Research On The Reaction Characteristics And System Of Methane Production By High Temperature CO₂/H₂O Co-electrolysis

Solid oxide electrolysis cell（SOEC）can directly convert H₂O and CO₂ into CH₄ by using renewable power,therefore,enables to synchronously utilize CO₂and store renewable power,as well as to promote the deep combination between renewable energy and natural gas network.In order to facilitate the application of the SOEC in the distributed generation system integrating renewable energy and natural gas,it’s of great importance to understand the reaction mechanism and the coupling mechanism of reaction and transfer process within the SOEC,as well as the transfer principles of mass flows and energy flows between the SOEC and other system components.This thesis carries out the research on the reaction characteristics and system of methane production by using solid oxide H₂O/CO₂ co-electrolysis cells by combining experimental test,kinetics calculation and numerical simulation.First,the patterned electrode is used to accurately control the electrochemical active interface to obtain the intrinsic kinetic parameters and speculate the electrochemical reaction mechanisms and corresponding rate-limiting steps of both H₂O electrolysis and CO₂ electrolysis.To understand the correlation between reaction mechanisms and intermediate elementary species,an elementary reaction kinetics model is established.Results show that the rate-limiting steps of reversible SOEC are the charge transfer step that produces the intermediate species OH^-（YSZ）and the one that consumes intermediate species CO（Ni）.The reaction characteristics of an SOEC unit is a comprehensive result coupling electrochemical reaction,chemical reaction as well as transfer processes of charge,mass and heat.To illuminate the coupling of reaction and transfer processes in an SOEC unit,a pressurized tubular SOEC reactor and a multi-physical tubular SOEC thermo-electric model are built.The tubular SOEC can stably operate at 4 bar and regulate the CH₄ production by thermo-flow design and pressurized operation.After optimization,a CH₄ production ratio as high as39.5%is obtained at-2 A.Besides,the dynamic couple of reaction and transfer processes is optimized to ensure the stable and efficient operation of the tubular SOEC driven by intermittent renewable power.The study on the SOEC unit offers basic data,reaction and transfer process coupling mechanism and stable operation principle for the system integration.Finally,a distributed generation system simulation platform is built to combine renewable energy and natural gas.The integration between H₂O/CO₂co-electrolysis and other system components is studied.Results show that integrating H₂O/CO₂ co-electrolysis and methanation into one reactor can improve the thermal coupling to enhance the system efficiency to 81.3%at 8.15bar,which is at least 3%higher than separate SOEC+methanation reactor PtM process.In the case with intermittent wind power penetration,the system performance can be improved from the perspective of system efficiency,wind power penetration,power supply stability and storage capacity by optimizing the ratio of wind power direct supply to wind power storage and combining SOECs and lithium-ion batteries.