Temperature profiles in a wood packed bed heated by hot combustion gases

Abstract

Axial and radial temperature profiles are presented for the initial sensible heating stage of wood carbonisation process in a packed bed. These profiles are important in promoting greater understanding of the heat transfer processes during the sensible heating stage and the mechanics of the gas flow through the bed. These data are also useful as an aid to the design of wood carbonisation retorts. Gas temperatures were measured in a cylindrical retort within which a randomly packed bed of green wood was heated by hot inert combustion gases. The bed dimensions were 0.62m in diameter and 1.28m in height. The temperatures were measured by thermocouples mounted at 0.46m, 0.8Sm and 1.28m heights and transversed across the bed diameter. The bed was heated by combustion gases fed from a charcoal combustion chamber. It was discovered that gas temperature drops through the bed were as much as 86% of the inlet temperature, and that the most significant temperature gradients occurred in the first one third of the bed; this has significant implications in the retort design. Other interesting features observed were that there was a strong local variation in temperature throughout a given horizontal section of the bed and a symmetrical radial temperature profiles across the bed. Furthermore, gas temperatures were significantly lower close to the retort wall than the centre. These phenomena were explained in terms of gas channelling and a wall effect which increased the resistance to flow there. A two-phase mathematical model was used to predict axial temperature variation, while an empirical equation was fitted to the radial temperatures. This same model although a one-dimensional axial one, was used with a number of simplifying assumptions, to estimate the radial temperature profiles. In general there was good agreement between the predicted and the experimental results. The maximum deviation was -20% but 90% of the predictions were within t7.SX. This good agreement implies that the mathematical nodel is physically sound, a significant finding since there is no evidence in the literature to suggest that a two-phase model exists that can predict both axial and radial temperature profiles simultaneously in a packed bed.