Review of Information Related to the Charring Rate of Wood

United States Department of Agriculture Review of Forest Service Forest Information Products Laboratory Related to the Research Note FPL-0145 Issued 1966 Charring Rate of Wood Slightly Revised 1980 SUMMARY Buildings constructed with heavy timber for posts, beak, girders, and flooring have often proven themselves fire resistant because of the slow rate of charring and loss of strength of wood under fire exposure. However, much of the information relating to fire endurance of such construction is empirical in nature. To remain competitive with other construction materials more precise data should be obtained upon which to design and predict fire endur ance of wood assemblies. A long-range program to obtain these data is proposed as part of the fire research studies at the U.S. Forest Products Laboratory. A research program was established in cooperation with the National Forest Products Association to determine the charring rates for wood and the effect of temperature, species, density, moisture content, and grain direction on these rates. The literature review presented here was made as an initial part of that study. Information is summarized on the mechanism of thermal degrada tion of wood, properties affecting rate of char, models and measured rates of char development, and heat liberation during combustion of wood. Related studies on the mechanism of thermal degradation of plastic polymers and analytical degradation models are also summarized. Contents Page Nomenclature . ii Subscripts . iv Introduction . 1 Studies on Degradation of Wood to Char . 2 Mechanism of Degradation to Char . 2 Comparative Char Resistance of Wood Species . 6 Models of Char Development . 6 Measured Rates of Char Development . 14 Properties Affecting Rate of Degradation to Char . 21 Heat Liberation and Absorption During the Combustion of Wood . 29 Studies on Degradation of Other Polymers to Char . 32 Mechanisms of Degradation to Char . 32 Degradation Models . 36 Summary of Findings . 45 Literature Cited . 46 Appendix . 54 FPL-0145 Nomenclature Symbol Description Dimensions A frequency factor, a material constant 1/sec. a,b proportionality constants ------------------ 3 C molar concentration g.-mole/cm. Cp specific heat at constant pressure cal./g. C. E activation energy cal./g.-mole F latent heat of fusion cal./g.-mole f numerical coefficient ------------------ H enthalpy cal./mole 2 h heat transfer coefficient cal./cm sec. °C. J Joule's mechanical equivalent of heat ergs/cal. K Arrhenius rate constant 1/sec. k thermal conductivity cal./sec. cm. °K. 2 m mass g. sec. /cm ¶m m rate of mass change g. sec./cm. ¶t n unit outer normal to closed volumetric surface ------------------ 2 q heat flux cal./cm. sec. R gas constant cal./deg. g.-mole S surface ------------------- s char or melt layer thickness cm. FPL-0145 ii Symbol Description Dimensions • s charring rate (¶s/¶t) cm. /sec. T temperature °C. t time sec. 3 V volume cm. v rate of char formation ------------------ mass average velocity cm./sec. absolute velocity cm./sec. W mass weight g. W net rate of material depletion or production g./sec. X material coordinate ------------------- 2 a thermal diffusivity cm. /sec. D change in enthalpy, heat of combustion cal./g.-mole D time period ------------------ e emissivity ------------------ 3 r density or specific gravity g./cm. 2 4 s Stefan-Boltzmann constant cal./sec./cm/ /°K f fraction of material ------------------- FPL-0145 iii Subscripts Symbol c char D dry d decomposed material dp depolymerization f fusion g gas i initial condition n index integer o initial condition (t = o) P plastic or polymer r residue u unreacted W wet w surface FPL-0145 iv REVIEW OF INFORMATION RELATED TO THE CHARRING RATE OF WOOD By E. L. SCHAFFER, Research Engineer 1 Forest Products Laboratory, Forest Service U.S. Department of Agriculture Introduction Wood is known to have low mass density and relatively low heat conduction properties. The char layer that is formed when wood is exposed to fire further lowers the effective heat transfer properties. Therefore, when massive wood sections, posts, beams, girders, and flooring are exposed to fire, these prop erties control the rate of char penetration or, equivalently, the temperature rise resulting in degradation of the wood at interior points. When a single massive section of wood is exposed to fire until sustained flaming exists on the surface, the combustible volatiles just below the wood sur face provide the fuel. However, as the surface acquires a thicker layer of char, the amount of heat transferred by surface flaming decreases as the liberation of volatiles decreases. In proportion to char depth and decreasing heat transfer, the amount of volatiles liberated from the uncharred wood then decreases until flaming stops. Hence, for the continuous degradation to char of a massive wood section, another source of impinging fire is required. This phenomenon is evi dent in the difficulty of burning large logs in a fireplace without sufficient kindling material. The depth of char on a massive wood section under continuous exposure to fire increases until the char layer itself begins to degrade at the surface. The thickness of the char layer is not of prime interest, but it influences the dimen sional rate at which the solid wood underneath is converted to char. 1 -Maintained at Madison, Wis., in cooperation with the University of Wisconsin. FPL-0145 The rate at which the cross-sectional dimensions of a wood structural mem ber decrease is of paramount interest in determining its load-carrying capacity. Of further interest, of course, is the effect of the transferred heat and moisture on the strength properties in the noncharred section. The rate at which polymers degrade to char is an important consideration in the space program. Nose cones of rockets are covered with a low thermally con ductive, but degradable material which acts as an insulative barrier for protec tion of instruments housed within the cone during leaving and reentering the earth's atmosphere. In order to provide enough insulation under this high- temperature exposure, the rate at which a material degrades to char must be known, and much has been accomplished in characterizing polymeric materials for this application. Much of the theory and information developed concerning the thermal degradation of polymers can also be applied in the study of the ther mal degradation of wood. This report summarizes what has been published in the literature about the combustion of wood and the rate of char development in both wood and other polymeric materials. A related report2 discusses experimental results and analysis of charring rates of wood investigated at the U.S. Forest Products Laboratory in cooperation with the National Forest Products Association. Studies on Degradation of Wood to Char Mechanism of Degradation to Char 3 Hawley (23) and others present a simplified description of the mechanism of the combustion of wood. The combustion of wood is usually initiated by destruc tive distillation of the wood, which produces both combustible and incombustible volatiles, and nonvolatile combustible charcoal. The actual combustion of the wood becomes the combustion of the products of this initial distillation. If com bustion on the surface of the wood continues, four degradation zones within the wood and parallel to the exposed surface may be described (13). Zone A, initial temperature to 200° C. (392° F.).--Wood begins to lose the water of constitution at about 105° C. (221° F.) (57). Weight loss proceeds slowly as nonignitible gases are liberated (19). The wood lignins and hemicellu loses undergo glassy transitions at temperatures ranging from 130° to 190° C. depending upon the moisture available, as water uptake lowers this transition temperature (20). 2 Schaffer, E. L. 1967. Charring rate of selected woods--transverse to grain. USDA FS Res. Pap. FPL 69. Forest Prod. Lab., Madison, Wis. 3 Underlined numbers in parentheses refer to Literature Cited at the end of this report. -2- FPL-0145 Zone B, 200° to 280° C. (392° to 536° F.).--Decomposition of hemicellulose begins at 200° C.4 Hydrolysis of cellulose and pyrolysis of lignin and cellulose follow in order. These reactions may be exothermic or endothermic depending upon the amount of oxygen present; a trace of oxygen may convert endothermic processes to exothermic ones. Figure 1 shows the nature of the combustion process in wood within this range in both an oxygen and a nitrogen atmosphere. 3 Wood cellulose softens at temperatures between 200° to 240° C. and this point is not markedly affected by the presence of moisture. Cellulose and hemicellulose darken simultaneously with the onset of cellulose softening, and weight losses of between 26 and 39 percent (average 35 percent) indicate that pyrolysis occurs here (20). Zone C, 280° to 500° C. (536° to 932° F.).--Tang 4 further indicates that a small wood specimen will rapidly begin to lose weight above 280° C., and if oxygen is present in the wood, the process will be highly exothermic; the peak of the exo therm being reached at about 315° C. (600° F.) (fig. 1). Hawley (23) generally describes the range from 280° to 320° C. as the area where wood undergoes destructive distillation. Gases pour forth from this zone toward the surface. If liberation is rapid enough, these gases will blanket the wood surface, thereby excluding oxygen and protecting the charcoal from burning oxidation; charcoal is therefore left to accumulate. The accumulating char will delay attainment of the exothermal point in the wood beneath, due to its lower thermal conductivity as compared to wood (13, 24). Zone D, above 500° C. (932° F.).--At 500° C. (incipient red heat) the charcoal glows and is consumed (13). When surface temperatures rise somewhat beyond 1,000° C. (yellowish-red heat), carbon is consumed at the surface as rapidly as the reaction zones penetrate into the piece (13, 40).

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