【作者】 于建国；

【导师】 翁史烈； 王玉璋；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2012， 博士

【摘要】 随着节能减排和环境保护的呼声日益高涨，以煤炭为能源主体的我国面临着前所未有的压力。但在相当长一段时间里，煤炭仍是我国的主要能源，因此，高效、清洁地利用煤炭资源，对缓解我国能源压力、提高能源利用效率和减少温室气体排放有着重要的意义。煤气化燃料电池燃气轮机混合动力循环（IGFC/GT）使煤炭清洁利用成为可能，具有良好的发展前景与研究空间。高温固体氧化物燃料电池（SOFC）是IGFC/GT的关键部件，其性能对系统性能起决定性作用，但同时煤基合成气SOFC还存在很多不完善之处有待研究。因此，本文结合实验与数值模拟的方法，系统地对SOFC单电池进行了性能研究与优化。设计搭建了研究多孔介质内传热性能的实验台，制备了不同孔隙率的Ni/YSZ多孔阳极实验件，分别在以二氧化碳和氮气为工质工况下，研究了不同电池运行温度、气体流速、孔隙率工况下多孔介质内的传热性能，归纳出多孔介质内考虑温度修正的有效导热系数公式。实验表明，多孔介质有效导热系数随温度的升高略有上升，近似与孔隙率成线性函数关系，在实验条件下气体流速对其影响非常小。同时，对于不同单组分气体，温度对导热系数的影响规律相似，但分别以二氧化碳、氮气为工质时，两者获得有效导热系数差别在3.2%以内。建立了煤基合成气SOFC单电池全三维数值分析模型，包括了流动传热、电化学、电流场等模型，同时具有以下特点：以分析三维情况下的气体扩散过程来计算浓度过电势，代替了简单的浓度过电势计算；考虑了包括氢气、一氧化碳、二氧化碳、水蒸汽、甲烷以及碳等化学组分在内的十多种化学反应，能很好的描述单电池内真实的化学环境；能有效预测碳沉积可能性及位置；采用温度相关的气体特性函数以及实验获得的多孔介质有效导热系数和化学反应有效动力学等模型。以公布的氢气和天然气为燃料的实验数据对模型进行验证，计算结果与实验数据相吻合。利用上述模型，研究了多个重要参数对SOFC性能的影响，并进行了性能优化，详细结论如下：1)分析了顺流时单电池的性能，得到了单电池内温度、组分、电流及电势等参数的详细分布。对电池内流动传热、电化学等变化规律有清晰的认识，由分析结果可见：单电池内最高温度出现在阳极出口靠近三相界面处；活化损失是SOFC的主要损失，在平均电流密度为13000Am^-2工况下，占据了Nernst电动势的59%，而欧姆损失仅占4%；电流密度在电解质层呈波浪形分布，主要是由电池几何结构所决定。2)在满足电池内温度梯度限制的情况下，最优空气过量系数主要是由平均电流密度决定的，最优空气过量系数随着平均电流密度的增加逐渐上升，但上升幅度逐渐缩小，而随着空气过量系数的增加，输出电压及功率密度随之降低。根据上述研究，给出最优空气过量系数与平均电流密度的关系式。3)通过改变燃料供应量来研究电池调峰调谷的性能，由研究可见：增加燃料供应量SOFC最大温差降低，但变化幅度较小，且燃料流量对最大温差的影响逐渐减弱；燃料利用率则随着燃料流量的增加而降低；燃料的增加有助于提高输出电压，也增加了其可用的用电负荷范围。4)通过改变平均电流密度来研究负载对电池性能的影响。由研究可见：随着负载的增加，SOFC内部最大温差逐渐增加，但当平均电流密度大于10000Am^-2时，最大温差变化较为接近，而SOFC入口附近温度梯度增加明显；欧姆过电势随着平均电流密度的增加而增加，但是在计算工况范围内，最大的欧姆过电势也未超过0.04V；给出相应的性能曲线及最佳的工作范围。5)建立了碳沉积可能性的预测模型，研究了煤基合成气中各组分不同含量工况下碳沉积的可能性和位置。由研究可见：一氧化碳含量的增加会减小SOFC碳沉积的范围，但是会增强局部碳沉积活性；较低的氢气含量会导致SOFC碳沉积可能性增强；二氧化碳对碳沉积影响不明显，较小的碳沉积活性主要集中在SOFC近入口附近；水蒸汽的增加对碳沉积有明显的抑制作用，但考虑到对SOFC输出电压的影响，水蒸汽含量应在合理范围；甲烷存在会引起明显的碳沉积，且随着含量的增加而增加，因此，在以煤基合成气为燃料时，应尽量降低甲烷含量。6)利用ANSYS软件，分析了上述工况下单电池内的热应力。由研究可见：在以煤基合成气为燃料情况下，SOFC内部热应力分布均匀；空气过量系数越高，平均电流密度越低，则热应力越小；微晶玻璃加陶瓷材料密封可以有效降低电池内热应力。更多还原

【Abstract】 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.更多还原