An analogue method of estimating wood pyrolysis rates

Abstract

A technique is described whereby the heating conditions experienced by particular pieces of wood in experimental fires can be reproduced in separate experiments; in the actual fire, it is not normally possible to establish the weight/time history of individual fuel elements, but the separate experiment is designed so that individual fuel elements are continuously weighed, hence their pyrolysis rates are determined. For this technique, it is assumed that if one ascertains the temperature distribution in a burning piece of wood, as a function of time and position, and reproduces that distribution in another piece of wood of identical physical properties, then one will reproduce the chemical reaction distribution, hence the total weight loss/time curve of the original piece of wood. An example of practical importance is considered—namely, the burning of wood which is being used as a lining to an inert wall of known thermal properties. In this example, heat-conduction theory shows that if the initial conditions of wood thickness, thermal properties, moisture content, etc., are reproduced and the rear face of the lining is in good thermal contact with the wall, then the temperature distribution within the wood during burning is reproduced if the surface temperature variation is reproduced. The technique therefore consists of measuring the surface-temperature variation of a part of the wood lining during a fire, and causing another specimen of wood to have this same surface temperature variation, while being continuously weighed. The apparatus used for this purpose is described. Tests are described which gave information on the validity of the assumptions and on the reproducibility of the technique. The applications and limitations of the technique are discussed.