(569fz) Experimental Investigations on LOHC Dehydrogenation Utilizing SOFC Exhaust Gas As Heat Source

This study aims to use the heat from Solid Oxide Fuel Cell (SOFC) exhaust gases to drive the dehydrogenation of Liquid Organic Hydrogen Carrier (LOHC) compounds.As depicted in Figure 1.1, the coupled LOHC-SOFC system comprises two units. The first unit, a dehydrogenation reactor, employs heterogeneous catalytic dehydrogenation to release hydrogen from Perhydro-Benzyltoluene (H12-BT), yielding Benzyltoluene. Heat for this endothermic process is supplied by the second unit, a SOFC. The SOFC converts the released hydrogen into electrical power and high-temperature exhaust gas. Utilizing the thermal energy from the SOFC waste heat holds considerable potential for significantly improving the efficiency of LOHC based hydrogen storage systems. [1,2]

Previous work by Preuster [3] and Peters [4] revealed high potential of integrating an LOHC-SOFC system. It has been demonstrated that the SOFC suffers no adverse effect when fueled by hydrogen saturated with LOHC -, even in long-term operation. Additionally, load changes and system start-up/shut-downs were supported by a combined LOHC-SOFC system. Mueller [5] further explored strategies for heat integration in LOHC dehydrogenation, examining various process variants. These options focused on integrating low-temperature heat, could provide supplementary flexibility to LOHC-SOFC coupling and would facilitating process integration. Ultimately, theoretical considerations indicated the feasibility of heat integration between the LOHC and SOFC unit.

The stack temperature of SOFC systems typically ranges between 600 and 1000 °C, depending on the cell type [6]. However, due to the internal heat integration of the SOFC to heat the fuel and air streams, the rejected heat has a significantly lower temperature.

The pre-commercial SOFC considered in this study, which is currently operated using 100 % natural gas, produces a usable hot exhaust gas at 200 °C. [7]These temperatures are significantly below the current optimum for the dehydrogenation of the used LOHC system. Due to low exhaust gas temperature effective utilization of the hydrogen storage capacity of the LOHC is not trivial from a thermodynamic point of view. [8] To overcome the limit of current LOHC systems, the SOFC system was modified to increase the exhaust gas temperature. Furthermore, we evaluated the LOHC system to expand its existing thermodynamic boundaries, by lowering the hydrogen partial pressure. Additionally, we propose an enhanced reactor design specifically for hot gas utilization. These approaches contribute to the feasibility of an exhaust heat-integrated LOHC-SOFC system.

An exhaust temperature of 350°C can be achieved by a more complex system design proposed in this paper. This adaptation makes an efficient utilization for the LOHC dehydrogenation feasible, without adversely affecting the internal conditions of the SOFC in terms of the inlet temperatures at the anode/cathode interfaces.

In cases where existing SOFC systems are not economical a way has been found to gain advantage from this by co-feeding NG into the dehydrogenation reactor. The presence of a second gas reduces the partial pressure of hydrogen during dehydrogenation, resulting in higher conversion rates at lower dehydrogenation temperatures. Experiments have demonstrated up to a 20 °C reduction in dehydrogenation temperature while maintaining a similar hydrogen release rate.

In conclusion, the findings indicate that adapting the SOFC and LOHC System enables an autothermal process where the endothermic LOHC dehydrogenation is coupled with an SOFC. These insights form the basis for adapting both the SOFC and LOHC unit designs for a coupled LOHC-SOFC system.

References

[1] Müller, Karsten, Simon Thiele, and Peter Wasserscheid. "Evaluations of concepts for the integration of fuel cells in liquid organic hydrogen carrier systems." Energy & fuels 33.10 (2019): 10324-10330.

[2] Li, Longquan, et al. "Assessing the waste heat recovery potential of liquid organic hydrogen carrier chains." Energy Conversion and Management 276 (2023): 116555.

[3] Preuster, Patrick, et al. "Solid oxide fuel cell operating on liquid organic hydrogen carrier-based hydrogen–making full use of heat integration potentials." International journal of hydrogen energy 43.3 (2018): 1758-1768.

[4] Peters, Roland, et al. "A solid oxide fuel cell operating on liquid organic hydrogen carrier-based hydrogen–A kinetic model of the hydrogen release unit and system performance." International journal of hydrogen energy 44.26 (2019): 13794-13806.

[5] Müller, Karsten, Tanja Skeledzic, and Peter Wasserscheid. "Strategies for low-temperature liquid organic hydrogen carrier dehydrogenation." Energy & fuels 35.13 (2021): 10929-10936.

[6] Wahl, Stefanie. Verfahrenstechnische Optimierung Und Leistungsskalierung Eines Festoxid-Brennstoffzellensystems Mit Hilfe Multiphysikalischer Modellierung Und Experimenteller Daten. 1. Auflage. Verl. Dr. Hut, 2015.

[7] Robert Bosch GmbH: Our fuel cell system as a reliable and efficient source of energy for industrial and commercial use. https://www.bosch-hydrogen-energy.com/sofc/applications/industry/. Accessed: 27.02.2024.

[8] Rüde, Timo, et al. "Benzyltoluene/perhydro benzyltoluene–pushing the performance limits of pure hydrocarbon liquid organic hydrogen carrier (LOHC) systems." Sustainable Energy & Fuels 6.6 (2022): 1541-1553.