Ammonia as a hydrogen energy carrier

The gravimetric H₂ densities and the heats of combustion of tanks stored ammonia (ammonia storage tanks) were similar to those of the liquid H₂ tanks at the weight of 20–30ton, although the gravimetric H₂ density of liquid H₂ is 100 wt%. The volumetric H₂ densities and the heats of combustion of ammonia storage tanks were about 2 times higher than those of liquid H₂ tanks at 1–4 × 10⁴ m³. Gray ammonia is synthesized from hydrogen through process known as steam methane reforming, nitrogen separated from air and Haber-Bosch process. Blue ammonia is the same as gray ammonia, but with CO₂ emissions captured and stored. Green ammonia is produced by reacting hydrogen produced by electrolysis of water and nitrogen separated from air with Haber-Bosch process using renewable energies. The energy efficiencies of gray, blue and green ammonia were better than those of liquid hydrogen and methylcyclohexane (MCH) with high H₂ density and similar to the efficiency of H₂ gas. The energy efficiencies of ammonia decreased in the order, gray ammonia > blue ammonia > green ammonia. The production costs of green hydrogen energy carried increased in the order, ammonia < liquid H₂<MCH. The amounts of energy consumption by N₂ production and Haber-Bosch process were below 10% compared with the value of H₂ production from water electrolysis.