Investigation of an Intermediate Temperature SOFC Operation in Series with a CPOx Reformer
Authors: Serincan Mustafa Fazil , Hickey Darren , Wang Honggang 
Published: September 2023

Solid oxide fuel cells are known for their fuel flexibility which makes them a great candidate for decentralized power generation and off-grid applications. Depending on the target application, there have been different system configurations such as steam reforming vs. CPOx reforming, external reforming vs. internal reforming, anode recirculation vs. single pass. The CPOx reforming in series with SOFC with single pass configuration opens up the intermittent power source market, due to its unique benefit of low cost, short startup time and easy maintenance. In this scheme fuel cell electrochemical performance is directly related to the syngas composition, i.e. H2 and CO fractions in the fuel stream, as well as any unreformed methane.

In this study a computational fluid dynamics (CFD) model has been developed to study the dynamic and steady state behavior of a commercial intermediate temperature SOFC stack operating in series with a CPOx reformer. The CFD model of the SOFC accounts for the charge, species, momentum and energy balances, whereas a reduced order model for the CPOx incorporates equilibrium thermodynamics. CPOx is an exothermic catalytic reaction with an overall balance of:

CH4+1/2O2 → 2H2+CO (1)

which is defined in stoichiometric ratios. In practice the air flow is regulated to control the reactor temperature and syngas yield. Hence, along with temperature and pressure the ratio of oxygen to carbon (O-C ratio) in the left hand side of Eq. 1 is a critical parameter for determining the overall performance and efficiency of the system. Moreover, the CPOx operation typically has different routes to achieve desired products which constitutes a system with various reaction equilibria [i]. However, apart from Eq.(1) the dominant reaction paths can be summarized as [ii]:

Reformation CH4+CO2 ↔ 2H2+2CO (2)

Water gas shift CO+H2O ↔ H2+CO2 (3)

Boudouard CO2 + C ↔ 2CO (4)

CH4 Combustion CH4+2O2 → 2H2O+CO2 (5)

CO Combustion 2CO+O2 → 2CO2 (6)

H2 Combustion 2H2+O2 → 2H2O (7)

Of these, combustion reactions can be practically considered as irreversible. Overall system equilibrium is a result of these reactions and is a function of temperature, pressure and O/C ratio. Any changes in these parameters shift the reaction equilibrium, hence the conversion efficiency and syngas yield. As a result, SOFC performance will be directly dependent on these parameters.

In this study different operating regimes of CPOX and SOFC have been investigated. The model is validated experimentally with polarization data and gas chromatography analysis. The model predictions show good agreement with the GC data for both the CPOX and SOFC outlet gas compositions.

Performance of the SOFC running on syngas obtained as a result of different CPOx equilibria is compared and a fuel cell efficiency map as a function of temperature and O-C ratio has been constructed. Also, dynamic response of SOFC to the changes in CPOx operating parameters is estimated with the transient simulations. Moreover, selectivity of SOFC toward H2 and CO has been investigated by outlining the different energy conversion paths inside the fuel cell electrode. It is found that SOFC at intermediate temperatures has an overall tendency for CO conversion due to enhanced WGSR reaction inside the electrodes although the standalone CO oxidation overvoltage is higher than that of H2 oxidation. Finally, SOFC performance is analyzed for injecting various amounts of unreformed CH4 into the syngas stream.

[i] Bjørn Christian Enger, Rune Lødeng, Anders Holmen, A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts, Applied Catalysis A: General 346 (2008) 1–27.

[ii] Ruoshui Ma, Bang Xu, Xiao Zhang, Catalytic partial oxidation (CPOX) of natural gas and renewable hydrocarbons/oxygenated hydrocarbons—A review, Catalysis Today 338 (2019) 18–30.

Figure 1