The homogeneous and heterogeneous chemistry of arsenic and mercury at post combustion conditions

Over the last two decades, the emission of trace elements such as arsenic and mercury from combustion of fossil fuel for electric power generation has become increasingly important because of the potential health risks associated with elevated concentrations of both elements in the environment. The potential environmental impact of these elements is dependent on the physical and chemical forms in which they are emitted. For this reasons, a comprehensive study of the heterogeneous and homogeneous chemistry of arsenic and mercury has been conducted at post combustion conditions. ^ To assess the partitioning of arsenic to the gas and solid phases under post combustion conditions, thermogravimetric experiments were first conducted with calcium-containing compounds. Experiments were conducted with typical flue gas arsenic concentrations and three calcium bearing solids: calcium oxide, mono-calcium silicate and di-calcium silicate to determine the chemical kinetics of the gas-solid surface reactions over a temperature range of 600–1000°C. On the basis of mass of arsenic reacted per mass of calcium in each solid, all three solid reactants captured arsenic at equivalent levels. Using the chemical kinetic rates obtained from these experiments, a partitioning model was developed. Calculations showed that the transformation of arsenic in combustion systems was influenced by many factors such the ash and calcium content of the parent coal. ^ To assess the impact of flue gas species on the homogeneous chemistry of mercury oxidation in the presence of chlorine-containing species at post flame conditions, bench scale experiment were conducted. Molecular chlorine (Cl2(g)) or and hydrogen chloride (HCl(g)) were used as chlorine-containing species and the effects of the flue gas species sulfur dioxide (SO2(g)), nitric oxide (NO(g)), methane (CH 4(g)), and oxygen (O2(g)) on the mercury oxidation were assessed. Data collected showed that mercury oxidation by Cl2(g) or HCl (g) was promoted when excess O2(g) was present in the flue gases whereas the effects of SO2(g) and NO(g) were dependent on their concentrations in the flue gases and on the concentrations of the chlorine-containing species. Methane addition to the flue gases either inhibited or did not affect mercury speciation depending upon the temperature of injection.