Ammonia as a hydrogen energy carrier

Abstract

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.