Peculiarities of sulphur functionalities in the Thracian coal province, Bulgaria

Abstract

The Thracian coal province in Bulgaria is comprised of three main coal basins, Maritza East, Maritza West and Elhovo, with extremely immature coals (Rr = 0.18–0.21%). These Miocene-Pliocene aged low rank coals (lignites) are characterized by high ash and sulphur contents, and low calorific values. Temperature programmed reduction/oxidation at atmospheric pressure (AP-TPR/TPO), coupled with on-line mass spectrometer(AP-TPR/TPO-MS) and with an off-line gas chromatograph-mass spectrometer (GC-MS), were applied to the qualitative and semi-quantitative analysis of sulphur functionalities in representative samples from each basin of the coal province. Carbonates and mineral sulphur were removed by preliminary treatment with diluted acids at mild conditions to prevent effects on the TPR kinetograms. This treatment mainly removed the mineral sulphur, and its effects on organic sulphur were insignificant. Most of the organic sulphur in the Thracian lignites occurs in thiophenic structures (~ 60%). The TPR-MS profiles showed the presence of alkyl and aryl thiols, dialkyl and aryl-alkyl sulphides and thiophenes. The main organic sulphur compounds registered by the off-line AP-TPR-GC-MS were highly volatile compounds (thiols, sulphides, disulphides), alkylated thiophenes, and a lesser proportion of benzothiophenes. The disulphides (-SS-) are relatively abundant in the Maritza East and Elhovo lignites. They are probably the main aliphatic sulphur species in the coal’s organic matter. Aliphatic chains with sulphur bridges are also principal structural functionalities in the Maritza West organic matter. Aromatic compounds with two aromatic rings and high degrees of substitution could be key organic structures in the studied lignites.