It is vital to find the optimal quantity of water added to the fuel in a biogas-fueled Solid Oxide Fuel Cell (SOFC). An under-optimal water quantity can impede steam reforming reactions, while an over-optimal quantity of water (which is equivalent to reduced biogas portion) may reduce the performance of the fuel cell due to a shortage of fuel. Water production in electrochemical reactions and the carbon deposition constraint add to the complexity of water quantity optimization. The present work develops a 3D numerical model to simulate a direct biogas-fueled SOFC to find the optimal steam/biogas (S/C) ratio. Several biogas mixtures (CH4/CO2 ratios) are analyzed. For each mixture, a wide range of S/C ratios is evaluated in a wide range of voltages. It is found that the polarization curves corresponding to different S/C ratios intersect each other for a given biogas mixture. In other words, an S/C ratio can be optimal at some voltages and non-optimal at others. Based on power density maximization as an explicit criterion, the optimal S/C ratio is found to be 0.75 at low CH4 molar fractions (~50%) and 1.25 at high CH4 molar fractions (~75%).
The ever-growing field of micro- and nanotechnology has a great deal of interest in simulating dynamic phenomena of multiscale systems. Hybrid approaches that produce a trade-off between accuracy and computational costs play a key role in this area. In this study, an improved hybrid continuum-atomistic model is proposed for the simulation of electroosmotic flows in nanochannels. The aqueous solvent phase is modeled by the continuum four-way coupled Navier-Stokes equations, while a Lagrangian approach is used for the ion transport. Different forces, including the drag, buoyancy, Brownian, electrostatic, and ion-ion/wall-ion collision, and torques, including the drag and collision, govern the motion of ion particles. The ion-ion/wall-ion collision is taken into account by a discrete phase model, and the electric field is derived by the Poisson-Boltzmann closure. Results of the model, such as the change in bulk velocity with surface electric charge density, are validated by several molecular dynamics simulations and experimental observations available in the literature. It is shown that the present hybrid model is capable of predicting the main features of the problem. Moreover, the significance of different forces and the other alternative for modeling the external electric field, i.e., the discrete Coulomb’s approach with the modified particle mesh Ewald boundary treatment, are also examined. The proposed model would be extremely useful for future studies on the electrokinetics in nanochannels, especially in more complex geometries where the molecular dynamics approaches are limited due to the computational costs.
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