Document 2R5k2mR54a8XryMNqXMa7QDER

326 CHAPTER 14 1954 Guide of the domestic, commercial and industrial type. No. 3 buckwheat anthra- ^ cite, or barley, has no application in domestic heating. ; *' Firing Methods for Bituminous Coal A commonly recommended procedure for firing domestic beating units, ' called the side-bank method, requires the movement of live coals to one side or the back of the grate, and placing the fresh fuel charge on the : opposite side. The results are a more uniform release of volatile gases, and the subjection of these gases to the high temperature of the red coals. If the fresh charge is covered with a layer of fine coal, still better results may be obtained because of slower release of volatile matter. Bituminous coal should never be fired over the entire fuel bed at one time. A portion of the glowing fuel should always be left exposed to i. ignite the gases leaving the fresh charge. \; The importance of firing bituminous coal in small quantities at short g intervals is discussed in a U. S. Bureau of Mines technical paper.1 Better .. . combustion is obtained by this method in that the fuel supply is maim tained more nearly proportional to the air supply. C If the coal is of the raking kind, the fresh charge will fuse into one solid -'!< mass which can be broken up with the stoking bar and leveled from 20 y min to one hr. after firing..depending on the temperature of the firebox.^ Care should be exercised w hen stoking not to bring the bar up to the surface of the fuel, as this will tend to bring ash into the high temperature,^ zone at the top of the fire, where it will melt and form clinker. The*; stoking bar should be kept as near the grate as possible, and should be X raised only enough to break up the fuel. With fuels requiring stoking it,j;. may not be necessary to shake the grates, as the ash is usually dislodged;/,' during stoking. / ' It is acknowledged that it may be difficult to apply the outlined^ methods to domestic heating boilers of small size, especially when frequenfe attendance is impracticable. The adherence to these methods insofar as|?j practicable, however, will result in better combustion. The output obtained from any heater with bituminous coal will usuallyg;: exceed that obtained with anthracite, since bituminous coal bums mogg rapidly than anthracite, and with less draft. Bituminous coal, howeverg will usually require frequent attention to the fuel bed. $ A Preventing Smoke In general, time, temperature and turbulence are the essential requitf^ ments for smokeless combustion. Anything that can be done to increase."' any one of these factors will reduce the quantity of smoke discharged^* Special care must be taken in hand-firing bituminous coals. - Itf Checker or alternate firing, in which the fuel is fired alternately !<W?j separate parts of the grate, maintains a higher furnace temperature and;' thereby decreases the amount of smoke. Coking and firing, in which the fuel is first fired close to the firing dqgg| and the coke pushed back into the furnace just before firing again, duces the same effect. The volatiles as they are distilled thus have 1 pass over the hot fuel bed where they will be burned if they are mixed witig sufficient air, and are not cooled too quickly by the heat-absorbing surfachS| of the boiler. rJ|| Steam or compressed air jets, admitted over the fire, create turbulent ; in the furnace and bring the volatiles of the fuel more quickly into contacts^) Fuels and Combustion 327 with the air required for combustion. These jets are especially helpful for the first few minutes after each firing. Frequent firings of small charges shorten the smoking period, and reduce the density. Thinner fuel beds on the grate increase the effective combustion space in the fur nace, supply more air for combustion, and are sometimes effective in reduc ing the smoke emitted, but care should be taken that holes are not formed in the fire. A lower volatile coal or a higher A.P.I. gravity oil always produces less, smoke than a high volatile coal or low A.P.I. gravity oil used in the same furnace and fired in the same manner. The installation of more modern or better designed fuel-burning equip ment, or a change in the construction of the furnace, will often reduce smoke. The installation of a Dutch oven, which will increase the furnace volume and raise the furnace temperature, often produces satisfactory results. In the case of new installations, the problem of smoke abatement can be solved by the selection of the proper fuel-burning equipment and furnace design for the particular fuel to be burned, and by the proper operation of that equipment. Constant vigilance is necessary to make certain that the equipment is properly operated. In old installations the solution of the problem presents many difficulties, and a considerable ' investment in special apparatus is often necessary. Lower rates of combustion per square foot of grate area will reduce the quantity of solid matter discharged from the chimney with the gases of combustion. The burning of coke, coking coal, and sized coal from which the extremely fine coal has been removed, will not, as a general rule, produce as much dust and cinders as will result from the burning of non-coking coals and slack coals when they are burned on a grate. Modern boiler installations are usually designed for high capacity per square foot of floor space, because such designs give the lowest cost of construction per unit of capacity. Designs of this type discharge a large quantity of dust and cinders with the gases of combustion, and if pol lution of the atmosphere is to be prevent^!, some type of dust and cinder catcher must be installed. Firing Methods for Semi-Bituminous Coal The Pocahontas Operators' Association recommends the central cone method of firing, in which the coal is heaped on to the center of the bed forming a cone, the top of which should be level with the middle of the firing door. This allows the larger lumps to fall to the sides, and the fines to remain in the center and be coked. The poking should be limited to breaking down the coke without stirring. Grates should be rocked gently. It is recommended that the slides in the firing door be kept closed, as the thinner fuel bed around the sides admits the required air. Firing Methods for Coke Coke ignites less readily than bituminous coal and more readily than anthracite, and burns rapidly with little draft. In order to control the air admitted to the fuel it is very important that all openings or leaks into *ehpit be closed tightly. A coke fire responds rapidly to the opening | the dampers. This is an advantage in warming up the system, but it so makes it necessary to watch the dampers more closely in order to pi'event the fire from burning too rapidly. In order to obtain the same terva] of attention as with other fuels, a deep fuel bed always should be aintained when burning coke. The grates should be shaken only