Document 820QLj16NkbzJLXVreE2KyDom

322CHAPTER 17 ____________ 1946 Guide of the coke as it is released, most of it remains on the hearth or grates and, as this ash layer becomes thicker with time, that portion exposed to the higher temperatures surrounding the retort fuses into a clinker. The temperature in the fuel bed, the chemical composition and homogeneity of the ash, and the time of heating govern the degree of fusion. , Most bituminous coal stokers of Classes 1, 2, 3 and 4 require manual' removal of the ash in clinker form. In the. underfeed`side-cleaning stokers the fuel is introduced at the front of the furnace to one or more retorts, and is advanced away from the retort as combustion progresses, while finally the ash is disposed of at the sides. This type of stoker is suitable for all bituminous coals while in the smaller sizes it is suitable for small sizes of anthracite. In this type of stoker the fuel is delivered to a retort beneath the fire and is raised into the fire. During this process the volatile gases are released, are mixed with air, and pass through the fire where they are burned. The ash may be continuously or periodically discharged at the sides." Automatic Fuel Burning Equipment 323 coking fuels but preferably for those, of low volatile content. Its grate action keeps the fuel bed broken up. thereby allowing for, free passage of air. Because of its agitating effect on the fuel it is not desirable for badly cliiikering coals. It usually should be provided with a.front arch to ignite the volatile gases. Combustion Adjustments The coal feeding rate and air supply to the stoker should be regulated so as to maintain a balance between the load demand and the heat liberated by the fuel. Under such conditions no manual attention to the fuel bed should be required, other than the removal of clinker in stokers which operate on this, principle of ash removal. As in all combustion processes, the maintenance of the correct pro portions of air and fuel is essential. It is desirable to supply the minimum amount of air required to properly burn the fuel at the rate of feed. While there may be only slight variations in the rate at which the coal is being fed due to variations in the size or density of the coal, there may The underfeed rear-cleaning stoker accomplishes combustion in much the same manner as the side-cleaning type, but consists of several retorts placed side by side and filling up the furnace width, while the ash disposal is at the rear. In principle, its operation is the same as the side cleaning underfeed type. % Overfeed flat-grate stokers receive fuel at the front of the grate in a layer of uniform thickness and move it horizontally to the rear of the furnace. Air is supplied under the moving grate to carry on combustion at a sufficient rate to complete the burning of the coal near the rear of the furnace. The ash is carried over the back end of the stoker into an ash pit beneath. This type of stoker is suitable for small sizes of anthra cite or coke breeze, and also for bituminous coals, the characteristics of which make it desirable to burn the fuel without disturbing it. This type of 'stoker requires an arch over the front of the fuel bed to maintain ignition of the incoming fuel. Frequently, a rear combustion arch is also required. In addition to the use of rocking grates, the overfeed inclined-grate stoker is provided with an ash plate on which ash is accumulated and dumped'periodically. This type of stoker is suitable for all types of .Fig. 10 Overfeed Spreader Stoker.(Pneumatic Type) be wide variations in the rate of air flow as the result of changes in fuel bed resistance. - These changes in resistance may be caused by changes in the porosity of the fuel bed due to variations in size or friability of the coal, ash and clinker accumulation, and variations in depth of the fuel bed. Because of this variable fuel bed resistance, many bituminous stokers, even in the smaller domestic sizes, incorporate air controls which. automatically, compensate for these changes in resistance and maintain a constant air fuel ratio. It is desirable on most stoker installations to provide automatic draft regulation in order to reduce air infiltration and provide better control during the banking, or off, periods of the stoker. The efficiency of com bustion. may be determined by analyzing the flue gases as explained in Chapter 16. Furnace Design Although there is considerable variation in stoker, boiler, and furnace design, the stoker , industry, from long-time experience,-has established certain rules for- the-proportioning of furnaces for domestic, and commercial stokers. The stoker installer and designer of stoker-fired equip ment should give careful consideration to these factors.