Characterization of sulfur functionalities of supercritical extracts from coals of different rank, using reductive pyrolysis

Abstract

The extracts obtained via supercritical fluid extraction (SFE) from three Polish coals of different rank, using toluene, toluene/2-propanol, and toluene/tetrahydrofuran (THF) mixtures, were subjected to reductive pyrolysis to determine their organic sulfur functionalities. The extraction yield was in the range of 11.4-43.1 wt %, depending on the type of solvent and coal. Toluene gave the highest extraction yield for medium-volatile bituminous coal, whereas a toluene/2-propanol mixture was the most effective in the case of lignite. The organic sulfur content in the resultant supercritical extracts amounted to 2.17-3.70 wt %. The atmospheric-pressure-temperature-programmed reduction technique coupled with potentiometric detection of H2S (APTPR) and mass spectrometry (AP-TPR-MS) were applied to monitor the sulfur-containing compounds. During this reductive pyrolysis, these compounds were hydrogenated to H2S at specific and discrete temperature intervals. The H2S AP-TPR recovery for supercritical extracts was in the range of 23-65 wt %. The results show that thiols, polysulfides, and/or elemental sulfur have the highest contribution to the detectable sulfur in supercritical extracts by AP-TPR. However, alkyl aryl sulfides, aryl sulfides, and thiophenes are also present in smaller amounts. A toluene/THF mixture, compared to the rest of solvents used, seems to have the greatest extraction ability toward thiols and polysulfides-containing coal fragments. It was proven that the higher rank coal subjected to the SFE, the higher the contribution of thiophenic sulfur into the H2S AP-TPR evolution profile in the range of 350-900 degreesC. The use of the AP-TPR setup coupled with a mass spectrometer instead of the potentiometric detection system has revealed the presence of alkanethiols, thiophene, and C-1- and C-2-thiophenes in the gaseous products of reductive pyrolysis. C-1-C-3 dibenzothiophenes, C-1-C-3 benzonaphthothiophenes, and C-1-C-2 dinaphthothiophenes can be observed in the evolved tar.