Temperature Profiles in a Wood Packed Bed Heated by Hot Inert Gases

Abstract

Presented in this paper are temperature profiles in a green-wood packed bed heated by hot inert gases for the sensible heating stage of wood carbonization. They give insight into the heat transfer during the initial carbonization stage and hence are useful for carbonization retort design.

Gas temperatures within a cylindrical bed, packed with green wood particles, were measured at different heights across the diameter by means of thermocouples. The wood bed was heated by inert hot gases from a charcoal combustion chamber. There was a high temperature drop which happenedmostly in the first one third of the bed. This was attributed to the high moisture content of the wood. The measured radial temperature variations showed a symmetrical profile between each side of the bed; the centre being the highest with strong local horizontal variations. This uneven heat distribution could affect the quality and uniformity of the charcoal produced. It was deduced that for uniform charcoal production, a short and wide bed was more appropriate.

An attempt was made to predict the axial temperatures profiles using the one-dimensional axial mathematical model, initially proposed by Schumann6 and extended by Rosen10 and Jeffreson111. The bed was physically modelled as thin independent elements similar to that proposed by Parti5. The predictions were within 14% of the measured values. Using the axial temperatures at every measurement height, an empirical relationship for radial temperatures was suggested.