Experimental study on CO and CO2 emissions from spontaneous heating of coals at varying temperatures and O2 concentrations

• Experiments on CO and CO2 emissions from spontaneous heating of coals at varying temperatures and O2 concentrations.
• CO/CO2 ratio increases with increase of initial temperature and becomes less significant for more reactive coal.
• There are no significant changes in CO and CO2 emissions for O2 concentration at or below 15%.
• The maximum CO production rate correlated well with the maximum center coal temperature rise.
• The CO/CO2 ratios become independent of the inlet O2 concentration as the initial temperature increases.

Laboratory experiments were conducted to investigate carbon monoxide (CO) and carbon dioxide (CO2) emissions from spontaneous heating of three U.S. coal samples in an isothermal oven at temperatures between 50 and 110 °C. The oxygen (O2) concentration of an oxygen/nitrogen (N2) mixture flowing through the coal sample was 3, 5, 10, 15, and 21%, respectively. The temperature at the center of the coal sample was continuously monitored, while the CO, CO2, and O2 concentrations of the exit gas were continuously measured. The results indicate that the CO and CO2 concentrations and the CO/CO2 ratio increased when the initial temperature was increased. As the inlet O2 concentration increased, the CO and CO2 concentrations increased, while the CO/CO2 ratios tended to converge to the same value. The ratio of CO/CO2 was found to be independent of coal properties, approaching a constant value of 0.2. The maximum CO production rate correlated well with the maximum coal temperature rise. The apparent order of reaction for coal oxidation was estimated to be between 0.52 and 0.72. The experimental results in this study could be used for early detection and evaluation of a spontaneous heating in underground coal mines.