.. WOOD rUCt AND WOOD STCViS liiiif 1942 life! I*r«ioi« r UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin 4- ; ^ \- WOOD FUEL A2JD WOOD STOVES The big increase in the demand for ccal required "by industry and the heavy load that the transportation systems are "bearing because of the war program have turned more attention to wood for fuel. This trend is reflected by the increase in the number of inquiries received at the Forest Products Lab- oratory on wood fuel and on the "burning of wood. As a fuel, wood has certain distinct advantages, particularly in regions where plentiful supplies are locally grown. It is cheap. It is easily handled. It burns well, with little smoke, provided certain simple rules are observed. It has a high heat producing potential. The accompanying table indicates the comparative heat values for wood and good coal; for example, a cord (i?C cubic feet) of white oak has slightly more heat value than a ton of good 7 coal. And, finally, a standard cord of hardwood burns down to only oO pounds of ashes, while a ton of hard coal leaves from 200 to 300 pounds. Heat Value of Wood In burning wood it is essential to have the wood as dry as possible in order to produce more heat per pound of fuel, to obtain a higher efficiency, and also to lessen the creosote trouble. Oven-dried wood has a calorific value of g,600 B.t.u. , and will yield about 7,250 B.t.u. per pound of fuel when burned under rather ideal conditions, whereas air-dried wood containing about 20 percent moisture will yield about 5.800 B.t.u., and green wood con- taining about 60 percent moisture will yield only about U,100 B.t.u. The table gives the approximate weights, heating value per cord, and the equivalent in heat value to ton3 of coal for different species of wood. Generally speaking, the heat value of thoroughly dry wood is in direct proportion to its weight per cubic foot. One pound of oven-dried wood substance of any wood has a calorific value of 3,600 B.t.u. The presence of other substances, such as oils, tannins, resin, and the like, somewhat changes the actual calorific value of the different woods. Resinous woods, such as pine, have a higher calorific value than nonresinous wood. In the table all hardwoods were assumed to have a calorific value of 8,600 B.t.u. per pound when oven dry and 5,800 B.t.u. available heat when containing 20 percent moisture. Similar heat values for white pine are 9»150 B.t.u. and 6,U00 B.t.u. Burning of Wood In the combustion of wood there are three principal phases. In the first phase the moisture is evaporated and driven off; in the second phase the volatile matter is distilled off. When heated to temperatures of approxi- mately 1,100° F. or greater, this volatile matter bursts into flames and R1279 "burns if mixed with siifficient air. In the third or final phase the fixed carbon biirns. All three phases of burning may, of course, go on at the same tine, but the first two stages are largely carried on when the fire is started or when new fuel is added. In the first two phases of burning, heat is consumed. The evaporation of 1 pound of water requires about 1,000 3.t.u. , and 1 pound of volatile natter absorbs about 200 B.t.u. when driven off. Since the volatile matter com- prises about bO percent of the weight of bone-dry wood, the amount of heat absorbed may be sufficient to cool the furnace when a fresh charge cf fuel is added. Volatile gas is not then maintained at approximately 1,100° F, and will not burn. Incomplete combustion of the volatile matter can, there- fore, constitute a big loss in efficiency. It is, moreover, troublesome because the gases that are not burned contain creosote, tarry substances, and acetic acid, which eat away the stove pipe, leak out of the joints, and stain material with which they come in contact. Due to its high percentage of volatile matter, wood has a long flame and therefore the combustion space above the fuel level must be greater than that for coal. When the air supplied through the grates passes through approximately 2 inches of glowing wood embers about all the oxygon in it is consumed or combined with the carbon to form carbon dioxide. This carbon dioxide is reduced to carbon monoxide as it passes through more glowing carbon. For this reason, according to some authorities, SC percent of the air needed should be supplied ever and around the fuel. To summarize the points that lead to complete combustion of wood: Sufficient air must be supplied to the fire and a large portion of this air should be supplied over or around the fuel. -This air must be mixed with the gases given off from the wood, and the temperature of these gases should be kept above 1,100° F. so that they will burn. Theoretically, it takes 6 pounds of air to burn 1 pound of bone-dry wood, but in actual practice more air is required because the air and the gases are not thoroughly mixed. Avoidance o f Creosote In severe cold weather there is generally less creosote trouble, for with a hot fire the gases are kept hotter than 1,100° F. and, when supplied with sufficient air, will burn. Also, with a hot fire, the flue gas temperatures are higher and acetic acid and creosote that are present because of incom- plete combustion may be carried out of the chimney in the form of a vapor. In order to prevent undue creosote trouble, it is best to burn dry wood in a hot small fire instead of a large smoldering fire. Refueling the stove at more frequent intervals with smeller amounts of fuel also helps. When a small amount of new wood is added less gas will be formed, the fire will not cool do\/n so much, and the gas will therefore burn more completely. Short stove pipes and insulated stove pipes will further help to keep the creosote and acetic acid from condensing. £1279 -2- Hogged Fuel B urners In the Pacific ITorthwest, many sawdust or hogged fuel timers have "been installed. This development has teen largely limited to the Northwest, for here hogged fuel is pluntiful and cheap; also, green Douglas-fir has a relative^ low moisture content — about 35 percent on an oven-dry "basin. Other greon native woods usually contain about tv/ice as much moisture. In large installations some type of automatic stoking is usually used. For heme heating with furnaces, the sawdust units are usually installed by removing the ash pit door and the grates, and the unit is aligned with this opening. The "burners consist of a hopper from which the fuel flows "by gravity on an inclined grate placed in front of a refractory-lined com- bustion chamber which is usually large enough so that ell the burning takes place in the combustion chamber. In all these units secondary air is intro- duced above or around the layer of fuel. The heat that is radiated back from the refractory lining is an aid in driving off the moisture and volatile gas, and in maintaining the temperature in the chamber above 1,100° F. so that the volatile gas can be burned more completely. Wood 3riquettos Briquettes made of wood waste make an excellent, clean, and convenient fuel. Since they are made of wood v/aste many sawmills, planing mills, and wood- working plants desire information on the advisability of converting their wood waste into briquettes. Unless they ha^e a large and constant supply of wood waste, are located in a region where coal is particularly high in price, and are in a position to pay royalties on machinery, briquetting does not rate as a highly attractive proposition. Wood briquettes are made of dried and ground-up sawmill or planing mill wastes. This material is fed into an automatic briquetting machine where the material is fed into a mold and then subjected to a pressure exceeding 10,000 ixiunds per square inch. The pressure and the heat developed are suf- ficient to plasticize the material partly and it forms a briquette that will stand, handling. If a binder wore used, lower pressures could be employed. Few suitable binders are cheap enough for this purpose, and the manufacturers of briquettes using no binders have been the most successful. Burning Wood with Coal In many rural sections wood is burned in the furnace in mild weather when a constant fire is not required or when the rate of burning can be held down and still give off sufficient heat. In severe cold the farmer either burns coal or a mixture of chunk wood and coal. The best wav to handle the com- bined fuel is to partly fill the furnace with chunk wood, then throw coal on top of the wood filling the spaces between the wood and the firepot and be- tween the piuces of \\rood, besides covering the wood. A bed of coal and wood will hold fire overnight. R1279 -3- Bur o -pean Slow-Combustion Stoves In the United States within the last few years considerable publicity has been given to the European slow-combustion stove. These stoves are of the hopper type with the grate at the bottom of the hopper. A damper in the ash pit controls both the primary and the secondary, air supply. The primary air passes up through the grate, and- supports combustion in a limited area above the grate. The secondary air is introduced in the flame area at the bottom of the combustion chamber, which is in front of the hopper. This combustion chamber is lined with fire brick, and provides for a long flame travel so that the gases formed can be more completely burned. The re- fractory lining is a!!. so an aid in promoting cleaner burning and in eliminat- ing much of the creosote trouble. Very high efficiencies have been claimed for these stoves. At the present time European slow-combustion stoves are not available in this country. Test of Si ow-Com bus tion S tove Tests have been run at one of the universities on one of these slow-combustion European stoves and on one American sto^e of similar design. At a low rate of burning with a flue gas temperature of 15S° F, , the European stove showed an efficiency of 79 percent based on the actual heat value of the wood. In an installation in a home such conditions would be impractical, for the low flue temperature would produce such a small draft and low rate of burning that the stove would produce too little heat, At higher flue gas temperatures with higher rates of burning, the efficiency of the stove would range from 59 to 7^ percent. The American stove of similar design had a range of efficiency of from Si to 72 percent for different rates of burning, American Wood-Burning Steves In the last few years several new wood-burning stoves have been introduced in the United States. Some of these stoves have incorporated the features of the European slow-combustion stove. These stoves of the hopper type have a combustion chamber either in front or behind the hopper where the second- ary air is introduced into the flame area. The majority of the new stoves are equipped with thermostats that automatically regulate the rate of burn- ing by controlling the inlet air, and in this way hold, the room temperature more consto.nt. All the new stoves claim high efficiency and less maintenance attention. Some of the stoves are also more attractive in appearance. The following are some of the American companies producing new and improved \tfood burning stoves: H. 3. Smith Company, Westfield, Mass. Riteway Products Company, Harrisonburg, Va. Ashley Automatic Wood Stove Company, Columbia, S. C. Shapleigh Hardware Company, 900 Spruce St., St. Louis, Mo. R1279 -U- Many of the installations of the old type of wood stove with a long stove pipe have as high an efficiency as the more recent stoves. In many cases the lon~ stove pipe gave off almost as much heat as the stove itself. A long stove pipe is, however, objectionable "because it generally leads to creosote trouble. In the new type of stove a short pipe can be used and still have good eff iciency. Many of the old stoves were designed to burn coal, which has a shorter flame than wood. If the height above the fuel bed is too small, the flames will shoot out into the stove pipe and much of the heat will go out the chimney. In the old airtight and barrel stoves, unless long stove pipes were used to radiate much of the heat, the efficiency would be around 50 percent. Research in Wood Fuel Due to the increased interest in wood fuel, several agencies in the Federal Government are carrying on studies designed to lower the cost of wood fuel by developing labor-saving methods and ^oy developing improved wood burning units. Experiments are being carried on with a portable hog to reduce the handling costs and also to put the wood in a form that lends itself to automatic firing. The handling of fuel wood in crates or in wired bundles is also being tried. The Forest Froducts Laboratory at present is experi- menting with a gang saw combined with an automatic splitter to produce cubes of wood. Wood in tho form of small cubes about the size of a fist can be more readily handled with belt conveyors, shovels, and spouts than chunk wood. The Laboratory is also working on stoves designed for this type of wood fuel with hopper feed to make firing easier and more convenient* R1279 -5- Approximate weights and heating value per cord of wood cont cining 80 cubic feet of solid roo d with 20 percent moisture^ and rith green wood Variet2 r of wood Weight per cord containing 80 cubic feet of solid wood in pounds Green 20 percent moisture Available- heat units per cord million B.t.u, Green 20 percent moisture Equivalent in heat value to tons of co all Green 20 percent moisture Ash Aspen Beech Birch, yellow Elm, American Hickory, shagbark Maple, rod Maple, sugar Oak, red Oak, white Fine, white 3,8^0 3.^0 } +,320 k , 5o0 ^,320 5,01+0 H.000 U.Ugo 5,120 5,0^0 2,880 3,^0 2,160 3,760 3,^80 2,960 U.2U0 3,200 3,680 3,680 3,920 2,080 16.5 10.3 17.3 17.3 20.7 15.0 18. k 17.9 19.2 12.1 20.0 12-5 21.8 21.3 17.2 2'4.6 18.6 21.3 21.3 22.7 13.3 0.75 •79 • 79 .65 .9^ .68 .8k .81 .87 • 55 0.51 • 57 •99 •97 • 78 1.12 .85 •97 •97 1.0U .60 —Percent moisture given in terms of oven-dry weight. 2 Available heat equals calorific value minus loss due to moisture, minus loss due to water vapor formed, minus loss due to l.eat carried away in dry chimney gas. Flue temperature taken as ^50 degrees. ITo excess air, 3 Heat value of coal under similar conditions taken as 11,000 B.t.u. This would require a good coal with a total heat value of about lU.OOO B.t.u, per pound of dry coal. HI279 UNIVERSITY OF FLORIDA lllJIJflllllttllllil 3 1262 08927 3311