Sulfur Transformation during Hydrothermal Dewatering of Low Rank Coal

Abstract

The presence of sulfur in coals has raised serious environmental issues, which are obstacles to large-scale utilization of coals. Hydrothermal dewatering (HTD) is a promising upgrading method for low-rank coals (LRCs) to significantly remove oxygen-containing groups and irreversibly decrease the inherent moisture content. To uncouple the complex behavior of sulfur evolution during HTD processing of lignite and to elucidate the main mechanism, this research experimentally studied the characteristics of sulfur transformation in a Chinese lignite from Xiaolongtan coal mine during HTD upgrading. Results reveal that the HTD upgrading of raw coal within the temperature range from 200 to 300 °C can obtain a desirably upgraded coal with higher calorific value and lower inherent moisture. Compared with raw coal, organic sulfur content decreased significantly, whereas sulfate sulfur content gradually increased after HTD. X-ray photoelectron spectroscopy results showed that HTD promoted aliphatic sulfur decomposition and the release of sulfur-containing gases. The released gases, such as H 2 S, reacted with the organic matrix of coals to form thiophenic sulfur. As a result, thermally stable thiophenic sulfur increased with increasing HTD temperature. The increase of sulfate sulfur content after HTD was attributed to the release of SO 2 . The calculation of the mass balance on the sulfur revealed that the vast majority of sulfur remained in upgraded coals, and only a minimal amount was released into gaseous and liquid products. The sulfur-containing gases remarkably increased with increasing HTD temperature, whereas the sulfur in the wastewater decreased.

Transformation of Sulfur during Co-Hydrothermal Carbonization of Coal Waste and Food Waste

“…This high concentration of organic sulfur in CW1 and CW2 gradually decreased with the increase in HTC temperature. Wu et al [15] observed similar phenomena during the hydrothermal dewatering of low-rank Xiaolongtan coal and concluded that the organic sulfur was released in the gaseous products. Researchers have reported a similar decrease in organic sulfur in the solid phase during hydrothermal carbonization of sewage sludge, which was possible because of the formation of SO 2 in the gaseous phase and sulfate ions in the liquid phase [16,17].…”

“…Although the increment was slow at 180, and 230 • C, the sulfate sulfur content increased significantly at 280 • C and reached a maximum of 11.2 and 13.5 mg/g hydrochar for CW1 and CW2, respectively. The conversion of organic sulfur into sulfate sulfur could be the reason for this increase [15]. Wang et al [18] observed a similar increase in sulfate sulfur while studying the transformation of sulfur impurities in lower rank coals.…”

Structure of Sewage Sludge-Clay Multiscale Composite Particles to Control the Mechanism of SO2 and H2S Gas Release

“…The mass loss process can be divided into three stages: (1) The drying and water evaporation stage from 50 °C to 200 °C, at this stage, the SS sample absorbs heating slowly and mainly loses mass due to water evaporation, and the mass loss is only 1.12%; (2) the pyrolysis and charring stages are from 200 °C to 600 °C, and among 200~400 °C is the pyrolysis stage, which is mainly associated with mass loss caused by the decomposition of unstable proteins and volatilization of organic matter [ 19 , 20 ]. In addition, 400~600 °C is the charring stage, with a mass loss of 11.44%, and the organic matter (aliphatic-S and aromatic-S [ 21 ]) in this section of SS will be slowly pyrolyzed until charring into char. Above all, the mass loss at this stage is the largest and the maximum mass loss is at temperature point 290 °C; (3) 600~1100 °C is the stage of decomposition of refractory organic and inorganic substances, in which the mass loss from 600~950 °C is due to the decomposition of residual volatile organic substances, sulfate and carbonate [ 21 , 22 ] in SS, while 950~1100 °C is associated with mass reduction because of the decomposition of char.…”

“…In addition, 400~600 °C is the charring stage, with a mass loss of 11.44%, and the organic matter (aliphatic-S and aromatic-S [ 21 ]) in this section of SS will be slowly pyrolyzed until charring into char. Above all, the mass loss at this stage is the largest and the maximum mass loss is at temperature point 290 °C; (3) 600~1100 °C is the stage of decomposition of refractory organic and inorganic substances, in which the mass loss from 600~950 °C is due to the decomposition of residual volatile organic substances, sulfate and carbonate [ 21 , 22 ] in SS, while 950~1100 °C is associated with mass reduction because of the decomposition of char.…”

A Probable Origin of Dibenzothiophenes in Coals and Oils

“…Dibenzothiophene and its derivatives are major polycyclic aromatic sulfur heterocyclics (PASHs) in coals, crude oils and sedimentary organic matter [1][2][3][4]. Due to the excellent micro-biological degradation resistance and a high thermal stability of DBTs [5][6][7][8][9], their geochemical parameters such as distribution and concentration have been widely applied to evaluate the maturity of coals and suggested as the depositional environment indicator of sedimentary organic matter as well as the tracer of hydrocarbon migration pathways [10][11][12][13][14][15][16]. In view of their valuable applications as geochemical markers, great efforts have been devoted to elucidating the origin of DBTs [17][18][19][20].…”