Fe-based Hexaaluminates As Oxygen Carriers For Chemical-looping Dry Reforming Of Methane

Chemical looping dry reforming of methane(CLDR)is a new technology for synthesis generation.Firstly,the methane reacts with the lattice oxygen to obtain for obtaining the high quality synthetic gas with H2/CO molar ratio of 2.Then the CO2 was used to recovery the lattice oxygen,which utilized the greenhouse gas CO2 resource.The oxygen carrier as the key of CLDR,the iron based oxygen carriers are considered to be the ideal active component for their high oxygen storage capacity,abundant reserves and environmental friendliness.The hexaaluminates material has excellent high temperature stability,redox property tunability and fast lattice oxygen diffusion due to their special crystal structure.So it can be used as the skeleton structure for the excellent oxygen carrier.This thesis was based on the Fe-based hexaaluminates oxygen carrier,whose feasibility was studied in the CLDR reaction.Then the effects of oxidation atmosphere,iron substitution and bimetallic substitution on the microstructure and reaction properties of Fe-based hexaaluminate were also studied.At the same time,the oxygen carrier was characterized by XRD,BET,H2-TPR,SEM and 57Fe Mossbauer spectroscopy.It was expected that the correlation between CLDR reaction performance and microstructure of iron-based hexaaluminate oxygen carrier could be established and provide theoretical guidance to the optimal design of oxygen carrier.Compared to the supported Fe/Al2O3 oxygen carrier,the Fe-base hexaaluminates oxygen carrier had more lattice oxygen which could effectively improve the conversion rate of methane,avoid methane cracking and a higher quality of syngas in the product gas.During the cyclic reaction,the skeleton structure of hexaaluminates was kept constant.There was no obvious phase separation and sintering phenomenon,which proved the superiority of the Fe-base hexaaluminates oxygen carrier in the CLDR reaction.The O6-Fe3+(Oh)lattice oxygen couldn’t be recovered in the atmosphere of CO2.And the oxygen vacancy was retained for improving the conversion of methane.The Os-Fe3+(Tr)and O4-Fe3+(Th)lattice oxygen were selectively restored by CO2 atmosphere,which significantly improved the selectivity of syngas and the economy of CO2 utilization.The effect of Fe substitution over the LaFexAl12-xO19 and BaFexAl12-xO19(when x=0～4)Fe-based hexaaluminate oxygen carriers during the CLDR reaction process was investigated.The increase of Fe substitution could effectively promote the formation of intact hexaaluminate crystal structure,and there wasn’t a obvious sintering phenomenon.The reaction performance of oxygen carrier increased with the addition of Fe and then decreased.When x=3,the oxygen carrier exhibited the best comprehensive reaction performance.The modification effect of bimetallic(Ni,Co,Mn)on iron-based hexaaluminate oxygen carriers was investigated.The oxygen carrier showed strong methane decomposition ability when the nickel ion was added to the lattice.For the manganese ion,the performance of oxygen carrier didn’t improve.When the cobalt ions are doped,the conversion rate of methane and the quality of synthetic gas improved greatly.