Modes and kinetics of CO2 and CO production from low-temperature oxidation of coal

Abstract

It is imperative to have an in-depth understanding of carbon oxide emission during low-temperature oxidation of coal not only for preventing fires in the coal industry but also for reducing emissions of hazardous gases. A set of modified isothermal batch reactors was designed to investigate the modes and kinetics of carbon oxide emission from different classes of coals (lignite, sub-bituminous coal, and bituminous coal) oxidized at temperatures below 200°C. Based on the effects of coal mass and oxygen consumption on CO2 and CO emissions during coal oxidation, it was found that carbon oxides are formed from the decomposition of not only surface oxides, caused by the reaction between coal and oxygen, but also inherent oxygen-containing groups in the coal matrix. The CO2 and CO emission modes are dependent on the types of coal. Owing to the linear relationship between the CO2 and CO emissions and the exposure times, the kinetics of carbon oxide emission was studied. These results clearly show that the low-temperature oxidation of coal can be separated into three stages with respect to the activation energies for CO2 or CO emission, and the activation energies of these three stages increased with increasing atmospheric temperature, which indicates that the reaction regime for CO2 and CO emissions switches during the spontaneous combustion of coal. Furthermore, the relationship between the activation energies for carbon oxide emission and the occurrence of oxygenated functional groups was also explored.