Hydrogen production by low-temperature oxidation of coal: Exploration of the relationship between aliphatic CH conversion and molecular hydrogen release

Molecular hydrogen, released by coal combustion, is closely related to mine safety. In this study, the evaluation of emission characteristics of hydrogen in the low-temperature (T ≤ 200 °C) oxidation process of coal was first introduced in the use of batch reactors. The effects of coal rank, coal temperature, and particle size on hydrogen release were systematically investigated. Results showed that the release of hydrogen at low temperature was linearly related to oxygen consumption and did not really depend on the mass of coal. Coal samples with higher rank and under the condition of higher temperature produced more amount of hydrogen. When the particle size was large, the rate of hydrogen release exhibited a dependence on particle size, while for the coal samples with very fine particles (less than 0.12 mm in diameter), it was much more difficult to release hydrogen and the rate of hydrogen release was independent of the particle size. Moreover, in-situ fourier transform infrared spectroscopy was employed to analyze aliphatic CH stretching vibration from 2800 to 3000 cm−1. The conversion rates of three types of CH groups during coal oxidation were analyzed and calculated. The results indicated that methylene group exhibited the highest conversion rate; however, hydrogen release was more likely to involve the conversion of hydrogen radicals from functional group containing methane bond. Finally, the intrinsic relationship between the conversion of aliphatic CH and the release of hydrogen was quantified based on theory of oxidation kinetics.