Should Green Ammonia be the Hydrogen Carrier of Today?

With decarbonisation on the agenda worldwide, engineers are looking for new ways to use hydrogen as an energy vector. Alex Howard and Jonathan Upton look at the less discussed technical challenges

IN RECENT years, the pursuit of clean and sustainable energy has led to an ever-increasing interest in hydrogen as a potential solution for decarbonisation. As the world embraces the hydrogen economy, green ammonia (NH₃) as a carbon-free molecule containing 17.8% hydrogen by weight has emerged as a promising contender. While ammonia’s potential as a hydrogen carrier is widely discussed, there are several lesser-known aspects that deserve attention. This article aims to shed light on these aspects, revealing green ammonia’s challenges, benefits, and potential to revolutionise the energy landscape.

Ammonia production and uses

Ammonia is one of the most widely used and important of all chemicals produced globally. Around 183m t/y were produced in 2020 with growth estimates reaching 333m t/y by 2050¹. This demand exists due to ammonia’s versatility and essential role in key industries such as agriculture, manufacturing, refrigeration, and water treatment. Beyond these, ammonia’s future role as a potential zero-carbon fuel or energy vector is strongly driving market growth estimates.

Ammonia is typically produced using the Haber-Bosch process, which involves the reaction of nitrogen and hydrogen gas in the presence of an iron catalyst, at high temperature and pressure. Nitrogen is typically sourced from atmospheric air using a cryogenic air separation unit (ASU) or pressure swing adsorber (PSA), and hydrogen obtained from a fossil-based feedstock which is converted to a CO/H2-rich syngas. Adjusting the syngas composition to maximise the hydrogen content required for ammonia synthesis produces significant quantities of carbon dioxide.

With this brief introduction, it is immediately clear that decarbonisation of ammonia production is not only important, but could also be superficially technically straightforward; a source of renewable power to run a conventional ASU or PSA, coupled with electrolyser units to produce hydrogen would result in green ammonia production. Such a scheme is shown in Figure 1 and is the approach that green ammonia megaprojects such as that of NEOM Green Hydrogen Company in Saudi Arabia are taking². Other entirely novel green ammonia synthesis technologies such as direct electrochemical synthesis are also receiving research and development attention.

Given its potential for zero-carbon production, ammonia is widely considered a future green fuel or energy vector. This potential can broadly be categorised as either:

• ammonia as a substitute fuel for modified combustion engines
• ammonia as a fuel for direct ammonia fuel cells, to produce electricity
• ammonia as a hydrogen carrier for use in an indirect ammonia fuel cell, or other uses