Transformation behaviors of C, H, O, N and S in lignite during hydrothermal dewatering process

Lignite resources are abundant in China, but high moisture contents limit their utilization. With a view to making optimal use of beneficial elements (C, H, O) and effectively control the release of harmful elements (N and S) in lignite during drying, two lignites, one from Inner Mongolia (IM) and one from Yunnan (YN) province, were hydrothermally dewatered (HTD) at 230 °C, 270 °C and 300 °C. The solid, liquid and gaseous products were characterized by proximate and ultimate analyses as well as Fourier transform infrared spectroscopy (FT-IR), chemical oxygen demand (COD) analysis, Raman laser analysis and gas chromatography. Based on these data, the transformation behaviors of C, H, O, N and S contained in the raw lignite as they partition into the HTD products (solid, liquid and gas) were investigated. Results show that the effects of HTD on the elemental transformations depend on the coal properties. HTD is more effective for YN lignite than for IM lignite. IM liquid products have lower COD contents, resulting from less dissolution of IM lignite. CO ₂ and CO are released during HTD and their amounts increase with temperature for both IM and YN lignites. High temperature leads to a significant loss of carbon in the solid product; this corresponds with increases of the carbon content in the gas and liquid phases. Hydrogen, nitrogen and sulfur in lignite are mainly transferred into the liquid phase. A large proportion of the oxygen-containing functional groups in the lignite decompose, causing its transfer out of the solid and into the gas and liquid phases.