Safety of Ammonia As Hydrogen and Energy Carriers

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Abstract

Ammonia (NH3) is liquefied at 1 MPa and 25 °C, and has a highest volumetric hydrogen density of 10.7 kg H2 /100L. It has a high gravimetric hydrogen density of 17.8 wt%. The heat of formation of NH3 is about 1/10 of combustion heat of hydrogen. NH3 has advantages as a hydrogen carrier for fuel cell vehicles and an energy carrier for power plants. In this research, the purpose is to figure out regulations for safety of NH3 in the world, and survey NH3 accident. We also characterize water as a NH3 absorbent.

Regulations for flammability and health hazard are defined in each region. Major standard regulations in each region are Japanese GHS (The Globally Harmonized System of Classification and Labelling of Chemicals), NFPA (National Fire Protection Association) 704 in USA, CLP (Classification, Labelling and Packaging of substances and mixtures) in Europe. We surveyed those regulations of NH3, gasoline (n-hexane), hydrogen, organic hydride (methylcyclohexane, toluene), natural gas (methane), propane in Japan, USA and Europe. We evaluated correlation between flammability and fuel properties. NFPA flammability of NH3 is low because of high flash point. The flammability increases with decreasing of the flash point, approaching to constant value 4. The CLP and Japanese GHS flammabilities also have similar trend except the value of NH3 in Japanese GHS. This is due to the fact that the classification of flammability obtained by Japanese GHS is defined by explosive limit in air. NFPA health hazards of NH3 and liquid H2 have high value 3 because of low 50% lethal concentration of rat etc. The NFPA health hazard is similar to that in CLP and Japanese GHS. The health hazards of hydrogen gas obtained by NFPA 704, CLP and Japanese GHS have low value in spite of the low 50% lethal concentration. Liquid hydrogen and hydrogen gas have different classification of health hazard in NFPA 704. Aquatic environmental hazards of NH3 obtained by Japanese GHS and CLP are large.

NH3 accident occurred in Japan 2014 is only in the plant facilities. The ratio of NH3 accidents divided by the high pressure gas accidents is about 10%. All accidents with NH3 are caused by leaks from valve, pipe and flange in the plant facilities. It is well known that large amount of water is a NH3 absorbent in the plant facilities for emergency. The solubility of NH3 in water is 50 g/100gH2O at 293 K under 0.09 MPa of NH3 vapor pressure. The solubility of NH3 is high, although NH3 vapor concentration and NH3 concentration in the water solution have similar high values at 298 K. NH3 storage materials for the vapor concentration (vapor pressure) lowering and the nitrogen concentration lowering in water will be required to reduce the effects on the environment.

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (funding agency : JST).

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