Drying and Pyrolysis of Logs of Wood

This work concerns log firing in small-scale boilers for house heating, which in Sweden meets a heat demand of 12 TWh a year. In boilers used for domestic central heating in the sizes of 10 to 30 kW the predominant combustion technique is grate firing. Prediction of the transient release of combustible gas from the log-fire in such boilers enables an optimised design of the combustion chamber to minimise emissions of unburnt and harmful emissions. The release of gas is governed by the heating of each log of wood. The heat flux to a log is given by the design of the furnace and by the surrounding logs. Exposure of a log to a heat flux will lead to evaporation of water (drying) and then to thermal degradation (pyrolysis) of the wood into volatiles (about 80%) and char (about 20%). Subsequently these constituents will burn. <p />In order to predict the release of gas from a log of wood under combustion conditions, drying and pyrolysis of one log of wood, idealised to a cylinder, are studied. The complex heating situation in a log-fire in a small-scale boiler is simplified to a constant furnace wall temperature. The progress of drying and pyrolysis was monitored from measurements of interior temperature distribution and mass loss. The heat transfer properties of wood and charcoal were investigated. Based on comparison of measured data and simulations, using a numerical model, the applicability of measured thermal properties, the submodel of pyrolysis and the anisotropy of wood permeability are discussed. The results suggest that modelling of drying and pyrolysis of thermally thick wood particles should include: <OL> <LI>The structural changes during pyrolysis causing growth of pores, formation of cracks and global shrinkage of the sample. <BR> <LI>A pyrolysis reaction mechanism that accurately predicts initiation of pyrolysis at slow heating rates, such as in the particle's interior. <BR> <LI>Drying governed by the liquid permeability in the longitudinal fibre direction at moisture contents above the fibre saturation point. <BR> <LI>Instant, non-thermal equilibrium release of gas from the particle at temperatures above 400 °C (after pyrolysis). </OL> <p />Due to the structural changes in conjunction with uncertainties in literature on thermal properties of wood and charcoal, it is recommended to use a simple approach, for instance, of constant thermal diffusivity. The application of the results of this study to other types of thermally thick wood particles needs data on thermal properties of wood and charcoal coupled to the structural variations in such particles