Insight into low-temperature co-pyrolysis of Qinglongshan lean coal with organic matter in Huadian oil shale

A blend of lean coal (LC) and oil shale organic matter (OSO) was co-pyrolyzed in a fixed bed in the temperature range of 350 °C–500 °C. Changes in specific volatile during the low-temperature co-pyrolysis (LTCP) at a molecular level were analyzed using gas chromatography and pyrolysis-gas chromatography with mass spectrometric detection. In-situ Fourier-transform infrared spectrometry was used to study the real-time changes in active groups during the LTCP. Electron spin resonance was used to reveal radical transfer between LC and OSO. Based on the above characterizations, a microscopic description of synergistic effects regarding product compositions at a molecular level during the LTCP is provided. The results showed that experimental yield for char of the blend was higher than the theoretical value, indicating that one synergistic effect of the co-pyrolysis was to increase the yield of solid product. Infrared spectroscopy and gas chromatography results showed that addition of OSO improved the release of aliphatic compounds, mainly alkanes and alkenes with 7–21 carbons, and CH4 from the lean coal pyrolysis, and inhibited the release of H2 and aromatic compounds with 1–4 rings. OSO has abundant aliphatic compounds that generate sufficient H radicals to stabilize the H-poor radicals produced from coal pyrolysis. The coupling of these two types of radicals, as confirmed by electron spin resonance, accounted for decreases in the yields of H2 and aromatic compounds, and increases in the yields of aliphatic compounds, CH4, and semicoke.