Driving sustainability through green ammonia production

Ammonia is a compound made up of just nitrogen and hydrogen and is vital for numerous everyday applications in the modern world. Most importantly, it plays a critical role in food production as the primary nitrogen source for fertilizers, enabling sufficient farming to feed the growing global population.

Beyond agriculture, ammonia is used extensively in several industrial process applications, including:

• Chemical production, providing a base for various products, including plastics, fibers, dyes and explosives.
• Pharmaceutical manufacturing, as a starting material for various active pharmaceutical ingredients and to control pH.
• HVAC, for refrigerant due to its efficient heat transfer properties.
• Food industry, as an additive and preservative.
• Water treatment, to purify water supplies by creating chloramines, which help facilitate organic disinfection.
• Cleaning products, as a key ingredient in many household agents due to its degreasing and disinfecting properties.
• Textile manufacturing, applied during the finishing process to improve fabric smoothness and dye affinity.

While ammonia is an invaluable chemical in many industries, conventional ammonia production methods have environmental impacts, encouraging the exploration of greener alternatives.

Ammonia is primarily produced using the Haber-Bosch process, whereby nitrogen is combined with hydrogen at high pressure and temperature in the presence of a catalyst.

The steps include:

• Nitrogen extraction: Nitrogen (N₂) is extracted from the air using an air separation unit through a process of liquefaction and distillation. Alternatively, pressure swing adsorption employs specialized adsorbent materials to selectively capture nitrogen molecules.
• Hydrogen production: The hydrogen (H₂) feed is commonly produced via steam methane or autothermal reforming.
• Ammonia synthesis: The purified nitrogen and hydrogen gases are mixed and passed over a catalyst - typically iron - at high pressure (> 200 atm/2,940 psi) and temperature (> 450 °C/842 °F), creating some elemental nitrogen and hydrogen, which combine to form ammonia (NH₃).
• Ammonia and residual hydrogen and nitrogen cooling: The gases leave the reactor at a temperature > 450 °C/842 °F and they are cooled down by a heat exchanger doubling as a waste heat boiler. This produces superheated steam, which is subsequently used in a power generation unit.
• Ammonia separation: The gas mixture is cooled to approximately -40 °C (-40 °F), causing the ammonia to condense so it can be separated due to its higher boiling point from unreacted nitrogen and hydrogen gases. These unreacted gases are recycled back into the process to complete their reaction.

The overall process is represented as: N₂ + 3H₂ ↔ 2NH3

The Haber-Bosch method is energy-intensive, and brown, black and grey hydrogen production processes contribute to greenhouse gas (GHG) emissions. This environmental impact is prompting a preference for green hydrogen from sustainable sources and for powering nitrogen production with renewables.

The production of hydrogen used to create ammonia accounts for approximately 1.8 % of global carbon dioxide emissions, a figure comparable to the entire aviation industry. In response to these environmental concerns, the concept of "green ammonia" aims to substantially reduce ammonia production’s carbon footprint by sourcing its component gases through net-zero processes, driven by renewable power sources.

The raw materials for green ammonia production consist of three simple needs: air, water and renewable energy to power the various processes.

Using cryogenic distillation or pressure swing adsorption, air separation units extract nitrogen from the atmosphere. Cryogenic distillation works by cooling air to extremely low temperatures, separating its component gases—including nitrogen, required for ammonia production—based on their different boiling points. Alternatively, pressure swing adsorption employs specialized adsorbent materials to selectively capture nitrogen molecules.

The primary designator for green ammonia is the use of green hydrogen, produced by renewably-powered water electrolysis, which creates zero emissions. This process entails splitting water molecules into their component hydrogen and oxygen atoms, whereby hydrogen gas is harvested as a feedstock for ammonia synthesis and benign oxygen is released into the air. Electrolysis takes place primarily using either alkaline electrolyzers - a mature and vetted technology - or proton exchange membrane (PEM) electrolyzers, known for higher efficiency and faster response times, but with higher costs.

The nitrogen and hydrogen are then reacted according to the Haber-Bosch process, but with a reduced carbon footprint. All processes along the path must be powered by renewable energy for the resulting ammonia to be classified as “green”.

Ammonia can also be decomposed into hydrogen through the process of ammonia cracking, although this reaction is quite energy intensive, requiring heat of 600 °C/1,112 °F in the presence of a catalyst. This process is used for some applications that require hydrogen because ammonia is easier to handle and store in bulk than pure hydrogen.

As with many low-carbon processes, scaling the production of green ammonia comes with challenges regarding infrastructure limitations and high production costs. First, the renewable energy sector - though growing rapidly - has yet to approach the level of grid integration and power storage of conventional systems, which is required to meet industrial and consumer product demands. Similarly, electrolyzers for green hydrogen production still require significant investments to scale up.

However, as sustainable technologies mature and economies of scale are realized, green ammonia production will become increasingly cost-competitive. The cost of renewable energy is on the decline in recent years and this trend is expected to continue, which will make green hydrogen and nitrogen more economically attainable.

Ammonia is also being piloted as a carbon-free energy carrier - and even for direct combustion in some applications - helping address the intermittency challenges of solar and wind power. Additionally, its high energy density has led to exploration as a fuel for shipping and long-haul transportation, traditionally carbon-heavy applications.

Irrespective of its value, ammonia is a hazardous chemical and must be handled cautiously. Inhalation of just 0.5 % ammonia-concentrated air can be lethal for humans. Its highly corrosive and toxic nature demands stringent safety protocols during production, transportation and use.

In addition to standard personal protective equipment and comprehensive employee training, leak detection systems help mitigate danger in the event of an unplanned containment breach. These systems are made up of electrochemical gas detectors, pressure transmitters and other advanced safety instrumentation and components.

Moreover, air separation units used to extract nitrogen from the air for ammonia production rely on instruments like TDLAS analyzers for purity analysis and pressure sensors for process control. In the water electrolysis process, conductivity sensors monitor electrolyte quality, ensuring optimal performance and longevity of the electrolyzer, while flowmeters help to regulate hydrogen and oxygen production.

The ammonia synthesis loop relies on a wide variety of instrumentation - including temperature sensors, pressure transmitters and gas analyzers - to optimize reaction conditions and ensure product quality. These instruments provide continuous data for both real-time control and historical analysis, empowering personnel to optimize the process and maximize efficiency, while maintaining safe operating conditions.

Ammonia decomposition requires mass flowmeters for precise gas delivery and optimal reaction rates, thermocouples for accurate temperature measurement and gas analyzers for monitoring product hydrogen stream composition to ensure purity.

While ammonia is incredibly valuable in numerous industrial processes that consumers often take for granted, the widespread adoption of green ammonia still faces barriers. Renewable energy and electrolysis infrastructure remains limited and investment costs are high. Additionally, the safe transportation and storage of ammonia requires careful consideration due to its hazardous nature, although approved methods are well-established.

Green ammonia’s primary benefit is decoupling ammonia production from fossil fuel dependence to reduce GHG emissions. As a result, it has attracted the attention of governments and industry alike and these stakeholders are strategically incorporating this product into sustainability plans. It provides a pathway to reduce the fertilizer industry’s carbon footprint and the myriad other sectors that rely heavily on ammonia.

Further research and development, along with continued innovation in production and utilization technologies, will be fundamental for driving green ammonia’s widespread adoption, as the world strives to achieve net-zero initiatives by mid-century.