Using coffee for cleaner fuel

 By Robin Wylie

Monoaromatic hydrocarbons are compounds which are commonly added to gasoline. In the past, such “aromatics” were derived from fossil fuels, however today there is a demand for greener sources. One potential source of aromatics is coffee. Humans have developed such a taste for this beverage that each year we throw away more than one million tonnes of coffee waste. Converting this discarded plant waste into aromatics could offer an environmentally friendly way to reduce petroleum consumption, and thereby reduce humanity’s CO2 emissions. In a new study, scientists from the US and China have developed a new process, involving “torrefaction”, which they show can reduce the production cost of coffee-derived aromatics by $150 per tonne. This development means that this green alternative to aromatic production could become an even more attractive prospect in the future, with the unlikely help of our favorite drink…

Your morning cup of Joe could be more useful than you thought. A new study has found an improved way to produce a vital kind of hydrocarbon from used coffee grounds.

Aromatic hydrocarbons, a group of chemicals which includes benzene (C6H6), toluene (C7H8) and xylene (C8H10), play a central role in modern life. These liquids, named for their distinctive smell, are used to make everything from electronic goods to penicillin. Nylon is partly derived from benzene, so there’s a good chance some of your clothes owe their existence to aromatics.

These compounds are also important from a green standpoint. As key ingredients in items such as household insulation foam and lightweight aircraft components, aromatics help increase energy efficiency and hence reduce CO2 emissions.

But despite their many benefits, aromatics come with a drawback. The vast majority of aromatics that are produced today are derived from crude oil – the burning of which, most countries now agree, needs to be dramatically reduced.

But there are greener sources of aromatic hydrocarbons. One of these is biomass. Renewable aromatics can be generated by rapidly heating plant material to temperatures of around 500°C (932°F), a process called “Catalytic Fast Pyrolysis.” And in the search for a green source of aromatics, researchers are now turning to one of the biggest sources of biomass humans produce – used coffee.

The researchers discovered that with torrefaction (which they found lowered the weight of the coffee grounds by 15%) the overall cost of producing aromatics from coffee was lowered from $1,423 per metric ton to $1,271 per metric ton - an 11% price drop

The United States produces around 1.5 million metric tonnes (1.65 million short tons) of coffee waste each year*, much of which is simply thrown away. Converting this waste into aromatic hydrocarbons, therefore, could create a much-needed renewable source of these compounds. The technology to generate aromatics from coffee already exists, but the high costs involved are currently stopping the process from being rolled out on a wide scale. But a new treatment process could help change that.

One of the barriers to producing aromatics from coffee is due to the cost of transporting the grounds. Haulage is expensive. This was the problem that a joint US-Chinese team of researchers set out to address, in a study published last month. In a world first, the team treated coffee waste using “torrefaction,” a heating process which removes oxygen from, and therefore also lightens, the plant material, making it cheaper to transport.

The researchers discovered that with torrefaction (which they found lowered the weight of the coffee grounds by 15%) the overall cost of producing aromatics from coffee was lowered from $1,423 per metric ton (the cost for untreated grounds) to $1,271 per metric ton – an 11% price drop.

This isn’t a huge saving – and probably won’t cause oil companies to flock to Starbucks’ trash cans just yet. But it is a promising step in the right direction. And an important reminder that, when we treat our trash in smarter ways, the benefits can be significant.

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.