Use of CO2 for enhanced carbon recovery in thermochemical processing of fruit peel waste

Abstract

To address a supply variability associated with biomass, utilization of food waste provides a practical approach to consistently obtain carbon–neutral energy resource. Despite the technical merits of conventional biological process, the inevitable generation of less useful by-products, such as carbon dioxide (CO2) and sludge, remains a primary constraint. As an alternative, thermochemical process, such as pyrolysis, presents a promising solution for the comprehensive valorisation of food waste. In this study, citrus peel, one of the fruit processing wastes, was used as a model food waste. To enhance the eco-friendliness of the pyrolysis system, CO2 was introduced as a medium gas given the uncertainty regarding its behaviour as an inert or reactive agent at the pyrolysis temperatures. The pyrolysis approach demonstrated that CO2 interacts with volatile pyrogenic products, resulting in an increased formation of carbon monoxide (CO). Notably, this enhancement was observed at ≥ 400 °C, suggesting that CO2′s reactivity is limited at lower temperatures. To maximize the utility of CO2, the pyrolysis setup was modified by incorporating a heating block at 600 °C and a nickel-based catalyst (Ni/Al2O3). While these adjustments accelerated the thermal cracking of citrus peel, test setup modification led to an aromaticity increase of the resulting pyrogenic products. However, integrating CO2 into the catalytic pyrolysis created favourable conditions for detoxifying these aromatic compounds and further boosting CO production. Syngas productivity from catalytic pyrolysis in the presence of CO2 was 35.2 wt%, showing 3.67 times increase over that from conventional pyrolysis. In conclusion, CO2-assisted catalytic pyrolysis of the citrus peel offers significant technical benefits: it enhances carbon recovery in the form of CO and reduces the formation of undesirable aromatic pyrogenic products.