The global push to decarbonize energy systems is driving the search for alternative fuels that can replace fossil sources without contributing to atmospheric carbon dioxide (\(\text{CO}_2\)). Ammonia (\(\text{NH}_3\)), a simple compound of nitrogen and hydrogen, has emerged as a leading candidate. While primarily used in fertilizer production, its chemical structure makes it a compelling, carbon-free energy carrier. Ammonia offers a pathway to store and transport hydrogen-based energy at scale, addressing a significant challenge in the shift to renewable power.
Ammonia’s Properties as an Energy Carrier
Ammonia is attracting interest because its physical properties offer advantages over hydrogen, particularly for storage and transport. The compound contains a high density of hydrogen atoms, resulting in a favorable volumetric energy density. Liquid ammonia holds \(3.83\) megawatt-hours of energy per cubic meter, which is higher than the \(2.64\) megawatt-hours per cubic meter stored in liquefied hydrogen.
The ease of liquefaction also makes ammonia an attractive fuel option. Ammonia can be stored as a liquid by cooling it to \(-33^\circ\) Celsius at atmospheric pressure, or by applying a moderate pressure of about \(7.5\) bar at room temperature. In contrast, hydrogen must be cooled to an extremely cryogenic temperature of \(-253^\circ\) Celsius to be liquefied, which is energy-intensive and costly. This easier storage allows ammonia to utilize much of the existing global infrastructure designed for handling pressurized liquids.
The most fundamental advantage is that ammonia contains no carbon atoms in its molecular structure. When pure ammonia is combusted, it cannot produce \(\text{CO}_2\), the primary greenhouse gas driving climate change. This positions the substance as a zero-carbon fuel at the point of use, making it useful for sectors that are difficult to electrify.
Converting Ammonia into Usable Energy
Energy can be extracted from ammonia through two distinct pathways. One method is the direct use of ammonia as a fuel in modified internal combustion engines or gas turbines, known as direct combustion. The primary challenge arises because the fuel contains nitrogen, which can lead to the formation of nitrogen oxides (\(\text{NO}_{\text{x}}\)), a regulated air pollutant.
\(\text{NO}_{\text{x}}\) forms when fuel-bound nitrogen reacts with oxygen during high-temperature combustion. To mitigate this, combustion systems use techniques like co-firing with a small amount of an easily ignitable fuel, such as hydrogen or diesel, to improve flame stability. Advanced abatement technologies, such as selective catalytic reduction (SCR) systems, are often required post-combustion to clean the exhaust gases.
The second method uses ammonia as a carrier for hydrogen, known as ammonia cracking. In this approach, \(\text{NH}_3\) is thermally decomposed back into hydrogen (\(\text{H}_2\)) and nitrogen (\(\text{N}_2\)) by passing it over a catalyst at high temperatures, typically between \(500^\circ\)C and \(800^\circ\)C. This endothermic reaction requires a significant energy input to break the chemical bonds.
The resulting high-purity hydrogen can then be fed into a fuel cell to generate electricity, bypassing combustion entirely. Although cracking consumes energy, this method avoids the \(\text{NO}_{\text{x}}\) emissions associated with direct combustion and allows end-users to leverage existing hydrogen fuel cell technology. This dual-use capability underscores ammonia’s versatility in the energy landscape.
Key Industrial Applications
Ammonia’s unique properties make it well-suited for applications requiring high energy density and long-distance transport. The maritime shipping sector is a promising area for deployment, driven by the need to decarbonize global trade. Because it is liquid at moderate conditions, ammonia storage systems on ships require less complex infrastructure than cryogenic hydrogen tanks, making it a viable fuel for long voyages.
Ammonia is also being considered for large-scale, stationary power generation. Gas turbines can be adapted to burn ammonia, allowing power plants to generate electricity without \(\text{CO}_2\) emissions. This application is relevant for stabilizing electrical grids that rely increasingly on intermittent renewable sources like wind and solar, providing reliable, on-demand power.
Beyond its direct use as a fuel, ammonia’s major application is its role as a global hydrogen transport medium. Since shipping pure hydrogen is difficult and expensive, it is more efficient to convert hydrogen into liquid ammonia, transport it via established global chemical shipping routes, and then convert it back to hydrogen through the cracking process. This allows regions with abundant renewable energy resources to export clean energy, creating a global supply chain for clean hydrogen.
Supply Chain and Safety Considerations
The environmental benefit of ammonia depends on how it is produced, creating a distinction between “grey” and “green” ammonia. Currently, the vast majority of ammonia produced globally is “grey” ammonia, synthesized using hydrogen derived from natural gas through processes like steam methane reforming. This conventional method is carbon-intensive, releasing approximately \(1.6\) to \(1.8\) tons of \(\text{CO}_2\) for every ton of ammonia produced.
For ammonia to fulfill its potential as a clean fuel, production must shift to “green” ammonia. This method uses hydrogen generated by water electrolysis powered exclusively by renewable electricity, such as solar or wind power. Because the process is powered by clean energy, green ammonia production results in near-zero \(\text{CO}_2\) emissions. While the cost of green ammonia remains higher than grey ammonia today, this differential is expected to narrow as renewable energy costs fall and carbon regulations are implemented.
Handling and storage represent a significant consideration, as ammonia is a hazardous substance. It is a colorless gas with a pungent odor, classified as toxic and corrosive. Direct exposure, particularly inhalation of concentrated vapors, can cause severe irritation, respiratory damage, and even death.
The risk profile necessitates specialized safety protocols and infrastructure modifications for widespread adoption. Storage tanks and transfer systems must be designed with corrosion-resistant materials, and facilities require robust ventilation and leak detection systems. Personnel who handle ammonia must undergo extensive training in emergency response and wear appropriate protective equipment to mitigate toxicity risks.