Recovering Heat When Generating Power

7/28/2019 Recovering Heat When Generating Power

1/8

RECOVERING HEAT WHEN GENERATING POWER Page 1 of 8

file://C:\Datain\Papers\HRSG\RECOVERING HEAT WHEN GENERATING POWER.htm 9/1/01

RECOVERING HEAT WHEN GENERATING POWER

V. Ganapathy Abco Industries

Intelligent use of heat-recovery steam generators (HRSGs) is vital for the efficient operation ofcogeneration plants, which furnish both thermal energy (usually in the form of steam) and electricenergy. HRSGs are similarly important in combined-cycle power plants, in which the thermal energy

rejected from the primary electric-power-generation step is harnessed (as discussed below) to produceadditional electrical energy.*

In these facilities, the HRSG is typically heated by gas-turbine exhaust, because both cogenerationand combined-cycle plants are likely to employ these turbines as their prime movers. Gas turbines aresimple and efficient, incur low installed cost per kilowatt, require relatively little space, can start upquickly, and require relatively little cooling water. Their electrical output ranges from 5 to 150 MW,so both large and small plants find applications for them.

Natural gas is the fuel most widely used for gas turbines in the U.S., whereas fuel oil is the main fuelin other countries. In either case, the turbine exhaust is clean and thus usually does not pose corrosion

or related problems for the HRSG.

In cogeneration plants, HRSGs generally convert this turbine-exhaust energy into low-pressuresaturated steam, around 10 to 300 psig, suitable for applications such as drying, process heating orcooling. In combined-cycle plants, by contrast, the HRSG generates high-pressure, high-temperaturesteam, usually exceeding 750 psi and 705 degrees F, which drives steam turbines to produceadditional electricity. If the cogeneration or combined cycle plant is large, the gas-turbine exhaustmight be used not only for steam generation at multiple pressure (including low-pressure steam fordeaeration) but also to heat condensate or heat-transfer fluids.

Due to the large mass-flows associated with gas turbines, water-tube rather than fire-tube HRSGs are

generally the choice (for more discussion about this tradeoff, see the article cited in the footnotepreceding). However, fire-tube boilers may be more economical if the installation is of low capacity.With water-tube designs, extended surfaces can be used to make the units compact.

Depending on the amount of steam to be produced, HRSGs for gas-turbine-exhaust applications maybe unfired, supplementary-fired or furnace fired. In the last-named two options, additional fuel issprayed into the turbine-exhaust stream. Note that this is in fact feasible without need for additionaloxygen, as the exhaust itself typically contains about 16% oxygen by volume.

Publication : CE

Date : February, 1993

Copyright : Copyright 1993 McGraw-Hill, Inc.

Volume : 100

Issue : 2

Page : 94

Section : ENGINEERING PRACTICE


2/8



The table gives the steam-generation capabilities of each of the three options. In a typical situation,simulation and temperature-profile analyses are carried out to determine which one is mostappropriate.

Unfired HRSGs

Typical temperature of gas entering an unfired HRSG ranges from 800 to 1,050 degrees F. In the caseof natural-circulation units (discussed below), two of the widely used configurations are the single-and two-gas-pass designs.

In the two-pass version, a horizontal baffle plate divides the evaporator into two portions. Theexhaust gases enter the lower portion of the shell side of the evaporator, make a 180-deg turn andthen flow across the top half of the evaporator.

This design requires relatively little floor space. It is ordinarily the choice when exhaust-gas flow isless than 200,000 lb/h and when steam generation at a single pressure is adequate.

Various gas inlet and exit configurations are possible. It is possible also to incorporate a steamsuperheater and a water-heating economizer.

When gas flow exceeds 200,000 lb/hr or when steam must be generated at multiple pressures, it ispreferable to use a single gas pass; otherwise the HRSG would have to be inconveniently high. Aside-benefit with single-pass HRSGs is that they are easier to outfit with catalytic reduction systems(discussed later) to remove NOx or carbon monoxide.

The type of circulation, whether natural or forced, also affects the boiler configuration. With naturalcirculation, the difference in thermal head between the comparatively cool water in the downcomerpipes and the hotter steam-water mixture in the evaporator is responsible for the circulation through

the system. Vertical tubes are used for the evaporator in such designs.

The circulation ratio (mass ratio of circulating steam-water mixture to generated steam) in such a unitis arrived at by balancing the thermal head available between the cooler and hotter columns of steam-water mixture against the losses in the system. It varies from about 10 to 40 depending on the systemlosses and configuration [2].

In the forced-circulation design, pumps circulate the steam-water mixture through horizontalevaporator tubes to and from the steam drum. Circulation ratio is selected to be in the range of 3 to10.

The forced-circulation approach, widely used in Europe, is more compact. However, savings in floorspace must be weighed against the capital costs, operating costs and reliability risks associated withthe pumps. The startup time for the two options do not differ significantly -- in both cases, the overallheat transfer rate is governed mainly by the (lower) gas-side coefficient, which is not significantlyaffected by the tube orientation.

Vertical tubes, as in natural circulation designs, provide a natural path for the steam bubbles. Withhorizontal tubes, on the other hand, separation of steam from the steam-water mixture poses a risk ofoverheating. This is particularly true when the heat flux is very high (as in the supplementary-fired


3/8



systems, discussed below). Operating personnel should periodically check for signs of departure fromnucleate boiling (boiling that involves distinct vaporization nuclei on the heating surface), especiallywith the horizontal-tube option.

Proper use of fins

As mentioned earlier, extended surfaces are used to make the unfired HRSG compact, whether it is ofnatural- or forced-circulation design. Fin densities range from 2 to 5 fins per inch, fin height from 0.5to 1.0 in., and fin thickness from 0.05 to 0.12 in. Both solid and serrated fins are widely used. Gaspressure drop across the tubes increases with increase in fin density or height.

A significant consideration is the maximum temperature the fin will incur, as this limits the materialsof fin construction that can be used. The maximum temperature will be at the fin tip. The thinner thefin, the higher this temperature will be, other things being equal. A large ratio of external to internalsurface (i.e., a high fin density) also increases the fin-tip temperature, which likewise raises that ofthe tube wall.

This effect is particularly pronounced when the tubeside coefficient is low, as it is in steam

superheaters. Accordingly, a low fin density is recommended for superheaters. On the other hand,economizers and evaporators can accommodate a higher density because the tubeside coefficient is

very large, on the order of 1,000 to 3,000 Btu/(ft2)(h)( degrees F) [2,3].

Choosing a large amount of fin surface does not automatically mean that more energy will betransferred -- as the engineer compares alternative designs having successively more fin area, itbecomes important to look not just at the area but instead at the product of area and overall heat-transfer coefficient. The gas-side heat transfer coefficient decreases as the ratio of external to tubeinternal surface increases [2,3].

The metal casing for unfired HRSGs is internally insulated with 4 to 5 in. of ceramic fiber insulationand is protected from the hot gases by a liner made of stainless steel, alloy steel or carbon steeldepending on the gas temperature. The design of this liner must allow for expansion.

The same approach can be used for the duct that leads from the turbine to the HRSG. However, somemanufacturers prefer to use alloy steel for the duct material and place the insulation on the outside.Though this reduces maintenance problems with the insulation liners, the expansion problems due tohigher casing temperature has to be dealt with.

Supplementary fired HRSGs

The term ``supplementary fired'' is used somewhat loosely, but it generally implies an HRSG that isoutfitted with a duct burner to raise the temperature of the entering gas from the turbine duct burnerand thus increase steam production. A supplementary-fired HRSG using such a burner can be seen inFigure 1.

The HRSG that is shown in Figure 1 has a single-pass design and generates steam at both high andlow pressure. Apart from having the duct burner, it closely resembles the single-pass unfired designsdiscussed on the previous page.

Indeed, the HRSG design for supplementary firing does not differ much from that for the unfired


4/8



HRSG. The main exceptions are the sizing of drums and piping to accommodate the larger steamflows, and the selection of tube and fin materials to accommodate the higher gas temperatures.Casing-design considerations limit the maximum gas temperature after firing to 1,700 degrees F --beyond this temperature, the liner material starts warping and thus exposes the bare insulation to thehot gases.

In line with the precautions discussed earlier for unfired HRSGs, the superheater and evaporatorportions of the supplementary-fired versions are designed with varying combinations of fin densitiesto minimize tube wall and fin tip temperature. For instance, one common arrangement employs a fewrows of bare (unfinned) tubes closest to the flame, then has a few rows of tubes with low fin density,followed finally by ones with higher density.

When specifying an HRSG, the engineer should make sure that the distribution of the gas flow acrossthe duct does not distort the flame pattern and thereby overheat the tubes or ductwork. Discussionswith the burner supplier regarding gas uniformity, distance from burner to HRSG surfaces and ductconfiguration are important in this vein.

Furnace firing

Depending on steam demand, the aforementioned simulations and temperature profile analyses maydictate a firing temperature over 1,700 degrees F. In that case, furnace-fired HRSGs may be used.

For temperatures up to 2,300 degrees F, a suitable arrangement consists of an HRSG equipped with aduct burner and employing a water-cooled, integral membrane wall (photo, p. 97). The evaporatorportion includes a radiant section that is followed by a convection section; after they leave theevaporator, the gases pass through an economizer.

As discussed under supplementary firing, the convection section is designed with a few bare tubesfollowed by finned tubes, with a varying fin configuration that takes into account the tube wall and

fin tip temperatures and heat flux inside the tubes. If desired, superheater tubes can be placed in thesame area as the convection tubes.

When the required firing temperature exceeds 2,300 degrees F, a register burner is required. Thisconsists of a burner that is outfitted with its own air chamber (wind box), of the type that is used inpackaged steam generators. Such a burner system can fire up to the adiabatic combustiontemperature, leaving a residual oxygen content of less than 3% (dry basis) in the exhaust. As seenfrom the table, one can maximize the amount of steam generation with such a design.

Firing and system efficiency

As indicated earlier, the decision between unfired, supplementary-fired or furnace-fired HRSGs isbased mainly on the quantity of steam required. It should be noted, however, that supplementary orfurnace firing in any case improves the energy efficiency of the operation; for instance, the efficiencyas defined by the American Soc. of Mechanical Engineers Power Test Code PTC 4.4 [7]. Theexplanation is straightforward: By consuming oxygen in the turbine exhaust, one is reducing theamount of excess air. [2,3]

In some installations, an HRSG must be able to operate even when the gas turbine is off. Such asystem requires a forced-draft fan to supply the air for combustion to the burner. With such fresh-air


5/8



operation, the burner duty is much higher, because the air temperature has to be raised from am- bientto the high firing temperature.

Isolating dampers are employed to shield the hot gases from the fan whenever the gas turbine isrunning. This mechanism should be designed to provide for quick switchover to the fan mode ofoperation.

Pressure drop

Whether the HRSG is unfired, supplementary fired or furnace fired, it is important to keep pressuredrop in mind when preparing the specifications. As a rule of thumb, each additional 4 in. w.c. (watercolumn) of pressure drop through the HRSG reduces the power output of the upstream gas turbine bynearly 1%.

HRSGs associated with relatively small gas turbines (up to about 10 MW) typically incur a pressuredrop of 5 to 6 in. Toward the other end of the scale, larger multiple-pressure units equipped withcatalytic reduction systems involve a drop as high as 12 to 14 in. At any event, every effort should bemade to minimize the figure.

Duct burners typically offer a low resistance to gas flow, on the order of 0.3 in. w.c. For registerburners, however, the figure is around 4 in.

Cheng cycle

An interesting application of HRSGs with gas turbine exhaust is the Cheng cycle, Figure 2. Inconventional gas-turbine-HRSG systems, whenever the steam demand falls off, one has to eitherbypass the exhaust gases to the stack using a diverter, or else vent (and thereby waste) the steam, oroperate the turbine at a lower electrical output (which may not be desired). The Cheng cycle avoidsthose drawbacks: The steam not required for the process is superheated and injected into the gas

turbine, thus increasing the electrical power output of the latter. The ratio of injection steam toprocess steam can be varied to suit the process requirements.

Since steam is being injected into the gas turbine, its purity is of utmost importance. Cheng-cycleunits accordingly employ a combination of internal and external steam separators to removenoncondensibles and other impurities and thus to achieve steam purity in the parts-per-billion range.

In a Cheng cycle installation, the steam generation can be increased by including a duct burnerbetween the superheater and evaporator. Since the amount of water vapor present in the turbineexhaust gases can be as high as 26% when operating in the steam-injection mode, with oxygen onlyin the range of 11 to 12% by comparison, an augmenting air fan is usually required to stabilize the

flame in the duct burner.

Since the increased electrical output is obtained by steam injection, there is no need for a steamturbine-condenser system. The Cheng cycle achieves the electrical power output of a comparablecombined cycle system or slightly more power, with less equipment and complexity. A two-gas-passHRSG that is intended for Cheng cycle usage appears in the photo on p. 94.

Catalytic reduction


6/8



Due to air-pollution regulations, catalytic reduction units are required on turbine systems in severalU.S. states, to limit nitrogen oxides and carbon monoxide in the exhaust gases. Since these unitsrequire a narrowly defined temperature range for efficient operation (e.g., 600 to 750 degrees F forcertain NOx catalysts), their placement within the HRSG is particularly important.

A typical HRSG exhibits variations in gas flow and exhaust-gas temperature over time, so numerousperformance runs must be made to determine the best location for the reduction unit. Often anevaporator may have to be split into two or more sections to achieve this objective.

Performance testing

In the same vein, bear in mind that the performance data generated during proposal stages of a HRSGproject may not cover all of the operating regimes that can be anticipated during the life of theequipment. A gas turbine may operate at different ambient conditions or load, so the exhaust-gasflowrate, composition and temperature may change, which in turn affects the HRSG performance [9].Also, the steam parameters may be different during plant commissioning.

Another uncertainty to be aware of is the error that is due to the instruments and measuringtolerances. It is very difficult, for instance, to measure the gas flow; in fact, the aforementionedASME PTC 4.4 code cites a 3 to 5% error in gas-flow measurement with different methods.

Also, depending on the duct size, a variation of 30 to 50 degrees F can be experienced in gastemperature across the duct cross section. Too, the exhaust-gas temperature could vary by 10 degreesF due to instrument tolerances.

Given these facts, one could argue that the HRSG can receive up 5% less mass flow at a 10 degrees Flower exhaust gas temperature than the design values and still show that the design flow andtemperature conditions are being met. As a result, the HRSG could be generating less steam even

though correctly sized for the design conditions.

Hence it is prudent for both the supplier and end user to arrive at a consensus on HRSG performance,operating regimes and testing procedures before installing the unit. Performance data of the HRSG atdifferent gas and steam parameters may be generated and discussed before testing.

*For an explanation of how HRSGs work, how they are employed during manufacture of ammonia,sulfuric acid and other chemicals, and how they can be used in incineration systems, see ``Effectiveuse of heat-recovery steam generators,'' CE, January, pp. 102-106. That article also providesguidelines on specifying HRSGs.

For information on suppliers of waste-heat-recovery boilers, see the Chemical Engineering BuyersGuide for 1993.

Edited by Nicholas P. Chopey


7/8



[Photograph]

Photograph: This steam

generator for

use in Cheng-cycle

units (see p. 97)

typifies the efficient

recovery of heat

in power plants.

[Illustration]

Table: As the table

shows, generators

can be unfired,

supplementary-fired

or furnace-fired (This table is not available electronically. Please see the March 8, 1993 issue.)

[Illustration]

Figure 1: The supplementary fired HRSG shown here has an overall configuration similar to that ofthe unfired version

LUTHER EASON

[Photograph]

Photograph: Boiler fabricated with water-cooled membrane wall is suitable for furnace firing

[Illustration]

Figure 2: Cheng ycle represents a notably versatile gas-turbine heat-recovery system

[References]

1. Ganapathy, V., Simplify heat recovery steam generator evaluation, Hydrocarbon Processing,March 1990.

2. Ganapathy, V., ``Applied Heat Transfer,'' Pennwell Books, Tulsa, 1982.

3. Ganapathy, V., Evaluate extended surface exchangers carefully, Hydrocarbon Processing,October 1990.

4. Ganapathy, V., ``Waste Heat Deskbook,'' Fairmont Press, Atlanta, 1991.

5. Ganapathy, V., Chart estimates supplementary fuel parameters, Oil and Gas Journal, June 25,1984.


8/8



6. Ganapathy, V., Program computes fuel input, combustion temperature, Power Engineering, July1986.

7. ASME Power Test Code PTC 4.4, ``Gas turbine heat recovery steam generators,'' American Soc.of Mechanical Engineers, New York, 1981.

8. ``HRSGs -- Software for simulation of design and off-design performance of HRSGs,''Ganapathy, V., available from author.

9. Ganapathy, V., How to evaluate HRSG performance, Power, June 1991.

10. Ganapathy, V., Heat recovery boilers -- the options, Chemical Engineering Progress, February1992.

[Biography]

For biography of author, see January issue, p. 106.

Recovering Heat When Generating Power

Documents

Transcript of Recovering Heat When Generating Power