Caffeinating the biofuels market: Effect of the processing conditions during the production of biofuels and high-value chemicals by hydrothermal treatment of residual coffee pulp

Abstract The manufacturing of coffee, one of the most popular beverages globally, renders enormous amounts of by-products and wastes, which may trigger severe environmental issues if not treated appropriately. The coffee pulp, resulting from the wet processing of coffee, is the predominant by-product, with around 10 Mt annually produced worldwide. For the first time, this work addresses the hydrothermal treatment of coffee pulp to produce biofuels and platform molecules, scrutinising the influence of the processing conditions (temperature, pressure, reaction time and solid/water ratio) on the process. This strategy allowed the transformation of coffee pulp into bio-crude and hydrochar in different yields (10-26 % and 10-42 %, respectively), depending on the conditions. The bio-crude included a pool of alkanes, carboxylic acids, ketones, phenols and nitrogen species, with varying quantities of C (54-71 wt.%), H (6-7 wt.%), O (18-34 wt.%) and N (3-5 wt.%) and a calorific value shifting from 23 to 32 MJ/kg. The hydrochar contained different proportions of C (57-72 wt.%), H (4-6 wt.%), O (20-35 wt.%) and N (2-3 wt.%) and had a calorific value between 22 and 29 MJ/kg. Process optimisation showed that up to 45 % of the coffee pulp could be simultaneously converted into energy-rich (29 MJ/kg), merchantable liquid (20 % bio-crude) and solid (24 % hydrochar) biofuels during the treatment of a 15 wt.% coffee pulp suspension at 320 oC and 162 bar for 1 h. At the same time, a bio-crude with a high proportion of profitable phenolic derivatives (42 %) can be attained in high yield (25 %) when a 5 wt.% suspension is treated at 280 oC and 120 bar for 2 h. These promising results, along with the bespoke nature of this hydrothermal treatment, are a landmark achievement for the economy and sustainability of coffee producer countries, thus representing a pioneering step change towards the sustainable management of early-stage coffee leftovers.