Recovery of Energy from Orange Peels Through Anaerobic Digestion and Pyrolysis Processes after d-Limonene Extraction

Currently, the citrus processing industry generates a voluminous waste stream in regions characterized by an important citrus production. This solid waste, mainly constituted by orange peel waste (OPW), represents more than half of the raw material, and it could be exploited for the production of bio-based products and bio-fuels, according to the waste hierarchy. d-limonene, the main component of essential citrus oils, is a high-added value molecule, which can be applied in several fields, ranging from cosmetics to the food industry. Because of the high moisture content in OPW (about 80%), anaerobic digestion (AD) can be considered a suitable technique for waste valorization. However, this process is challenging, due to the potential toxicity of d-limonene on fermentative bacteria. For this reason, it is necessary to remove this high-added value compound in advance. After the recovery of d-limonene via solvent extraction, the porous residual matrix, after exposure to the atmosphere for some days at room temperature, releases the moisture content, thus making pyrolysis a valid alternative for the recovery of energy