Modes of Occurrence of Chromium and Their Thermal Stability in Low-Rank Coal Pyrolysis

Chromium (Cr) and the emission of its compounds into the environment have caused long-term environmental contamination. In this study, the modes of occurrence of Cr in low-rank coal and their thermal stability in pyrolysis were investigated by sequential chemical extraction (SCE), single-component samples (SCS) pyrolysis, and thermochemical equilibrium simulation. The results showed that organic matter, aluminosilicate, and carbonate were the dominant modes of occurrence of Cr in low-rank coal. The modes of occurrence and chlorine (Cl) content affected the volatilization of Cr in coal. The characteristic release temperature range of Cr bounded to aluminosilicate was >600 °C and 400–600 °C for Cr bounded to a disulfide. Cr bounded to organic matter almost released completely before 600 °C. Cl enhanced the volatility of Cr and reduced its release temperature in Cr bounded to aluminosilicate. The simulation showed the content of gas products was very low, mainly chlorides. While the content of solid products, sulfides, and oxides, was much higher than gas products, showing their high thermal stability. The sulfides and oxides in chars were closely related to the carbonate and aluminosilicate bound form of Cr. The results of the equilibrium simulation were consistent with the experimental results.