Trace Metal Emissions from Combustion Systems

摘要

The combustion of pulverized coal for electric power generation remains a source of trace metal emissions worldwide. Efforts to develop strategies for reducing these emissions are challenging, however, because of incomplete fundamental understanding of trace metal chemistry in combustion systems. To fill this gap, fundamental experimental studies of trace metal vaporization, gas phase chemistry, gas-solid reactions, and condensation are required, as are models describing these phenomena. Using As and Hg as examples, trace metal vaporization, gas phase chemistry, and gas-solid reactions are described. For arsenic, a dynamic partitioning model that examines competing rates of surface reaction and condensation is developed to provide improved estimates of As stack emissions. For Hg, experiments demonstrating that mercury oxidation by Cl_(2(g)) or HCl_((g)) is promoted when excess O_(2(g)) is present in flue gas, whereas the effects of SO_(2(g)) and NO_((g)) are dependent on their concentrations and those of Cl species, are used to develop a homogeneous gas phase kinetic model that can predict Hg speciation and emissions. These results are then used to identify gaps in understanding of other important trace elements that must be resolved to reduce emissions of these potentially harmful species.