Biomass is abundant, widely available, and potentially carbon-neutral, but raw plant material generally has high moisture content, low energy density, uneven composition, and strong water affinity. Torrefaction—a thermochemical pretreatment conducted at approximately 200–300 °C in an oxygen-limited environment—can improve biomass energy density, grindability, storage stability, and transportability. Previous studies have established that temperature, residence time, feedstock type, and particle size affect torrefaction products. However, comparatively little is known about how heat moves through large biomass pieces such as chips, briquettes, and densified cylinders, where temperature differences between the surface and core may produce uneven treatment and inconsistent fuel quality.
A study (DOI: 10.48130/scm-0026-0012) published in Sustainable Carbon Materials on 16 March 2026 by Yogesh Patil's team, Zhejiang University, reports that particle diameter governs internal heating more strongly than length, while temperature and residence time control thermal uniformity and moisture resistance.
The researchers prepared cylindrical birchwood particles in nine geometries by combining diameters of 15, 25, and 35 mm with lengths of 30, 40, and 50 mm. They inserted K-type thermocouples at the surface, middle, and core of each particle to record temperature changes during treatment. Samples were placed in a horizontal quartz-tube furnace, purged with nitrogen for 30 minutes at 1 L/min, and heated from 25 °C at 10 °C/min. Torrefaction was then performed at either 250 or 300 °C for 30 or 60 minutes. The team also recorded mass yields and immersed selected 30-mm-long raw and torrefied samples in water, weighing them after 24, 48, and 72 hours to assess hydrophobicity. Temperature measurements revealed that increasing either particle diameter or length slowed heating at the middle and core, whereas surface temperatures changed relatively little with geometry. Diameter had the stronger effect because widening a cylinder substantially increases the radial distance over which heat must travel. Raising the diameter from 15 to 35 mm therefore delayed core heating and enlarged the temperature difference between the core and surface. Curves also showed a plateau near 100 °C, corresponding to the energy consumed by internal water evaporation. This plateau lasted longer in larger particles and was most pronounced at their cores. At 250 °C, a 30-minute residence time was insufficient for the middle and core of a 35-mm-diameter, 50-mm-long particle to reach the target temperature. Extending treatment to 60 minutes allowed temperatures at all three locations to converge, producing a more uniform thermal distribution. Treatment at 300 °C accelerated heating but generated an internal temperature overshoot of approximately 20–30 °C, particularly at the core. The researchers attributed this effect to exothermic reactions associated with biomass decomposition and to restricted heat dissipation through the developing char layer. Increasing torrefaction severity also reduced mass yield as more volatile matter was removed. Water-absorption experiments showed that most uptake occurred during the first 24 hours and that samples treated at 300 °C absorbed less water than those treated at 250 °C or untreated birchwood.
Overall, the study demonstrates that particle geometry cannot be separated from operating conditions when designing biomass torrefaction processes. Large-diameter particles require sufficient residence time to prevent under-treated cores, while high temperatures may create local overheating through exothermic reactions. By connecting geometry with internal temperature development and water resistance, the work provides a practical basis for improving the consistency of torrefied chips, briquettes, pellets, and other solid fuels.
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References
DOI
10.48130/scm-0026-0012
Original Source URL
https://doi.org/10.48130/scm-0026-0012
About Sustainable Carbon Materials
Sustainable Carbon Materials (e-ISSN 3070-3557) is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.