技術論文とプレゼンテーション

Hydrogen has long been recognized as an environmentally friendly and viable alternative fuel for powering vehicles or applications. Consequently, it is crucial to examine and assess different strategies as well as technologies, which support the hydrogen infrastructure. While hydro-gen is an environmentally friendly fuel, the storage of hydrogen has often proven to be either difficult or economically not valuable. For this reason ammonia is often considered, due to its higher volumetric energy density and better storage conditions. Ammonia (NH₃) can be “cracked” into hydrogen (H₂) and nitrogen (N₂), wherein the ammonia conversion usually is being driven by a catalyst at high temperatures. For this purpose usually a single zone ammonia reactor is used, wherein ammonia gas meets a fixed bed catalyst at roughly 650 °C to 850 °C.

While single ammonia reactors are a good practice to convert ammonia into hydrogen, practically PEM fuel cells react poorly to possible residual ammonia in the created hydrogen. In order to reduce the residual ammonia in the ammonia cracking process, the ZWOC project was created. The project aims to develop a novel two-zone ammonia cracker for the efficient and cost-effective production of hydrogen. The zones differ in the used catalyst as well as the used temperature. In order to achieve a high conversion rate and a very low residual ammonia con-tent in the product gas (<100 ppm for PEM fuel cell applications) usually a temperature of approximately 850 °C is necessary due to the chemical equilibrium of the cracking reaction. By integrating a two-zone cracker, the main amount of ammonia can be converted at material-friendly low temperatures of 400 °C to 500 °C with a precious metal catalyst (ruthenium). The residual ammonia content is then to be minimized in the second zone with a temperature resistant base metal catalyst (nickel) at higher temperatures of approximately 650 °C. Due to the high cost of the precious metal catalyst, an optimization approach is used to determine an optimal reactor geometry that minimizes both heating and investment costs while maximizing conversion.

For this purpose, the COMSOL CFD package, the porous media package, the chemical reaction package as well as the heat transfer package was used. The first step was to create a simulation model, which is able to mirror the experimental setup of the nickel and ruthenium catalysts. The second step was to simplify the simulation model to a still sufficient physical approximation in order to minimize the simulation time. The last step was an inclusion of the LiveLink for Matlab® and the simulation model, wherein an adjusted particle swarm optimizer (PSO) was solving for a global minimum. During the PSO each particle is the same simulation setup with different geometry parameters. Consequently, throughout the optimization and simulation process an optimal geometry for the two-zone ammonia reactor was found.