Structural evolution in middle rank coals based on the characterization of FTIR and carbon isotope of alkane gas
Several coalification jump points exists in coal metamorphism,in which alkane gas formed through thermal degradation and pyrolysis.In order to study the chemical structure evolution characteristics of middle-rank coals,coal seams in Xishan coalfield were collected and the structural evolution was analyzed by Fourier infrared spectroscopy.The results show that carbon isotopes values range from-57.20‰ to-31.41‰,in which ethane isotopes ratio changes weakly than methane.The hydroxyl groups in samples are dominated by self-hoofed hydrogen bonds,and the aromatic hydrocarbon structures are dominated by tri-substituted benzene rings.The aromatic carbon ratio changes weakly while the aromatic ring condensation degree shows a trend of increasing-decreasing-increasing.Aliphatic hydrocarbons are dominated by methylene content,and the branching degree parameter has decreased-increased-decreased during the evolution.Coupling with the characterization of carbon isotopes of methane,structural evolution in middle rank coals included five stages.The first stage(0.65%<Ro<1.10%)was dominated by thermal degradation and decreasing of δ13C1 values.Thermal degradation and polycondensation were cooperated together in the second stage(1.10%<Ro<1.30%)and δ13C1 values became heavy.The third stage was characterized by the polycondensation and formation of many alkene gases.The congruence effect was prepared when the polycondensation ended in the fourth stage(1.50%<Ro<1.80%)and fifth stage(1.80%<Ro<2.0%),in which δ13C1 values were reducing since the adsorption effect of nanopores.The analysis of the structural evolution in middle-rank coals and carbon isotope changes are of great significance for guiding the exploration,development and clean utilization of coalbed methane.
medium rank coalFTIRchemical structurealkane gascarbon isotope
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