Numerical Analysis of Process Dynamics of Thermochemical Water Splitting

Kurzfassung

The project HYDROSOL-3D aims to design a 1MWth plant based on a thermo-chemical water splitting cycle. The cycle works as follows: In a first step (about 900 °C), water and metal oxide (MO) react in such a way that the metal oxidizes and thus takes up oxygen from the water, leaving hydrogen as a product gas: H2O + MO_RED --> H2 + MO_OX In a second step, the oxidized metal oxide is reduced at higher temperatures (about 1300 °C); oxygen leaves the material and can be carried out by an inert gas flushing stream. This step regenerates the metal oxide so it can be used for water splitting again: MO_OX --> MO_RED + O2 To get a realistic overview of an achievable efficiency of the complete hydrogen generation process it is important to critically examine all loss mechanisms. Some of these can only be studied thoroughly using dynamic modeling approaches. Therefore, a dynamic process model is being established in the commercial software Aspen Dynamics. The core part of the process model is the reactor which had to be created from the ground up (using Aspen Custom Modeler software), since it is not a standard unit operation available in Aspen. Model validation could be carried out based on test campaign results from predecessor project phases. Other loss mechanisms do not need numerical techniques, but can be analyzed based on thermodynamic considerations. One of these is the loss due to preparation of nitrogen which is used as flushing gas in the process. This flushing nitrogen is required to be of high purity for chemical regeneration of the metal oxide to take place. The minimum work needed to supply the nitrogen is calculated by thermodynamic analysis and different scenarios of purity requirements are compared.