When wood is heated to produce biochar, it does not shrink uniformly. Instead, it contracts differently across and along its natural grain. This uneven shrinkage shapes the internal structure of biochar and can influence its performance in environmental applications. Yet the mechanisms behind this process have remained poorly understood.
Now, researchers from Kyoto University in Japan have uncovered how the breakdown of wood's structural components, together with naturally occurring metal ions, determines its shrinkage behavior during carbonization. Published in the journal Biochar , the findings provide new insights into how biochar structures could be controlled through wood selection and chemical treatment.
"Wood is not a uniform material. Its internal structure and chemical composition determine how it responds to heat," said corresponding author Professor Haruo Kawamoto of Kyoto University. "By understanding these interactions, we can move toward more precise control of biochar formation."
Wood cell walls contain three major components: cellulose, hemicellulose, and lignin. Cellulose provides a strong supporting framework, while hemicellulose helps connect and stabilize this structure. When wood is heated, these components decompose at different temperatures, causing the material to contract.
To investigate this process, the researchers examined ten wood species, including five softwoods and five hardwoods . They also modified the samples by introducing different exchangeable ions, including sodium, potassium, magnesium, and calcium. Using thermal analysis and continuous imaging, the team monitored how each sample shrank across the grain and along the wood fibers as temperatures increased.
The results revealed two distinct mechanisms controlling wood shrinkage .
Shrinkage across the grain, known as radial shrinkage, began primarily when hemicellulose started breaking down. In contrast, shrinkage along the grain, or longitudinal shrinkage, was closely associated with cellulose degradation. The most rapid longitudinal contraction occurred after substantial cellulose decomposition, typically at temperatures 10 to 20 °C above the peak thermal decomposition temperature.
Metal ions also played a surprisingly important role. Sodium and potassium generally caused shrinkage to begin at lower temperatures by accelerating the thermal breakdown of wood components. In hardwoods, hydrogen and magnesium treatments increased radial shrinkage in certain species, while calcium often reduced it.
Wood anatomy provided another explanation for the differences. Hardwoods generally exhibited greater radial shrinkage than softwoods , partly because of differences in the arrangement and shape of specialized cells called ray parenchyma.
These findings suggest that controlling both the chemical environment and anatomical characteristics of wood could help researchers influence the structure of the resulting biochar.
"Our findings show that the shrinkage of wood during carbonization follows predictable chemical and structural processes," Kawamoto explained. "This knowledge may help guide the selection of biomass and treatment conditions for producing biochar with desired properties."
Biochar is increasingly investigated for applications such as pollutant adsorption, catalysis, and soil improvement . Its internal pores and structural stability can strongly influence its suitability for these uses.
Although further research is needed to translate these findings into practical production strategies, the study establishes a scientific foundation for more targeted and efficient biochar design , supporting the development of higher-value materials from renewable woody biomass.
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Journal Reference: Wang, Y., Eljamal, R., Yokoyama, M. et al. Cell-wall chemistry governs anisotropic shrinkage during wood carbonization: roles of polysaccharide degradation and exchangeable cations. Biochar 8 , 144 (2026).
https://doi.org/10.1007/s42773-026-00661-5
About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
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Biochar
Experimental study
Cell-wall chemistry governs anisotropic shrinkage during wood carbonization: roles of polysaccharide degradation and exchangeable cations
21-Oct-2026