The Quest To Replace Liquid Fuels
As renewable energies progress, some limitations are becoming clearer. The intermittency of renewables requires the presence of energy storage. This could be in the form of batteries, as we explored in our article “The Future Of Energy Storage – Utility-Scale Batteries Tech”.
However, some forms of energy consumption are very resistant to electrification. For example, long-distance naval shipping, or air freight.
Even trucking has so far failed to be converted to EVs, due to the weight of batteries required; as well as limitations in the battery production capacity, which has delayed for many years the Tesla Semi.
Largely, these limitations stem from the fact that gasoline/diesel/kerosene are extremely energy-dense, much more than the best batteries.
So the idea of a liquid fuel that would be an alternative to petroleum products is highly attractive. One idea is biofuels, which essentially produce the same product as fossil fuel, but from renewabl sources. It is another idea we explored in our article “Algal Biofuel: The Next Energy Revolution?”.
Another idea is to use the most abundant atom in the universe, hydrogen, to store energy. However gaseous hydrogen has some limitations, from the difficulties of producing it economically, to the energy costs of liquefying, transporting, and storing it.
Hydrogen atoms do not need to be in the form of H2 gas to be storing energy. Another extremely abundant element, nitrogen, forming 4/5th of our atmosphere, can help.
Ammonia As An Energy Carrier
Simple Chemistry
Ammonia, or NH3, is a fertilizer and can be burned or oxidized to produce nitrogen and water.
NH3 +O2 → N2 + H2O
So it is somewhat similar to hydrogen combustion, in that it only produces harmless byproducts, at least in ideal conditions (more on that later).
The difference with hydrogen is that ammonia is a lot larger molecule than H2, and a lot more stable as well. This makes its transportation and storage a lot easier. mmonia is also almost 50% more energy-dense than liquid hydrogen.
Source: Kleinman Center For Energy Policy
Hydrogen liquefaction wastes 44.7% of the energy it contains, as it requires cooling at -253°C (-423°F). Keeping it liquid leads to more losses, increasing with the duration of storage, potentially up to 79% losses for seasonal storage.
“Ammonia, on the other hand, can be liquefied by either cooling it below -33°C (at atmospheric pressure) or pressurizing it above 7.5 bar (at 20°C)—significantly more achievable conditions than those required for hydrogen. This process can be close to 99% efficient”.
Source: Kleinman Center For Energy Policy
Existing Infrastructure
Ammonia is not a rarely produced chemical, with it being a key component in the production of fertilizer, but also plastics and explosives. This means there is already an existing industry and supply chain for the mass production of ammonia, although somewhat dependent on fossil fuels for now. It also makes it a ell-understood and efficient process. Overall, ammonia is the second most highly produced chemical in the world.
Ideally, an ammonia economy would rely on so-called green ammonia, generated from renewable energy. This distinguished it from other types of ammonia:
Grey/brown ammonia: produced from fossil fuels.
Blue ammonia: produced from fossil fuels, but with carbon capture.
Pink ammonia (sometimes also called yellow ammonia): produced from nuclear energy.
Turquoise ammonia: produced from the pyrolysis of methane. This breaks down methane into hydrogen and solid carbon, with the hydrogen later converted to ammonia. The solid carbon can be stored or used for applications like carbon fibers.
And while a lot less extensive than the fossil fuels network, there is no less than 5,000 km (3,100 miles) of ammonia pipeline in the US (and 490,000 km of high-pressure natural gas pipelines). With ammonia being non-corrosive and not damaging steel pipes like hydrogen (“embrittlement ”), new pipeines could be relatively inexpensive.
This would nevertheless require massive investment in both production capacity and pipelines. To replace just half of the global natural gas consumption would require multiplying by 20 the global ammonia production.
Depending on the exact solution adopted, ammonia could work as an energy storage and transfer system with an efficiency (returned energy) ranging from 84%-38%.
Source: Kleinman Center For Energy Policy
The Limitations of Ammonia
The problem with using ammonia directly for energy is that combustion is seldom a perfectly efficient process. When ammonia is impartially burned, it produces NOx gases, which are toxic, as well as greenhouse gases (300x more potent than CO2).
Several solutions have been proposed, including:
Burning green ammonia in a fuel blend, in combination with fossil fuel.
Burning green ammonia in a fuel blend, in combination with hydrogen.
Converting liquid ammonia to compressed hydrogen directly on the storage sit, and on-demand, like at a fuel station. A process named “cracking”.
Using dedicated fuel cells, like for hydrogen, to directly generate electricity and power an electric motor.
Using catalyst to destroy NOx before they are released. This could cause some problems as these catalysts often are extremely expensive metals like Palladium, Platinum, and Rhodium, which are already used to reduce NOx emissions in fossil-fuel-powered vehicles. A purely ammonia-based combustion would require a lot more of it.
As NOx are powerful greenhouse gases, making sure we don’t see them replacing CO2 is a must to justify transitioning to an ammonia economy.
Ammonia As An Hydrogen Carrier
As mentioned above, ammonia (NH3) can be “cracked” back into nitrogen (N2) and hydrogen (H2). This means that even if the issue of NOx emission proves unsolvable, there is still potential for an ammonia economy.
In this context, ammonia is used directly for storage, transportation, as well some niche applications. And gt turned into hydrogen in a gaseous form (even if compressed) to power vehicles through direct combustion or fuel cells.
This allows the decarbonized economy to enjoy a few key benefits of ammonia’s physical and chemical characteristics:
Lower energy losses during the production of the liquid fuel, while still using green energy to do so.
Low-cost renewable energy during periods of overproduction (strong sun & wind) can be used to reduce the overall fuel production costs.
The possibility of multi-month storage, allowing for resiliency in the energy system, is somewhat of a must for mobility. For example, a hurricane knocking off the power grid would not stop the supply chain, ambulances, and cars from running, as it might for a fully EV-centric system.
The multi-month storage also allows for excess production in sunny or windy months to be turned into fuel supply for months with lower renewable energy production.
So it is possible that talking of an ammonia economy or a hydrogen econmy might be a little misleading.
A more likely scenario is a mixed ammonia-hydrogen economy, with each technology leveraged to perform best on its strong points:
Ammonia for transportation, long-term storage, and “soaking up” the surplus energy of sunny or windy days.
Hydrogen for direct consumption, short-term storage, and applications requiring quick & high energy output, like steel production or jet engines.
This is a scenario where nuclear power plants could also play a role in decarbonization, even without mass adoption of EVs, thanks to “pink ammonia” providing a low-carbon source of liquid fuel.
The Path To An Ammonia Economy
The road map for ammonia adoption is a little harder to determine. Some research estimates that an ammonia-driven economy, using mass production of green ammonia (2nd generation), is unlikely before 2030.
In this scenario, more efficient ammonia production relying on direct electroreduction (3rd generation) would take over only at the end of the 2030s, ue to the technology being today in its early stages, especially efficient electrocatalysts.
