Pathways for Production of CO2 and CO in Low-Temperature Oxidation of Coal

Oxidation of a bituminous coal has been studied using an isothermal flow reactor operated at temperatures between 60 and 90 °C, and packed with coal particles smaller than 853 μm in diameter. The time-dependent rates of production of CO2 and CO during experiments were obtained simultaneously from the measurement of CO2 and CO concentration at the reactor's exit, just after the onset of oxidation of the coal, using a dual-column micro gas chromatograph. The ratio of the rates of production of CO2 and CO was high at the beginning of an experiment, but decreased to reach a steady state after 10 hours or so. Experimental data indicated that this ratio, at steady state, depends on temperature, and is independent of the size of coal particles and oxygen concentration in the oxidizing medium. The latter discovery has led to formulation of a new mechanism for low-temperature oxidation of coal, which is consistent with the existing spectroscopic data. It is proposed that carbon oxides are produced via the direct burnoff reaction and the decomposition of stable oxygenated complexes, such as carboxyl and carbonyl species. The decomposition of the initial chemisorption intermediates liberates carbon dioxide. For the first time, the paper reports an estimate of the activation energy for the direct burnoff reaction, which ranges between 62.3 and 70.1 kJ/mol for the present coal. The proposed mechanism in conjunction with the activation energy for the direct burnoff reaction could be used for predicting the emission of CO2 and CO from low-temperature oxidation of coal in practical applications.