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• Coal pyrolysis in O 2 /CH 4 atmosphere can improve tar yield at atmospheric pressure. • The contents of aliphatics, aromatics and alcohols in tar were improved. • CH 3 radicals from methane-oxygen reaction can directly bond to coal radicals. In this paper, coal pyrolysis in O 2 /CH 4 atmosphere was studied by an atmospheric fixed-bed reactor. The effect of reaction temperature on yield, fraction distribution and composition of tar was explored to understand formation of tar during coal pyrolysis. The results show that the addition of O 2 to CH 4 can enhance tar formation and tar yield reaches 18.15 wt.% at the O 2 /CH 4 ratio of 15:85 and temperature of 650 °C, which is about 1.24 and 1.7 times as those in CH 4 and N 2 atmosphere, respectively. The fraction distributions of tar were obtained with simulated distillation, which indicates the addition of O 2 to methane can improve the content of asphalt at 550 °C and the contents of anthracene oil and asphalt at 600 °C. The contents of naphthalene oil, wash oil, anthracene oil and asphalt from coal pyrolysis at 650 °C are improved by using O 2 /CH 4 as pyrolysis atmosphere. The results of gas chromatography – mass spectrometry / flame ionization detector show that the addition of O 2 to CH 4 is conducive to formation of aliphatics, aromatics and alcohols, but unbeneficial to formation of phenols compared with that in pure CH 4 atmosphere. Lower aromaticity and longer alkyl-substituents of tar from O 2 /CH 4 atmosphere than that from N 2 atmosphere indicate that CH 3 from methane-oxygen reaction directly combine with coal radicals to increase tar yield and affect the fraction distribution and composition of tar. [ABSTRACT FROM AUTHOR]

[Display omitted] • H 2 can react with stable radicals during coal pyrolysis without catalyst at 200 °C. • H-atoms are introduced into tar mainly by radicals induce effect in coal pyrolysis. • Some of non-volatile radicals can abstract hydrogen atoms from H 2. In this work, the mechanism of coal hydropyrolysis was investigated by using deuterium tracer method. To figure out how hydrogen reacts with coal at the temperature below 400 °C, chars from coal pyrolysis in D 2 and N 2 atmospheres when the pyrolysis temperature is at 150 to 400 °C were measured by isotope ratio mass spectrometry (IRMS) and electron paramagnetic resonance spectroscopy. The results show that H 2 can react with stable radicals in coal during coal pyrolysis without catalyst at temperature as low as 200 °C. To obtain deep insight on which hydrogen transfer pathways affect pyrolysis product distributions of coal, IRMS combined with elemental analyzer and 2H NMR were used to obtain quantitative and qualitative date of deuterium in products. The results suggest that hydrogen atoms are introduced into tar mainly through radicals induce effect during coal pyrolysis without catalyst. And some of non-volatile radicals can abstract hydrogen atoms from H 2. Hydrogen atom of H 2 can incorporate all structural groups in the tar during coal pyrolysis, and it incorporates preferentially into 1-and 2-ring aromatic carbon and aliphatic carbon in the β or further positions of aromatic ring. [ABSTRACT FROM AUTHOR]