What is wood pyrolysis?

Wood is thermally degradable and combustible material. The solid fuel combustion process involve major four processes: evaporation of moisture, volatile release/char formation (pyrolysis), char oxidation and volatile combustion (oxidation). Pyrolysis is an important phenomenon in combustion of wood. Pyrolysis refers to release of volatile from wood leaving behind char. As soon as volatile are released, they are subjected to oxidation resulting in a exothermic reaction.

Pyrolysis of wood is typically started at 200^oC and last till 450^oC.  Pyrolysis of wood is essentially an endothermic process. Heat transfer from a heat source causes temperature of wood to rise. As temperature of wood increases, wood starts to show some physical and chemical behavior. Chemical bonds begin to break at temperature >100^oC. Between 100 and 200^oC, wood becomes dehydrated and releases water vapor and other noncombustible gases. From 200 to 300^oC, wood components undergo serious pyrolysis and significant amount of combustible gas (especially CO and tar) are released. Around 300 to 450^oC, highly flammable volatiles are produced. In this range, pyrolysis of wood results in overall low char residue. At temperature > 450^oC, all volatiles are emitted leaving behind char. Flow of released volatile towards unpyrolysed fuel result in heat transfer between two heating the fuel and subsequently cooling volatile. Thus, secondary pyrolysis also begins due to these interactions.

These volatiles and char when oxidized release significant amount of heat. These heat would be used for pyrolysis process. Thus wood burning process initially requires a heat source and later can sustain itself if proper flame temperature is maintained.