The effect of reducing conditions and temperature on mercury release from coal

Coals contain small quantities of mercury that have the potential to be emitted to the atmosphere during thermal processing. While significant research effort has improved knowledge of mercury emissions from direct combustion of coal in power stations to produce electricity, much less effort has been devoted to mercury behavior from coal under reducing conditions, typical for smelting and metal processing. This article discusses laboratory investigations of mercury release from coal during heating under reducing conditions. Two coals, one of Australian and one of United States origin, were subjected to thermal treatment to temperatures ranging between 100 and 800°C. The level of mercury was monitored using cold-vapor atomic fluorescence spectrometry. The largest fraction of mercury was released at temperatures of up to 500°C, however some strongly bonded mercury was retained in coal samples even at temperatures of 600°C. Reducing atmospheric conditions revealed a likely delay to higher temperatures in the release of mercury from coal. Speciation of mercury in coal was further determined with a six-stage sequential selective extraction method. Mercury reaction mechanisms occurring during the heating of coal under both reducing and oxidizing conditions relevant to direct ironmaking processes are discussed.