Effect of reaction conditions on structure and thermal reactivity of asphaltenes from mild liquefaction of Hami coal

Asphaltenes including asphaltene and preasphaltene (PAA) are the important products of the direct coal liquefaction process, whose structure and property are essential for the high-efficiency liquefaction and the subsequent utilization. The structure and property of PAA are closely related to the liquefaction conditions. Therefore, in this work, the effects of liquefaction temperature, residence time, pressure, and the ratio of solvent to coal on the structure and property of PAA obtained from mild liquefaction of Hami coal (named as HMPAA), were investigated in a batch autoclave with tetrahydronaphthalene as solvent. The structure features of HMPAA obtained under different conditions were characterized by elemental analysis, infrared spectroscopy and solid-state ¹³C-NMR. Thermal reactivity of HMPAA and the evolution curves of gas product during pyrolysis were analyzed by TG-MS. The results showed that the yield of HMPAA increased with decreasing reaction temperature, increasing pressure, decreasing residence time and increasing the ratio of solvent to coal. The highest HMPAA yield was 35.0% at 340 ℃, 3 MPa, residence time of 1 h, and the ratio of solvent to coal of 2∶1. The carbon structure of HMPAA consisted of aliphatic carbon and aromatic carbon, while the latter accounted for about 80%. Increasing liquefaction temperature was favorable to the formation of HMPAA with higher aromaticity. The aromatic condensation degree of HMPAA increased with extended residence time. The aromaticity and aromatic condensation degree of HMPAA decreased with the increase of the ratio of solvent to coal. The liquefaction pressures examined in this work had little effect on the structure and property of HMPAA. The pyrolysis of HMPAA started at about 250 ℃ and the peak temperature of maximum weight loss was between 400 ℃ and 500 ℃, and the final weight loss was over 40%. The thermal reactivity of HMPAA increased with decreasing liquefaction temperature and increasing the ratio of solvent to coal, and the change of pressure had little effect on the thermal reactivity of HMPAA.