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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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Auxiliary power units (APUs) running on carbon-containing fuels provide solutions for high-efficient power supply and emission reduction. In order to ensure safe operation, unwanted degradation caused by carbon depositions should be avoided or controlled. Early detection allows trigerring of counteractions and avoids rapid deterioration of the cell performance as well as mechanical degradation of the cell microstructure and it is presented here. This study also shows that carbon does not only block the active catalyst sites and porous gas channels thus deteriorating the cell performance, but massive carbon depositions lead to destruction of the lattice YSZ-structure. In order to prolong the lifetime of the investigated cells, carbon-dioxide is tested as a possible agent for gasification of deposited carbon. Supplying with CO 2 gasifies carbon to carbon-monoxide, but experimental investigations show further degradation of the cell performance and a reduced methane conversion rate. A method, which represents a combination of CO 2 fed to the anode and an overvoltage applied to the cell, enabled complete performance regeneration in a cell-protecting manner. Solid oxide fuel cells are high-efficient devices, which convert the chemical energy of gaseous fuels directly into electrical energy without additional conversion steps. The heat losses occurring during the operation at temperatures between 600-1000• C can be used as a high-quality heat energy. SOFCs offer a great fuel flexibility and compared to other fuel cell types they can internally reform hydrocarbons. Internal conversion of carbon-containing fuels over nickel as the most widely used catalyst tends to promote carbonaceous deposits on the anode surface as well as inside the anode, causing deactivation of the fuel cells.1-3 The usage of other alternative materials such as Cu-, Gdand Sm-stabilized ceria-based materials or conducting oxides is possible to suppress carbon depositions, but Ni still offers a considerably better conductivity and electrocatalytic performance. 4-7Carbon deposition.-To date many researchers have investigated SOFC anode degradation. Khan et al.8 made a short review of Ni-YSZ degradation mechanisms, classifying these into four types: nickel coarsening, coking, redox instability and sulfur poisoning. A detailed analysis of challenges for nickel steam-reforming catalyst for industrial application can further be found in study from Sehestad.9 However, regarding operation of Ni-YSZ based SOFCs, if the cell is operated under expected conditions, which exclude sulfur in the fuel and the presence of oxygen on the anode side, redox instability and sulfur poisoning can be excluded as possible degradation phenomena. Nickel coarsening is a process where metal powder begins to sinter and small particles grow in size. It reduces the cell conductivity and worsens the cell performance. The sintering of nickel is also known to prone carbon formation, which requires prevention of this mechanism in order to reduce carbon build. 9 Coking can lead to an i...