Leveraging Ammonia for Clean Energy
As the United States looks to transition our energy economy away from fossil fuels and towards renewable energy resources, we will face the need to effectively move power, as we wrote about recently, and increase our capacity to use renewables through energy storage. Energy can be stored in a variety of ways — as chemical energy in fuels like hydrogen or in batteries, as kinetic energy in things like flywheels, as potential energy in hydro storage or gravity storage, or even as heat (i.e. thermal storage). In order to find the most cost-effective and scalable storage medium, some engineers are now looking to a chemical that is already being produced at a rate of 235 million tons per year, and one that already helps sustain the world’s growing population: ammonia.
As we’ll explore, ammonia production and utilization can be CO2-free, but its use for energy applications remains a relatively new area of research, and the promising technologies enabling its use must be further developed before ammonia can safely and cost-effectively play a significant role in shifting the country away from fossil fuel dependence.
About Ammonia
Ammonia (NH3) is a nitrogen fertilizer typically produced via the Haber-Bosch process, invented in 1909, which combines hydrogen and nitrogen under high pressures. The hydrogen input today is hydrocarbon-based and is most commonly produced by steam reformation of methane from natural gas. Yet green ammonia uses hydrogen produced by renewables-powered electrolysis of water, then is stored as a liquid fuel for various applications.
Interestingly, ammonia contains twice as much hydrogen as liquid hydrogen by volume. It is also easily liquefied at a pressure of 8.6 bar, so it requires relatively little energy to transport and store.
However, despite ammonia’s benefits, there are various obstacles preventing its widespread implementation in energy storage. It is hazardous, and can burn human eyes and skin. Though CO2 emission free, combustion of ammonia emits NOx, a human respiratory system irritant which contributes to the formation of acid rain, particulate matter, and ozone. Additionally, the high ignition temperature makes ammonia difficult to burn. Researchers at Cardiff University have taken steps to mitigate this risk by demonstrating that a richer air/fuel ratio can significantly lower the NOx emissions, and further research is needed to scope the engine modifications required for clean combustion of ammonia.
Production of Ammonia
Ammonia was produced by just 16 companies at 35 plants in 16 states in the US in 2020, and all plants are located in the middle of the country. About 60% of total U.S. ammonia production capacity is in Louisiana, Oklahoma, and Texas due to their large reserves of natural gas and proximity to fossil fuel transport infrastructure.
Ammonia is currently produced on a very large scale: it is responsible for up to 2% of global energy consumption, as it requires high temperatures between 400-600°C and pressures between 200-400 atm. Furthermore, the Haber-Bosch process results in emissions of more than two tons of CO2 per ton of ammonia, globally accounting for over 500 million tons of CO2 emissions annually. These astounding statistics underline the need for alternative, sustainable ammonia production.
At present, green ammonia production costs 2-4 times as much as conventional ammonia production. Another version, “blue ammonia”, is produced conventionally by Haber-Bosch while its CO2 emissions are captured and stored (akin to “blue hydrogen”). The only currently active blue ammonia plant in the US is the Dakota Gasification Plant in North Dakota, which pipes CO2 emissions to nearby oil fields for enhanced oil recovery.
Visual of green ammonia production from renewable energy powered electrolysis
Source: ScienceMag.org
Analogous to renewable energy powered electrolysis, other emerging technologies such as membrane reactors or ceramic reverse fuel cells can drastically reduce process emissions, but membrane reactors sacrifice efficiency (while reverse fuel cells sacrifice speed) when compared to Haber-Bosch. For example, non-thermal plasma-assisted synthesis allows the reaction to occur at far more moderate conditions and is conducive to localized production, but advancements are still required in catalysis and process engineering to prevent back-reactions from occurring.
Further research and development of these and similar technologies to realize an ammonia production process that is reasonably efficient and fast will be necessary to achieve a significant scale of green ammonia. The U.S. has already made strides in this direction, as the country has recently seen an increase in ammonia production and yet a decrease in the carbon intensity of that production in recent years due to low natural gas prices prompting more gas-fed production plants (which are less carbon intensive than coal plants).
Uses for Ammonia
Although over 80% of ammonia is used as fertilizer today, ammonia has the potential to play a significant role in the decarbonization of the gas industry. For starters, ammonia can be used as a standalone fuel; recent technological advances in solid oxide fuel cells and polymer electrolyte membranes have allowed fuel cells to convert chemical energy in ammonia directly to electricity without dissolving the cell membrane. Despite their potential, a review article published by Frontiers in Energy Research points out that more research is needed before solid oxide fuel cell technology can be commercially scaled to be competitive with the Haber-Bosch process.
Another method of utilization, decomposition into nitrogen and hydrogen, occurs via a high-temperature reaction known as “cracking.” Cracking can be free of carbon dioxide emissions and low-energy with the right metal catalyst. This CSIRO Energy study finds that cracking ammonia into H2 is about 75% efficient, and that producing ammonia from renewable hydrogen is about 60% efficient (for an end-to-end efficiency of only about 45%). Inefficiencies of liquefying hydrogen are less substantial, resulting in an energy penalty of just over 30% of its energy content along with boil-off losses during transportation; however, ammonia is much safer and easier to transport than hydrogen due to its density and the fact that hydrogen causes embrittlement of steel that endeavors to contain it. Finally, today’s transport infrastructure is much more established for ammonia than hydrogen, especially when considering long distances.
Today’s Technical Examples
Several companies have begun to invest in ammonia and pioneer renewable-hydrogen-to-ammonia innovation. One of the largest-scale examples is Air Products’ ammonia plant in Saudi Arabia, to be commissioned in 2025, which will take advantage of the consistent Saudi desert sun and winds to electrolytically produce hydrogen and nitrogen and feed them through the Haber-Bosch process, making 1.2 million tons of ammonia each year. This ammonia is destined to be re-converted back to hydrogen in processing units at bus and truck depots.
Elsewhere in Norway, Yara is looking to convert its ammonia plant to source its hydrogen from water electrolysis powered by the Norwegian grid (which runs on 98% renewable energy). The project is planned for completion in 2026, with the ammonia mostly designated for use as fuel. Yara has a few pilot projects in the works in Norway, Australia, & Netherlands.
Following suit, CF Industries in Louisiana, the world’s largest ammonia producer, plans to install a renewable energy & electrolysis-based system that will produce 20,000 metric tons per year of green ammonia. It is expected to cost between $400 million and $450 million and eyes completion in 2023. CF Industries will also be implementing carbon capture and sequestration across its production facilities to enable blue ammonia production.
On the hydrogen production side, U.S.-based Bettergy is developing a catalytic membrane reactor to allow point-of-use hydrogen production from ammonia, as opposed to production and transport of hydrogen. These innovations to minimize emissions in generating hydrogen from ammonia are necessary if ammonia is to enable a faster transition to a no-carbon gas economy.
Finally, as far as direct use, Canadian Hydrofuel Inc. designs dual fuel injection systems to allow gas and diesel engines to also run on ammonia, but their technology still depends on green sources of ammonia to truly be considered a low-carbon alternative.
Where do we go from here?
The United States could look to the Midwest, which has plenty of existing ammonia production and transport infrastructure in place (the DOE claims that pipeline operation is generally safe and cost effective) and relatively strong renewable energy potential for future ammonia projects. Integrating renewables into ammonia production and expanding today’s ammonia pipeline system to serve new energy markets could be a viable pathway towards decarbonization and resiliency while minimizing new infrastructure and undue investment. For example, GenCell, a manufacturer of ammonia fuel cells, markets their solution as “ideal for use in remote locations that lack grid access or where the grid is unstable and backup power is frequently required.” As the country makes a larger shift towards cleaner gas solutions and electrification, ammonia production, transport, storage and utilization could be a viable solution worth developing in parallel with end-to-end hydrogen solutions elsewhere.
As Air Products, CF, Yara and others have already realized, producing ammonia by water electrolysis is a promising avenue through which we can reduce emissions, and could allow ammonia to play a major role in the future of gas. There is much opportunity for innovation surrounding ammonia: in green ammonia production, ammonia fuel cells, and hydrogen generation from ammonia. The next 5-10 years will shed important light on our ability to clear these technological hurdles that still prevent large-scale deployment.