Oil can be extracted from coffee grounds by soaking them in an organic solvent, before being chemically transformed into biodiesel via a process called 'transesterification', according to researchers from the Centre for Sustainable Chemical Technologies at University of Bath.
As part of the study, the researchers made biofuel from ground coffee produced in 20 different geographic regions, including caffeinated and decaffeinated forms, as well as Robusta and Arabica varieties.
"Around 8 million tonnes of coffee are produced globally each year and ground waste coffee contains up to 20 per cent oil per unit weight," said Dr Chris Chuck, Whorrod Research Fellow from the Department of Chemical Engineering.
"This oil also has similar properties to current feedstocks used to make biofuels. But, while those are cultivated specifically to produce fuel, spent coffee grounds are waste. Using these, there's a real potential to produce a truly sustainable second-generation biofuel," Chuck said.
The research found that there was a reasonably standard composition and little variation in the relevant physical properties of the fuels, irrespective of the source.
This means that all waste coffee grounds are a viable feedstock for producing biodiesel.
"The yields and properties of biodiesel can differ depending on the growth conditions of current biodiesel feedstocks, sometimes causing them to fall out of specification," Chuck added.
"The uniformity across the board for the coffee biodiesel fuel is good news for biofuel producers and users," Chuck said.
The researchers suggest that while coffee biodiesel would be a relatively minor part of the energy mix, it could be produced on a small scale by coffee shop chains to fuel vehicles used for deliveries.
These same delivery vehicles could be used to collect spent coffee grinds and take them to a central biodiesel production facility to be processed.
"We estimate that a small coffee shop would produce around 10kg of coffee waste per day, which could be used to produce around 2 litres of biofuel," said Rhodri Jenkins, a PHD student in Sustainable Chemical Technologies and first author of the study.
"There is also a large amount of waste produced by the coffee bean roasting industry, with defective beans being thrown away. If scaled up, we think coffee biodiesel has great potential as a sustainable fuel source," Jenkins said.
The study was published in the ACS Journal Energy & Fuels.