Evolution Charateristics Of Chemical Structure Of Medium-rank Coal And Its Controlling Mechanism On Micropores

CBM resources are rich in Shanxi province,accounting for about one third of the country,of which about 45%are of medium-rank coal.However,the reform and drainage of coal reservoir have become urgent problems to be solved,owing to the low CBM output and quick decline,low efficiency during CBM well production.Therefore,the understanding of the physical and chemical structures of medium-rank coals and their coupling relationship from the molecular scale can provide a theoretical basis for the development and drainage of CBM.In this paper,the Fourier transform infrared（FTIR）,solid carbon nuclear magnetic resonance(¹³C-NMR),X-ray diffraction（XRD）,micro-Raman and high-resolution transmission electron microscopy（HRTEM）were used to analyze the evolution of molecular structure and aggregate structure of medium-rank coals(R_o,max=0.68%～1.98%)from different coalfields in Shanxi province.In addition,low-temperature liquid nitrogen adsorption and carbon dioxide adsorption were applied to obtain the characteristic of mesopores and micropores.The relationship between structure evolution and micropores was analyzed in detail,and the control mechanism of micropores by microscopic molecular structure and aggregation structure was revealed.The following research results were obtained:（1）The molecular structure evolution of medium-rank coals has a consecutiveness and abrupt.The results of FTIR showed that the aromaticity I₁,I₂and condensation DOC₂increase with increasing R_o,maxfrom 0.68%to 1.20%.The long aliphatic chain and aliphatic ring formed,and the carbonyl structure also decreased rapidly.When R_o,maxvaried from1.20%to 1.50%,I₂and DOC₂increased slightly,and reached a large value at R_o,max=1.50%.The aliphatic structure was first enriched and then decreased with increasing R_o,max.The breaking or aromatization of aliphatic substances between the aromatic structure resulted in the strengthening of branched chain degree.In the stage of R_o,max>1.50%,the loss of the methyl led to the weakening of the degree of branched chain,the variation of aromatic structural parameters is consistent with the first stage,and condensation of aromatic structure increase in this stage.The structure evolution of ¹³C-NMR results was consistent with that of FTIR:In stage of R_o,max=0.68%～1.20%,the shedding of aromatic alkyl structure resulted in the rapid increase of aromatic carbon and protonated aryl carbon,which showed in the parameters of f_a^S,f_al,f_al^Hand f_al*decreased,f_aand f_a^Hincreased.At stage of R_o,max=1.20%～1.50%,the shedding of aromatic alkyl structure still caused the increase of aromatic carbon and protonated aryl carbon,but the increase rate was slower than the previous stage.When R_o,maxranged from 1.50%to 1.98%,the shedding of aliphatic structure slows down,and the increase in aromatic degree is mainly related to the enrichment of aromatic bridge carbon,and the condensation of aromatic increases in coals.Around R_o,max=1.20%and R_o,max=1.50%,the structural parameters abrupted in response to the second and third coalification jumps,respectively.The second jump was related to the enrichment aliphatic structure,and the third jump was mainly the result of enhanced condensation.（2）There are three main stages in the change of aggregate structure in medium-rank coals.When R_o,maxranged from 0.68%to 1.30%,L_c,L_a,G-D and A_D/A_Gincrease sharply and the content of short fringes decrease,the content of intermediate fringes and long fringes increase.The shedding of oxygen-containing functional groups and aliphatic structure in this process,which provides new active sites,leading to the growth of short fringes to intermediate fringes,and the increase of interlayer spacing,stacking of aromatic layer and degree of order.In addition,coal sample with R_o,max=1.16%has the more long aliphatic chains or aliphicyclic substances,which leads to steric hindrance and the temporary increase in d₀₀₂.With R_o,maxranging from 1.30%to 1.50%,d₀₀₂,L_c,L_a,G-D and A_D/A_Gdecreased,the content of short fringe increased,and the content of intermediate fringe and long fringe decreased.At this stage,the cross-linked structure between large aromatic rings broke to form short fringes.The existence of these short fringes hindered the stacking and growth of aromatic layers,and the degree of order decreased accordingly.In the third stage(R_o,max>1.50%),due to the enhancement of condensation,the content of more ordered rings increases,the stacking and size of aromatic layer increases,and the degree of order increases,which is shown in the rapid increase of G-D,L_cand L_a,the decrease in G-FWHM and d₀₀₂,and the increase in intermediate fringes and long fringe content.Among them,the turning points of L_cand L_aare around R_o,max=1.30%lagged behind 1.20%of d₀₀₂,which is related to the second coalification jump.During the jump process,the steric hindrance adjustment first causes the change of layer spacing and order degree,and then the aromatic structure begins to grow.The transition of R_o,max=1.50%is in response to the third coalification jump,which is related to the formation of short fringes by the breaking of the cross-linked structure.（3）The molecular structure of coal controls micropores mainly in three stages:In first stage(R_o,max=0.68%～1.30%),the micropore volume decreases slightly,and H_al/H,f_al,f_al^Hand f_al*decreased rapidly.The transition from aliphatic structure to aromatic structure leads to the decrease of micropore volume in coal macromolecular structure network.The coal samples near R_o,max=1.20%have lower micropore volume,which is related to more aliphatic chain and the blocking of asphaltene At the stage of R_o,max=1.30%～1.50%,the micropore volume continues to decrease,and there is no obvious relationship between the micropore volume and the molecular structure parameters of coal samples in this stage.The DQ coal sample with R_o,max=1.50%has a very low micropore volume,which is related to the blocking of the highest one adjacent hydrogen per aromatic ring.When R_o,maxis larger than 1.50%,the micropores volume is positively correlated with aromaticity,five adjacent hydrogen per aromatic ring,hydroxy-πhydrogen bond and f_a^H,indicating that the increase in aromatic structure leads to micropores in coal,and micropores are formed along with the edge of protonated aromatic carbon tructure.（4）The effect of aggregate structure on micropore volume main depends on the arrangement of aromatic structure.When R_o,maxis smaller than 1.30%,the micropore volume decreased with the increase of L_aand A_D/A_G,and the orientation of fringe gradually improved,that is,the fringe growth process resulted in the decrease of 0.4～0.5nm and>0.8nm micropore.With R_o,maxranging from 1.30%to 1.50%,L_a,L_cand A_D/A_Gdecreased,and the content of short fringes increased.Meanwhile,these new short fringes filled the micropore space and caused the rapid decline of micropore volume.In the last stage(R_o,max=1.50%～1.98%),the short fringes continued to grow to the intermediate fringes and long fringes,L_aand L_cincreased rapidly,and the orientation of the fringe increased,indicating that the system space would be readjust during the process of fringe growth,providing more 0.4～0.5nm and0.5～0.8nm micropore space,and also providing growth space for the next fringe growth.（5）The size of micropores extracted from HRTEM lattice fringe images ranged from 0.4nm to 1.2nm,and micropores>0.8nm are always found in the regions without the basic structural units（BSUs）in lower-rank coals.The micropores in coal are mainly0.5～0.8nm,which appear near the intermediate fringes and increase with increasing coal rank.The compatibility of HRTEM and CO₂adsorption methods for micropores with size ranging from 0.5～0.8nm.Micropores of 0.4～0.5nm are mostly developed at regions stacked with long fringes in the higher-rank coals,and its content increases rapidly when R_o,max>1.30%.