Internal-heating pyrolyzer produces cleaner, agriculture-ready biochar

Using reed straw pellets as feedstock, the team found that the system could produce biochar with low toxic risk, strong functional properties, and relatively high energy recovery.

Biochar has attracted growing attention as a carbon-rich material produced by slow pyrolysis in oxygen-limited conditions. It can help stabilize carbon for long periods, reduce carbon dioxide emissions, and improve soils because of its porous structure and nutrient-retention potential. At the same time, biochar production can generate harmful contaminants such as polycyclic aromatic hydrocarbons, making cleanliness an important factor in real-world application. Previous studies have mostly focused on batch slow pyrolysis with external heating, while less is known about continuous, internally heated moving-bed systems, even though these systems may offer better heat transfer, lower costs, and stronger industrial potential.

A study (DOI:10.48130/bchax-0026-0011) published in Biochar X on 13 March 2026 by Hongbin Cong’s team, Key Laboratory of Energy Resource Utilization from Agriculture Residue, Ministry of Agriculture and Rural Affairs, points to a practical route for optimizing industrial biochar production, especially for agricultural uses such as soil improvement, carbon sequestration, and cleaner biomass utilization.

The researchers used reed straw pellets to test an internally heated cigar-type slow pyrolysis system designed as a quasi-moving-bed reactor. In this setup, part of the biomass is burned to provide heat for pyrolysis, while switching the air inlets and gas outlets enables either updraft or downdraft operation. The team examined three pyrolysis temperatures—550, 600, and 650 °C—and also adjusted air distribution rate and cooling mode to compare high and low airflow as well as water cooling and air insulation. The resulting biochars were assessed using a wide range of indicators, including fixed carbon, ash, atomic ratios, specific surface area, pH, cation exchange capacity, electrical conductivity, PAH concentration, toxic equivalence quantity, and energy conversion efficiency. The results showed that some conventional properties remained relatively stable, while others were strongly affected by operating conditions. Fixed carbon content ranged from 38.46% to 44.02%, and the H/C and O/C ratios indicated good long-term carbon stability. Among all indicators, specific surface area was especially sensitive to pyrolysis conditions and increased with temperature. At 650 °C, biochar produced under downdraft, low air distribution, and air insulation showed surface area values 1.46, 2.26, and 3.00 times higher, respectively, than those produced under updraft, high air distribution, and water cooling. By contrast, updraft operation, high air distribution, and water cooling generally improved cation exchange capacity, an important trait for retaining nutrients in soil. Cleanliness was another key finding. PAH concentrations ranged from 0.03 to 0.44 mg/kg, and toxic equivalence values were only 0.39–5.68 µg/kg, both far below the relevant international limits cited by the authors. Still, PAH levels rose significantly under high air distribution and water cooling. At 550 °C, high airflow produced 2.66 times more PAHs than low airflow, while at 650 °C, water cooling generated 6.89 times more PAHs than air insulation. Meanwhile, syngas heating value increased with temperature, and average energy conversion efficiency reached about 75.31%.

Overall, the study shows that internally heated slow pyrolysis can produce biochar that is both functionally useful and environmentally clean, but performance depends strongly on process design. Downdraft operation favored specific surface area, while updraft operation offered some advantages in cation exchange capacity, energy efficiency, and production capacity. The authors conclude that future optimization should balance biochar quality with conversion efficiency to match different agricultural applications and support larger-scale industrial deployment.

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References

DOI

10.48130/bchax-0026-0011

Original Source URL

https://doi.org/10.48130/bchax-0026-0011

Funding information

This work was supported by the National Key Research and Development Project (Grant No. 2024YFD170110401).

About Biochar X

Biochar X is an open access, online-only journal aims to transcend traditional disciplinary boundaries by providing a multidisciplinary platform for the exchange of cutting-edge research in both fundamental and applied aspects of biochar. The journal is dedicated to supporting the global biochar research community by offering an innovative, efficient, and professional outlet for sharing new findings and perspectives. Its core focus lies in the discovery of novel insights and the development of emerging applications in the rapidly growing field of biochar science.