Used coffee grounds. If they don't end up in the trash, they can be used in peeling cosmetics or as a garden fertilizer. Now there is a chance that they can be used as... fuel for the fireplace or grill. A Warsaw University of Technology spin-off company is working on processing coffee grounds into briquettes.
Researchers from the Lodz University of Technology together with colleagues from universities in Germany and Ukraine developed a method to minimize the risk of blackout, i.e. a long-term interruption in the supply of energy in a significant area. They publish their findings in Nature Communications.