Effects of thermal dissolution in different solvents on structural characteristics and pyrolysis behaviors of lignite

Abstract

Thermal dissolution (TD) of Xilinhaote lignite (XL) in n-hexane, toluene and methanol were carried out, and then the residues of thermal dissolution (RTDs) and XL were pyrolyzed at a low temperature of 550 °C to explore the effect of TD on structural characteristics and pyrolysis behaviors of XL. Results show that RTDs present higher aromatic carbon content (fa), lower aliphatic carbon (fal) and oxygen-linked carbon content (fO), indicating that the molecular structure of XL becomes more ordered after TD, especially that in n-hexane. Among three RTDs, the RTD in methanol obtained at 290 °C (RTDM,290) exhibits the largest aromatic cluster size. The influences of TD at 290 °C on the pyrolysis behaviors of XL are remarkable. Compared with XL , the pyrolysis of RTDs obtained at 290 °C (RTD290s) present higher char yields, lower tar, gas and water yields. The pyrolysis of RTD in n-hexane obtained at 290 °C (RTDH,290) exhibits extremely low tar yield, suggesting that the low molecular weight compounds (LMWCs) isolated from XL during TD can transport into tar and/or act as hydrogen donors to stabilize the radical fragments during pyrolysis processes. Although the methanol soluble portion (SP) yield is up to 9.45% at 290 °C, the tar yield of RTDM,290 is almost equal to that of XL, indicating that the swelling effect of XL in methanol is significant and it is conducive to the production of tar.