Dynamic Energy Supply By a Pilot Scale Liquid Organic Hydrogen Carrier Unit

Liquid Organic Hydrogen Carriers (LOHCs) can store hydrogen in a chemically bound form at ambient conditions. If there is a need for energy, a dehydrogenation reactor is used to release hydrogen at temperatures at around 580 K. To function as a storage for electrical energy, the chemical reactor has to be supplemented by further equipment. In terms of efficiency heat recovery is crucial, because of the endothermal nature of the reaction. Furthermore, depending on the reconversion technology, hydrogen has to fulfil certain purity demands. Using a PEM fuel cell as a hydrogen reconversion system, its dynamics are limited by an insufficient supply with hydrogen in times of increased electric demand. There are two major process parameters that can be used to increase the hydrogen production rate in the dehydrogenation reactor.

Volume flow of the reactants: increasing the pump flow rate is a fast and dynamic possibility, but results in lower LOHC discharge levels and efficiency due to reduced residence time.

Temperature: increased temperature enhances the hydrogen release, as the reaction responds quite fast to temperature changes. However, dynamics in practice suffer from the thermal inertia of the system.

If hydrogen release and electrical power production do not match, the hydrogen pressure in the LOHC system is changing. Since there is a free gas volume between the fuel cell and the chemical reactor, the hydrogen gas phase can serve as an interim energy buffer. This free volume is constituted not only by the tubing, but also by the heat exchangers and hydrogen purification units. Thus, the LOHC system is able to cover dynamic electrical energy demand, which would exceed the dynamic capability of the dehydrogenation reactor.

To characterize the dynamics of the whole LOHC process, a pilot scale reaction system based on dibenzyltoluene was combined with a heat integration, a hydrogen purification system, a PEM fuel cell and different electricity consumers. In this contribution the dynamic performance of such a system is discussed.