Vacuum-Assisted Microwave Pyrolysis of Rice Husk for the Production of High-Heating Value Biochar for Practical Energy Applications

Abstract

The improper disposal of rice husk sparks interest in its potential as a sustainable bioenergy resource. This study investigates the biochar production from rice husk using the vacuum-assisted microwave pyrolysis (VMP) process. The experiments are conducted at different microwave powers of 360, 540, and 720 W for durations of 30, 45, and 60 min. A central composite design (CCD) with response surface methodology (RSM) is used to optimize the process parameters to maximize the high heating value (HHV) of biochar. The optimized HHV comes out to be 21.71 MJ/kg at 694 W and 56.06 min. The maximum biochar yield of 72.31 wt.% with an energy yield of 85.22% is achieved at 360 W for 30 min, whereas the highest HHV of 21.65 MJ/kg and energy density of 1.51 are achieved at 720 W for 60 min. The biochar is characterized through proximate and elemental analyses, Fourier Transform Infrared (FTIR) spectroscopy, and x-ray fluorescence (XRF), while energy properties are assessed through HHV, energy yield, and energy density. The FTIR analysis reveals the presence of functional groups such as hydroxyl (O–H), carbonyl (C═O), methyl (CH3), and aromatic C–H bonds, which enhance the energy characteristics of biochar. Techno-economic analysis shows that the VMP process achieves the shortest payback period (∼0.46 year) compared to grid-based (∼0.49 year) and solar-powered microwave systems (∼1.8 years).