The chemistry of arsenic and mercury under post-combustion conditions

Coal combustion remains an important source of toxic metal emissions. Dilute concentrations and poor understanding of their chemistries hinders reduction of these emissions. This thesis examines the chemistry of the trace metals arsenic and mercury in coal combustion systems to help provide guidance toward improving emission control strategies.;While studies in the literature examine coal arsenic chemistry, vaporization of arsenic, interaction between arsenic and solid materials, and partitioning of arsenic in fly ash are not well understood. Synthetic char combustion experiments showed that gas-solid reactions of arsenic and calcium likely occur externally of char particles and the reactions rates reported in the literature are slower than that which occurs during combustion. As hypothesized, As4O6 was only found in the sub-micron fly ash, likely from condensation.;This information was used in a mathematical model of arsenic partitioning in fly ash. Simulations show that arsenic in sub-micron fly ash particles is present from condensation of As4O6, while arsenic in super-micron fly ash particles is present from arsenic that remains un-vaporized during the combustion process.;Mercury studies in the literature address aspects of coal mercury chemistry, however, the effects of flue gas constituents on mercury oxidation are not well understood. In homogeneous experiments, mercury oxidation levels increased with increasing HCl concentrations, while oxidation in the presence of increasing SO2 concentrations remained consistent. When both HCl and SO 2 were present, mercury oxidation was enhanced at an HCl concentration of 200 ppmv and inhibited at an HCl concentration of 555 ppmv.;In heterogeneous experiments, mercury oxidation was enhanced from the homogeneous experiments at high hematite particle concentrations in the presence of HCl alone and in the presence of SO2 alone at a concentration of 500 ppmv. Experiments conducted in the presence of montmorillonite particles showed that increased oxidation was not due solely to injected particles.