01 POWER ISLAND / 01 CCPP / V. Ganapathy-Industrial Boilers and HRSG-Design (2003)
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11.Fuel analysis should be provided for a fired HRSG or boiler. Also, the cost of fuel helps to determine if a design can be optimized by using a larger boiler and smaller fuel consumption or vice versa.
12.If the boiler is likely to operate for a short period only or weekly or is being cycled, then this information should also be given. Frequent cycling requires some considerations in the design to minimize fatigue stresses. Provisions for keeping the boiler warm during shutdown may also be necessary.
In addition, local code requirements, site ambient conditions, and constructional features, if any, should be mentioned.
REFERENCES
1.V Ganapathy. Simulation aids cogeneration system analysis. Chemical Engineering Progress, October 1993.
2.V Ganapathy. Evaluating gas turbine heat recovery boilers. Chemical Engineering, Dec 7, 1987.
3.V Ganapathy. Evaluating waste heat boiler systems. Plant Engineering, Nov 22, 1990.
4.V Ganapathy. HRSG features and applications. Heating, Piping, Air-Conditioning, January 1989.
5.V Ganapathy. Simplify HRSG evaluation. Hydrocarbon Processing, March 1990.
6.V Ganapathy. Fouling—the silent heat transfer thief. Hydrocarbon Processing, October 1992.
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3
Steam Generators
INTRODUCTION
Steam generators, or boilers as they are often called, form an essential part of any power plant or cogeneration system. The steam-based Rankine cycle has been synonymous with power generation for centuries. Though steam parameters such as pressure and temperature have been steadily increasing during the last several decades, the function of the boiler remains the same, namely, to generate steam at the desired conditions efficiently and with low operating costs. Low pressure steam is used in cogeneration plants for heating or process applications, and high pressure superheated steam is used for generating power via steam turbines. Steam is used in a variety of ways in process industries, so boilers form an important part of the plant utilities. In addition to efficiency and operating costs, another factor that has introduced several changes in the design of boilers and associated systems is the stringent emission regulations in various parts of the world. As discussed in Chapter 5, the limits on emissions of NOx; CO; SOx, and particulates have impacted the design and features of steam generators and steam plants, not to mention their costs. Today’s cogeneration systems and power plants resemble chemical plants with NOx; SOx, and particulate control systems forming a major portion of the plant equipment. Oiland gas-fired packaged boilers used in cogeneration and combined cycle plants have also undergone significant changes during the last few decades. Selective catalytic reduction
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systems (SCRs) are used even in packaged boilers for NOx control, adding to their complexity and costs.
Steam pressure and temperature ratings of large utility boilers have been increasing in order to improve overall plant efficiency. Several supercritical plants have been built during the last decade. There have been improvements in the design of packaged boilers too. Figure 3.1 shows the general arrangement of a packaged steam generator. The standard refractory-lined packaged boilers of the last century are being slowly replaced by custom-designed boilers with completely water-cooled furnaces (Fig. 3.2). The air heater that was once an integral part of oiland gas-fired boilers is now replaced by the economizer, which helps to
FIGURE 3.1 Package water tube boiler. (Courtesy of ABCO Industries, Abilene, TX.)
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FIGURE 3.2 Completely water-cooled furnace design. (Courtesy of ABCO Industries, Abilene, TX.)
lower NOx levels. To improve efficiency, a few plants are even considering the use of condensing economizers.
Though pulverized coal–fired boilers form the backbone of utility plants, fluidized bed boilers are finding increasing application when it comes to handling solid fuels with varying moisture, ash, and heating values; they also generate lower emissions of NOx and SOx. Oiland gas-fired fire tube boilers (Fig. 3.3) are widely used in small process plants for generating low pressure saturated steam. Though different types of boilers are mentioned in this chapter, the emphasis is on the oiland gas-fired packaged water tube steam generator, which is fast becoming a common sight in every cogeneration and combined cycle plant.
BOILER CLASSIFICATION
The terms boiler and steam generator are often used in the same context. Boilers may be classified into several categories as follows:
By Application: Utility, marine, or industrial boiler. Utility boilers are the large steam generators used in power plants generating 500–1000 MW of
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FIGURE 3.3a Fire tube boiler—wetback design.
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FIGURE 3.3b Fire tube boiler—dryback design.
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electricity. They are generally fired with pulverized coal, though fluidized bed boilers are popping up in some plants. Utility boilers generate high pressure, high temperature superheated and reheat steam; typical parameters are 2400 psig, 1000=1000 F. A few utility boilers generate supercritical steam at pressures in excess of 3500 psig, 1100=1100= 1100 F. Double reheat cycles are also in operation. Industrial boilers used in cogeneration plants generate low pressure steam at 150 psig to superheated steam at 1500 psig at temperatures ranging from 700 to 1000 F.
By Pressure: Low to medium pressure, high pressure, and supercritical pressure. Process plants need low to medium pressure steam in the range of 150–1500 psig, which is generated by field-erected or packaged boilers, whereas large utility boilers generate high pressure (above 2000 psig) and supercritical pressure steam.
By Circulation Method: Natural, controlled, once-through, or combined circulation. Figure 3.4 illustrates these concepts. Natural circulation is widely used for up to 2400 psig steam pressure. There is no operating cost incurred for ensuring circulation through the furnace tubes, because gravity aids the circulation process. Controlled and combined circulation boilers use pumps to ensure circulation of a steam–water mixture through the evaporator tubes. Supercritical boilers are of the oncethrough type. It may be noted that once-through designs can be employed at any pressure, whereas supercritical pressure boilers must be of a once-through design.
By Firing Method: Stoker, cyclone furnace, fluidized bed, register burner, fixed or moving grate.
By Construction: Field-erected or shop-assembled. Large industrial and utility boilers are field-erected, whereas small packaged fire tube boilers up to 90,000 lb=h capacity and water tube boilers up to 250,000 lb=h are generally assembled in the shop. Depending on shipping dimensions, these capacities could vary slightly.
By Slag Removal Method: Dry or wet bottom, applicable to solid-fuel-fired boilers.
By Heat Source and Fuel: Solid, gaseous, or liquid fuels, waste fuel or waste heat. Waste heat boilers are discussed in Chapter 2. The type of fuel used has a significant impact on boiler size. For example, coal-fired boiler furnaces are large, because a long residence time is required for coal combustion, whereas oiland gas-fired boilers can be smaller, as shown in Fig. 3.5.
According to Whether Steam is Generated Inside or Outside the Boiler Tubes: Fire tube boilers (Fig. 3.3), in which steam is generated outside the tubes, are used in small plants up to a capacity of about 60,000 lb=h
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FIGURE 3.4 Boiler circulation methods. (a) Natural; (b) forced circulation; (c) once-through; (d) once-through with superimposed circulation. 1, Economizer; 2, furnace; 3, superheater; 4, drum; 5, orifice; 6, circulating pumps; 7, separator.
FIGURE 3.5 The impact of fuel on furnace size.
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of saturated steam at 300 psig or less; they typically fire oil or gaseous fuels. Water tube boilers, in which steam is generated inside the tubes, can burn any fuel, be of any size, and operate at any pressure but are generally economical above 50,000 lb=h capacity. See Chap. 2 for a comparison between fire tube and water tube waste heat boilers.
STEAM PRESSURE AND BOILER DESIGN
The energy absorbed by steam is distributed among feedwater heating (sensible heat), boiling (latent heat), superheating, and reheating functions. The distribution ratios are a function of steam pressure, as can be seen from steam tables or from Fig. 3.6. If the latent heat is large as in low pressure steam, a large furnace is required for the boiler; as the pressure of steam increases, the latent heat portion decreases and the superheat and reheat energy absorption increases. The boiler design accordingly varies with large surface areas required for the superheaters and reheaters and a small furnace with little or no convective evaporator surface in particular. The sensible heat, which is absorbed in the economizer, is also high at high pressure. The distribution of energy among the various surfaces—the furnace, evaporator, superheater, reheater, and economizer—is somewhat flexible, as will be shown later, but it must be emphasized that steam pressure plays a significant role in determining the sizes of these surfaces.
FIGURE 3.6 Distribution of energy in boilers as a function of steam pressure.
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In natural circulation units the density differential between the cooler water in the downcomers and the less dense steam–water mixture in the riser tubes of the furnace provides the hydraulic head for circulation of the steam–water mixture through the evaporator tubes. The circulation ratio, CR, which is the ratio of the mixture flow to steam flow, could be in the range of 6–8 in high pressure boilers. In packaged boilers operating at low steam pressure, say 150– 1000 psig, the CR could be higher, ranging from 10 to 20. Note that we are referring to an average value. The circulation ratio will differ for each parallel circuit, depending on its length, tube size, heat flux, and static head available, as discussed in Q7.29. The controlled circulation boiler is operated at a slightly higher steam pressure, around 2500–2600 psig, and flow is ensured through the furnace tubes by a circulating pump; which forces the boiler water through each circuit. The circulation ratio is preselected in the range of about 2–4. This is done to reduce the operating cost associated with the circulating pumps; also, the use of carefully selected orifices ensures the flow of the steam–water mixture through each circuit. Hence a low CR is used in these systems. The once-through unit with superimposed circulation requires the circulating pump during start-up and at low loads when flow through the circuits is not high and later switches to the once-through mode at higher loads.
PACKAGED STEAM GENERATORS
Packaged boilers are widely used in cogeneration and even in combined cycle plants as auxiliary boilers providing steam for turbine sealing and steam for other uses when the gas turbine trips and the HRSG is not in operation. These boilers are generally shop-assembled and custom-designed. Typically, boilers of up to 250,000 lb=h capacity can be shop-assembled and larger units are field-erected. Steam parameters vary from 150 psig saturated to 1500 psig, 1000 F. They typically burn natural gas, distillate fuel oils, and even heavy residual oils. Widely used methods for NOx control are low-NOx burners, flue gas recirculation, and selective catalytic reduction systems (SCRs). Carbon monoxide catalysts are also used if required. Emission control methods are discussed in Chapter 4.
Packaged boilers could be further classified as D, A, or O-type depending on their construction, as shown in Fig. 3.7. In the A- and O-type boilers, the flue gases exit the furnace and then make a 180 turn, split up into two parallel paths, and flow through the convection section, then recombine to flow through the economizer. Using a convective superheater in this type of boiler is tricky, because it has to be split into two halves. A radiant design may be located at the furnace exit, but it operates in a harsh environment as discussed later.
D-type boilers are widely used in industry. The flue gases generated in the furnace travel though the furnace, make a turn, and go through the convection
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