Identifying modes of occurrence of mercury in coal by temperature programmed pyrolysis

Abstract

Understanding the modes of occurrence of mercury in coal provides information needed to evaluate effectiveness of mercury emission control by pretreatment of coal prior to coal combustion. Two coals of different rank, lignite, and bituminous coal, were investigated to identify the modes of occurrence of mercury in coal by density separation followed by acid extraction procedure and temperature programmed pyrolysis. The bituminous coal was divided into five density fraction between <1.4 and >1.8 g/cm 3 by separation in a heavy-liquid mixture. The density-fractionated fractions were extracted by HNO 3 solution. The treated samples and the lignite were pyrolyzed at a ramp rate of 1 °C/min from room temperature to 660 °C to investigate the mercury release behavior during temperature programmed pyrolysis of coal. The results showed that each mode of occurrence of mercury has its characteristic temperature range for mercury release: Hg 0 at <150 °C, HgCl 2/organic-bound Hg at 150–250 °C, HgS/silicate-bound Hg at 250–400 °C, and pyrite-bound Hg at 400–600 °C. It is inferred that HNO 3 extraction and density separation are effective for the removal of pyrite-bound Hg and then the coal can be pyrolyzed at 400 °C to remove mercury in other forms.