Effect of Thermal Severity on the Structural Evolution and Stability of Rice Husk Biochar

This study evaluates the effect of thermal severity on the yield, structural evolution, and stability of biochar produced from rice husk via controlled pyrolysis. The raw biomass, containing 1.67 wt% moisture, 60.94 wt% volatiles, and 21.66-wt% fixed carbon, proved highly suitable for thermochemical conversion. Using a 3 × 3 factorial design (500–700 °C; 30–60 min), variance analysis revealed temperature as the dominant variable (F = 67.15; p < 0.001; ηp2 = 0.88), alongside a significant temperature time interaction (F = 6.19; p = 0.003). Maximum biochar yield occurred at 500 °C and 60 min (59.4 ± 2.0 wt%), whereas heating to 700 °C reduced yields to 41.8–44.0 wt% via enhanced devolatilization and secondary cracking. Structurally, the biochar developed a predominantly mesoporous matrix with a maximum BET surface area of 56.23 m2/g and ~4.9 nm average pore diameters. Furthermore, FTIR and DSC analyses demonstrated that higher thermal severity reduced oxygen-containing functional groups while increasing thermal stability and aromatic reorganization. Ultimately, 700 °C (at the 45 min residence time evaluated for porosimetry) provided the greatest structural development (maximum BET surface area), while 700 °C/60 min provided the highest thermal-oxidative stability, and lower severities favored biochar yield, revealing a trade-off between mass recovery and structural/stability performance. These findings establish that thermal severity dictates the physicochemical evolution of rice husk biochar, offering vital criteria for optimizing energy and environmental applications.