The word equation for wood combustion is that wood in the presence of oxygen and high heat combusts to produce carbon dioxide, water vapor, heat and ash residue. Wood combustion gives off heat because it is an exothermic reaction.
Wood is made up of cellulose, an organic compound that is a polymer or macromolecule. Thermal decomposition of cellulose is the reason why wood ignites and combusts under the influence of heat. Another name for thermal decomposition is pyrolysis.
In the pyrolysis process, the application of high heat to wood causes the cellulose to begin its decomposition. The decomposing products react with the oxygen in air to produce more heat and a chain reaction that culminates in the release of the products carbon dioxide, water in gaseous form, ash residue and heat.