Multi-response optimization for the production of Albizia saman bark hydrochar through hydrothermal carbonization: characterization and pyrolysis kinetic study

Abstract

In this study, Albizia saman bark hydrochar (ASB-HC) as a solid biofuel was generated by hydrothermal carbonization (HTC). Response surface methodology (RSM) identified the importance of variables and their interactions to conduct the experiments. The effects of the reaction temperature (180–200 °C), residence time (2–4 h), and stirring speed (400–600 rpm) on ASB-HC yield (wt. %) and higher heating value (HHV) were analyzed and optimized for these factors and responses using the Box–Behnken design (BBD) of RSM. FTIR, TGA, SEM-EDX, and HHV analyses were done to characterize and understand the effects of HTC on Albizia saman bark (ASB-raw) and ASB-HC in an optimized state. In this optimized state, the carbonization temperature, residence time, and stirring speed were 180 °C, 4 h, and 600 rpm, respectively, with 69.89% yield and 18.59 MJ/kg HHV. The influence of temperature on hydrochar production compared to the time and stirring speed was evident in the investigation. The HHV of ASB-raw was 15.8 MJ/kg which increased to 18.59 MJ/kg at ASB-HC. The proximate and ultimate analysis revealed that ASB-HC had higher fixed carbon but there was less oxygen and volatile matter content than ASB-raw. The obtained activation energy indicated the slower reaction rate of hydrochar degradation. This investigation provides practical insights about hydrochar properties, process conditions, and thermal degradation kinetics on hydrothermal treatment for bark feedstock.