Suppression of the spontaneous combustion of upgraded brown coal

Brown coal is characterized by a high moisture and volatile matter content, but its heating value per unit weight is low. To use it as a substitute fuel for bituminous coal, it must be dried and carbonized to increase its heating value to a level comparable to that of the bituminous variety. During this upgrading process, careful consideration must be given to its combustion efficiency while retaining a certain proportion of volatile matter. However, the upgraded coal containing volatile matter poses a considerable risk of spontaneous combustion, preventing long-distance transportation and long-term storage. This study proposes a straightforward method to suppress this spontaneous combustion by stabilizing the aliphatic hydrocarbons. Quantum chemical calculations have shown that the carboxyl and ester groups produced during the oxidation of methyl groups in the upgraded coal have a high chemical stability. Based on these results, Loy Yang coal, a representative brown coal from Victoria, Australia, was upgraded at 430 °C, and its oxidative behavior was evaluated using FT-IR, nuclear magnetic resonance, thermogravimetry/differential thermal analysis, and gas chromatography. The results confirmed that oxidizing the upgraded coal at 240 °C effectively reduced its tendency for spontaneous combustion.