Pyrolysis of tetralin liquefaction derived residues from lighter density fractions of waste coals taken from waste coal disposal sites in South Africa

Abstract

The lighter density (<1.5 g/cm3) fractions produced from two waste coals sampled from the waste coal disposal sites at thermochemical plants situated in South Africa were used as feed materials for liquefaction with tetralin. The liquefaction residues from the lighter density and untreated lighter density fractions were used in pyrolysis experiments. Pyrolysis of the lighter density fractions was carried out in a Fischer Assay oven at 750 and 920 °C under an argon atmosphere. Advanced analytical techniques (gas chromatography–mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy) were employed to characterize the pyrolysis products. Also, the lighter density fractions, liquefaction residues, and their chars were examined using conventional and advanced analytical techniques. The pyrolysis char yields of the liquefaction residues ranged between 74% and 76%, and those of the coal float fractions ranged between 67.0 and 71.5%. Gas pyrolysis yields ranged between 16.0% and 20.0% for the residues and between 14.5% and 18.4% for the lighter density fractions, while the pyrolytic water and the tar products of the lighter density fractions were slightly higher than those of the liquefaction-derived residues. The proton NMR analysis of the tars from the residues shows marginally higher amounts of aromatic protons than those of the lighter density fractions. Chars which were generated after pyrolysis of the liquefaction-derived residues show higher porosity values than those from the pyrolysis of the lighter density fractions. The differences in the porosities are attributed to the opening of pores and extraction of some lower molecular mass aliphatic species from the coal matrix during liquefaction. The pyrolysis products distribution and characterization of the products showed that the residues (waste material) generated after tetralin liquefaction of the float fractions from the float–sink experiments of waste coals may be utilized for thermochemical processes (pyrolysis).