Pyrolysis of spent coffee grounds using a screw-conveyor reactor

• Pyrolysis offers a waste-to-energy opportunity for spent coffee grounds.
• Pilot-scale demonstration of an extruder-type screw-conveyor reactor
• Highest bio-oil yield at lowest residence time and 505 °C pyrolysis temperature
• Bio-oil enriched in higher-value fatty acids

The fast pyrolysis of spent coffee grounds using a compact, transportable, screw conveyor reactor for producing bio-oil was studied. A two-factor, five-level, central composite response surface experiment was completed to formulate a statistical model that relates reactor temperature (429–550 °C) and residence time (23–42 s, controlled by the screw rotation rate) to bio-oil yield and quality. Regression analysis of model fits with experimental data showed that temperature and residence time had a significant effect (p < 0.05) on bio-oil yield. Highest bio-oil yield (61.8%) was observed at 500 °C while the highest char yield (20.6 % w/w) was produced at the lowest temperature of 429 °C. Bio-oil yields increased with screw speed and decreasing residence time. The model predicted a maximum liquid yield of 61.7% and an accompanying char yield of 17.1% at 505 °C (778 K) and 70 rpm. In addition to containing oxygenated organic compounds typical of bio-oils, spent coffee bio-oil also contains more hydrophobic compounds (> 20% peak area) such as fatty acids, fatty acid esters, medium-chain paraffins, olefins, and caffeine. Because of the abundance of spent coffee grounds and the quality of its bio-oil, this waste stream offers potential as a valuable bioenergy feedstock.