Fe-based Hexaaluminates As Oxygen Carriers For Methane Chemical-looping System

The chemical-looping combustion of methane?CLC?is a new type technology with low-energy and no CO₂ emission,and the chemical-looping dry reforming of methane?CLDR?technology is based on the CLC technology and uses greenhouse gas CO₂ for producing high quality synthesis gas.Both the CLC and CLDR systems include a reduction reactor and an oxidation reactor.The circulation of the oxygen carrier between two reactors is the key to the operation of the above systems.The iron is considered to be an ideal active component because of their abundance,low cost,and low toxicity.Hexaluminate is considered as one of the most promising high-temperature catalytic materials due to its unique layered crystal structure.In this dissertation,based on the hexaaluminate,Sn-substituted hexaferrate and different metal ions substitued iron-based hexaaluminate were prepared by adjusting the amount of iron substituted and substitution of different metal ions,and their performance in CLC and CLDR systems was investigated.The oxygen carriers were characterized by XRD,SEM,TEM,et al.It is expected to establish the correlation between CLC or CLDR reaction performance and microstructure of iron-based hexaaluminate oxygen carriers to provide theoretical guidance for the design of oxygen carriers.Based on BaFe₁₂O₁₉ hexaferrite,the effect of the amount of Sn substitution on the structure and the performance of oxygen carriers was investigated.It was found that Sn substituted samples also formed the MP-type hexaaluminate structure.Sn⁴⁺ entered into the hexaaluminate lattice to compensate for the lack of positive charge,and inhibited the formation of the BaFe₂O₄ spinel.In addition,Sn-substituted samples exhibited higher CH₄ conversion,CO₂ selectivity,and the actual amount of converted oxygen during the redox cycle,and with the increase of the amount of Sn substitution,the trend of the CLC performance increasing with the increase of the number of cycles became more and more obvious.For Ba Fe11.4Sn0.6O19 sample,the CH₄ conversion and the actual amount of converted oxygen during redox cycles increased fastest,and CO₂ selectivity remained at 100% during the whole redox process.Investigate the effect of different calcination temperature?900-1200??on structure and CLDR performance of the La-Fe-Al-t?La:Fe:Al=1:3:9?oxygen carrier.It was found that thesamples calcined at 900 and 1000 ? underwent phase separation during the redox process,and the presence of metallic Fe and FeAl₂O₄ led to CH₄ pyrolysis.Pure hexaaluminate samples calcined at 1100 and 1200 ? exhibited good crystal phase stability during the redox process,but an excessively high calcination temperature?1200??caused sintering,and thus lattice oxygen was inhibited from the bulk phase to surface movement resulting in carbon deposition.To be note,only the La-Fe-Al-1100 sample calcined at 1100 ? with a moderate crystal size single-phase hexaaluminate structure exhibited excellent reactivity and stability for producing syngas with desirable H2/CO ratio?²?during ten CH₄/CO₂ redox cycles.Based on BaFe₃Al₉O₁₉ samples,the effect of the introduction of trace Ce⁴⁺,Si⁴⁺ and Zr⁴⁺ions on the structure and CLDR performance of BaFe_2.8M_0.2Al₉O₁₉ was investigated.It was found that only Ce⁴⁺substituted sample was still dominated by hexaaluminate.The introduction of Si⁴⁺ inhibited the release of lattice oxygen of oxygen carriers.The introduction of Ce⁴⁺and Zr⁴⁺ improved the oxygen mobility and the amount of available lattice oxygen for fresh samples.However,for Zr⁴⁺substituted sample,the diffraction peaks of FeAl₂O₄ continuously increased during redox cycle,and it inhibited the release of lattice oxygen.It was worth noting that only the BaFe_2.8Ce_0.2Al₉O₁₉ sample exhibited excellent performance in the CLDR process for producing high quality syngas due to the synergistic effect between its metals.