Study On Methane Chemical Looping Reforming By One-step Synthesis Of Highly Active Composite Oxygen Carriers

Fossil fuels will still bean important part of the energy system for a period of time in the future,however,in the face of increasingly serious environmental problems,how to make efficient and green use of natural gas-based fossil fuels is particularly important.The chemical looping reforming technology can not only provide a new way for the utilization of greenhouse gases,but also maximize the energy conversion efficiency,and has a broad prospect.However,the preparation of excellent oxygen carriers is an important factor restricting this technology.Among them,Ce-based oxygen carrier has become one of the hottest oxygen carrier materials studied in recent years due to its unique structural advantages.But,easy sintering at high temperature has become an urgent problem to be solved.The introduction of dopants is an effective strategy.Based on this,the Ce-based oxygen carrier was systematically studied by thermodynamics,kinetics,and combined experiments and characterizations（XRD,H₂-TPR,SEM,BET）.The contents are as follows:In chemical looping reforming technology,CH₄ is the most widely used fuel.Firstly,thermodynamic calculation and equilibrium composition calculation of the reactions of Ce-Fe oxide,Ce-Co oxide,Ce-Ni oxide,Ce-Cu oxide,and Ce-Mn oxide in the fuel reactor were carried out by thermodynamic software.Investigate the possibility of the cerium-based oxygen carrier reacting in thermodynamics,and calculate the oxidizing order and reaction performance of different metals reacting with CH₄ according to the thermodynamic data.The best dopant is screened out theoretically,which plays a guiding role in the actual experiment.Secondly,the best oxygen carrier（Ce₉Co₁Od）selected by thermodynamics was used to investigate the influence of preparation methods（hydrothermal method,solution combustion method,co-precipitation method,ball milling method）on the chemical looping reforming of methane.From the perspective of CH₄ conversion,the oxygen carrier prepared by the solution combustion method has better chemical looping reforming performance.Next,a series of Ce_xCo_1-xOd（x=1,0.9,0.8,0.7,0.6,0）oxygen carriers were prepared by solution combustion method,and the corresponding activity evaluation was carried out at a constant temperature of 850°C using a fixed bed device.After evaluation,it was found that Ce₉Co₁Odhad better partial oxidation activity,and through characterization,it was concluded that Co³⁺not only improved the catalytic and oxygen-carrying capacity of CH₄,but also lowered the reduction temperature.Then the calcination temperature（650°C,750°C,850°C,950°C）,fuel type（citric acid,glycine,urea,ethylene glycol）and fuel addition amount（30wt%,50wt%,70wt%,90wt%）were investigated in detail.It was finally determined that the Ce₉Co₁Odoxygen carrier prepared by calcination at 750°C with a mass fraction of 70%citric acid had the best effect.Although the addition of Co played a good role in promoting the cerium-based oxygen carrier,the sample was seriously deactivated at the later stage of the experiment（CH₄conversion rate decreased rapidly）and the CO selectivity was not very high（about 80%）.Based on this,in order to make the oxygen carrier show better reaction performance,further optimization of cerium-cobalt oxygen carrier by adding support（CNTs,coconut shell,activated carbon,ED）under optimal conditions and changing the amount of support added（5wt%,10wt%,15wt%,20wt%）.It was found that the addition of 10wt%coconut shell further improved the grain size,reduction temperature and microstructure of the oxygen carrier,so that the samples maintained high activity throughout the reaction time.Finally,in order to further understand the reaction mechanism of the oxygen carrier,the CH₄ reduction kinetics of the oxygen carrier was studied through a fixed bed.The results show that the two-dimensional phase boundary control model has the best fitting effect among the three typical models（diffusion control model,phase boundary control model,nucleation-nuclei growth model）,and the activation energies of Ce O₂ oxygen carrier,Ce₉Co₁Odoxygen carrier and 10wt%CS Ce₉Co₁Odoxygen carrier under this model were obtained by fitting to be 128.73 k J/mol,91.18 k J/mol and 46.82 k J/mol,respectively,which is also corresponds with the reactivity sequence of the oxygen carriers.