Study On Multi-physics Modeling And Long-term Performance Of The Integrated System Of Compact Methane Steam Reformer And Solid Oxide Fuel Cell

As a pillar of global trade,the maritime industry provides key support for the development of the world economy but also brings serious pollution emissions.In the face of increasingly stringent emission regulations,it is becoming more difficult for traditional internal combustion engine to meet the requirements of energy use while the electric propulsion plant is enjoying a renaissance.Wherein,solid oxide fuel cell(SOFC),with higher power density and thermal efficiency as well as the adaptation to shipboard natural gas,is considered one of the most promising candidates.The adoption of natural gas-fueled SOFC hedges the high cost of obtaining and storing hydrogen for marine application,but also brings the long-term performance issue such as performance degradation and thermal stress.Meanwhile,the power generation within SOFC is a complex process coupling heat and mass transfer,chemical reaction,electrochemistry and mechanics.Conventional experimental tests can’t be used to clarify the corresponding internal mechanism and long-term behavioral characteristics.Therefore,in present work,a numerical simulation method is adopted to unclear the multi-field coupling mechanism and long-term performance characteristics occurring in SOFC.A transient three-dimension numerical model coupled electrochemical reaction,thermochemical reaction,transport of ion,electron,heat,mass and momentum,as well as thermal stress is developed.After checking the validation of the numerical simulation,the multi-physics analysis and longterm performance evaluation of SOFC for ship power design are carried out.More specifically,the following work is carried out in present paper:(1)Performance degradation caused by carbon deposition is one of the major obstacles to the commercial application of natural gas-fueled SOFC.Based on the established transient multi-physics model coupled transport of ion,electron,heat,mass and momentum,as well as carbon deposition effect,the spatial-temporal evolution of carbon deposition occurring within SOFC and the corresponding interaction mechanism with long-term performance are elucidated.It is found that the deposited carbon occurs more easily around the inlet region of direct internal reforming SOFC.The deposited carbon would adsorb onto the solid surface of porous electrode and occupy its void volume,causing the decrease of porosity and permeability.The permeability of the porous electrode near the SOFC inlet decreases by two orders of magnitude after 20000 hours of steady-state operation.The rapid development of carbon deposition and the decrease in local porosity and permeability are concentrated in the first 4000 hours of operation.This results in the occurrence of deactivation zone with zero current density within electrode inlet and the tendency to expand towards the outlet,which further deteriorates the output performance of solid oxide fuel cell.(2)Methane reforming and cracking reaction occurring near the fuel inlet of SOFC is one of the main reasons for the stress concentration,the imbalance of internal heat supply and demand,as well as the deterioration of carbon deposition.In present work,the integrated system of compact methane steam reformer(MSR)and SOFC is proposed by utilizing the exhaust gas of SOFC and adopting the design of modular microreactor.And the thermal stress model is further introduced to evaluate the thermo-electro-chemo-mechanical behavior of SOFC considering the integrated system design.The main and secondary factors affecting the performance degradation of SOFC is quantitatively analyzed.The results show that the outlet temperature of MSR,pre-reformed methane level and the operating voltage are the main factors affecting the performance degradation of SOFC.Increasing the operating voltage of SOFC,improving pre-reformed methane level of MSR and reducing its temperature can effectively reduce the carbon deposition and improve the performance degradation of SOFC.However,an overdesigned MSR(fully pre-reformed fuel condition)would result in the appearance of extreme high temperature within SOFC and lead to a higher thermal stress.(3)The transient multi-physics model coupled catalytic reforming and combustion reactions,transport of heat,mass and momentum,as well as carbon deposition effect,for MSR is developed.The corresponding long-term performance evaluation of MSR is then conducted.It’s found that the stress concentration appears at the interface between substrate plate and catalyst layer.This implies the underlying delamination and fracture between the catalyst layer and substrate.While the performance degradation of MSR is relatively less severe than that of SOFC in simultaneous operation because of its lower operating temperature.The methane conversion rate of MSR decreased from 50.7% to 47.4% after 4000 hours of operation,and further decreased to 28.3% after 20000 hours of operation.(4)The spatial-temporal evolution of thermal stress occurring within SOFC is elucidated based on the transient multi-physics model.The corresponding failure probability of brittle ceramic components is also evaluated by Weibull failure analysis approach.It is found that,from the perspective of the spatial distribution,thermal stress always follows the general trend that the value of outlet region is larger than that of the inlet and the bound region under ribs is larger than that of the free region.The thermal stress within the electrolyte is the largest while that of the porous anode electrode is the smallest.The thermal stress within SOFC at different spatial locations decreases first and then increases over time.Moreover,due to the lowest Weibull modulus and the second highest first principal stress,mechanical failure is most likely to occur in porous cathode electrode.(5)Functional graded electrode is introduced to relieve the thermal stress occurring within SOFC.The corresponding thermal stress evaluation of SOFC with functionally graded electrodes is then conducted.Based on the volume fractions-mixture combined rule calculation model,the thermal physical properties and mechanical parameters of functionally graded electrodes are clarified.It’s found that the functionally graded electrodes can be used to reduce the thermal stress within SOFC and eventually improve the mechanical reliability of the integrated system.It is also clarified that the corresponding mechanism is mainly from the functionally graded design which completes the smooth transition of the thermal expansion coefficient and Young’s modulus of positive-electrolyte-negative electrodes.