In-depth temperature and smoke production of charring wood under a constant external heat flux

Abstract

The combustion characteristics of charring wood have been studied experimentally in a well-ventilated environment of a smoke chamber. A numerical simulation has also been performed for a limited case, with the Fire Dynamics Simulator, to estimate the burning environment. A horizontally placed specimen (ponderosa pine) with a moisture content of 0% or 20% is exposed to a radiant flux (25 kW/m2), with or without flaming ignition. Simultaneous measurements of the specimen’s in-depth temperature and the mass loss determine the charring front (rate) at 300 °C and the gasification rate, respectively. These condensed-phase conditions relate directly to real-time variations of gas-phase quantities: the specific optical density of smoke and the concentrations of toxic gases measured by a Fourier transform infrared gas analyzer. In-depth temperature trends are similar whether the flame exists, whereas the smoke and toxicants’ concentrations are substantially different. After the charring front moves through the specimen, the oxidative pyrolysis continues under the irradiation at high temperatures (up to ∼550 °C). Carbon monoxide and acrolein are produced continuously throughout the test, and the results indicate strong correlations. Although char formation of wood is favorable for fire safety, consequent incomplete combustion produces smoke and toxicants.

Fire Risk Rating Assessment for Wood Specimens Coated with Flame Retardant

“…Smoke is a combustible gas resulting from the thermal decomposition of wood to form polycyclic aromatic hydrocarbons that produce char upon flame combustion. The unburned char escapes as smoke in the flame combustion region due to radiant cooling, where it burns incompletely [15]. The smoke measurement test method using a cone calorimeter is based on Beer-Bouguer-Lambert's law, which generally states that the intensity of transmitted light decreases exponentially with distance [14].…”