Pyrolysis- the breakdown of wood or combustible material via the application of heat0
Definition:
The chemical alteration of a solid fuel, from the action of heat alone, in the absence of oxygen and hence, without burning. The products of pyrolysis are gases, vapors and charcoal.
Notes about pyrolysis and spontaneous combustion of wood:
Once sufficient moisture has been removed from the wood, Wood molecules begin to break down due to a process known as pyrolysis, a chemical alteration of fuel molecules as a result of the application of heat. This initial wood pyrolysis, the second identifiable stage in the burning of wood, is crucial to the combustion process since most of the hydrocarbons produced by pyrolysis are in the form of gases or very small droplets of liquid tar. These substances, commonly known as hydrocarbon volatiles, contain roughly 30-50% of the potential energy found in the wood. In order for combustion to occur at this stage, oxygen must become well mixed with the gases given off by the pyrolyzing wood.
The mixture must be hot enough to cause the molecules to collide energetically with one another. In order to achieve complete combustion of solid fuel, temperatures in the combustion chamber must reach 1100 degrees F. Since it is unlikely that the entire firebox will reach this temperature, oxygen must mix with the hydrocarbon volatiles at the hottest area of the firebox. Close to the burning wood. In addition, sufficient oxygen must be allowed into the firebox to provide for oxidation reactions with as many of the fuel molecules as possible.
When wood forming or framework is left on the underside of your firebox or hearth extension, it is potentially subject to pyrolysis, and in time after all moisture and other chemical compounds are driven out of the wood due the application of heat, you are left with charcoal which may ignite spontaneously with temperatures as low as 235 degrees.
