Performance And Degradation Mechanism Of SOFC Based On Symmetrical Double-sided Cathodes Fueled With Silmulated Coke Oven Gas

The utilization of coke oven gas(COG)is important for the production of iron and steel industry.Of all the traditional applications,COG power generation by the heat–electricity conversion is a large proportion,which follows the Carnot cycle,but subjected to low efficiency and high pollution.Solid oxide fuel cells(SOFCs)can directly convert the chemical energy of COG into electricity,and is a type of clean and highly efficient power generation device.In this work,the application of COG in SOFC was firstly explored,focusing on the performance of SOFC based on symmetrical double-sided cathodes(DSC-SOFC)using simulated COG as fuels.Contributing factors,mechanism,and the characteristics of COG applying in DSC-SOFC at high temperature were carried out.The direct internal reforming of COG in DSC-SOFC was studied,along with the processes of carbon deposition and sulfur poisoning in the DSC-SOFC.Finally,the strategies of property recovery and performance improvement of DSC-SOFC using COG as direct internal reformed fuel were investigated.The main results are as follows:1.DSC-SOFC with high anti-failed load and strong redox stability is developed,and it is found that symmetry and high thickness ratio are two important internal conditions for DSC-SOFC to have good structural stability.The research results showed that the anti-failed load of DSC-SOFC before reduction was more than 1000N,which was more than 10 times that of T-SOFC.After reduction,the anti-failed load of DSC-SOFC was more than 850 N,which was more than 25 times that of T-SOFC.The mechanical properties of DSC-SOFC are more conducive to the assembly of SOFC stack.Both experimental research and theoretical simulation showed that during the reduction and oxidation cycles,the overall structure and electrolyte structure of DSC-SOFC remained intact and stable,which made DSC-SOFC more potential for application in extreme environments and very suitable for power generation process with COG.2.The critical humidification conditions for the stable operation of DSC-SOFC using COG as fuel is found by studying the carbon deposition process during the direct internal reforming of COG in DSC-SOFC,which indicates that the carbon deposition process is affected by the water vapor concentration and the distribution of carbon deposition reactions.When the ratio of H₂O/CH₄ was about 1,the power density of the cell decreased from 0.23 Wcm^-2 to 0.19 Wcm^-2,which meant that H₂O/CH₄=1 was not only the critical point of anodic carbon deposition but also the mutation point of discharge performance.The existence and action process of critical point was verified by thermodynamic calculations.The results of microstructure and carbon deposition distribution displayed that carbon deposition of COG in DSC-SOFC at high temperature mainly came from reactions of methane pyrolysis and carbon monoxide disproportionation.The former occurred near the anode surface,while the latter existed around the triple-phase boundary.The performance of DSC-SOFC fueled with COG under different H₂O/C ratio was measured.The results showed that when H₂O/C was greater than or equal to 1.79,DSC-SOFC can run stably for 240 hrs with no obvious attenuation.3.The effection and mechanism of COG humidification ratio on the discharge performance of DSC-SOFC using H₂S-containing COG as fuel are explored,and found that the formation of nickel sulfide is the fundamental reason for the degradation of DSC-SOFC performance.The peak power density of DSC-SOFC decreased from 0.12 Wcm^-2 to 0.07 Wcm^-2 by the direct internal reforming of COG containing H₂S.With the continuous injection of H₂S,every time the performance of DSC-SOFC was worse than before slightly.OCV of DSC-SOFC decreased continuously for the cumulative effection of H₂S toxicity till the humidification temperature came to 61?(20.6%by volume of water vapor),a critical inflection point.When H₂S-containing H₂ or H₂S-containing COG was used as fuel gas,sulfur poisoning to DSC-SOFC contained two stages of performance attenuations,one was the rapid attenuation at the beginning,and the other was the slow attenuation during long-term operation.The analysis of sulfur poisoning of DSC-SOFC using COG as fuel showed that the redistribution of nickel in DSC-SOFC anode caused by the formation,migration and decomposition of nickel sulfur compounds was an important reason for the performance degradation of DSC-SOFC.4.Through the applications of atmosphere selection,interface improvement,and current control,the discharge performance of DSC-SOFC is restored and improved.The results of electrical performance test and impedance analysis indicated that all changes in conditions affected the kinetic process of cell reactions and resulted in the changes of impedance.After sulfur poisoning,the performance of DSC-SOFC can be partially recovered by adjusting working temperature,running current,and fuel treatment.The OCV of sulfur-poisoned DSC-SOFC can be partially restored by increasing operating temperature and highering current.The power density of DSC-SOFC at 750?increased by nearly 60%with the transient injection of CH₄,and the improved cell kept on running stably for close to 20 hours.The DSC-SOFC had the best discharge performance when the termipoints of current and voltage were respectively located on the opposite sides.In addition,the discharge performance of DSC-SOFC can be greatly improved by increasing the contact area between electrodes and interconnecters.As a result,at 750?the power density of DSC-SOFC increased by about 40%.