Shipping hydrogen over long distances requires efficient and safe carriers, with liquefied hydrogen (LH2), ammonia, and liquid organic hydrogen carriers (LOHCs) being the primary options. Each method has distinct environmental implications, including CO2 emissions from carrier production, energy losses during transport, and lifecycle impacts. Comparing these to conventional marine fuels such as heavy fuel oil (HFO) and marine gas oil (MGO) reveals trade-offs in emissions, energy efficiency, and sustainability.
Liquefied hydrogen is energy-intensive to produce, requiring temperatures below -253°C to maintain its liquid state. The liquefaction process consumes approximately 30% of the hydrogen's energy content, leading to significant energy losses. During transport, boil-off losses occur due to imperfect insulation, typically ranging between 0.2% and 0.5% per day. If the boil-off gas is not recovered, it contributes to indirect emissions, as hydrogen leakage has a global warming potential (GWP) over a 100-year horizon due to its interactions with atmospheric methane. However, if the boil-off is combusted or re-liquefied, emissions can be mitigated.
Ammonia, a hydrogen derivative, is easier to transport than LH2 due to its higher boiling point (-33°C) and established maritime infrastructure. However, most ammonia today is produced via the Haber-Bosch process using grey hydrogen from steam methane reforming (SMR), which emits approximately 1.8-2.0 tons of CO2 per ton of ammonia. If renewable energy powers ammonia synthesis, emissions drop significantly. Ammonia combustion in marine engines produces nitrogen oxides (NOx), a potent greenhouse gas and air pollutant, though catalytic converters can reduce this. Unlike LH2, ammonia does not suffer from boil-off losses, but its toxicity requires stringent safety measures to prevent environmental contamination.
LOHCs, such as toluene-methylcyclohexane or dibenzyltoluene, store hydrogen chemically and release it through dehydrogenation. These carriers are liquid at ambient conditions, simplifying transport. However, the dehydrogenation process requires high temperatures (250-300°C), consuming additional energy. The lifecycle emissions of LOHCs depend on the energy source for hydrogenation and dehydrogenation. If renewable energy is used, emissions are minimal, but fossil-based energy increases the carbon footprint. LOHCs also have lower hydrogen storage densities (around 6-7% by weight) compared to ammonia (17.6%), leading to higher transport volumes and associated energy costs.
Conventional marine fuels, such as HFO and MGO, have well-documented environmental impacts. HFO combustion emits high levels of sulfur oxides (SOx), particulate matter, and CO2—approximately 3.1 kg of CO2 per kg of HFO burned. MGO has lower SOx emissions but similar CO2 output. The International Maritime Organization’s (IMO) 2020 sulfur cap reduced SOx emissions, but CO2 remains a challenge. Alternative fuels like liquefied natural gas (LNG) emit less CO2 (around 2.75 kg per kg of LNG) but suffer from methane slip, a potent short-term greenhouse gas.
A comparative analysis of emissions per unit of delivered hydrogen energy highlights key differences.
| Carrier | CO2 Emissions (kg CO2/kg H2) | Energy Loss (%) | Key Environmental Concerns |
|------------------|-----------------------------|----------------|-------------------------------------|
| LH2 | 5-10 (liquefaction) | 30-40 | Boil-off losses, high energy input |
| Grey Ammonia | 10-12 (production) | 20-25 | NOx emissions, toxicity |
| Green Ammonia | <1 (renewable-powered) | 20-25 | Lower emissions, NOx control needed |
| LOHCs | 8-15 (fossil-based) | 25-35 | Low H2 density, dehydrogenation energy |
| HFO | 25-30 (combustion) | 5-10 | SOx, particulate matter, high CO2 |
| MGO | 23-28 (combustion) | 5-10 | Lower SOx, still high CO2 |
Boil-off in LH2 shipping presents a unique challenge. If vented, hydrogen contributes to indirect warming effects. If burned, it produces water vapor, a less harmful byproduct but still an energy loss. Ammonia avoids boil-off but introduces NOx risks. LOHCs trade energy density for ease of handling, requiring more trips or larger ships for equivalent hydrogen delivery.
The lifecycle impacts extend beyond direct emissions. LH2 requires specialized cryogenic tanks made of rare materials, increasing embodied carbon. Ammonia production relies on hydrogen feedstock, so unless green hydrogen is used, upstream emissions remain high. LOHCs need energy-intensive dehydrogenation plants at the destination, adding to their footprint.
In contrast, conventional fuels have simpler supply chains but higher combustion emissions. The shift to hydrogen-derived marine fuels must address these trade-offs. Green ammonia and LOHCs produced with renewable energy offer the lowest lifecycle emissions, while LH2 remains constrained by energy-intensive liquefaction.
Future improvements in insulation, boil-off recovery, and renewable-powered synthesis could reduce the environmental footprint of hydrogen shipping. However, each carrier’s viability depends on infrastructure development, regulatory support, and cost reductions in clean hydrogen production. Until then, the transition from conventional fuels will require balancing emissions, energy efficiency, and safety across the supply chain.
The environmental assessment underscores that no hydrogen carrier is without trade-offs. While LH2, ammonia, and LOHCs offer pathways to decarbonize shipping, their full lifecycle impacts must be minimized through renewable energy integration and technological advancements. Compared to conventional marine fuels, hydrogen-based options can significantly reduce CO2 emissions if produced cleanly, but challenges in energy losses and secondary pollutants remain critical hurdles.