Effect of tetrahydrofuran extraction on lignite pyrolysis under nitrogen

•THF extraction of lignite affects its pyrolysis product distribution.•Gas from residue pyrolysis has higher content of CO and CO2 but lower CH4.•Tar from pyrolysis of extraction residue has more light components than that from raw coal.•Tar from pyrolysis of extraction residue has higher fa and lower HR than that from raw coal.•Extraction can improve the activity of O in coal and turn it into phenolic OH.

Based on the host-guest model of coal, the fixed and mobile phase of coal which can be separated by solvent extraction will present different characteristics during thermal conversion. The pyrolysis of Xilinguole and Huolinhe lignites and their extraction residues with tetrahydrofuran were investigated at 450–700 °C under N2 atmosphere. The results indicate that the temperature at the maximum tar yield of extraction residue is lower than that of coal. Compared with raw coal, residue pyrolysis process exhibits lower water yield, higher gas yield and similar char yield. Similar H2 yield, more CO and CO2 yields but less CH4 yield are obtained during residue pyrolysis than those in coal pyrolysis. GC analysis of tar shows that contents of phenols and mononuclear aromatics from residue tar are higher than those from raw coal tar. 1H NMR and 13C NMR of tar indicate that the tar from residue has higher aromaticity factor than that from coal. The total acidity and acid distribution in raw coal, extraction residue and their chars obtained at 600 °C show that phenolic OH is the main oxygen contained group in extract. The activity of O in coal could be improved by extraction, and more O will be translated to OH during pyrolysis.