Some Considerations Pertaining to the Problem of Wood-Burning

Abstract

Analyses appearing in the literature on combustion of cellulosic fuels assume that the energy transfer in the fuel interior is adequately described by an unsteady state conduction equation provided with an Arrhenius source. It has been discovered experimentally by x-ray photography that even though in certain zones of space and time the above assumption is valid, there exist other zones in which several ‘secondary’ physico-chemical effects of energy evolution and transfer become non-negligible and take a dominating role in the control of the burning rate. The present paper gives a critical summary of the ‘secondary’ processes of energy and mass evolution and transport in wood-burning, in an attempt to formulate them in such a way that a meaningful quantitative handle is possible. An equation is presented to calculate the specific gravity of moist wood as a function of the porosity, ambient temperature and humidity. The fibrous nature of the wood is demonstrated. A thorough discussion of the pre-decomposition endothermic process is presented. A critical survey of the literature regarding the kinetics and energetics of thermal decomposition of pure as well as contaminated α-cellulose, lignin and wood, is presented. Much information is available in the literature regarding either the ‘burning’ or ‘charring’ rate of woody solids exposed to intense healing—convective and/or radiative. This information along with our own measurements on free convective burning of a-cellulose cylinders, is utilized to establish the importance of the internal convection due to the outflow of pyrolysis gases produced in the interior of the solid. The analysis indicates that the internal convection would influence the burning rate of large specimens to a stronger degree. Furthermore, as the outward convective flux approaches the inward conductive flux in magnitude, a quasi-steady state burning can be expected so that the burning rate of isolated specimens would tend to be less violently time-dependent than what a pure conduction model would suggest. Thermal diffusivity measured along the grain of α-cellulose is found to be two to three times larger than the transverse values. The average of the present measurements on α-cellulose cylinders is in good agreement with the values reported in the Smithsonian Tables for various woods. A simple linear model to compute the thermal properties of partially charred woody solids is proposed. Such a model, in view of the success demonstrated in the fields of ablative cooling and drying wet porous solids, seems to be a good start to understand the much needed order-of-magnitude information about thermal conductivity and specific heat as dependent upon the extent of charring. The pyrolysis gases flowing out through the hot char layers of the decomposing cellulose specimen, undergo exothermic reactions catalyzed by the porous char. The kinetics of such surface reactions are briefly discussed.