Seeing biochar form in 3D: scientists track a single walnut shell particle as it turns into carbon

A study has captured, in real time and in three dimensions, how a single piece of biomass changes as it is heated into biochar, offering fresh clues for designing more effective carbon materials for water cleanup, soil improvement, catalysis, and climate-related applications.

Biochar, a porous carbon-rich material made by heating biomass in limited oxygen, is widely studied for environmental uses because its internal pores and surface chemistry can help trap pollutants, store carbon, and support chemical reactions. But one major challenge remains: scientists still do not fully understand how those pores form and move inside a particle while biochar is being produced.

In a study published in Biochar, researchers used synchrotron-based x-ray micro-computed tomography to watch individual walnut shell particles during pyrolysis, the thermal process that converts biomass into biochar. The team continuously imaged particles about 1 to 2 millimeters in size as they were heated to 575 °C in an argon atmosphere. The imaging reached a resolution of about 0.82 micrometers, allowing the researchers to track both the shrinking particle and its changing pore network.

“Biochar performance depends strongly on its pore structure, but until now we have had limited direct evidence of how that structure develops inside a particle during pyrolysis,” said corresponding author Roberto Volpe. “By watching a single particle transform in 3D, we can begin to connect processing conditions with the internal architecture that makes biochar useful.”

The researchers compared untreated walnut shell particles with particles that had been washed in water before pyrolysis. The difference was striking. The untreated sample showed about 30 percent volume shrinkage, while the washed sample shrank by about 70 percent. The untreated particle also swelled between 200 and 300 °C, a behavior the authors link to the softening of native biopolymers associated with lignin. This swelling was not observed in the water-washed sample.

Water washing also changed how pores developed inside the particle. The study found that washed samples developed porosity toward the center of the particle about 3.5 times faster than unwashed samples. At high temperature, the washed particle reached a more uniform pore distribution throughout its volume, which may be important for applications where internal surface access controls performance.

To quantify this internal pore redistribution, the researchers introduced a purpose-defined parameter, Λ, which tracks whether pores are concentrated closer to the particle surface or toward the center. The results showed a linear temperature dependence, rather than a faster exponential trend. According to the authors, this provides evidence that heat and mass transport limitations strongly influence the chemical reactions occurring inside the evolving porous particle.

“These observations help explain why mild pretreatments, even something as simple as water washing, can alter the final structure of biochar,” Volpe said. “That knowledge could help researchers and engineers tune biochar production for specific environmental technologies.”

The work also addresses a broader challenge in biomass pyrolysis research. Many models assume simplified particle shapes or uniform structures, but real biomass particles are irregular, chemically complex, and full of evolving pores. By providing direct, time-resolved 3D evidence, the study gives modelers new experimental data for improving predictions of how biomass decomposes and how biochar structures emerge.

The authors note that the current results are limited to the micro-CT resolution used in the study, meaning smaller nanoscale pores were not fully captured. Future studies combining micro-CT, nano-CT, in situ x-ray diffraction, and x-ray fluorescence could further reveal how minerals and carbon structures evolve during pyrolysis.

Overall, the study shows that watching biochar form from the inside out can provide a pathway toward precision-engineered biochars, where particle structure is designed for targeted environmental functions.

=== 

Journal Reference: Salinas-Farran, L., Mosonik, M.C., Jervis, R. et al. Tracked evolution of single biochar particle’s morphology during pyrolysis in operando x-ray micro-computed tomography. Biochar 6, 86 (2024).   

https://doi.org/10.1007/s42773-024-00374-7  

=== 

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. 

Follow us on Facebook, X, and Bluesky.