Research On Symmetric Double Cathode Solid Oxide Fuel Cell With Direct Internal Reforming Of Biomass Gas

Solid oxide fuel cells（SOFCs）are generators that converts chemical energy into electrical energy.They are considered a promising alternative energy technology due to their clean,efficient,and adaptable fuel capabilities.However,the high cost of hydrogen transportation and storage when using pure hydrogen as fuel has been a major bottleneck restricting the application of SOFCs.In contrast,biomass gas resources are wide and renewable,and the production of biomass energy will not cause carbon dioxide emissions in the atmosphere.Methane,in particular,is considered an ideal hydrogen carrier and fuel for SOFCs due to its abundance and ease of transport.Direct internal reforming allows hydrocarbons to be directly supplied to the SOFC anode as fuel,without requiring additional equipment which has the advantage of low cost and high efficiency.During the operation of SOFC,due to the high operating temperature and small cell size,the distribution of key parameters and the characteristics of mass and heat transfer inside the cell are difficult to be observed directly by experiments.Therefore,in this paper,the flat tube symmetrical double cathode SOFC was taken as the research object,and the methane steam was used as the direct internal reforming fuel（DIR-DSC）,and examined the distribution of the internal physical field of DIR-DSC through numerical simulation.At the same time,the influence of different working conditions on the heat and mass transfer process in DIR-DSC were investigated.Finally,the scheme of optimizing the flow channel structure was proposed.The paper mainly carried out the following research work:（1）A three-dimensional numerical model coupling chemical reaction,electrochemical reaction,mass transfer,charge transfer and heat transfer was established.The validity of the model was verified by comparing with the experimental results.（2）The distributions of gas composition,temperature,reaction rate and current density inside the methane steam reforming DIR-DSC were calculated.The results show that the Z-shaped flow channel structure of DIR-DSC cathode and anode leads to the uneven distribution of fuel and oxidant flow between sub-channels.The uneven distribution of gas components further leads to the irregular chemical and electrochemical reactions inside the DIR-DSC,and the temperature distribution is also affected.（3）The effects of different operating voltages on DIR-DSC performance were studied.The results suggest that reducing the working voltage can increase the current density of DIR-DSC.When the S/C ratio is 2 and the inlet gas temperature is 1023 K,the peak power density of DIR-DSC reaches 305.1m Wcm^-2 at 0.5 V.At low voltages,when the conversion rate of methane tends to 100%,the total heat absorption of the DIR-DSC is almost unchanged,while the heat generated by the electrochemical reaction increases sharply in a quadratic relationship with the overpotential.Combined with the influence of the anode current collecting position,the low temperature zone caused by the endothermic reforming reaction at the anode inlet is replaced by the high-temperature zone.（4）The effects of different inlet temperatures on DIR-DSC performance were investigated.The results show that increasing the gas inlet temperature can increase the chemical reaction rate,electrochemical reaction rate and ion conduction rate,thereby significantly improving the performance of DIR-DSC.When the gas inlet temperature increases from 973 K to 1073 K,the power density of DIR-DSC increases from 206.81m Wcm^-2 to 405.32 m Wcm^-2.However,when the gas inlet temperature exceeds 1073 K,the concentration loss of DIR-DSC at low voltage increases significantly,and the temperature gradient of the anode support layer grows,which is not conducive to the long-term operation of DIR-DSC.（5）The effects of different S/C ratios on the heat and mass transfer process in DIR-DSC were studied.The higher S/C ratio will dilute the methane concentration,decreasing the H₂ concentration and H₂ generation rate generated by the MSR reaction,which reduces the electrochemical reaction rate and ultimately reduces the output performance of DIR-DSC.When the working voltage is less than 0.6 V,the maximum rate and average temperature of WGS reaction in DIR-DSC decrease with the increase of S/C ratio.Conversely,when the voltage exceeds 0.6 V,the maximum rate of the WGS reaction and the average temperature of DIR-DSC increase with the growth in S/C ratio.（6）By simulating the three models of changing the anode Z-type flow channel to I-type,changing the cathode Z-type flow channel to triple-parallel serpentine,and increasing the coverage area of the cathode flow channel to cathode,their effects on DIR-DSC performance were studied.The results show that the improvement of mass transfer process reduces the concentration polarization loss of DIR-DSC due to insufficient supply of reaction gas at low voltage,and the electrochemical power density of the DIR-DSC increases.When the voltage is 0.5 V,the anode I-type flow channel increases the electrochemical power density of DIR-DSC by 14.12 m Wcm^-2,and the cathode triple-parallel serpentine flow channel increases the electrochemical power density of DIR-DSC by 25.034 m Wcm^-2.Increasing the coverage area of the cathode channel on the cathode surface is equivalent to increasing the electrochemical reaction area,eliminating the huge oxygen concentration difference and current density difference formed by the mass transfer effect in the cathode and electrolyte layer,and significantly improving the performance of the battery.The electrochemical power density of DIR-DSC increased by32.12 m Wcm^-2,which is the most obvious improvement of DIR-DSC performance among the three different channel structures.（7）When H₂is used as fuel,the electrochemical reaction occurs directly in DSC,which obtain a larger current density than the DIR-DSC.DSC does not need to produce H₂ by endothermic reforming reaction,so there is no low temperature region below the operating temperature（1023 K）,but the temperature gradient in electrolyte of DSC is obviously larger than that of DIR-DSC.It indicates that the direct internal reforming of fuel can effectively alleviate the temperature gradient in DSC,which is beneficial to the long-term operation of the DSC.