In the present work the processes occurring in the structures of slowly pyrolysed beech wood char during thermal gasification have been investigated. Emphasis was put on physical changes and gas transport properties during conversion. The highly anisotropic structure of wood was preserved in its char. Direct observation of the microscopic char structures during gasification in CO2 at 735 QC showed that the basic structure was nearly intact up to degrees of conversion of 0.6—0.7. Uni-axial measurements of diffusion coefficients and permeabilities with a Wicke-Kallenbach cell revealed differences of 3—4 orders of magnitude between the longitudinal and other directions in freshly pyrolysed beech wood char. Diffusion in the longitudinal direction of the beech wood char before gasification corresponded to direct, unobstructed diffusion through its vessel cells. Radial and tangential diffusion were limited by Knudsen diffusion through the pits in the wood cell walls for degrees of conversion by gasification up to at least 0.5. A computer model of slab gasification based on the diffusion measurements successfully predicted the mass loss rate during diffusion-limited gasification of beech wood char slabs, and confirmed that radial and tangential diffusion in beech char were limited by Knudsen diffusion.
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Henriksen, Ulrik Birk, Jensen, Anker Degn, Qvale, Einar Bjørn