Study On Negative Pressure Drying And Rare Earth Catalytic Pyrolysis Of Low Rank Coal And Respective Mechanism

At present,there are some challenges in regards of the pyrolysis of Low-rank coal(LRC)under medium/low temperature.For instance,the low tar yield,high heavy components production,oil/dust separation,and pipeline clogging are all problems need to be solved,and metal catalytic pyrolysis is considered to be one of the methods to solve these problems.Otherwise,in order to prepare coal for pyrolysis,drying is a necessary procedure,which also needs to be optimized in terms of the low heat transformation efficiency,high dust explosion and other shortcomings in traditional drying technologies.Considering the literatures related to the catalytic pyrolysis of LRC and the rare earth feature,this thesis proposes an alternative method to solve the above challenges for LRC utilization with drying under negative pressure and catalytic paralysis with rare earth as catalyst for LRC utilization.Drying experiment under negative pressure was conducted in the vacuum drying oven with Hongce long-flame coal as the selected coal sample.The influence of the drying temperature and pressure on the LRC drying behavior,shrinkage,fragmentation and subsequent pyrolysis were studied,and the negative pressure drying mechanism was discussed in this paper.CeO2 and La2O3 were respectively selected as catalysts for LRC pyrolysis and the influence of catalyst on coal pyrolysis were analyzed in terms of the yield of products and their respective characteristics.In order to insight the mechanism,the statistical constitution analysis method,quantitative XRD,Raman,FTIR and GC were used to insight the microcrystalline structure and functional groups of the semicoke,and the escape rule of each component of the pyrolysis gas,the temperature programmed pyrolysis was implemented in a thermogravimetric analyzer in order to clear and definite the influence of rare earth oxide on coal pyrolysis reactivity and the apparent activation energy.Finally combine with coal pyrolysis mechanism reported in the literature and the characteristics of the rare earth oxides to speculate the catalytic pyrolysis mechanism of rare earth oxides.The research results of LRC drying test under negative pressure showed that:with lower drying pressure and higher temperature,the greater the instantaneous dehydration rate and its extreme value is,the final moisture content will be smaller,The Page model can be used to describe the negative pressure drying process;The volume shrinkage rate has a strong positive correlation with coal fragmentation rate,volume shrinkage is an important factor causing coal fracture;After drying under negative pressure,the specific surface area,the surface fractal dimension and the number of the small hole is increased,infrared spectrum peaks remain the same,the pyrolytic weight loss increased by 0.2%,negative pressure drying is conducive to the follow-up pyrolysis process.Reasoning based on the radical ball physics model combining the Kelvin formula,Fick's first law of diffusion,Knudsen's diffusion law and transition diffusion region proves that the removal mechanism of the internal water is "evaporation-water vapor diffusion”.The experiment results of tube furnace pyrolysis show that CeO2 intensifies the coal pyrolysis,reduces the carbocoal yield and increases the gas and the tar yield;while La2O3 increase the yields of the semicoke and gas,but reduces the tar yield.CeO2 makes the tar heavy,conversely,La2O3 makes it light.Considering the composition of the tar,pyrolysis of the coal sample with La2O3 under 550? is optimum,the light component of the tar accounts for 54.96%with tar yield at 4.725%as the lowest,the gas heating value is 14.44 MJ/kg.Considering the tar yield,the optimum condition is 550?with CeO2 as catalyst.Specifically,the total tar yield is 5.94%with light component at 36.65%as the lowest,the gas calorific value is 18.52 MJ/kg.Synthetically considering the yield and composition,the pyrolysis of the coal sample without rare earth under 550? is the best with tar yield 5.385%and proportion of light component at 47.54%,the gas calorific value was 12.23 MJ/kg.The better was CeO2 catalytic pyrolysis under 600? with tar yield 5.485%and light component accounted for 5.485%,the gas calorific value was 14.87 MJ/kg.The research results of catalytic pyrolysis mechanism with rare earth as catalyst show that CeO2 promotes the aromatization and graphitization of the carbocoal.Otherwise,La2O3 hinders the aromatization and graphitization of carbocoal.The proposed catalytic intensity index s was appropriate to display the catalysis strength of the catalyst under arbitrary temperature.CeO2 can catalyze coal pyrolysis in the range of 200? to 855? with best performance at about 460?,and can reduce the apparent activation energy of pyrolysis.La2O3 can catalyze coal pyrolysis in the range of 304?to 334? but hinder pyrolysis under other temperature,CeO2 can catalyze the bond-breaking of side chain in addition to promoting the rupture of bridge bonds between the basic structural unit of coal,it may be associated with the oxygen storage/release capacity of CeO2.La2O(CO3)2 not La2O3 works in coal pyrolysis,which promotes the removal of side chain of aromatic structure especially the carboxyl to produce more gas including CO2.The action mechanism of La2O3 may be that:La2O3 is first converted into La2O(CO3)2 under lower temperature followed by La(III)chelating with carboxyl and hydroxyl in coal,then the free radical fragments crosslink each other with La(?)as the link point under higher temperature,get bigger,so they can not be volatilized as tar or gas.In addition,La2O3 promotes the generation of H2,C2H2 and C2H4 in 350?-450?,this implies that La2O(CO3)2 catalyzes dehydrogenation reaction of alkane and olefin under low temperature.This study enriches the theory and research methods for coal drying and coal catalytic pyrolysis,and it is a reference for the further research of conventional pyrolysis,catalytic pyrolysis,catalytic gasification and catalytic combustion.