Generation of ultra-clean fuel from Victorian brown coal – Synchrotron XANES study on the evolution of sulphur in Victorian brown coal upon hydrothermal upgrading treatment and thermal pyrolysis

This paper reported the speciation of sulphur in two parent Victorian brown coals and their upgraded derivatives through the use of direct, non-destructive synchrotron-based S K-edge XANES. Victorian brown coal is the single largest source for power generation in Victoria, Australia. To date little has been known about its sulphur speciation. The results here demonstrated the predominance of organosulphur in the forms of thiophene and mono-/poly-sulphide in Victorian brown coals, irrespective of coal mining location and particle size. The sulphidic sulphur in the raw coal was readily cleaved and released as gas upon hydrothermal leaching in citric acid at the temperatures up to 200 °C. The use of concentrated 5 M nitric acid for acid leaching caused the oxidation of a portion of sulphide and thiophene into organic sulphate which bears an XANES position between inorganic sulphate and sulphone. Thermal pyrolysis is more effective in reducing sulphur in char, resulting in an overall sulphur content as low as 0.1 wt.% for the char produced at 800 °C. This is due to the rapid decomposition of organic sulphide commencing at 400 °C. The mobilisation rate of thiophene associated with aromatic carbon varied greatly with coal type. For the sulphur released upon thermal pyrolysis, a portion of it was also captured by inherent alkali and alkaline earth metals forming sulphate particularly in the 800 °C char. A prior acid washing of coal favoured eliminating the formation of inorganic sulphate in the resulting char sample. However, the remaining sulphur in the coal was evolved to distinct structure that has a rather stronger resonant scattering, or in a formation of acid-insoluble sulphate caused by the oxidation/reformation of organic matrix.