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FIGURE 5-16. Dry-bottom furnace and burner configurations: (a) horizontal (front or rear); (b) opposed horizontal; (c) tangential (or corner firing); (d) opposed inclined; (e) single U-flame; (f) double U-flame. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

Flame temperatures in the pulverized coal-fired units are typically around 2750°F. Heat is lost primarily by radiation in the furnace to the waterwalls and superheater/reheater tubes suspended in the furnace, and the temperature of the flue gas exiting the furnace is typically 1850°F.

Horizontal and opposed horizontal furnaces (Figure 5-16a,b) are usually fired by circular burners spaced uniformly across the width of the furnace on the front or rear wall or on both front and rear walls. Each burner has its own flame envelope, and the firing system can be designed so that an individual burner may be placed in service, adjusted, or removed from service independently of the other burners. In front or rear wall firing, the burners are arranged in such a way as to promote turbulence. In opposed firing, the burners in opposite walls of the furnace impinge their flames against each other to increase turbulence [7].

In tangential and, to a lesser extent, opposed inclined furnaces (Figure 5-16c,d), the burner turbulence is replaced by the overall furnace turbulence. In these furnaces, a single flame envelope promotes combustion stability and avoids the high flame temperatures that tend to favor NOZ

formation. In addition, the burners in the tangential furnace, where the fuel and air are admitted at all four corners and at different levels of the furnace and the burners, can be tilted upward or downward by 20° from the horizontal, thereby changing the temperature of the flue gas by as much as 150°F [7]. This allows for changing the combustion volume of the furnace to control superheat and reheat temperatures.

Single and double U-flame furnaces (Figure 5-16e,f) are used for firing difficult-to-ignite and slowly burning fuels such as anthracite and coke. In these designs, the fuel is fired downward, and radiation from the rising portion of the flames and from the burners in the opposite arch (in the double U-flame units) assists in maintaining a stable flame over a wide load range.

Wet-Bottom Firing Early wet-bottom furnaces (i.e., in the 1920s) were simply open, single-stage furnaces with burners located close together and near the furnace floor to achieve the high temperatures necessary for melting the ash [2]. The furnace type used was usually one of those shown in Figure 5-16a-d, which was modified to accommodate the molten ash, and it satisfactorily used favorable coals where limited turndown was required. For coal ash that is difficult to melt and when a larger turndown range is required, two-stage designs have been developed. Examples of two-stage, slag-tap firing are shown in Figure 5-17 [2]. The primary advantage of wet-bottom furnaces

FIGURE 5-17. Furnace and burner configurations for two-stage slag-tap firing. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

is easier ash handling and disposal; however, the disadvantages of using wet-bottom furnaces have led to its decline in the United States. These disadvantages include lower boiler efficiency through sensible heat loss of the slag, less fuel flexibility, higher incidences of ash fouling and external corrosion of pressure parts, decreased average steam generator availability, and higher levels of NOZ emissions [2].

Cyclone Furnaces Cyclone-furnace firing, shown in Figure 5-18, is a form of two-stage, wet-bottom design although some do not classify it as suspension firing because a large portion of the fuel is burned on the surface of a moving slag layer [2]. In cyclone firing, one or more combustors are mounted on the wall of the main furnace. Most cyclone furnaces in the United States are fired with coal crushed to about 1/4-inch top size while foreign practice uses partially pulverized coal (e.g., 25% finer than 200 mesh [74 ^m]). In the screened-furnace type, the gases exiting the cyclone pass through a small chamber and slag screen before entering the main furnace. This design has been largely replaced by the open-furnace arrangements as larger units have been developed. The development of the horizontal cyclone furnace occurred rapidly in the United States in the mid-1940s, and B&W was the leader in this technology development. The interest in the cyclone furnace is due to the several good features that it has, including a very high rate of heat production

FIGURE 5-18. Horizontal cyclone furnace arrangements: (a) screened furnace; (b) one-wall open furnace; (c) opposed open furnace. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

FIGURE 5-18. Horizontal cyclone furnace arrangements: (a) screened furnace; (b) one-wall open furnace; (c) opposed open furnace. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

(i.e., up to 500,000 Btu/hr-ft2 compared to 150,000 and 400,000 Btu/hr-ft2 in dry-bottom and slag-tap furnaces, respectively); high flame temperatures (~3000°F) to melt the ash sufficiently; ability to utilize coarser particles than pulverized coal-fired units, which results in lower system costs because pulverizers are not required; and the ability to be designed to use almost any coal type as well as opportunity fuels such as tires, petroleum coke, and others. The fuel characteristics of greatest interest in cyclone firing are the ash fusibility and viscosity of the ash lining the walls of the cyclone. The composition of the ash must be such that the ash will melt, coat the walls of the cyclone, and tap (i.e., be fluid and exit steadily) from the cyclone. In addition, the moisture content of the fuel, such as in lignites, is important because high moisture fuels will consume heat while the moisture is being evaporated, which can affect temperature of the cyclone and hence the fluid behavior of the slag. As previously mentioned, the elevated temperatures produced in wet-bottom furnaces result in the generation of high levels of NOx. Because of this, the use of cyclone furnaces in future installations is unlikely, and more attractive alternatives are pulverized-coal and fluidized-bed systems.

Influence of Coal Properties on Utility Boiler Design

The design of a utility steam-generating plant requires a technical and economic evaluation. Parameters that must be considered to arrive at a final design include the heat release rate, fuel properties (e.g., ash fusion temperatures, volatile matter, ash content), percentage of excess air, production of emissions (e.g., NOZ), boiler efficiency, and steam temperature [14], with the most important item to consider being the fuel burned [8]. This section discusses the influence of coal properties on boiler design, specifically as they relate to suspension firing, as this is the primary combustion technique used by the electric-generating industry today.

The coal properties that influence the design of the overall boiler system include but are not limited to coal and ash and handling, coal pulverizing, boiler size and configuration, burner details, amount of heat recovery surface and its placement, types and sizing of pollution control devices, and auxiliary components such as forced and induced-draft fan sizes, water treatment, and preheaters. The discussion in this section focuses on the influence of coal properties on furnace design consideration.

Furnace Design

Furnaces for burning coal are more liberally sized than those for gas or fuel oil firing, as illustrated in Figure 5-19 [8]. This is necessary to complete combustion within the furnace and to prevent the formation of fouling or slagging deposits. A furnace is designed to take advantage of the high radiant heat flux near the burners [14]. Because the flue gas temperature at the exit of the boiler (i.e., entrance to the convective section) must be at least 100°F

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