Effects of iron catalyst and atmosphere on sulfur transformation during pressurized low-temperature pyrolysis of Baishihu coal

A vitrinite-rich low rank coal, Baishihu (BSH) coal with moderate sulfur content was treated by dehydration and crushing. The treated samples were pyrolyzed in an alloy tubular reactor under 2 MPa. Influence of iron catalyst and atmosphere on sulfur transformation during pressurized low-temperature coal pyrolysis was investigated. Molecular composition of sulfur compounds in tar was characterized by gas chromatography with sulfur chemiluminescence detector (GC-SCD) combined with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Sulfur K-edge XANES was used to study sulfur molecular structure after pyrolysis. Sulfur compounds in BSH coal are predominantly S₁ class species in branch chain of the coal. Elemental sulfur in catalyst enters the tar and forms mercaptan or thioether compounds during pyrolysis. Iron catalyst promotes activation of hydrogen atoms in coal and contributes to hydrogenation saturation and cracking of aromatic sulfide in tar. The catalyst preferentially captures H₂S to increase content of pyrite in char and inhibits formation of sulfate. Under H₂ atmosphere, significant decrease of thiophene compounds is observed with catalyst coupled with decrease of sulfoxide compounds.