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

The firing temperature T is obtained as follows:

9:86 106 ¼ 140;000 0:3 ðT 980Þ

or

T ¼ 1215 F

Thus, by using a few simple concepts, preliminary information about the HRSG may be obtained. However, a complete temperature profile analysis requires a computer program such as the HRSG simulation software.

8.36

Q:

Can we assume that a particular exit gas temperature can be obtained in gas turbine HRSGs without doing a temperature profile analysis?

A:

No. It is not good practice to assume the HRSG exit gas temperature and compute the duty or steam generation as some consultants and engineers do. The problem is that, depending on the steam pressure and temperature, the exit gas temperature will vary significantly. Often, consultants and plant engineers assume that any stack gas temperature can be achieved. For example, I have seen catalogs published by reputable gas turbine firms suggesting that 300 F stack gas temperature can be obtained irrespective of the steam pressure or parameters. Now this may be possible at low pressures but not at all steam conditions. In order to arrive at the correct temperature profile, several heat balance calculations have to be performed, as explained below.

It will be shown that one cannot arbitrarily fix the stack gas temperature or the pinch point.

Looking at the superheater and evaporator of Fig. 8.13,

Looking at the entire HRSG,

Blowdown was neglected in the above equations for simplicity. Dividing Eq. (78) by Eq. (79) and neglecting variations in Cpg, we have

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Factor X depends only on steam parameters and on the approach point used. Tg3 depends on the pinch point selected. Hence if Tg1 is known, Tg4 can be calculated.

It can be concluded from the above analysis that one cannot assume that any HRSG exit gas temperature can be obtained. To illustrate, Table 8.27 shows several operating steam conditions and X values and exit gas temperatures. As the steam pressure or steam temperature increases, so does the exit gas temperature, with the result that less energy is transferred to steam. This also tells us why we need to go in for multiple-pressure-level HRSGs when the main steam pressure is high. Note that even with infinite surface areas we cannot achieve low temperatures, because this is a thermodynamic limitation.

Example 1

Determine the HRSG exit gas temperature when the gas inlet temperature is 900 F and the steam pressure is 100 psig sat.

Solution. X ¼ 0.904. Saturation temperature ¼ 338 F. Hence with a 20 F pinch point, Tg3 ¼ 358 F, and tw2 ¼ 323 F with a 15 F approach point,

Example 2

What is Tg4 when steam pressure is 600 psig and temperature is 750 F?

TABLE 8.27 HRSG Exit Gas Temperaturesa

aBased on 15 F approach point, 20 F pinch point, 900 F gas inlet temperature, and no blowdown. Feedwater temperature is 230 F. Similar data can be generated for other conditions.

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So a 300 F stack temperature is not thermodynamically feasible. Let us see what happens if we try to achieve that.

Example 3

Can you obtain a 300 F stack gas temperature with 900 F inlet gas temperature and at 600 psig, 750 F, and 15 F approach temperature?

Solution. X ¼ 0.7728. Let us see, using Eq. (80), what Tg3 results in a Tg4 of 300 F, because that is the only unknown.

ð900 Tg3Þ=ð900 300Þ ¼ 0:7728; or Tg3 ¼ 436 F

which is not thermodynamically feasible because the saturation temperature at 615 psig is 492 F! This is the reason one has to be careful in specifying HRSG exit gas temperatures or computing steam generation based on a particular exit gas temperature.

Example 4

What should be done to obtain a stack gas temperature of 300 F in the situation described in Example 3?

Solution. One of the options is to increase the gas inlet temperature to the HRSG by supplementary firing. If Tg1 is increased, then it is possible to get a lower Tg4. Say Tg1 ¼ 1600 F. Then

This is a feasible temperature because the pinch point is now (595 7 492) ¼ 103 F. This brings us to another important rule: Pinch point and exit gas temperature cannot be arbitrarily selected in the fired mode. It is preferable to analyze the temperature profiles in the unfired mode and evaluate the off-design performance using available simulation methods discussed earlier.

Example 5

If gas inlet temperature in Example 1 is 800 F instead of 900 F, what happens to the exit gas temperature at 100 psig sat?

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Solution.

800 358 ¼ 0:904

800 Tg4

or Tg4 ¼ 312 F versus the 300 F when the inlet gas temperature was 900 F. We note that the exit gas temperature increases when the gas inlet temperature decreases, and vice versa. This is another important basic fact.

Once the exit gas temperature is arrived at, one can use Eq. (79) to determine how much steam can be generated.

8.37

Q:

How can HRSG simulation be used to optimize gas and steam temperature profiles?

A:

HRSG simulation is a method of arriving at the design or off-design performance of HRSGs without physically designing them as shown in Q8.34. By using different pinch and approach points and different configurations, particularly in multipressure HRSGs, one can maximize heat recovery. We will illustrate this with an example [12].

Example

A gas turbine exhausts 300,000 lb=h of gas at 900 F. It is desired to generate about 20,500 lb=h of superheated steam at 600 psig and 650 F and as much as 200 psig saturated steam using feedwater at 230 F. Using the method discussed in Q8.34, we can arrive at the gas=steam temperature profiles and steam flows. Figure 8.15 shows results obtained with HRSGS software. In option 1, we have the high pressure (HP) section consisting of the superheater, evaporator, and economizer followed by the low pressure (LP) section consisting of the LP evaporator and economizer. By using a pinch point of 190 F and approach point of 15 F, we generate 20,438 lb=h of high pressure steam at 650 F. Then, using a pinch point of 20 F and approach point of 12 F, we make 18,670 lb=h low pressure steam. The stack gas temperature is 370 F. In option 2, we have the HP section consisting of the superheater and evaporator and the LP section consisting of only the evaporator. A common economizer feeds both the HP and LP sections with feedwater at 375 F. Because of the larger heat sink available beyond the LP evaporator, the stack gas temperature decreases to 321 F. The HP steam generation is adjusted using the pinch point to make 20,488 lb=h while the LP steam is allowed to float. With a pinch point of 20 F, we see that we can make

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Figure 8.15 Optimizing temperature profiles.

22,400 lb=h in comparison with the 18,673 lb=h earlier. The ASME system efficiency is much higher now. Thus by manipulating the HRSG configuration, one can maximize the heat recovery.

8.38

Q:

How is the HRSG efficiency determined according to ASME Power Test Code 4.4?

A:

The efficiency E is given by

energy given to steam=water=fluids

E ¼ gas flow inlet enthalpy þ fuel input on LHV basis

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To evaluate the efficiency, the enthalpy of the turbine exhaust gas should be known. The Appendix gives the enthalpy based on a particular gas analysis. Fuel input on LHV basis should also be known if auxiliary firing is used.

In Q8.37 the efficiency in the design case is

E ¼ ð21:4 þ 3:84Þ 106 ¼ 0:715; or 71:5% 140;000 242

If steam or water injection is resorted to, then the gas analysis will change, and the enthalpy has to be computed based on the actual analysis.

The HRSG system efficiency in gas turbine plants will improve with the addition of auxiliary fuel, which increases the gas temperature to the HRSG and hence increases its steam generation. There are two reasons for this.

1.Addition of auxiliary fuel reduces the effective excess air in the exhaust gases, because no air is added, only fuel. Hence the exhaust gas loss in relation to steam production is reduced.

2.With increased steam generation, usually the HRSG exhaust gas temperature decreases. This is due to the increased flow of water in the economizer, which offers a larger heat sink, which in turn pulls down the gas temperature further. In gas turbine units, the gas flow does not vary much with steam output as in conventional steam generators, which accounts for the larger temperature drop.

More information on HRSG temperature profiles can be found in Chapters 1 and 2.

Table 8.28 shows the performance of an HRSG under various operating conditions. Case 1 is the unfired case; cases 2 and 3 have different firing conditions. It can be seen that the system efficiency is higher when more fuel is fired, for reasons explained above.

TABLE 8.28 Data for Supplementary-Fired HRSG

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8.39a

Q:

In some cogeneration plants with gas turbines, a forced draft fan is used to send atmospheric air to the HRSG into which fuel is fired to generate steam when the gas turbine is not in operation. What should the criteria be for the fan size?

A:

The air flow should be large enough to have turbulent flow regimes in the HRSG and at the same time be small enough to minimize the loss due to exiting gases. If the air flow is high, the firing temperature will be low, but the system efficiency will be lower and the fuel input will be higher. This is illustrated for a simple case of two fans generating 250,000 and 210,000 lb=h of air flow in the HRSG. The HRSGS program was used in the simulation. See Table 8.29.

It can be seen that though the firing temperature is higher with the smaller fan, the efficiency is higher due to the lower exit gas losses considering the lower mass flow and exit gas temperature. It should be noted that as the firing temperature increases, the exit gas temperature will decrease when an economizer is used. Also, with the smaller fan the initial and operating costs are lower. One should ensure that the firing temperature does not increase to the point of changing the basic design concept of the HRSG. For example, an insulated casing design is used up to 1700 F firing temperature, beyond which a watercooled membrane wall design is required. See Chapter 1.

8.39b

Q:

How is the performance of an HRSG determined in fresh air fired mode?

A:

In this example, a multiple pressure HRSG with a common economizer is simulated in the design unfired mode and we are predicting its performance in the fired mode with fresh air firing using the HRSGS program.

TABLE 8.29 Fresh Air Firing Performance

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This is a three pressure level HRSG with HP steam at 600 psig, IP steam at 200 psig, and LP steam at 10 psig. (HP ¼ high pressure, IP ¼ intermediate pressure, LP ¼ low pressure.) A common economizer feeds the HP and IP steam. Once the pinch points for the HP, IP, and LP evaporators are suggested, the program arrives at the steam flows and temperature profiles as shown in Figs. 8.16a and 8.16b. The flow through the common economizer is arrived at after a

FIGURE 8.16a Unfired multipressure HRSG temperature profile.

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FIGURE 8.16b Fresh air fired temperature profile.

Copyright © 2003 Marcel Dekker, Inc.

few complex iterations. Figure 8.16a shows the design mode results from the HRSGS program.

In the off-design or fired mode, fresh air is used instead of turbine exhaust. The air flow used is close to the design exhaust gas flow. We input the ambient air flow and the desired HP steam flow, and the program asks for fuel analysis and automatically arrives at the firing temperature. The off-design performance is shown in Fig. 8.16b.

The efficiency according to ASME Power Test Code 4.4, US values of each surface in both the design and off-design modes may also be seen, as well as the exhaust gas analysis after combustion.

This is yet another example of how simulation may be used to perform various studies without a physical design of an HRSG. Consultants and planners of cogeneration or combined cycle projects should find this a valuable tool.

8.40

Q:

How do we evaluate alternative HRSG designs if the operating costs are different?

A:

Let us consider the design of two HRSGs, one with low pinch and approach points (and hence more surface area and gas pressure drop), called design A, and another with higher pinch and approach points, called design B, which costs less. These HRSGs operate in both unfired and fired modes for 50% of the time. In the fired mode, both HRSGs generate 70,000 lb=h of steam; in the unfired mode, design A naturally generates more steam. Table 8.30 shows the performance of the HRSGs in unfired and fired modes.

Let fuel cost $3=MM Btu (LHV). Cost of steam ¼ $3.5=1000 lb and electricity ¼ 6 cents=kWh. Assume that an additional 4 in.WC of gas pressure drop is equivalent to a 1% decrease in gas turbine power output, which is a nominal 8000 kW. The HRSG operates in unfired and fired modes for 4000 h=y each.

Design A has the following edge over design B in operating costs. Due to higher steam generation in unfired mode:

ð50;000 47;230Þ 3 40001000 ¼ $33;240

Due to lower fuel consumption:

ð22:55 19:23Þ 3:5 4000 ¼ $46;480

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