Pore structure of selected Chinese coals with heating and pressurization treatments

Pore structure of Chinese coals with heating and pressurization treatments was studied using small angle X-ray scattering (SAXS), N-2 adsorption/desorption isotherms and scanning electron microscope (SEM). SAXS was performed for some samples after heat treatment at seven elevated temperatures from 25 to 250 degrees C at 0 MPa and for other samples with hydrostatic pressure treatment at 0, 5, 10, 15 and 20 MPa at the room temperature. The results show that N-2 adsorption isotherm together with SAXS could be a comprehensive method to evaluate the pore shape and the pore size distribution: the pore shapes are generally spherical for low rank coal and they are mainly ellipsoidal for high rank coal. All these measurements were then interpreted using the fractal theory to reveal relationship between surface fractals and coal rank, and the evolution of surface fractals under heating and pressurization treatments. The results show that surface fractal dimension (Ds) changes with different treating temperature and pressure and maximum vitrinite reflectance (R-o,R-m). Especially in the bituminous stage, Ds shows an increasing trend with R-o,R-m under varied temperatures. Moreover, Ds shows an increasing trend with increasing temperature before 200 degrees C, and a decreasing trend after 200 degrees C. Furthermore, the results show that Ds has a more complex relationship with R-o,R-m under varied treating temperature than that under varied treating pressure.