Effects of Hydrothermal Modification on Sulfur Release of Low-Quality Coals During Thermal Transformation Process

In this paper, the effects of hydrothermal modification on sulfur-containing pollutants, such as sulfur dioxide (SO₂) and carbonyl sulfide (COS), during coal pyrolysis and combustion, have been investigated. Three typical Chinese low-quality coals, Zhundong, Yimin, and Zhaotong coal (ZT), have been treated by hydrothermal modification at final modification temperatures of 200 °C, 250 °C, and 300 °C. Coal pyrolysis and combustion experiments using raw coal and modified coals were performed using a tube furnace. Results showed that SO₂ and COS emission were suppressed after hydrothermal modification in the pyrolysis process. Lower emission of both SO₂ and COS were also achieved when final hydrothermal modification was increased, this was attributed to the loss of aliphatic sulfur, e.g., sulfoxide, sulfone, and thiother, during the modification process. For ZT, hydrothermal modification also caused a delay in the release of sulfur-containing gases. In combustion experiments, hydrothermal modification reduced the SO₂ emission for Yimin coal, but for ZT, the SO₂ release amount almost doubled, and for Zhundong coal (ZD), it also increased, after hydrothermal modification. Hydrothermal modification also caused a delay in peak SO₂ emission during the combustion of ZT; this is attributed to conversion of sulfur containing structures to stable aromatic compounds through hydrothermal modification.