Degradative solvent extraction of low-rank coal: Role of water on pyrolysis mechanism of low-rank coal in a highly-dispersed medium

Abstract

A degradative solvent extraction (DSE) method was proposed to upgrade low-rank coals (LRC) for their cascaded utilizations in a highly-dispersed medium. The derived products exhibited well improved properties in comparison to the raw LRCs. Previously, the LRCs were pre-dried to eliminate the potential impact of the varied water content of raw LRCs, but in light of the abundant water in fresh coals, the hydrolyzing effect of water on coals under heating, and the energy cost to pre-dry the LRCs, it is consequently essential to clarify the effect of water so as to verify the necessity of pre-drying process. In this study, the roles of inherent water and extra-added water were respectively investigated, using dried, raw and wet coals from two typical LRCs. The results show that increasing the moisture content of raw LRCs contributed to the extraction ability of DSE method without noticeably changing the elemental composition, chemical structure and thermal decomposition behavior of extractable products, thus pre-drying of LRCs before DSE treatment was proven unnecessary. Additionally, the roles of the inherent water were concluded as: 1) leading to the formation of stable covalent bond during drying process; and 2) acting as H donor to promote extraction ability, while the added water can only function as H donor. Since excessive water will increase the pressure during DSE treatment and lead to the waste water treatment, the LRCs with a water content between 10% and 30% were consequently recommended for the practical application of DSE treatment.