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334 CHAPTER 13 1951' Guide the thickness of the bed of fresh coal, is severe enough to blow open the doors and dampers of the furnace. Where frequent attention can be given and care exercised in manipulation of the grates, this fuel can be burned satisfactorily without the aid of any special equipment, except small mesh grates. In general, it will be found more satisfactory with buckwheat coal to maintain a uniform heat output and, consequently, to keep the system warm all the time, rather than to allow the system to cool off at times and then to attempt to bum the fuel at a high rate while wanning up. A uni form low fire will minimize the clinker formation and keep any clinker formed in an easily broken up condition so that it readily can be shaken through the grate. Forced draft and small mesh grates or, for greater convenience,' domestic stokers are frequently used. Buckwheat anthracite No. 2, or rice size, is used principally in stokers of the domestic, commercial and industrial type. No. 3 buckwheat anthracite, or barley, has no application in domestic heating. FIRING METHODS FOR BITUMINOUS COAL A commonly recommended procedure for firing domestic heating 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 ignite the gases leaving the fresh charge. The importance of firing bituminous coal in small quantities at short 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 main tained more nearly proportional to the air supply. If the coal is of the caking kind, the fresh charge will fuse into one solid mass which can be broken up with the stoking bar and leveled from 20 min to one hour after firing, depending on the temperature of the firebox. Care should be exercised when 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 raised only, enough to break up the fuel. With fuels requiring stoking it 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 frequent attendance is impracticable. The adherence to these methods, insofar as practicable, however, will result in better combustion. The output obtained from any heater with bituminous coal will usually ' exceed that obtained with anthracite, since bituminous coal bums more rapidly than anthracite, and with less draft. Bituminous coal, however, will usually require frequent attention to the fuel bed. yuels arid Combustion 335 Preventing Smoke In general, time, temperature and turbulence are the essential require ments for smokeless combustion. Anything that can be done to increase any one of these factors will reduce the quantity of smoke discharged. Especial care must be taken in hand-firing bituminous coals. Checker or alternate firing, in which the fuel is fired alternately on 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 door and the coke pushed back into the furnace just before firing again, pro duces the same effect. The volatiles as they are distilled thus have to pass over the hot fuel bed where they will be burned if they are mixed with sufficient air, and are not cooled too quickly by the heat-absorbing surfaces of the boiler. Steam or compressed air jets, admitted over the fire, create turbulence in the furnace and bring the volatiles of the fuel more quickly into contact 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.7, gravity oil always produces less smoke than a high volatile coal or low A.P.7, gravity oil used in the same furnace and fired in the same manner. The installation of more modem 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. Modem boiler installations are usually designed for high capacity per square foot of ground area, 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 prevented, some type of dust and cinder catcher must be installed. FIRING METHODS FOR SEMI-BITUMINOUS COAL .The Pocahontas Operators' Assodalio'n recommends the central cone method of firing, in which the coal is heaped on to the center of the bed