Fractal characteristics and reactivity evolution of lignite during the upgrading process by supercritical CO2 extraction

In this study, an upgrading process of lignite with supercritical CO2 extraction under rather mild temperatures was studied as a possible method for producing clean solid fuel with less moisture and enhanced combustion reactivity. The fractal characteristics and reactivity evolution of two typical lignites during the upgrading process by supercritical CO2 extraction have been studied in present work. Two typical lignites and one subbituminous coal were extracted in different conditions by a semi-continuous supercritical CO2 extraction device and the evolution of coal pore structure was investigated by N2 and CO2 adsorption/desorption isotherms. Two fractal dimensions D1 and D2, at relative pressures 0-0.45 and 0.45-1, respectively, were calculated by the Frenkel-Halsey-Hill model. The combustion reactivity of raw and upgraded coals was studied by a thermogravimetric analyzer. The results indicate that the N2 quantity adsorbed in pores of upgraded lignite increases with the increase of extraction time and temperature which is consistent with the variation of specific surface area and total pore volume of upgraded lignite. The mesopores and micropores of upgraded HB lignite are developed whereas macropores and mesopores of upgraded ZT lignite are enlarged after supercritical CO2 extraction. It was found, D1 values of upgraded HB lignite decrease whereas D2 values increase slightly. Meanwhile, the D1 and D2 values of upgraded ZT lignite are all decreased. The D1 values are mainly affected by the influence of mesopores and macropores on specific surface area and D2 values are mainly affected by the effects of fine mesopores and micropores on total pore volume. We found that D1 values have a positive relationship with the average pore diameter whereas have a negative relationship with specific surface area and total pore volume. D2 values have a negative relationship with average pore diameter whereas have a positive relationship with specific surface area and total pore volume for the raw and upgraded lignite. The combustion reactivity of upgraded lignite is enhanced after supercritical CO2 extraction due to the improvement of specific surface area and total pore volume.