Can airplanes fly on coal?

Firing up the jet engines with a coal-based fuel is the goal of a $4.56 million project unveiled by those wild and wacky researchers at the US Defense Advanced Research Projects Agency.

The agency is looking to develop technology that can demonstrate economical and environmentally friendly conversion of coal-to-liquid (CTL) fuels primarily for aviation purposes. Not an easy task. First of all, the current process is expensive: DARPA says the current cost of implementing a 100,000 barrel-per-day (bpd) CTL facility would cost well over $6 billion and result in a per gallon cost of about $4.50. Oh, and by the way, the US Department of Defense (DOD) currently uses on average over 300,000 barrels of petroleum based liquid fuels per day.

Then there's the Carbon Dioxide problem. The environmental cost of fuels derived from coal results in a net increase of 80% or more in terms of carbon dioxide emissions as compared to the use of petroleum based fuels, DARPA said. While the use of proposed carbon capture technologies might mitigate the increased carbon dioxide emissions, the scalability, reliability, and economics of these technologies are still very much unknown, DARPA said.

Then there's the large amount of water needed. Existing CTL technologies use about one kilogram of water for each kilogram of coal, DARPA said. This is due to the fact that coal, while high in carbon content, is comprised of only 5% hydrogen. In contrast, JP-8 jet fuel has closer to 15 % hydrogen composition by weight. Water is a natural source for hydrogen (11% hydrogen by weight); however the water based reactions for converting coal to liquid fuels result in the excess carbon and oxygen reacting to produce the additional unwanted carbon dioxide that is a major disadvantage of existing CTL technologies, DARPA said.

So what does DARPA expect to develop? Its requirements include:

  • a facility that would cost about $1.5 billion
  • a resource able to produce up to 100,000 bpd
  • a resulting cost per-gallon of about $3
  • 0% carbon dioxide contribution
  • less than a 235 kg of water use for every barrel produced

Given the abundance of US coal reserves, it is reasonable to assume that coal derived fuels could play an increased role in meeting future energy needs, DARPA said. Today, producers of liquid fuels from coal are capable of converting 120,000 metric tons of coal into approximately 150,000 bpd of oil. In addition, the United States has over 275 billion tons of estimated coal reserves.

DARPA is seeking to reduce the military's reliance on petroleum-based fuels to power their aircraft, ground vehicles, and ships. The agency also has a BioFuels program is working to develop an affordable surrogate for JP8 derived from oil-rich crops such as rapeseed, other plants, algae, fungi, and bacteria. Initial efforts in the BioFuels program have already delivered BioFuel samples that have passed the key JP8 initial qualification tests - these are BioFuels whose performance is indistinguishable from petroleum-based JP8. The BioFuels program is expanding the development of processes for cellulosic and algal feedstocks with the ultimate objective of providing for an affordable, significant, and diverse supply of military jet fuel.

The Air Force earlier this year successfully flew a B-1B aircraft at supersonic speed using an alternate fuel in a flight over the White Sands Missile Range in Texas and New Mexico. The fuel, a 50/50 blend of synthetic and petroleum gas, is being tested as part of an ongoing Air Force program to use a fuel produced in the US, the Air Force said.

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