Comparative study on the chemical structure characteristics of lump coal during superheated water vapor pyrolysis and conventional pyrolysis

Underground coal thermal treatment is an in-situ pyrolysis process that converts underground coal into synthetic liquid and gaseous fuels using either nitrogen or superheated water vapor as the heat-and-mass-transfer medium. Therefore, comparing the effects of both pyrolysis methods on the coal pyrolysis characteristics is essential. The present study examines proprietary superheated water vapor pyrolysis and conventional pyrolysis of Tashan lump coals. The functional groups and carbon microcrystal structures of the solid residue were detected using Fourier transform infrared, Raman, and X-ray diffraction spectroscopies. Volatile components were quickly removed from coal in the 350°C-500 °C temperature range and were more effectively removed by superheated water vapor than by conventional pyrolysis. Increasing the temperature decreased the percentages of aliphatic structures, CO, C–O, and aromatic CC groups, increased the percentage of –OH groups, and increased and then decreased the percentage of aromatic C–H groups. The superheated water vapor facilitated the removal of aliphatic structures and increased the content of aromatic C–H groups in the solid residue, but did not significantly change the evolution of oxygen-containing functional groups. Increasing the temperature of the superheated water vapor increased the degree of ordering of the carbon microcrystalline structure in the coal while gradually decreasing the aromatic stacking height. At temperatures between 300 °C and 500 °C, the superheated water vapor more effectively increased the aromaticity and rank of the coal than conventional pyrolysis.