Ammonia revisited: carbon-free fertilizer, hydrogen carrier and fuel—from alleviating famines to contributing to a carbon free planet

Ammonia synthesis technologies are known and reliable. With more than 100 years of history, they have been optimized and are now considered safe. They are preceded by a robust supply chain that is now being integrated and adapted to take green or blue hydrogen to produce green ammonia. The most widely used process to produce ammonia is the Haber-Bosch (HB) process.

Traditionally, ammonia has been primarily produced by integrating either the steam methane reforming (SMR) process or the coal gasification or fuel oil oxidation processes. This produces the needed H₂ using the HB process. The SMR process generates more carbon dioxide (CO₂) compared to any other chemical synthesis process. It emits around 2.4 tonnes of CO₂ per tonne of product, more than steel (1.4 tonnes of CO₂ per tonne of steel) and 0.6 tonnes of CO₂ per tonne of produced concrete. The SMR process only accounts for ~1.3% of global CO₂ emissions each year.

Decarbonization will include producing green hydrogen and, as a result, will transform the SMR process-based units worldwide.

In 2021, 235 million tonnes of ammonia were produced worldwide, making it the second highest-produced chemical commodity. It is extensively used for fertilizer, in air conditioning and refrigeration systems for large building units, in manufacturing explosives and textiles, in the pharmaceutical industry and as an absorption agent in acid gas removal. It accounts for around 2% (8.6 EJ) of the planet’s total net anthropogenic energy production.

And there are further emerging uses for ammonia:

• as an energy source in power generation, whether by direct combustion in gas turbines or as cracked ammonia in alkaline and proton exchange membrane (PEM) fuel cells.
• as fuel for use in direct combustion engines, solid oxide fuel cells or PEM fuel cells.
• as a medium for heat transfer.

Direct emissions from ammonia production currently equal two thirds of Canada’s CO₂ footprint.

Comparable to H₂ and methanol, ammonia is also a substance for energy storage. Abundant energy can be stored over long periods, allowing for optimized energy distribution. Ammonia can be in liquid form at -33°C at atmospheric pressure or at room temperature with pressures of around 10 bar, which is easier than with hydrogen. Liquid ammonia has a high H₂ content of 17.8% by weight and an energy density of 13.77 MJ/L at 20^oC and 8.6 bar. It is also comparatively more stable and convenient to transport than methanol.

Liquid and gaseous ammonia have a well-established and high-capacity infrastructure for production and distribution, including tankage, pipelines, tank trucks, bunker shipping, overseas fleets and more. It also has well-defined regulations and a good safety history spanning more than a century. Ammonia is also far less flammable when exposed to air than is natural gas, methanol, hydrogen or gasoline vapours. While it is considerably more toxic than the aforementioned substances, safety protocols and best practices for its handling are already well established in the relevant industries.

Ammonia is mainly produced at commercial scale using the HB process, with technologies being developed to optimize it. This process produces ammonia from atmospheric nitrogen (N₂) and H₂ using a metal catalyst at high temperature and pressure. Traditionally, H₂ is extracted from fossil fuels, as explained above.

The HB process is well established. It consumes 8 MWh of energy per tonne of ammonia produced, with the natural gas reforming process for H₂ production (where applicable), and accounts for ~75% of total energy demand. The remaining 25% is needed during ammonia synthesis, gas compression and ammonia separation. The H₂ generation process accounts for 90% of CO₂ emissions involved in the HB process, which, over the course of its evolution, has been subject to development and optimization that have increased its efficiency.

Recent trends show that producing green hydrogen for all energy requirements is the only viable way to achieve deep decarbonization. The process uses water electrolysis and, ideally, renewable electrical sources to prevent the emissions generated by the SMR process.

Another step to improve efficiency is by using water electrolysis technology based on solid oxide cells (SOC), recovering the heat released from the synthesis reaction to perform steam-based electrolysis.

Alternative approaches to the HB process include electrochemical ammonia production and non-thermal plasma synthesis. These are promising technologies, but still need to reach commercially viable applicability.

With regard to the source of H₂, ammonia is also referred to by a colour code. So far:

• Brown ammonia is produced through the HB process using grey or brown H₂, both of which imply fossil fuels as feedstock. Grey H₂ is from natural gas using the SMR process, while brown H₂ is created with coal gasification. Both H₂ colours emit CO₂ during production and release it into the atmosphere. Brown ammonia accounts for most of the ammonia produced since the HB process started being implemented.
• Blue ammonia is produced through the same methods as brown ammonia, but carbon capture and storage are incorporated into the traditional process to prevent CO₂ from escaping into the atmosphere.
• Green ammonia is produced using renewable, CO₂-free energy sources, without the use of hydrocarbons. The H₂ it uses as its prime raw material is produced through water electrolysis using 100% sustainable electricity sources. Green ammonia production also requires air separation units (ASUs) to produce N₂.

A side issue is the volatilization of NH₃ from agricultural systems and is the major anthropogenic source of atmospheric NH₃, accounting for 10–30% of fertilizer nitrogen and animal excreta nitrogen. The issue becomes a valid concern when a fraction of the NH₃ can transform into N₂O, another green house gas but with way more potential than CO₂.

A circular approach to green ammonia production should also include the recovery of emitted ammonia based on direct capture approaches.

Ammonia production is currently a large contributor to global CO₂ emissions due to its strong dependence on fossil fuels and the removal of all carbons in CO₂ form. Major energy requirements, along with most CO₂ emissions, occur from the H₂ production process, upon which ammonia synthesis relies. Green ammonia is therefore only feasible if H₂ production is made fully dependent upon renewable energy sources. With that condition met, ammonia is a carbon-free alternative to hydrocarbons as an agent for carrying and storing energy.

Existing technologies, infrastructure and policies make ammonia convenient and safe to transport. Moreover, increased research into energy efficiency is accelerating the development of technologies for green ammonia production. Areas of further research in the H₂ supply chain include cost reduction and better performance for electrolysers, H₂ storage and the transportation infrastructure.