Evaluation of thermochemical routes for the valorization of solid coffee residues to produce biofuels: A Brazilian case

• Published in: Renewable & sustainable energy reviews 2021-03, Vol.137, p.110585, Article 110585
• Main Authors: Mendoza Martinez, Clara Lisseth, Saari, Jussi, Melo, Yara, Cardoso, Marcelo, de Almeida, Gustavo Matheus, Vakkilainen, Esa
• Format: Article
• Language: English
• Subjects: Coffee residues; Energetic routes; Mass and energy balances; Thermochemical; Value-added products
• ISSN: 1364-0321; 1879-0690
• DOI: 10.1016/j.rser.2020.110585

Coffee production in Brazil creates significant amounts of residues. The goals of this study are to evaluate the characteristics of these residues (parchment, husk, pulp, spent grounds, silverskin and defective beans); to discuss their potential for conversion to improved biofuels via thermochemical methods; and to develop mass and energy balances of the processes to help determine the value of residues for direct combustion, fast and slow pyrolysis, gasification, hydrothermal carbonization and torrefaction. Particularly the pulp, but also husk and parchment, are characterized by high moisture, as well as high contents of cellulose (41–64%) and hemicellulose (27–35%). These residues are suitable for several conversion routes, albeit with the drawback of drying need for the dry methods. The ash of these also creates a risk of fouling, corrosion and agglomeration with high-temperature and fluidized bed technologies. The silverskin and some of the defective beans are available at lower moisture. The spent coffee grounds appear a particularly advantageous residue for energy use: while moisture varies, the roasted product dries easily and has the highest heating value of the residues. For defective beans, little thermochemical treatment data is available. Among the technologies, for wet feedstocks hydrothermal carbonization has the advantage of post-conversion drying. Gasification appears advantageous for parchment with a high syngas yield and heating value. Fast pyrolysis of biomass suffers from the oxygen content of the liquid, requiring additional treatment; slow pyrolysis may be more appropriate. In conclusion, coffee residues have potential as feedstocks for a number of thermochemical conversion processes. [Display omitted] •Brazil is the largest producer and exporter of coffee worldwide.•Coffee agroindustry is responsible for a considerable side stream of residues.•Agricultural policy in Brazil has increasingly focused on sustainable development.•Thermochemical technologies are potential conversion routes for energy generation.