Liquid Organic Hydrogen Carriers (LOHCs) present a promising solution for transporting hydrogen over long distances via maritime routes. Unlike liquefied hydrogen (LH2) or ammonia, LOHCs enable hydrogen storage and transport using existing oil-based infrastructure, offering distinct advantages in safety, handling, and scalability. This article examines the chemical processes, carrier materials, and onboard storage systems involved in LOHC-based hydrogen transport, comparing them with LH2 and ammonia for maritime applications.
### Chemical Processes: Hydrogenation and Dehydrogenation
LOHCs store hydrogen through a reversible chemical reaction. The process involves hydrogenation at the production site and dehydrogenation at the destination.
**Hydrogenation:**
Hydrogen is chemically bonded to an organic carrier molecule under elevated pressure (30–50 bar) and temperature (150–300°C) in the presence of a catalyst, typically platinum, palladium, or ruthenium-based. For example, dibenzyltoluene (DBT), a commonly used LOHC, undergoes hydrogenation to form perhydro-dibenzyltoluene (H18-DBT), storing up to 6.2 wt% hydrogen.
**Dehydrogenation:**
At the destination, the hydrogenated LOHC is heated (250–300°C) over a catalyst to release hydrogen. The dehydrogenated carrier (e.g., DBT) is then shipped back for reuse. This closed-loop system minimizes waste, though energy input is required for both reactions.
### Carrier Materials
Dibenzyltoluene is widely studied due to its stability, low toxicity, and compatibility with conventional fuel infrastructure. Other candidates include toluene, methylcyclohexane, and N-ethylcarbazole, each with varying hydrogen storage capacities (5–7 wt%) and thermal properties. DBT stands out for its high boiling point (390°C), reducing evaporation losses during transport.
### Onboard Storage Systems
LOHCs are stored in standard fuel tanks, similar to diesel or heavy fuel oil, eliminating the need for cryogenic or high-pressure systems. Key considerations include:
- **Temperature Management:** Heating systems maintain LOHCs in liquid form, typically requiring insulation but not extreme cryogenics.
- **Catalyst Integration:** Dehydrogenation reactors onboard or at port must efficiently extract hydrogen with minimal energy loss.
- **Safety Measures:** LOHCs are less flammable than LH2 or ammonia, but precautions against leakage and thermal degradation are necessary.
### Comparison with LH2 and Ammonia
#### Safety
- **LOHCs:** Non-toxic, non-explosive, and liquid at ambient conditions, reducing risks associated with high-pressure or cryogenic storage.
- **LH2:** Requires cryogenic temperatures (-253°C), posing risks of boil-off and embrittlement. Leaks can lead to flammable gas clouds.
- **Ammonia:** Toxic and corrosive, requiring stringent handling protocols. Leaks pose health and environmental hazards.
#### Cost
- **LOHCs:** Lower infrastructure costs due to compatibility with existing tankers and ports. Energy penalties for hydrogenation/dehydrogenation increase operational costs.
- **LH2:** High liquefaction energy (30–35% of hydrogen’s energy content) and specialized cryogenic tanks escalate costs.
- **Ammonia:** Lower synthesis costs than LH2 but requires cracking or direct use, adding complexity.
#### Scalability
- **LOHCs:** Leverages global oil shipping infrastructure, enabling rapid deployment. Limited by dehydrogenation efficiency and catalyst longevity.
- **LH2:** Scalability hindered by energy-intensive liquefaction and scarce cryogenic transport vessels.
- **Ammonia:** Benefits from established production and transport networks but faces regulatory hurdles due to toxicity.
### Energy Efficiency and Environmental Impact
LOHC systems incur energy losses of 30–40% during hydrogenation and dehydrogenation, comparable to ammonia cracking (20–30%) but higher than LH2 boil-off losses (0.2–0.3% per day). However, LOHCs avoid the carbon emissions associated with ammonia synthesis (if derived from fossil fuels) and the extreme energy demands of LH2 liquefaction.
### Operational Challenges
- **Dehydrogenation Latency:** Hydrogen release is slower than LH2 or ammonia cracking, potentially delaying supply chains.
- **Catalyst Degradation:** Repeated cycles degrade catalysts, requiring replacement and increasing costs.
- **Purity Requirements:** Fuel cells demand high-purity hydrogen, necessitating additional purification steps post-dehydrogenation.
### Conclusion
LOHCs offer a pragmatic pathway for maritime hydrogen transport, balancing safety, cost, and scalability. While less energy-efficient than LH2 or ammonia in theory, their compatibility with existing infrastructure and lower regulatory barriers position them as a viable near-term solution. Advances in catalyst technology and dehydrogenation efficiency could further enhance their competitiveness in the emerging hydrogen economy.