Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas

Walters, Kevin M.; Dean, Anthony M.; Zhu, Huayang; Kee, Robert J.

doi:10.1016/s0378-7753(03)00536-6

Cited by 76 publications

(61 citation statements)

References 17 publications

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“…Only a few studies have been published on the gas-phase kinetics within the anode channels of SOFC [79][80][81][82][83][84]. Walters et al [79] predicted the propensity of coke formation in the anode channels of SOFC fuelled with dry natural gas by applying a simple gas-phase kinetics model of CH 4 pyrolysis and oxidation, and relating the rate of formation of cyclic hydrocarbon species to the propensity of carbon deposition. Sheng et al [80] demonstrated that substantial gas-phase chemistry occurs in SOFC fuelled directly with n-butane by using a complex kinetics model to predict the conversion of butane pyrolysis.…”

Section: Kinetics In the Anode Compartmentmentioning

confidence: 99%

Direct Utilization of Liquid Fuels in SOFC for Portable Applications: Challenges for the Selection of Alternative Anodes

2009

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Solid oxide fuel cells (SOFC) have the advantage of being able to operate with fuels other than hydrogen. In particular, liquid fuels are especially attractive for powering portable applications such as small power generators or auxiliary power units, in which case the direct utilization of the fuel would be convenient. Although liquid fuels are easier to handle and transport than hydrogen, their direct use in SOFC can lead to anode deactivation due to carbon formation, especially on traditional nickel/yttria stabilized zirconia (Ni/YSZ) anodes. Significant advances have been made in anodic materials that are resistant to carbon formation but often these materials are less electrochemically active than Ni/YSZ. In this review the challenges of using liquid fuels directly in SOFC, in terms of gas-phase and catalytic reactions within the anode chamber, will be discussed and the alternative anode materials so far investigated will be compared.

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Section: Kinetics In the Anode Compartmentmentioning

confidence: 99%

Direct Utilization of Liquid Fuels in SOFC for Portable Applications: Challenges for the Selection of Alternative Anodes

2009

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“…Gupta et al 9 reported that at temperatures below 800°C and residence time of a few seconds, only a few percent of the methane fuel is reacted in homogeneous reactions. Walters et al 10 have shown through simulation that the temperature needs to approach 900°C for gas-phase chemistry to play a substantial role for SOFCs using natural gas as a fuel.…”

Section: Modelmentioning

confidence: 99%

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Hao

Goodwin

2007

J. Electrochem. Soc.

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A two-dimensional numerical model of a single-chamber solid oxide fuel cell ͑SCFC͒ operating on hydrocarbon fuels is developed. The SCFC concept is a simplification of a conventional solid oxide fuel cell in which the anode and cathode are both exposed to the same premixed fuel-air mixture, and selective catalysts promote electrochemical oxidation of the fuel at the anode and simultaneous electrochemical oxygen reduction at the cathode. Optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming-shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including yttria-stabilized zirconia vs Ce 0.8 Sm 0.2 O 1.9 electrolytes, the effects of mixed conductivity, and the optimal fuel-to-air ratio are explored. A single-chamber fuel cell ͑SCFC͒ is one in which the anode and cathode are both exposed to the same premixed fuel-air stream, and selective electrocatalysts are used to preferentially oxidize the fuel at the anode and reduce oxygen at the cathode. The latest studies of SCFCs demonstrate great improvements in power density and reduction in operating temperature with innovations in material and system design.1-4 While the efficiency is typically lower than that of conventional dual-chamber solid oxide fuel cells ͑SOFCs͒, SCFCs do not require seals, and allow a very simple gas manifold design. For some applications, notably small-scale power generation, the mechanical simplicity of an SCFC design may make it more attractive than a dual-chamber SCFC, even with a lower efficiency.The need for selective electrocatalysts has several implications for the design of an SCFC. First, an SCFC must operate at a temperature low enough that the catalysts maintain some degree of selectivity; this typically limits the temperature to below 700°C, which is significantly lower than that of conventional SCFCs with an yttria-stabilized zirconia ͑YSZ͒ electrolyte. For this reason, SCFCs demonstrated to date have used ceria-based electrolytes, rather than YSZ.Another implication of the need for selective electrocatalysts is that an SCFC is unlikely to run well, if at all, on hydrogen. Any catalyst that promotes electrochemical oxidation of hydrogen, or electrochemical reduction of oxygen, would very likely promote direct catalytic combustion if exposed to a hydrogen-air mixture. This problem can be dealt with by using a hydrocarbon fuel instead of hydr...

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“…Applying equilibrium calculations to reforming reactions may be straightforward but is often misleading as reported by Walters, Dean et al (2003), requiring the kinetics of the various reactions to be considered. However, as soon as the step is taken to include the kinetics a large number of reactions all occurring simultaneously must be considered.…”

Section: Discussionmentioning

confidence: 99%

The effect of current density and temperature on the degradation of nickel cermet electrodes by carbon monoxide in solid oxide fuel cells

Offer

Brandon

2009

Chemical Engineering Science

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The oxidation of dry Carbon Monoxide (CO) in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) has been studied using a three electrode assembly. Ni/CGO:CGO:LSCF/CGO three electrode pellet cells at 500, 550 and 600C were exposed to dry carbon monoxide for fixed periods of time, at open circuit and under load at 50 and 100 mA cm -2 , in an aggressive test designed to accelerate electrode degradation. It is shown that if the anode is kept under load during exposure to dry CO, degradation in anode performance can be minimised, and that under most conditions the anode showed significant irreversible degradation in performance after subsequent load cycling on dry H 2 . Only at 500C and at 100 mA cm -2 was the degradation in performance after operation on dry CO and subsequent load cycling on dry H 2 within the background degradation rates measured. Where anode performance was compromised, this appeared to be caused by a reduction in the exchange current density for hydrogen oxidation, and the relatively large degradation after load cycling on dry H 2 was primarily caused by an increase in the series resistance of the anode. It is suggested that this increase in series resistance is associated with the removal of carbon deposited in the non-electrochemically active region of the electrode during operation on dry CO, and that operation under load inhibits carbon deposition in the active region. IntroductionInvestigations of Ni cermet anodes operating on CO or CO/CO 2 mixtures have been reported by a number of authors (Mizusaki, Aoki et al. 1990;Eguchi, Setoguchi et al. 1993; 2.Mizusaki, Tagawa et al. 1994;Yoshida, Yamamoto et al. 1994;Aaberg, Tunold et al. 1996;Holtappels, De Haart et al. 1999;Matsuzaki, Hishinuma et al. 1999;Matsuzaki and Yasuda 2000;Lauvstad, Tunold et al. 2002;Weber, Sauer et al. 2002;Jiang and Virkar 2003;Gemmen and Trembly 2006;Sukeshini, Habibzadeh et al. 2006). However these have so far been limited to YSZ containing anodes, and temperatures of 700C and above. Furthermore, thermodynamics predict that in both dry fuels and at temperatures of 600C and below, which are of interest for IT-SOFC operation, carbon deposition is a significant problem. This requires dilution of carbon containing fuel streams with the products of oxidation, or oxygen.The steam to carbon ratio is often used to predict the propensity of carbon deposition under such conditions, most often presented in the form of C-H-O diagrams as shown by . However, thermodynamic predictions are only applicable in ideal conditions, and in an operating fuel cell the reaction kinetics must also be considered. In addition it is also known that; maintaining a fuel cell at reasonable currents can have a protective effect by reducing carbon deposition in the electrochemically active region of the anode (Weber, Sauer et al. 2002;Gunji, Wen et al. 2004); and that novel anode materials can help prevent carbon deposition demonstrated by Brett, Atkinson et al. (2005); and even that some carbon deposition can be beneficial as...

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Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas

Cited by 76 publications

References 17 publications

Direct Utilization of Liquid Fuels in SOFC for Portable Applications: Challenges for the Selection of Alternative Anodes

Direct Utilization of Liquid Fuels in SOFC for Portable Applications: Challenges for the Selection of Alternative Anodes

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

The effect of current density and temperature on the degradation of nickel cermet electrodes by carbon monoxide in solid oxide fuel cells

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