Effect of solvent swelling with different enhancement methods on the microstructure and pyrolysis performance of Hefeng subbituminous coal

Abstract

Hefeng subbituminous coal (HSBC) was swelled with different enhancement method, including mechanical activation (THSBC2), ultrasonic radiation (THSBC3) and microwave radiation (THSBC4), with isometric N-methylpyrrolidone (NMP) and 1-ethyl-3-methylimidazole acetate (IL) mixture as the solvent (the un-enhancement coal was marked as THSBC1). Pore structure and pyrolysis performance of each sample were discussed. Results showed that the five samples presented abundant micropores, mesopores and some macropores from N2 adsorption-desorption profiles. In lower P/P0 range, the pore type was mainly semi-open pore closed at one end that did not affect the hysteresis loop of the samples; in higher P/P0 range, the pore types were mainly open pore (including cylindrical pore with two open ends and parallel plate pore with four open sides), ink-bottle pore and semi-open pore closed at one end. Hysteresis loops of THSBC3 and THSBC4 were similar to THSBC1, and the pore types were similar to some extent. The hysteresis loop in higher P/P0 range of THSBC2 was not obvious, indicating that there were more semi-open pores closed at one end (with no ink-bottle pore). Pore structure and pyrolysis kinetics of each sample were investigated, and fractal dimension and kinetic parameter were calculated by Frenkel-Halsey-Hill model and Coats-Redfern model, respectively. Results demonstrated that the fractal dimensions D1 (related to surface roughness) and D2 (related to complexity of inner pore structure) were closely related to the pore structure. Compared to THSBC1, the value of D1 is larger for THSBC2, presenting more obvious surface roughness. While the complexity of the pore structure of the five samples is little different, showing very close D2. In addition, the change of pyrolysis activation energy was more obvious for the sample of THSBC2 (29.027 kJ∙mol−1), 4.75% less than THSBC1 (30.482 kJ∙mol−1), which might be attributed to the violent collision process caused by mechanical activation. HSBC presents higher solid yield during pyrolysis, up to 64%, and the solid yield of others samples is around 60%. Compared with HSBC, the gas yields of other four samples have little change, while the liquid yields are increased, indicating that the swelling treatment can promote the formation of tar.