FIGURE 5-19. Effect of fuel type on furnace sizing assuming a constant heat input. H, distance between centerlines of lowest hopper headers and furnace roof tubes; W, width; D, depth. (Adapted from Singer [8]. With permission of ALSTOM Power, Inc.)

below the ash-softening temperature (which varies from ~2000 to 2500°F), the radiant heat-transfer surface in a coal-fired boiler must be increased by 15% or more in order to achieve a steep reduction in temperature from the flame temperature of ~2750°F.

The convective section of the boiler is designed to extract the maximum amount of heat from the partially cooled flue gas exiting the boiler. The flue gas velocity should not exceed about 60 ft/sec in the convective section, when firing coal, to minimize erosion of the tubes from the fly ash [14]. Sootblowers are required in coal-fired power plants to keep the heat-transfer surface clean.

The furnace size and shape must allow for adequate coal residence time within the furnace to achieve complete combustion. Sufficient heat must be contained in the flue gas exiting the boiler to enable efficient designs of superheater, reheater, economizer, and air heater heat-transfer surfaces. The flue gas exiting the stack must be low to minimize heat losses from the system but must be above the dew point of the acid gases so metal corrosion is not experienced.

In general, ignition stability in a pulverized coal-fired furnace varies directly with the ratio of volatile matter to fixed carbon [14]; hence, coals such as anthracite are typically fired in U-flame furnaces. Coals with higher volatile matter can be more easily burned in suspension, which allows for a lower furnace temperature but requires a larger furnace heat-release area. Also, coals with higher volatile matter tend to have lower ash fusion temperatures (i.e., higher fouling and slagging tendencies), thereby requiring lower furnace temperatures [14].

Figure 5-19 illustrates the relationship between furnace exit gas temperature requirements and heat release rates for typical coals as compared to natural gas and fuel oil. To avoid slagging problems, a lower heat release rate is required for a coal having a relatively low ash fusion temperature than for a coal having a higher ash fusion temperature. The furnace exit gas temperature is primarily a function of heat release rate, which is the available heat divided by the equivalent water-cooled furnace surface [14].

Ash Characteristics

Coal is a very heterogeneous substance, and the mineral matter distributed throughout the coal exists in various forms, compositions, and associations and cannot be simply represented by composition (i.e., elemental oxides) and a single set of melting temperatures (i.e., ash fusion temperatures). Empirical indices have been developed using coal ash chemistry; however, they are successfully used only part of the time and are not applicable to ranks of coal beyond that for which they were developed. Some of these historical indices that are important when evaluating coal ash behavior, as they affect furnace slagging and fouling on both the furnace walls and convective surfaces (which vary among boiler manufacturers), include but are not limited to ash fusibility temperatures, dolomite concentrations, total ash concentration and composition, and base-to-acid, iron-to-calcium, and silica-to-aluminum ratios [8,27,28]. These parameters indicate the slagging and fouling potential of an ash. In addition, slag viscosity is an important parameter for cyclone furnace operation.

Slag Viscosity The viscosity of the slag formed from the coal ash is an important parameter for cyclone-fired furnaces. Slag will just flow on a horizontal surface at a viscosity of 250 poises [6]. The temperature at which this viscosity occurs (T250) is used as the criterion to determine the suitability of a coal for a cyclone furnace. The T250 can be either calculated from a chemical analysis of the coal ash or, more preferably, determined experimentally using a high-temperature viscometer, and a value of 2600° F is considered maximum. Coals with a slag viscosity of 250 poises at 2600° F or lower are considered candidates for cyclone furnaces provided the ash analysis does not indicate excessive formation of iron or iron pyrites [6]. The T250 index is used for all ranks of coal.

In dry-bottom furnaces, the formation of slag must be avoided so as not to adversely affect the unit's operation. A relationship between furnace slagging and T250 has been developed [27]:

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