Optimization of pyrolysis conditions for Catha edulis waste-based biochar production using response surface methodology

Catha edulis (Khat) waste (KW) is one of the challenging waste managements in Ethiopian urban areas. While biochar from other biomass sources has been studied, the effect of pyrolysis conditions on Catha edulis waste-based biochar yield and quality remains unexplored. Therefore, this study aims to optimize the biochar production process from Catha edulis waste for high yield and desirable characteristics. The KW and biochar were characterized using FTIR, BET, proximate analysis and other key parameters. The results indicated that KW possesses favorable properties for thermochemical conversion, with low ash content (4.35% wt. dry basis) and significant organic constituents (46.89% cellulose, 28.53% lignin, 19.62% hemicellulose, 4.96% extractives). The effect of pyrolysis process variables embracing reaction temperature, reaction time, and particle size on biochar yield and quality was optimized using response surface methodology (RSM) coupled with central composite design (CCD). The biochar was desirably characterized by a pH of 8.96, fixed carbon of 60.08%, ash content of 10.55%, and a yield of 45.12% at the optimum production processes of 390 °C, 44 min, and 0.7 mm particle size. Moreover, the study found that pyrolysis temperature was the most influential factor across all responses (yield and quality). Consequently, the biochar (yield and quality) was significantly (p < 0.05) influenced by pyrolysis temperature. In conclusion, the study inferred that KW holds substantial potential for biochar production with remarkable soil amendment characteristics.