Internally fired boilers, such as the firebox and packaged types, are self-contained and require no additional setting. Externally fired boilers
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require special consideration in terms of furnace construction, partic-ularly since each installation is designed to meet specific plant requirements and space availability.
The horizontal-return tubular boiler (see Fig. 2.5) is supported by the furnace walls. It is mounted on lugs set on rollers, permitting the boiler to move longitudinally. An improved method of installation is shown in Fig. 2.6. Here, the refractory walls do not carry the weight of the boiler.
Expansion and contraction for water-tube boilers are taken care of in a number of ways: (1) by suspending the drums and headers from slings attached to overhead columns, (2) by supporting the drum at the end, on columns or overhead beams, and (3) by anchoring the lower drum at the floor level, permitting expansion upward.
In the past, refractory arches frequently were installed in furnaces equipped with chain-grate stokers. Their primary purpose was to assist in maintaining stable ignition with a reduction in smoke emis-sion. Such arches were difficult to maintain, resulting in frequent replacement that required outages of boiler units. These arches have largely been replaced by water-cooled arches or by small snub-nose refractory arches, also water-cooled. Over-fire air jets are provided to improve combustion efficiency.
The vertical boiler (Fig. 2.25) is a two-drum three-gas-pass water-tube boiler, with side waterwalls. The steam drum is supported on steel beams, while the lower (mud) drum is suspended from the inclined-vertical tubes. It is fired by a spreader stoker. (Refer to Chap. 5.) Note the fly ash reinjection at the rear of the furnace. Its purpose is to improve the boiler efficiency by burning the unburned combustibles that have fallen into the hopper at the boiler outlet.
Until the 1920s, any increased steam requirements were met with increasing the number of boilers in a system. In order to reduce costs, attempts were made to increase the size of the boilers. These boilers were basically refractory-lined furnace designs, and methods of firing coal, primarily on stokers, were no longer adequate.
The use of pulverized coal became the answer to the requirements of high combustion rates and higher boiler steam capacities. The refractory furnace was no longer adequate, and the water-cooled furnaces were developed. These furnace designs eliminated the problem with the rapid deterioration of refractory walls because of the molten ash (slag) that formed on the hot walls. The water-cooled walls also lowered the tem-perature of the gases leaving the furnace. This not only improved the heat-absorption capability of the boiler but also reduced the accumula-tion of ash (slagging) in the convecaccumula-tion heating surfaces of the boiler.
Water-cooled furnace walls not only reduced maintenance on the furnace and fouling of the convection heating surfaces (and thus
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minimized forced boiler outages), but the wall also absorbed heat, which helped to generate more steam. As a result, boiler tube bank surface was reduced because of this additional steam-generating sur-face in the furnace. In order to obtain a higher cycle efficiency, feed-water and steam temperatures were increased, with an increase in steam pressure, and this further reduced boiler tube bank surface.
However, it was replaced with an additional superheater surface.
As a result, boilers designed for steam pressures above 1200 psig consist of basically furnace water walls, superheaters, and supple-mental heat recovery equipment of economizers (for heating feedwater) and air heaters (for heating combustion air). Boilers designed for lower pressures have a considerable amount of steam-generating surface in boiler banks in addition to the water-cooled furnace.
Most modern boiler furnaces have walls that are water cooled. This not only reduces maintenance on the furnace walls but also serves to reduce the temperature of the gas entering the convection bank to the
Figure 2.25 Vertical boiler with a dump grate spreader stoker.
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point where slag deposits and superheater corrosion can be controlled by soot blowers.
Furnace wall tubes are spaced on close centers to obtain maximum heat absorption. Tangent tube construction, used on earlier designs, has been replaced with membrane walls (see Fig. 2.30) in which a steel bar or membrane is welded between adjacent tubes.
The two-drum boiler shown in Fig. 2.26 is of waterwall construction with membrane walls. A radiant-type superheater is located in the furnace. The boiler is fired primarily by coal, wood, bark, or other solid fuels on a traveling-grate spreader stoker, with auxiliary burners (gas or oil) located in the rear wall providing flexibility for supplementary fuel firing.
The single-elevation top support ensures an even downward expan-sion without differential stresses or binding. The drum rests on over-head steel beams, and the superheater is hung from slings. Boilers of this type are largely prefabricated, with the furnace walls built in panel sections. Later the panels are welded together to form the membrane wall furnace sections. These units are carefully built
Figure 2.26 The Stirling SS boiler with a traveling-grate spreader stoker. (Babcock & Wilcox, a McDermott company.) Ch02_Lammers_1418466 10/8/04 11:26 AM Page 78
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under controlled shop conditions, for ease of erection, requiring a min-imum amount of time to assemble.
This is a single-gas-pass boiler, and therefore, no baffling is required.
There are no local areas of high-velocity products of combustion to cause tube erosion. When required to reduce unburned carbon, fly ash return from the last pass of the boiler is by gravity to the rear of the grate. Over-fire air jets are provided to improve combustion. These units are generally available in capacities of 60,000 to 400,000 lb/h of steam, pressures of 160 to 1050 psi, and temperatures to 900°F.
The Stirling Power Boiler (SPB) shown in Fig. 2.27 is designed with a controlled-combustion-zone (CCZ) furnace to provide better mixing of the fuel and air. This design is used primarily in the firing of waste fuels, particularly bark and refuse-derived fuel (RDF), a solid fuel that is processed from municipal solid waste (MSW). Figure 2.27 shows a boiler designed to burn RDF on a spreader stoker, with auxil-iary natural gas burners, in a waste-to-energy plant.
Furnace heat release is expressed in Btu per hour per cubic foot (Btu/h/ft3) of furnace volume. The permissible heat release varies
Figure 2.27 Controlled-combustion-zone (CCZ) furnace design. (Babcock & Wilcox, a McDermott company.)
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with design, depending on whether the furnace is refractory-lined or water-cooled, the extent of water cooling, heat transfer, and the type of fuel burned. High furnace heat release is usually accompanied by high furnace temperatures. When coal low in ash-fusion temperature is being burned, the ash adheres to the refractory surface, causing ero-sion and spalling. The ash also may adhere to the heating surfaces, reducing the heat transfer and frequently fouling the gas passages with a loss in boiler capacity and efficiency. For the refractory-lined furnace, high furnace heat release is more severe than for waterwall installations.
As noted previously, because refractory walls were unable to meet the severe service conditions to which they were subjected, waterwalls were introduced, even for the smaller boiler units. Excessive mainte-nance and outage of equipment are thus avoided, and the addition of waterwalls increases the boiler capacity for a given furnace size.
The first application of furnace water cooling was the installation of the water screen when burning pulverized coal. This screen consisted of a series of tubes located above the ashpit and connected to the boiler water-circulating system. Its purpose was to reduce the temperature of the ash below its fusion point; thus slagging was prevented.
The waterwall was added next. In replacing the refractory walls, the added heating surface increased the boiler output, and with the elimination of refractory maintenance, boiler availability was improved.
The amount of water cooling that can be applied is determined in part by combustion conditions to be experienced at low steam capacity, since excessive cooling reduces stability of ignition and combustion efficiency.
Therefore some furnaces are partially water cooled, or the waterwall is partially insulated; each design is based on experience.
Details of wall construction are illustrated in Figs. 2.28 and 2.29;
tube construction with full and partial stud tubes is shown. The studs are used to anchor the refractory in place, while tie bars hold the tubes in line.
Various types of wall blocks are used. The choice is determined by their individual capacity for heat conductivity and by the varying con-ditions to which they are exposed in different parts of the furnace.
The blocks may be rough faced or smooth, of bare metal or refractory faced. Depending on known heat-transfer coefficients, blocks are applied to meet design specifications and to limit the heat input to the tubes in order to prevent overheating and other problems.
Special attention must be given to wall sections subjected to flame impingement, to tube bends, and to division walls and slag screens subject to the blast action of the flame. Special refractory materials provide protection against molten slag and erosion. The arrangement of studs and the extent of refractory covering are modified to meet the specific requirements of the individual furnace and the type of fuel
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burned. In operation, any excess refractory is washed away until a state of thermal equilibrium is reached because of the cooling effect of the studs. Fully studded tubes are used to assist ignition and to pro-mote complete combustion for sections of the furnace where maximum temperatures are desired. Partially studded tubes are usually used in cooler zones of the furnace and where more rapid heat absorption is advantageous.
Over the years, efforts have been directed to the reduction of air infiltration into the boiler setting in order to improve unit efficiency while maintaining boiler capacity. The use of waterwalls with welded outer casings has reduced this leakage considerably. The pressurized furnace was the next step. It uses an all-welded casing behind the tube enclosure, the insulation being located behind the casing.
However, on pressurized units, flue gas can still leak through the walls to cause overheating of the inner and outer casings. Such leakage causes flue gas and fly ash to enter the casings. The flue gas may be sat-urated with sulfur, resulting in corrosion of the casings. On balanced-draft designed units, air infiltration was a problem that reduced boiler efficiency. The membrane wall construction was developed to solve these problems (Fig. 2.30). Tightness is accomplished by welding a bar between the tubes, insulation being placed behind the tubes, with casing or lagging on the outside.
Figure 2.28 Water-cooled furnace wall construction. (Babcock & Wilcox, a McDermott company.)
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For boilers such as those shown in Figs. 2.9 and 2.10, the bulk of the heat absorbed was the result of convection and conduction; only the lower rows of tubes received heat by radiation. The number of square feet of heating surface was then used to determine the capacity of the unit, approximately 10 ft2 of heating surface being consid-ered capable of generating 34.5 lb/h of steam “from and at 212°F”
Figure 2.29 Block-covered wall showing method of clamping blocks on tubes.
(Babcock & Wilcox, a McDermott company.)
Figure 2.30 Membrane wall construction with block insulation and metal lagging. (Babcock & Wilcox, a McDermott company.)
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feedwater temperature. (See Sec. 3.8 for an explanation of this mea-surement.) Where waterwalls comprise the greater portion of the heating surface, receiving most of the heat by radiation, the previous standard cannot be applied. Therefore, for modern units, boiler perfor-mance and steam capacity are calculated by the designer based on design performance data and experience with similar units in the field.
When pulverized coal is being fired, difficulty may be experienced with deposits of furnace slag. This is especially troublesome when the coal contains ash having a low fusion temperature. The slag becomes very hard and difficult to remove, especially when it is attached to the brickwork. Furthermore, a portion of the refractory is frequently removed along with the slag, thus increasing the maintenance cost.
Furnaces can be designed to burn coal of any fusion range. If the ash is removed in the dry state, the unit is referred to as a dry-bottom furnace. Or for low-fusion-ash coal, the unit may be designed to remove the ash in liquid form; the unit is then called a wet-bottom furnace. The liquid ash can be removed on a continuous basis. Here the molten ash collects on the furnace floor, is made to flow over a weir located in the floor of the furnace, and drops into a bath of water below. Later the ash is removed from the hopper hydraulically. Or the molten ash may be permitted to remain and collect on the furnace floor to be tapped off at intervals. On being discharged, the molten ash encounters a jet of high-velocity water; the chilling of the ash causes it to break up into a fine granular form for ease of disposal.
Wet-bottom furnaces (often called slag-tap furnaces) have been used for both pulverized coal and cyclone firing systems. Cyclone furnaces were developed to burn crushed coal and to form a molten sticky slag layer. They were designed to burn coals that were not well suited to pulverized coal firing.
The use of slag-tap boilers designed for pulverized coal firing declined in the early 1950s when there were significant design improve-ments in dry-bottom units and these minimized the ash-deposition problems. However, slag-tap boilers with cyclone furnaces continued to be used until the mid-1970s, when environmental restrictions imposed limits on NOxemissions that mandated that NOxbe controlled.
The slag-tap boilers operated at high furnace temperatures, and this resulted in high NOxformation. Cyclone furnaces still remain in oper-ation; however, other means for controlling NOxformation have to be incorporated as part of the boiler design.
All modern pulverized coal-fired boilers use the dry-bottom arrangement. In a dry-bottom boiler, approximately 70 to 80 percent of the ash is entrained in the flue gas and carried out of the furnace.
This portion of the ash is known as fly ash. Some of the fly ash is col-lected in hoppers that are arranged under the economizer and air
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heater, where the coarse particles of the fly ash fall out of the flue gas stream when changes in flue gas direction occur. The remaining fine particles of the fly ash are captured subsequently by either a precipi-tator or bag filterhouse as part of the environmental control system.
The 20 to 30 percent of ash that is not part of the fly ash is removed through a hopper at the bottom of the furnace.