Complete utilization of wet spent coffee grounds waste as a novel feedstock for antioxidant, biodiesel, and bio-char production

Abstract

Due to the limitation of resource, non-edible feedstock has been gaining more attraction as the future feedstock for bio-based industries. Spent coffee grounds (SCGs) are the waste from coffee process which contained various high value products. The purpose of this study is to investigate the possibility of utilizing wet SCGs completely through three processes, antioxidant extraction, biodiesel production, and bio-char production. A semi-continuous process, with four extraction column reactors and peristaltic pumps, was used for extracting antioxidants from wet SCGs with methanol as solvent. Subsequently, potassium methoxide was fed into the reactors containing SCGs residue remaining after antioxidant extraction to produce biodiesel via in-situ transesterification. The defatted SCGs obtained after biodiesel production was further transformed to bio-char via slow pyrolysis. Antioxidant compounds with phenolic content of 10.3 mg gallic acid equivalents/g dried SCGs and antioxidant activity of 48.8 mg butylated hydroxyanisole equivalents/g dried SCGs could be extracted. Moreover, free fatty acid and moisture contents were reduced to levels suitable for in-situ transesterification. A biodiesel yield of 131.1 mg biodiesel/g dried SCGs was achieved, which was exceeded the maximum yield based on calculation. The bio-char produced from defatted SCGs had a heating value of 26.0 kJ/g and was smokeless, which satisfies the Thai community product standard (Per 238/2547). According to inventory analysis, the heat energy from bio-char product was far sufficient to support the whole processes.