Occurrence Modes of Chromium and Lead in Chinese Low-Rank Coal and Effects on Their Volatilization During Pyrolysis Process

The release of heavy metal elements (HMEs) in coal have drawn special attention because of the risk of toxicity to both human health and the environment. In the present study, the minerals distribution and occurrence modes of Chromium (Cr) and Lead (Pb) in low-rank coal were studied by sequential chemical extraction (SCE), and to obtain the single-component (SC) samples. The volatilization behavior of Cr and Pb during pyrolysis process was investigated through fixed-bed reactor for both raw coal and single-component (SC) samples at temperatures 400 to 800°C. The results showed that aluminosilicate and carbonate (containing monosulfide, sulphate, phosphate and oxide) were the dominant minerals in coal. Aluminosilicate, organic matter and carbonate (containing monosulfide, sulphate, phosphate and oxide) were the main occurrence modes of Cr in low-rank coal, while Pb occurred mainly in carbonate. The volatilization characteristics of Cr and Pb in coal were not only dependent on their own thermochemical property, but also on their occurrence modes. The volatilization temperature of organic-bound mode Cr and Pb was 400-600 °C, aluminosilicate-bound mode was >700 °C and disulfide-bound mode was 400-600 °C. FactSage simulation indicated that the chlorides of Cr and Pb were the dominant compounds in 100-800 °C.