A review on enhanced biofuel production from coffee by-products using different enhancement techniques

Coffee by-products are a renewable, plentiful, cost-effective, and mostly untapped resource that could be used as a biofuel feedstock. However, the energy efficiency and biofuel yields are mostly determined by the biofuel production technologies. Pretreatment procedure, hydrolysis methods, fermentation methods, oil to biodiesel conversion techniques, binders employed, applying pressure and temperature are the main factors to improve the biofuel yields from coffee by-products. This paper examines state-of-the-art methods for increasing biogas, bio-ethanol, biodiesel, briquettes, and pellets outputs from coffee by-products. Pretreatment and co-digestion of coffee by-products with other low carbon to nitrogen ratio animal manure boost the biogas yield of coffee by-products, which is also discussed. A yield of bio-ethanol from coffee by-products was also improved using advanced pretreatment procedures, production processes, and the use of genetically modified yeast strains that ferment the majority of sugar monomers. Additionally, oil extraction methods from spent coffee grounds were reviewed, as well as optimizing biodiesel yield from spent coffe grounds oil. The process of making briquettes and pellets, as well as the types of binders utilized, are discussed. The main novelty of this review is on improving biofuel yields such as biogas, bio-ethanol, biodiesel, briquettes, and pellets from the entire dry cherry coffee beans processing residues, wet coffee (coffee pulp or peeled) beans processing residues, and optimizing oil and biodiesel yield from spent coffee grounds.