Investigation And Optimization Of Planar Anode Supported Solid Oxide Fuel Cell Using Syngas As Fuel

With the requirements of energy saving, emission reduction and environmentprotection, China confronts great pressures with coal as main energy source. In quite along period of time, coal will still be the dominant energy source in China, thus, it isimportant to use coal efficiently and cleanly. Integrated Coal gasification and fuel cellgas turbine combined cycle (IGFC/GT) is one of the promising systems for its highenergy transform efficiency. This system has broad developing prospect andresearching space. Solid oxide fuel cell (SOFC) is the key device in IGFC/GT hybridsystem and should be fully investigated. Thus, by the methods of experiment andnumerical analysis, the performance and optimization of single SOFC areinvestigated.In this work, an experimental bed was designed and built to be used to study theeffective thermal conductivity in porous media. The Ni/YSZ experimental sampleswith different porosities (0.2348,0.3471and0.4178) were made by the process of ballmilling, drying, pressing, sintering and reduction. The effective thermal conductivityin porous media is obtained with various operating temperature, gas flow rates andporosities, when carbon dioxide and nitrogen are supplied, respectively. From theexperiment results, the effective thermal conductivity in porous media increases withincrease of operating temperature and is proximately linear with the porosity. Theeffects of operating temperature on the effective thermal conductivity are similar withdifferent gas. The difference of the effective thermal conductivities is less than3.2%when carbon dioxide and nitrogen are supplied in the experiment, respectively.The fully3-dimensional numerical models are developed to analyze theperformances and characteristics of SOFC. In the models, the diffusion of species iscalculated by Darcy's model with molecular diffusion and Knudsen diffusion. Mainchemical reactions considering equilibrium constants and kinetic rates are consideredto describ the real chemical environment. The carbon deposition in SOFC could bepredicted. With hydrogen and methane as fuel, the results calculated by these modelsat the same experiment conditions meet the measured data. The models are validated.Using the above models, the effects of key parameters on the performance ofSOFC are analyzed. From the results, some conculsion could be draw as follows:1) At coflow, the detailed distributions of main parameters, such as temperature,species and current, are obtained. Form the result, the highest temperature region isnear the fuel outlet in anode. The dominant loss is activation overpotential, which iseven59%of Nernst potential with average current density of13000Am^-2. The currentdensity changes wavely in electrolyte layer. And the distribution is determined by thestructure of SOFC. 2) Under the limitation of temperature gradient (1000Km^-1) in SOFC, theoptional range of air excessive coefficient is mainly deterimined by the averagecurrent density. The optimal air excessive coefficient increases with increase ofaverage current density. However, the output voltage and power density decrease withincreasing air excessive coefficients. The function of air excessive coefficient to theaverage current density is obtained.3) In order to analyzing the off-design performance of SOFC, the effects of thesupplied fuel flow rate are investigated. The increase of syngas flow rate may makethe max temperature difference in SOFC reduce. The reduction is not obvious,especially at large air excessive coefficient, but it leads to the increment of theactivation overpotential. The fuel utilization coefficient reduces with increase of fuelflow rate. Totally, the increase of fuel flow rate makes the output voltage enhance atthe same current density and enlarges the range of operating current density.4) The effect of average current density is used to simulate the effects of the loadchange on the performance of SOFC. With the increase of the load, the maxtemperature difference and the ohmic overpotential in SOFC are increasing as well.However, when the average current density is higher than10000Am^-2, the maxtemperature difference in SOFC almost unchanges. But the temperature gradient nearthe SOFC inlet increases gradually. In all calculation conditions, the max ohmicoverpotential is not larger than0.04V in this work.5) The model of carbon deposition is introduced. The activity and position ofcarbon deposition are investigated. From the calculation results, the reduction of COmolar fraction may inhibit the carbon deposition, and narrow down the region ofcarbon deposition in SOFC. But it will enhance the activity of carbon deposition nearthe fuel inlet. The activity of carbon deposition will increase when the H2molarfraction decreases. The effect of CO₂molar fraction is unobvious, and the carbon maybe deposited in the anode near the fuel inlet with decrease of CO₂. The addition ofwater will inhibit carbon deposition effectively. However, considering the reduction ofoutput voltage, water should be added properly. The increase of CH₄molar fractionmay lead to carbon deposition significantly, thus, methane in syngas should beremoved as clear as possible.6) The thermal stress is investigated under the above operating conditions usingANSYS software. The distribution of thermal stress is nearly uniform with syngas asfuel. And when the air excessive coefficient is larger and average current density issmaller, the thermal stress is lower. The seal of glass-ceramics can reduce the thermalstress effectively.