Hydrogen carriers such as ammonia, liquid organic hydrogen carriers (LOHCs), and liquid hydrogen (LH2) are critical for enabling the hydrogen economy by facilitating storage and transportation. Each carrier presents distinct risk profiles across transportation, storage, and conversion phases. A comparative analysis using standardized metrics reveals key differences in safety, handling requirements, and operational hazards.

### **Transportation Hazards**

**Ammonia (NH3)**
Ammonia is widely transported via pipelines, ships, and trucks, leveraging existing infrastructure due to its established use in fertilizer production. Its risks include high toxicity, requiring stringent leak prevention measures. Inhalation of even low concentrations (300–500 ppm) can be fatal. However, ammonia’s high boiling point (-33°C) allows storage at moderate pressures (10–15 bar) or refrigerated conditions, reducing energy costs compared to LH2. Maritime transport of ammonia has a long safety record, with incidents primarily involving leaks during loading/unloading rather than catastrophic failures.

**Liquid Organic Hydrogen Carriers (LOHCs)**
LOHCs, such as dibenzyltoluene, are transported in liquid form at ambient conditions, eliminating cryogenic or high-pressure risks. Their primary hazard is flammability, similar to diesel fuel, but toxicity is low. Spills are less volatile than ammonia or hydrogen, simplifying containment. However, LOHCs require dehydrogenation at the destination, involving high temperatures (250–300°C) and catalysts, introducing thermal and chemical hazards. Truck transport of LOHCs has shown fewer acute risks than ammonia or LH2, but long-term exposure studies are limited.

**Liquid Hydrogen (LH2)**
LH2 is transported at cryogenic temperatures (-253°C), requiring specialized insulated tanks to prevent boil-off and pressure buildup. Risks include extreme cold burns, embrittlement of materials, and rapid evaporation leading to flammable gas clouds. Truck and maritime transport of LH2 have recorded incidents involving tank failures due to thermal stress. The 1983 NASA LH2 truck explosion demonstrated the potential for rapid phase transitions causing overpressure ruptures. Modern systems incorporate pressure relief valves and vacuum insulation, reducing but not eliminating risks.

### **Storage Hazards**

**Ammonia**
Storage requires corrosion-resistant materials (stainless steel) due to ammonia’s reactivity with copper and zinc. Large-scale storage in salt caverns or pressurized tanks is feasible, but leaks can lead to toxic plumes. Detection systems (e.g., electrochemical sensors) are mandatory. Unlike hydrogen, ammonia does not embrittle steel, simplifying infrastructure.

**LOHCs**
Storage is comparable to conventional fuels, using standard tanks at ambient conditions. Fire risk is mitigated by low vapor pressure, but decomposition at high temperatures can release hydrogen prematurely. Long-term stability is proven, with negligible degradation over multiple cycles.

**Liquid Hydrogen**
Cryogenic storage demands vacuum-insulated tanks to minimize heat ingress. Boil-off rates range from 0.1–0.5% per day, requiring venting or reliquefaction. Hydrogen’s low ignition energy (0.02 mJ) increases explosion risks if leaks occur. Material compatibility is critical—carbon steel becomes brittle, necessitating austenitic stainless steel or aluminum alloys.

### **Conversion Hazards**

**Ammonia Cracking**
Ammonia must be decomposed into hydrogen and nitrogen at 600–900°C using catalysts (e.g., ruthenium). Risks include high-temperature corrosion, ammonia slip (unreacted NH3 contaminating hydrogen), and nitrogen oxide formation if combustion is incomplete.

**LOHC Dehydrogenation**
Endothermic dehydrogenation requires significant energy input, with risks of thermal degradation and catalyst poisoning. Benzene derivatives in some LOHCs raise carcinogenicity concerns during handling.

**Liquid Hydrogen Revaporization**
LH2 must be warmed to gaseous form, requiring heat exchangers. Rapid expansion can cause pressure surges, while impurities (e.g., air ingress) may form explosive mixtures.

### **Standardized Risk Metrics**

A comparative assessment using key metrics:

| Carrier | Toxicity | Flammability | Storage Pressure | Temp. Requirements | Leak Risk | Conversion Complexity |
|------------------|----------|--------------|-------------------|---------------------|-----------|-----------------------|
| Ammonia | High | Moderate | Moderate | Moderate | High | High |
| LOHCs | Low | Moderate | Ambient | Ambient | Low | Moderate |
| Liquid Hydrogen | Low | High | Low (cryogenic) | Extreme | High | Low |

### **Case Studies in Transportation**

**Maritime Ammonia Transport**
A 2021 study of ammonia carriers found that 85% of incidents involved minor leaks during transfer, with no major fatalities due to strict ventilation protocols.

**LOHC Truck Transport**
German trials from 2018–2022 recorded zero hazardous material incidents in over 10,000 km of LOHC transport, attributed to inert handling conditions.

**LH2 Truck Incidents**
The 2019 LH2 trailer leak in California resulted in a temporary road closure due to a hydrogen cloud, though no ignition occurred. Investigations highlighted valve fatigue as a failure mode.

### **Conclusion**

Ammonia poses high toxicity risks but benefits from mature handling protocols. LOHCs offer safer transport and storage but introduce complexity in dehydrogenation. Liquid hydrogen’s extreme conditions demand advanced engineering controls to mitigate cryogenic and flammability hazards. Selection of a carrier depends on balancing infrastructure readiness, safety thresholds, and end-use requirements. Standardized risk assessments must factor in lifecycle handling to ensure safe scaling of hydrogen logistics.