Volatilization properties and environmental impact of chromium and lead during low-rank coal pyrolysis

Study utilized sequential chemical extraction to analyze the distribution and occurrence forms of Chromium (Cr) and Lead (Pb) in two low-rank coals, and obtain the rich-component (RC) samples. The volatilization characteristics of Cr and Pb during coal pyrolysis were investigated through both raw coal and RC samples in a fixed-bed reactor at 400 ~ 800°C. It was revealed that the occurrence mode of Cr was predominantly associated with aluminosilicate, organic matter, and carbonates, whereas Pb was mainly found in carbonates. The volatility characteristics of Cr and Pb in coal depend on individual thermochemical characteristics and occurrence modes. The volatilization temperature range for both the disulfide and organic mode of Cr and Pb was between 400°C and 600°C, but that of the aluminosilicate exceeded 700°C. Environmental evaluation demonstrated that pyrolysis reduced the potential environmental impact of trace elements (TEs) in raw coal while enhancing the stability of TEs in the resulting char. At 800°C, the total potential ecological impact factor (RI) of TEs in Xinjiang coal decreased from 998.75 to 1.08. While for Yunnan coal, RI decreased from 497.98 to 0.62.