Jet fuel grows on gum trees

Many greenhouse gas-emitting processes can now be replaced by low or zero carbon emission alternatives, but how about your holiday to Bali?

Jetting off on holidays might be one of our favourite pastimes, but it’s also a major contributor to climate change. Modern jet aircraft are not able to fly with renewable fuels such as bioethanol or biodiesel because they contain less energy than standard jet fuel, so new approaches are needed if we want to reduce carbon emissions from the aviation industries.

The solution could be found inside the leaves of Australia’s iconic gum trees.

The essential oils found inside the leaves of gum trees are made up of a cocktail of chemicals. The majority of these chemicals are a type called ‘terpenes’, such as eucalyptol which gives eucalyptus oil its famous scent.

But the oils from some gum trees contain different terpenes. Oil from lemon-scented gum, for example, contains mostly citronellal, which is used in insect repellents and perfumes.

Other gum trees contain very high amounts of yet another group of terpenes called pinene and limonene. Recent research has shown that these terpenes can be converted into a high energy fuel using a special catalytic process.

The resulting biofuel has similar chemical and physical properties to jet fuel, except that it contains slightly more energy per litre. This means that by replacing a portion of standard jet fuel with this new renewable jet fuel, planes will actually be able to carry more passengers or freight, or fly longer distances.

So the potential exists to produce jet fuel from gum trees, but will it take away good agricultural land, similar to bioethanol and biodiesel production? And how much do we need to grow to make an impact?

No, the type of gum trees we are talking about grow in the dryer parts of the country, areas that are no good for agriculture. They are called ‘mallees’ and are currently used to produce eucalyptus oil. Every year or two, they can be harvested by chopping them off just above the root. They will regrow from root structures called lignotubers. They grow in a bushy form that can best be described as a ball of leaves containing large amounts of terpenes.

Australia’s advantage is that there are hundreds—almost 800 in fact—species of gum trees and they harbor both a great amount of genetic diversity and grow in every imaginable niche or environment.

These gum trees have not been domesticated in the ways that food crops such as corn or wheat have. Without any domestication, the gum trees can produce about 200 kg of oil per hectare every year. Just by selecting the best individual trees, it is estimated that this could be increased to over 500 kg of oil per hectare every year. But to make this jet fuel economically viable, further improvements are needed.

Among individuals of the same species, the amount of oil found in the gum leaves varies hugely: up to 30-fold. Most of this variation is caused by genetic differences, such as mutations, and not environment. Due to recent improvements in DNA sequencing technology we can now look under the hood at these genetic differences and determine which ones cause some individuals to produce more oil than others.

We can now understand what causes the huge differences of oil in the leaves among individuals. Using this knowledge we hope to be able to increase the amount of oil per hectare and year to beyond 1000 kg.  

Worldwide, over 20 million hectares of land are used to grow gum trees for the production of pulp, paper and timber. Hypothetically, if these 20 million hectares of gum trees were used for the production of jet fuel, at 500 kg oil per hectare and year, it could substitute 10 percent of the global aviation fuel consumption. Doesn’t sound like much? Well, it would reduce greenhouse gas emissions by 22 million tons each year! With improved crops this could be further increased.

But will jet exhaust in the future smell like eucalyptus oil? No, the exhaust will still consist of CO2 and water. The CO2 however has been recycled through photosynthesis in the gum trees.

So let’s stop digging for oil and grow it on trees instead!