Properties of lignite and key factors determining the methane adsorption capacity of lignite: New insights into the effects of interlayer spacing on adsorption capacity

Abstract

The lignite reservoir and related parameters that determine the adsorption capacity remain largely unexplored and not understood. The coal composition, pore characteristics, mineral matter, coal rank, and molecular structure of lignite were systematically studied, and the influence of various factors on the adsorption capacity was analyzed. Lignite samples were separated into bright and dull types by handpicking. The adsorption capacity of dull coal is always higher than that of bright coal. Based on this fact, the differences between types of lignite were further clarified to determine the effects of various factors on the adsorption capacity of lignite. The differences in the lithotypes of lignite are related to the maceral composition and mineral composition. The adsorption capacity of lignite was significantly affected by the maceral composition. The adsorption capacity increased with an increase in the huminite content. The difference in the adsorption capacities of huminite and inertinite may be attributed to differences in the chemical structures. The interlayer spacing of huminite is larger than that of inertinite at lower rank levels. The large interlayer spacing in lignite may provide considerable storage space for methane molecules. Moreover, the effect of the dilution of mineral matter on the adsorption capacity of lignite is less significant than that of medium-high-rank coal. Clay minerals in lignite can also contribute a nonnegligible adsorption capacity, especially for smectite, whose adsorption capacity is even higher than that of the organic matter in lignite.