Study On Petrographic And Geochemical Characteristics Of Coal Pyrolysis And Gasification Products

In this dissertation,the topic of“Study on Petrographic and Geochemical Characteristics of Coal Pyrolysis and Gasification Products”is set up.Based on coal geology,thermal behaviour characteristics of macerals and elements during coal pyrolysis and gasification process were studied using an optical microscope(white light,fluorescent light,cross-polarization with a 1-lambda gypsum plate),a Hamamatsu 928A photomultiplier,Fourier transform infrared spectroscopy(FTIR),solid carbon-13 nuclear magnetic resonance(¹³C NMR)and scanning electron microscopy combined with an energy dispersive detector(SEM-EDX),etc.,with a view to provide scientific guidance and theoretical support for the utilization of coals according to their characteristics and quality.(1)A computer-controlled Leco 701 thermogravimetric analyser was used to perform pyrolysis simulation experiments on the twelve coals varying from lignite to anthracite.Four groups of parameters were selected,as follows:500?/30 min(500?final pyrolysis temperature with 30 min residence time),750?/30 min(750?final pyrolysis temperature with 30 min residence time),950?/30 min(950?final pyrolysis temperature with 30 min residence time),and 950?/2 h(950?final pyrolysis temperature with 2 h residence time);they are intended to represent low-temperature,medium-temperature,and high-temperature pyrolysis,respectively.An optical microscope(assisted cross-polarization with a 1-lambda gypsum plate),and a microphotometer were used to study the petrological characteristics of char/coke affected by coal rank and maceral composition.Char is the pyrolysis product of the non-caking coals,and the carbon forms under the microscope can be observed as four types:mass,block,fusinite,and debris.The carbon forms in char are greatly affected by the maceral composition of the raw coals.The volume percentage of the mass is well correlated with the sum of volume percentages of the densinite/collodetrinite,attrinite/vitrodetrinite and semifusinite in the corresponding raw coal(R is about 0.900),and the volume percentage of the block is well correlated with the sum of volume percentages of the textinite/telinite,ulminite/collotelinite,corpohuminite/corpogelinite and micrinite(r is about 0.900).On the condition of the high-temperature pyrolysis,the volume percentage of the block decreased as the residence time increased from 30 min to 2h,which indicated that the previous macerals of the block react faster compared to the mass.Semicoke and coke are the pyrolysis products of the caking coals.There are pores observed in the pyrolysis cokes,and the carbon forms of the cokes are greatly affected by the coal rank.The anisotropic domain size increased as coal rank increasing.From the medium volatile bituminous coal to the low volatile bituminous coal,the dominated carbon forms of the corresponding cokes changed from the circular to the ribbon.On the condition of high-temperature pyrolysis,the volume percentage of the fine anisotropic domain size decreased as the residence time increased from 30 min to 2h,which indicated that they are more susceptible to reaction compared to the coarse ones.The random reflectance of carbon forms is used to characterize the optical characteristics of pyrolysis char/coke.Samples with low dispersion of random reflectance are mostly optically isotropic under the microscope,and samples with high dispersion are mostly optically anisotropic under the microscope.Under the four pyrolysis conditions,the pyrolysis char of the lignite,sub-bituminous coal C and high volatile bituminous coal C are all optically isotropic;the low-temperature pyrolysis product of the high volatile bituminous coal A is optically isotropic,while the anisotropic began to appear in its medium and high-temperature pyrolysis products;the random reflectance of the pyrolysis coke derived from the medium and the low volatile bituminous coals was greatly discrete,showing strong optical anisotropy under the microscope;The random reflectance of the semianthracite changed the least during the pyrolysis processes compared to others,while the dispersion of the anthracite random reflectance became smaller and smaller as the final pyrolysis temperature and the residence time increased.(2)Fourier transform infrared spectroscopy and solid-state ¹³C nuclear magnetic resonance spectroscopy tests were performed on the twelve coals and the corresponding48 pyrolytic char/cokes to study the coal structure evolution affected by the coal rank and the maceral composition.In general,as the final pyrolysis temperature and the residence time increases,the condensation degree of the aromatic rings in one group of the samples(coal and corresponding pyrolysis char/coke)increases,and the length of the aliphatic chain decreases,but the structure thermal evolution of the coals is hardly affected by the coal rank.For the pyrolysis char with similar random reflectance,there has no similar structures between them.The ¹³C NMR results indicate that the graphitization occurred in the high-temperature pyrolysis char/coke,but CH₂/CH₃,the FTIR structural parameter,indicates that there are still methyl or methylene functional groups existing in the high-temperature pyrolysis products.However,the tendency to break of the two functional groups is different.Compared with the coal rank,the thermal evolution of the coal structure is greatly affected by the maceral composition.During pyrolysis process,coals with the same metamorphic degree but different maceral composition show different structure evolution,the high vitrinite coal resulted in the pyrolysis char/coke having a high degree of the aliphatic chains or the high CH₂/CH₃ value,while the high inertinite coal resulted in the pyrolysis char/coke having a high degree of the aromatic ring condensation.(3)Petrographic and trace element geochemical characteristics of the gasification samples were studied to classify the residual carbon forms in the gasification residues and to analyse the potential utilization of the residues.The feed coals used for the entrained-flow gasification process are high volatile bituminous coal C,after coal gasification,eight carbon forms could be observed in the residues using an optical microscope,they are tenuinetwork,crassinetwork,inertinite,fusinite-like,secretinite,mixed dense,unaltered macerals and crassisphere.The FTIR results of the concentrated residual carbon show that the residual carbon wrapped with the glassy substance of the OMB and the GSP residues has a higher aromatic degree after coal gasification,while the residual carbon adsorbed on the surface of the OMB and GSP residues has some aliphatic chains.The carbon content of the residual carbon concentrated from the OMB-CR is the highest(79.90%,C_d),and the FTIR of it indicates that there exists methyl symmetric stretching vibration.Different potential utilization methods for the different gasification residues are proposed combining the trace element geochemical characteristics of the residues and petrographic characteristics of them.(4)A comprehensive evaluation of the REY in the gasification residues,based on four parameters(outlook coefficient(C_outl),REY_def,rel-C_outl graph,REO,and relative enrichment factor(EF))indicate that the residues could be regarded as promising REY raw materials for economic exploitation,as the REO average contents in the GE-CRA,GSP-CR and GSP-FR were relatively high and the highest one was higher than 600?g/g.Compared to the GSP residues,REY in the GE residues are enriched.The modes of occurrence of the REY in one subset of feed coals and corresponding residues were determined.However,considering the heterogeneous composition of gasification residues and calculated mass balance deviation(MBD)of the REY for chemical extraction,regression analyses(?REY)of selected major elements(Si,Al,Ca,Fe and Mn)were also performed.The results indicate that REY in the GE residues are associated with carbonates and Fe-Mn oxides,but with aluminosilicate as well.In the GSP residues,REY is primarily associated with aluminosilicates.