The environmental footprint of hydrogen carriers such as ammonia and liquid organic hydrogen carriers (LOHCs) is a critical consideration in assessing their viability within a sustainable energy system. Both carriers serve as intermediate solutions for hydrogen transport and reconversion, but their life cycle impacts differ significantly due to variations in production pathways, energy requirements, and emissions profiles. This analysis focuses on synthesis, transportation, and reconversion, excluding storage-specific aspects.
### Synthesis Phase
Ammonia (NH3) is primarily synthesized via the Haber-Bosch process, which combines nitrogen from the air with hydrogen under high pressure and temperature. The hydrogen feedstock is typically derived from steam methane reforming (SMR), coal gasification, or electrolysis. The Haber-Bosch process is energy-intensive, with a global average energy consumption of approximately 30-35 GJ per ton of ammonia. When SMR-derived hydrogen is used, the carbon footprint ranges from 1.5 to 2.5 tons of CO2 per ton of ammonia. Electrolysis-based ammonia can reduce emissions significantly if renewable electricity is used, but the energy demand remains high due to the inefficiencies of nitrogen fixation.
LOHCs, on the other hand, rely on aromatic compounds such as dibenzyltoluene or toluene, which undergo hydrogenation to store hydrogen chemically. The hydrogenation process requires hydrogen input, usually sourced from the same methods as ammonia. The energy intensity of LOHC hydrogenation is lower than ammonia synthesis, typically around 10-15 GJ per ton of hydrogen stored. However, the carbon footprint depends heavily on the hydrogen production method. If renewable hydrogen is used, emissions can be minimal, but fossil-based hydrogen results in 8-12 kg of CO2 per kg of hydrogen stored in LOHCs.
### Transportation Phase
Ammonia has a high volumetric hydrogen density (121 kg H2/m³ at 10 bar), making it efficient for long-distance transport. Shipping ammonia via tankers emits approximately 30-50 g CO2 per ton-kilometer, depending on vessel efficiency and fuel type. Pipeline transport is also feasible, with emissions similar to those of natural gas pipelines when considering compression energy. However, ammonia’s toxicity necessitates stringent safety measures, increasing operational complexity.
LOHCs are transported in liquid form at ambient conditions, eliminating the need for cryogenic or high-pressure containment. Their energy density is lower than ammonia (around 60 kg H2/m³), leading to higher transport volumes for the same hydrogen quantity. Shipping emissions for LOHCs are comparable to those of conventional liquid fuels, at 40-60 g CO2 per ton-kilometer. Truck and rail transport are also viable but less efficient than ammonia for large-scale shipments due to lower energy density.
### Reconversion Phase
Releasing hydrogen from ammonia requires cracking, which involves heating the compound to around 400-600°C in the presence of a catalyst. This process consumes 8-12 GJ per ton of hydrogen recovered, with associated emissions of 0.5-1 ton CO2 per ton of hydrogen if fossil energy powers the heating. Advanced methods using renewable heat or membrane reactors can mitigate these emissions but remain under development.
LOHC dehydrogenation is endothermic, requiring temperatures of 250-300°C and catalytic assistance. The energy demand is slightly lower than ammonia cracking, at 6-10 GJ per ton of hydrogen, but the process often yields trace contaminants that require purification. Emissions range from 0.3 to 0.8 tons CO2 per ton of hydrogen, depending on the heat source. Unlike ammonia, LOHCs can undergo multiple hydrogenation-dehydrogenation cycles, though degradation over time affects long-term efficiency.
### Comparative Life Cycle Assessment
A holistic comparison reveals trade-offs between the two carriers:
- **Energy Efficiency**: Ammonia synthesis and cracking are more energy-intensive than LOHC hydrogenation and dehydrogenation. However, ammonia’s superior transport density can offset losses in long-distance scenarios.
- **Emissions**: Both carriers exhibit similar emission profiles when using fossil-based hydrogen. With green hydrogen, ammonia’s higher process emissions give LOHCs a slight edge.
- **Infrastructure Compatibility**: Ammonia leverages existing fertilizer industry infrastructure, whereas LOHCs integrate with liquid fuel logistics. Retrofitting needs vary by region.
- **Safety and Handling**: Ammonia’s toxicity requires specialized handling, while LOHCs are benign but face challenges in hydrogen purity post-dehydrogenation.
### Conclusion
Ammonia and LOHCs present distinct environmental footprints across their life cycles. Ammonia excels in transport efficiency but suffers from high synthesis and cracking emissions. LOHCs offer lower process energy demands but incur higher transport emissions due to lower energy density. The optimal choice depends on the hydrogen production source, transport distance, and available infrastructure. Both carriers require further optimization to minimize their environmental impacts in a decarbonized energy system.