Exothermal Decomposition Temperature of Wood Impregnated with Flammable Organic Liquids KENNETH A. KOBE AND FRED L. GOIK University of Washington, Seattle, Wash. The exothermal reactions in the thermal decomposition of wood are distributed over a considerable range of temperature and depend upon the rate of heating. An apparatus and procedure have been developed which give reproducible results and allow the study of the effect of various materials on the exothermal decomposition temperature. The moisture in the wood has no effect on the exothermal decomposition temperature. Gasoline, kerosene, and light oil lower the exothermal decomposition temperature, heavy oil causes no change, and creosote increases it.
determine if the exothermal decomposition temperature of the wood is changed by this treatment and the general effect of flammable organic liquids.
Effect of Temperature on Wood Wood decomposes when heated in the absence of air. At temperatures as low as 100" C., discoloration can be noticed. From 200" C. upward the reactions are speeded up until a t 275" C., according to Bunbury ( I ) , "there is a sudden and very rapid rise in the velocity of decomposition, considerable quantities of heat being evolved." If we examine the curves shown by Bunbury we may see that any point which might be termed "exothermic" occurs between 100" and 200" C. instead of 275 " C. Apparently what Klason, and others have done is to heat the retort to 300" C. and maintain the retort temperature constant; they have shown the exothermic nature of the reactions by plotting the temperature of the wood
M
OST of the literature on the exothermal decomposition of wood is a result of observations made during the destructive distillation of wood. The publication on wood distillation in which detailed temperature figures were first given was that of Violett ( 3 ) . He published a table showing the percentage residue from charges of wood after they had been heated to temperatures ranging from 160" C. to the melting point of platinum. Chorely and Ramsay (2) also measured wood distillation temperatures and found that in the vicinity of 280" C. the reaction became exothermic. Klason, von Heidenstam, and n'orlin (1) confirmed the exothermic character of the reactions and established the temperature a t which exothermal decomposition begins as being 275" C. Pasquier (4) found the decomposition reaction of spruce wood to become exothermic a t 275" C. Pasquier further determined that the treatment of wqod with certain metallic salts raised the exothermal decomposition temperature and that certain other chemicals when soaked into wood have a tendency to increase the amount of heat liberated by the exothermal decomposition of the wood. This work was conducted to determine the effect on the exothermal decomposition temperature caused by the impregnation of spruce wood with flammable organic liquids such as gasoline, kerosene, light motor oil, heavy motor oil, creosote, and dioxane. In a recent legal case the point was made that, wood so impregnated had a much lower exothermal decomposition temperature due to the presence of such flammable organic liquids impregnated in the wood. Because of the widespread use of creosoted wood, it is important to
FIGURE 1. DIAGRAM OF RETORT
on the same axis as the retort temperature. That the reactions are exothermic is clearly shown by the fact that the wood temperature rises above the retort temperature. This procedure may be utilized by calling the lowest temperature a t which the wood will rise above the retort temperature, the "exothermic point."
Experimental Method The retort used in this work is shown in Figure 1: Retort E consists of steel shafting, 2 X 5 inches (5.1 X 12.7 cm.), in which a hole, F , 1 x 41/2inches (2.5 x 11.4 om.), is drilled. The outer end of the hole is threaded t o take a plug, B, 1171
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31, NO. 9
perature was reproducible within 5". The data on a specimen of spruce as received is shown in Figure 2, upper section. I m p r e g n a t i o n of t h e wood was carried out by immersing a number of test s p e c i m e n s in the liquid. The oils and creosote were heated to lower the viscosity. When cut in half, a test specimen would show uniform staining from the liquid. The data are given in Table I; percentage gain in weight on impregnation and loss on heating are on the as-received basis. During the runs on the untreated wood it was noted that smoke, which escaped 0 IO 0 10 20 through the hole in the TIME IN MINUTES TIME IN MINUTES plug, f i r s t a p p e a r e d a t about 200" C. Shortly after FIGURE 2. EXOTHERMAL DECOMPOSITION TEMPERATURE OF IMPREGNATED WOOD the first a p p e a r a n c e of smoke, tar distilled out. I n all the runs on the treated in which is drilled a thermocouple opening, C. The lower and samples the first characteristic noted was that the impregnatupper Portion of hole F was filled with steel, leaving a rectangular ing agent tended to distill out a t low temperatures. As the opening 1 X inch (2.5 X 0.95 cm.) into which the test specitemperature increased, considerable volumes of smoke from men of wood fitted tightly. Retort E fitted tightly into a sheet the impregnating agents were evolved. In the cases of the metal sleeve, H , which was covered with mica and wound with resistance wire to form heating element A . This was placed inheavily impregnated samples, the preliminary heating was side a can, K , and insulated with magnesia, J , except at the outer accompanied by crackling and popping. end where the can cover, G,left a dead air space, I. More resistance wire at this end of the heating element gave a uniform temperature along the retort. The temperature of the wall was measured in two places with thermocouples D. The temperature DECOMPOSITION TEMPERATURE OF TABLEI. EXOTHERMAL of the wood was measured by a thermocouple which was placed IMPREGNATED WOOD in the center of the test specimen. The heating rate was Wt. Wt. controlled by variable resistances in series with the heating eleTreatment before after wt. Temp. ment. The retort was charged with a weighed Diece of spruce wood of such a shape that $ fitted snugli against the walls of the container so that heat was transferred by conduction. The wood was drilled to a depth of 21/2 inches (6.4 cm.) and of a diameter just large enough to receive the thermocouple. When the retort and wood were slowly heated so that their temperatures were nearly equal, the temperature of the wood became greater than that of the retort wall, but there was never a sudden liberation of heat from the wood to give a break on the rate-of-temperature-rise curve for the wood. The temperature a t which the wood became equal to the retort wall varied between 250" and 300" C. By increasing the rate of heating, more heat was given off from the wood per unit of time and the slope of the wood temperature curve was greater. For these reasons a standard procedure was adopted for the heating of retort and wood. The retort was heated rapidly to approximately 250" C. and then held constant until the wood temperature was within 20" of the wall temperature. The rheostats were then adjusted until the rate of heating caused a temperature rise in the retort Of 2" per were taken every minute on each of the three thermocouples. In the first few minutes of the run the temperature of the wood rose above the retort wall temperature, increased a t the same rate as the wall temperature, and then Started to drop to the temperature of the wall, Wall and wood temperatures were plotted was taken against time, and theintersection Of the as the exothermal decomposition temperature. This tern-
of Spruoe Sample
As reoeived Oven-dried Dioxane Gasoline Kerosene Light oil, Heavy oil Creosote
Treatment
Treatment
Grams
Grams
4.538 4.945 4.456 4.215 5.044 4.198 4.542 4.430
4:5'i5 5.325 4.587 6.262 5.935 7.065 6.765
2in
after Charring
Loss
%
of Deoompn.
26 29 47 54 23 31 20 33
273 276 275 263 266 267 272 284
Grams
... -8.1 19 8.1 24 41 56 52
3.348 3.535 2.350 1.955 3.875 2.900 3.779 2.965
c.
Moisture has no appreciable effect on the exothermal decomposition temperature, since the oven-dried sample decomposes within 3" C . of the sample containing 10 per cent moisture. The removal of moisture with dioxane and the impregnation of the sample with this liquid which itself contains 36 per cent oxygen has no effect on the exothermal point. The volatiIe liquids, such as gasoline and kerosene, decrease the exothermal point which increases with the boiling point of the liquid. Creosote increases the exothermal temperature.
Literature Cited (1) Bunbury, H. M., "Destructive Distillation of Wood," pp. 76-86, New York, D. Van Nostrand Co., 1923. (2) Chorely and Ramsay, J. Soe. Chem,.,Ind., 11, 395 (1892). (3) H a w k , L. F., "Wood Distillation, pp. 52-4 New York, Chemical Catalog Co., 1923. (4) Pasquier, p. M., Univ. Wash., thesis in &em. eng., 1932. PREBENT before ~ D the Division of Cellulose Chemistry a t the 96th Meeting of the Amerioan Chemioal Sooiety, Milwaukee, wis.
