Systematic Study On Ceramic Foam Enhanced Solar Methane-assisted Thermal Reduction Of CO₂

Solar thermochemical reduction of carbon dioxide,driven by high-level solar energy,can simultaneously recover the two main greenhouse gases in the atmosphere,namely CO₂ and CH₄,and convert them into syngas—the main components are H₂ and CO—or liquid fuels.This technology has the dual advantages of clean utilization of renewable energy and carbon energy recovery,promising to become a future clean energy technology to achieve the goal of“carbon neutrality”in 2060.However,due to the late start of solar thermochemical reduction of CO₂ technology,the work in multi-physics coupling characteristics analysis,preparation of efficient catalytic materials,and experimental system integration is still incomplete,which greatly limits the industrial application of this technology.In this regard,this paper proposes the reaction concept of"methane-assisted thermal reduction of CO₂"based on the technical advantages of the three existing thermochemical CO₂ reduction reaction systems.The laws of energy conversion,the screening and preparation methods of oxygen carriers,and the application test of the experimental system were carried out,respectively.The main contents and conclusions are as follows:A convective-radiative-chemical multi-physics coupled model based on the assumption of local thermal non-equilibrium was constructed to more accurately predict the thermochemical properties and energy conversion mechanism of the foam structural reactor.Through specific examples,the applicability and deviation of different momentum dissipation models and two-phase heat transfer coefficient models in the current study are discussed,while the necessity of thermal non-equilibrium assumption and medium radiative transfer for the middle-high temperature thermochemical modeling process is emphasized.Based on the developed multi-field coupling model,the influence of key operating conditions on the thermochemical properties of the reactor,as well as the regulation of the porous ceramic foam on the photo-thermal-chemical energy field in the reactor was revealed.Aiming at the problem of carbon deposition,which is easy to cause catalyst deactivation,a mitigation method combining operating parameter optimization and pore structure adjustment was discussed.In addition,based on the regulation effect of foam ceramics on multi-physics fields,a heat and mass transfer enhancement strategy of hierarchical pores and multi-layer structure is proposed,which can increase the theoretical efficiency by more than 10%.In terms of reaction mechanism,combined with the gas-phase reaction and surface reaction mechanism in the thermochemical process,the complete gas-solid coupling reaction kinetic model of iron-based oxides is established.The mechanism is simplified by DRGEP algorithm and sensitivity analysis,which reduces the calculation amount by about 90%.The analysis results of the reaction mechanism indicated the optimal reaction temperature for the methane-assisted thermal reduction of CO₂,and revealed the important role of methane-assisted gas in reducing the reaction temperature and increasing the production of syngas.To find the best oxygen carrier with high catalytic activity,high selectivity,and excellent cycling stability at the lower reaction temperature,seven basic oxygen carriers,and four doped materials were tested and evaluated by thermogravimetric analysis and other characterization methods.Based on this,a high-performance nano-oxygen carrier material with Co-Ni bimetallic active components was prepared.Then,innovatively using ethanol as a solvent,a supported-structural catalyst with a micro-millimeter-scale dual-scale pore structure was prepared.The experimental results illustrate that the prepared material achieves a CO yield of up to 42 m L min^-1 g^-1,a single-cycle CO production of 10.5 L,and a CO₂ conversion of94%at a reaction temperature of only about 1000 K,after 24 hours of reaction without obvious inactivation.In addition,different from previous studies,this material exhibits higher catalytic activity during isothermal cycling,which effectively reduces the irreversible heat loss during the reaction.Finally,an 18-kW solar thermochemical experimental system was designed and built to carry out experimental tests under the two-step redox cycle and methane-assisted reaction system.The results indicate that the addition of methane auxiliary gas can reduce the reaction temperature by about 300 K.At the same time,the CO₂ conversion is doubled,and the CO₂ production in a single cycle is increased by 14 times,confirming the remarkable advantages of the proposed methane-assisted thermal reduction of CO₂ reaction system in practical applications.In terms of energy efficiency,the thermal efficiencies of the reactor under these two reaction systems were 44%and 68%,while the highest energy conversion efficiencies were 0.89%and 4.1%,respectively.After analysis and comparison,the size of the designed reactor limits the load of the internal oxygen carrier,which is one of the key factors affecting the energy efficiency of the current system.