Ammonia cracking by microwave plasma under reduced pressure

The cracking of pure ammonia by microwave discharges under reduced pressure (typically 50–500 mbar) is studied as a hydrogen formation process. The lowest energy cost associated with the production of 1 kg of hydrogen is 33 kWh ( i.e. 13 % energy efficiency for a dissociation ratio of 28 %). With the same process, NH 3 can be fully dissociated at a greater energy expense (50 kWh/kg H2 ). The plasmas produced have electron densities of no more than 5 × 10 11 cm −3 at electron temperatures close to 0.5 eV. We show that the molecule is dissociated thermally and by electron collisions. Some of the waste heat lost in this process can be recovered in post-discharge by activating surface reactions. With flowrates exceeding 10 L per minute, it is plausible that the process could be even more efficient by reducing the gradients that are responsible for the loss of energy efficiency. • Microwave discharges are showed to be efficient sources for ammonia dissociation. • The overall energy cost is about a factor of three with respect to thermal cracking. • Microwave plasmas are among the best plasma sources to crack ammonia. • The role of surface processes at moderate pressure is not negligible. • The management of gradients in the plasma is key in the overall process efficiency.