Speciation and mass-balance of mercury from pulverized coal fired power plants burning western Canadian subbituminous coals

Abstract

This report summarizes the results of a study carried out on six pulverized coal-fired power plants in western Canada burning subbituminous coal for the mass-balance and speciation of mercury. The main objectives of this study were to: determine the total gaseous mercury (TGM) emitted from stacks of power plants using the Ontario Hydro method; identify the speciation of emitted mercury such as metallic (Hg⁰) and gaseous elemental (GEM) mercury; and perform mass-balance calculations of mercury for milled-coal, bottom ash, electrostatic precipitators (ESP) fly ash and stack-emitted mercury based on three tests. Sampling of mercury was carried out using the Ontario Hydro method and mercury was determined using the USEPA method 7473 by cold vapor atomic absorption (CVAAS). The sample collection efficiencies confirmed that both oxidized and the elemental mercury had been successfully sampled at all power plants. The total gaseous mercury emitted (TGM) is 6.95–15.66 g h⁻¹ and is mostly in gaseous elemental mercury (GEM, Hg⁰) form. The gaseous elemental mercury is emitted at a rate of 6.59–12.62 g h⁻¹. Reactive gaseous mercury (RGM, Hg²⁺) is emitted at a rate of 0.34–3.68 g h⁻¹. The rate of emission of particulate mercury (Hg^p) is low and is in the range 0.005–0.076 g h⁻¹. The range of mass-balances for each power plant is more similar to the variability in measured mercury emissions, than to the coal and ash analyses or process data. The mass-balance calculations for the six power plants, performed on results of the three tests at each power plant, are between 86% and 123%, which is acceptable and within the range 70–130%. The variation in mass-balance of mercury for the six power plants is mostly related to the variability of coal feed rate.