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

FIGURE 2.15 Waste heat boiler in carbon black plant. (Courtesy of ABCO Industries, Abilene, TX.)

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also equipment such as scrubbers downstream of the boiler. When fouling sets in, the steam generation decreases and the gas pressure drop increases over a period of time. There are a few ways to infer if the fouling has become severe:

1.The exit gas temperature from the boiler will increase over a period of

time; if, say, the normal exit gas temperature from the convection bank is 550 F and we observe 570–600 F for the same load, then we can infer that fouling has set in. Fouling deposits build up over heat transfer surfaces (whether inside or outside), and the fouling factor increases exponentially and then tapers off as shown in Fig. 2.16. With periodic cleaning some of the deposits are removed, which decreases the fouling factor, but a base layer builds up and increases the exit gas temperature and decreases the boiler duty. A complete shutdown and cleaning may help restore the original boiler performance or close to it.

2.The gas pressure drop across the convection section increases. If the fan power consumption increases over a period of time, then one can infer that there is some blockage of the gas path and that fouling has set in.

3.Steam generation naturally decreases with fouling.

4.Superheated steam temperature, if a superheater is present, has to be looked at carefully, because fouling in different sections may be different, and one cannot conclude that there is fouling at a given surface without having data on the gas inlet and exit temperatures and steam inlet and exit temperatures and flows. Sometimes steam-side fouling is caused by deposition of salts from steam. Steam-side fouling can increase the tube wall temperatures and cause overheating as discussed in Q8.13. Steam-side fouling is more critical in finned water tube boilers, as discussed in Q8.24.

One has to shut down the boiler and perform an investigation if fouling is severe. Normal fouling may be acceptable between maintenance shutdowns. Heat transfer calculations backed up with field data and tube wall temperature

FIGURE 2.16 Fouling in waste heat boilers versus time.

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measurements can also show if the fouling is on the gas side or steam side or both. With gas-side fouling the tube wall temperatures will not increase, whereas with steam-side fouling the tube wall temperatures can increase significantly. With a combination of gasand steam-side fouling, the measurement of operating data on each side, followed by elaborate calculations, can reveal the extent of fouling.

Because both fire tube and water tube boilers are used in HRSGs, a few guidelines on their sizing are in order.

FIRE TUBE BOILER DESIGN CONSIDERATIONS

The sizing procedures for fire tube boilers are discussed in Q8.10. It may be noted that the tube size plays a significant role in minimizing the length of the boiler. With small gas flows, one may consider multi-gas-pass design, which can reduce the overall length. Tube sizes vary from 1.5 to 2.5 in. OD; smaller tubes generally have lower tube wall temperatures and also require less surface area and shorter tube length. Hence a comparison of surface areas of two or more designs should be made with caution. Heat fluxes are quite low in fire tube boilers owing to low gas-side heat transfer coefficients, an exception being gas streams in hydrogen plants, as discussed earlier. SA 178a carbon steel tubes are typically used for evaporators handling common flue gases. In reformed gas applications, T11 or T22 tubes are preferred. Gas pressure drop can range from 3 to 6 in. WC in flue gas heat recovery boilers and about 1 psi in high gas pressure applications such as reformed gas boilers.

Boiler circulation may be checked using methods discussed in Q7.32. With poor water quality, fouling and scale formation are of concern, and tube wall temperatures can increase significantly with scale thickness as discussed in Q8.13.

Elevated steam drum design is generally used if the steam purity has to be less than 1 ppm. External downcomers and risers help cool the tubes and tube sheet by circulating the water–steam mixture over them. If the flue gas temperature is below 1500 F, then an elevated drum design may be dispensed with and a single-shell fire tube boiler may be used. The steam purity without internals is low, on the order of 5–15 ppm, which may be adequate for low pressure process heating applications.

Owing to the large inventory of water, fire tube boilers respond slowly to load changes compared with water tube units. However, the pressure decay on loss of heat input will also be smaller.

WATER TUBE BOILER DESIGN CONSIDERATIONS

The design procedure for waste heat boilers is quite involved. With a given set of inlet gas conditions such as flow and temperature, we have to see how the various

Copyright © 2003 Marcel Dekker, Inc.

heating surfaces respond. The surfaces could consist of bare or finned tubes. The superheater could have one or more stages; a screen section may or may not be used. Import steam could come from another boiler to be superheated in the boiler in question, or saturated steam may be drawn off the steam drum for deaeration or process purposes. The feedwater temperature or steam pressure could vary depending on plant facilities.

Before attempting to evaluate the performance of a complete waste heat boiler, one must first know how to obtain the performance of individual components such as the superheater, evaporator, and economizer by using the number of transfer units (NTU) method or through trial and error. This is discussed in Q8.29 and Q8.30. Once we know how to evaluate the performance of each surface, evaluating the overall performance of a waste heat boiler is simple. Figure 2.17 shows the logic for a simple waste heat boiler consisting of a superheater, evaporator, and economizer. A few iterations may be required, because we have to first assume a steam flow and completely solve all the other sections and then check on whether the assumed steam flow was fine. A

FIGURE 2.17 Logic used for evaluating HRSG performance.

Copyright © 2003 Marcel Dekker, Inc.

computer program is required, because these calculations become tedious with two-stage superheaters with attemperation, a combination of bare and finned tubes in evaporators, and the use of import or export steam, to mention a few variables. Also the incinerator may operate at different combinations of gas flows, temperatures, and gas analysis. The performance has to be checked at different operating points before finalizing it.

Figure 2.18 shows the printout of results for a water tube waste heat boiler for a gas turbine exhaust consisting of a furnace section, a screen section, a twostage superheater, an evaporator consisting of bare and finned tubes, and a finned tube economizer. In the unfired mode this HRSG makes about 45,000 lb=h of steam. The turbine exhaust enters the HRSG at 980 F, which is raised to 2175 F by the burner located at the HRSG inlet to generate 150,000 lb=h of steam at 620 psig and 750 F. The oxygen content has decreased from 15% to 8.39% by volume and the burner duty is 123 MM Btu=h on LHV basis. The gas temperature drops to 2063 F in the furnace section and is cooled to 1852 F in the screen section before entering the superheater. The gas pressure drop in the HRSG is about 6 in. WC. To this must be added the burner, selective catalytic reduction (SCR), and duct losses. The printout also shows the tube wall temperatures, fin tip temperatures, heat transfer coefficients at various sections both inside and outside the tubes, and the gasand steam=water-side pressure drops. The amount of spray water used for attemperation is also computed. Several variables can be changed to check the effect on performance. The evaporator uses different fin configurations. This is done to minimize the heat flux inside the evaporator tubes and also the tube wall and fin tip temperatures. The boiler duty is 177 MM Btu=h. The fuel used is typically natural gas.

Boiler tube sizes typically range from 1.5 to 2.5 in. and fin density can vary from 2 to 6 fins=in. depending upon the design. Bare tube boilers are used in dirty gas applications. Sometimes multipass designs offer a compact design. Whereas with finned tubes, both in-line and staggered arrangements are used, an in-line arrangement is generally used with bare tubes because it is inefficient to use a staggered arrangement, as discussed in Q8.22. Tube spacing can vary depending on gas velocity, dirtiness of the gas stream, and heat transfer considerations. A radiant furnace is also used if the incoming gas is at a high temperature and has the potential to cause slagging problems. Superheaters can be of bare tube or finned tube design, depending upon the gas temperature and cleanliness. Generally a low fin density is preferred for superheaters owing to the low heat transfer coefficient inside tubes, as discussed in Q8.22 and Q8.27. Superheater tubes can be vertical or horizontal depending on size or layout considerations. Economizers are of bare tube design in dirty gas applications and use finned tubes in clean gas applications. In sulfuric acid plants, a few suppliers use cast iron gilled tubes.

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FIGURE 2.18 Printout of HRSG performance.

Copyright © 2003 Marcel Dekker, Inc.

PREDICTING HRSG DESIGN AND OFF-DESIGN

PERFORMANCE USING HRSG SIMULATION

It is possible to predict the performance of water tube HRSGs in clean gas applications by using a simulation process instead of physically designing the unit. Thus anyone familiar with heat balances such as consultants and those planning cogeneration or combined cycle plants can obtain a good idea of the performance of the HRSG under various modes of operation. This information may be used to arrive at the HRSG configuration and optimize the major parameters for the steam system. Several ‘‘what if’’ scenarios may be looked at. The performance of an existing HRSG may also be evaluated to see if its performance is reasonable, as discussed in Q8.45. Though simulation may be used for any clean gas convective type of HRSG, it is particularly useful in gas turbine applications, because the HRSG designs involving multiple-pressure, multiple-module designs are more complex as discussed in Chapter 1.

Because of the large amount of exhaust gases and the low inlet gas temperature of an HRSG, one cannot arbitrarily assume an exit gas temperature and compute the steam generation. The problem of evaluating steam generation and temperature profiles gets complicated further as it is not often possible to recover a substantial amount of energy from the exhaust gases with steam at a single pressure level. Multiple-pressure steam generation with split modules alone can optimize energy recovery, makes the task of performing energy balance calculations very tedious. Gas and steam temperature profiles and hence energy balance in an HRSG are governed by what are called pinch and approach points (Fig. 2.17) Q8.34 and Q8.37 explain this in greater detail.

Basically, we estimate the term UA, the product of the overall heat transfer coefficient and the surface area, for each heating surface in the design mode and then correct it for the effect of gas flow, temperature, and analysis. Using this corrected UA, one can use the NTU method to evaluate the performance of any exchanger and then the overall performance.

The HRSGS Program

I have developed a simulation program called HRSGS to perform these complex design and off-design performance calculations. Basically the desired HRSG configuration is built up by using the six basic modules shown in Fig. 2.19. By using the common economizer or common superheater concept, one can configure complex multiple-pressure HRSGs, as shown in the examples in the figure. Up to 10 modules or nine pressure levels can be evaluated. The program automatically arrives at the firing temperature and the fuel requirement if the desired steam quantity is known with both turbine exhaust and fresh air cases. It checks for steaming in the economizer and handles import or export steam from evaporators as illustrated by a few examples in Chapter 1 (See p. 26 and 36). It

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FIGURE 2.19 How basic modules may be combined to arrive at complex HRSG configurations.

Copyright © 2003 Marcel Dekker, Inc.

computes the ASME efficiency and prints out the US values for each surface in the design and off-design modes as shown in several examples in Chap. 1.

The simulation program is generally used for convective-type HRSGs and waste heat boilers, which operate on clean gas streams. If a radiant furnace is used, there will be some variation between the actual and predicted values. Because of the large fouling factors involved in dirty gas applications, the heat transfer coefficients cannot be corrected for off-design conditions accurately; hence there will be some deviation between predicted and actual performance if this is used in, say, municipal waste applications. For more information about the program, please contact the author at v_ganapathy@yahoo.com or visit the web site http://vganapathy.tripod.com/boilers.html.

SPECIFYING WASTE HEAT BOILERS

The following points may be considered while developing specifications for heat recovery applications.

1.Because there are numerous applications of heat recovery, it is always good practice to start off the specifications by describing the process that generates the flue gases, because that gives an idea of the nature of the gas stream. With a clean gas stream, finned tubes could be used to make the boiler design compact, whereas a dirty gas with slagging potential must have bare tubes, with provision for cleaning the surfaces. Process gas applications such as hydrogen plants or sulfuric acid plant boilers require exit gas temperature control systems.

2.Desired steam purity should be mentioned, particularly if the steam generated is used in a gas or steam turbine. Also, based on load swings, one could arrive at the proper size for the steam drum.

3.The extent of optimization required and the cost of fuel, electricity, and steam should be indicated. For example, simply stating the inlet

gas conditions and steam parameters may not be adequate. If design A cools the gas to, say, 450 F and design B cools it to, say, 400 F by using a larger boiler at higher cost, how is this to be evaluated? Also, if for the same steam parameters, one design has 6 in. WC pressure drop and another has 4 in. WC, is there any way to evaluate operating costs? Such an indication in the specifications will help the designer to review the design and balance the installed and operating costs.

4.Space availability and layout considerations should be indicated. Sometimes a boiler is built before the builder finds out that it has to be located inside a building that has already been constructed.

5.The steam system should be clearly described. Often only the makeup water conditions are given without an indication of where the steam to

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the deaerator comes from. If the steam is taken from the boiler itself, then the design is likely to be affected, particularly if a superheater is present. Hence a scheme showing the complete steam–water system for the plant will be helpful. In waste heat boilers, sometimes import steam from another source is superheated in the boiler. This affects the superheater and boiler performance, particularly when the import steam supply is reduced or cut off.

6.Often feedwater is used for desuperheating steam to control its temperature. This water should have zero solids and should preferably be demineralized. Softened water will add solids to the steam if used directly as spray, so one may have problems with solid deposits, fouling, and overheating of superheater tubes and possible deposition of solids in the steam turbine blades. If demineralized water is not available and that is so stated up front, the designer could come up with a sweet water condensing system to obtain the desired spray water for steam temperature control (see Chap. 3). The feedwater analysis is also important because it affects blowdown rates.

7.Gas flow should be stated in mass units. Often volumetric units are given and the writer of the specifications has no idea if it is actual cubic feet per minute or standard cubic feet per minute; then without the gas analysis, it is difficult to evaluate the density or the mass flow. The ratio between standard and actual cubic feet per minute of flue gas could be nearly 4 depending on the gas temperature. The problem is resolved if the flue gas mass flow is given in pounds per hour or kilograms per hour.

8.Flue gas analysis is important. We have seen that the presence of water vapor or hydrogen in flue gases increases the heat transfer coefficient and also affects the specific heat and temperature profiles of the gas. The presence of corrosive gases such as hydrogen chloride, sulfur trioxide, and chlorine suggests the possibility of corrosion. The boiler duty for the same gas temperature drop and mass flow could be different if one designer assumes a particular flue gas analysis and another designer assumes another. Hence flue gas analysis should be stated as well as the gas pressure. High gas pressure, on the order of even 1–2 psi, affects the casing design and cost.

9.With HRSGs, one should perform a temperature profile analysis before arriving at the steam generation values. As shown in Q8.36, assuming an exit gas temperature and computing HRSG duty or steam generation on that basis can lead to errors.

10.Emission levels of NOx; CO, and other pollutants required at the exit of the HRSG or waste heat boiler should be stated. In such cases, information on pollutants in the incoming gases should also be given.

Copyright © 2003 Marcel Dekker, Inc.