Study on structure and properties of remained solids from liquefaction of Naomaohu coal in tetrahydronaphthalene

To understand the main reactions in direct coal liquefaction and to find out the temperature at which condensation reactions become significant, the liquefaction of a vitrinite-rich coal, Naomaohu (NMH), is studied at 380–460 °C in tetrahydronaphthalene (THN). The main focuses are on the trends of products yield and evolution of solid residues’ composition and structure. It is found that NMH is very active at 420 °C with a conversion of >90 wt% and an oil-plus-water yield of 39 wt% in 40 min. The reaction, especially the condensation behaviors at temperatures lower than 420 °C are different from those at higher temperatures as indicated by the trends in products yield and in the residue’s morphology and properties. The residues obtained at 440–460 °C contain mesophase pitch spherules. The changes in residues’ volatiles content, H/C and O/C ratios, FTIR and Raman spectra, and ESR parameters are slow at 380–420 °C but much fast at 440–460 °C. These different trends are clearly evidenced by plotting the residues’ characterization parameters against the decreasing residue yield during the reaction.