One of the largest weaknesses for the U.S. military is its dependence on fuel. During the Global War on Terror, fuel convoys were easy targets for insurgent attacks. They will likely be targeted in future conflicts as well. This supply chain vulnerability is critical, since without access to fuel, military aircraft cannot maintain air superiority, a major requirement for winning wars.
Last week, the Air Force Operational Energy office awarded a contract to the energy company Twelve to develop the technology necessary to produce jet fuel from air. While this may sound unrealistic, the process is actually fairly straightforward. Indeed, Twelve has already shown that this process can be done at small levels. However, there are significant challenges involved in using this process at scale.
The inherent reason behind this contract is straightforward: Sustainability will play a key role in future conflicts. All of the powerful new weapon systems under development are useless without energy. And that energy typically comes from burning jet fuel shipped from the United States and then convoyed to a forward deployed location. Given the vulnerability of these supply chains, there is a push to provide “sustainability at the edge,” such that items for resupply are produced locally. For example, the U.S. Army plans to provide 3D printing capabilities to soldiers on the front lines to reduce the need for spare parts. However, there are few options for replacing fuel, especially for aircraft.
The Navy took the lead in developing alternatives to allow for the localized production of jet fuel. A consortium made up of the University of Rochester, the University of Pittsburgh, the Naval Research Lab, and OxEon Energy have been working over the last decade to develop a catalyst that can extract carbon dioxide from seawater, which can then be converted to jet fuel. The project is ongoing, with the catalyst having shown proof-of-concept. The Air Force contract with Twelve follows a similar science, but extracts the carbon dioxide from air rather than seawater.
The science behind this conversion can be explained by the chemical equation that underlies the combustion process:
fuel + oxygen -> carbon dioxide + water + energy
This equation gives that under the right conditions, a mixture of fuel and oxygen will combust into carbon dioxide and water, releasing a large amount of energy. At the risk of over-simplifying the chemistry, this equation also states that if energy is added to a mixture of carbon dioxide and water, it can be turned into fuel and oxygen. In reality, this process is significantly more complicated, and requires an intermediate step, where the carbon dioxide and water are converted into carbon monoxide and hydrogen, commonly referred to as syngas. Syngas can then be reformed into a variety of fuels, including gasoline, diesel, or jet-fuel. Countries such as South Africa rely heavily on this process for producing automotive fuels from coal.
The Air Force’s attempt to produce jet fuel from air has been promoted as a game-changing technology, since all it requires is access to renewable energy and air. The issue arises with how much renewable energy and air would be required for this process to be practical.
In terms of the amount of energy, it would take approximately 28 square meters of standard solar panels to provide enough energy to produce 1 gallon of fuel in a day. This assumes access to direct sunlight and an ideal chemical conversion. A C-130 transport plane has a fuel capacity of 6,700 gallons, equating to a solar array that is approximately the size of 38 football fields. Note that the actual size of the solar array would be significantly larger since the conversion process is far from ideal. The similar process used for converting water to hydrogen fuel is 80 percent efficient; with this chemical process being more complex, a lower efficiency is expected.
Given that space is limited in forward deployed areas, the most viable “renewable” energy option is a small nuclear power plant. Indeed, the Navy project leverages the nuclear reactor aboard an aircraft carrier for its seawater to fuel program. Further, the Department of Defense has projects underway for the development of mobile nuclear power options with power up to 5 MW. Unfortunately, even one of these nuclear reactors running constantly for 24 hours, would only be able to produce half the fuel necessary to fill a C-130.
A second issue is the amount of air required for this system. Since carbon dioxide only makes up 0.053 percent of air, 13,000 cubic meters of air is required to produce a gallon of jet fuel. This equates to 100 million cubic meters of air to fill a C130. While air is very abundant, a larger issue arises from fouling. When processing this much air, pollutants in the air can damage the sensitive catalyst materials that extract the carbon dioxide.
These issues with the amount of energy and air required are fairly large. Meanwhile, other options are more reasonable, especially synthetic aviation fuel. These fuels can be generated from a range of sources including waste oils and agricultural products, both of which are readily available around the globe.
As the U.S. Air Force evaluates its future fuel needs it is good that it’s considering novel solutions. Unfortunately, although the generation of jet fuel from air is technically feasible at small levels, production at the scales necessary for the Air Force is not realistic.
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