On the Combustion of Wood I: A Scale Effect in the Pyrolysis of Solids

Abstract

Experiments on well-ventilated cross-piles of wood sticks burning in room air show that the instantaneous mass of the pile normalized with respect to the initial mass is proportional to time/(individual stick thickness)^1.6. Experiments conducted by the present authors on isolated α-cellulose cylinders burning free convectively in room air indicate that while the normalized instantaneous mass correlates with (t/d ^1.6), the internal temperature profiles correlate with (t/d ²) —t and d respectively denoting time after ignition and specimen diameter. In this paper an approximate analysis is presented to explain this behavior as a consequence of the interaction of heat conduction and finite rate kinetics of thermal decomposition. If M_i is the initial mass (gms), α is thermal diffusivity of the wood species (cm²/sec) and the stick size d is in centimeters, it is shown that the rate of burning can be estimated as 0.9268M _iα^0.8/d ^1.6gm/sec.