Fine Gas Purification Approaches for High Purity Hydrogen Production from Ammonia

This review aims to evaluate the state-of-the-art in purifying hydrogen from green ammonia. Green ammonia contains 17.8% of H₂ and enables efficient liquid storage and transport of hydrogen produced from renewable electricity. Back-conversion of ammonia to hydrogen may be carried out by several routes, whereby thermal ammonia cracking is one of the most efficient approaches. In the conversion of ammonia at high pressures, it is only possible to achieve complete conversion at very high temperatures (≈800 °C). Therefore, the separation of ammonia in the low percentage range (e.g., <0.001%) is an important task. For mobility applications wherein, polymer electrolyte membrane fuel cells are to be fed with cracked H₂ (from NH₃), fine purity must be achieved, in agreement with ISO 14687-2. In such a context, ammonia is particularly important, with a permissible concentration of 0.100 ppm. The main feasible routes for ultrafine purification are presented, namely, 1) permeation membranes and 2) absorbents, as well as a third route via 3) biological scrubber, and discussed with regard to efficiency, availability, and cost. Current technology gaps are also identified and an outlook is given toward new innovative approaches.