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

Abstract

In this paper, coal pyrolysis in O2/CH4 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 O2 to CH4 can enhance tar formation and tar yield reaches 18.15 wt.% at the O2/CH4 ratio of 15:85 and temperature of 650 °C, which is about 1.24 and 1.7 times as those in CH4 and N2 atmosphere, respectively. The fraction distributions of tar were obtained with simulated distillation, which indicates the addition of O2 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 O2/CH4 as pyrolysis atmosphere. The results of gas chromatography – mass spectrometry / flame ionization detector show that the addition of O2 to CH4 is conducive to formation of aliphatics, aromatics and alcohols, but unbeneficial to formation of phenols compared with that in pure CH4 atmosphere. Lower aromaticity and longer alkyl-substituents of tar from O2/CH4 atmosphere than that from N2 atmosphere indicate that CH3 from methane-oxygen reaction directly combine with coal radicals to increase tar yield and affect the fraction distribution and composition of tar.