Investigation On The Chemical Structural Evolutionary Model And Its Genesis Of Thermally Altered Coals And Their Maceral Groups

The research on thermally altered coal is beneficial to the safe mining of thermally altered coal,and also helpful to inspire and guide the technology of coal utilization.However,on the one hand,most of the previous studies on thermally altered coal have focused on coal petrology,geochemistry,mineralogy,etc.,while the evolution of its chemical structure is relatively weak;on the other hand,the heterogeneity of thermally altered coal is strong and its composition is extremely complex.Therefore,in this thesis,we aimed to research the thermally altered coals from both vertically and horizontally.The special pathways of the elemental evolution of the thermally altered coals are explained and discussed,and the micro-crystalline structure and chemical structure evolution stages of the thermally altered coals are discussed.The characteristics and causes of the four carbon nano-structure in the thermally altered coal were further revealed.a.The elemental evolution of thermally altered coal has three different evolutionary tracks.Track 1 is similar to the evolutionary track of classical track,i.e.,an arc segment with the hydrogen to carbon atom ratio of about 0.7 and the oxygen to carbon atom ratio of about 0.05 as the inflection point.The slope of this arc segment indicates that the evolution before the inflection point is dominated by deoxygenation,while the evolution after the inflection point is dominated by dehydrogenation.The linear shape of track 2 indicates that the points on track 2 undergo dehydrogenation and deoxygenation processes in a more constant ratio than a two-stage process in which deoxygenation is dominant and then dehydrogenation is dominant.The thermally altered coal samples on track 3 obviously have higher oxygen content,and the track exhibits an evolutionary shape of increasing and then decreasing oxygen content from the starting range to the end range,indicating that the most serious anomalies do not occur in the part near the beginning and end of evolution,but in the middle part of the track.b.The evolution of micro-crystalline structure of thermally altered coal has gone through three stages,from mainly longitudinal stacking to mainly lateral enlargement to both.The first stage is the nucleation stage,in which a large number of nuclei are formed and a large number of small-sized micro-crystalline structures are formed.The second stage is the lateral extension stage,in which the size of micro-crystalline structures in thermally altered coal increases but the number and height of stacking decreases and the layer spacing is enlarged.The third stage is the directional adjustment stage,in which the adjacent micro-crystalline structures are merged into larger-sized micro-crystalline structures by the effect of stacking.c.The chemical structure evolution of the thermally altered coals also shows a three-stage evolution trend.NMR and high-resolution transmission electron microscopy analysis of a series of thermal-altered coals from the Shuoli coal mine shows that the chemical structure of the thermally altered coals undergoes a three-stage evolution of condensation,aromatization,and condensation again,which corresponds to the evolution of the micro-crystalline structure in the thermally altered coals.d.As the samples gradually approach the igneous intrusions,the content of the vitrinite gradually decreases,while the natural coke component gradually increases,and natural coke components such as pyrolytic carbon and mosaic structure can be observed,indicating that the samples are gradually coking.Inertinite does not remain inert all the time,and in the samples near the intrusions,the reflectance of inertinite begins to increase,and the optical anisotropy gradually also increases.The content of liptinite is very low,and the meta-sporinite and meta-cutinite can be occasionally seen.The natural coke component of Shuoli thermally altered coal is mostly found in circular mosaics,with occasional basic anisotropy and pyrolytic carbon,and the content of fine-circular mosaics increases significantly in the early stage,while the content of medium-and coarse-circular mosaics increases in the middle stage,and the content of mosaics does not increase in the later stage.e.After micro-Raman spectroscopy analysis and in situ ultra-thin section-high resolution transmission electron microscopy analysis,it was found that the changes in petrological characteristics are correlated with the chemical structural evolution.The size of the aromatic lamellae increased from SL5-1 to SL5-2 in the vitrinite(and circular mosaics),providing conditions for the formation of mesophase spheres,and thus many fine-circular mosaic structures were formed in this range.While from SL5-2to SL5-4,the main feature of the chemical structure change was the formation of new aromatic structures by aromatization,and the newly formed aromatic structures promote the growth of mesophase spheres,so the increase of fine-circular mosaic structures in the range from SL5-2 to SL5-5 slows down,while the content of medium-and coarse-circular mosaic structures increases significantly in this range.With the aromatization of non-aromatic substances,the system also gradually solidifies,so the increase of mosaic structures was not observed in the subsequent period.f.The fusinite contains more substituents and amorphous carbon,which hinder their anisotropy development.These structures decrease when samples closed to intrusions,which is synchronous with the development of anisotropy and the increase of reflectance.This is also the reason for the anisotropic fusinite observed under the microscope,i.e.,the cross-linked structure in the fusinite was broken and the aromatic structure was reoriented.g.There are four types of carbon nano-structures in thermally altered coal,namely,disordered aromatic cluster structure,aromatic concentric ring structure,more ordered turbostratic carbon structure and ordered graphite-like structure.Among them,the disordered aromatic cluster structure is mostly attributed to the cross-linked structure in inertinite and the still disordered part in vitrinite.The concentric ring structure and the ordered turbostratic carbon structure were believed that the two are homologous,both are the evolution of the mesophase spheres in the mosaic structure.The author believes that the cause of the ordered graphite-like structure is related to the endogenous stress,and the in situ heating experiments show that the presence of surface stress is conducive to the formation of graphite-like structures,and the pore fracture edges,which are prevalent in the thermally altered coals,and the boundaries between macerals will provide conditions for surface stress.