To explore the reaction mechanism of spontaneous coal combustion and the indicators for forecasting this risk, reaction characteristics of free radicals and functional groups during low-temperature oxidation of coal with different ranks were analyzed by electron spin resonance (ESR) and Fourier transform infrared (FTIR) spectroscopy. Combined with gas chromatography of CO, other indicators besides the main indicator gas CO were researched to forecast spontaneous coal combustion. The results showed that with increasing oxidizing temperature in the range of 30 degrees C-230 degrees C, the production of free radicals changed from slow to rapid after being oxidized and heated above characteristic temperatures, depending on the coal grade. The oxygen-containing functional groups in coal of all ranks mainly include -OH, C=O, C-O and -COOH, whose reaction trends varied widely. The concentration and production rate of free radicals and the C=O functional group in coal can be regarded as the leading indicators. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.