air heater powermag 101 - coal

8/10/2019 Air Heater Powermag 101 - Coal

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Power 101: Improving the Performance of

Boiler Auxiliaries, Part I

12/01/2010 | By Nenad Sarunac

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Editors note:In this second installment of our Power 101 series (the first installment was Flue

Gas Heat Recovery in Power Plants, published in three parts:Part I,Part II,andPart III), weexamine boiler auxiliary components that may be secondary to the design of the boiler itself but

which are absolutely essential to its operation. We begin this multi-part series with an

introduction to the air preheater (APH) performance calculations. In Part II, well examine

performance degradation caused by corrosion and fouling as a result of coal combustion flue

gas constituents plus the effects of ammonia and sodium bisulfite injection for SO3mitigation. InPart III, well look at options for improving APH performance. Future articles will center on

improving the performance of other boiler auxiliaries.

The Ljungstrom (or Rothemuhle) air preheater (APH) transfers sensible heat in the flue gasleaving the boiler to the combustion air, using regenerative heat transfer surfaces in a rotating(Ljungstrom) APH or stationary (Rothemuhle) heat transfer matrix. Increasing the temperature of

the incoming combustion air increases boiler efficiency and reduces the plant heat rate (Figure

1). The increase in boiler efficiency is significant: a 25F change in flue gas temperature leavingthe APH results in an approximately 1% (relative) change in net unit heat rate, with a

corresponding reduction in CO2emissions.
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1.The effect of APH gas outlet temperature on net unit heat rate. Source: Energy Research

Center, Lehigh University (ERC)

The flue gas temperature leaving the economizer (and entering the APH) is typically reduced

from 600F to 800F to 275F to 350F at the APH outlet. Combustion air is heated from about 80F

to 500F to 750F, depending on the coal heat and moisture content. In theory, only the primary air(the air used to dry the coal in the pulverizers and convey the pulverized coal to the burners)

could be heated. Fuel can be combusted without preheating secondary and tertiary air. However,

there is considerable advantage to the furnace heat transfer process by heating all of thecombustion air: It increases the rate of burning and helps increase furnace flame temperatures.

The Ljungstrom APH operates by continuously revolving the heat transfer matrix through thegas (absorbing heat energy) and air streams (releasing heat energy) with a rotational speed in the

1 rpm to 3 rpm range. The Rothemuhle (or stationary plate) APH uses a stationary heat transfer

matrix with a rotating air/flue gas hood (Figure 2).


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2. The Rothemuhle regenerative APH. Source: ERC

APHs can be designed as bi-sector or tri-sector types. In a bi-sector APH, total combustion air

(secondary and primary air) flows through the air sector of the APH (Figure 3a). In a tri-sectordesign, the primary air flows through a separate primary air sector, while the secondary air flowsthrough the secondary air sector (Figure 3b). A tri-sector design is used for boilers firing high-

moisture or low-rank fuels where hotter primary air is needed for evaporation of coal moisture in

mills.

3.Bi-sector (left) and tri-sector (right) APH configurations. Source: ERC

Due to a difference in static pressure between the flue gas and combustion air streams,

combustion air can leak into the flue gas stream. The APH leakage ( APH,L) is typically in the 8%


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to 12% (by weight) range for Ljungstrom APHs. For the Rothemuhle APH, air leakages in the

25% to 30% range are not uncommon.

Leakage paths for a tri-sector APH are more complex, compared to a bi-sector APH. In a tri-

sector APH, primary air (PA) leaks into the flue gas and secondary air (SA) streams, while SA

leaks into the flue gas stream. Leakage occurs both on the cold end (CE) and hot end (HE) sidesof the APH. Due to large difference in pressure between the PA and SA streams, as well as the

PA and flue gas streams, leakage in a tri-sector APH is higher than in a bi-sector APH. Its also

important to note that for a tri-sector APH, the APH leakage, as defined by Equation 1 (all of theperformance calculation equations referenced are included in an Appendix at the end of this

article), does not include the PA to SA leakage and therefore is not an accurate measure of total

air leakage. Furthermore, the PA to SA leakage cannot be measured either by the oxygen rise or

tracer gas techniques but has to be calculated using information on pressure difference betweenthe PA and SA streams and assumed clearances.

Air leakage for a tri-sector APH can be determined by a computer model, such as that presented

in Figure 4. For this 700-MW power plant firing bituminous coal, leakages are calculated byusing information on static pressure differences between the PA, SA, and flue gas streams,

assuming two-thirds of the leakage occurs at the hot end of the APH. The air to flue gas leakageis 12% (by weight). As discussed earlier, this leakage does not include the PA to SA leakage. For

the given static pressure distribution around the APH, a calculated value of the PA to SA leakage

is 5.5%.

4.The Engineering Research Center at Lehigh University has written a computer model to

calculate air and gas leakage for a tri-sector APH. Source: ERC

Air leakage also affects the flow rate of PA, SA, and flue gas through the APH heat transfer

matrix and, thus, affects the APH capacity rate ratio (CR, known in the industry as the X ratio).


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This parameter has a significant effect on APH performance. The CE leakage affects the

temperature of the flue gas leaving the APH. The flue gas temperature, downstream of the APH,

after adiabatic mixing between the flue gas and air leakage streams (297F in this example), islower compared to the flue gas temperature measured at the APH outlet (305.4F in this

example).

Major components of a vertical-shaft bi-sector Ljungstrom APH and leakage paths are presented

in Figure 5. In this arrangement, the flue gas flows downward, while combustion air is flowing

upward. The rotor, packed with heat transfer surface, is supported through a lower bearing at theCE of the APH and guided through a guide-bearing assembly located at the top or HE of the

APH.

5. This illustration of a bi-sector Ljungstrom APH includes seals and leakage paths. Source: ERC

The conventional rotor sealing system contains of leaf-type labyrinth seals bolted to the rotor

radial members at the HE and CE. The radial seals press against radial plates, located at the HEand CE. Axial seals are positioned along the periphery of the rotor. For the vertical-shaft

Ljungstrom, where the rotor is supported from below, most of the leakage occurs at the HE of

the APH. As a rule of thumb, approximately two-thirds of the air leakage occurs at the HE of the

APH (Figure 5). For a horizontal-shaft APH, approximately two-thirds of air leakage occurs atthe CE.

The transfer of heat from the hotter flue gas stream to the colder air stream creates temperature

gradients, which cause thermal distortions throughout the structural members. The relative


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distortion of the various components affects clearances between the seals and sealing surfaces.

Therefore, control of APH leakage is not an easy task, especially in case of a Rothemuhle APH.

The heat transfer surface of Ljungstrom- or Rothemuhle-type air preheaters uses a combination

of flat or formed steel sheets with corrugated, notched, or undulated ribbing, forming

longitudinal (axial) passages within the APH (Figure 6). The heat transfer surfaces are designedfor high heat transfer and low pressure drop. As an approximate rule, 1 inch in height of standard

Ljungstrom heat transfer surface recovers as much heat as 2 feet of surface in a tubular heater

with equivalent resistance to flow. The compact arrangement of metal sheets permits a largeamount of heat transfer surface to be placed in a relatively small rotor. Normally, the heat

transfer surface is divided into two or more axial layers. Advantages resulting from layering the

surface include these:

A field-removable CE section, usually 12 inches in length (or height), that can be easily

replaced in case of corrosion damage or severe plugging.

Different heat transfer surfaces can be used in different layers to satisfy site-specific

requirements. In addition, different-gauge materials can be used in each layer, with theCE layer typically manufactured of the heaviest gauge material.

Each layer can be constructed of a different material to accommodate fouling andcorrosion conditions that vary with flue gas temperature.

6.Fl at, notched, corrugated, and undulated steel plates are used to form APH heat transfer

surfaces (top). The shape shown on the bottom left is typically used in the cold end layer of the


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APH. The bottom right configuration is traditionally used in the intermediate and hot end layers.

Source: ERC

Traditionally, APH design used four axial layers: cold end, cold intermediate, hot intermediate,

and hot end layer. Later, the four-layer design was simplified to a three-layer design. For the

APHs operating downstream of a selective noncatalytic reactor or selective catalytic reactor forNOxcontrol, where ammonia slip and formation and deposition of ammonium bisulfate within

the APH is a problem, a two-layer APH design is used. The two-layer design uses a deep CE

layer manufactured of low-alloy Corten steel or enameled carbon steel that was developed tofacilitate better cleaning. The two-layer APH design is used by the electric utility industry with

varying degrees of success.

Estimating APH Air Leakage

Air preheater leakage not only affects performance of the APH, but it also has a detrimental

effect on unit performance. Air leaking from the combustion air to the flue gas stream is not

participating in the combustion process, and it represents an additional burden on the forced draft(FD) and induced draft (ID) fans. As air leakage increases, the FD fan needs to handle an

additional volume of air to supply combustion air to the boiler. Additionally, the ID fan power

increases, because the fan has to handle leakage air in addition to the combustion products (fluegas). As a result, station service power increases, causing an undesirable increase in net unit heat

rate.

The effect of APH air leakage on net unit heat rate and station service power is presented in

Figure 7 for a bi-sector APH and a 400-MW power plant firing bituminous coal. The inlet guide

vane flow control was assumed for FD and ID fans in this example.

7.The effect of APH air leakage on net unit heat rate and station service loads. Source: ERC


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Changes in net unit heat rate and station service power can be developed as functions of the APH

air leakage. The 6% APH leakage is used as a reference point, because it represents the design

value used by APH manufacturers. As APH leakage increases, the net unit heat rate penaltyincreases linearly. A 10 percentage point increase in APH leakage (for example, an increase from

6% to 16%) increases station service power by about 11% and increases the net unit heat rate by

approximately 0.25%. Eliminating APH leakage by using gas-to-gas heat exchangers or heatpipe heat exchangers will improve net unit heat rate by approximately 0.15%.

The heat rate penalty due to the APH air leakage is higher for tri-sector APHs, where high-pressure PA from the primary air sector leaks into the SA and flue gas streams, resulting in

increased PA fan power requirements, in addition to increased FD and ID fan power. A

comparison of the heat rate penalty for power plants employing bi-sector and tri-sector APHs is

presented in Figure 8.

8.The effect on plant heat rate caused by APH air leakage for bi-sector and tri-sector APHs.Source: ERC

Besides affecting station service power and net unit heat rate, APH air leakage has a negative

effect on performance of the air pollution equipment by reducing treatment (residence) time due

to a higher flow rate of flue gas. Also, air leakageincluding the APH and boiler in-leakagehas a negative effect on the post-combustion CO2capture system and its efficiency. The size

(and cost) of the post-combustion CO2capture system is directly proportional to the flow rate ofthe flue gas that needs to be treated. In addition, efficiency of the post-combustion CO2capture

system is affected by the concentration of CO2in the flue gas, which decreases as air leakage

increases.


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The effects of APH air leakage on the flue gas flow rate and CO2concentration in the flue gas

are presented in Figures 9 and 10 for a bi-sector APH installed on a typical 400-MW power plant

firing bituminous coal. The flow rate of flue gas leaving the APH increases linearly as the APHair leakage increases. For high levels of APH leakage, typical of Rothemuhle APHs, the increase

in flue gas flow rate is significant. In this situation, the concentration of CO2in the flue gas

decreases with air leakage. For high levels of APH leakage, the decrease in the CO2concentration is significant.

9.The effect on flue gas flow caused by changes in APH air leakage. Source: ERC


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10.CO2concentration in the boiler flue gas decreases as APH air leakage increases. Source:ERC

In the case of an oxy-combustion retrofit, any air leakage will have a severe negative effect on

plant efficiency, retrofit cost, and CO2purity and will have to be eliminated. This will require

zero-leakage heat exchangers when firing in the oxy-fuel mode.

Air Preheater Seals

The APH leakage usually drifts (slowly increases) with time, unless a catastrophic seal failure

occurs (Figure 11). Most new APHs, after initial commissioning or when set up correctly after anoutage, can achieve leakage levels that are close to design values. However, after a period of

operation (two or more years), the leakage will increase, typically from 6% to 12% on the

secondary APHs, and to between 12% and 25% (or higher) on primary air preheaters. Similarincreases in air leakage are expected for tri-sector APHs.


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11. The APH air leakage will drift up quickly after installation. Source: "Modernization of

Rotating Matrix Regenerative Air Preheaters," Howden Power Ltd., presented at PowerGen

1996.

In addition to higher FD and ID power requirements and higher net unit heat rate, thedeterioration in leakage performance can have other serious consequences. These changesinclude reduced downstream temperatures (because of the dilution effect), causing a potential for

increased acid deposition and corrosion of the downstream ductwork and increase in the FGD

booster fan power. In severe cases, derating the unit might be required due to fan limitations.

Adjustable Sector Plate. Manually adjustable sector plates are used to minimize APH leakage,

as APH seals wear over time. An improved version of this concept involves actuator-adjusted

radial and axial sealing plates (Figure 12). Sliding arrangement of the top and bottom radialsector plate primary seals allows movement of the sealing surfaces. These seals are prone to wear

and erosion by fly ash in the flue gas stream. Although initially these seals showed good sealing

characteristics, after years of operation, seal wear contributed to air leakage around the sectorplates. As a direct consequence of the leakage, deposition of fly ash behind the sector plates

caused reduced sector plate travel that caused the plates to jam in position. Similar problems

occur on the axial seal plates. The best way to solve the problem is complete removal of the

adjustment to the sealing surfaces. During the APH retrofit, the sector and axial sealing plates areset to accommodate all clearance gaps for all operational duties and are welded in position,

eliminating the need for future adjustment or maintenance.


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12.Adjustable seal plate design. Source: ERC

Radial and Axial Seals.Radial seals are used to minimize the direct air to gas leakage thatoccurs at the center section or sealing surfaces in the APH. The seals are attached to each

diaphragm (stay, splitter, or radial division plate) of the rotor to maintain close contact with the

sealing surface (that separates the air and gas streams) and to axial seal holding bars. Single-leafsealing strips, fitted to the radial division plates and axial seal holding bars, provide a sharp edge

that allows fine adjustment for proper seal setting.

Early experiences showed that two-leaf type seals showed better initial sealing characteristicswith adjustable mechanisms (adjustable sector plate), but because of this adjustment they were

prone to wear. Laboratory tests found that once worn, two-leaf seals were no longer any better

than an equivalent single-leaf seal. With the modifications to fixing adjustable sealing surfaces,the single-leaf seal was found to be more effective.

There are many designs of single-leaf seals. For example, radial seals offered by Paragon

Airheater Technologies are designed to act like a spring and maintain a positive contact between

the seal and the sealing surface (Figure 13). The spring force prevents the seal from opening due

to high differential pressure across the hot and cold ends of the APH. The bellows architectureenables the seal to "self-adjust" over time as wear occurs on the sealing edge, or during APH

turndown. The seal replaces the OEM seals and attaches to each diaphragm, utilizing the existing

holding hardware and the existing bolt holes on the diaphragms. The seals are installed andadjusted to maintain the appropriate force and clearance between the seal and sealing surface.


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13.Single-leaf adjustable radial seal. Source: Paragon Airheater Technologies

Traditional APH designs have either 12-sector rotors with 30 sector plates or 24-sector rotorswith 15 sector plates, which means that only one radial sealing strip is passing under the sector

plate at a time. Damage, corrosion, and erosion of these seals dramatically increases the leakage.

Theory shows that leakage is directly proportional to the square root of the driving pressure

differential, so it follows that if the number of seals under the sector plate is doubled, effectivelyhalving the pressure differential acting on each sealing surface, the radial leakage will be

decreased by approximately 30%. The double-sealed APH rotor design is presented in Figure 14.


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14.Double-sealed APH rotor design compared to the older single-sealed rotor design. Source:

ERC

Circumferential and Bypass Seals. Circumferential seals prevent air and gas from bypassing

the heating surface through the space between the rotor and the housing shell. These seals areretained by mounting angles bolted to the circumference of the rotor. Circumferential seals wear

against the sealing rings, which are welded to the APH housing.

Bypass seals differ from circumferential seals by reducing the quantity of air flowing over the

outer rim of the rotor, as the task of reducing axial leakage is handled by the axial sealing

system. Bypass seals are mounted on the housing and seal against the rotor-mounted T-bar.

Circumferential and bypass seals are installed at the hot and cold ends of an APH (Figure 15).

Bypass reduces APH performance because the air and flue gas bypassing the APH rotor are notparticipating in heat exchange. As a result, the temperature of combustion air leaving the APH is

lower, while the temperature of the flue gas is higher, compared to the zero-bypass case.


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15.Circumferential and bypass seals on an APH. Source: ERC

The circumferential and bypass seals offered by Paragon Airheater Technologies are composed

of two interlocking members (Figure 16). The tabs and slots interlock, providing seal tabs from

being torn lose in the event that the APH rotor comes in contact with the seal or a foreign object,such as a clinker lodged between the rotor and the seal (Figure 17). The neck (narrow) sections

of the seal do not overlap, providing greater flexibility for each of the tabs. With standard

circumferential and bypass seal designs, it is very common for seal tabs to be torn away, thusexposing large gaps that provide leakage paths around the APH rotor.

16.Interlocking design of circumferential and bypass seals. Source: Paragon Airheater

Technologies


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17. Interlocking process of sealing members. Source: Paragon Airheater Technologies

The use of advanced and self-adjusting seals can reduce APH leakage by a factor of two,resulting in fan power reduction, net power output increase, and reduction in operating costs.

However, even with advanced seal designs, APH leakage should be checked on an annual basis.

Seals need to be adjusted or replaced to keep APH air leakage and air and flue gas bypass low.

The oxygen rise method is commonly used for determining air leakage. The tracer gastechniques are accurate, but very expensive.

It should be noted that internal bypass of combustion air and flue gas can also occur as aconsequence of excessive clearances between individual APH baskets. This situation might

occur after an APH basket replacement if new baskets are slightly undersized (Figure 18). A

careful inspection of APH basket size is very important. Equally important is detailed inspectionof the re-basketed APH to ensure there are no gaps or clearances between the baskets.

18.The location of internal bypass through clearances between APH baskets is shown by thearrows. Source: ERC


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The Effect of Boiler Leakage on APH Performance. As the air in-leakage in a boiler convective

pass increases, the value of excess O2measured in the economizer gas exit duct increases. This

increase is sensed by the combustion control system and interpreted as an increase in airflowrate. To maintain the O2setpoint, the combustion control system reduces the flow rate of

combustion air. A lower flow rate of combustion air results in a lower furnace excess air (excess

O2) level, increased CO emissions, increased levels of unburned fuel in fly ash, higher furnaceexit gas temperature, increased slagging and fouling rates, and higher net unit heat rate.

To illustrate the effect of boiler convention pass leakage on APH and unit performance,calculations were performed for a bituminous coal-firing 400-MW unit configured with a bi-

sector APH. The excess O2setpoint of 3.5% was maintained at the economizer gas exit. The

results are presented in Figures 19 to 22.

19.The effect of boiler convection pass leakage on excess oxygen levels in the flue gas. Source:

ERC


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20.Boiler convection pass leakage will change the air and flue gas flow rates. Source: ERC

21.APH capacity rate ratio as a function of boiler convection pass leakage. Source: ERC


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22. The change in flue gas temperature at the APH outlet and net unit heat rate as a function ofboiler convection pass leakage. Source: ERC

The furnace excess O2level decreases linearly with boiler convention pass leakage, as the excess

O2setpoint is maintained at the economizer gas outlet. As the boiler convection pass leakage

doubles from the reference value of 5%, the furnace excess O2decreases from 2.7% to 1.9%(Figure 19). Corresponding changes in combustion air and flue gas flow rate are presented in

Figure 20. An increase in boiler convection pass leakage from the reference value of 5% to 10%

results in about a 4.4% decrease in airflow. The flue gas flow rate increases slightly because of

the higher unit heat rate.

These changes in air and flue gas flow rates result in a change in the APH capacity rate ratio, CR,which decreases as boiler convection pass leakage is increased (Figure 21). The capacity rate

ratio has a large effect on thermal performance (effectiveness, ) of an APH and represents the

ratio of capacity rates (product of specific heat and mass flow rate) of air to flue gas streams. A

lower CRvalue, therefore, corresponds to a lower flow rate of air through the APH and highertemperature of flue gas leaving the APH. Higher flue gas exit temperature, in turn, results in

higher unit heat rate.

Changes in flue gas temperature at the APH exit and net unit heat rate are presented in Figure 22

as functions of the boiler convection pass leakage. Both increase linearly with the increase in airleakage. The net unit heat rate increases by 0.4 percentage point as the boiler convection passleakage increases from 5% to 10%. For best unit performance, it is important to maintain boiler

tightness and keep convection pass leakage at a minimum.

It is recommended that annual checks of boiler tightness be performed. The convection pass

leakage is determined by measuring excess O2level at the furnace and economizer exit planes. A

traverse is needed at both locations to account for spatial stratifications. A water-cooled high-


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velocity thermocouple (HVT) probe is used for excess O2measurement at the furnace exit plane.

The alternative includes temperature mapping by a permanently installed multi-grid acoustic or

tunable laser measurement system. Unfortunately, very few plants are equipped with suchsystems, so manual HVT traverses are needed.

APH Corrosion and Fouling

Operation of a regenerative APH represents a compromise between performance and

maintenance. As the temperature of the flue gas leaving the APH decreases, unit performance

improves (Figure 1). Lower flue gas temperature, however, results in the lower temperature ofheat transfer surfaces and, as a result, the cold end of the APH typically operates below the acid

dew point. As temperature decreases below the acid dew point, sulfuric acid condenses in the

flue gas stream, forming mist, or on metal surfaces, forming a liquid, hydroscopic, and sticky

layer. The water vapor and fly ash from the flue gas are attracted to this initial layer, formingdilute (very corrosive) sulfuric acid and ash deposits on the heat transfer surface.

Depending on the coals mineral content, these deposits can be removed easily (as is the case forbituminous coals). On the other hand, western coals and lignites have a high alkaline content,

causing calcium and magnesium oxides from the fly ash to sulfate on the APH furnace, forming

hard-to-remove deposits. Other APH fouling mechanisms include deposition of the ammoniumbisulfate, injected additives, nitric acid, and ammonium nitrate. The latter could be a

consequence of ammine-enhanced fuel-lean gas reburn. The effect of fouling of CE passages in

two APHs is presented in Figure 23.

23.Examples of fouled APH cold end passages. Source: ERC

In case the deposits cannot be removed by the APH sootblowers, APH water washes are needed

to restore the APH pressure drop (Figure 24). Scheduled and unscheduled outages for APH waterwashing reduce unit availability.

The full-load operating data for a 650-MW unit (Figure 24) show that as a consequence of CEplugging, the secondary APH gas-side pressure drop doubled: It increased from about 7 inches to

14 inch w.g. in less than one year.


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24.Variations of the gas-side pressure drop for the primary and secondary APHs are illustrated

for a 650-MW coal-fired plant. The primary APHs are not water-washed because they operate

with higher exit flue gas temperature compared to the secondary APHs. Owing to the higher exit

temperature, the light deposition on the primary APH has caused a gas-side pressure dropincrease of 2 inches to 3 inches w.g. over a five-year period. Source: ERC

If severe plugging occurs, the unit might become fan-limited, causing a unit load derate until theAPH is washed and the APH pressure drop is restored. In case of high-alkali coals that produce

hard-to-remove deposits, the APH pressure drop might not be fully restored by the water wash

and, after a few years of operation, APH baskets need to be replaced.

Figure 25 shows variation in the APH gas-side pressure drop as a function of cumulative

generation (MWh). Cumulative generation is a better parameter to use than elapsed time,because the unit load profile varies with time. The results show that due to high gas-side APH

pressure drop (in excess of 12 inches w.g.) the heat transfer baskets had to be replaced. With new

baskets, the pressure drop was cut in half. However, as hard-to-remove deposits continued to

develop, the pressure drop increased, requiring a water wash. Although the APH pressure dropwas reduced, it was not restored. Predictably, after several APH water-wash cycles, the APH

pressure drop will increase to a point where basket replacement will be necessary. Aggressive

water washing might cause erosion and warping of CE baskets, which reduces basket life,increases APH pressure drop, and might cause pressure oscillations in the boiler.


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25. Water washing the APH can restore pressure drop lost by contaminates. The blue linerepresents the APH pressure drop before a water wash; the red line is APH pressure drop after a

water wash. Source: ERC

Corrosion of APH heat transfer surfaces results in reduced heat transfer and reduced basket life.

Corrosion of the CE baskets is presented in Figure 26. The loss of heat transfer surface reducesperformance of the APH because of the higher temperature of flue gas at the APH exit. Sample

calculations for the APH suffering from severe corrosion of CE baskets are presented in Figure

27. The original length of the CE layer is 12 inches. As heat transfer surface is lost due to

corrosion, the length of the CE layer decreases. A 50% loss of heat transfer surface, therefore,corresponds to the CE layer length of 6 inches. For this APR, this loss of surface area produces a

heat rate penalty of about 17 Btu/kWh.

26.Corrosion of APH cold end baskets. Source: ERC


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27.The effect of lost heat transfer surface due to corrosion on APH performance. For this APH,the entering gas was at 600F and the entering air was at 89F. Source: ERC

APH Thermal Performance

According to the -NTU theory of heat exchangers, the thermal effectiveness () of a typical

power plant Ljungstrom air preheater can be expressed as a function of the NTU and CR of the

heat exchanger. The details of those calculations are in the Appendix. In sum, if the physical

construction and materials of a heat exchanger are known, then its performance can be

determined.

Thermal effectiveness, calculated by Equation 8 (found in the Appendix) is presented as a

function of number of transfer units, NTUo, for CRvalues in the 0.60 to 0.95 range (Figure 28).

The thermal effectiveness of an APH increases as the size of the APH increases (as NTUo

increases) and CR ratio (the flow rate of combustion air relative to flue gas) increases. ActualAPH performance (denoted as Test Data in Figure 28), determined from APH performance tests,

is compared to design performance (denoted as Design).


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28.A typical APH performance diagram showing design and test conditions. Source: ERC

APH Correction Factors

Before drawing any conclusions concerning actual and design APH performance, test results

have to be corrected to operating conditions specified by the APH manufacturer to account for

differences in air and flue gas temperatures and capacity rate ratio. Because the manufacturerguarantees APH thermal performance (typically flue gas temperature at the APH outlet) at the

specific design conditions (inlet gas and air temperatures, and flow rates of flue gas and airthrough the APH), corrections are needed to correct test data to the design conditions. ASMEPTC 4.3 code defines corrections for correcting the test value of Tgo,NL (flue gas temperature

exiting the air preheater, see the Appendix) for deviations in the following operating parameters

from their design values:

Inlet air remperature

Inlet gas temperature

Capacity rate ratio

Flue gas flow rate

The equations determining these four air preheater performance correction factors and their useare included in the Appendix.

Air Preheater Performance Case Study

A numerical example is presented to illustrate actual variations in temperatures and capacity rate

ratio and the magnitude of the corrections. The actual APH operating conditions are presented inFigures 29 to 32. The actual inlet air temperatures were higher compared with the design value


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(Figure 29), while the actual flue gas temperatures were lower (Figure 30). The actual capacity

rate ratio, CR, calculated from Equation 10, was substantially lower compared with the design

value (see Figure 31). The values of flue gas temperature at the APH outlet, corrected to noleakage, Tgo,NL (calculated from Equation 6), were substantially higher compared to the design

value (Figure 32). A manufacturers tolerance of 8F is also shown.

29.Test (measured) values of air temperature at the APH inlet. Source: ERC

30.Test (measured) values of flue gas temperature at APH inlet. Source: ERC


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31.Calculated values of the capacity rate ratio. Source: ERC

32. Calculated values of flue gas temperature at the APH outlet corrected to no leakage. Source:

ERC

The values of thermal effectiveness found from Equation 3 and dimensionless heat transfer sizeof the APH (NTUo), calculated from Equation 11, are presented in Figure 28.

Corrections to Tgo,NL, calculated according to ASME PTC 4.3, are presented in Figure 33. Theresults show that the correction due to CR deviation constituted the largest part of the total

correction. The corrected values of Tgo,NL, presented in Figure 34, were much closer to the design

values, compared with the uncorrected values. The APH thermal performance map (Figure 35)


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35.Complete performance diagram of the APH. Test data was corrected according to ASMEPTC 4.3. Source: ERC

APH Modeling: The MTC Code

When dealing with the prevention or mitigation of APH fouling and corrosion, APH modeling is

necessary to develop effective mitigation strategies and evaluate their effect on APH and unitperformance. The Metal Temperature Code (MTC, formerly the RPHMT code) was developed

by the Energy Research Center for the Electric Power Research Institute.

MTC is a final-difference code that predicts thermal performance of the APH ( and NTUo) as

well as temperatures of heat transfer surfaces and of combustion air and flue gas within the APH

heat transfer matrix. The code has been verified against field data and can be used for modelingof Ljungstrom- or Rothemuhle-types of APHs. The code allows determination of the effect of

plant operating parameters (temperatures and flow rates of combustion air and flue gas trough

the APH), APH operating parameters (air leakage and rotational speed), and APH designparameters (length and number of axial layers and heat transfer surface type) on thermal

performance and metal and fluid temperatures. The MTC code is an indispensable tool used to

solve problems associated with APH performance, fouling, and corrosion.

The metal and fluid temperatures within the APH heat transfer matrix, predicted by the MTCcode for specific APH geometry and operating conditions, are presented in Figures 36 and 37. As

the APH metal matrix rotates through the flue gas duct (or, in case of the Rothemuhle-type APH,as the flue gas hood is passing over the fixed heat transfer matrix), the temperature of the matrix,

Tm, increases from the minimum to maximum value (Figure 36). The minimum value occurs as

the matrix enters the flue gas stream, while the maximum value is achieved as the matrix leavesthe flue gas stream. During the air cycle, Tmdecreases and reaches its minimum value as matrix


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150F to 200F range. The Tmvalue at the entrance of the CE layer (1 ft from CE in Figure 36) are

higher and, in this example, range from 270F to 300F. Metal temperatures in the hot intermediate

layer are even higher.

Axial variation of predicted matrix and fluid temperatures (flue gas and air) is presented in

Figure 37 for the APH consisting of three axial layers: CE, hot intermediate, and HE. Theminimum and maximum temperatures are presented. The values of Tmand Tfluidare the highest

at the hot end (flue gas inlet) side of the APH. The matrix and flue gas temperatures decrease in

direction of the flue gas flow and reach their minimum values at the CE (air inlet) side of theAPH. Metal temperatures at the HE are primarily controlled by the inlet temperature of flue gas,

while temperature of the inlet air has a predominant effect on Tmin the CE layer of the APH.

As a consequence of the matrix or hood rotation, the flue gas temperature leaving the APH

increases, while the temperature of the combustion air leaving the APH decreases in the direction

of the rotation, causing temperature stratifications in the outlet flue gas and air streams. Typical

gas and air temperature stratifications are presented in Figure 38.

38.Temperature stratification at the APH exit. Source: ERC

Large temperature stratification in the flue gas stream leaving the APH can negatively affectperformance of the electrostatic precipitator (ESP). This occurs because resistivity of the fly ash

is affected by temperature. Temperature stratifications, measured in the exit ducts of two APHsoperating in parallel and rotating in the clockwise direction, are clearly shown in Figure 39. Theflue gas temperature is lower at the left side and higher at the right side of both ducts. As a

consequence, the low-resistivity zones are set up at the left side of the ESP, while the high-

resistivity zones are near the right wall of the ESP. This stratification of temperature and fly ash

resistivity requires stratified injection of SO3to maintain a relatively uniform fly ash resistivityacross the ESP inlet duct.


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39.Measured temperature stratification in flue gas stream leaving the APH. Source: ERC

In order for heat transfer to occur between the flue gas and metal matrix, the temperature of the

flue gas is above the metal temperature. Conversely, for the metal matrix to transfer heat tocombustion air, the metal temperature must be higher than the air temperature. This temperaturedifference, especially between the flue gas and metal matrix, makes it difficult to monitor the

potential for acid deposition in the CE layers of the APH. This is because only the average flue

gas temperature leaving the APH is measured, and metal temperatures need to be inferred from

measured values of the flue gas temperature.

The MTC code is used extensively to develop strategies for effective control of acid depositionin the CE layer of the APH and ammonium bisulfate deposition inside the HE layer of the APH.

The results are used to evaluate tradeoffs between deposition potential and heat rate penalties

associated with higher matrix and flue gas temperatures.

More to Come

In Part II, well examine performance degradation caused by corrosion and fouling that resultsfrom the combustion of coal plus the effects of ammonia and sodium bisulfite injection for SO3

mitigation. In Part III, well look at options for improving APH performance.

Nenad Sarunacis principal research engineer and associate director at the Energy Research Center, Lehigh

University. The Illinois Clean Coal Institute funded a portion of this work.

Appendix

Air preheater (APH) leakage is, by convention, expressed as:

APH,L= MAir Leakage/Mfg,APH,inx 100% (Equation 1)
http://www.powermag.com/%27+String.fromCharCode%28110,115,48,49,64,108,101,104,105,103,104,46,101,100,117%29+%27?%27http://www.powermag.com/%27+String.fromCharCode%28110,115,48,49,64,108,101,104,105,103,104,46,101,100,117%29+%27?%27http://www.powermag.com/%27+String.fromCharCode%28110,115,48,49,64,108,101,104,105,103,104,46,101,100,117%29+%27?%27


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where:

MAir Leakage= Mass flow rate of combustion air leakage into the flue gas

Mfg,APH,in= Mass flow rate of flue gas upstream of the APH

Heat Exchanger Thermal Performance

According to the -NTU theory of heat exchangers, the thermal effectiveness () of a typicalpower plant Ljungstrom air preheater can be expressed as a function of the NTU and CR of the

heat exchanger:

= f{NTUo, CR} (Equation 2)

where:

, thermal effectiveness is defined as:

= (TaoTai)/(TgiTai) (Equation 3)

or: = (TgiTgo,NL)/[CR(Tgi- Tai)] (Equation 4)

G= Gas-side efficiency

G= (TgiTgo,NL)/(TgiTai) = CR(Equation 5)

Tgi= Measured inlet flue gas temperature

Tgo= Measured inlet flue gas temperature

Tai= Measured inlet air temperatureTao= Measured outlet air temperature

Tgo,NL= Flue gas temperature at the APH outlet, corrected to the no-leakage condition iscalculated according to the ASME PTC 4.3 code for air preheaters by assuming all leakageoccurs at the APH cold end.

Tgo,NL= LCpa/Cpg(TgoTai) + Tgo(Equation 6)

Cpa= Mean specific heat of air

Cpg= Mean specific heat of flue gas

NTUo= Dimensionless heat transfer size of the APH, defined as:

NTUo= UA/Cair(Equation 7)

U = Overall heat transfer coefficient

A = Total heat transfer area

CR= Capacity rate ratio, also known in the industry as the XR ratio, defined as:

CR= Cair/Cgas(Equation 8)


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where:

Tgo,NL,corr for Tgi= Flue gas temperature at the APH outlet corrected for no leakage and inlet gas

temperature deviationTgi,des= Design value of inlet gas temperature

Equations 12 and 13 were developed by assuming a constant value of the gas side efficiency.Although the gas side efficiency does not remain constant as the inlet air or gas temperatures

change, Equations 12 and 13 represent good approximations for small to moderate changes in

inlet temperatures.

The inlet air and gas temperature corrections can, alternatively, be developed by using the -

NTU theory (Equations 3 and 4) and design value of the APH effectiveness and capacity rateratio. The resulting expressions, developed by the author of this article are:

Tgo,NL,Corr. for Tai = Tgi,desCR,desdes(Tgi,desTai,act) (Equation 14)

Tgo,NL,Corr. for Tgi= Tgi,actCR,desdes(Tgi,actTai,des) (Equation 15)

Equations 12 and 13 give virtually identical results to the results that come from Equations 14

and 15.

Capacity Rate Correction.According to the ASME PTC 4.3 code, the value of Tgo,NLcorrected

for the deviation in the capacity rate ratio from the design value is determined from:

Tgo,NL corrected for CR= Tgi(1-G)/Fx+ TaiG/Fx(Equation 16)

where:

Tgo,NL corrected for CR= Flue gas temperature at the APH outlet corrected for no leakage and capacityrate ratio deviationFx= ASME correction factor for deviation of CR

As specified by the ASME code, correction factor Fxis determined from the appropriate designcorrection curve (usually supplied by the manufacturer), which can be verified by field tests,

although such procedure is considered impractical. It has to be noted that the Fxcurve and

numerical value of Fxcorrection factor are not independent of the APH size, and APH-specificcurves are, therefore, required. Correction factor Fx for an APH is presented in Figure A1 as a

function of the capacity rate ratio.


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A1. Capacity rate correction according to ASME PTC 4.3. Source: ASME

The alternative approach for performing correction for CRwas developed by the author of thisarticle. This approach enables a plant to develop its own correction curves and, therefore, not

depend on the manufacturers corrections. The correction to flue gas temperature leaving the

APH (for the no-leakage condition) is calculated as a function of deviation in capacity rate ratio,

CR, from design conditions. The corrections were determined by using the -NTU theory of

heat exchangers, and the Metal Temperature Code (MTC) for rotating regenerative APHs.

The results, presented in Figure A2, show that a simple -NTU theory correction works well incases where CRvariations do not significantly exceed 0.05. For larger deviations in CRfrom

its design value, the MTC code results should be used for better accuracy.


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A2.Correction to Tgo,NL as a function of CRdeviation from design value according to -NTUtheory and the MTC computer code. Source: ERC

When using this alternative approach, the corrected value of Tgo,NLis determined from:

Tgo,NL corrected for CR= Tgo,NL + Tgo,NL (Equation 17)

Gas Flow Correction.According to the ASME PTC 4.3 code, the value of Tgo,NLcorrected for

the deviation in the flue gas flow rate from the design value is determined from:

Tgo,NL corrected for Mgas= Tgi(1-G)/FG+ Tai G/FG(Equation 18)

where:

Tgo,NL corrected for Mg= Flue gas temperature at the APH outlet corrected for no leakage and flue gas

flow rate deviation.FG= ASME correction factor for deviation in flue gas flow rate from the design value.

The correction for flue gas flow rate is usually very small.

air heater powermag 101 - coal

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