Valorization of solid digestate through biochar production for toluene adsorption and enhanced energy recovery as solid recovered fuel

Abstract

Anaerobic digestion (AD) transforms organic waste into biogas, aligning with circular economy principles; however, its byproduct, digestate, poses waste management challenges due to variable composition and potential heavy metal contamination. This study proposes an innovative dual-purpose approach to convert waste digestate and air pollutants into energy. Solid digestate was pyrolyzed to produce high-performance biochar for toluene adsorption and subsequent use as solid recovered fuel (SRF), effectively valorizing waste materials. Unmodified and potassium carbonate (K₂CO₃)-modified biochars were produced via pyrolysis at temperatures ranging from 500 to 800 °C, and their physicochemical properties were thoroughly analyzed. K₂CO₃ modification significantly enhanced biochar porosity, reduced ash content, and increased toluene adsorption capacity from 81.63 mg/g to 963.94 mg/g, surpassing commercial activated carbons. Moreover, the calorific value of biochar ranged from 10.66 MJ/kg to 33.56 MJ/kg prior to adsorption, with an additional increase of 5 %–33 % after adsorption, meeting SRF criteria. This dual process effectively converts both waste and air pollutants into energy resources. Economic analysis demonstrated that the production costs of modified biochar are considerably lower than those of commercial activated carbons, offering a cost-effective solution for industrial applications. These findings demonstrate that converting digestate into K₂CO₃-modified biochar not only enhances toluene adsorption but also provides an efficient pathway for waste valorization and energy recovery as solid recovered fuel. By integrating pollution control with resource utilization, this approach offers a practical strategy for managing digestate while mitigating VOC emissions.