Apr 28 2022

Sustainable Jet Fuels

As we try to transition as much as possible away from fossil fuels, jet fuel remains a tricky problem to solve. Jets, like rockets, require a fuel with a high energy density (energy per volume) and specific energy (energy per mass). For rockets the specific energy is far more important – mass is everything, due to the rocket equation – and so perhaps the ideal fuel for rockets is hydrogen, because it is so light. For jets, however, energy density is also very important because there is only so much volume in the fuel tanks, and making them bigger can be counterproductive.

Jet fuel has another requirement. O-rings are used to seal metal-on-metal joints in the fuel tank and engine. In order for these O-rings to work optimally they have to swell during engine operation, a property known as seal-swell. Currently aromatics are added to jet fuel because they cause the seals to swell. In fact, this has been a huge hindrance to the use of replacement and more sustainable jet fuels, because they lack adequate seal-swell properties.

A new innovation, however, may have solved this problem. The researchers developed a lignin-based jet fuel additive (LJF) with several desirable properties. Here’s the highlight: “A new LJF is reported primarily composed of C6-C18 mono-, di-, and tri-cycloalkanes.” They tested a 10% blend of their new LJF with conventional jet fuel and it increased the fuels energy density and specific energy, both highly desirable effects. But perhaps more importantly, the LJF additive had great seal-swell properties. It could therefore replace the aromatics in the jet fuel.

This is important for the project of creating sustainable jet fuels because the aromatics have a significant negative impact on the environment. They produce a lot of soot when burned and contribute to contrails.  Contrails contribute to global warming, perhaps even more than the CO2 released by burning jet fuel. Reducing or eliminating soot from aromatics and the resulting contrails could significantly reduce the contribution to AGW from jet travel. As a blend the LJF, which is a biofuel, displaces fossil fuel, and by making the fuel more energy dense reducing overall fuel use.

The goal now is to continue testing, also with higher blend ratios, up to 50%. Of course, the other big question with biofuels is now efficient is their production? If you are using land and fossil fuel-based fertilizer to grow the feedstock for biofuels, you lose the advantage. In the study the authors used agricultural waste, which is better than using feedstock grown exclusively for the purpose. Although “waste” is always a relative designation. What would have otherwise happened to that waste? Would it have been used to feed animals, or make organic fertilizer to be put back into the soil?

Also, how will the process of scaling up the production of the LJF work? Is there enough agricultural waste available? How energy intensive and efficient is the process of making the LJF? Once we get to an industrial scale, we will need to do a net energy efficiency analysis – how much total energy goes into making the LJF (and how much carbon is produced) and how much energy do we get out of it. In other words, what will its net energy efficiency and carbon efficiency be? This is why I have not been very enthusiastic about biofuels in general. Their margins are thin, and they may not be ultimately worth the land and water use required to make the feedstock.

Researchers are working on solutions, such as growing feedstock in vats, harvesting them from the ocean, or using otherwise true biological waste. But I’m not holding my breath that biofuels are going to be a significant contributor to displacing fossil fuels. But jet fuel has always been the exception to my general pessimism about biofuels. For practical reasons jets will require high energy density and specific energy. Hydrogen doesn’t work (the energy density is too low) and we are nowhere near the required energy density for storing electricity for electric engines. We will be burning energy dense jet fuel for the foreseeable future (at least for long-distance travel, which is the primary niche of jet travel anyway).

Jet biofuel is therefore the most likely option to displace the use of fossil fuel in jets. I did not appreciate before researching this news item that eliminating the need for the aromatics is perhaps just as important. This new LJF blend can perhaps do both. Even if it will not eliminate the fossil fuel component, significantly reducing it, and reducing the contribution of contrails to AGW, could be significant.

It’s also interesting to think that to some extent our technology has gone full circle. As industrial technology developed, we turned primarily to biological sources for oil and fuel products. We burned wood for fuel, whale oil in lamps, and obtained pitch, tar and resin from distilling wood or other organic substances. England, which was trying to have a massive navy to control its empire, and heat its increasingly populated cities, essentially ran out of wood. Coal solved their problem, even though it burned dirtier and choked their cities with pollution. But this kicked off the industrial revolution. Then crude oil was discovered, and the petrochemical industry took off. Having massive amounts of high-energy oil just sitting in the ground was jus too cost effective and convenient to ignore.

Now we are trying to partly get back to a biofuel-based world for our oils and fuel and bring an end to the era of fossil fuels. It remains to be seen how long this will take and what the consequences will be.

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