Study of Radiant Smoldering Ignition of Plywood Subjected to Prolonged Heating Using the Cone Calorimeter, TGA, and DSC

Abstract

Maple plywood samples were heated in an oven at 180°C to a residual weight of 70%, 50%, and 30% of the virgin weight. The virgin plywood and the thermally pretreated samples were tested in a cone calorimeter over the range of 6 to 15 kW/m 2 without the use of an ignitor. Tests were conducted until smoldering/glowing combustion was observed or for eight hours if ignition was not observed. The extent of thermal pretreatment of the plywood had no measurable effect on the minimum heat flux for smoldering ignition. At heat fluxes above the minimum for ignition, ignition times were reduced for thermally pretreated samples relative to virgin wood. This is consistent with the thermal pretreatment achieving the charring required as the first step in smoldering ignition and the reduction in thermal inertia resulting from charring. Samples of the virgin and pretreated wood were subjected to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The chemical kinetics analysis of the virgin and pretreated samples showed no change in the chemical mechanism. The thermal pretreatment simply resulted in the samples being further along the reaction pathway at the start of the ignition test. The results are not consistent with the formation of a more reactive char species commonly referred to in the fire investigation lore as "pyrophoric carbon."

Ignitibility of structural wood products exposed to embers during wildland fires :

“…Under radiant heat fluxes, the ignition temperature is significantly affected by the moisture content [49]. Earlier studies [35,41,46,49,109,110] have shown that the samples containing moisture have increased ignition temperature and reduced smoldering velocity. Kuznetsov and Filkov [46] quantified the effect of moisture content on the ignition of various wood species and reported that an increase in moisture content in wood by a factor of 10 resulted in increasing its ignition time by more than 50 %.…”

“…This process absorbs heat which, in a dry sample, would otherwise pyrolyze the fuel. Swann et al [41] studied the effect of preheating on the smoldering ignition of maple plywood. They concluded that the preheating of the wood had little effect on the minimum heat flux for ignition and the ignition temperature; however, the time to ignition was markedly reduced by the prolonged preheating that essentially removed moisture from the plywood.…”

“…Several researchers [38,39,41,42,44,45] [42,44] reported that glowing ignition temperature increased from 317 °C to 400 °C as the incident heat flux increased from 10 kW/m² to 30 kW/m². Depending on the incident heat flux, surface temperatures from 200 °C to 400 °C were reported by other studies [35,39,42,44,45].…”

“…For determining smoldering ignition in wood, Gratkowski et al [39] considered four criteria including 1) glowing of sample, 2) temperatures over 400 °C at the surface or within the sample, 3) evidence of smolder propagation wave, and 4) visual observation of decomposition of the sample and presence of residual white ash. Several researchers have reported glowing as a primary criterion for smoldering ignition while others verified smoldering combustion by post-test observation of significant consumption of the sample or the presence of residual white ash [41]. Continuous smoking which is indicative of sustained combustion within the pyrolyzing material has also been used as visual evidence of smoldering [21].…”

“…Ember ignitability of wooden substrates is a complex problem as it requires knowledge of thermal characteristics of embers and also the material properties of the substrate fuel that control its thermal, physical, and chemical response to heating, in addition to environmental conditions. Extensive literature [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] is available on pyrolysis and combustion of wood exposed to radiant heating; however, systematic theoretical studies on wood ignition by embers and its subsequent burning are inadequate. Limited empirical data available on the ember ignition of the solid wood substrate are not sufficient to predict ignitability and sustained smoldering.…”