Experimental investigations on microstructure and adsorption property of heat-treated coal chars

The relationship between microstructure of coal char and its methane adsorption property was investigated in this work. The coal was heated at different temperatures 200, 400, 600, 800 degrees C under nitrogen atmosphere, and the microstructure of each sample was analyzed using nitrogen adsorption, scanning electron microscopy, X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis. To determine the extent of ordered structures, the following parameters were discussed: interlayer spacing, d(002), from the XRD profiles, and I-D/I-G, the ratio of the intensity of D-type structures (defect structures) to the intensity of G-type structures (graphite structures), confirmed by Raman spectroscopy. After heat treatment, the crystalline structure of coal chars was more compact, and as an overall trend, the degree of graphitization was declined. Moreover, the pore structure had changed significantly and the microporosity was further developed, with the coal chars structure rearranged. Methane adsorption measurement was performed at 298 K with the pressure range of 0-5.0 MPa. The adsorption results were well described by Langmuir model, for the correlation coefficients were more than 0.99. It was demonstrated that a more developed microporosity was associated with the higher adsorption capacity of methane. Moreover, when the surface area and pore volume of the samples were considerable, the prominent role of affecting the methane uptake was the crystalline structure and the degree of graphitization. In conclusion, the microstructure of coal chars, not just the pore structure, but also the crystalline structure and the degree of graphitization, should be taken into account for methane adsorption. (C) 2013 Elsevier B.V. All rights reserved.