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486 CHAPTER 34 1959 tiple retort design. It is used in some of the largest indus trial plants and central power stations. Zoned air control has been applied to these stokers, both longitudinally and trans versely of the grate surface. The overfeed flat grate stoker is represented by the vari ous chain- or traveling-grate stokers. Another distinct type of overfeed fiat-grate stoker is the spreader type in which coal is distributed either by rotating paddles (Fig. 5) or by air over the entire grate surface (Fig. 6). This type of stoker is adapted to a wide range of fuels and has a wide application on small sized fuels, and on fuels such as lignites, high-ash coals, and coke breeze. The overfeed inclined-grate stoker operates on the same general combustion principle as the flat-grate stoker, the' main difference being that rocking grates, set on an incline, are provided in the former to advance the fuel during com bustion. Combustion Process In anthracite stokers of the Class 1 underfeed type, burn ing takes place entirely within the stoker retort. The refuse of combustion spills over the edge of the retort into an ash pit or receptacle from which it may be removed either manually or automatically. Anthracite for stoker firing is usually pea, buckwheat, rice, or barley size (see Chapter 33 for a description of size). Because the majority of the small bituminous coal stokers operate on the underfeed principle, a general description of their operation is given. When the coal is fed into the retort, it moves upward toward the zone of combustion and is heated by conduction and radiation from the burning fuel in the combustion zone. As the temperature of the coal rises, it gives off moisture and occluded gases, which are largely noncombustibles. When the temperature increases to around 700 or 800 F the coal particles become plastic, the degree of plasticity varying with the type of coal. A. rapid evolution of the rambustible volatile matter oc curs during and directly after the plastic stage. The distil lation of volatile matter continues above the plastic zone where the coal is coked. The strength and porosity of the coke formed will vary according to the size and character istics of the coal. While some of the ash fuses into particles on the surface of the coke as it is released, most of it'remains on the hearth or grates and, as tins ash layeT 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, and 3 require maiyud removal of the ash in clinker form. In the underfeed side-cleaning stokers the fuel is intro duced at the front of the furnace to one or more retorts, and is advanced away from the retort as combustion progresses, Fig. 3 .... Underfeed Screw Stoker, Hopper Type, Class 2 or 3 while finally the ash is disposed of at the tides. 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 tides. The underfeed rear-cleaning stoker accomplishes combus tion in much the same manner as the tide-cleaning type, but consists of several retorts placed tide by tide 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 thicknes and move it horizon tally 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 ashpit beneath. This type of stoker is suitable for small sizes of anthracite 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, and frequently a rear combustion arch. Overfeed inclined-grate stokers are provided with rocking grates and ash plates on which ash is accumulated and dumped periodically. This type of stoker is suitable for all types of coking fuels, but preferably for those of low volatile content. Its grate action keeps the fuel bed broken up, thereby allowing free passage o! air. Because of its agitating effect on the fuel, it is not desirable for badly clinkering coals. It usually should be provided with a front arch to ig nite 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 Automatic Fuel Burning Equipment 487 For burning rates from 100 to'1200 lb coal per hour . H m 0.03 B + 24 (2) where demand and the heat liberated by the fuel. Under such con ditions, no manual attention to the fuel bed should be re quired, 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 proportions 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 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 re sistance, many bituminous stokers, even in the smaller do mestic sizes, incorporate air controls which automatically compensate for these changes in resistance and maintain a constant air-to-fuel ratio. The efficiency of combustion may be determined by analyzing the flue gases,, as explained in Chapters 33 and 44. It is desirable on most stoker installations to provide auto matic draft regulation in order to reduce "air infiltration and provide better control during the banking, or off, periods of the stoker. It is imperative that adequate combustion air be supplied and, therefore, it is usually necessary to provide a direct outdoor opening for this purpose. (See Chapter 33.) H = minimum setting height, inches, measured from dead plates to crown sheet for steel boilers. For east-iron boilers height may be % H. B " burning rate coal per hour, pounds. Standards for minimum firebox dimensions and base heights have been formulated by the Stoker Manufacturers Association as shown in Fig. 7/ In considering these recommendations, it should be under stood that they show the average recommended minimum. There are many factors affecting the proper application of stokers to various types of boilers and furnaces, and, in cer tain instances, setting height or firebox dimensions shown in the standards may be modified without impairing perform ance. Such modification will depend upon the experience of the installer or designer with a particular stoker, the type of fuel used, and the construction of the boiler or furnace. Installation of stokers (particularly Smaller sizes) on the side of the boiler or furnace may facilitate ^linker removal. Rating and Sizing Stokers The Stoker Manufacturers Association*' * has suggested a method for rating solid fuel stokers as follows: Load (Btu per hour)________________ Stoker burning rate Heating value of coal (Btu per pound) required (pounds of X overall efficiency of stoker per hour) and boiler or furnace In determining the total load placed on a stoker-fired boiler by a steam or hot water heating system, a piping and pickup factor of 1.33 is commonly used in rising the stoker, but this factor should be increased at times due to unusual conditions. Controls The control of coal-fired equipment is discussed in the section Controls for Automatic Fuel-Burning Equipment. DOMESTIC OIL BURNERS Combustion Chamber Design The Stoker Manufacturers'"Association has published standard recommendations on setting heights for stokers having capacities up to 1200 lb of coal per hour/ The empirical formulas for ...determining these setting heights are: For burning rates up to 1001b coal per hour B = 0.1125 B + 15.75 (1) An oil burner is a mechanical derice for the oxidation of hydrocarbon liquids (fuel oil) and air under controlled con- r9- 2------ Underfeed Stoker, Bin Feed Type, Class l COAL DISTRIBUTING BAR fig. 4 .... Underfeed Side Cleaning Stoker . fN.1^. permit Mtabetary pericmnw but tbeue dimension*ere |4fend Fig. 7 .... Suggested Minimum firebox Dimensions and Base Heights*