Study On Reaction Kinetics Of Synthetic Gas Produced By Solar-driven High-temperature Thermochemical Process Based Ferrite

In recent years,much attention has been paid to efficient solar energy conversion technology,especially that using CO₂ as raw material to convert solar energy into fuels to meet the increasing global energy demands in the perspective of reducing greenhouse effect and environmental pollution hazards.Photochemical,electrochemical,and high-temperature thermochemical are considered as state-of-the-art technologies for producing energy-rich chemical that are compatible with our energy infrastructure today,such as gasoline,jet fuel and hydrogen.Among these processes,the solar thermochemical fuel conversion technology is one of the most attractive research areas for CO₂ depletion and turning CO₂ emission into value-added products such as solar fuels and chemical commodities.In this thesis,a combination of numerical simulation and experimental research is carried out to study the solar thermochemical reacting system for hydrogen and synthetic gas production from CO₂.The species reactivity is limited to the formation of H₂ and CO which are important feedstocks for the fuel cells and synthetic fuels production such as synthetic solar hydrocarbon fuels,methanol,and other chemical products.A two-step solar thermochemical reacting system of Fe-based oxide redox is developed for H₂/CO yield via H₂O/CO₂-splitting.Heat and mass transfer exhibit a strengthening law regarding the thermochemical reaction mechanisms and kinetics of two-step solar thermochemical looping reforming of Fe-based oxide redox cycles within the solar thermal reactor.Analysis of Fe₃O₄ redox cycles via H₂O/CO₂-splitting shows that the parameters in term of temperature,pressure,and mixing ratio of H₂O/CO₂??_g?have great influence on the production rate and final composition of the synthetic gas.The optimum operating conditions for hydrogen-rich syngas production are obtained when?_g?28?2at 1600 K and 20 atm.To deeply understand the reaction mechanism of inner working of the chemistry models,detail chemical reaction mechanisms describing H₂ and CO formation are conceptually structured in a hierarchical manner with the assistance of reaction paths diagram.H₂ and CO formation are essentially based on oxygen exchanges capability and the reactivity of short-lived radical species such as H,O,C and OH at the interface of iron oxide surface.The species reactivity in gas-FeO-Fe reaction is limited by the rates of lattice oxygen extraction and diffusion from gas species to the iron bulk surface to replenish the extracted oxygen.The main Fe bulk phase of iron is not completely transformed to the fully oxidized Fe₃O₄ phase because of the restricted oxygen transfer to Fe possibly.Moreover,the study on high surface temperature gas-solid?Fe₃O₄ and H₂O/CO₂?interfacial reaction characteristics indicates that radiation heat transfer and temperature distributions are important factors that affect solar-to-chemical energy conversion in the solar thermochemical reactor.During the process of solar thermochemical reacting systems?STRS?,the reaction extent is favored by higher axial temperature distribution and the convective heat flux which enhanced gas-solid contacting time thereby resulting in higher heat and mass transport.A novel laboratory scale solar thermochemical reactor using agglomerated solar energy to drive high-temperature thermal chemical reaction is developed.Then,the effects of geometric parameters,boundary conditions,and radiation heat transfer models on the thermal performance of the reactor and heat and mass transfer enhancement strategy are sufficiently investigated by adopting the radiation heat transfer models,including P1 approximation,finite volume discrete ordinate method?fvDOM?approximation,surface-to-surface radiation model,and Rosseland approximation for radiation heat transfer.The incident radiation intensity distribution and quality are obtained in the inner cavity of the reactor.Both experiment and numerical results indicate that the temperature distribution resultes in incident radiation intensity distribution throughout the inner cavity of the reactor.It is observed that more heat flux is applied to the reactor,more the reactor is heating up.The influence of structural parameters in term of diffused irradiance intensity,the mass flow rate,heat transfer coefficient,quartz glass and inner cavity wall surface emissivity,the porosity and extinction coefficient that could affect heat transfer and fluid flow performance of the proposed solar cavity receiver are sufficiently investigated.It is found that the substantial drops in temperature are mainly attributed to the thermal losses by radiative,convective and conductive heat transfer.Moreover,the radiative heat transfer and thermal characteristics of different foam-type RPC structures,including SiC,CeO₂,FeAl₂O₄,NiFeAlO₄,Fe₃O₄/SiC,and NiFe₂O₄/SiC are compared.The mass flow rate and foam structural parameters,including the permeability,pore mean cell size,and extinction coefficients have significantly affected the axial temperature distribution,pressure drop,heat transfer,and fluid flow characteristics.Integrated porous structure to the solar receiver could maximize the incorporation of redox powder in the reacting medium,lower the temperature drop,and enhance the thermal performance of the thermochemical reacting system.Among the materials investigated,SiC structure can be considered as candidate material in the case where more heat flux and high axial temperature distribution is needed.However,Fe-based oxide coated Al₂O₃ structure could be considered regarding heat transfer enhancement along with the catalyst activity of oxygen carriers for solar thermochemical reacting system performance.Besides,experiment and numerical simulation are conducted on two-step solar thermochemical looping reforming of Fe₃O₄ redox cycles.It is found that the key performance of two-step CH₄-Fe₃O₄ redox for higher H₂ and CO production relied on the efficiency of methane and oxidizer?H₂O and CO₂?conversions.Moreover,the solar thermochemical reacting system of NiFe₂O₄ redox cycles combined with CH₄ partial oxidation indicated that the synergistic effect of the reactivity of FeO-Fe and Fe/Ni exhibited a very promising strategy for producing 45%of syngas with2.54 ratios of H₂:CO at the first step and 55%of syngas with 2.34 ratios of H₂:CO at the second step at 437.69 kW/m² of applied solar flux.Moreover,the oxidation temperature,operating pressure and the concentration of oxidizing species have strong impacts on the oxidation kinetics.In the perspective of CCST,developing state-of-the-art technologies for CO₂ utilization?CU?is a big challenge for the global community regarding CO₂ emission control and utilization.Analysis of CO₂utilization into synthetic gas?H₂+CO?in the context of carbon capture and utilization?CCU?technologies indicated that the reactor has the potential to utilize by?60%of CO₂ captured with 40%of CH₄ co-fed into syngas?72.9%of H₂ and27.1%of CO?at 741.31 kW/m² of solar flux.Besides,the mixture gas inlet velocity,operating pressure and CO₂/CH₄ feeding ratio are crucial to determining the efficiency of CO₂ utilization to solar fuels.Catalytic CO₂-reforming of CH₄ to chemical energy is a promising strategy for an efficient utilization of CO₂ as a renewable carbon source.Heat transfer characteristics and reaction mechanisms and kinetics of the thermal cracking process based on advanced redox oxide material NiFeAlO₃ RPC structure are analyzed for efficient CO₂ utilization into syngas production.The benchmark experimental system,including NiFeAlO₃ RPC structure,and the reaction mechanisms are described and the effects of Alumina catalyst supported Fe-Ni bimetallic on CH₄-assisted CO₂ utilization are investigated.The concentration of lattice oxygen in the pores of NiFeAlO₃ catalyst significantly improved the yield of syngas by decreasing carbon deposition and promoting the partial oxidation of CH₄.The key factors affecting the production of synthetic gas are NiFeAlO₃ weight loading,the ratio of Ni/Fe,and the ratio of CO₂/CH₄.Carbon deposit could be significantly decreased by resulting in higher syngas production by using 0.72 of Ni/Fe ratio and 60%of CO₂.The yield of CO could be further improved by considering the reverse water gas shift?RWGS?and Boudouard reactions.Ni-Fe-Aluminate RPC can be considered as a cheaper alternative redox oxide material for CO₂ utilization to synthesis gas which can be however converted to liquid hydrocarbon fuel or energy-rich chemical products.The research results of this thesis have important theoretical significance and practical application values for the solar thermal chemical fuel conversion technology that uses the heat of concentrating solar energy to drive the high-temperature thermochemical cycle.