A simpler route to biodiesel from used coffee grounds

The world produces almost 10 million tons of waste coffee grounds every year. Researchers have now discovered an efficient way to turn that waste into a green fuel. Their simple one-step process, outlined in the Journal of Environmental Chemical Engineering, would save time and the cost of producing biodiesels from coffee.

Biofuels are mostly made from sugarcane, corn, and palm oil today; sugary plants are used to make ethanol while palm oil is converted to biodiesel. But growing these plants for fuel can cause deforestation and compete with agriculture for land and water. Using wastes such as restaurant grease is a more sustainable alternative.

Coffee grounds are a good feedstock for fuels because of their high calorific value. Recycled coffee is already used to make fire logs and composite materials for furniture. And researchers have recently come up with ways to turn coffee waste into water-purifying and carbon-capturing materials.

The known method to convert spent coffee to biodiesel involves two steps. The first involves extracting the oils in coffee grounds. This is done by mixing the grounds with the solvent hexane and then cooking the mixture at 60°C for 1–2 hours to extract the oils. The hexane is evaporated to leave behind the oils. In the second step, methanol and a catalyst are added to create biodiesel. Glycerol, which is also created as a by-product, has to be separated from the biodiesel.

A team of chemical engineers and environmental scientists from Lancaster University in the UK streamlined the process by eliminating the need for solvent-based oil extraction. They mixed coffee grounds with methane and a catalyst to make biodiesel directly. The key was to use the right concentration of sodium hydroxide and adjust the methane-to-oil ratio during production.

The researchers found they could extract the same amount of oil from spent coffee as the old method in just ten minutes, as opposed to an hour or two. By significantly cutting the time and cost needed to extract the oils for biofuel, the process could make spent coffee grounds a much more commercially competitive source of fuel, lead author Vesna Najdanovic-Visak said.

The method could yield around 800,000 tons of biodiesel, or over 286 million gallons, a year. That is, if spent coffee grounds could be all collected and processed. Right now, most coffee waste goes to the landfill.

Source: Najdanovic-Visak V et al. Kinetics of extraction and in situ transesterification of oils from spent coffee grounds. Journal of Environmental Chemical Engineering. 2017.

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