Identification of environmental aspects of citrus waste valorization into D-limonene from a biorefinery approach

Abstract

Citrus waste (CW) consists mainly of peels and pressed pulp (seeds and segment membranes), which represent approximately 50% of the fruit weight. In the framework of bioeconomy, this type of waste could serve as a valuable raw material for the manufacture of biofuels and other commodities. Specifically, CW comprises high levels of pectin, cellulose, hemicellulose and soluble sugars, and to lesser extent essential oils. d-limonene is a terpenic compound and main component of essential oils in CW. In the present study, alternative and low environmental impact technologies for the valorization of CW are evaluated in the framework of a biorefinery strategy. Although d-limonene is the target product, a “closing the loop” strategy has been incorporated obtaining biogas and digestate as co-products from the residual side-streams. Plant-wide simulations are performed according to four different scenarios (Scenario I to Scenario IV) comprising four sections: pre-treatment, extraction, purification and anaerobic digestion. The main differences between the scenarios correspond to the extraction (hydrodistillation, cold pressing and solvent extraction) and purification stages. Scenarios III and IV use ethanol and hexane, respectively, as extractive agents. The results show that Scenario I has the worst environmental profile, while Scenario II has the best profile of all the scenarios analysed. In addition, purification stage is the main responsible for the highest environmental burdens in scenarios I, II and IV due to the energy required. In Scenario II, pre-treatment and extraction steps are identified as hotspots, due to the high electricity requirements. There is therefore still room for improvement, and future research should focus on optimising the purification stages to reduce the impacts of those scenarios with poorer environmental profiles.