Ammonia (NH3) may reduce CO2 emissions from the marine industry as it is carbon-free and more energy-dense than hydrogen or batteries. Pure ammonia exhibits poor combustion characteristics, so several researchers have already operated small-scale internal combustion engines (ICEs) with ammonia-hydrogen blends. However, nitrogen oxides NOx and unburned ammonia are major objections that require the after-treatment of exhaust gases. Experimental studies with solid oxide fuel cell (SOFC) stacks up to 1 kW demonstrated that SOFCs can convert ammonia to electricity with the highly endothermic ammonia cracking reaction at the fuel electrode or before the SOFC as an intermediate step. Although SOFCs tend to be more efficient and emit fewer NOx compared to ICEs, their power density, dynamic behaviour and investment costs are inferior. Different authors came up with integrations of mostly methane-fuelled SOFCs with gas turbines or internal combustion engines.

This work presents an ammonia-fuelled SOFC--ICE hybrid system in which hydrogen off-gas from an SOFC enhances ammonia combustion in an ICE. 0-D components models are developed in an integrated model to quantify the effect of different parameters on fuel efficiency, power density, and heat management.

The hybrid system simulations vary three parameters, all in three different ways. First, the SOFC provides either 25, 50 or 75% of the hybrid system power, resulting 25/75, 50/50 or 75/25 SOFC/ICE power splits. Secondly, either 0, 50 or 100% of the hydrogen in the SOFC is obtained from external cracking before the stack. Thirdly, the share of hydrogen in the ammonia-hydrogen ICE fuel blends is varied: 20, 30 and 40%. This leads to 27 distinct simulations, all at 5000 A/m2 SOFC current density. Besides, current density is varied from 2500 and 5000 to 9000 A/m2 in the 50/50 SOFC/ICE power split configuration.

Full internal cracking of ammonia in the SOFC results in an electrical efficiency of up to 58% and 54% (based on the lower heating value) for a 75/25 and 50/50 power split, respectively, but it leads to heat management failures in the 25/75 power split; where 50% external cracking results in 47% efficiency. Complete external instead of internal cracking decreases efficiency by at most 10 percent points (75/25 power split), because additional fuel is needed heat to the external cracker. Moreover, external cracking results in increased power consumption by the cathode air blower. The ICE efficiency, 41% (standalone system) or 43% (being part of the hybrid system), is outperformed in all 27 simulations mentioned above. The after-treatment model predicts the formation of highly unwanted N2O. The computed SOFC--ICE specific volume is 2, 3 or 4 times larger than an ICE, but less fuel tank space required could cancel this out.

This research indicates that an SOFC reduces fuel consumption compared to an ICE, but using hydrogen off-gas with ammonia in an ICE is advantageous for power density and efficiency. Furthermore, operational parameters of the SOFC can be adjusted in favour of the system efficiency until there is too little electrochemical waste heat compared to the heat absorbed by ammonia cracking in the SOFC. Amongst many other topics, future research should include part load and dynamic operation because of the expected benefits of the hybrid system.