Ignition of Wood: A Review of the State of the Art

Abstract

This review encompasses the available practical and experimental data on the ignition of solid wood. Only solid, natural wood is considered, not sawdust, chips, or products that have been treated with fire retardants or other substances, nor is the ignition of living trees. Panel products such as plywood or particleboard have ignition properties very similar to solid wood, so the solid-wood results will generally be applicable to them. Wood may ignite by flaming directly, or it may ignite in a glowing mode, which may or may not be followed by flaming. It is shown that the ignition temperature is around 250oC for wood exposed to the minimum heat flux possible for ignition, and that it invariably ignites, at least initially, in a glowing mode under these conditions. The ignition temperature rises rapidly as the heat flux is increased. Piloted ignition at heat fluxes sufficient to cause a direct-flaming ignition normally occurs at surface temperatures of 300 – 365oC. Autoignition temperatures at fluxes higher than minimum are essentially unknown. No theory is available which encompasses the possibility of glowing, glowing followed by flaming, or direct-flaming ignition modes. Most published studies have dealt with radiant or radiant+convective heating, and knowledge is extremely poor for ignition from direct contact by hot bodies or by flames. A speciesindependent correlation is derived for the radiant, piloted ignition of thermally-thick wood, but the fit is only fair. The minimum flux for ignition is 4.3 kW m -2 , based on a single study; most reported tests have been much too brief to produce useful data on this point.