Late Permian bark coals and barkinite:A review of structure and reaction

Since typical bark coal in Leping,Jiangxi province was first reported by Chinese scholar Xie Jiarong,bark coal and barkinite has attracted widely attention because of its special chemical properties. Typical bark coals were mainly found in the late Permian of Southern China. Some important research progress has been made on the chemical properties of bark coal,however,there is lack of systematically researches on its structural characteristics and thermal reactivity,which directly affect the rational use of bark coal. In this paper,by combining a large number of previous references and the research data from our group,the structure and thermal reactivity of bark coal and barkinite were carefully summarized and analyzed. Chemical compositions,organic geochemical characteristics,chemical structure characteristics,and thermal reactivity of bark coal and barkinite were discussed significantly. The definition of bark coal has been re-analyzed,and the relationship between chemical structure of bark coal and plants precursors were presumably established,instructing that bark coal has at least four special properties in elemental composition:high hydrogen content,high H/C atomic ratio,high volatile matter,and high sulfur content. The structure of bark coal mainly discussed the physical structure and chemical structure,instructing that the most significant chemical structural feature of bark coal is rich in aliphatic structures. The morphological textures of barkinite evolve from fibrous structure to irregular network structure as the Ro increase from 0.67% to 1.12%,and the orientation arrangement seems to be enhanced with more organized microscopic structures,similar with the structural evolution characteristics of vitrinite. Bark coal and barkinite has some abnormal thermal behaviors:intense thermal decomposition,super-high Giesler fluidities behavior,high hydrocarbon-generated abilities,and high liquefaction performance. Finally,there are some suggestions for the further utilization of bark coal,and the problems that need to be solved in the next study of bark coal and barkinite are also pointed out.