Experimental Study On The Evolution Of Char Structure And Reactivity During Coal Catalytic Hydrogasfication Under High Pressure

Coal-fired power generation is one of the major sources of pollutant and carbon dioxide emissions.The near-zero emission technology is a feasible solution for solving the environmental pollution from coal combustion.Physical-chemical properties of coal can directly affect the design and stable operation of the hydrogasifier.Therorfore,it is necessary to study the evolution characteristics of structure and reactivity of coal during hydrogasification for improving our understanding of the mechanistic aspects of hydrogasification and quantifying the kinetics of specific reaction processes for wider application.Firstly,Raman statistical analysis method was developed for analyzing the carbon skeleton structure.The difference between Fourier Raman spectroscopy(FT-Raman)and Confocal Raman spectroscopy(Confocal-Raman)for determining the carbon skeleton structure of char in hydrogasification process were analyzed.Results showed that compared with Confocal-Raman,the laser wavelength of FT-Raman was longer,and the signal noise ratio(SNR)of FT-Raman spectra obtained was much lower.Signal of FT-Raman was selective for different char samples.The FT-Raman SNR of a higher degree graphitization char samples obtained was higher than lower graphitization degree samples.For FT-Raman experiment,one test needed thousand number of scans for improving SNR.Confocal Raman has no selectivity for different char samples,the SNR has nothing to do with the graphitization degree of sample.The results of Raman and XRD correlation analysis showed that the Confocal-Raman based on Raman statistical method was more superior in the measurement of carbon skeleton of char compared with FT-Raman.Secondly,the hydrogasification of chars produced from two different pyrolysis heating rate was conducted in temperature up to 850 ℃ and in the pressure of 5 MPa using a high pressure fixed bed reactor.The evolution characteristics of char structure and reactivity during hydrogasification process were investigated.The reaction parameters on hydrogasification reaction rate were also analyzed.The kinetics of coal char under high hydrogen pressure hydrogen was determined based on the L-H mechanism.The experimental results showed that the char hydrogasification process could be divided into two stages: an initial stage with sharp decline in hydrogasification rate and a stable stage with slower decline one.The sharp decline in the reaction rate was attributed to the release of functional groups remaining in the char,and the slow reduction in the reaction rate in the latter stage was associated with the slower reaction of the residual aromatic carbon and hydrogen.The hydrogasification reactivity was proportional to the hydrogen pressure.The development of pore structure during char hydrogasification can be classified into three stages: micropore formation,pore coalescence,and pore collapse.Good linear correlations were found between the hydrogasification reaction rate and the skeleton structure of char.The amorphous carbon content in char were gradually reduced and graphitization degree were increased as hydrogasification proceed,which led to the gradual decrease in reaction rate of the char during hydrogasification.The pore structure of the rapid pyrolyzed char was relatively developed,the active groups and the amorphous carbon content in char were higher than that of the slow pyrolyzed char,and thus the hydrogasification reaction rate of rapid pyrolyzed char in the first stage was the higher than slow pyrolyzed char.Thirdly,the catalytic hydrogasification reactivity of coal char was investigated under high pressure(5 MPa)using alkali metal and alkaline earth metal(AAEM)compounds as catalysts.Analysis of char structural evolution was performed to further explore the catalytic mechanism.The results showed that the CaO,CaCO3,Na2CO3,and K2CO3 catalysts obviously promoted the hydrogasification reactivity of char and the order of catalytic efficiency was K2CO3 > Na2CO3 > Ca O > CaCO3.The catalytic reaction of CaO only occurred on the char surface.CaO catalyst inhibited the aromatic ring condensation of the char structure during hydrogasification.The alkali catalysts changed the char hydrogasification pathways.Na2CO3 or K2CO3 was reduced to metallic Na or K and diffused into the char under hydrogen atmosphere,enhancing the cracking of large aromatic ring systems into small ones,which were the active species for hydrogasification.Compared with alkali earth catalyst,alkali metal catalyst has good mobility.The catalytic reaction of alkali catalyst occurred simultaneously on the surface of and inside the char.Therefore,the Na2CO3 and K2CO3 displayed higher catalytic activities than CaO in hydrogasification.Finally,the method of gas chromatography combined mass spectrometer(GC-MS)technology was established to precisely analyze the composition of the complex hydrocarbon gas formed in coal hydrogasification process.The effects of alkali metals and alkaline earth metals on the gas formation characteristics in coal hydrogasification were investigated.Calcium-based additives promoted the secondary cracking of tar,thus resulting in the increase in gas formation.The catalytic effect of CaO on coal hydropyrolysis under high pressure conditions was not obvious.CaO promoted the char hydrogasification reaction.Alkali catalyst can enhanced the decomposition and polycondensation of coal macromolecular structure.The catalytic reaction of alkali catalyst occurred both in hydropyrolysis and char hydrogasification process.The amount of methane formed can be significantly increase as the addition amount of alkali metal catalyst increases.