Experimental Study on the Molecular Hydrogen Release Mechanism during Low-Temperature Oxidation of Coal

Abstract

Although H2 gas is used in coal mines as an important indicator to reflect the state of coal spontaneous combustion, the gas production of H2 at low temperature has been scarcely reported in the literature. In this paper, the modes and release mechanism of molecular hydrogen were investigated for three different coal ranks below 200 °C. Batch reactor tests were performed in combination with chromatographic analysis of the coal oxidation process. The experimental results showed that molecular hydrogen release mainly originated from coal oxidation rather than thermal decomposition of inherent hydrogen-containing groups. The amount of hydrogen released increased with the coal rank. The H2 release process during low-temperature oxidation typically proceeds in two phases, namely H2 slow release (T < 100 °C) and H2 accelerated release (T > 100 °C) phases. Experiments with model compounds revealed aldehyde compounds to noticeably produce H2. Coal plays a positive role in promoting the aldehyde groups to release H2 and CO2, but an opposite trend was observed in the case of CO. As revealed by Fourier transform infrared (FTIR) spectroscopy, the amount of aliphatic structures significantly decreased with the oxidation intensity, and a drastic increase in the aldehyde content was found at temperatures above 120 °C. Additionally, the path for the formation of H2 during low-temperature oxidation of coal was provided.