Evidence obtained by gas chromatography-mass spectrometry of conversion of alkanes into aromatic compounds during coal pyrolysis

Tars obtained under different temperature and pressure conditions from a type III-S coal were analysed by high-resolution capillary gas chromatography. The tar components were identified by means of a selective mass detector. Quantitative analysis was also carried out using a flame ionization detector. It was shown that the tar obtained at low temperature (500°C) and pressure (0.1 MPa) has a higher content of n-alkanes and a lower content of aromatic compounds than those obtained at high temperature (700°C) and pressure (1 MPa). This fact is presented as evidence of the conversion, by cyclization secondary reaction, of alkanes into aromatic compounds. Most of the aromatic compounds identified are alkyl derivatives of naphthalene and benzothiophene. The presence of heterocyclic sulphur compounds in the high-temperature tars shows that sulphur released from coal during pyrolysis is involved in the cyclization reactions of the n-alkanes. Empirical models correlating the alkane, naphthalene and phenanthrene yields with the temperature and pressure of pyrolysis were obtained.