Impact of direct and indirect heating on cassava rhizome biochar and pyroligneous acid characteristics using an integrated fixed bed carbonizer and cross-draft gasifier-burner

This study provides a direct comparison of cassava rhizome pyrolysis under direct- and indirect-heating modes using an integrated fixed-bed carbonizer and cross-draft biomass gasifier–burner system. Process performance, product distribution, product quality, and greenhouse gas emissions were evaluated. At similar average reactor temperatures of 553 and 550°C, direct heating reduced the processing time from 370 to 240 min, corresponding to a 35.1% reduction, and decreased firewood consumption from 56.00 to 15.00 kg, equivalent to a 73.2% reduction. Biochar productivity increased from 1.13 to 2.82 kg h⁻¹, while biochar yield remained comparable at 24.3–25.0 wt.%. Direct heating increased pyroligneous-acid yield from 34.8 to 43.8 wt.%, an increase of 9.0 percentage points, and reduced pyrolysis gas yield from 40.2 to 31.9 wt.%. The produced biochar contained 76.01–77.00 wt.% carbon and exhibited iodine numbers of 195–220 mg g⁻¹, indicating potential for deodorizing and other non-fuel applications, although its ash content exceeded the requirements for cooking and grilling charcoal. Direct heating also reduced total greenhouse gas emissions from 101.99 to 27.32 kg CO₂-eq and specific emissions from 9.19 to 2.85 kg CO₂-eq kg⁻¹ biochar, corresponding to a reduction of approximately 69.0%. Compared with traditional kilns requiring 72–84 h, the direct-heating system completed the process within 4 h while maintaining a comparable biochar yield. Overall, direct heating improved process intensity, fuel efficiency, and environmental performance under the tested system configurations.