| The reserves of natural gas are abundant,and the CO2 emission in the process of natural gas utilization is lower compared with coal and oil.The utilization of natural gas can alleviate the exhaustion of coal and oil and the greenhouse effect.Methane is the main component of natural gas.The reforming of methane converts methane into high-quality syngas（the mixture of CO and H2,and H2/CO is 2）,which is an high-efficiency and low-energy approach of methane utilization.Compared with other methane reforming processes,chemical looping methane reforming has obvious advantages.Chemical looping methane reforming（CLMR）includes chemical looping steam methane reforming（CLSMR）,chemical looping dry reforming of methane（CLDRM）and chemical looping partial oxidation of methane（CLPOM）.In this work,lanthanum-manganese perovskite oxygen carrier was obtained by means of A and B site substitution and surface modification of LaMn O3+δperovskite,and then their performance and mechanism of CLMR were investigated.The LaMn O3+δperovskites with doping of Fe,Co and Ni were used as oxygen carriers of CLSMR.The tests results suggested that the introduction of Fe,Co and Ni increases surface active sites of methane activation,improving the syngas generation rate and oxygen release rate.The promoting effect increases with the doping proportion,and follows the order Ni>Co>Fe.The CLSMR performance of LaMn O3+δperovskites can be improved by the double adjustment of Co and Sr doping.The doping of Co provides active sites for methane activation on the surface of oxygen carrier particles,which improves the reactivity and methane conversion,but promotes carbon deposition.On the other hand,the doping of Sr increased the oxygen vacancies and enhanced the oxygen anion migration rate in the bulk of oxygen carrier particles.As a result,the carbon deposition is inhibited,which delays the occurrence of carbon formation,lengthens the methane reforming period,and increases the yield of effective syngas.The appropriate doping proportion of Co and Sr is beneficial to the matching of the oxygen anion conduction of in the bulk and the methane activation on the surface of the perovskite particles,so that the oxygen supply meets the oxygen demand.The substitution proportion of Co and Sr could be set in range of 0.4-0.5 and 0.2-0.4,respectively.Ni doped LaMn O3+δoxygen carriers were used as oxygen carriers for CLPOM.The introduction of Ni greatly enhances the release of the selective oxygen and the generation rate of syngas,leading to high syngas yield,methane conversion and syngas selectivity.Even at lower temperatures,Ni doped LaMn O3+δoxygen carriers also exhibited excellent performance compared with other oxygen carriers.In addition,superior stability and cyclic performance were obtained in the 20-cycles successive redox tests with Ni doped LaMn O3+δoxygen carriers.Al doped LaMn O3+δperovskites was were used as oxygen carriers for CLSMR.The incorporation of Al ions into LaMn O3+δlattice can stabilize the perovskite structure and improve the symmetry of crystal structure.This weakens the bond energy of Mn-O bond and the binding to lattice oxygen,promotes the formation of oxygen vacancy,and increases the release rate of selective oxygen and the syngas yield.No carbon deposition occurred in the reduction stage when the Al doping proportion is low,and pure hydrogen can be produced in the oxidation stage.The CLSMR performance of LaMn0.7Co0.3O3+δoxygen carrier can be improved by modification of LaMn1-yAlyO3+δ.The modification of LaMn1-yAlyO3+δdecreases the release rate and amount of unselective oxygen,increasing CO selectivity.The 5A/C30ba oxygen carrier with 5%modification of LaMn1-yAlyO3+δpromoted the release rate of selective oxygen.The modification of LaMn1-yAlyO3+δsignificantly enhanced the resistance towards carbon deposition of oxygen carriers,which decreases the carbon deposition rate and amount,delays the time of carbon deposition,and extends the duration of methane reforming stage in the reduction stage. |
PDF Full Text Request