Characterization of mineral composition and its influence on microstructure and sorption capacity of coal

• Demonstrate mineral content, occurrence modes, and composition.
• Analyze the relation among mineral, microstructure, and gas sorption capacity.
• Explain the impacts of mineral occurrence and composition on gas sorption capacity.

Mineral matter is widely accepts as one of the important factors influencing the gas sorption capacity of coal. By analyzing ash content and Langmuir volume of coals, studies have reported positive, negative, and poor impacts of mineral matter on sorption capacity without convincing reasons explaining the contradictory results. This paper proposes a new analysis method to correlate minerals and gas sorption capacity by connecting the mineral matter compositions to sorption capacity through the variations in the microstructure. In addition to mineral content, mineral occurrence modes and compositions were also studied to investigate their relations with gas sorption capacity.A total of 22 coal samples are used to interpret the characterization of minerals, including mineral content by proximate analysis, mineral occurrence mode by scanning electron microscope (SEM) and energy-dispersive X-ray spectrometer (EDX) analyses, and mineral compositions by X-ray powder diffraction (XRD) analysis. Low-temperature nitrogen adsorption and high pressure methane adsorption analyses of selected samples are applied to characterize microstructure and gas adsorption capacity.We found that mineral content, occurrence mode, and composition are three controlling factors that together determined the influence of mineral matter on gas sorption capacity. In fact, some factors have potential for both positive and negative influence. This is why both negative and positive influences have been previously observed. The direction and magnitude of influence depends on the relative weights of the driving factors. For samples in this study, clay mineral content showed the strongest positive relation to SBET, total VBJH, and VL, compared to total mineral and brittle mineral content. The relation of other minerals to SBET, total VBJH, and VL is weak. The final result indicated that mineral matter had a positive influence on gas sorption capacity.