Ammonia pyrolysis and oxidation chemistry

Ammonia has been essential to human activities for centuries. It is widely used as feedstock for fertilizers, industrial chemicals, and emission after-treatment systems. Owing to its properties, ammonia has garnered interest as a carrier for hydrogen in energy applications. It can be generated from carbon-free emission sources and pyrolyzed to produce pure hydrogen for various applications. The combustion of ammonia for power generation has been previously reviewed in this journal besides several aspects of ammonia oxidation chemistry, as it relates to emission after-treatment and reburn systems. However, the pyrolysis and oxidation chemistry of ammonia requires further elucidation to improve its use as a hydrogen carrier and as a fuel for combustion systems. This article provides an in-depth review of ammonia pyrolysis and oxidation chemistry in noncatalytic and catalytic systems. The catalytic pyrolysis chemistry of ammonia to produce pure hydrogen is reviewed to understand catalyst and reactor requirements for scaling up this technology. The combustion properties of ammonia as a pure fuel and in mixtures, including ignition, flame propagation, and extinction characteristics; its pyrolysis and oxidation reactions; and its potential to produce pollutant emissions are extensively reviewed. Ammonia combustion reaction mechanisms are reported based on results from pyrolysis and oxidation reactors, shock tubes, rapid compression machines, and research engines. The experimental work is complemented by the development of detailed combustion models via chemical kinetic and quantum chemistry simulations. Herein, recent results on ammonia pyrolysis and oxidation chemistry are introduced and summarized by highlighting the pertinent aspects of this rich and rapidly increasing body of information.