Converting coffee silverskin to value-added products by a slow pyrolysis-based biorefinery process

This work aims to transform coffee silverskin (CSS), the only waste from the coffee roasting process, that worldwide amounts to about 76 million kg/year, into value-added products within an integrated slow pyrolysis process. The study, performed at 280 °C, 400 °C and 500 °C, determined the potential applications of the resulting fractions. Biochar has been studied as an adsorbent of organic pollutants in water, using methylene blue (MB) and methyl orange (MO), which are respectively cationic and anionic aromatic dyes, as model compounds, and with 400 °C biochar giving the highest removal values, at 98% with MB and 40% with MO. Moreover, CSS biochar could be used to obtain renewable energy from its combustion, with 22.6–24.2 MJ/kg calorific values. The liquid fraction could be a potential source of caffeine, among phenolics, with 400 °C aqueous phase presenting the highest concentration of caffeine (14.3 g/L). Concerning the gas fraction, it could be used to obtain heat for biomass drying before pyrolysis. Hence, use of the pyrolysis products as described would allow zero-waste to be achieved in the coffee roasting industry, thus promoting the green and circular economy and production of green chemicals and materials in a biorefinery context.