Structural Evolution And Hydrocarbon Generation Mechanism Of Hydrogen-rich Coal

Coal has a strong heterogeneity,it is very difficult to clarify its physical and chemical structure.Based on a large number of relevant data at home and abroad,this paper studies residue of different temperatures obtained by thermal simulation experiments of hydrogen-rich coal.The scanning electron microscope,small-angle X-ray scattering and high-resolution transmission electron microscopy are used to study the morphology and distribution characteristics of micro-nano pores in hydrogen-rich coal?the characteristics of macromolecular stacking structure?difference of chemical structure of microscopic components and the infrared spectroscopy parameters of vitrinite changing with coal rank.Finally,the structural evolution model of hydrogen-rich coal was constructed by combining the thermal simulation liquid product evolution curve,and the hydrocarbon generation mechanism of hydrogen-rich coal was explained.The results are as follows:1.Thermal simulation experiments show that hydrogen-rich coal has a very high hydrocarbon generation potential.400? is the boundary line of total hydrocarbon yield change.The total hydrocarbon yield change before 400? is consistent with liquid hydrocarbons yield,and the total hydrocarbon yield change at 400-600? is consistent with gaseous hydrocarbons yield.Before 300?,liquid hydrocarbon yield gradually rises and reach the peak of oil production at 300?.In this stage,the liptinite are pyrolyzed to produce oil;at 300-400?,the yields of liquid hydrocarbons and total hydrocarbons decrease rapidly,while the yields of gas hydrocarbons rises slowly,and liquid hydrocarbons cyclize into aromatic transition compound.At 400-450?:gas hydrocarbons yield rise rapidly,aromatic transition compound pyrolyzes into gas.At 450-550?:gaseous hydrocarbons yield rises again rapidly,and the aromatic ring of the vitrinite condenses and demethylate,leading to gas generating.2.Under the scanning electron microscopy,the original sample's structure is relatively dense,and a small number of primary pores are developed.The primary pores are mainly formed by dehydration and condensation caused by external forces;at 250?,the lipitinite pyrolyzes,causing a large number of nanopores;at 300?,the lipitinite is mostly pyrolyzed and nanopores gradually enlarge to macropores,while the vitrinite develops a small number of nanopores;450?:the vitrinite forms a large number of nanopores due to hydrocarbon generation.The small-angle scattering of the residue of hydrogen-rich coal at different temperature shows that the proportion of fine mesopores is the largest in the pores of 0-60nm,and the volume percentage accounts for 45.51%?66.96%.As the temperature of the thermal simulation increases,the maximum volume pore size and number of micropores increase gradually,and the growth speed is increasing.The number of fine mesopores and intermediate pores gradually decrease,and the growth speed is increasing.After 450?,the rapid change of the maximum volume pore size,the percentage of fine mesopores and mesopores is related to the polycondensation of hydrogen-rich coal.3.The size of the aromatic ring of hydrogen-rich coal increases with the increase of thermal simulation temperature:At 250?(Ro=0.68),coal mainly consists of naphthalene ring and 2*2 size aromatic ring.At 600?(Ro=2.88),the content of naphthalene ring is almost zero,while 3*3 and 4*4 size aromatic rings become the main structure of coal.The change of the aromatic ring with the thermal simulation temperature is mainly divided into two stages.The first condensation stage:250?-400?,the naphthalene ring decreases sharply,while the 2*2,3*3 size aromatic ring increases sharply;in this stage the naphthalene mainly condense to 2*2,3*3 type aromatic ring of larger size;the second condensation stage:400?-600?,when the 2*2 size aromatic ring decreases sharply,the aromatic ring of 3*3 or more increases sharply.Big.Small-sized aromatic rings are gradually condensed into large-sized aromatic rings.The direction of aromatic streak of hydrogen-rich coal changes from disorder to order with the increase of thermal simulation temperature.The interlayer spacing of aromatic stripe decreases with the increase of thermal simulation temperature,and the decrease of layer spacing can also be divided into two stages.350-400?:the layer spacing is sharply reduced with strong condensation,which is related to the detachment of aliphatic side chains of vitrinite,liquid hydrocarbon cyclization and aromatization that forms transition state substance;450-550?:the layer spacing decreases and is related to the vitrinite's condensation which results in demethylation and gaseous hydrocarbons..4.The liptinite mainly composed of long aliphatic chain and has great potential for hydrocarbon generation.The main structure of the inertinite is aromatic ring,which has the weakest hydrocarbon generation potential;the hydrocarbon generating potential of vitrinite is between the chitin and the inertinite.Aliphatic hydrogen reduces rapidly at 300-400?.In this stage the aliphatic side chains of vitrinite are detached,generating gaseous hydrocarbons..The aromatic hydrogen content increased at first and then decreased.The increase of aromatic hydrogen was related to the pyrolyzes of aliphatic side chains of vitrinite,which generated gaseous hydrocarbons.The reduction of aromatic hydrogen was related to the intense polycondensation reaction of the vitrinite at high temperature.The aromaticity and condensation degree of the vitrinite measured by infrared parameters increased with the increase of temperature.5.The evolution of hydrogen-rich coal structure is mainly divided into five stages:?before 300?,there is almost no change in the content of aliphatic hydrogen and aromatic hydrogen in the vitrinite,and the fluorescence disappears as temperature rises,while a large number of pores develop in the liptinite.?300-400?,the aromatic hydrogen of the vitrinite sharply decrease while its aliphatic side chain pyrolyzed into gas,the content of aromatic hydrogen increase rapidly,the interlayer spacing decrease rapidly with strong polycondensation,and the liquid hydrocarbons rapidly decrease by forming transition state substance in cyclization or aromatization reactions.?400-450?,there is almost no change in vitrinite's aliphatic hydrogen?aromatic hydrogen and interlayer spacing.At this stage,the thermal decomposition of transition materials mainly produces gaseous hydrocarbon.?450-550?,the content of aliphatic hydrogen is almost 0,and the aromatic hydrogen decrease to the minimum amount rapidly,the layer spacing also reduce rapidly.the polycondensation effect is obvious at this stage,and the gaseous hydrocarbon productivity index is once again rising rapidly.This stage is mainly caused by a large amount of polycondensation reaction in the vitrinite which is demethylated and generating methane.?550-600?,the morphological group has almost zero aliphatic hydrogen content,and the vitrinite has almost no change in aromatic hydrogen content,gas-liquid hydrocarbons and total hydrocarbon yield.At this stage,the chitin and the vitrinite in the hydrogen-rich coal are basically cracked completely.