London buses need coffee too!

We all have those days when we feel like coffee is the one and only fuel capable of keeping us going to the bitter end. As of November, we are not alone in this dependence on the coffee bean for energy – London’s famous red buses are now using oils from leftover coffee grounds as fuel.

Introducing Biofuels

When oil is extracted from leftover coffee grounds and used to drive buses it is known as a biofuel. Biofuels, as opposed to fossil fuels which come from matter which has been dead for millions of years, are fuels derived from recently living matter. They are pretty exciting stuff and offer an alternative to our depleting reserves of fossil fuels.

Brilliantly, biofuels are much less damaging to the environment than fossil fuels as they can be easily and quickly replaced and, in the case of leftover coffee, are really widely available. Yet another huge advantage of biofuels is that the machinery of the buses using these fuels doesn’t have to be changed at all – it’s exactly the same as using diesel fuel.

The Science Behind The Magic

Coffee is between 9% and 20% oily compounds such as Cafestol and Kahweol. Most of this is removed when coffee is brewed, but a significant amount remains in the leftover grounds, which can be removed using hexane extraction.

Hexane is a simple hydrocarbon with 6 carbons and 14 hydrogens. Hexane is a nonpolar molecule, meaning charge is distributed evenly across the molecule, and you don’t have particular bits with net positive or negative charge. Water, on the other hand, is polar. Polar solvents can dissolve other polar molecules – for example things like sugar and salt (polar) easily dissolve in water. Hexane on the other hand, is a nonpolar solvent, meaning nonpolar compounds like oils dissolve in it.

When hexane is mixed with coffee grounds, the nonpolar oils dissolve into the hexane, giving a mix of hexane and oil. As we mentioned earlier, hexane is a very simple molecule, giving it a low boiling point. The hexane can therefore be boiled off, leaving pure oil. The hexane can be condensed and then reused.

With all the interest surrounding this area, researchers worldwide are on the hunt for efficient and effective ways to make biofuels. Some bacteria, like E. coli, can be engineered to produce fats which can be used as biofuel. E.coli normally produce lipids in very small amounts, but can be engineered to produce much more.

However, researchers are encountering a common problem – the fats produced by bacteria are unbranched and therefore can pack together closely, meaning they solidify at low temperatures and become waxy. This means they can’t be used in a conventional engine.

Genetic engineering may be the answer to this problem. An engineer at Washington University in St. Louis has stitched together bits of DNA from different bacteria to change the way E. coli makes fats. By inserting new genes using a gene editing technique called CRISPR, the E.coli ‘s fatty acid pathway has been changed so that it creates a biofuel which is freeze resistant and stays liquid.

Biofuel: The Perfect Solution?

So what’s the catch? Why do we not find coffee oil being pumped to to all our petrol stations?

Although biofuels seem like a saviour, it is not the perfect solution. There are some problems with growing plants to be used to make biofuels – as there is a finite amount of space for growing crops, biofuel crops will inevitably end up competing for space with food crops. If biofuels can bring more profit to farmers (e.g. by being sold to rich customers abroad), why would they use their fields to grow the food that the locals need to survive?

As the world population grows and more and more food is needed, ever more land is needed to grow food crops. If much of the arable land is taken up by biofuel crops, this may lead to deforestation to create space to grow food. This removes carbon sinks so less CO2 is removed from the atmosphere. We have, therefore, come full circle. A more green alternative to fossil fuels will create it own environmental and atmospheric damage.

Waste Not, Want Not

However, if biofuels are created using leftover food which would otherwise go to waste it doesn’t carry these problems with it – coffee grounds will do much more good powering a bus than in landfill. And it isn’t just coffee, many food types are now being used as fuels – from soybeans to cooking oil, and beef tallow to potatoes. We can make a clean fuel from waste which is almost unlimited – in the UK we consume a whopping 500,000 tonnes of coffee per year.

So will biofuels be propelling us forward into the future? Many large companies seem to think so – Qantas has announced plans for the first biofuel powered flight between the US and Australia just a year from now. Qantas are partnering with a Canadian agriculture-tech company to create an aviation fuel crop which can grow successfully in climates where food crops normally fail, so it won’t compete with food sources. Qantas has already operated flights in Australia running on fuel partly derived from cooking oil.

So can innovations like the bean powered buses change the way we see waste? With our reliance on fossil fuels seeming to draw near to an end, maybe using food waste is the next step. It is clear that we need to find new ways to power our energy-hungry lifestyle and it is not clear which new technology will emerge as the powerhouse of years to come. It is clear, however, that invention, originality and creativity are what will drive us forward to the next age.

About our writer

Naomi Foster is in her second year studying Engineering Science at St. Anne’s College, Oxford. Naomi was a Mentor for Cambridge Immerse in 2017.

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