Theories of the combustion of wood and its control

As a rule wood does not burn directly (4 4 , 7 9 , 1 1 4 ). –2 It first undergoes thermal degradation, or pyrolysis -some of the products of which are combustible gases, vapors, or mists. Under appropriate conditions the products may be set afire and, if enough of their heat of combustion is retained by the wood to maintain the pyrolysis, the burning may continue of its own accord until the wood has been consumed except for inorganic products left as ash. Ordinarily wood is set afire by bringing to bear enough heat to start active pyrolysis, and then applying a pilot flame or other source of high temperature to the combustible gaseous products after they have escaped and become mixed with air. In the absence of a pilot flame, much more heat must be supplied before the pyrolysis products will take fire spontaneously. The minimum rate of heating necessary for ignition by pilot flame is of the order of 0.3 calorie per square centimeter per second, but for spontaneous ignition it is of the order of 0.6 calorie per square centimeter per second (6 8 ).