Comprehensive study on microwave pyrolysis process variables and operating modes for optimized biochar production

This study comprehensively investigates microwave-assisted pyrolysis of agroforestry waste into quality biochar through systematic evaluation of process variables, operating modes, and quantification techniques to address key challenges for production optimization. Building on this, 92 systematic experiments were conducted across various agroforestry residues, evaluating more than ten control parameters classified by their impact on yield and quality: primary (power, time, temperature, heating-rate, feedstock), secondary (moisture content, particle size, sweep-gas flow, susceptor use), and tertiary (reactor configuration, control modes). Four operating modes were investigated: constant power with/without high-temperature alarm, fixed temperature, and controlled heating-rate; and two novel metrics (carbonized amount and absolute yield) were studied alongside traditional metrics to more accurately quantify biochar production and quality. Microwave power and residence time emerged as the primary drivers of yield and carbonization, while heating rate and target temperature acted as fundamental dependent factors. Constant-power operation without alarm achieved the highest reproducibility and absolute yield (up to 33.85 %), whereas controlled ramping produced biochars with HHV > 30 MJ/kg and fixed carbon > 70 %. Under optimal conditions of 500 W, 20-40 min residence time, 400-500 °C, 10-15 % moisture content, and < 3.15 mm particle size, energy efficiency reached up to 54.1 % while maintaining superior biochar quality. This comprehensive framework enables tunable, scalable microwave-pyrolysis protocols for sustainable biochar production from biomass wastes.