Theoretical performance analysis of ethanol-fuelled solid oxide fuel cells with different electrolytes

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Abstract

The theoretical performance of ethanol-fuelled solid oxide fuel cells (SOFCs) with oxygen ion conducting and proton conducting electrolytes are presented in this paper. It was reported in a previous work that an SOFC with a proton conducting electrolyte (SOFC-H+) offers higher efficiency than an SOFC with an oxygen ion conducting electrolyte (SOFC-O2−). However, the study was based on the same steam-to-hydrocarbon feed ratio. Our previous work demonstrated the potential benefit of the SOFC-O2− over the SOFC-H+ in terms of a lower requirement for steam in the feed. Therefore, in this article, this benefit is taken into account in the performance comparison. Influences of mode of operation (i.e. plug flow (PF) and well-mixed (WM)) on the performance of SOFCs were also investigated. In the PF mode, two feeding patterns (i.e. co-current (Co) and counter-current (CC)) were considered.

The results show that theoretical SOFC efficiencies depend on the type of electrolyte, mode of operation, inlet H2O:EtOH ratio and fuel utilization. Although it was found that the feeding pattern has an influence on EMF distribution along the cell, the average EMF is not affected. At the best conditions for each type of SOFC, it was observed that SOFC-O2− yields a maximum efficiency at the minimum inlet H2O:EtOH ratio which is the limit for carbon formation for each value of fuel utilization. On the other hand, in SOFC-H+, optimum inlet H2O:EtOH ratios are higher than the limit of carbon formation. At the optimum conditions, the rank of the various SOFCs is as follows: SOFC-H+(PF) > SOFC-O2−(PF) > SOFC-H+(WM) > SOFC-O2−(WM) over the temperature range (1000–1200 K). No difference in SOFC efficiency between both feeding patterns was observed. It is clear from our theoretical studies that the SOFC-H+(PF) is the most promising SOFC system.

Introduction

Fuel cells are currently regarded as the most promising technology for conversion of chemical to electrical energy. Solid oxide fuel cells (SOFC) have attracted considerable interest as they offer the widest range of potential applications, possibility in operation with an internal reformer, and possessing a high system efficiency. Many fuels have been suggested for use in SOFCs; however, among these, ethanol is considered to be an attractive green fuel because of its renewability from various biomass sources including energy plants, waste materials from agro-industries, forestry residue materials, and even organic fractions from municipal solid wastes. They also offer advantages related to natural availability and safety in storage and handling.

There are a number of studies published dealing with the use of ethanol for fuel cells. Ethanol was found to provide higher electrical and overall efficiency than methane in a direct internal reforming molten carbonate fuel cell (DIR-MCFC) [1]. Thermodynamic analysis of an indirect internal reforming molten carbonate fuel cell (IIR-MCFC) revealed that among different fuels (i.e. methane, methanol, and ethanol), ethanol presented the highest power density and the highest cell voltage. At a constant power density, ethanol allows the system to operate close to its thermal equilibrium better than does methanol but not as well as methane [2]. Tsiakaras and Demin [3] investigated performances of SOFCs fuelled by products from different ethanol processing; i.e. steam reforming, dry reforming, and partial oxidation with air. The product from ethanol steam reforming showed the highest maximum efficiency. Performances of external reforming SOFCs (ER-SOFC) fed by different fuels, e.g. methane, methanol, ethanol, and gasoline, were compared within a temperature range of 800–1200 K [4]. It was observed that at low temperatures, methane required a lower inlet steam:fuel ratio to prevent unfavorable coke formation than did methanol and ethanol. Nevertheless, at high temperatures the steam:fuel ratio at the limit of coke formation for ethanol was the same as for methane.

Although two types of electrolytes are possible for the SOFC operation, an oxygen ion conducting electrolyte is more commonly used than a proton conducting electrolyte. Until now, there are very few studies related to the use of the proton conducting electrolytes in SOFCs in the open literature [5], [6], [7], [8]. In addition, all the studies of the ethanol-fed SOFCs employed only the oxygen ion conducting electrolyte. Demin et al. [7] reported an interesting result that an SOFC with a proton conducting electrolyte (SOFC-H+) provides higher efficiency than an SOFC with an oxygen ion conducting electrolyte (SOFC-O2−) for the system fed with methane. The comparison study was based on the same steam:methane feed ratio for both SOFC-O2− and SOFC-H+. It was demonstrated in our previous work that the steam requirement of SOFC-O2− is lower than that of the SOFC-H+ because water produced from the electrochemical reaction of hydrogen appears in the anode chamber [9]. Therefore, when the benefit from the lower steam requirement in SOFC-O2− is taken into account, it is unclear whether the SOFC-H+ still shows better performance than the SOFC-O2−.

In this study, the theoretical performance of ethanol-fuelled SOFCs with two electrolytes in different modes of operation (i.e. plug flow (PF) and well-mixed (WM)) were investigated. Two feeding patterns of the PF mode (i.e. co-current (Co) and counter-current (CC)) were also considered. The efficiencies of SOFC-O2− and SOFC-H+ were compared, taking into account the benefit from the lower steam requirement for SOFC-O2−. This is important in determining whether future SOFCs should be based on the use of the proton conducting electrolyte.

Section snippets

Theory

The reaction system involving the production of hydrogen via ethanol steam reforming can be represented by the following reactions [1]:C2H5OH + H2O = 4H2 + 2COCO + H2O = H2 + CO2CO + 3H2 = CH4 + H2O

Previous results confirmed that a gas mixture at thermodynamic equilibrium contains only five components with noticeable concentration, e.g. carbon monoxide, carbon dioxide, hydrogen, steam, and methane [10], [11].

Two types of solid electrolytes can be employed in the SOFC, i.e. oxygen ion and proton conducting

Results and discussion

The influences of the mode of operation (plug flow and well-mixed), feeding pattern (co-current and counter-current) and type of electrolyte on the partial pressure of each component along the cell are studied. Fig. 1 shows the anode components’ partial pressure at different fuel utilizations (Uf) defined as the moles of hydrogen consumed by the electrochemical reaction divided by the maximum number of moles of hydrogen produced from ethanol (6 mol of hydrogen:1 mol of ethanol). The inlet H2

Conclusions

Thermodynamic analysis of ethanol-fuelled SOFCs using proton and oxygen ion conducting electrolytes in different modes of operation (plug flow and well-mixed) and feeding patterns (co-current and counter-current) has been presented in this article. At stoichiometric inlet H2O:EtOH ratios, the SOFC-H+(PF) provides the highest EMF and efficiency among various electrolytes and modes of operation. In order to compare the theoretical performances of SOFCs with different electrolytes, the benefit of

Acknowledgements

The support from the Thailand Research Fund, Commission on Higher Education, and National Metal and Materials Technology Center are gratefully acknowledged.

References (13)

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    Lin and Beale [9] studied the performances of SOFC considering simplified as well as detailed transport models for H2 fuel using commercially available package on computational fluid dynamics and found that the results of these two approaches show remarkable similarity in terms of temperature, current density and species mass fraction distribution. Jamsak et al. [10] reported the theoretical performance of ethanol-fuelled solid oxide fuel cells (SOFCs) with oxygen ion-conducting and proton-conducting electrolytes and demonstrated the possible benefit of oxygen ion-conducting electrolytes over proton conducting electrolytes in terms of lower steam requirement in the feed. Haynes and Wepfer [11] developed a model considering only ohmic and concentration polarization losses for analyzing the power generation trends of Siemens Westinghouse's tubular SOFCs.

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