Sustainable energy recovery from mixed agri-food waste by hydrothermal carbonisation: Mechanistic evaluation of the evolution of product characteristics

Abstract

To address aggravating climate concerns and enhance bioenergy production, mixed agri-food waste (MAFW) was upgrade into high quality solid biofuel using hydrothermal carbonisation (HTC), demonstrating the potential to convert low-quality biowaste into valuable bioenergy resource. The influence of reaction temperature (190–230 °C), residence time (1–5 h) and solid loading (5–20 %) on the evolution of hydrochar and process water (PW) characteristics were systematically investigated to gain mechanistic insight into the HTC process of MAFW. In addition, an assessment of the valorisation option(s) for MAFW PW was conducted to guide the development of a conceptual HTC-based biorefinery system for sustainable bioenergy production. Results demonstrated that high HTC reaction severity (increased reaction temperature and residence time) enhanced the fuel characteristics of the hydrochar except for nitrogen content, which increased by ⁓86 % at the most severe HTC operating condition. Furthermore, the concentration of organic compounds in the PW showed a significant decrease as the reaction severity increased, except for total phenol content which showed a consistent increase. Meanwhile, the concentration of organic compounds in the PW increased as the solid loading increased. Overall, MAFW PW is rich in organic matter but relatively low in nutrients. Based on the analysis of the hydrochar and PW characteristics at different HTC operating conditions, it was revealed that dehydration, decarboxylation, aromatisation and Maillard reaction constitute primary reaction mechanisms influencing the HTC conversion of MAFW. Although higher reaction severity yielded a high degree of coalification in MAFW hydrochar, a medium reaction severity was conducive for producing high-quality solid biofuel. Additionally, medium solid loading yielded ample solid yield and a moderate organic matter concentration in the PW. To efficiently valorise the MAFW PW, a multistage process involving the combination of wet oxidation, anaerobic digestion and nutrient recovery was proposed. The findings presented in this study offer valuable referential information for improving the fuel quality of hydrochar obtained from MAFW and guiding the development of a sustainable strategy to harness the energy potential of MAFW via HTC.