Experimental exploration of processes for deriving multiple products from spent coffee grounds

Abstract

Valorizing spent coffee grounds (SCG) through hydrothermal liquefaction (HTL) has great potential for biocrude production. To improve its economic viability, an integrated biorefinery route for SCG valorization was proposed in this study. High-value polyphenols and polysaccharides were extracted using water under ultrasound assistance. A central composite design was applied to jointly maximize the yields of both polyphenols and polysaccharides. Under the optimal conditions: a solid:water ratio of 1:23, an ultrasound power of 213 W, and an extraction time of 30 min at the temperature of 68 °C, the polyphenols yield was 1.0 wt.% (10 mg gallic acid equivalent/g SCG) and polysaccharides’ yield was 1.1 wt.% (11 mg glucose equivalent/g SCG). The biochemical compositions of raw and post-extraction SCG were characterized and exhibited appreciable difference, implying the impact of ultrasound-assisted extraction on feedstock composition. HTL of both raw and post-extraction SCG were conducted under three different temperatures (270 °C, 295 °C & 320 °C). It was found that 295 °C was the most desirable temperature, resulting in a biocrude yield of 31.1 wt.% and 27.7 wt.% for raw and post-extraction SCG, respectively. As for the biocrudes’ chemical composition, ester is the main component and increasing HTL temperature favored the fatty acid formation over ester, consequently led to more formation of amides.