| Solid oxide fuel cell (SOFC) is one kind of fuel cells with ceramic materials as electrolyte, which can work between600-1000℃. There are many advantages associated with SOFCs which has been recognized as one of the most promising fuel cells in the future, such as high efficiency, low pollution, high power output, flexible fuel supply, simple operational principles. Anode-supported plannar SOFC with thick anode layer can work in relative lower temperature. A3D steady CFD model of a single cell unit has been developed to evaluate the multi-physics processes in SOFCs, including methane steam reforming (MSR), water-gas shift reaction (WGSR) and electrochemical reactions, and the transport processes of mass, momentum, heat and charge (ion/electron). The commercial CFD code Ansys/Fluent12.1has been employed for numerical simulation calculation, and the experimental data of current density, voltage, power density have been used to validate the models and the calculation results. A parameter study is conducted to analyse the effects on the reactions and the transport processes by changing some conditions, such as temperature, porous structures, fuel compositon and gas channel arrangment in SOFCs. Similarly, a3D steady CFD model is developed for the SOFC anode by implementing the42-step elementary reaction mechanism to evaluate the catalytic reactions between the gases and the catalyst (Ni) in the anode porous structure, based on the internal reforming reactions coupled with mass and heat transfer processes.According to the prediction of the globle gas reaction model in the single cell, the temperature distribution increases along the main flow direction from the inlet, and a maximum value occurs at the electrolyte near the outlet. The internal reforming reactions, which provide H2and consume H2O and heat from the electrochemical reaction near the electrolyte, are stronger in the anode layer close to the fuel channel. The conversion rate of CH4is about59%, while the molar fraction of hydrogen XH2decreases by10%at800℃, which means that the electrochemical reaction is stronger than internal reforming reactions; Regarding to the electrochemical reaction which occurrs in the active layer near the electrolyte, its thickness is about20um in the anode side and10um in the cathode side. The distribution of the current density and the potential is not even, which has been affected by the gas channel, temperature and gas distribution; The total potential losses is0.3V, in which the active overpotential in the anode and the cathode side is about70%and20%, respectively, when the operational voltage is set to0.7V. The current density Iion and Ic increase37.5%and28.6%, respectively, when the temperature is50℃higher. It means that the reforming and electrochemical reactions are stronger; When the permeability is10times bigger, the current density increases about53%, the active overpotential decreases40%, and the thickness of reaction region decreases, the rate of MSR decreases, working temperature becomes high (6-10℃), and the rate of WGSR increases when the molar fraction of CH4has been decreased to10%; There are four major surface species identified, including Nis(65%), COS(26%), Hs(7%) and Os(1%), as predicted by the catalytic surface reaction model; high operating temperature and permeability can improve the reactions towards to the desperation reactions to release more Nis while consume more Hs and COS; It is also found that the temperature, gas species can be affected by each elementary reaction steps. |
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