Lignite sulfur transformation during the supercritical water gasification process

•Sulfur forms in solid, liquid and gaseous products were clarified for lignite gasification in supercritical water.•Transformation paths of sulfur during lignite gasification in supercritical water were deduced.

Gasification of lignite in supercritical water (SCW) is considered to be a potential clean technology for generating energy by utilizing lignite. An important step for the implementation of this technology is to understand the reaction mechanisms and emission characteristics resulting from pollutants in lignite, which, however, are yet to be investigated. Considering the importance of sulfur pollutants in the utilization of coal, we have studied the transformations of sulfur compounds in lignite under the reaction conditions of lignite gasification in SCW. The studies were carried out in a batch autoclave, with a pressure range of 19–25 MPa, a temperature range of 450–600 °C, and an equivalence ratio of oxygen between 0.0 and 0.4. Known forms of sulfur in solid, liquid, and gaseous phases were analyzed, and reasonable reaction mechanisms are discussed. Pyrite (FeS2) in lignite transformed to FeS under the experimental conditions in the present study. Organic sulfur was present in very high quantities in raw lignite, and 80% of it decomposed upon gasification in SCW. Quantities of sulfate in solid residues exceeded that of raw coal when the equivalence ratio (ER) was higher than 0.3. SO42− and S2O32− were the main sulfur-containing species in the liquid fractions, while S2−, HS−, SO32−, and HSO3− were present only in minor quantities. H2S was the only detected sulfur compound in gaseous fractions. Transformation pathways of sulfur during lignite gasification in SCW were also deduced. These results could form the framework for the evaluation of sulfur emissions and their potential control in the implementation of this technology.