Transporting hydrogen over long distances presents significant challenges due to its low energy density in gaseous form and the complexities of handling cryogenic liquid hydrogen. Liquid Organic Hydrogen Carriers (LOHCs) have emerged as a promising solution, offering a balance between energy density, safety, and infrastructure compatibility. This article examines the role of LOHCs in long-distance hydrogen transport, comparing them with ammonia and liquid hydrogen alternatives, while addressing infrastructure needs, energy efficiency, and logistical constraints.
LOHCs are organic compounds that can reversibly absorb and release hydrogen through chemical reactions. They store hydrogen in a liquid state at ambient conditions, eliminating the need for high-pressure or cryogenic storage. Common LOHC candidates include toluene-methylcyclohexane, dibenzyltoluene-perhydro-dibenzyltoluene, and naphthalene-decalin systems. These carriers enable hydrogen to be transported using existing liquid fuel infrastructure, such as tanker ships, railcars, and trucks, with minimal modifications.
The infrastructure requirements for LOHC transport are less specialized than those for liquid hydrogen or ammonia. Since LOHCs are stable at room temperature and pressure, they can be stored and moved using conventional liquid hydrocarbon handling systems. Ports, pipelines, and storage tanks designed for oil derivatives can often be repurposed for LOHCs, reducing capital expenditures. However, hydrogenation and dehydrogenation plants are necessary at loading and unloading points, adding complexity to the supply chain. These facilities require catalysts and heat input, typically between 200-300°C for hydrogen release, which impacts overall energy efficiency.
Energy density is a key advantage of LOHCs. While liquid hydrogen boasts a higher gravimetric hydrogen density (around 70.8 kg/m³), its volumetric energy density is compromised by the need for cryogenic temperatures (-253°C). LOHCs, in contrast, achieve practical hydrogen densities of 50-60 kg/m³ without extreme cooling, making them more efficient for bulk transport. Ammonia (NH3) has a higher hydrogen density (121 kg/m³) but introduces toxicity concerns and requires cracking to extract hydrogen, which consumes additional energy.
Logistically, LOHCs offer safety benefits. Unlike ammonia, which is toxic and corrosive, or liquid hydrogen, which poses explosion risks and requires stringent insulation, LOHCs are non-toxic, non-flammable in their hydrogenated state, and compatible with conventional safety protocols. This reduces regulatory hurdles and insurance costs. However, the dehydrogenation process is endothermic and slow, requiring significant energy input and reducing round-trip efficiency to around 60-70%, compared to 70-80% for ammonia cracking and 80-90% for liquid hydrogen evaporation.
Ammonia transport has distinct advantages, including higher hydrogen density and established global shipping infrastructure. Ammonia is already traded internationally, with dedicated terminals and pipelines in place. However, its toxicity demands specialized handling and emergency measures, while cracking ammonia back into hydrogen requires temperatures above 600°C, often powered by fossil fuels, undermining carbon neutrality. Liquid hydrogen, while pure and simple to dehydrogenate, suffers from boil-off losses (0.2-0.3% per day) and high energy costs for liquefaction (30-40% of hydrogen’s energy content).
The economic viability of LOHCs depends on scale and distance. For distances exceeding 1,000 km, LOHCs can compete with liquid hydrogen and ammonia due to lower transport costs per unit of energy. However, the need for hydrogenation/dehydrogenation facilities at both ends increases capital costs, making shorter routes less attractive. Ammonia may be preferable for very long maritime routes where its energy density offsets cracking costs, while liquid hydrogen is optimal for applications requiring high-purity hydrogen without secondary processing.
Material compatibility is another consideration. LOHCs interact with standard steels and polymers used in fuel logistics, whereas ammonia requires stainless steel or aluminum to prevent corrosion, and liquid hydrogen demands advanced composites or austenitic steels to withstand cryogenic conditions. This further simplifies LOHC adoption within existing supply chains.
In summary, LOHCs provide a pragmatic medium for long-distance hydrogen transport, particularly where repurposing liquid fuel infrastructure is feasible. Their energy density and safety profile make them competitive, though energy losses during hydrogenation and dehydrogenation remain a drawback. Ammonia suits high-volume, long-haul maritime transport despite toxicity, while liquid hydrogen is niche, reserved for applications where purity and rapid deployment outweigh cost penalties. The choice between these carriers hinges on specific route economics, existing infrastructure, and end-use requirements.