Modeling and parametric simulations of solid oxide fuel cells with methane carbon dioxide reforming

https://doi.org/10.1016/j.enconman.2013.02.008Get rights and content

Abstract

A two-dimensional model is developed to simulate the performance of solid oxide fuel cells (SOFCs) fed with CO2 and CH4 mixture. The electrochemical oxidations of both CO and H2 are included. Important chemical reactions are considered in the model, including methane carbon dioxide reforming (MCDR), reversible water gas shift reaction (WGSR), and methane steam reforming (MSR). It’s found that at a CH4/CO2 molar ratio of 50/50, MCDR and reversible WGSR significantly influence the cell performance while MSR is negligibly small. The performance of SOFC fed with CO2/CH4 mixture is comparable to SOFC running on CH4/H2O mixtures. The electric output of SOFC can be enhanced by operating the cell at a low operating potential or at a high temperature. In addition, the development of anode catalyst with high activity towards CO electrochemical oxidation is important for SOFC performance enhancement. The model can serve as a useful tool for optimization of the SOFC system running on CH4/CO2 mixtures.

Highlights

► A 2D model is developed for solid oxide fuel cells (SOFCs). ► CH4 reforming by CO2 (MCDR) is included. ► SOFC with MCDR shows comparable performance with methane steam reforming SOFC. ► Increasing CO electrochemical oxidation greatly enhances the SOFC performance. ► Effects of potential and temperature on SOFC performance are also discussed.

Introduction

Solid oxide fuel cells (SOFCs) are very promising electrochemical devices for stationary power generation [1]. Working at a high temperature (i.e. 673–1273 K), SOFCs have a few advantages: (1) use of low cost catalyst (i.e. Ni as anode) due to fast electrochemical reaction rate; (2) relatively low activation loss compared with low temperature fuel cells; (3) potential for combined heat and power (CHP) cogeneration as the waste heat from the SOFC stack is of high quality and can be recovered; and (4) fuel flexibility – high working temperature enables direct internal reforming of hydrocarbon fuels or thermal decomposition of ammonia, thus SOFCs can make use of alternative fuels, including hydrogen, methane, coal gas, bio-ethanol, ammonia, dimethyl ether (DME), and other hydrocarbon fuels [2], [3], [4], [5], [6], [7]. The fuel flexibility feature makes SOFCs unique compared with low temperature fuel cells, such as proton exchange membrane fuel cells (PEMFCs), which require very pure hydrogen as fuel [8].

Methane is a widely studied fuel for SOFCs as it’s the major component of natural gas and a key component of towngas and biogas. For CH4 fed SOFCs, steam reforming of CH4 is needed as the direct electrochemical oxidation of CH4 in SOFCs is still very difficult [9], [10]. Extensive experimental and modeling studies have been performed to understand the methane internal steam reforming (MSR) and water gas shift reaction (WGSR) kinetics in SOFCs and their effects on SOFC performance [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. For describing the reaction kinetics of MSR and WGSR, global reaction schemes are widely used due to their easy implementation and less computational time [11], [12], [13], [14], [15], [16], [17]. Detailed elementary reaction schemes are also employed for internal reforming SOFCs [18], [19], [20], [21], [22], [23]. These studies showed that the inclusion of MSR and WGSR greatly influences the transport process and electrochemical performance of the cell. In addition to MSR, methane carbon dioxide reforming (MCDR) has also demonstrated to be feasible for SOFCs [24]. However, there is still no systematic modeling on SOFCs with MCDR. It’s still unclear on whether the performance of SOFC with MCDR is comparable to that of SOFC with MSR. It is also not fully understood on how the inclusion of MCDR can affect the SOFC performance and how to improve the SOFC performance by adjusting the operating conditions. In this study, a 2D numerical model is developed to characterize the transport and reaction phenomena in SOFCs with CO2/CH4 mixture as a fuel. The effects of various operating parameters on the cell performance are investigated.

Section snippets

Model development

The working principles and computational domain for SOFCs with CO2 reforming of CH4 are shown in Fig. 1. Consistent with the previous studies on SOFC with MSR [17], [25], the computational domain in the present study contains the interconnector, anode gas channel, porous anode layer, dense electrolyte, porous cathode layer and the air channel. In operation, CO2/CH4 gas mixture with a molar ratio of 1:1 is supplied to the anode gas channel and air is supplied to the cathode gas channel. In the

Numerical methodologies

The boundary conditions of the 2D model have been reported in the previous publications [25], [43]. The governing equations are discretized and solved with the finite volume method (FVM). The pressure and velocity are coupled with the SIMPLEC algorithm [42]. The iteration scheme is shown in Fig. 2. The program starts from initialization. Initial pressure, temperature, velocity, gas composition, etc., are assigned to the whole computational domain. Based on the initial data, the chemical model

Results and analysis

In this section, simulations are performed to investigate the effects of various operating parameters on performance of SOFCs with MCDR. The values of input parameters are summarized in Table 2. More detailed information about the parameters can be found from the previous publications [25], [29]. As SOFCs are typically operated at a potential of 0.5–0.8 V, simulations are performed for operating potential of 0.5 V and 0.8 V. For SOFC with steam reforming of CH4, usually 30% pre-reformed CH4 fuel

Conclusions

A 2D numerical model is developed to characterize the performance of SOFCs fed with CO2 and CH4 mixtures. The model fully considers the mass transport and heat transfer, the chemical reaction kinetics, and electrochemical reaction kinetics.

Simulations are performed at various inlet temperatures, CO electrochemical oxidation rates, and operating potentials. The computed CO2 reforming reaction rate is consistent with the literature data. The MCDR reaction rate is the highest near the inlet while

Acknowledgements

This research was supported by a Grant (Project Number: PolyU 5238/11E) from Research Grant Council (RGC) of Hong Kong.

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