Comparative analysis of moisture transport in conventional to high-temperature convective wood drying using X-ray computed tomography

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Abstract

Drying wood, a hygroscopic, porous, and anisotropic material, involves complex coupled heat and mass transfer processes that make it the most time-consuming and energy-intensive operation in the wood industry. Increasing the drying temperature above 100 °C under atmospheric conditions, referred to as high-temperature drying (HTD), can accelerate the drying process but risks defects if not controlled. This study quantifies moisture transport under three drying schedules in a lab-scale convective kiln using in-situ X-ray computed tomography (CT) and evaluates whether HTD can shorten the process without visible cracking. Three drying schedules were tested: conventional, moderate-, and high-temperature. Moisture contours derived from CT image processing were validated against gravimetric measurements with less than 3.1% error. HTD reduced the drying time from 19 h to 13.5 h relative to conventional drying. CT-derived moisture contours revealed rapid free-water evaporation followed by bound water removal in HTD, driven by convective and conductive heat transfer, respectively. Higher temperatures shortened the capillary phase and advanced diffusion onset. HTD showed the steepest and highest drying rates, indicating enhanced moisture diffusivity, even at low moisture contents. The maximum surface-to-core temperature difference was 3 °C in moderate-temperature drying and 11 °C in HTD. Moisture gradients rose with higher temperature and lower moisture content, tripling between conventional and HTD schedules. Post-drying CT confirmed a visually defect-free specimen, demonstrating that the applied HTD schedule can significantly shorten drying time without causing surface cracking.

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