Effect of O2/CH4 atmosphere on tar production during coal pyrolysis.

• 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]