Ammonia has emerged as a promising hydrogen carrier due to its high hydrogen density, ease of storage, and established transportation infrastructure. A life cycle assessment (LCA) of ammonia as a hydrogen carrier involves evaluating its environmental and energy impacts across production, storage, transportation, and end-use stages. This analysis contrasts ammonia with other hydrogen carriers, such as liquid organic hydrogen carriers (LOHCs), liquid hydrogen, and metal hydrides, to assess its viability in a decarbonized energy system.
### Production Phase
Ammonia is primarily produced via the Haber-Bosch process, which combines nitrogen from the air with hydrogen derived from steam methane reforming (SMR), coal gasification, or electrolysis. The energy intensity and emissions of ammonia production depend heavily on the hydrogen feedstock and energy source.
- **SMR-Based Ammonia**: Conventional ammonia production relies on hydrogen from SMR, emitting approximately 1.6–2.4 kg CO₂ per kg of ammonia. The process consumes 28–32 GJ per ton of ammonia, with natural gas as the primary energy source.
- **Electrolysis-Based Ammonia**: If renewable electricity powers electrolysis, emissions drop significantly. However, the energy requirement remains high (35–40 GJ/ton) due to the inefficiencies in electrolysis and Haber-Bosch synthesis.
- **Alternative Production Methods**: Emerging methods, such as electrochemical ammonia synthesis, aim to reduce energy use but remain at laboratory scale.
Compared to other carriers, ammonia production is more energy-intensive than LOHCs (e.g., methylcyclohexane) when derived from fossil fuels but can be cleaner with renewable inputs. Liquid hydrogen production via electrolysis requires less energy per kg of H₂ but faces higher storage and transport losses.
### Storage and Transportation
Ammonia’s advantages become evident in storage and transport. It liquefies at -33°C or under moderate pressure (10 bar at 25°C), making it easier to handle than liquid hydrogen (-253°C).
- **Energy Use**: Ammonia storage requires minimal energy compared to cryogenic hydrogen storage, which demands continuous refrigeration. LOHCs need additional energy for hydrogenation and dehydrogenation.
- **Infrastructure Compatibility**: Existing ammonia shipping and storage infrastructure reduces capital costs. In contrast, liquid hydrogen requires specialized cryogenic tanks, and LOHCs rely on modified petroleum infrastructure.
- **Losses**: Ammonia experiences negligible boil-off losses, unlike liquid hydrogen, which can lose 0.5–1% per day due to evaporation.
### End-Use and Conversion
The end-use phase involves extracting hydrogen from ammonia or using ammonia directly. Two primary methods exist:
1. **Ammonia Cracking**: Thermal cracking splits ammonia back into hydrogen and nitrogen at 400–600°C, consuming 12–15% of the energy content of hydrogen. Catalysts can lower the temperature but add cost.
2. **Direct Ammonia Utilization**: Fuel cells and combustion systems can use ammonia directly, avoiding cracking losses. However, NOx emissions from combustion require mitigation.
Compared to alternatives:
- LOHCs require dehydrogenation at 250–300°C, with energy penalties of 20–30%.
- Liquid hydrogen is ready-to-use but suffers from evaporation losses.
- Metal hydrides release hydrogen at low temperatures but have low gravimetric density.
### Emissions and Environmental Impact
A full LCA reveals trade-offs between ammonia and other carriers:
- **Carbon Emissions**: SMR-based ammonia has higher CO₂ emissions than electrolysis-based hydrogen or LOHCs from renewable sources. However, renewable ammonia can achieve near-zero emissions.
- **NOx Emissions**: Ammonia combustion or cracking may produce NOx, requiring catalytic reduction. Liquid hydrogen and LOHCs do not have this issue.
- **Energy Efficiency**: The round-trip efficiency (production to end-use) of ammonia is 40–50% with cracking, compared to 50–60% for LOHCs and 60–70% for liquid hydrogen (assuming renewable inputs).
### Comparative Analysis with Other Carriers
The table below summarizes key LCA metrics for ammonia versus other carriers:
| Carrier | Production Energy (GJ/ton H₂) | Storage Conditions | Round-Trip Efficiency | CO₂ Emissions (kg/kg H₂) |
|------------------|-------------------------------|--------------------------|------------------------|--------------------------|
| Ammonia (SMR) | 120–150 | -33°C or 10 bar | 40–50% | 10–12 |
| Ammonia (Electrolysis)| 140–170 | -33°C or 10 bar | 45–55% | <1 |
| Liquid H₂ | 90–110 | -253°C | 60–70% | <1 (renewable) |
| LOHCs | 130–160 | Ambient | 50–60% | 2–5 (fossil-based) |
| Metal Hydrides | 100–120 | <100°C | 60–75% | <1 (renewable) |
### Conclusion
Ammonia presents a viable hydrogen carrier with strengths in storage and transport but faces challenges in energy-intensive production and cracking. When produced from renewable sources, its environmental impact is competitive with other carriers. For long-distance transport and large-scale storage, ammonia’s existing infrastructure and stability offer distinct advantages. However, liquid hydrogen and LOHCs may outperform ammonia in applications requiring high-purity hydrogen or lower-temperature utilization. The optimal carrier depends on specific use cases, energy sources, and infrastructure availability. Future advancements in ammonia cracking and green production methods could further enhance its role in the hydrogen economy.