CFD Modeling: Different Kinetic Approaches for Internal Reforming Reactions in an Anode-Supported SOFC

Fuel cells are electrochemical devices that convert chemical energy into electricity. Solid oxide fuel cells (SOFCs) are a particularly interesting type because they can reform hydrocarbon fuels directly within the cell, which are possible thanks to their high operating temperature. The purpose of this study is to develop an anode-supported SOFC theoretical model, to enhance the understanding of the internal reforming reactions and their effects on the transport processes. A CFD approach, based on the finite element method, is implemented to unravel the interaction between internal reforming reactions, momentum, heat and mass transport. The three different steam reforming reaction rates applied were developed and correlated to experimental studies found in the literature. An equilibrium rate equation is implemented for the water-gas shift reaction. The result showed that the reaction rates are very fast and differ quite a lot in the size. The pre-exponential values, in relation to the partial pressures, and the activation energy was affected the reaction rate. It was shown that the anode structure and catalytic composition have a major impact on the reforming reaction rate and on the cell performance. The large difference between the different activation energies and pre-exponential values found in the literature reveals that several parameters probably have significant influence on the reaction rate. As the experiments with the same chemical compositions can be conducted on a cell or only a reformer, it is important to reflect over the effect this has on the kinetic model. To fully understand the effect of the parameters connected to the internal reforming reaction, micro scale modeling is needed

Modeling Analysis of Different Renewable Fuels in an Anode Supported SOFC

Background. It is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and possibility to use renewable fuels. Fuels, such as biogas, can be produced locally close to the customers. The improvement for fuel cells during the last years has been fast, but the technology is still in the early phases of development, however the potential is enormous. Method of approach. A CFD approach (COMSOL Multiphysics) is employed to investigate effects of different fuels such as biogas, pre-reformed methanol, ethanol and natural gas. The effects of fuel inlet composition and temperature are studied in terms of temperature distribution, molar fraction distribution and reforming reaction rates within a singe cell for an intermediate temperature solid oxide fuel cell (IT-SOFC). The developed model is based on the governing equations of heat-, mass- and momentum transport, which are solved together with global reforming reaction kinetics. Results. The result shows that the heat generation within the cell depends mainly on the initial fuel composition and the inlet temperature. This means that the choice of internal- or external reforming has a significant effect on the operating performance. Conclusions. The anode structure and catalytic characteristic have a major impact on the reforming reaction rates and also on the cell performance. It is concluded that biogas, methanol and ethanol are suitable fuels in a SOFC system, while more complex fuels need to be externally reformed

Comparison of Humidified Hydrogen and Partly Pre-Reformed Natural Gas as Fuel for Solid Oxide Fuel Cells applying Computational Fluid Dynamics

A three-dimensional computational fluid dynamics (CFD) approach based on the finite element method (FEM) is used to investigate a solid oxide fuel cell (SOFC). Governing equations for heat, gas-phase species, electron, ion and momentum transport are implemented and coupled to kinetics describing electrochemical as well as internal reforming reactions. The model cell design is based on a cell from Ningbo Institute of Material Technology and Engineering in China and the electrochemical area-to-volume ratios are based on experimental work performed at Kyushu University in Japan. A parameter study is performed focusing on the inlet fuel composition, where humidified hydrogen, 30 % pre-reformed natural gas (as defined by IEA) and 50 % pre-reformed natural gas (as defined by Kyushu University) are compared. It is found that when 30 % pre-reformed natural gas is supplied as fuel the air mass flow rate is halved, compared to the case with humidified hydrogen, keeping the inlet and outlet temperatures constant. The current density is decreased but the fuel utilization is kept at 80 %. It is found that the cathode support layer has a significant oxygen gas-phase resistance in the direction normal to the cathode/electrolyte interface (at positions under the interconnect ribs), as well as an electron resistance inside the cathode (at positions under the air channel) in the same direction. The methane steam reforming reaction is shown, both according to the experiments and to the models, to proceed along the main flow direction throughout the cell

SOFC Modeling from Micro- to Macroscale: Transport Processes and Chemical Reactions

The purpose of this work is to investigate the interaction between transport processes and chemical reactions, with special emphasis on modeling mass transport by the Lattice Boltzmann method (LBM) at microscale of the anode of a solid oxide fuel cell (SOFC). In order to improve the performance of an SOFC, it is important to determine the microstructural effect embedded within the physical and chemical processes, which usually are modeled macroscopically. Without detailed knowledge of the transport processes and the chemical reactions at microscale it can be difficult to capture their effect and to justify assumptions for the macroscopic models with regard to the source terms and various properties in the porous electrodes. The advantage of an anode-supported SOFC structure is that the thickness of the electrolyte can be reduced, while still providing an internal reforming environment. For this configuration with an enlarged anode, more detailed knowledge of the porous domain in terms of the physical processes at microscale is called for. In the first part of this study, the current literature on the modeling of transport processes and chemical reactions mechanisms at microstructural scales is reviewed with special focus on the LBM followed by a report on the emphasis to couple conventional CFD to LBM. In the second part, two models are described. The first model is developed at microscale by LBM for the anode of an SOFC in MATLAB. In the LB approach, the main point is to carefully model the diffusion and convection at microscale in the porous region close to the three-phase-boundary (TPB). The porous structure is reconstructed from digital images, and processed by Python. The second model is developed at macroscale for the whole unit cell. For the macroscale model the kinetic model is evaluated at smaller scales to investigate if any severe limiting effects on the heat and mass transfer occur. LBM has been found to be an alternative method for modeling at microscale and can handle complex geometries easily. However, there is still a need for a supercomputer to solve models with several physical processes and components for a larger domain. The result of the macroscale model shows that the three reaction rate models are fast and vary in magnitude. The pre-exponential values, in relation to the partial pressures, and the activation energy affect the reaction rate. The variation in amount of methane content and steam-to-fuel ratio reveals that the composition needs a high inlet temperature to enable the reforming process and to keep a constant current-density distribution. As experiments with the same chemical compositions can be conducted on a cell or a reformer, the effect of the chosen kinetic model on the heat and mass transfer was checked so that no severe limitation are caused on the processes at microscale for an SOFC. For future work, macroscale and microscale models will be connected for the design of a multiscale model. Multiscale modeling will increase the understanding of detailed transport phenomena and it will optimize the specific design and control of operating conditions. This can offer crucial knowledge for SOFCs and the potential for a breakthrough in their commercialization

Evaluation of Lattice Boltzmann Method for Reaction-Diffusion Process in a Porous SOFC Anode Microstructure

In the microscale structure of a porous electrode, the transport processes are among the least understood areas of SOFC. The purpose of this study is to evaluate the Lattice Boltzmann Method (LBM) for a porous microscopic media and investigate mass transfer processes with electrochemical reactions by LBM at a mesoscopic and microscopic level. Part of the anode structure of an SOFC for two components is evaluated qualitatively for two different geometry configurations of the porous media. The reaction-diffusion equation has been implemented in the particle distribution function used in LBM. The LBM code in this study is written in the programs MATLAB and Palabos. It has here been shown that LBM can be effectively used at a mesoscopic level ranging down to a microscopic level and proven to effectively take care of the interaction between the particles and the walls of the porous media. LBM can also handle the implementation of reaction rates where these can be locally specified or as a general source term. It is concluded that LBM can be valuable for evaluating the risk of local harming spots within the porous structure to reduce these interaction sites. In future studies, the information gained from the microscale modeling can be coupled to a macroscale CFD model and help in development of a smooth structure for interaction of the reforming reaction and the electrochemical reaction rates. This can in turn improve the cell performance

Lattice Boltzmann Method for Water-Splitting over Nanorrods with Emphasis on Reactive Mass Transport in 3D

Lattice Boltzmann method (LBM) is an alternative to conventional CFD to capture the detailed activities of the transport processes at microscale. Here LBM is used to model the hydrogen production by splitting water by incident sunlight over water covered Si-nanorods. The purpose of this study is, by a 3D microscale model, to investigate the transport and the formation of the hydrogen bubbles by electrochemical reactions. An ordered array of nanorods is created where each rod is 10 μm high and 10 nm in diameter. The 3D model is simulated using parallel computing with the program Palabos. A multicomponent reaction-advection-diffusion transport for 3 components is analyzed with electrochemical reactions and this process is further coupled with the momentum transport. It has here been shown that LBM can be used to evaluate the microscale effect of electrochemical reactions on the transport processes. An increased Bond number increase the bubble flow through the nanorod domain. A decreased contact angle facilitates the disconnection of the bubble to the nanorod at the top surface. The collection of the hydrogen bubbles at the top surface of the nanorods will be facilitated by an easy disconnection of the bubbles