Experimental operation, modelling and simulation of solid oxide cell reactors with multiple stacks for process systems

Abstract

Solid Oxide Cell (SOC) reactors are highly efficient electrochemical energy converters. They can be operated in electrolysis mode (SOEC) to produce chemical feedstocks and in fuel cell mode (SOFC) to convert chemical energy into electricity. These characteristics enable them to meet the challenges of the energy transition and the increasing penetration of renewables, such as intermittency and integration into specific industrial sectors and processes. SOC reactors can therefore play a central role in the energy system of the future. Today’s large-scale SOC reactors are composed of multiple stacks/subreactors, resulting in a modular design. However, such an arrangement leads to several operational challenges for further scaling and operation. The objective of this dissertation is to establish a general understanding of the operational behavior of large SOC reactors and to contribute to the deployment of SOC reactors in the future energy system by developing generic scaling, operation and control strategies. In this work, above objective is addressed by experimental and numerical studies of SOC reactors with multiple stacks. The approach is based on the construction and operation of test rigs to study large reactors, the development and application of a simulation framework and a strong interaction between these two. The experiments demonstrated the general operational behavior and provided parameterization as well as validation data for the simulations. These, in turn, supported the experimental investigations, for example, by providing estimates of operating parameters for specific operating points. Finally, the simulations were used to develop operation and control strategies that were iteratively improved by using feedback from the experiments. A unique test rig for the investigation of SOC reactors with multiple stacks was built, which contains a blower for off-gas recirculation at SOC reaction temperature and a pressure vessel for operation under pressure. This pressurized reactor test rig was used to study a modular 30 kW SOFC reactor with multiple stacks. In addition, a simulation framework for the study of process systems with SOC reactors was created. The simulation framework has two unique features. First, it allows modelling of complete SOC reactors consisting of multiple stacks, pipes, manifolds, thermal insulation, and thermal interaction between all these components. Second, the framework provides the capability for transient simulation of not only the SOC reactors, but also all the required BoP components. Both the test rig and the simulation framework were used to develop generic strategies for reliable operation. In a measurement campaign with the pressurized SOC reactor test rig, fuel gas, reactant conversion, and pressure were varied in stationary and transient experiments. The experimental results showed that the operating conditions of the individual stacks of large SOFC reactors vary largely due to flow distribution and heat losses. Methods for the investigation of these critical characteristic parameters were derived from the experimental results. Furthermore, the impact of pressurization and fuel gas recirculation on the SOFC reactor was analyzed. These experimental investigations showed the need to understand the behavior of large SOFC reactors with multiple stacks to increase the performance and robustness of complete process systems. Therefore, the simulation framework was applied to an entire SOC reactor consisting of multiple stacks, pipes, manifolds, and thermal insulation. After experimental validation on stack and reactor level, the model was used to investigate the fundamental behavior of the SOC reactor and its individual stacks in fuel cell and electrolysis mode. Subsequently, the simulation framework was applied to develop operational and control strategies. An example that also provides generic conclusions, is the model of a megawatt scale flexible electrolysis system consisting of twelve reactors with a nominal load of 80 kW. The model was used to define crucial and efficient operation points and to establish transitions between these by comparing different strategies and control approaches. The simulation results showed that systems with SOCs can be operated more transiently than usually assumed. For instance, the start-up time from a hot standby point was reduced by 80 %, while at the same time the temperature gradients were significantly reduced. Furthermore, by taking advantage of the modular nature of state-of-the-art reactors, fast power modulation was achieved. In addition to the study of electrolysis systems, operating strategies for fuel cell operation were developed with a focus on the challenges that arose from the experiments with the pressurized SOC reactor test rig. A control strategy was developed for sub-reactors with separate electrical channels but shared reactant processing units. It uses the operating parameters of each stack in each sub-reactor to improve power and efficiency. This was successfully tested on the pressurized SOC reactor test rig. In addition, a feed forward temperature control was developed and experimentally validated, resulting in a significantly improved controller and the possibility of faster power ramps. A unique test rig was operated for the first scientific investigations focusing on SOC reactors with multiple stacks, and a simulation framework was developed to study large SOC reactors in process systems. Both activities contributed to a better understanding of large reactors and to the integration and operation of SOC reactors in the future energy system. In the process, unique experimental results were obtained and operating as well as control strategies were developed.