Sulfur Transformation Characteristics and Mechanisms during Hydrogen Production by Coal Gasification in Supercritical Water

Coal supercritical water gasification (SCWG) is famous for generating clean gas without SOₓ pollutant. Study of sulfur transformation characteristics can provide the basis of sulfur removal during hydrogen production by coal gasification in supercritical water (SCW) at the source. In this work, two coals produced from Linfen and Zhangjiamao in China (hereinafter to be referred as L-coal and Z-coal), were chosen as experimental feedstocks to investigate sulfur transformation characteristics during hydrogen production by coal gasification in SCW (550–750 °C, 20 min, 25 MPa). Sulfur transformation pathway and sulfur forms in the products were complex but detected comprehensively. H₂S was the only gaseous product instead of SOₓ, whereas SO₄²– was the main liquid–sulfur product. Inorganic and organic sulfur compounds were used to investigate sulfur transformation mechanisms. H₂S had three sources as follows. First, among inorganic sulfur of raw coal, FeS₂ (Pyrite) was chemically stable in SCW lacking of hydrogen. When FeS₂ was in hydrogen atmosphere, H₂S was generated and FeS₂ was converted to Fe₁–ₓS and Fe₃O₄ under SCW. Second, H₂S came from unstable sulfate minerals such as FeSO₄ which may decompose and be converted to Fe₃O₄. Third, organic sulfur, especially thiophene sulfur transformed to H₂S. The two sulfur products H₂S and SO₄²– depend on H or OH free radical in SCW. More H free radical provided a reducing environment of SCW to generate H₂S at higher temperatures, whereas more OH radical provided an oxidizing environment of SCW to generate SO₄²– at lower temperatures, but the final trend was generating H₂S when coal gasified completely at a higher temperature. The results of this study may provide an experimental basis of solving the SOₓ emission from coal at the source and demonstrate a promising clean utilization way of coal.