National Workshop on Boiler Corrosion, 11-121h April, 1995, NhML Jamshedpur, INDIA

FIRESIDE CORROSION AND EROSIONPROBLEMS IN COAL BASED POWER PLANT

A.S. KHANNACorrosion Science & Engineering

Indian Institute of Technology, Bombay - 400 076

Abstract

High temperature corrosion problems in coalloil basedpower plants are discussed. The main corrosion problems inpower plants are the waterlsteam side corrosion of the innerwall tubes of the boiler and the oxidation/sulphidation and hotcorrosion of outer walls of hoiler tubes. The former is wellunderstood and can he controlled by controlling the chemistryof the feed water, the fireside corrosion is, however, leastunderstood and is a function of several variables, both operat-ing and material. These are discussed in detail with some of thedata from the failure of plants as a result of fireside corrosionand erosion.

Introduction

High-temperature corrosion playsan important role in the selection ofmaterials for construction of industrialequipments. The principal modes ofhigh-temperature corrosion, frequentlyresponsible for component failure are:oxidation. sulphidation, nitridation,carburization, hot corrosion and flyasherosion. Oxidation most often partici-pates in the high-temperature corrosionprocess, regardless of the predominantmode of corrosion. In fact, alloys oftenrely upon oxidation reaction to developa protective oxide scale to resist varioushigh-temperature corrosion. Environ-ment plays an important role in all typesof corrosion attacks. The environmentsin coal/oil based power plants containseveral corrosive species which mayresult in "oxidizing" (very high oxygenactivity) or "reducing" (very low oxygen

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activity due to combustion under stoichio-metric or sub-stoichiometric conditions)atmosphere. The reducing atmosphere isgenerally more corrosive for corrosionmodes, such as sulphidation, and ash/saltdeposit corrosion. Moreover, the fuelcontaminants can form ash/sal deposits onmetal surfaces during high-temperatureexposure which play significant role in thecorrosion process. For example, sulphurfrom the fuel and NaCl from the ingestedair may react during combustion to formsalt vapou r, suc h a s Na2SO4 which at lowertemperatures, deposits on the metalsurfaces, resulting in accelerated corro-sion, caused by the chemical reaction be-tween the protective oxide scale and thesalt deposit, leading to the breakdown ofthe scale. Such a corrosion processoccurring in the presence of salt deposits istermed as "Hot Corrosion". Thepresence of large particles of quartz, FeS2or Al203 in the flyash causes yet another

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problem known as "Flyash Erosion".Fireside oxidation/hot corrosionand flyasherosion are the major problems incoalfiredboilers. Boiler statistics1'1 for coal-basedpower plants indicate that flyash erosionaccounts for about 28% of waterwall tub-ing failures and about 22% of superheaterand reheater failures; fireside corrosioncauses0.5 and 1% of failures, respectively,in these areas. Problems with boiler pres-sure parts account for an equivalent avail-ability loss of around 7.5% (waterwall andeconomizer tubes: 4% and superheater/reheatertubes: 2.2%). However, thedeter-mination of the availability loss, due to thecauses mentioned above, is currently diffi-cult because boiler tube damage and fail-ure occur essentially in all regions of theboiler. Waterwall s suffer fai l ures from bothfireside corrosion and flyash erosion, asdosuperheaters and reheaters, where aseconomizers usually suffer only erosionfailures because of their position in theboiler. The objectives of this paper are toassess the importance, as related toavailability, of boiler tube failues due tocorrosion and to discuss the causes andmechanisms thereof. Attempts have alsobeen made to examine the limitations ofthe current preventive and controltechnologies, and to provide direction fortheir improvement, more efficientimplementation , or development of newtechnologies.

Corrosion Problems in Coal/Oil BasedPower Plants

A fossil-fired steam power plant isillustrated in Fig. 1. Three fl uid flow loopscirculate through the system: fuel-air,water-steam, and condenser cooling. Inthe fuel-air loop, the fossil fuel is burnt inair and transfers its heat to a series of heat

exchangers. In the water-steam loop, cleanfeedwater is converted into superheatedsteam in a boiler, which expands through aseries of turbines, converting its heat intomechanical energy. In the condenser-cooling loop, cold water is passed throughthe condenser and can be recirculated oris exhausted back to the source of thecooling water. Each fluid loop possessesits unique corrosion problems.

The fossil fuel is burnt in a very largechamber constructed of water walls(consisting of vertical or spiral tubes weldedtogether in a web), where the feedwater isheated. In subcritical boilers, the saturatedsteam is superheated in tubular heatexchangers. In supercritcal boilers, theliquid becomes superheated vapour with-out undergoing a phase change. The boilerfluid may become acidic or caustic,depending on the presence of corrosiondeposits and flow interruptions. Underacidic conditions, the steel boiler tubesmay be hydrogen embrittled; undercaustic conditions, the tubes may becaustic gouged.

The corrosion in a steam boiler maybe represented by the equation:

3Fe(s) + 4H20(1 or g) = Fe3 04 + 4H2(g).

From the corrosion point of view , a boileris nothing but a thin film of magnetic ironoxide supported by steel . This oxide filmiscontinuously damaged and repairedduring boileroperation , with simultaneousproduction of hydrogen . The superheaterand reheater tubes suffer from steamoxidation of the inner surfaces and hotcorrosion of the outer surfaces . The fire-side corrosion is a typical problem. Incoal-fired boilers , it exhibits a maximumrate at 700 to 750C, where the corrodent is

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a liquid, and decreases to a minimum athigher temperatures. Fig. 2 shows the mainparts of a boiler susceptible to hotcorrosion problems. There are essentiallythree distinct high-temperature corrosionproblems in coal- fired boilers :

Water-solubleNa+K(Wt.%)

Corrosiveness

0.50.5-1.01.0

LowMediumHigh

a. Fireside Corrosion of Superheater/Reheater and Waterwalls,

b. Flyash Erosion of Waterwalls andSuperheater/Reheaters,

c. Steamside Oxidation in the Super-heater/Reheaters.

Causes and Mechanisms of High-Temperature Corrosion

When coal particles are introduced intothe flame, the moisture and the volatilespecies are driven off, the fixed carbon inthe individual particles begins to burn.The contained mineral matter may bemelted or vaporized, and is largelyoxidized. The sulphur-containingcompounds in the coal (such as FeS) areconverted to oxides such as Fe2O3, K2O,Na2O, SO2 and SO3. The relativeproportions of SO2 and SO3 in the flamedepend on the available oxygen and thetemperature. SO2 is thermodynamicallyfavoured at higher temperatures (>70('C);the formation of SO3 can be catalyzed bycertain metal oxides, so that the proportionof SO3 in flue gas may increase down-stream of the burners. Thus the gaseousspecies released, as the coal passes throughthe flame, contain potential corrodents suchas sulphur, vapour ofalkalimetal salts, andchlorine compounds (mostly HCI). Thequality of coal used is very important.Raask^2J has proposed a simple three-category ranking of the corrosiveness ofcoals based on the sum of the percent-ages of water-soluble sodium andpotassium in the coal:

A correlation has been found betweenthe corrosion rate of superheater/reheatertubes and the chlorine content of thecoal[3).Chlorine (more than 0.2 wt %) has beenfound to promote the release of both Naand K into the flame , and acts as a strongcatalyst for the molten trisulphate attack.There is also evidence that HCI formed inthe flame can destroy the Fe2O3 layer on asteel surface, thereby exposing it toadditonal oxidative attack[41.

Fireside Corrosion Problems

The fireside corrosion of various com-ponents of a coal-tired boilermay be attrib-uted to the following:

1. Reducing (sub-stoichiometric) condi-tions caused by impingement of in-completely combusted coal particlesand flames,

2. Accelerated oxidation from overheat-ing, and

3. Molten salt or slag-related attack.

The fireside corrosion is generallylocalized to regions on the walls near theburners. The thick, hard, external scalesformed often exhibit cracks which resemblean alligator hide as shown in the Fig.3.Reducing atmospheric corrosion can re-sult due to direct reaction of the waterwalltubes with a sub-stoichiometric gaseousenvironment containing sulphur, or withpartially combusted char containingFeS,. The reducing conditions have twomain effects on corrosion. First, they tend

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to lower the melting point of any depos-ited slag, increasing its ability to dissolvethe normal oxide scales, and second, thestable gaseous sulphur compounds underthese conditions include H255-'1, which ismore corrosive than SO2 that predomi-nates under oxidizing conditions. Fig.4shows the cross-section of a 1.25 Cr-alloywaterwall tube removed from a boilerwhere it was exposed to "reducing"atmosphere corrosion. The scale is amixture of sulphide and oxide, whichsuggests that the conditions in thecombustion gas at the waterwall wereclose to those suggested by the Fe 304/FeS boundary in the phase stabilitydiagrams shown in Fig.5.

Overheating of the superheater isrelated to the poor design of the boiler,when slagging problems are experienced.Change in parameters such as the feed rateof coal to attain the desired steamtemperature can cause overheating.Overheating of the reheaters can occurin rapid startup situations, when thecombustion gas temperature at thereheater reaches its maximum valuebefore full steam flow through the reheateris achieved. The overheating leads toaccelerated oxidation of both the firesideand the steamside surfaces of the tubes toproduce thickened, hard (alligator hide)scales . Above 570C, a very non-protective scale of wustite (FeO) can beformed on iron which leads to the onset ofrapid oxidation.

Mechanism of Molten Salt or Slag-Related Attack

Molten salt or slag-related attack takesseveral forms. Local disruption of thenormal oxide film on the wall tubes canlead to either accelerated oxidation, or to

oxidation/sulphidation attack due tosulphur species in the slag. Alkalisulphates deposited on the waterwallsmay react with SO2 or SO3 to formpyrosulphates such as K2 S207 & Na2S,07,or possibly complex alkali-irontrisulphates, the latter compounds beingformed in thicker deposits after long timesat about 482CC81. The K2SO4-K2S2O7 sys-tem forms a molten salt mixture at 407Cwhen the SO3 concentration is 150 ppm.The above mechanism maybe depicted bythe following sequence of reactions:

K2SO4 + SO4 (or SO) -> K 2S207 (407C,150 ppm SO3)

(K2SO4+K2S2O7) -Molten Salt Mixture,K2S2O7 + 3Fe -a Fe2O3 +K2SO4

By such a mechanism, the pyrosulphatecan react aggressively with any protectiveiron oxide scales on the tubes, and lead toaccelerated wastage through fluxing of theoxides and attack of the substrate metal.The corresponding sodium system canbecome liquid at 400C with about 2500ppm of SO3. Such high concentrations ofSO3 is possible in the stagnant regionsbeneath deposits, so a similar attack byNa2S2O7 may occur when a high-sulfurcoal produces combustion gases contain-ing high levels of sulphur oxides. Rei&1has pointed, however, that the levels ofSO3 present at this location in a boilerburning a typical coal are such that K2S2O7is unlikely to be found at temperaturesabove about 510C, and Na2S2O7 only upto about 400C.

Deposit-related molten salt attack ofthe pendant tubes concerns the develop-ment of conditions beneath a surfacedeposit which are conducive to theformation of a low melting salt of the type

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(Na, K)3 Fe(SO_4)3. Catalytic oxidation ofSO2 in stagnant zones beneath a layer ofdeposit can lead to nearly equilibriumlevels of SO3, so that conditions arefavourable forthe formation oftrisulphatesin deposits up to about 704C. Above thistemperature, the required SO3 concentra-tions cannot be sustained, and thetrisulphates become unstable, decompos-ing to the alkali sulphates which are solid.There is wide acceptance that compoundsof this type play a critical role in thecorrosion of superheater tubes.

Deposits on the superheater tubes areusually found to be tightly bonded to thetubes at the room temperature. They typi-cally consist of three distinct layersH"'l:

i. A hard, brittle and porous outer layer,which is the bulk of the deposit and hasa composition similar to the boilerflyash.

ii. A white intermediate layer. When thislayer has a chalky consistency, corro-sion is found to be mild or non-existent.When it is fused and semi-glossy,corrosion is found to be severe.Compounds identified in this layerinclude complex alkali sulphates, landthe alkali-iron trisulphates.

iii. A black, glossy inner layer, composedprimarily of oxides and sulphides ofiron.

The typical appearance of a corrodedsuperheater tube is illustrated In Fig.6. Tilethickened, non-protective scale formedbeneath such deposits comprises of mi xedlayers of iron oxides and sulphides as isevident from Fig.7.

Fireside Erosion Problems

Light erosion damage is usually main-

fested by polishing of the affected surface.The eroded area is often quite clean andfree of deposits. Thinned and flattenedareas result from more severe erosion.Erosion tends to be localized to particularareas of the boiler, and to part icular parts ofa given tube bank . Fig.8 shows the typicalappearance of flyash erosion damage onall economizer tube.

The factors in coal which contribute tofireside erosion problems are largeparticles of dense minerals such as quartz,or FeS2, and those mineral constituentswhich may be converted during combus-tion to hard/abrasive compounds such asalumina and silica based oxides.

System variables (such as flyashparticle velocity and the angle of impinge-ment), and operating variables (whichdefine the size, shape, hardness andnumber density of the flyash particles), arevery important. The rate of erosion loss isusually found to be proportional to:

Q the impact velocity raised to a powerbetween 2 and 4, and

0 the number of individual impacts bythe flue gas.

Erosion damage is, therefore , a poten-tial problem at any point on the fireside ofthe boiler where the ash -laden flue gascontacts boiler tubes or internal supportstructures at velocities and with particleloadings above some minimum values.Flyash erosion of waterwall tubes isgenerally encountered in the areas aroundthe top of the rear wall of the furnace,where the flue gas is turned to flow throughthe rear pass. Erosion results largely fromturbulence created by the change in flowdirection, and by flow around pendant tubebundles.

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t 1 t Rn.lwl i ---f T' '

r ^^^r rrs F. r .^t.a,r S^ am.o,r.^sprec^D.,a,D.

InO^. ,ray t

Fig. I : The schematic of a coal-fired power plant

.LPF.... f .r[I, .

CCL ECII("N

F II

,1. :NL I At ;PF ".(.t'4 .'

FurUJAL [ wAl . 'I,R[5

wunST ON EIDE WALLS

Fig. 2 : The schematic of a coal-fired boiler showingthe main parts susceptible to hot corrosion

''if,41ATER:

^^'.urrIMF a'tnr,

r1FAr11

--(-- n nr.nr.E :,nu , r) Nn17SI0N

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Other type of erosion in the regions ofthe waterwalls occurs from ash or slagentrained by wall blowers, or possibly bydirect impingement of the flames.

The erosion is rarely the cause of thetube fai lures in outlet superheater/reheatertubes. Erosion can occur in these areaswhen the gas velocity is locally increasedabove the design level (I 5-20m/s). Block-age of the normal gas flow path by slag orash deposits can result in channeling of theflue gas through the tube banks, with rapidthinning of the tubes.

Flyash Properties

can cause the tube failure in 10,000-50,000hrs., which is in good agreement withpractical experiences (usual design veloci-ties are 15-20 m/s). Kratina^121 has pro-posed an erosion prediction method basedon "Coal Erosiveness Factor (CEF) whichis determined as follows:

CEF= 8.25/HHV x (% ash) x a, (% erodents),,

where 8.25 is a constant related to unit heatinput, HHV is fuel heat value (in B ThU)and a, is the erosion index of the erodentspresent. The erosion index is determinedas follows:

Raask1"I developed the concept of an"Erosive Index" for coals relating the ero-sivity to the quartz content of the flyash,which he has defined as:

Erosion by flyashINDEX(I) = ----------------------------------

Erosion by equal weight of100 m quartz particles.

The value of I was found to range from0.2 to 0.4. From experimental results, ero-sion rate of mild steel by 100 m quartzgrains is given as:

a.(% erodents)=a,(%quartz) + a2(%S'02)+ a3(% A12O3) + a4(% Fe2O).

Interpretation of CEF value for tube bankvelocities in the range of 16 to 21 m/s areas follows :CEF Predicted ErosionValue0 - 0.5 No erosion problem0.5-1.0 Mild to persistent erosion1.0-1.5 Serious to very serious erosion1.5-more Severe erosion

Control of Hot Corrosion/ErosionProblems in Coal-Fired Boilers

W =9.5x10-10xW xU2.5m

where W = weight of the metal eroded,Wm = weight of impacting particles (Kg),U= velocity of the particles (m/sec).

The corresponding expression for erosionby flyash is:

W =9.5x 10"xIx W xU2.5m

Using these calculations, one can saythat at velocities around 35 m/s, erosion

The complete el i rnination of oxidationand hot corrosion/erosion problemsencountered in coal-fired boilers is notpossible considering the complexity of theenvironments and the mechanismsinvolved. However, with proper selectionof contruction materials and design ofcomponents, modification of operatingconditions, use of good quality coals, andapplications of various heat-resistantcoatings, the problems can be effectivelychecked to a great extent.

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Fig. 3 : The thick, hard scale on waterwall tubesshowing the cracks which resemble an alligator hide

Fig. 4 : Cross- section of the corroded face of a waterwall tube subjected to a"reducing" atmosphere . X-ray maps show that the inner scale contain iron and

chromium from the alloy, and sulphur.

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From the discussion in precedingsections, it is clear that waterwalls, super-heater/reheater and economizer tubes ofthe boiler suffer from the severe attacks ofsteam oxidation of the inner surfaces, andfireside hot corrosion/flyash erosion ofthe outer surfaces. To combat theseproblems mainly two types of strategiesare adopted: (i) Modifications inOperation Parameters, (ii) Use ofCorrosion/Erosion-Resistant Materialsand/or Coatings.

The waterside corrosion problems inboilers can be controlled mainly by form-ing a good magnetite coating on the nor-mally used steel walls of the boiler and itsmaintenance. The appied coating shouldbe least permeable to water, because wateris the species responsible for the continu-ous damage of the thin magnetite filmformed on the steel during the operation.Purity of feed water is also an importantfactor from the corrosion point of view.Hence, the chemistry of feed water shouldbe monitored and efforts be made to re-duce the quantity of dissolved gases andsalts in it.

Fireside Corrosion"Reducing" condi-tions near the water walls can be counteredby adjusting the air and fuel distributionto burners to promote better mixing of airand fuel and more uniform combustionconditions. Air blanketing, that is, intro-ducing flow of air along the walls throughopenings in the membrane betweenwaterwall tubes, can also be effective.Fireside hot corosion of superheaters andrepeaters can be checked by reducing thelevels of the chemical species in the coal.This is done by blending of coal types andcoal washing which improve thestoichiometric balance of the coal and airflow to each burner. Standard coal washing

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can remove approximately one-half thesulphur and alkali metal content of thecoal. Blending a known corrosive coalwith another also helps to produce a lesscorrosive ash.

The corrosion of superheater/reheatertubes can be controlled also by limiting themaximum temperature of steam generatedto 538C or, in some cases, 556C. More-over, the tube metal temperature should bemaintained in a regime where the rate ofcorrosion from alkali-iron sulphate-typeattack is considerably less than the maxi-mum possible; this is illustrated in Fig. 9.However, in normal circumstances free-dom to change the operating conditions isquite limited; and sometimes the changesare not very effective. Thus the abovemodifications in operational parametersdo not provide a long-term solution tocorrosion problems.

Use of More Corrosion ResistantMaterials

Direct replacement of tubes or theapplication of acorrosion resistant alloy asa coating on the affected tube are twoapproaches. The former is probably themost satisfactory solution, e.g., replacingthe tubes with high chromium alloys, suchas stainless steels. But it also involves costsseveral times those of the original tubes.Moreover, such a replacement is not verysatisfactory for waterwall corrosion, asaustenitic stainless steels are prone to beattacked by chlorides. Existing lowchromium ferritic steels are quite satis-factory. Thus a replacement which satis-fies the waterwall corrosion and for whichthe existing chromium concentrationis sufficient, an outer Cr-rich alloy isneeded to combat fireside corrosion. The

" I4Nn l ^^+1111A1111^14^1111lAt11lNB 11AI ^II^IPI^IA^NIIi IPl ^Y

A.S. KHANNA

..60,

1 .,o. 1 ..,o,

e .e ^o ro _e

I

1Ie M M .. 1e '. e

i! Po 1l...y

Fig. 5 : Phase stability diagrams at 482C indicating the stable corrosionproducts as a function of percentage stoichiometric combustion air

Fig. 6 : Typical cross- sectional appearance of asuperheater tube which has fireside corrosion

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materials choice is probably betweenthicker-walled carbon steel tubes and tubeswith a coextruded outer layer of a highchromium alloy such as AISI310orlncoloy671(50Cr-50Ni). Cladding a tube with anouter layerof acorrosion-resistant alloy byco-extrusion can provide a more cost-ef-fectivesolution. Forexarnpie, co-extrudedtubes of type 310 SS on mild steel can bean economically viable replacement forbase-loaded units1' . Increased corrosionresistance can also be achieved through theenrichment of surface to be protected withelements suchaschromium oralurninium.The use of surface nitriding has also beenproposed, based on laboratory results114'.Chromized and aluminized tubes madeusing pack cementation method are goodoptions for control of corrosion) 15I.

use of corrosion-resistant alloys for someof the tube rows in the superheaters andreheaters is recommended . Compositematerials, where a corrosion resistant alloyis clad over the load-bearing alloy by co-extrusion , have been found to be costeffective in such applications . Inconel671(Ni-48Cr) and AISI 310 (Fe-20Ni-25Cr) can provide superior corrosion re-sistance to the other austenitic materialsqualified for use in this applicationst161.

Coatings of the type discussed forwaterwall tube protection have not provedeffective in this case. There is little re-ported experience with coatings on super-heaters and reheaters in recent years, eventhough some of the modern coatings (suchas FeCrAIY-types) may be effective.

Flame-or plasma-spraying is also usedto apply corrosion-resistant alloys intowaterwall tubes. The alloys used aretypically based on aluminium, oron chem-istries similar to those of the corrosion-resistant alloys with high chromiumcontents or chromium and aluminiumcontents. Plasma coating of AISI 310stainless steel powder can be effectivelyemployed. To encounter corrosion of thesuperheaters or reheaters, attachement ofshields to the leading edges of the tubeaffected, can be an effective measure.Shields are strips of metal which areattached to the tube by means of tack-welded straps. The shields must be resis-tant to high-temperature oxidation, and areusually made from alloys such as AISI309(25% Cr). It is also desirable that thesuperheater and reheater tubes that havethe highest metal temperature should befabricated of stainless steels such as AISI304, 321, 347.

In boilers operating at 566C steam,

Very little is known regarding the useofadd 1 tives to combat corrosion in coalfiredboilers. However , Mg-Based additives,such as, MgO and Mg(OH)2 have beentried in some cases to prevent corrosionby sulphuric acid condensation in thecold end of the boiler . Also, Rahmelt17reported that addition of either Mg orCa-sulphate reduced the corrosion of stain-less steels caused by K2SO4 . CaO has alsobeen used in some cases.

Flyasla Erosion

Changes in operational parameters,such as washing and blending of the coalsto lower the specific ash content of thecoal, changing the fineness of the coal todecrease erosivity of the coal, and theregular sootblowing to prevent pluggingof loose deposits can reduce flyash erosionof the boiler components.

The typical coal fineness specificationis 70 percent passing through 200 mesh.

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Fig. 7 : Details of the deposit and scale on afireside corroded superheater tube

Fig. 8 : Typical appearance of flyash erosion damage on an economizer tubes

-60Ch,Cm. 11'. aII ev i c SIni

40s

^I

Il C, -h h

zo ^

34 ^vj VI:70Ci : 1001 11 ^(AI i l ^`Cr .1C

4N r^Oe.^^rt C Il)

Fig. 9 : The bell-shaped curves showing theeffect of metal temperature on corrosion rate

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A change in the fineness of the coal to lessthan 0.5 percent remaining on a 50 meshsieve has been reported to be effective inreducing erosion ^i81. The effect is possiblydue to adecrease both in the residence timefor complete combustion and the numberof impingement ofchar on thewalls. Othermodifications include: reducing the bulkgas velocity by use of lower excess air andoperating at reduced load, and control ofthe gas flow across the boiler section toeliminate localized turbulent region. Toreduce local high velocities, baffles havebeen used with mixed resultst'91. The othermeausres to combat wastage due to ero-sion are the attachernent of shields, andpad-welding. The shields are essentiallythe same as those used to combat firesidecorrosion of the superheaters and reheaters.

In general, pad-welding should bediscouraged, since it is one of the causes ofa large number of repeated failures. Itshould only be used in an emergencysituation. However, the combined use ofpad-welding and shielding can be a goodapproach to protect less accessible areas ofthe tube banks in which erosion damagehas occurred.

Use of Materials and Coatings

Materials with demonstrated erosionresistancecan be used as sleeves or shieldsin critical areas, but few instances of theextensive use of such metallic materialshave been reported. Pourable refractoriesare sometimes overlaid on componentssuch as headers which are exposed to anerosive gas flow, but the experiencesuggests that such coatings must berenewed at every opportunity. However,embedding of economizer elbows inrefractory cement has proved effective insome cases.

Conclusions

The following conclusions are drawnon the basis of the above discussion:

1. Fireside corrosion and flyash erosionare the major problems for the powerplants using coal which has high sul-phur (> 2.5 wt.%), alkalies(>0.5 wt.%)and chlorine (>0.2% wt.%), or whichcontains high percentage of erosiveminerals such as quartz in the ash.

2. Flyash erosion appears to be moresignificant problem than firesidecorrosion.

3. The problems requiring most attentionare:

i. erosion of back pass superheater/reheater and economizer tubebundles by flyash, and

ii. corrosion of waterwalls by the lowoxygen, high sulphur conditions.

4. The problems of fireside corrosion andflyash erosion are mainly tackled byextensive maintenance; though per-manent and long-term measures, suchas the use of more corrosion resistantalloys or cladding for fireside corro-sion, and gas flow modifications by theuse of screens/baffles for flyash ero-sion, have also been taken at manyplaces.

5. R & D effort is needed to:

i. incorporate permanent solution tofireside corrosion problems eitherby the application of proper coat-ing of waterwall/superheater tubesby a high chromium alloy or by

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using co- extruded tubes or tubesof better materials, and

ii. establish correlation from actualfailure analyses.

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13.T.Flatley, E.P. Latham and C.W.Morris, "Co- extruded Tubes ImproveResistance to Fuel Ash Corrosion inU.K. Utility Boilers", Materials Per-formance, 20, 12-17 (1981).

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A.S. KHANNA

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