Performance comparison of the mass transfer models with internal reforming for solid oxide fuel cell anodes Shuping Wang, William M. Worek ⇑ , W.J. Minkowycz Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W Taylor Street, Chicago, IL 60607, United States article info Article history: Received 5 January 2012 Accepted 7 March 2012 Available online 27 April 2012 Keywords: Solid oxide Fuel cell Internal reforming Model abstract In this work, models describing multicomponent gas diffusion process in an electrode of a porous solid oxide fuel cell (SOFC) anode coupled with internal reforming reactions were developed. The perfor- mances of three different types of models, the dusty-gas model (DGM), the binary-friction model (BFM) and the cylindrical pore interpolation model (CPIM), were compared in 1D. All these models take into account Knudsen diffusion and molecule–molecule diffusion can be used in transition region which is generally the case in a SOFC electrode. The developed models are able to predict the fuel components’ molar fraction distributions in the anode electrode, and the concentration overpotential. They are capable of simulating the internal reforming process for hydrocarbon fuel, such as natural gas, with kinetic mod- els considering both methane-steam reforming (MSR), and water–gas shift reaction (WSR). The effects of pressure gradient, pore size, current density, are studied. It was found that three models give similar results in difference cases using the same ‘‘tuned’’ tortuosity factor (s 2 ). The difference caused using the isobaric assumption is negligible for the H 2 –H 2 O–Ar and CO–CO 2 system, expect at small pore sizes (under 1 lm) and high current density (above 1 A/cm 2 ). For a system fed with hydrocarbon fuel, the iso- baric assumption will change the molar fraction distribution by up to 10% for different gas mixture com- ponents for the CPIM and the BFM, and up to 25% for the DGM at small pore sizes. However, the reaction rates for both MSR and WSR remain the same when the pressure variation is neglected. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Due to its high energy conversion efficiency and fuel flexibility, SOFCs are considered to be one of the most promising technologies for future central and distributed power generation systems. SOFCs typically operate at a temperature between 600 °C and 1000 °C. The high operation temperature makes internal reforming a feasi- ble option so that they can directly operate using a hydrocarbon fuel such as natural gas and syngas. In addition, the high quality exhaust gas makes it an ideal candidate to form a bottoming cycle for a cogeneration system. It has been demonstrated that an atmo- spheric pressure tubular SOFC operate with a combined heat and power systems (CHP) can achieve electrical efficiencies greater than 45% and energy efficiencies near 75% [1]. A planar anode-supported SOFC cell is essentially a sandwiched structure consists of two bi-polar plates, anode and cathode elec- trodes, and an electrolyte layer in the middle. The anode electrode thickness is typically in the range of 500 lm to 1000 lm, the elec- trolyte layer thickness is about 10 lm. For the cathode electrode, its thickness is around 50 lm [2]. The electrical conductivity for the electrolyte material (typically YSZ) is much lower than the other materials used in a SOFC cell, and it increases as the opera- tion temperature decreases. So, using a thick electrode to bear the mechanical stress and a thin electrolyte layer, the operating temperature can be lowered to the range of 500 °C to 800 °C with- out significantly increasing the overall ohmic overpotential. Anode-supported configurations are usually considered superior than cathode-supported ones. It has been shown [3], the cathode concentration overpotential is negligible compared to the one at the anode. When a hydrocarbon fuel is used, internal reforming reactions take place in the anode channel and at the anode elec- trode. The multicomponent gas transport process, coupled with chemical reaction in the anode, is much more complicated and important than the gas transport process occurs in the cathode where only O 2 and N 2 participate. As a result, many works have focused on modeling the gas transfer process on the anode side [4–8]. Some investigators have also considered internal reforming in their simulation [9–11]. It was reported, the diffusion is the dominant transport mecha- nism in the porous electrode [4]. In the SOFCs’ porous anode electrode, the Knudsen number, which is defined as the ratio of 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.03.024 ⇑ Corresponding author. Tel.: +1 (630) 240 6590. E-mail address: wworek@uic.edu (W.M. Worek). International Journal of Heat and Mass Transfer 55 (2012) 3933–3945 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt