Pyrolysis Of Zhundong Coal And CO₂ Gasification Of Char With Emphasis On A Lkali Metals Transformation And Mass Transfer Process

Zhundong coal is rich in reserves,with low sulfur content and high reactivity.As a typical high sodium coal,it has unique pyrolysis and gasification properties,among which the migration and transformation of alkali and alkaline earth metals are the difficulties.The reduction of greenhouse gas emissions is related to the future and fate of human living environment.The coal gasification process using CO2 as gasification agent is more economical than the conventional carbon capture technology.Its transfer characteristics directly affect the reaction efficiency and product composition.The pyrolysis and gasification behaviors of Zhundong coal are studied in this thesis.Catalytic pyrolysis of Zhundong coal over three metal chlorides(NaCl,CaCl2,and FeCl3)is investigated and the pyrolysis mechanisms regarding the cleavage of bonds are proposed.It is explored for the effect of pyrolysis atmosphere on the morphology,structure and gasification reactivity of Zhundong coal char,and kinetics are analyzed during catalytic gasification with alkali and alkaline earth;metals.Moreover,the existence of active intermediates and catalytic performance ofNa2CO3 for char gasification are revealed.The gasification characteristics and mass transfer are further discussed in the process,in order to provide basic data support for the development of Zhundong coal gasification technology.Catalytic pyrolysis of Zhundong coal over three metal chlorides(NaCl,CaCl2,and FeCl3)is investigated via a thermogravimetric analyzer and a fixed bed reactor.The structural features of the chars are examined and the pyrolysis mechanisms regarding the cleavage of bonds are proposed.The results demonstrate that all three chlorides have a benefcial effect on the production of CH4/H2 and are ranked according to catalysis ability as follows:NaCl>FeCl3>CaCl2;the opposite effect is observed for the production of CO.The addition of metal chlorides results in a highly rough surface and a low degree of graphitization.Moreover,the sample with added FeCl3 has a more porous and disintegrated structure than those with added NaCl and CaCl2,and the pores and cracks are caused by gas release during the transformation process of FeCl2 to Fe2O3.In terms of the catalytic mechanism,FeCl3 differs from NaCl and CaCl2 in coal pyrolysis.FeCl3 is converted to Fe2O3 to promote the formation of free radicals during the pyrolysis process,while Na/Ca species are involved in a repeated process of bond formation and bond breakage between the species and the coal matrix.Chars derived from Zhundong coal are prepared in a fixed bed reactor under CO2 and N2 atmospheres.Besides,the correlations of structure and gasifcation reactivity of char samples are also investigated.The results reveal that char yield during devolatilization under CO2 atmosphere is relatively lower than that under N2 atmosphere,which could be attributed to heterogeneous reactions of CO2 with tar and char under CO2.The char formed in CO2 atmosphere exposes more pores in char matrix accompanied by the rough surface,resulting in a higher specifc surface than that obtained under N2.Additionally,the char formed under CO2 atmosphere has lower degree of graphitization and higher ratio of small to large aromatic ring systems,compared to that obtained under N2.As a result,the char prepared under CO2 is more reactive.The homogeneous and heterogeneous reactions of CO2 with gases,tar and char occur when CO2 is introduced in the pyrolysis process.Moreover,the introduction of CO2 changes morphology of char,since CO2 participates in reaction process on the char surface,resulting in promoting the gasifcation reactivity.The effects of alkali and alkaline earth metals(AAEMs)on gasification of char derived from Zhundong coal are investigated using a thermo-gravimetric analyzer.Additionally,the characteristics of chars are analyzed by X-ray fluorescence and scanning electron microscopy with energy dispersive spectroscopy.The results show that the solution with 0.1 mol/L sulfuric acid can remove most of AAEMs from Zhundong coal char,and the carbon conversion increases as the temperature increases.The Zhundong coal char possesses a higher gasification reactivity than the acid washing Zhundong coal char.It is indicated that AAEMs have catalytic effect on CO2 gasification of char derived from Zhuandong coal.For the kinetic analysis,the volumetric reaction model reveals a proper description among the three models(volumetric reaction model,random pore model,shrinking core model),and the coal chars present a compensation effect.Besides,the detailed correlation of two chars is lnk0=0.10 EA-1.77(Zhundong coal char)and lnk0=0.10 EA-2.85(the acid washing coal char),respectively.It indicates that AAEMs can change the reaction pathway in gasification process.The catalytic effect of Na2CO3 on the CO2 gasification of Zhundong coal char is analyzed using the thermogravimetric analyzer together with mass spectrometer.The weight loss and gas product release of graphite/Zhundong coal char loaded with Na2CO3 are studied in inert atmosphere,and the migration and transformation process of catalytic precursor is explored.The results indicate that the Na2CO3 induces the promotional effect on the CO2 gasification of the Zhundong coal char.Furthermore,the addition of Na2CO3 contributes to the formation of disordered and amorphous carbon structure.The presence of O-containing functional groups in coal char affects the interaction between Na2OO3 and carbon.These groups are in favor of the stabilization of sodium on the char surface,promoting the formation of active complexes.Na2CO3 reacts with coal char to form complexes containing O and Na.These complexes make it easy for CO2 to attack the char,thereby prompting the gasification reactivity.Mass transfer and reaction characteristics of Zhundong coal char are explored during CO2 gasification.CO2 gasification of Zhundong coal char with different particle sizes in the temperature range of 850?-1000? is studied by thermogravimetric analyzer,and the evolution of coal char structure in the gasification process is tested by means of scanning electron microscopy and physisorption apparatus.The results show that the scale has influence on heat and mass transfer,volatile matter and pore structure during coal pyrolysis.The initial gasification rate of coal char decreases with the increase of particle size.When the particle size is larger and the gasification temperature is higher,the effect of internal diffusion on coal char gasification is more significant.The calculated efficiency factor can evaluate the effect of internal diffusion on the initial gasification rate,but it is not applicable to the whole gasification process.The gasification reactivity of char produced by fast pyrolysis is higher than that of char produced by slow pyrolysis.Coal char with high gasification reactivity is susceptible to internal diffusion effects.