High Iron Content Oxygen Carrier For Methane-steam Chemical Looping Hydrogen Generation

Chemical looping hydrogen generation（CLHG）technology is a new type of hydrogen generation technology that combines the preparation of high purity hydrogen and the internal separation of carbon dioxide.Oxygen carriers are the focus of current research efforts in CLHG.Iron-based oxygen carriers are of interest in chemical looping system due to their high oxygen carrying capacity and significant cost-effectiveness.However,Fe₂O₃ shows a rapid decrease in reactivity and poor stability due to sintering when used in chemical looping processes.Loading it on inert carrier can effectively improve its cycling stability,but it will inevitably reduce the content of active component Fe,thus reducing its oxygen carrying capacity.In addition,the phase separation of the iron-based oxygen carrier and the inert carrier is easy at high temperature,so the loading capacity of Fe is usually less than 42 wt%.Hexaaluminates have excellent high temperature stability,resistance to sintering and thermal shock,and the lattice mosaicism of Fe³⁺induced into hexaaluminates overcomes the reduced stability due to high Fe content.The oxygen carriers were also characterized by combining H₂-TPR and XRD to establish an effective correlation between performance and microstructure.Firstly,an orthogonal experimental design was used to investigate the effect of four factors,namely preparation method,calcination temperature,reaction temperature and reaction time,on the high iron content hexaaluminate oxygen carrier BF12 in the CLHG process and to explore the relationship between the structure of the oxygen carrier and the reaction performance.The results showed that:1.The influence of four factors was:reaction temperature>calcination temperature>preparation method>reduction time.2.The optimized results are as follows:the reaction temperature was 900℃,the calcination temperature was900℃,the preparation method was co-precipitation method,and the reduction time was 20min..Next,BaFe_11.4M_0.6O₁₉（M=Ce,Sn,Co,Ni）oxygen carriers were prepared by co-precipitation method based on BaFe₁₂O₁₉ with the introduction of trace amounts of tetravalent ions(Ce⁴⁺,Sn⁴⁺)and divalent ions(Co²⁺,Ni²⁺).The results showed that the reaction performance of the oxygen carriers with methane was improved to some extent when the BaFe₁₂O₁₉ oxygen carriers were doped with Ce⁴⁺,Sn⁴⁺,Co²⁺and Ni²⁺ions,respectively,with the cerium-doped oxygen carriers exhibit the best methane reaction performance and hydrogen production performance.The loading of cerium was further examined and it was found that an increase in the loading of cerium contributed to its improved performance.Finally,thermodynamic calculations of CLHG were carried out using Aspen Plus software based on the Gibbs free energy minimization principle.A CLHG system with BaFe₁₂O₁₉ as the oxygen carrier and methane as the fuel was developed.The influence of feed ratio,temperature and pressure on the process performance in the fuel reactor,steam reactor and air reactor were investigated through sensitivity analysis tools.The heat transfer network of the system was designed and its energy analysis was carried out.The preferred feed ratios in the fuel,steam and air reactors were 3.8,1.1 and 1.2,the preferred temperatures were 900,500 and 500℃ and the preferred pressure was 1 atm,respectively.Under these conditions,100%CH₄ conversion,92.4%CO₂ selectivity,29.5%water vapor conversion and high purity hydrogen（100%）can be obtained.In addition,carbon capture,energy efficiency and chamber efficiency can be achieved at 85.3%,91.3% and 77.2% respectively.