Environmental and economic sustainability assessment of biofuels from valorising spent coffee grounds

This study quantifies and compares the life cycle environmental and economic sustainability of five most common and widely researched valorisation routes for spent coffee grounds (SCGs): i) fuel pelletizing; ii) combination of transesterification and pyrolysis; iii) pyrolysis; iv) combination of transesterification and fermentation; and v) anaerobic digestion. The scope of the study is from cradle-to-gate and the functional unit is defined as the “treatment of 1 t of SCGs”. The results indicate that fuel pelletizing is the best option from the environmental point of view, with all 19 impacts considered being net-negative. Pyrolysis and anaerobic digestion are the second-best options with 18 net-negative impacts, followed by the combination of transesterification with pyrolysis, with 13 net-negative impacts. Transesterification and fermentation is the worst option for 16 categories, including climate change, due to the lowest credits from the products produced. The results of the economic sustainability assessment reveal that all options earn profits and are economically feasible. Pyrolysis is the most economically sustainable alternative with a profit of £172/t SCGs, which is 2–3 times higher than for the other options. Transesterification and fermentation is again the worst option but still earning a profit of £58/t. Applying multi-criteria decision analysis reveals that pyrolysis is the best and transesterification and fermentation the least sustainable option. Therefore, coffee producers, local authorities, and other relevant stakeholders could consider prioritising pyrolysis for achieving both economic and environmental benefits. Fuel pelletizing, transesterification & pyrolysis, and anaerobic digestion should also be considered as they too offer significant sustainability benefits.