Humans drink approximately 400 billion cups of coffee annually, leaving behind 18 million tonnes (19.8 million tons) of wet coffee grounds, roughly the weight of the three Great Pyramids of Giza. These grounds, which mostly end up in landfill, have the potential to be fuel. However, their moisture poses a significant challenge. Scientists at the Korea Institute of Geoscience and Mineral Resources (KIGAM) have devised a technique to turn still-wet coffee grounds into high-grade biofuel in as little as 90 seconds.

The technology, which the researchers describe as a world-first flame plasma pyrolysis (FPP) technology, blasts the wet grounds with extremely hot plasma flames (800–900 °C, or 1,472-1,652 °F) that rapidly vaporize the moisture, creating a 'pop-corn' effect that transforms the grounds into a porous-structured bio-char.

The resulting product has fuel performance comparable to anthracite charcoal, according to the researchers. In addition to eliminating the predrying step required by other processes, the KIGAM process uses moisture advantageously, turning it into a steam-activation agent that accelerates reactions and improves product quality.

The fuel potential of spent coffee grounds has always been known. However, their high moisture content has always been a barrier to economically turning them into fuel. The challenge is the energy intensive and time-consuming process of predrying the grounds before they can be processed.

The researchers’ method, detailed in the journal Chemical Engineering, completely eliminates the drying process, turning moisture to an advantage. The technique treats coffee grounds containing roughly 55% moisture, still quite wet. During the process, the plasma flame, created by the combustion of liquefied petroleum gas (LPG) and compressed air, converted the wet grounds into dry, porous biochar within 90 seconds, with an 83.3% reduction in mass.

The resulting bio-char demonstrated a heating value of 29 MJ/kg, meaning that burning 1 kg of the fuel will provide 29 MJ of thermal energy. For context, wood has a heating value of 15–20 MJ/kg.

Beyond its high energy content, the biochar also showed significant improvements in quality and environmental performance. Fixed carbon content nearly tripled, rising from 15.6% to 46.2%, meaning a greater proportion of the material is converted into long-lasting, energy-rich carbon that burns more efficiently.

The process also completely removed sulfur compounds, preventing emissions of sulfur oxides that contribute to acid rain and air pollution. Meanwhile, the material's surface area increased from just 1.5 to 115.4 m²/g, giving it properties similar to those of activated carbon and opening the door to applications beyond fuel, including water purification, air filtration, and industrial adsorption. The plasma process also produced very little smoke or tar, reducing harmful secondary pollutants typically associated with biomass conversion and making the technology a cleaner way to produce renewable solid fuel.

Speed is another huge part of the process’s appeal. Conventional methods for converting biomass into solid fuel, such as hydrothermal carbonization and torrefaction, can take anywhere from 30 minutes to six hours. KIGAM’s FPP process does the job in 90 seconds, making it up to 240 times faster than the conventional methods.

The researchers also say the system avoids one of the usual drawbacks of plasma-based treatment. Instead of relying on electricity hungry plasma equipment, it generates plasma through LPG combustion and compressed air, reducing energy demand while still delivering the extreme heat needed for rapid conversion.

The biggest advantage, however, may be that the feedstock goes in wet, removing the drying stage could cut both energy use and cost. This brings us to the point that while the project focused on coffee grounds, the technology can be applied to a wide range of high-moisture organic waste, including food waste, agricultural residues, and even sewage sludge.

"This technology presents a new paradigm in which waste is no longer viewed as a disposal problem but as a valuable energy resource," said Dr. Taejun Park, lead author of the study. "We plan to expand the technology to various types of high-moisture organic waste and further optimize the process for industrial-scale commercialization."

The process is also compact, allowing for deployment in on-site waste-to-energy systems.

Source: KIGAM via EurekAlert