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Research Papers

Carbon Deposition Simulation in Porous SOFC Anodes: A Detailed Numerical Analysis of Major Carbon Precursors

[+] Author and Article Information
C. Schluckner

Institute of Thermal Engineering,
Graz University of Technology,
Inffeldgasse 25/B,
Graz 8010, Austria
e-mail: christoph.schluckner@tugraz.at

V. Subotić, C. Hochenauer

Institute of Thermal Engineering,
Graz University of Technology,
Inffeldgasse 25/B,
Graz 8010, Austria

V. Lawlor

AVL List GmbH,
Hans-List-Pl. 1,
Graz 8020, Austria

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received July 13, 2015; final manuscript received October 1, 2015; published online November 12, 2015. Assoc. Editor: Rak-Hyun Song.

J. Fuel Cell Sci. Technol 12(5), 051007 (Nov 12, 2015) (12 pages) Paper No: FC-15-1053; doi: 10.1115/1.4031862 History: Received July 13, 2015; Revised October 01, 2015

Solid oxide fuel cells (SOFCs) can be operated on a wide range of fuels, including hydrocarbons. Such a fuel supply includes the risk of carbon formation on the catalytically active nickel centers within the porous anodic substrate. Coking is very critical for the reliability and durability of the SOFCs. Thus, within this study, coking propensity of the most prominent carbon containing fuels was analyzed by thermodynamic equilibrium calculations for two fundamentally different types of carbon and detailed transient numerical simulations based on heterogeneous reforming kinetics. This approach is new to the literature and reveals the strengths and weaknesses of both fundamentally different approaches. It was shown that in thermodynamic equilibrium calculations, carbon formation is most likely due to pure methane. Carbon monoxide will form the least amounts of carbon in typical SOFC temperature ranges. Furthermore, based on a validated computational fluid dynamics (CFD) simulation model, detailed heterogeneous reaction kinetics were used to directly simulate elementary carbon adsorbed to the reactive substrate surface. The amounts, spatial and temporal distribution, of carbon atoms within the porous structure were identified between 600 °C and 1000°C for a broad steam-to-carbon ratio range of 0.5–2. It was shown that most carbon is formed at the beginning of the substrate. A key finding was that steady-state results differ greatly from results within the first few seconds of fuel supply. An increment in temperature causes a significant increase in the amount of carbon formed, making the highest temperatures the most critical for the SOFC operation. Moreover, it was shown that mixtures of pure methane deliver the highest amounts of adsorbed elementary carbon. Equimolar mixtures of CH4/CO cause second highest surface coverages. Pure carbon monoxide blends led to least significant carbon formations. This work has shown the important contribution that thermodynamic equilibrium calculation results, as well as the outcomes of the detailed CFD simulations, allow to identify suitable operating conditions for the SOFC systems and to minimize the risk of coking on porous anodes.

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Figures

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Fig. 1

Schematic of the 2D flow channel and the computational grid (not to scale)

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Fig. 2

Equilibrium calculations from different fuels

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Fig. 3

Carbon formation from different carbon containing fuels

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Fig. 4

Carbon boundaries for different carbon containing fuels

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Fig. 5

Polarization curves for a synthetic diesel reformate (cf. Fig. 5b for inlet composition): (a) simulated and measured polarization curves of different cell types and (b) experimental and numerical data of anode outlet gas composition for the ASC experiment at 800 °C (cf. Fig. 5a). Markers represent experimental data, and dashed lines represent simulated data calculated by means of this simulation model. Depicted data are taken from Refs. [48] and [50].

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Fig. 6

Major carbon precursors due to different species, S/C ratios, and temperatures at steady state: (a) surface carbon C(s) from CH4, (b) surface carbon C(s) from CO, and (c) surface carbon C(s) from equimolar CH4/CO blends

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Fig. 7

Carbon formation activities for different carbon precursors: (a) thickness-averaged carbon formation activities αI for pure CH4/H2O and equimolar CH4/CO/H2O mixtures at 600 °C, (b) thickness-averaged carbon formation activities αII for pure CH4/H2O, CO/H2O, and equimolar CH4/CO/H2O mixtures at 600 °C, and (c) thickness-averaged carbon formation activities αI for pure CH4/H2O and equimolar CH4/CO/H2O mixtures between S/C1 and S/C2 for 800 °C and 1000 °C

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Fig. 8

Steady-state CFD simulation results: (a) thickness-averaged surface coverage of C(s) for 600 °C, (b) surface coverage contours of C(s) at 600 °C, and (c) thickness-averaged surface coverage of C(s) for S/C0.5 for pure CH4/H2O mixtures at temperatures from 600 °C to 1000 °C

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Fig. 9

Transient-averaged surface coverages of C(s) at the active nickel centers: (a) thickness-averaged surface coverage of C(s) for 600 °C, (b) thickness-averaged surface coverage of C(s) for 800 °C, (c) thickness-averaged surface coverage of C(s) for 1000 °C, and (d) transient surface coverage contours of C(s) at 1000 °C for CH4 at S/C = 0.5

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