Oxygen chemisorption and low-temperature oxidation behaviors of sub-bituminous coal

The effective utilization of sub-bituminous coals should be extended as an energy source for power generation because of their low cost and long minable years. However, sub-bituminous coal may undergo spontaneous combustion owing to high reactivity with oxygen in low-temperature air compared with bituminous coal. Therefore, the objective of this study is to elucidate the low-temperature oxidation behaviors of sub-bituminous coal. In particular, transition from oxygen chemisorption to low-temperature oxidation of sub-bituminous coal has been experimentally examined in detail. Isothermal gravimetric analyses are conducted for several bituminous and sub-bituminous coal samples at temperatures varying between 356 K and 476 K in oxygen-enriched air. The chemical compositions of emission gas are continuously monitored under low-temperature oxidation conditions. The changes in the chemical structure of the coals are analyzed using Fourier transform infrared (FTIR) spectrometry. The experimental results show that the mass of all tested coals, including those of bituminous and sub-bituminous coals, increases at temperatures below 426 K as a result of oxygen chemisorption. At 476 K, the mass of sub-bituminous coal increases slightly with low H-2 and CO2 emissions at the beginning and subsequently decreases with high CO and CO2 emissions. This mass change indicates that the transition from oxygen chemisorption to low-temperature oxidation occurs during this period. In addition, the FTIR spectra of sub-bituminous coals show the existence of carbonyl groups even in raw coal samples. These carbonyl groups are considered to contribute to the formation of CO2 and H-2 from the beginning of reactions, triggering the transition from oxygen chemisorption to low-temperature oxidation.