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The Fuel Value of Wood 



Wood is by far the most widely used fuel. In the country and in 

 the newer or less accessible towns it usually provides the only source 

 of heat. Of the twenty billion cubic feet of forest material con- 

 sumed annually in this country, fire wood demands one-third and 

 comprises one-fifth of the total valuation. The principles governing 

 the fuel value of wood and the methods of determining it should 

 prove of interest. They are of value, for instance, in determining 

 the relative eflBciency and costs of different fuel for home or factory 

 use. 



There are various ways of determining the heating or calorifie 

 power of wood. One is by burning equal quantities under a boiler and 

 noting the amount of steam produced. While this may seem the most 

 practical method and the one which would naturally first suggest 

 itself, there is always the danger that the results will not prove of 

 general application. For a given problem, however, they give the 

 best satisfaction. 



A common laboratory method of determining the calorific power of 

 combustibles consists in burning a very carefully ^^•eighed quantity 

 of the substance enclosed in a small metallic vessel which could be 

 immersed in a receptacle containing a weighed quantity of water, 

 suitably protected against radiation by a jacket of non-conducting 

 material. The inner vessel containing the substance to be tested is 

 provided with an inlet tube for the purpose of supplying sufficient 

 quantity of pure oxygen, and an outlet pipe coiled through the water 

 forming an exit for the products of combustion. Ignition is best 

 effected by means of an electric spark. The rise in temperature of 

 the water gives a measure of the heating value of the substance under 

 test since different materials will raise the temperature of the water 

 different amounts, depending on their fuel values. Such an instru- 

 ment is called a calorimeter. 



Heat ia measured in units. The British thermal unit (B. T. U.) 

 commonly used in England and this country is the quantity of heat 

 necessary to raise the temperature of one pound of water one degree 

 Fahrenheit. In Europe, where the metric system is in common use, 

 the heat unit is the calorie, and is the quantity of heat necessary to 

 raise the temperature of one kilogram of water one degree centi- 

 grade. Thus the calorie is equal to nearly four (3.968) British ther- 

 mal units. 



If the chemical composition of a combustible is known the theoret- 

 ical calorific power can be computed. In 100 pounds of absolutely 

 dry wood, the composition is approximately as follows: 



Carbon 49 pounds 



Oxygen 44 pounds 



Hydrogen pounds 



Ash 1 pound 



This holds fairly constant for all species regardless of weight, ex- 

 cept as modified by infiltrated substances such as gums, pigments, 

 resins, tannin, etc. 



As a result of experiment it is known how many thermal units are 

 evolved for every pound of hydrogen and every pound of carbon con- 

 sumed. If a substance contained only carbon and hydrogen it would 

 simply be necessary to find the calorific value of each and add to- 

 gether. Wood, however, contains oxygen and it has a greater affinity 

 for the hydrogen than it does for the carbon, so, instead of carbon 

 dioxide (CO.^) being formed, the result is water (II.O). The combi- 

 nation is in the proportion of 8 to 1 by weight, that is, 8 pounds of 

 oxygen combine with one pound of hydrogen, forming 9 pounds of 

 water. Dividing the quantity of oxygen present by 8 will give the 

 hydrogen necessary to satisfy it, the remainder being left available 

 for fuel. 



The amount of water contained in fuel wood is an important con- 

 sideration since the heat required to evaporate it is lost for other 

 purposes. The water in green wood often makes up h,alf of the total 

 weight, especially in sap-wood. After such wood is thoroughly air- 

 seasoned there would remain about 20 per cent of water. If kiln- 

 dried, from '2 to 5 per cent of water would remain in the wood, and 

 if exposed to the air this would be increased by absorption (hygro- 

 scopically) from 10 to 15 per cent, depeading upon the humidity. 



—32— 



A lunidred weight of wood as sold on the market contains about 

 25 pounds of water, 74 pounds of wood substance and 1 pound of 

 ashes. These 74 pounds are made up of 37 pounds of carbon, 4.4 

 pounds of hydrogen and 32 pounds of oxygen. The oxygen combines 

 with the hydrogen as above stated in the proportion of 8 to 1, pro- 

 ducing 36 pounds of water and leaving four-tenths of a pound of 

 hydrogen to produce heat. The total amount of water to be evap- 

 orated becomes 25 plus 36, or 61 pounds; the amount of wood sub- 

 stance left available for heat production is 37.4 pounds out of the 

 original 100 pounds. 



To raise a pound of water from the temperature of the air, say 70 

 degrees P., to the boiling point (212 degrees F.) will require 142 heat 

 units, a heat unit being the amount of heat required to raise a pound 

 of water one degree in temperature. To convert this into steam will 

 require 966 heat units more (the latent heat of steam), making a 

 total of 1,108 for each pound of water. This does not take into 

 account the heat required to raise the steam to the temperature of 

 the flue gases. 



It is evident theu that the greater the amount of water present the 

 more the heat available for other purposes is cut down. Thus only 

 about one-half of the weight of the wood substance is heat producing 

 while every pound of water combined in the wood requires 1,108 units 

 of heat to evaporate it, and this diminishes the value of the wood as 

 fuel. Hence, under the most favorable circumstances, the heating 

 efficiency of wood, with 25 per cent moisture, will be less by one- 

 fourth of 1,10S units, or 277 units, than that of dry wood. If the 

 percentage of water content is 20 instead of 25 the reduction would 

 be one-fifth of 1,108, or 221.6, and so on. 



In the ordinary stove or other small apparatus the evil effect of 

 moisture in the wood is very much increased since combustion is ma- 

 terially interfered with. Unsound wood has a reduced heating ability, 

 as a portion of the cell walls is removed by decay. 



The heating power or fuel value of wood bears a direct ratio to 

 the specific gravity of the dry material. By specific gravity is meant 

 the ratio of the weight of a given volume of wood to that of an equal 

 volume of water. Water weighs a little over 62 pounds per cubic foot 

 and wood, weighing 31 pounds per cubic foot when perfectly dry, is 

 said to have a specific gravity of .50, and so on for other weights. 



Tho specific gravity of wood substance itself, that is tho actual 

 material of the cell and fiber walls, is about 1.6 ; that is, is more than 

 half again as heavy as water. This is true whether the wood in ques- 

 tion is white pine, Cottonwood, hickory, or ash. The reason that dry 

 woods of equal volume may show great disparity in weight is due to 

 the relative amounts of wood substance and air in them. A heavy 

 wood is dense and has few and small cavities, or else such as did exist 

 have become filled up with gums and resins. A light wood, on the 

 other hand, has a large proportion of its volume made up of cavities 

 and spaces which in a dry condition are without weight. The rea- 

 son any wood floats in water is because of the buoyancy of the im- 

 prisoned air; when this is replaced by water the wood becomes 

 waterlogged and sinks. 



Theoretically equal weights of wood substance will give the same 

 amount of heat regardless of the species. In other words a hundred 

 pounds of absolutely dry cottonwood should furnish as much heat as 

 a hundred pounds of hickory. In reality the varying forms of tissue 

 found in the different species, the addition of resin, gums, tannin, 

 oils and pigments as well as the water present in varying amounts 

 causes different woods to have different heating values. The pres- 

 ence of rosin in wood increases the heating power materially, the 

 results of numerous tests showing a difference ranging up to 12 per 

 cent or more. 



In the practical use of wood for fuel one is not so much interested 

 in the fact that thoroughly lignified tissue has the same heating value 

 for all species of trees as he is in knowing the unifonnity and dura- 

 tion of the heat produced. Softwoods give a quicker fire and more 

 flame than hardwoods and for that account have a special advantage 

 for kindling purposes. The denser and non-resinous hardwoods, which 



