AMMONIA CRACKING BY ATMOSPHERIC PRESSURE NH3 PLASMAS FOR HYDROGEN PRODUCTION

Abstract

Ammonia plays a crucial role in the energy sector, particularly as a more easily transportable hydrogen carrier. Japan, for example, uses it to produce the hydrogen needed for its energy mix. At present, its production is based on the Haber-Bosch process, which operates continuously and is not suited to intermittent energy sources such as solar or photovoltaic energy. Using microwave plasma to decompose ammonia could offer a viable, catalyst-free alternative that is compatible with localized and alternative energy production. In this study, we investigated the dissociation of NH3 in a microwave plasma operating at 2.45 GHz. A literature review revealed that the decomposition of ammonia has often been studied in high dilution with carrier gases such as nitrogen or argon. However, the fundamental properties of the ammonia molecule and the radicals it generates (NH, NH2) remain incomplete, which complicates the development of satisfactory models for ammonia-rich plasmas. The experiment required the design of a special reactor equipped with a resonant cavity device and advanced diagnostic equipment. An ammonia plasma under reduced pressure (10 to 500 mbar) was generated using a 2.45 GHz microwave source, with strict precautions to ensure the safety of operators. The gases were analyzed using infrared absorption and gas chromatography, enabling precise dissociation rates to be obtained. The study revealed that dissociation increases with power and pressure but decreases with gas flow rate. Rotational temperatures of around 2,500 K and an electron temperature of around 0.5 eV were determined. The results obtained show NH3 conversion of up to 97% with an energy efficiency of 8%, and up to 14% with a conversion of 28%. Although these performances are promising, they are still inferior to thermal processes assisted by catalysts. Process efficiency is highly dependent on the management of thermal and electronic gradients within the microwave cavity. Finally, the influence of the fused silica reactor tube surface was demonstrated, showing an improvement in NH3 dissociation thanks to heterogeneous interaction with the radical species. A transition to atmospheric pressure operation could further improve the energy efficiency of the process, making it more competitive with thermal approaches while avoiding the use of catalysts. However, the viability of this technology will require ammonia to be produced from decarbonized sources to ensure a favorable carbon balance.