Study On Physicochemical Property Evolutions And Upgrading Mechanism Of Low Rank Coal And Biomass During SCCO₂/C₂H₅OH Extraction Process

Low rank coal is abundant in China which accouts for 45.68%of China's coal reserves while the total biomass potential in China is 756 million tons of standard coal equivalence per year.Therefore,exploitation and utilization of low rank coal and biomass would be the inevitable requirement to meet China's energy needs and promote economic development.However,utilization of low rank coal and biomass is limited by its natural defects of high water content and oxygen content,low calorific value,difficulty in transportation and storage.Hence,it is of great significance to realize clean and efficient utilization of low-rank coal and biomass resources through upgrading and modification.In this dissertation,the solvent extraction and upgrading process of low rank coal and biomass is the research object.The drying kinetic of low rank coal by supercritical carbon dioxide(SCCO2)extraction,the physicochemical properties evolution of low rank coal and biomass and the interaction mechanism between low-rank coal and CO2 molecules are studied from the characterization of macroscopic properties,model description of drying process and micro interaction mechanism.The drying kinetic of low rank coal by SCCO2 extraction is described through establishing a model.The effects of extraction temperature and time on physicochemical properties of upgraded lignite were investigated.The physicochemical properties evolution of upgraded lignite and the correlation between different physicochemical property parameters were summarized and the upgradation mechanism of lignite was analyzed and verified by quantum chemical calculation.Finally,the thermal dissolution experiment of low rank coal and biomass were conducted with SCCO2-C2H5OH mixed solvent.The effects of extraction temperature and time,initial CO2 pressure,and ethanol/sample ratio on extraction rate were investigated.The extraction liquid components were analyzed and the solid properties of the residual were characterized.(1)Researches on drying kinetic of low rank coal showed that,SCCO2 as a medium for lignite drying had the advantages of high moisture diffusion coefficient and fast drying rate.The moisture diffiusion coefficients of ZT lignite ranged from 0.9951×10-8to1.6135×10-8m2/s whereas ranged from 0.9499×10-8 to 1.4649×10-8 m2/s for HLBE lignite under 62-102?and 30 L/h flow rate of SCCO2.It was bigger than moisture diffusion coefficients by fluidized bed drying at the same temperatures which applied air and N2.The activation energy of SCCO2 extraction was 11.6218KJ/mol and 10.5561KJ/mol for ZT and HLBE lignite,respectively.It was smaller than other drying method,indicating that the moisture was easier to transfer in SCCO2 extraction process.(2)Physicochemical properties variation of low rank coal during SCCO2 extraction process was characterized.Fixed carbon content of upgraded coal increased and oxygen to carbon atomic ratio(AO/C)of upgraded coals decreased.The pore structure of upgraded lignite was improved,and the improvement of pore structure was proportional to the extraction time and temperature within a certain range.The aromaticity of upgraded lignite increased,the content of oxygen-containing substances in upgraded lignite decreased,and the reactivity of upgraded lignite was greatly enhanced.But little variation of pore structure and reactivity occurred on subbituminous coal,this may be related to the difference in coal rank.(3)The relationship between pore structure parameters,fractal dimension of upgraded low rank coal and extraction conditions,and the relationship between fractal dimension and pore structure parameters,average reaction rate of upgraded low rank coal during SCCO2 extraction were systematically investigated.The mesopore and micropore structures of upgraded HLBE lignite were developed during SCCO2 extraction,while the proportion of macropore structure was reduced.The mesopore and macropore structures of upgraded ZT lignite developed during extraction,and the proportion of macropore structure increased,thismay result from the ash content difference in ZT and HLBE lignite.Fractal dimension D1 value of upgraded HLBE lignite decreased whereas fractal dimension D2 increased,but D1 and D2 values of upgraded ZT lignite both deceased.Dwas positively correlated with average pore size and negatively correlated with specific surface area and pore volume.D2 was negatively related with average pore size and positively related with specific surface area and pore volume.(4)The mechanism of lignite pore structure improvement by SCCO2 extraction was discussed.There were three kinds of interactions between CO2 molecules and lignite model compounds,C…C interaction,C…O interaction and H…O interaction.Among them,C…O interaction was the dominated interaction.The interaction between CO2 molecules and C atoms of benzene ring in lignite model compound was a force of bond strength.The interaction between CO2 molecules and oxygen-containing functional groups(carboxyl,carbonyl,hydroxyl,and ether-oxygen bonds),hydrogen atoms was a force of hydrogen bonds and dispersion interaction.The order of interaction strength between oxygen-containing functional groups and CO2 molecule was shown as follows:alcohol hydroxyl>carboxyl=carbonyl=ether oxygen bond.(5)The effect of material properties on supercritical CO2 extraction was investigated.With the decrease of volatile matter content,the effect of supercritical CO2 extraction will be weakened,and the suitable extraction temperature will be decreased,that is,lignite extraction and upgrading effect>sub-bituminous coal extraction and upgrading effect>bituminous coal extraction and upgrading effect>anthracite extraction and upgrading effect;Meanwhile,the suitable extraction temperature of petroleum coke and anthracite<suitable extraction temperature of bituminous coal<suitable extraction temperature of sub-bituminous coal<suitable extraction temperature of lignite.(6)Thermal dissolution experiments of sawdust,straw,green algae and HLBE lignite were carried out with SCCO2-C2H5OH mixed solvent.Results showed that,higher extraction temperature and initial CO2 pressure,longer extraction time,bigger ethanol/sample ratio,and smaller particle size of materials would increase the extraction yield.Among them,extraction temperature was the most important factor affecting extraction yield.Extract liquid of sawdust,straw,green algae and lignite contains abundant esters and phenolic substances.Volatiles content and oxygen-containing substance decreased while fixed carbon content,HHV value and aromatic substances content increased for biomass and lignite extraction products after thermal dissolution.The Carbon microcrystalline structure of biomass generated dxuring thermal dissolution process and its carbon structure order increased.Thermogravimetric analysis results showed that,the thermal stability of biomass extraction products was enhanced,its TG curve moved towards high temperature zone and its gasification reactivity decreased compared with raw biomass.Gasification reactivity of anthracite and petroleum coke was greatly enhanced by mixing biomass extraction products with anthracite and petroleum coke.