Pilot scale study of trace element vaporization and condensation during combustion of a pulverized sub-bituminous coal

Trace metal emissions from coal-fired power plants are largely associated with the fly ash. The work reported here is part of a larger effort to develop a fundamental model for transformation of trace metals in coal to air toxic emissions from coal-fired power plants. Because the time-temperature history of the combustion gases determines the condensation behavior of gaseous species, experimental study of these phenomena require a realistic post-combustion environment. Pilot-scale combustion of a Powder River Basin coal was conducted using realistic post-combustion conditions. Trace element distributions were measured in the submicron fly ash at the inlet to an electrostatic precipitator. Flame temperature had a dramatic impact on the amount of certain trace elements such as As and Se in the submicron ash, indicating that these elements vaporize during the combustion process. The amount of vaporization was not sensitive to coal grind. There is evidence for the reaction of As and Se vapors with the large (supermicron) ash in the post-combustion flue gas via a surface reaction. The correlation between arsenic and calcium in the ash suggests the formation of calcium arsenate. No such correlation was observed between selenium and calcium.