The Second Order Reaction Mechanism And Kinetic Process Of Hydrocarbon Formation Of Coal

Thermal simulation experiment has widely been used for the simulation of the petroleum formation of sedimentary organic matters under geological conditions.Based on the time-temperature compensation principle,traditional researches used to apply the laboratory results to geological conditions directly,while the reliability of such methods has been questioned a lot recently.More and more phenomena demonstrate that the hydrocarbon generation process of coal is not simply consisted of a certain number of parallel and independent first order reactions,but a complicated kinetic process including different components interacting each other.While limited to the complexity of the solid phase chemical structures of coal,research progresses on the mechanisms of chemical reactions as well as kinetic principles behind such phenomena are rather slow.As the most important driving force of petroleum formation,researches on the influence of temperature on hydrocarbon formation of coal as well as the kinetic mechanisms of chemical reactions behind should be meaningful to the theoretical research on hydrocarbon formation of coal and the application of pyrolysis results to geological conditions.In this paper,the isothermal semi-open system pyrolysis was adopted to simulate the hydrocarbon generation process of coal under different heating temperatures.The yields as well as components of pyrolysis products have been subjected to detailed analysis to study the associations and differences between hydrocarbon formation process of coal under different temperatures.Afterwards,microscopic fluorescence as well as Fourier transform infrared spectroscopy analyses were conducted on solid phase products to analyze the thermal evolution characteristics of macerals and chemical structures of coal.On this basis,the mechanisms of chemical reactions for different components at different temperatures were subjected to detailed analysis.Thereby,the differences between chemical reaction mechanisms of hydrocarbon formation at different temperatures could be revealed.Combining theoretical analysis,we try to discuss the kinetic mechanisms behind these phenomena.The major conclusions of this paper include the following five aspects:(1)With increasing experimental temperature,a sharp increase of over 25%of the maximum gas formation occurs at 550?.However,the residual hydrocarbon generation potential within pyrolysis residues measured by Rock-Eval demonstrates that the residual potential within the 450-500? pyrolysis products is very limited and far from enough to be the major source of the additi gas formation at 550?.Therefore,it could be deduced that the additional hydrocarbon formation during higher-temperature pyrolysis(>500?)cannot be attributed to the successive thermal degradation of less mature pyrolysis products generated in lower-temperature experiments.Higher order reaction pathways varying with the heating temperature during isothermal pyrolysis should be responsible for this phenomenon.Besides,the compositional and isotopic characteristics of products produced during higher-temperature pyrolysis also show obvious deviations to the evolution laws of such characteristics during lower-temperature pyrolysis.(2)Microscopic as well as microscopic fluorescence analyses on pyrolyzed coal samples demonstrate that the thermal evolution characteristics of different macerals show great differences.The hydrocarbon formation of coal before 325? is dominated by formation of oil products via the depolymerization of exinites.Afterwards,the oil products begin to generate gaseous hydroc though further degradation.The vitrinite and inertinite are relatively stable under low tern conditions.Until 400? the vitrinite is gradually involved in hydrocarbon formation via thermal cracking.As could be seen under the microscopic,massive pore structures appear on the surface of vitrinite due to hydrocarbon formation.With respect to the inertinite,alterations of those structures only turn up when the experimental temperature is higher than 500?.Moreover,the number of structures on the surface of vitrinite also shows large increase at this stage.(3)The thermal evolution characteristics of the contents of functional groups as well as chemical structures have been subjected to detailed FTIR analysis.It has been revealed that the transferable hydrogens needed for the generation of saturated hydrocarbons by aliphatic radicals are mainly derived from two pathways:the aromatization of aliphatic structures or the condensation between aromatic structures.The differences between reaction pathways of hydrocarbon formation at different experimental temperatures mainly reflect on the actual origin of transferable hydrogens.The transferable hydrogens were mainly derived from the aromatization of aliphatic structure during lower temperature pyrolysis and dehydrogenation condensation reactions between aromatic structures during higher temperature pyrolysis.(4)Rock-Eval analysis was conducted on pytolysis residues.It seems that aromatization reactions would cause the formation of structures with higher thermal stability,leading to the shift of bulk petroleum formation rate curves to higher temperature stage.The total hydrocarbon generation potential as well as the aromatization degree of aliphatic structures during the primary isothermal pyrolysis and the following secondary open system pyrolysis have been subjected to detailed calculation and analysis.The total hydrocarbon generation potential has been found to be largely dependent on the aromatization degree of aliphatic structures.Higher aromatization degree will result in higher loss of hydrocarbon generation potential.(5)Combining the analysis results of the gaseous,liquid and solid phase products,the reaction kinetic characteristics of different components of coal were subjected to theoretical analysis.The kinetic mechanisms of the second order reaction mechanism of coal were discussed and applied to study the application of pyrolysis results to geological conditions.