Superheater fireside depositsand corrosion in kraft recovery boilers

ABSTRACTSuperheater fireside deposits are formed by the impingement of particles ofblack liquor residue entrained in combustion gase-s (carryover), and bycondinsatioir of chemicals vaporized in the lower furnace (vaporization-condensation). Deposits are principally sodium, sulfate, and carbonate, butthere are also significant concentrations of chloride and potassium. Theenrichment of chloride and potassium salts depends strongly on thetemperature of the deposit, with Iower temperatures favoring enrichment andhigher temperatures resulting in depletion. Enriched deposits have lowermelting points, of which the first melting point and the radical melting pointare particularly important. The former allows accelerated corrosion caused bythe liquid phase, and the latter is the temperature at which both slagging andmore aggressive liquid-phase corrosion occurs. Laboratory corrosion resultsshow that potassium can cause an enhanced attack on mild steels as well assuperalloys. Enrichment of chloride and potassium salts in fireside depositsand thus increased corrosion of superheater tubes is not limited to the closedcycle mill.

KEYWORDSSuperheaterRecovery furnacesDepositsPotassiumSodium chlorideCorrosionKratt mil ls

Douglas W. Reeve, Hoc Nghia Tran, and David BarhamDepartment of Chemical Engineering,and Applied Chemistry, University of Toronto, Toronto,Ont . , Canada

The closed-cycle mill makes possiblethe elimination of all aqueous effluentfrom bleached kraft pulp mills (l).Bleach plant effluent is used in brownstock washing and cooking l iquor prep-aration, introducing sodium chloride(the spent bleaching chemical) into thepulping chemical recovery cycle. Thesodium chloride is then removed bywhite l iquor evaporation in a saltrecovery process (2).

The world's first closed-cycle kraftmill at Great Lakes Forest Products inThunder Bay, has been recoveringbleach plant effluent since March of1977. Severe superheater corrosion wasdiscovered in the mill after only 15months of operation, prompting thisstudy. In response to the severe super-heater corrosion, the mill init iated theextensive repairs required, lowered theoperating steam temperature, tempo-rarily suspended bleach plant effluentrecovery to minimize chloride concen-tration in the superheater slag, andfostered a collaborative investigation

of materials of construction (3) and ofsuperheater f ireside deposits and theirrole in corrosion (4,5).

Chloride (6) and potassium (Z-9)accumulate in all tightly closed pulpingchemical recovery systems, not just theclosed-cycle mill. Chloride and potas-sium are enriched by vaporization inthe upper sections of the furnace,significantly altering the compositionof superheater deposits. Lower meltingpoints can result and may lead to severecorrosron.

Experience in theclosed-cycle mill

The recovery boiler in the mill inThunder Bay is a Combustion Engi-neering V2R Type rated at 900 psig,900"F, and 3 mill ion lb/day of solidsfired (6201kPa,482"C, and 1.4 mill ionkg/dav). The superheater is a pendanttype with three stages, constructed ofcarbon steel ranging in grade up to T-22 (2.25% Cr, 17o Mo).

A tube leak in June 1978 led to thediscovery of severe corrosion in thehottest sections of the third stage and,to a lesser extent, in the second stage.The corrosion was worst on one side ofthe boiler, indicating that the tempera-tures were higher on that side. Thiswas later confirmed by steam and fluegas temperature measurement. At thebottom of the third-stage steam outletleg, comosion had produced extensivethinning and a very rough surface, asshown in Fig. 1.

The mill responded as follows. Thesuperheater outlet temperature wasdecreased from 850-875 to 700-750'F(454-468 to 371-399'C). Bleach planteffluent was suspended and later l im-ited to control NaCl to less than 30elliter in white liquor. The damagedplatens in the second and third stageswere replaced. The first stage wasremoved to lower the operating tem-perature in the second stage. The cross-furnace temperature difference wasinvestigated, and means of eliminating

Reprinted from Tappi, The Journal of the Technical Association of the Pulp and Paper Industry.Vol. 64, No. 5, May 1981, Copyright, 1981 by TAPPI, and reprinted by permission of the copyright owner.

it were evaluated. The lower bends ofthe outlet leg in the third stage werechanged to Incoloy 800 (32o/oNi,2lo/oCr,460/oFe,1.57o Mn, 1% Si, 0.6% Al).

Closed-cycle operations were resumedin January 1979. Chloride concentra-tions are closely controlled, and steamtemperature is being raised very grad-ually as corrosion is closely monitored.

Ghloride and. potassiumaccumulationIn the pulping chemical recovery cycle,minor components, such as chlorideand potassium, accumulate dependingon the input rate and the degree ofclosure or bleed rate. Sodium chlorideinput of up to 20 kglton of pulp withseaborne logs has led to accumulationto over 100 g/liter of NaCI in whiteliquor (6). In the closed-cycle mill,40-100 kg of NaCl per ton of pulp enterthe liquor cycle, but the salt recoveryprocess controls accumulation to about30 g/liter of NaCl in white liquor. Allkraft chemical recovery cycles havesome input of chloride from water,makeup chemicals, etc., and thus ac-cumulation results.

Presently, 4-8 g/l iter of NaCl inwhite liquor is not uncommon. Accu-mulation may also be expressed on amolar basis, i.e., Cl/Na X 1007o. (Theauthors suggest calling it "salinity.")The salinity of the previously men-tioned white liquor concentration is4-8o/o. ln the c losed-cycle mi l l thesalinity of white liquor is l2-I8o/o.

Similarly, there is always some inputof potassium into the l iquor cycle,chiefly with the wood. Potassium con-centration in wood varies from 0.03 to0.30% or from 0.6 to 6 kglton of pulp ( Z).Some hardwoods and the bark inwhole-tree chips are particularly richin potassium.

Potassium accumulation, generally,has reached only 2.5-3.5% K/K * Na.Thus, with the trends towards increas-ing c losure of l iquor systems andincreased use of hardwoods and whole-tree chipping, it is probable that potas-sium concentrations of 10-20% KIK +

Na will not be uncommon. Some datafor chloride and potassium concentra-tions in liquor cycles are given in TablesI and II.

Ghloride and potassiumenrichment

Of the smelt chemicals present in thelower section of the kraft recoveryboiler, sodium chloride, potassium chlo-ride, and potassium hydroxide areamong the most volati le (6,7). As aresult, these components are vaporized,only to be condensed in cooler regionsin the upper furnace. Thus, the depositsin the upper furnace can be enriched inchloride and potassium.

The data in Tables I and II show theenrichment that occurs in the precipi-tator dust. The chloride enrichmentfactor, from white liquor to dust, is anaverage of.2 for all data in Table LEarlier investigators have found en-richment factorsof 2.2 and 2.5 (6), and,at very low concentrations of chloride((5% salinity), 3.5-4.0 (smelt/dust) (i0).The potassium enrichment factor is anaverage of 1.8, white liquor/dust. Theonly other data reported is an averageenrichment factor of 1.6 for severalFinnish mills (9).

With a modest degree of closure,accumulation in the liquor cycle to 57oon a molar basis (K/K * Na or CllNa)wi l l occur , and enr ichment in thefurnace by a factor of 1.8-2.0 will resultin enr iched deposi ts in the uppersections of the furnace having 107opotassium and chloride concentrations.

Thus, the problems of enriched, low-melting-point superheater deposits arenot limited to the closed-cycle mill butexist in all mills with moderately closedliquor cycles.

Superheater depositsComposltlonAs wil l be discussed in more detaillater, there are two basic mechanismsfor formation of deposits in the super-heater: carryover and vaporization-condensation. As a result, the composi-

l. Close-up photograph of superheatercorrosion (lower bends, steam outlet leg,f inal superheater section).

tion of superheater deposits can varyfrom highly enriched in chloride and/orpotassium to non-enriched depositsderived from carryover. Examples ofthe types of deposits possible are shownin Table III.

FormatlonInspection and chemical analysis of thedeposits formed on air-cooled probesreveal that they are formed by twodifferent mechanisms. Burned blackliquor particles are entrained in theupward-fl owing combustion gases, andthey strike the superheater surface,forming a hard, thick layer. The secondmechanism is vaporization in the lowerfurnace, followed by condensation incooler regions in the upper furnace.The vapor diffuses inward throughcracks in the outside deposits andcondenses on the cooler surface of thesuperheater tubes, forming a layer ofwhite powder on the tube surface. Thiswhite powder is enriched in chlorideand potassium salts, quite similar to

l. Chloride around the recovery furnace,CllNa mole o/o-sali nity

l l. Potassium around the recovery furnace,K/K + Na mole o/o

C/osed- lnlandcycle mill mill

West Coast mills

B lack l iquor , f i red 17 .2 8 .6 27 .5 17 .1 11 .0Smel t 15 .3 25 .9 17 .1 9 .9Green l iquor 15 .7 7 .0 25 .1 16 .5 11 .3Whi te l iouor 12 .5 7 .O 24.6 17 .9 10 .8Precipitator dust 3'1 .2 2O.8 30.6 22.4 17.6

Enr ichment Rat ioDust /b lack l iquor 1 .9 2 .4 1 .1 1 .3 1 .6Dust /wh i te l iquor 2 .5 3 .0 1 .2 1 .2 1 .6

CC/osed- lnland

cycle mill mill

West Coast mills

B lack l iquor , f i red 2 .1 '1 .6 2 .9 1 .7 1 .5Smel t 2 .2 3 .1 2 .1 1 .9Green l iquor 2 .2 2 .3 2 .5 1 .9 1 .7Whi te l iquor 2 .4 2 .7 2 .9 2 .2 2 .1Precipitator dust 4.3 5.0 6.0 5.5 2.8

Enr ichment Rat ioDust /b lack l iquor 2 .2 3 .1 2 .1 3 .2 1 .9DusVwhi te l iquor 1 .7 1 .9 2 .1 2 .5 1 .3

1 1 0 Vol.64, No.5 May 1981 I Tappi

Cold end

furn"Nwal l l

Deposit A' zrLJCGFlue gas

Cross.section AA'

B'

Cross-section BB'

B

White powderFused to metalVoidFused exterior

B'

2. Close-up photograph of 800-hr probedeposit.

dust captured by the electrostatic pre-cipitator but with a higher carbonateconcentration. Figure 2 shows the twodifferent types of deposits very clearly.

Figure 3 illustrates the formation ofthe deposits on an air-cooled probe. Atthe cold end of the probe, the depositsare thicker and more porous than atthe hot end, where they are fused andcontinuous.. As the deposits mature,they sinter'or fuse and grow thicker.The outside surface, as a result, be-comes further and further removedfrom the cooling steam until it is nolonger cooled below the deposit fusionpoint. The surface is then molten, andthe deposit stops growing. The lowerthe steam temperature (all other factorsbeing equal), the thicker the depositmay become.-

Figure 4 shows the composition ofthe deposit through its cross-section.The inside layer was enriched in chlo-ride and potassium salts compared tothe outside layer, in agreement withthe fact that the temperature washigher on the outside and lower on theinside. The presence of the chlorideand potassium salts in lower concentra-tion in the outside layer than in thesmelt from which they were derivedindicates depletion of the highly volatilesalts as a result of vaporization at hightemDerature.

3. Deposit formation on the air-cooled probe.

Thermal propertles

Thermal properties of the kraft re-covery boiler deposits were intensivelydiscussed in a previous paper (d). Itwas pointed out that even a smallamount of sodium chloride and potas-sium can dramatically alter the thermalpropertiesof the deposits. Experimentson the effect of sodium chloride on themelting points of Naz SOa-NazCOg mix-tures (NazSOa/NazCOe molar ratio :4) showed that addition of up to 33 mole% NazClz resulted in depression of thecomplete melting point from 1560'F(849"C) to about 1150'F (621'C). De-pression of the first melting point to1150oF, however, occurred rapidly asthe first few mole 7o of NazClz wereadded. Substitution of sodium ions bypotassium ions lowered the meltingpoints further. Substitution of 20 moleo/o KzCOt for the equivalent amount ofNazCOs in a mixture containing 35.0mole 7o of NazClz, 32.5 mole o/o of

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ciz+YY

6

4

2

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4. Composit ion through the deposit cross-section.

NazCOs, and 32.5 mole % of NazSOalowered the complete melting pointfrom 1157 to 1050"F (625ta 566'C) andthe first melting point from 1140 to965'F (616 to 5lE"C). Sodium chlorideand potassium are also known to lowerthe melting points of the smelt in thekraft recovery boiler (11).

Cone slumping tests were performedon non-enriched and enriched denositstaken from kraft recovery boileri. Theresults are shown in Fig. 5 (5). It shouldbe noted that the difference in firstmelting points between the non-en-riched and enriched deposits is small,

lll. Kraft recovery superheater deposit compositions

Non-enriched,open

inlandmill

Enriched

Closed-cyclemiII

WestCoastmill

Hardwoodwhole-treechip mill

Samp le no .NaCl, o/o

Na2CO3, o/o

NaOH, o/o

Na2SOa, o/o

Insolubles, o/o

Cl lNa (molar) , o/o

K/K + Na (molar) , o/o

Reference

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62.01 . 8

25.5

c22.52.5

072.00.4

26.87.5(5)

D3.75.5

85.5

4.4' :

.0 .

Tappi I May 1981 Vol.64, No.5 1 1 1

Non-Enriched

Enilched

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995 1085 1105 1275

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1000 1100 1200 1300 1400 1 500

TEMPERATURE, OF

t. Deposi t mel t ing and detormat ion.

SAMPLE

G. Superheater deposit melting.

whereas the difference in the firstdeformation points, radical deforma-tion points, and complete melting pointsis very large. These melting pointswere determined based on the appear-ance of the cone, which correlated withthe amount of the l iquid phase in thecone (5).

Theoretical study of the relationshipbetween temperature and the amountof l iquid phase in synthetic depositsconsisting of pure compounds NaCl,NaCOs, and NazSOr showed that bothnon-NaCl-enriched (5 mole 70 of Na2C12)and NaCl-enriched (20 mole 7o of NazClz)synthetic slags have an identical firstmelting point at 1160oF (627'C) butshow a great difference in the amountof liquid phase at this temperature;13% for the non-enriched and 63% forthe enriched slags. However, moreimportantly, radical deformation con-sistently occurs when the l iquid phaseis about 707o. Thus, starting from thefirst melting point, the amount of liquidphase in the non-enriched slag mustchange from 13% to 70o/o in order toreach the radical deformation point,while the enriched slag needs to changeonly from 630/o to 70olo. The effect of theNaCl on the depression of the deforma-tion points is much greater as NaClcontent increases toward the ternaryminimum point of the Naz Clz-NazCOa-NazSOa system. For instance, an en-riched slag containing 23 mole 7o ofNazCls also has a first melting point ofabout 1160'F (627"C). However, at thistemperature the amountof liquid phaseis about 7 2o/o, larger than that requiredfor radical deformation to occur. Thisresults in the simultaneous occurrenceof the first melting, first deformation,and radical deformation points of thehighly enriched slags at 1160"F (627'C).

Depression of the radical deformationpoint of the enriched deposits has animportant impact. Above this tempera-ture, the deposits have enough liquidphase to fluidize, causing the super-heater slagging.

Figure 6 shows the comparison of themelting points of some deposits taken

112

from various kraft recovery boilers.The compositions of the deposits aregiven in Table III. Deposit "A" is atypical, non-enriched deposit from anopen cycle mill, "B" and "C" are chlo-ride- and potassium-enriched depositsfrom a closed-cycle mill and a Westcoast mill, respectively, and "D" ispotassium-enriched dust from a millpulping high proportions of hardwoodand whole-tree chips. The non-enricheddeposit"A" showed the highest meltingpoints. The enrichment in chloride andpotassium salts in "B" and "C" loweredthe melting points dramatically. De-posit "D," enriched only in potassiumsalts, had a very low first melting pointbut had a very high radical deformationpoint and complete melting point. Thelarge depression of the first meltingpointof"D" is probably the resultofthepresence of small amounts of chloride(4.4 mole 70 of Na2clz) and otherimpurities coupled with a large amountof potassium salts.

The results in Fig. 6 show that potas-sium on its own depresses the firstmelting point but does not cause anysignificant depression of the radicaldeformation or complete melting pointsof the deposits.

Laboratory studies oncorrosionVery little work on the corrosion ofsuperheater tubes in the kraft recoveryboiler has been reported. A recentpaper by Morris, Plumley, and Roczni-ak (3) describes the results of tempera-ture-controlled probe studies on th"wasting of various superheater materi-als in open-cycle, and seawater-exposedkraft recovery boilers. Based on theirdata, the wasting of the 2-in. (51-mm)OD mild steel tubes (T-11 and T-22)exposed for 1000 hr in the previouslymentioned boilers is plotted againstthe metal skin temperature. The resultsare shown in Fig. 7. It is apparent thatthe wastage in the closed-cycle boilerwas slightly less severe than in theseawater-exposed boiler. Wastage in

7. Carbon steel corrosion on air-cooledprobes.

the open-cycle boiler was less severecompared with the other two. The sig-nificant increase in wastage at around950"F (537"C) is of interest. An Arrhe-nius plot of these results shows a l inearrelationship between log (weight loss)and 1/T, suggesting that a temperatureeffect is the major cause ofthe increasedcorrosion [up to the data l imit of 1000oF(537'C)l and not a change in corrosionmechanism. However, the lack of datafor the non-enriched boiler, particularlyat temperatures higher than 930'F(499"C), makes it difficult to evaluatethe results further.

In order to examine the corrosivenessof the superheater deposits, simplecrucible tests were performed in air ontwo typical superheater m aterials T-22carbon steel and Incoloy 800. The pre-liminary results showed that (a) thepresence of potassium up to 10 mole %in the synthetic slag caused an acceler-ated corrosion of both T-22 and Incoloy800 at 1040'F (560'C), (b) the corrosionrateof T-22 was much higher than thatof Incoloy 800, and (c) the corrosionlayer formed on T-22 metal surfacewas less adherenttothe metal comparedto that on the Incoloy 800.

SummaryThe present work has demonstratedtwo different mechanisms for the for-mation of superheater deposits in thekraft recovery boiler. Carryover formsthe hard and thick outside layer which

N

E

uioJ

FT

E=

T E M P E R A T U R E . O F

Vol.64, No.5 May 1981 I Tappi

is not enriched in chloride and potas-sium salts, while vaporization-conden-sation is responsible for the formationof the inside white powder layer, whichis highly enriched in these salts.

The enrichment of chloride andpotassium salts results in a drasticdepression of the melting points of thedeposits, for which the depression ofthe first melting point and the radicaldeformation point are of particularinterest. The former results in theenhanced corrosion caused bythe liquidphase, while the latter may causeincreased slagging and the thinning ofdeposits.

Studies using air-cooled probes in-serted in the superheater environmentshowed that the presence of potassiumsalts in the synthetic slag caused en-hanced corrosion of both T-22 andIncoloy 800 superheater materials. Ithas been demonstrated that the corro-sion of mild steels T-11 and T-22 was

greater in the closed-cycle mill than inan open-cycle mill, but the corrosionwas not quite as severe as that in a WestCoast mill (g).

.The problems of enriched; low-melt-ing-point superheater deposits andaccelerated corrosion are not limited tothe closed-cycle mill but to all millswith moderately closed liquor cycles orwith high salt input.

Llterature clted1. Reeve, D.W., Rowlandson, G., Kramer,

J.D., and Rapson, W.H., Tappi62(8):51(le7e).

2. Isbister, J.A., Rae, R.G., Reeve, D.W.,and Pryke, D.C., Pulp Paper Con.80(6):T174 (1979).

3. Morris, K., Plumley, A.L., and Rocz-niak, W. R., paper presented at the ThirdInternational Symposium on Pulp andPaper Industry Corrosion Problems,Atlanta, jointly sponsored by CPPA,TAPPI, and NACE, May 5-8, 1980.

4. Reeve, D.W., and T!an, H.N., Universityof Toronto Report, January 1980.

5. Reeve, D.W, Tran, H.N., and Barham,D., Pulp Paper Can (in Press).

6. Reeve, D.W., Pulp Paper Con. 77(8):T136 (1980).

?. Gilbert, A.F., Ph.D. thesis, Universityof Toronto, Ont., Canada, 1976.

8. Gilbert, A.F., and Rapson, W.H., PulpPapar Can.81(2): T 3 (1980).

9. Keitaanniemi, O., and Virkola, N.,Paperi Puu. 6O(9):507 (1978).

10. Warnqvist, B., and Norrstrom,H.,Tapyi59(11): 89 (1976).

11. Shivgulam, N., Barham, D', and RaP-son, W.H., Pr.clp Paper Can.8O(9\:T282(197e).

This work has been generously supported by ErcoIndustries, Ltd., Great Lakes Forest Products,Ltd., and the Natural Sciences and EngineeringResearch Council'

Based on a paper published in 1980 Pulping Con-

Jerence Proceedin'g s, a T APPI PRE S S publ ication.

Tappi I tlay 1981 Vol.64, No.5 1 1 3