Optimization of Drum-Kiln Pyrolysis for Woody and Agricultural Biomass: Influence of Temperature and Residence Time on Biochar Yield and Ash Content

The thermochemical conversion of lignocellulosic biomass into biochar represents an effective strategy for the sustainable management of agricultural residues and the production of stable carbon-rich materials for soil improvement. Low-cost drum-kiln systems provide a practical and decentralized approach for biochar production, particularly in rural agricultural settings. However, the influence of operational parameters on biochar yield and composition requires further evaluation. In this study, the effects of pyrolysis temperature (350–550 °C) and residence time (30–120 min) on biochar yield and ash formation were investigated using a factorial experimental design. Six biomass feedstocks-Conocarpus erectus, eucalyptus branches, acacia pruning residues, rice straw, wheat straw, and pigeon-pea stalks-were characterized through proximate analysis to assess feedstock variability. Optimization experiments were conducted using Conocarpus erectus as a representative woody biomass under controlled drum-kiln pyrolysis conditions. The results showed that biochar yield decreased from 39.8 % at 350 °C and 30 min to 21.0 % at 550 °C and 120 min, while ash content increased from 3.2 % to 11.8 % with increasing pyrolysis severity. The reduction in biochar yield was attributed to enhanced volatilization of organic components at higher temperatures, whereas the increase in ash content resulted from the concentration of inorganic mineral constituents within the char matrix. Optimal operating conditions were identified at 375–400 °C with residence times of 45–60 min, providing a favorable balance between biochar yield, ash content, and physicochemical stability. Overall, the study demonstrates that optimized drum-kiln pyrolysis can produce efficient and stable biochar, offering a cost-effective and sustainable approach for agricultural residue management and decentralized biomass valorization.