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

FIGURE 3.7 A-, D-, and O-type boiler configurations. 1, Burner; 2, steam drum; 3, mud drum.

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bank and then through the economizer to the stack. The gas flow is not split into two parallel paths as in the A- or O-type designs. If a superheater has to be located in the convection bank, the D-type design is the most convenient, because there is no concern with maldistribution in gas flow between parallel paths as with the O- and A-type boilers, which may lead to thermal performance issues. However, the O- and A-type boilers are more suitable as mobile units, because they have balanced weight distribution; rental boilers, which move from location to location, are generally of A- and O-type designs.

The gas-fired O-type boiler shown in Fig. 3.8 is another variation of packaged boiler design. In this boiler the flue gases do not make a turn at the furnace end; the gases flow straight beyond the furnace to a convection section consisting of bare and finned tubes; the finned tubes make the convection section compact, thus reducing the overall length of the boiler. The advantage of this design is that the width required is not large, because the width of the furnace determines the width of the unit, whereas in a typical O- or A-type boiler the width of the furnace is added to that of the convection bank, making it difficult to ship the boiler to certain areas of the country or the world. Also, a convective type of superheater can be easily located behind a screen section. The advantages of the convective superheater over a radiant design are discussed later.

A recent application for packaged boilers has been in combined cycle plants. These plants require steam for turbine sealing purposes when the HRSG trips, and they need it at short notice, say, within 5–15 min. Packaged boilers with completely water-cooled furnace designs are well suited for fast start-ups, as discussed later.

Very high steam purity as in utility plants can be obtained in packaged boilers through proper design of steam drum internals. Depending on the application, steam purity in the range of 30–100 parts per billion (ppb) can be

FIGURE 3.8 A gas-fired O-type package boiler with extended surfaces. (Courtesy of ABCO Industries, Abilene, TX.)

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achieved. Packaged boiler designs have evolved over the years and have adapted well to the needs of the industry.

Standard Boilers

Standard boilers, which are pre-engineered packages, are inexpensive and are used in applications that are not very demanding in terms of process or emission limits. Decades ago, various manufacturers had developed so-called standard designs for boilers of 40,000–200,000 lb=h capacity with fixed dimensions of furnace, tubes, tube spacing, lengths, and surface areas. If someone wanted a boiler for a particular capacity that was not listed, the next or closest standard model would be offered. Standard models are less expensive than custom designs because no engineering is required to design and build them. It must be borne in mind that these designs were developed 30–50 years ago when the concept of flue gas recirculation and low-NOx burners were unheard of. They also had a lot of refractory in their design—on the floor, front walls, and rear walls—because completely water-cooled furnace designs had not yet been developed. The concerns with refractory-lined boilers are discussed later. However, emission regulations are forcing suppliers to custom design the boilers.

As discussed in Chapter 4, the effect of flue gas recirculation and changes in excess air levels have to be reviewed on a case-to-case basis depending on the NOx and CO levels desired. Hence standard furnace dimensions may or may not be suitable for a given heat input, because the flame shape varies according to the NOx control method used. Flame lengths with low-NOx burners can be wider or even longer than with regular burners. Hence the use of low-NOx burners makes it difficult to select a standard boiler that meets the same need and is also an economical option. The furnace size could be compromised, which may result in flame impingement concerns with the burners used, or the gas pressure drop across the convection surfaces could be very large due to the flue gas recirculation rates used; the efficiency also could be lower due to the higher exit gas temperature associated with the larger flue gas flow. The operating cost due to a higher gas pressure drop is discussed below and in Chapter 4.

Often gaseous and oil fuels are fired at excess air ranging from 10% to 20%; flue gas recirculation could be in the range of 10–35%, depending on the NOx level desired. In a few boilers, 9 ppmv NOx has been achieved with the burner operating at 15% excess air and 35% flue gas recirculation rate on natural gas firing. Thus it is possible to have a ‘‘standard’’ steam generator handling nearly 30–40% more flue gases than it was designed for in the good old days when 5–10% excess air was used without gas recirculation: A 100,000 lb=h standard boiler could be operating at gas flow conditions equivalent to those of a 140,000 lb=h boiler if it is not custom-designed. Of course, one could select a larger standard boiler, but it may or may not meet all the requirements of furnace

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dimensions, because developers of standard boilers generally increase furnace lengths for higher capacity but not the width or height, due to shipping constraints, particularly when the capacity is large. However, standard boilers are useful where one is not concerned about optimizing all the parameters such as efficiency, gas pressure drop, and emission levels and low initial cost is a primary objective.

Packaged steam generators of today are custom-designed with an eye on operating costs and emissions. The furnace design also has undergone major design innovations, the completely water-cooled furnace (Fig. 3.2) being one of them. This design offers several advantages over the refractory-lined boilers designed decades ago.

Advantages of Water-Cooled Furnaces

Water-cooled furnaces have a number of advantages over other types:

1.The front, rear, and side walls are completely water-cooled and are of membrane construction, resulting in a leakproof enclosure for the flame, as shown in Fig. 3.2. The entire furnace expands and contracts uniformly, thus avoiding casing expansion problems. When refractory is used on the front, side, or rear walls, the sealing between the hotter membrane walls and the cooler outer casing is a concern and hot gases can sometimes leak from the furnace to the outside. This can cause corrosion of the casing, particularly if oil fuels are fired.

2.Problems associated with refractory maintenance are eliminated. Also, there is no need for annual shutdown of the boiler plant to inspect the refractory or repair it, thus lowering the cost of owning the boiler.

3.Fast boiler start-up rates are difficult with refractory-lined boilers because of the possibility of causing cracks in the refractory. However, with completely water-cooled furnaces, start-up rates are limited only by thermal stresses in the drums and are generally quicker. The tubes may be welded to the drums instead of being rolled if the start-ups are frequent. With boilers maintained in hot standby conditions using steam-heated coils located in the mud drum, even 10–15 min start-ups are feasible. With a separate small burner whose capacity is 6–8% of the total heat input in operation during boiler standby conditions, the boiler can be maintained at pressure and can be ramped up to generate 100% steam within 3–5 min.

4.Heat release rate on an area basis is lower for the water-cooled furnace by about 7–15% compared to the refractory-lined boiler. Some gasfired boilers designed decades ago still use refractory on the floor; replacing this with a water-cooled floor will increase the effective heating surface of the furnace and lower the heat flux inside the tubes

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even further. The furnace exit gas temperature also decrease slightly due to the increased effective cooling surface of the furnace. A lower furnace exit gas temperature decreases the radiant energy transferred to a superheater located at the furnace exit and thus reduces the potential for superheater tube failures. A lower area heat release also helps reduce NOx, as can be seen from the correlations developed by a few burner suppliers.

5.Reradiation from the refractory on the front wall, side walls, and a floor increases the flame temperatures locally, which results in higher NOx formation. Of the total NOx generated by the burner, a significant amount of NOx is formed at the burner flame base, so providing a cooler environment for the flame near the burner helps minimize NOx to some extent.

6.Circulation was one of the concerns about the use of refractory on the floor of even gas-fired boilers because the D tubes are longer than partition tubes of the dividing wall. Heat fluxes in packaged boilers are generally low compared to those of utility boilers. To further protect the floor and roof tubes, a small inclination to the horizontal is used; also, considering the low steam pressure, tube-side velocities, heat flux, and steam quality, departure from nucleate boiling (DNB) has never been an issue, as evidenced by the operation of hundreds of boilers at pressures as high as 1000–1500 psig. The tube-side velocities are also adequate to ensure that steam bubbles do not separate from the water. Hence refractory is not required on the floor or front or rear walls for oil and gas firing.

7.Packaged boilers use economizers as the heat recovery equipment instead of air heaters, which only serve to increase the flame temperature, thus increasing the NOx formation. The gasand air-side pressure drops are also higher with air heaters, thus adding to the fan size and power consumption. The heat flux inside the furnace tubes is also reduced owing to the smaller furnace duty.

Custom-Designed Boilers

Custom-designed boilers, as the term implies, are designed from scratch. Based on discussions with the burner supplier and the level of NOx and CO desired, one first selects the type of burner to be used and the emission control strategy. A few options could be considered:

Use a large amount of flue gas recirculation (FGR) and a low cost burner, which results in higher operating costs; one may use a large boiler with a wide convection bank to minimize gas pressure drop.

Use an expensive burner, which uses fuel or air staging methods and requires little or no flue gas recirculation. A few burners can guarantee

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about 20–30 ppmv NOx (at 3% oxygen dry) on gas firing. Installation and operating costs associated with FGR are minimized.

One can also consider the possibility of using a selective catalytic reduction (SCR) system along with a less expensive burner, which has a low to nil FGR rate.

Steam injection may also be looked into, and the cost of steam versus FGR may be compared.

Depending on the NOx and CO levels desired and the fuel analysis, the solution may vary from case to case, and no obvious solution exists for every situation. Thus one arrives at the best option from an emission control viewpoint and then starts developing the boiler design using the excess air and FGR rates for the fuels in consideration; the furnace dimensions to avoid flame impingement on the furnace walls are then arrived at. Assuming a specific exit gas temperature, the boiler efficiency calculations are done to arrive at the air and flue gas flow rates and the amount of flue gas recirculated. This is followed by an evaluation of furnace performance and design of the heating surfaces. The exit gas temperature from the economizer is arrived at and compared with the assumed value; efficiency is recalculated using the computed exit gas temperature, and revised air and flue gas flows are obtained. (Air and flue gas quantities depend on the amount of fuel fired, which in turn depends on efficiency.) Another iteration starting from the furnace is done to fine-tune the performance. The superheater performance is evaluated at various loads to determine whether the surface areas are adequate.

If different fuels are fired, these calculations are carried out for all the fuels. Efforts are then made to reduce the fuel consumption and also lower the fan power consumption, which are recurring expenses, by fine-tuning the design of the evaporator and economizer. A large economizer may be used to improve the boiler efficiency if the duration of operation warrants it. The designer also has the ability to change the dimensions of the convection section—for example, the number of tubes wide, length, tube spacing, or even tube diameter—to come up with low gas pressure drop and hence low fan operating cost as shown below. Based on partial load performance and gas temperature profiles, bypass dampers may be required if an SCR system is used. Hence it is likely that the steam parameters of several boilers could be the same but the designs different due to the emission control strategy used and degree of custom designing. A computer program is used to perform these tedious calculations.

Example 1

A 150,000 lb=h boiler firing standard natural gas and generating saturated steam at 285 psig with 230 F feedwater uses 15% excess air and 15% flue gas recirculation. The exit gas temperature is 323 F. Compare the performance of a standard boiler with that of a custom-designed unit. The flue gas flow through the

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boiler is 184,300 lb=h. With 80 F ambient temperature, the efficiency is 83.38% HHV.

The results of the calculations are shown in Table 3.1. The following points may be noted from this table:

1.The efficiency is the same in both designs because the exit gas temperature and excess air are the same. Also, the furnace dimensions are the same. Hence the furnace exit gas temperature is the same in both designs.

2.The convection sections are different. In the standard boiler, we used a standard tube spacing of 4 in. In the custom-designed unit, we reduced the surface area significantly by using fewer rows and also made the convection bank tube transverse spacing 5 in. This reduces the gas pressure drop in the convection bank by 4 in. WC. It also reduces the

duty of the evaporator section, as can be seen by the higher exit gas temperature of 683 F versus 550 F.

3.We added a few more rows to the economizer in the custom-designed unit and made its tubes longer to obtain the same exit gas temperature and also to handle the additional duty. Economizer steaming is not a

TABLE 3.1 Reducing Boiler Gas Pressure Drop Through Custom Designing

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concern in packaged boilers due to the small ratio of flue gas to steam flows (this aspect is discussed later). Hence we can absorb more energy in the economizer, which is a less expensive heating surface than the evaporator. The overall gas pressure drop saving of 4 in. WC results in a saving of 31 kW in fan power consumption (see Example 9.06b for fan power calculation). If energy costs 7 cents=kWh, for 8000 h of operation per year the annual saving is

31 0:07 8000 ¼ $17;360:

This is not an insignificant amount. Simply by manipulating the tube spacing of the convection bank, we have dramatically reduced the fan power consumption and the size of the fan. Also the boiler cost for the two designs should be nearly the same because the increase in economizer cost is offset by the smaller number of evaporator tubes, which reduces the material costs as well as labor costs. To improve the energy transfer in evaporators one can also use finned tubes if the boiler is fired with natural gas or distillate fuels. For example, if we desire good efficiency but do not want an economizer because of, say, shorter duration of operation or corrosion concerns, we may consider using extended surfaces in the convection bank to lower the evaporator exit gas temperature by about 40–100 F, which improves the efficiency by 1–2.5% compared to a standard boiler.

4.Another important point is that surface areas should be looked at with caution. One should not purchase boilers based on surface areas, which is still unfortunately being done. It is possible to distribute energy among the furnace, evaporator, and economizer in several ways and come up with the same overall efficiency and fan power consumption and yet have significantly different surface areas as shown in Tables 3.1 and 3.2.

Comparing Surface Areas

Example 2

This example illustrates the point that surface areas can be misleading. A boiler generates 100,000 lb=h of saturated steam at 300 psig. Feedwater is at 230 F, and blowdown is 2%. Standard natural gas at 10% excess air is fired. Boiler duty ¼ 100.8 MM Btu=h, efficiency ¼ 84.3% HHV, furnace backpressure ¼ 7 in. WC

It is seen from Table 3.2 that boiler 2 has about 10% more surface area than boiler 1 but the overall performance is the same for both boilers in terms of operating costs such as fuel consumption and fan power consumption. Also the

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TABLE 3.2 Comparison of Boilers with Same Efficiency and Backpressure

aDuty is in MM Btu=h, fin dimensions in inches, heat transfer coefficient in Btu=ft2 h F.

energy absorbed in different sections is different, hence comparing surface areas is difficult unless one can do the heat transfer calculations for each surface.

It has become a common practice (with the plethora of spreadsheet users) to compare surface areas of boilers and generally select the design that has the higher surface area. Surface areas should not be used for comparing two boiler designs for the following reasons:

1.Surface area is only a part of the simple equation Q ¼ UA DT, where U ¼ overall heat transfer coefficient, A ¼ surface area, DT ¼ log-mean temperature difference, and Q ¼ energy transferred. However, the Q and DT could be different for the two designs at different sections as shown in the above example. Hence unless one knows how to compute U; A values should not be compared.

2.Even if DT remains the same for a surface, U is a function of several variables such as the tube size, spacing, and gas velocity. With finned tubes, the heat transfer coefficient decreases as fin surface area increases, as discussed in Q8.19. Hence unless one is familiar with

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all these issues, a simplistic tabulation of surface areas can be misleading.

EFFECT OF STEAM PRESSURE ON BOILER DESIGN AND PERFORMANCE

Another example of custom designing is shown in Example 3. In this example, we are asked to design a boiler for a lower pressure of operation for the first few years with the idea of operating at a higher steam pressure after that.

Example 3

An interesting requirement was placed on the design of a boiler. The 175,000 lb=h boiler was to generate steam at 150 psig and 680 F for the first few years and then operate at 650 psig and 760 F. The piping and superheater changes had to be minimal when the time came for modifications.

Operating a steam generator at two different pressures is a challenging task, particularly when a superheater is present. The reason is that the large difference in specific volume of steam affects the steam velocity inside the superheater tubes and the steam-side pressure drop, which in turn affect the flow distribution inside the tubes. The ratio of specific volume between the 150 and 650 psig steam is about 4. Hence for the same steam output, we could have a 4 times higher steam velocity at the lower pressure if the flow per tube were the same. Also, if the pressure drop at 650 psig were, say, 30 psi, it would be about 120 psi at the lower operating pressure if flow per tube were the same. Hence it was decided to manipulate the streams and steam flows as shown in Fig. 3.9.

In the low pressure operation, there would be two inlets to the superheater from opposite ends of the headers as shown in Fig. 3.9a. This would make the velocity and pressure drop inside the tubes more reasonable. The total length of tubing traveled by steam in the low pressure option would be nearly half that of the high pressure case, which also reduces the pressure drop. Part of the steam is in parallel flow and part in counterflow. At high gas temperatures, as in this case, the difference in performance between parallel and counterflow superheaters is marginal.

In the high pressure case, all the steam flows through the superheater tubes in counterflow. Because the specific volume is small, the steam can flow as shown with a reasonable steam velocity and without increasing the pressure drop. The performance in both, cases is shown in Table 3.3. Thus with a minimal amount of reworking, the piping could be changed when high pressure operation is begun. The superheater per se was untouched, and only the nozzle connections were redone. This boiler will be in operation for several years. If custom designing were not done, the capacity at low pressure mode would have to be limited to

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