The study seeks to reduce emissions in the shipping industry by exploring a propulsion system based on hydrogen internal combustion engines (H2ICEs) and liquid organic hydrogen carrier (LOHC) technology. While both technologies are strong contenders for future propulsion systems, their combined use has not yet been extensively researched. However, this combination could be both feasible and advantageous, and similar integrations have been studied for other hydrogen power systems.
A literature review is conducted on the integration of LOHC reactors with proton exchange membrane fuel cells (PEMFCs), solid oxide fuel cells (SOFCs), and H2ICE, specifically focusing on the hydrogen carrier H18-DBT. H18-DBT can store 6–8 wt.% hydrogen and comes close to meeting energy density requirements, although dehydrogenation losses are significant. Decreasing dehydrogenation losses is possible through waste heat recovery (WHR). DBT is favored for its low flammability and toxicity and high commercial availability, but environmental and viscosity issues remain.
The study identifies gaps in the literature by examining hydrogen carrier-based power systems, focusing on heat utilization methods for dehydrogenation, and quantifying the potential of WHR to improve system efficiency and exercise. This is especially relevant for H18-DBT due to its high dehydrogenation enthalpy.
For SOFC systems, the literature review concludes that direct heat transfer from high-temperature exhaust gases significantly enhances system efficiency and energy. Given an optimized design, WHR from SOFCs can fully support the dehydrogenation energy requirement under dynamic loads, which benefits overall system performance. In contrast, for PEMFC systems, WHR poses challenges due to the low-grade heat available, but preheating DBT before reactor entry improves efficiency. However, existing integration studies for PEMFCs may be overly optimistic, as they do not account for energy destruction at higher current densities. Nonetheless, coupling PEMFCs with LOHC reactors remains feasible and beneficial for system efficiency.
The integration of H2ICE with LOHC reactors and WHR appears promising based on the availability of high-temperature exhaust gases, similar to SOFC systems. The literature on port fuel injection (PFI) and direct injection (DI) H2ICE technologies shows potential for efficiency gains through WHR from exhaust gases and coolant.
To assess the efficiency improvements from WHR in H2ICE and their overall feasibility, this study proposes a system design and develops a model that simulates mass and energy balances for all operating points of an H2ICE. This model allows for comparison with other potential propulsion systems. A conceptual system-level model is created using MATLAB to analyze the WHR and coupling potential of H2ICE with LOHC reactors. The model uses two empirical engine models (for DI and PFI, respectively) and two reactor modeling approaches: a basic thermodynamic model and a kinetic model.
The model iterates until the reactor and H2ICE operating points are feasible with respect to the mass balances and determines the optimal reactor setting requiring minimum heat for dehydrogenation. It also calculates the mass flow for a hydrogen burner to estimate the additional hydrogen needed for complete dehydrogenation if WHR is insufficient. Available exergy and heat fluxes are calculated to provide a comprehensive review of the energy balance and WHR effectiveness.
Understanding the mass and energy balance is crucial for assessing the operational capabilities of these systems. The model reveals that while WHR integration in H2ICE is beneficial, it is not sufficient to sustain dehydrogenation at all operating points for H18-DBT.
The results indicate that coupling the dehydrogenation reactor with H2ICE is feasible and yields efficiency gains through WHR from exhaust gases and coolant flow. WHR integration with the thermodynamic reactor model reduces hydrogen combustion in the burner by 40% to 60%, increasing overall efficiency by 18.75%. In a 1D heterogenous model, the results were more promising: above 2,500 RPM, enough heat was available to sustain the dehydrogenation reaction. Nevertheless, concerns remain about the 1D model’s analytical derivation equation, as it may underestimate the dehydrogenation heat requirements.