Waste Coffee Grounds as Biodiesel Feedstock; Potential for 340M Gallons Per Year of Coffee Biodiesel
Waste coffee grounds can provide a cheap, abundant, and environmentally friendly source of biodiesel fuel, according to a study by researchers at the University of Nevada-Reno. According to the USDA, world coffee production is 16.34 billion pounds per year; the scientists estimated that spent coffee grounds can potentially add 340 million gallons of biodiesel to the world’s fuel supply. A paper on the work was published online in the ACS Journal of Agricultural and Food Chemistry.
Spent coffee grounds contain between 11-20 wt.% oil. The process developed by Mano Misra, Susanta Mohapatra, and Narasimharao Kondamudi for extracting that oil for subsequent transesterification yields 10-15% oil depending on the coffee species (Arabica or Robusta).
As an additional advantage, the biodiesel derived from coffee oil possesses better stability than biodiesel from other sources due to its high antioxidant content (which hinders the rancimat process). The solid waste remaining after oil extraction can be utilized as compost, as a feedstock to produce ethanol, and as fuel pellets.
For their study, the team collected Starbucks’s (Reno, Nevada) “grounds for your garden” spent coffee grounds. They dried the grounds in an over overnight at 50° C to remove moisture (mostly 50-60 wt%) and then refluxed them with low-boiling organic solvents such as n-hexane, ether, and dichloromethane to extract the oil from the coffee particles. The oil was separated from the solvents using a rotary evaporator, and the solvents were reused in the next batch of extraction.
More polar solvents extracted greater amounts of FFA and, in turn, more crude oil yields (13.4% hexane, 14.6% diethyl ether, and 15.2% dichloromethane), which caused a decrease in pH. Hexane extraction resulted in a more neutral pH (6.8) of the extracted crude oil, and was selected as a suitable solvent for the oil extraction process.
Free fatty acids (FFAs) present in the crude oil were separated by converting them into soap by mixing a basic solution with the extracted oil. Soap was removed from the pure oil by centrifuging the mixture.
GC-MS and HPLC analyses indicated that the coffee biodiesel consisted of both saturated (51.4%) and unsaturated (48.6%) esters. ASTM analysis confirmed that this biodiesel can be used industrially as an alternative to diesel. This can add approximately 340 million gallons of biodiesel to the world’s fuel supply.—Kondamudi et al. 2008
The reserachers plan to develop a small pilot plant to produce and test the experimental fuel within the next six to eight months.
Narasimharao Kondamudi, Susanta K. Mohapatra, and Mano Misra (2008) Spent Coffee Grounds as a Versatile Source of Green Energy. J. Agric. Food Chem., Article ASAP doi: 10.1021/jf802487s