Ammonia for Efficient CO2-Free Power Generation in SOFC Systems

To respond to the constantly growing demand for green hydrogen to decarbonize energy supplies and industrial processes such as steel production, a hydrogen core network was decided by the German federal government in 2024. As a more easily storable hydrogen derivative with a higher volumetric energy density, ammonia can contribute to the decarbonization of the energy supply of companies and municipalities that are not connected to the hydrogen core network. SOFC systems are suitable for using ammonia as fuel. Due to the high operating temperatures, ammonia will be cracked into hydrogen in the SOFC system itself and therefore can directly be used to generate electricity. Pre-treatment of ammonia prior the system to form hydrogen can therefore be completely avoided. The challenge in the development and operation of efficient NH 3 SOFC systems is the thermal management. With knowledge of the cracking reaction kinetics, the location of the partial cracking reaction can be adjusted, and efficient operation can be achieved by coupling the heat sources in the system with the heat sink due to the cracking reactions. In this work, a process calculation model based on energy and material balances was used to calculate theoretical system efficiencies of stationary operation points in a NH 3 SOFC system with internal heat utilization (see Figure 1). The stack calculation is based on a 0D-model of a Fraunhofer IKTS MK35x stack, which is parameterized with real stack performance data. [ [1] ] The goal of the analysis is to improve the system electrical efficiency. Therefore, the gross electrical efficiency was calculated, as well as the net efficiencies in consideration of the main internal power consumption (BoP). The inverter losses are also considered. Ammonia conversion rates are defined for the stack [formular 1] and the cracker [formular 2], in order to vary the amount of stack internal cracking. The comparison was realized in the model with a 40-cell 1 kW el SOFC stack fixed at 80% fuel utilization and 30,267 A / 0,238 A/cm². Variable parameters are the air flow rate, air blower efficiency (22,5%, 15% and 7,5%) and ammonia conversion rates. The inverter efficiency is assumed to be 95%, which results in nearly equal inverter losses for all the 1kW el rated operation points. Figure 2 shows the performance comparison between an operation point using H 2 /N 2 as reference fuel (OP1), fully pre-cracked ammonia (OP2) and partially stack internal ammonia cracking (OP3). Beside the higher efficiency due to the internal ammonia cracking, ammonia can also further improve the system net electrical efficiency. When shifting the ammonia cracking reaction partially towards the stack in OP3, the endothermal reaction affects the thermal balance in the stack. Consequently, less air flow used for cooling the stack is necessary, reducing the electrical power consumption of the air blower. Thus, the net system electrical efficiency is higher in OP3 compared to the operation points OP1 and OP2. The ammonia conversion rate in the stack in OP3 is determined to for operational reasons. To ensure stable operation conditions, the limit for the minimum air ratio is set to 2 for the stack. The minimal decrease of gross electrical efficiency in OP3 compared to OP2 stems from a slightly lower Nernst voltage in the calculation model, which is caused by the reduced oxygen partial pressure when using less air flow. But this effect on net electrical efficiency is rather small compared to the electrical power needs of BoP components. The theoretical analysis also shows that there is sufficient heat production in the system to cover all heat sinks in the rated operation points. Therefore, no electrical heaters are necessary, if the heat sinks and sources are thermally well integrated. Experimental work is done to partially validate the theoretical results. In this contribution, testing results will be shown to underline the theoretical findings. In summary, using ammonia leads to efficient operation in a SOFC system with MK35x stacks and the implementation of ammonia fueled SOFC systems is envisaged. Further coupling of the cracking reaction partially to the heat sources in the system reduces the overall system power losses. Regarding the implementation, future challenges are the development and compact integration of the hot components to ensure good thermal management in several operation points as well as for the startup. [1] reference: https://iopscience.iop.org/article/10.1149/10301.0307ecst formular1: X_(NH_3,stack)=(Δn ̇_(NH_3,stack))/n ̇_(NH_3,system,in) formular 2: X_(NH_3,cracker)=(1-X_(NH_3,stack) ) Figure 1 Figure 2