hg oxidation. ^ To refine rate constants of the Hg oxidation mechanism, rate constants of homogeneous gas phase reactions between Hg and Cl species were calculated using theoretical tools of quantum chemistry and transition state theory. Such rate constants eliminate empiricism and allow for the development of a complete heterogeneous reaction set and associated rate constants. Several quantum calculation methods (HF, MP2, MP4, B3LYP, QCISD, QCISD(T)) were used for the electronic structure calculations by employing ECP basis sets for Hg and extensive all-electron basis sets for Cl, O, and H. Transition state for each reaction was determined by using the method/basis set combination that provided the best agreement (within 4% relative error) with experimentally measured properties of bond length, vibration frequency and reaction enthalpy. The QCISD/QCISD(T) methods were found to provide an incorrect temperature dependence for the three body Hg/Cl recombination reaction. The theoretical rate constants for the eight reactions were typically lower in magnitude than the corresponding empirical rate constants and were within the collision limit for all reactions.