Stack-Scale Modeling of Ammonia-Fueled Solid Oxide Fuel Cell

Abstract

Ammonia is one of the promising fuels that can be used as a hydrogen source for solid oxide fuel cells (SOFC) due to its high energy density, high hydrogen content, no carbon emission at the consumption site, easy storage and transportation, and potential for low cost owing to a well-developed infrastructure. Ammonia-fueled SOFCs have been studied through several experimental works at both cell and stack levels, while a limited number of numerical models at cell levels have been devoted to this subject. Both experimental and numerical studies show that the ammonia decomposes chemically in the fuel support layer at the common operating temperatures of the SOFCs due to the catalytic role of the nickel particles for the ammonia decomposition.Here, we present the first Multiphysics model of an ammonia-fueled SOFC stack, which couples and solves the transport equations of mass, momentum, species, charges, and heat. The model includes the whole stack with manifolds, active domain, headers, and sealings. For the layered domains of the stack, a homogenization approach is used, which replaces them with an equivalent domain and solves for the effective variables [1-3]. Several research groups and stack manufacturers have been using this homogenization approach to model SOFC stacks due to the advantage of reasonable computational expenses and so runtimes. This makes systematic studies of operating conditions, design changes, and even degradation at the stack-scale computationally feasible [1].An ammonia decomposition reaction rate model has been developed and validated thoroughly in [4]. The model shows that the ammonia decomposition occurs over the thickness of the fuel support layer, depending on the amount of ammonia in the fuel channel and the operating temperature. With the homogenized model, the variations over the thickness of the electrode are effectively described by a well-accepted 0D model developed, see e.g. [5].Here, we present such a 0D model that has been validated with a 2D through-plane model of the fuel side of a single cell. We show that the 0D model can describe the effective penetration depth of the ammonia in the fuel support layer through the balance of the fluxes to/from the support layer. The developed relation for the effective ammonia penetration depth in the support layer approximates well the one obtained from the detailed 2D model. A similar idea can be used to find the effective penetration depth of methane in the support layer for the case of a methane-fired SOFC and facilitate its stack-scale modeling with the homogenized model.Ammonia is added to the stack-scale model with the 0D model, which correctly describes the decomposition rate per unit area. The model reproduces well-reported trends from experiments with ammonia-fueled SOFC stacks. Internal cracking of the ammonia is advantageous from a cost/efficiency perspective as the heat consumption of the cracking process facilitates stack cooling; however, depending on the conditions of operation, the ammonia decomposition could be too fast, making a sharp temperature drop at the inlet. This leads to increased thermal stresses in the region and increases the risk of mechanical failure. The model is used to investigate the effects of the operating conditions, e.g. temperature and flow rates of the fuel and air, and flow configuration on the temperature gradients at the inlet to identify safe operating conditions.