Investigation on chemical structure and hydrocarbon generation potential of lignite in the different pretreatment process

Abstract Combined with the analysis of FT-IR, lignite was treated by conventional flue gas (FTD), hydrothermal (HTD), and tetralin solvent thermal pretreatment (TTD) at different temperatures. A series of infrared parameters were obtained, and the evolution of chemical structure and hydrocarbon generation potential for lignite were studied. The results showed that the HTD, TTD and FTD processes reduce the water holding ability of lignite, and in general, the order of the dewatering effect is HTD > TTD > FTD. The FTD process promotes more efficient the increase of carbon content and energy density, and the upgraded coal likely to be suitable as power fuel. For lignite, the hydrogen-enrichment parameters are not an effective means to predict hydrocarbon generation potential, because the structural characteristics are not dominant during the process of hydrocarbon generation. The aromatization occurred during the FTD, HTD, and TTD processes, and has an important contribution towards enhancing the aromaticity. The order of aromatization of lignite for the three pretreatment processes is FTD > HTD > TTD. Conversely, the upgraded coal achieved by TTD process maintains the relative integrity of organic structure and high reactivity of lignite. HTD and TTD processes improve the H/C atom ratio of lignite, which provides more hydrogen free radicals to produce chemical products. Thus, the upgraded coal is suitable as feedstock of coal pyrolysis and liquefaction. Moreover, the upgraded coal achieved by TTD process retains high reactivity and relative integrity of organic structure compared with the HTD process. Thus, it has a much better chance of producing value-added chemical products.