01 POWER ISLAND / 01 CCPP / V. Ganapathy-Industrial Boilers and HRSG-Design (2003)

FIGURE 1.12b HRSG performance at 40% load of gas turbine.

arrive at the operating points at various loads. Because of the large variations that occur in drum pressure during sliding pressure operation, the drum level controls should be pressure-compensated.

As an example, using the HRSG simulation program, the effect of steam pressure on a single-pressure unfired HRSG was evaluated; the results are shown in Table 1.5. Note that when multiple-pressure HRSGs are involved, the

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TABLE 1.5 Effect of Steam Pressure on HRSG Performancea

aFeedwater temperature ¼ 230 F, heat loss ¼ 1%, blowdown ¼ 1%.

performance of a given module is affected by the module preceding it, so unless the configuration is known it is difficult to make generalized observations.

In the case for which data are given in Table 1.5, the HRSG was designed to generate steam at 1000 psia and 800 F and the off-design performance was evaluated at selected pressures.

The steam flow decreases as the pressure increases due to the higher saturation temperature, which limits the temperature profiles.

The exit gas temperature increases as the pressure increases, again due to the higher saturation temperature.

The steam temperature does not vary by much.

The duty or energy absorbed by steam decreases as pressure increases due to the higher exit gas temperature.

AUXILIARY FIRING IN HRSGs

Supplementary firing is an efficient way to increase the steam generation in HRSGs. Additional steam in the HRSG is generated at an efficiency of nearly 100% as shown in Q8.38. Typically, HRSGs in combined cycle plants are unfired and those in cogeneration plants are fired. The merits of auxiliary firing in HRSGs are discussed in Q8.38. Figure 1.13 shows the arrangement of a supplementary-fired HRSG, which can handle a firing temperature of about 1600 F. Typically, oil or natural gas is the fuel used. Figure 1.14 shows a furnacefired HRSG, which can be fired up to 3000 F. The superheater is shielded from the flame by a screen section. The furnace should be large enough to enclose the flame. In furnace-fired HRSGs even a solid fuel can be fired and the HRSG design approaches that of a conventional steam generator. Water-cooled membrane walls ensure that the casing is kept cool. A large amount of steam can be generated in this system. Table 1.6 compares the features of unfired, supplementary-fired, and furnace-fired HRSGs.

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FIGURE 1.13 Multipressure supplementary-fired HRSG.

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FIGURE 1.14a Furnace-fired HRSG arrangement.

Combined Cycle Plants and Fired HRSGs

It is generally believed that combined cycle plant efficiencies with fired HRSGs are lower than those with unfired HRSGs. The reason is not the poor performance of the HRSG. In fact, a fired HRSG by itself is efficient. However, the large losses associated with the Rankine cycle, particularly when the steam turbine power is a large fraction of the overall power output, distorts the results slightly as the following example shows.

FIGURE 1.14b Photograph of a furnace-fired ABCO HRSG in a cogeneration plant.

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TABLE 1.6 General Features of Fired and Unfired HRSGs

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Example 1

A combined cycle plant uses a fired HRSG. The gas turbine used is LM 5000. At 59 F,

Exhaust gas flow ¼ 1,030,000 lb=h at 800 F.

Gas analysis, vol%: CO2 ¼ 2.8, H2O ¼ 8.5, N2 ¼ 74.4, O2 ¼ 14.3 Power output ¼ 35 MW; heat rate ¼ 9649 Btu=kWh

Steam turbine data:

Inlet pressure ¼ 650 psia at 750 F Exhaust pressure ¼ 1 psia

Efficiency ¼ 80%, dropping off by 2–3% at 40% load. HRSG data:

230 F feedwater, 2% blowdown, 1% heat loss Steam is generated at 665 psia and 750 F.

The HRSG generates 84,400 lb=h in the unfired mode and a maximum of 186,500 lb=h when fired up to 1200 F. The HRSG performance was simulated by using the HRSGS program. The system efficiency in both cogeneration and combined cycle mode are calculated as follows:

Gas turbine fuel input ¼ 35;000 9649 ¼ 337:71 MM Btu/h,

lower heating value (LHV) basis.

Cogeneration mode efficiency at 900 F, from first principles (or fundamentals) ¼

ð35 3:413 þ 129:9Þ 100 ¼ 67:9% 337:71 þ 29:6

where 129.9 MM Btu=h is the HRSG output and 29.6 MM Btu=h is the HRSG burner input in LHV (lower heating value basis).

Combined cycle mode efficiency:

ð35 þ 12:1Þ 3:413 100 ¼ 43:8% 337:71 þ 29:6

where 12.1 MW is the power output from the steam turbine.

Table 1.7 shows the results at various HRSG firing temperatures. Cogeneration plant efficiency improves with firing in the HRSG as

discussed earlier. The combined cycle plant efficiency drops only because of the lower efficiency of the Rankine system as the proportion of power from the Rankine cycle increases. The HRSG, as can be seen, is efficient in the fired mode with a slightly lower stack gas temperature.

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TABLE 1.7 Cogeneration and Combined Cycle Efficiency with Fired HRSG

aBoiler duty is the energy absorbed by steam, MM Btu=h. bBurner duty is the fuel input to HRSG, MM Btu=h, LHV basis.

Generating Steam Efficiently in Cogeneration Plants

Today’s cogeneration plants have both HRSGs and packaged steam generators. To generate a desired quantity of steam efficiently, the load vs. efficiency characteristics of both the HRSG and steam generator should be known. Although the generation of steam with the least fuel input is the objective, it may not always be feasible, for reasons of plant loading, availability or maintenance, However the information is helpful for planning purposes [13].

To explain the concept, an HRSG and a packaged boiler both capable of generating up to 100,000 lb=h of 400 psig saturated steam on natural gas are considered. In order to understand how the cogeneration system performs, one should know how the HRSG and the steam generator perform as a function of load. Figure 1.15 shows the load vs. efficiency characteristics of both the HRSG and packaged boiler. The following points may be noted.

1.The exit gas temperature from the HRSG decreases as the steam generation is increased. This is due to the fact that the gas flow remains the same while the steam flow increases, thus providing a larger heat sink at the economizer as discussed earlier. On the other hand, the exit gas temperature from the steam generator increases as the load increases because a larger quantity of flue gas is handled by a given heat transfer surface.

2.The ASME HRSG efficiency increases as firing increases as explained in Q8.38. The range between the lowest and highest load is significant. The steam generator efficiency increases slightly with load, peaks around 60–75%, and drops off. The variation between 25% and 100% loads is marginal. This is due to the combination of exit gas losses and casing heat losses. The casing loss is nearly unchanged with load in Btu=h but increases as a percentage of total loss at lower loads. The

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FIGURE 1.15 Load versus efficiency characteristics of HRSG and steam generator.

flue gas heat loss is lower at lower loads due to the lower exit gas temperature and mass flow.

Performance calculations were done at loads ranging from 25% to 100% for both the steam generator and the HRSG. Results are presented in Tables 1.8 and

TABLE 1.8 Steam Generator Performance at Various Loadsa

aSteam pressure ¼ 400 psig; feedwater ¼ 230 F, blowdown ¼ 5%. Fuel: natural gas. C1 ¼ 97; C2 ¼ 2; C3 ¼ 1 vol%

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TABLE 1.9 HRSG Performance at Various Loadsa

aSteam pressure ¼ 400 psig; feedwater ¼ 230 F; 5% blowdown. Fuel input is on LHV basis.

1.9. Additional performance calculations may also be done for intermediate steam generation values. Table 1.10 presents the total fuel required for a given total steam output and shows the split between the boiler and HRSG steam generation.

It is obvious that the HRSG should be used first to make any additional steam, because its fuel utilization is the best. However, if for some reason we cannot operate the HRSG, then information on how the total fuel consumption varies with the loading of each type of boiler helps in planning. For example, if 100,000 lb=h of steam is required, the steam generator can be shut off completely and the HRSG can be fully fired; the next best mode is to run the HRSG at

TABLE 1.10 Fuel Consumption at Various Loads

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75,000 lb=h and the boiler at 25,000 lb=h or in that range. A similar table may be prepared if there are multiple units in the plant, and by studying the various combinations a plan for efficient fuel utilization can be developed. Note that a typical packaged boiler generates steam at about 92% efficiency on LHV basis, whereas it is nearly 100% if the same amount of fuel (gas or oil) is fired in an HRSG.

Cogeneration Plant Applications

The steam parameters of combined cycle and cogeneration plants differ significantly.

Combined cycle plants typically use unfired HRSGs and generate multiple- pressure-level steam with a complex arrangement of heating surfaces to maximize energy recovery. Fired HRSGs in combined cycle plants are often the exception to the rule owing to their impact on cycle efficiency as discussed above.

In cogeneration plants, a large amount of steam is required and hence supplementary or furnace-fired HRSGs are common. With a high gas inlet temperature, a single-pressure HRSG can often cool the gases to a reasonably low temperature, so single-pressure steam generation is often adequate.

In cogeneration plants, saturated steam is often imported from other boilers to the HRSG to be superheated; steam may also be exported from the HRSG to other plants.

Combined cycle plant HRSGs often operate at steady loads, cogeneration plant steam demand often fluctuates and is a function of the process. Given below is an example of an HRSG simulation in a cogeneration plant. Note the effect on steam temperature with and without the export steam.

Example 2

Exhaust gas flow from a gas turbine is 250,000 lb=h at 1000 F. Gas analysis in percent by volume (vol %) is CO2 ¼ 3, H2O ¼ 7, N2 ¼ 75, and O2 ¼ 15. Superheated steam is generated at 600 psia at 875 F, and about 20,000 lb=h of saturated steam is required for process, which is taken off the steam drum. Predict the HRSG gas=steam profiles. Use 20 F pinch and approach points, 230 F feedwater, and 1% blowdown and heat loss.

In the off-design mode, process steam is not required. Steam pressure is 650 psia. Determine the HRSG performance. Steam temperature is uncontrolled.

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