effects of chemical composition on the removability of ......various amounts of potassium, probe...

2001 TAPPI JOURNAL PEER REVIEWED PAPER JUNE 2001/VOL. 84: NO. 61

2001 TAPPI JOURNAL PEER REVIEWED PAPER

EFFECTS OF CHEMICAL COMPOSITION ON THE REMOVABILITYOF RECOVERY BOILER FIRESIDE DEPOSITS

Xiaosong Mao, Honghi Tran and Donald E. Cormack

Pulp & Paper Center andDepartment of Chemical Engineering & Applied Chemistry

University of TorontoToronto, ON, Canada

ABSTRACT

Effective removal of fireside deposits by sootblowers in a kraft recovery boiler is criticallyimportant for maintaining stable operation of the boiler. The removability of recovery boilerdeposits was studied under simulated conditions in the laboratory using an entrained flow reactor(EFR) coupled with an air jet blow-off apparatus. Synthetic carryover particles were fed into theEFR controlled at 800oC. The resulting deposits, which collected on an air-cooled probe at theEFR exit, were removed with a high-pressure air jet. The results show that the required peakimpact pressure (PIP) of the jet to remove a deposit increased markedly with an increase in thechloride content of the deposit, and, to a lesser extent, with an increase in potassium andcarbonate content. The combined effect of these components was significantly greater than thesum of the effects of individual components. The results also show that deposits were moredifficult to remove as the tube surface temperature increased.

KEY WORDS

Recovery Boiler, Fireside Deposits, Removal, Sootblower, Peak Impact Pressure, Chloride,Potassium, Carbonate

INTRODUCTION

The accumulation of fireside deposits on heat transfer surfaces in the upper furnace of kraftrecovery boilers is an inevitable process. If not removed, deposits can drastically reduce theboiler thermal performance, and, in severe cases, completely plug the flue gas passages and leadto unscheduled boiler shutdowns. Effective deposit removal is, therefore, critically important forensuring maximum boiler thermal efficiency and for maintaining stable boiler operation.

During boiler operation, deposits are typically removed by sootblowers which blast depositsperiodically with high-pressure steam. The ability of a sootblower to remove deposits from tube

2001 TAPPI JOURNAL PEER REVIEWED PAPER JUNE 20012

Gas burner

Samplefeeder

Probe

Furnace9 m

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surfaces depends on two main factors: the peak impact pressure (PIP) of the sootblower steamjet, and the strength of the deposits. In order to remove deposits, the PIP must be greater than thedeposit mechanical strength, particularly the adhesion strength at the deposit-tube interface.

While much work has been done to examine how deposits form and grow, and to determineimportant factors that influence the rate of deposit formation [1], only limited studies have beencarried out to date on deposit strength and removability [2,3,4]. These studies showed that thedeposit strength is strongly related to deposit porosity, composition and temperature as well asthe substrate temperature. However, the correlation between deposit strength and removability bysootblowers, the interaction between sootblower jets, tubes and deposits, and the main boileroperating parameters that affect deposit removal are not well understood. The lack of practicalinformation on deposit removability is primarily due to the difficulty of conducting tests in anoperating recovery boiler where the environment is extremely hostile and uncontrollable.Meaningful data may not be obtained with conventional laboratory test methods because of thedifficulty of either preparing appropriate deposit samples or reproducing a deposit-tube bond thatresembles conditions found in recovery boilers.

The availability of the Entrained Flow Reactor (EFR) at the University of Toronto (Figure 1) hasmade it possible to produce carryover deposits on a cooled tube surface, and to remove themunder conditions similar to those found in recovery boilers [5]. A systematic study has beenconducted to examine the effects of various parameters on deposit removal, including carryoverparticle size, composition and temperature, deposit thickness and temperature, tube surfacetemperature, gas velocity and temperature, and jet impact angle.

This paper discusses results of the study on the effects of composition of carryover particles andtube surface temperature on the removability of the deposits and the implications on recoveryboiler operation.

Figure 1. The Entrained Flow Reactor (EFR) at University of Toronto.


CARRYOVER DEPOSITS

Carryover deposits, which are formed as a result of the inertial impaction of physically entrainedsmelt and/or partially burned black liquor particles on tube surfaces, are the most troublesometype of deposits observed in recovery boilers [1]. Carryover deposits typically form in largequantities in the superheater region due to the high flue gas temperature. They consist mainly ofsodium sulphate (Na2SO4) and sodium carbonate (Na2CO3), and together with small amounts ofsodium sulphide (Na2S), sodium chloride (NaCl), and potassium salts. Depending on thecomposition and temperature, carryover deposits may be dense, hard and difficult to remove.

EXPERIMENTAL PROCEDURE

In this study, synthetic carryover particles of known composition were prepared from mixtures ofpure chemicals, Na2SO4, K2SO4, Na2CO3 and NaCl, which were melted, cooled, ground andsieved into a 150 to 300 Φm size range. The particles were introduced continuously for 20minutes at a feed rate of 2.5 g/min into the top of the EFR, in which, they were heated as theypassed through the reactor. An air-cooled probe, made from stainless steel 304 tubing with anouter diameter of 2.5 cm (1”), was placed horizontally at the EFR exit to collect the heatedparticles. The EFR was controlled at 800oC, the gas velocity at the exit was about 1.8 m/s, andthe average particle velocity was estimated to be 2 m/s. The probe surface temperature wascontrolled at 400oC (752oF) in most experiments. In tests where the effect of probe surfacetemperature was examined, it was set at a fixed temperature between 200oC and 500oC byregulating the amount of air flowing through the probe.

The removability of the deposit collected on the probe surface was determined using an air jetblow-off apparatus mounted near the probe (Figure 2). Compressed air was passed through abrass nozzle with an exit diameter of 7.35 mm to produce an air jet that simulates the fullyexpanded sootblower jet in recovery boilers. The air pressure at the nozzle inlet was 5.5Mpa. Thepeak impact pressure (PIP) of the jet as a function of distance between the nozzle and the depositsurface was pre-determined using a Pitot tube. It was also calculated based on the air pressureand the nozzle geometry. Figure 3 shows the comparison between the measured and calculatedPIP values obtained in this work.

Figure 2. EFR coupled with air jet blow-off apparatus for deposit removal tests.


Figure 3. Peak impact pressure (PIP) of the air jet used in this work (inlet pressure=5.5MPa).

After the deposit was collected, the particle feeding was stopped, the probe was rotated so thatthe deposit was perpendicular to the nozzle axis. At this point, the deposit did not accumulateany further, although the probe was still surrounded by the hot gas. The probe was then anchoredrigidly so that it would not move when struck by the jet. A blow was initiated by turning the airvalve on for 1 second. After each blow, the appearance of the deposit was photographed andcarefully inspected for signs of deposit removal. If there was no indication of removal, the nozzlewas moved closer to the deposit to increase the PIP, and the blowing process was repeated until asmall piece of deposit was removed from the probe surface. Using the relationship between thePIP value and distance in Figure 3, the minimum PIP required to remove the deposit wasdetermined, and used to indicate the removability of the deposit.

EFFECT OF CHLORIDE

While chloride is known to greatly increase the stickiness, and hence, the deposition rate ofcarryover particles [1,5,6], how it affects the removability of deposits is not well understood.Figure 4 shows the correlation obtained in this work between the chloride content of thesynthetic deposits and the minimum PIP required to remove the deposits from the probe surfaceat 400oC (752oF). The deposits in this case contained only Na2SO4 and NaCl; i.e. there was nopotassium or carbonate.

At a chloride content below 5 mole% Cl/(Na+K), the minimum PIP required to remove depositswas low, <0.05 MPa and did not change appreciably with the chloride content. Above 5 mole%Cl/(Na+K), however, the PIP increased markedly with an increase in chloride content, reaching1.2 MPa at 10 mole% Cl/(Na+K) and 1.7 MPa at 20 mole% Cl/(Na+K). These findings are notsurprising, since increasing the chloride content results in a larger amount of molten phase in theparticles, which consequently form deposits that are denser, more tenacious and more difficult toremove. This is confirmed in Figure 5 by the SEM (Scanning Electron Microscopy) photographsof deposits. Particles containing 5 mole% Cl/(Na+K) appear to have insufficient molten phase tobind together to form a strong deposit (Figure 5a), whereas particles containing 10 mole%Cl/(Na+K) appear to be fused together to form a strong deposit (Figure 5b).


Figure 4. Effect of chloride content on the minimum peak impact pressure (PIP) required toremove deposits. No potassium, no carbonate, probe surface temperature: 400oC(752oF).

(a) (b)Figure 5. SEM photographs of the probe deposits containing (a) 5 mole% Cl/(Na+K) and (b)

10 mole% Cl/(Na+K) with no potassium and no carbonate. Probe surfacetemperature: 400oC (752oF).

EFFECT OF POTASSIUM

The effect of potassium was studied by adding K2SO4 to the mixtures of Na2SO4 and NaClduring the particle preparation. The results are shown in Figure 6. For deposits that contain littlechloride, 2 mole% Cl/(Na+K), and no carbonate, a very low PIP was required even when thepotassium content was as high as 20 mole% K/(Na+K). However, for deposits that contained ahigher level of chloride content, e.g. 5 mole% Cl/(Na+K), increasing the potassium contentmarkedly increased the PIP required to remove the deposits. This effect of potassium on depositremoval is clearly depicted in the photographs of deposits taken after the blow-off tests (Figure7). For deposits with no potassium (0 mole% K/(Na+K)), the air jet was able to readily remove alarge piece of deposits. However, as the potassium content increased, the jet could remove onlysmall pieces of deposits despite its higher PIP.


Figure 6. Minimum PIP required to remove deposits containing 2 mole% Cl/(Na+K) and 5mole% Cl/(Na+K), no carbonate and various amounts of potassium. Probe surfacetemperature: 400oC (752oF).

0 mole% K/(Na+K)

10 mole% K/(Na+K)

Figure 7. Appearance of deposits after the blow-off tests. 5 mole% Cl/(Na+K), no carbonate,various amounts of potassium, probe surface temperature: 400oC (752oF).

SEM photographs of these deposits show that deposits with a high potassium content are denser,and fuse or sinter more than deposits with a low potassium content (Figure 8a and b). The effectcan be attributed to the ability of potassium to drastically lower the first melting temperature ofthe particles [7]. For deposits that contained low chloride, 2 mole% Cl/(Na+K), the amount ofmolten phase resulting from the presence of chloride was small. Therefore, even when thepotassium content was as high as 20 mole% K/(Na+K) the amount of molten phase in theparticles was insufficient to make any difference in deposit tenacity (Figure 8c).


(a) 5 mole% Cl /(Na+K) (b) 5 mole% Cl/(Na+K)0 mole% K/(Na+K) 10 mole% K/(Na+K)

(c) 2%Cl/(Na+K)20%K/(Na+K)

Figure 8. SEM photograph of deposits containing various amount of chloride, potassium,and no carbonate. Probe surface temperature: 400oC (752oF).

EFFECT OF CARBONATE

Figure 9 shows the effect of carbonate content on deposit removal. In this test, depositscontained 5 mole% Cl/(Na+K) and no potassium; the probe surface temperature was controlledat 400oC (752oF). The minimum PIP required to remove deposits increased only slightly withincreasing carbonate content up to 60 mole% CO3/(Na2+K2). The results are explainable sincecarbonate has little effect on the liquid content compared to chloride and potassium [1], and has amuch smaller effect on the deposit first melting temperature compared to potassium [7].


Figure 9. Minimum PIP required to remove deposits containing 5 mole% Cl/(Na+K), nopotassium and various amounts of carbonate. Probe surface temperature: 400oC(752oF).

COMBINED EFFECT

Note that the above effects of potassium and carbonate on deposit removal were obtained forsynthetic deposits that contained some chloride, either with potassium only, or with carbonateonly. Since both potassium and carbonate have an effect on the deposit first melting temperature[7], the results may be different if both potassium and carbonate are present, as they always are inreal carryover deposits.

A further study was carried out to examine the combined effect of potassium and carbonate ondeposit removal (Figure 10). For deposits containing 2 mole% Cl/(Na+K), increasing thepotassium and carbonate contents made deposits slightly more difficult to remove. For depositsthat contained 5 mole% Cl/(Na+K), adding carbonate to 20 mole% CO3/(Na2+K2) withoutadding potassium increased the PIP requirement only slightly, while adding potassium to 10mole% K/(Na+K) without adding any carbonate increased the PIP requirement to 0.47 MPa.However, when both carbonate, 20 mole% CO3/(Na2+K2), and potassium, 10 mole% K/(Na+K),were added, a much greater PIP was required, 0.79 MPa. It appears that the combined effect ofchloride, potassium and carbonate on deposit removability is much greater than the sum of theeffects of the individual components.


Figure 10. Comparison of minimum PIP required to remove deposits of varyingcompositions. Probe surface temperature: 400oC (752oF).

EFFECT OF PROBE SURFACE TEMPERATURE

Probe surface temperature was also found to have a significant effect on deposit removal (Figure11). For deposits that contained 5 mole% Cl/(Na+K), the PIP required for deposit removalincreased slightly with an increase in probe surface temperature up to 500oC (932oF). Fordeposits that had a higher chloride content, 10 mole% Cl/(Na+K), the PIP was low at probesurface temperatures below 300oC (572oF). Above this temperature, the required PIP increasedmarkedly, reaching a maximum at 500oC (932oF) and then decreasing sharply at highertemperatures. The drastic decrease in PIP required for deposit removal at temperatures higherthan 500oC (932oF) was probably due to the thermal shock and the sudden shrinkage of thedeposit when it was cooled from liquid to solid by the air jet.

Figure 11. Effect of probe surface temperature on PIP required to remove depositscontaining 5 and 10 mole% Cl/(Na+K), with no potassium and no carbonate.


Similar trends were obtained up to 500oC for deposits that contained potassium (Figure 12) andcarbonate (Figure 13). The effect of these components on PIP, however is much smaller thanthat of chloride.

Figure 12. Effect of probe surface temperature on PIP required to remove depositscontaining 5 mole% Cl/(Na+K), no carbonate and various potassium levels.

Figure 13. Effect of probe surface temperature on PIP required to remove depositscontaining 5 mole% Cl/(Na+K), no potassium and various carbonate levels

IMPLICATIONS

The results of this study suggest that deposit chemistry have a strong effect on depositremovability by sootblowers in recovery boilers. The effect appears to be related to the liquidcontent of carryover particles before they strike the tube surface. The higher the liquid content,the denser the deposit, and the more tenacious it becomes. It has been shown that the liquid


content is affected by two factors: temperature and composition of the particles, particularly thechloride content [1,6].

Although potassium has little effect on deposit stickiness and the rate of deposition [1], it appearsto have a significant effect on deposit removal for deposits that have a chloride content higherthan 5 mole% Cl/(Na+K). Since potassium lowers the first melting temperature of carryoverparticles, particles with a high potassium content will contain a liquid phase at a lowertemperature. This means that there will be a greater chance for liquid phase to be present inparticles when they strike the tube. Once the deposit has formed, there will also be a greaterchance for it to sinter and to become hard and resistant to sootblowing [2]. Therefore, depositsare more difficult to remove in boilers that burn high potassium liquor. However, the effect issmall if the deposits contain little chloride.

Since carryover particles usually contain a large amount carbonate, 15 to 30 wt% CO3 or 30 to 60mole% CO3/(Na2+K2), particularly when the boiler is highly overloaded, a small variation incarbonate content will have little effect on deposit removal. Nevertheless, in overloaded boilers,massive carryover deposits have often been found to form in the superheater region and theycannot be removed readily. The difficulty in deposit removal is more likely due to the high fluegas temperature and the high concentration of molten or partially molten carryover particles inthe flue gas that accompany overloading, than due to the high carbonate content of the particles.

The increase in the PIP required for deposit removal with increasing probe temperature meansthat superheater deposits in boilers operating at high superheated steam temperatures are moredifficult to remove due to the higher tube surface temperatures. Since the superheater tubesurface temperature typically varies from about 270oC (520oF) at the primary superheater inlet toabout 480oC (900oF) at the secondary or tertiary superheater outlet, deposits on the secondary ortertiary superheater may be more difficult to remove than those on the primary superheater.Furthermore, the combined effects of particle temperature and tube surface temperature ondeposit removal suggest that the configuration of superheater steam flow also has an importanteffect. In boilers with a counter-flow superheater design, deposits are more difficult to remove inthe region near the superheater entrance, while they are easier to remove in the region near theboiler bank inlet, compared to deposits in the same regions of boilers with a co-flow superheaterdesign.

In the generating bank and economizer regions where the tube surface temperature is typicallymuch lower than 300oC (572oF); deposits should be readily removed, regardless of theircomposition.

CONCLUSIONS

A systematic study was performed to examine the effect of composition on deposit removal,using an Entrained Flow Reactor (EFR) coupled with an air jet blow-off apparatus. The resultsshow that:

• Chloride not only has a significant effect on the rate of deposit formation but also makes thedeposit much more difficult to remove.


• Of the main components in the deposit, chloride has the most significant effect on depositremoval followed by potassium. Carbonate has only a small effect on deposit removal.

• The combined effects of chloride, potassium and carbonate on deposit removal is muchgreater than the sum of the effect of individual component.

• Increasing the tube surface temperature makes deposits more difficult to remove.

ACKNOWLEDGEMENTS

This work is part of the research program on “Improving Recovery Boiler Performance,Emission and Safety” jointly supported by ABB Alstom Power Inc., Ahlstrom Corporation,Aracruz Celulose S.A., Babcock & Wilcox Company, Boise Cascade Corporation, BowaterCanada Inc., Clyde-Bergemann Inc., Champion International Corporation, Daishowa-MarubeniInternational Ltd., Domtar Inc., Domtar-Eddy Specialty Papers Ltd., Georgia Pacific Corporation,International Paper Company, Irving Pulp & Paper Limited, Kvaerner Pulping Technologies,Potlatch Corporation, Stora Enso Research AB, Votorantim Celulose e Papel, WestvacoCorporation, Weyerhaeuser Company, Willamette Industries Inc., and by the Natural Sciencesand Engineering Research Council of Canada (NSERC) through its Industry-Oriented Research(IOR) Grant Program.

REFFRENCE

1. Tran, H. N., “Kraft Recovery Boilers - Chapter 9: Upper Furnace Deposition and Plugging”,edited by T. N. Adams, Tappi Press (1997).

2. Piroozmand, F., Tran, H. N., Kaliazine, A. and Cormack, D. E., “Strength of Recovery BoilerFireside Deposits at High Temperatures”, Proceedings of 1997 Tappi EngineeringConference, Tappi Press (1997).

3. Tateishi, M., Tokunaga, K., Arakawa, Y., and Maeda, T., “Plugging Prevention of RecoveryBoiler By Improving Ash Properties with Potassium Removal Equipment”, Proceedings ofInternational Chemical Recovery Conference, Tappi Press, Volume 2, p.581-598 (1998).

4. Kaliazine, A., Cormack, D. E., Ebrahimi-Sabet, A., and Tran, H. N., “The Mechanics ofDeposit Removal in Kraft Recovery Boilers”. Journal of Pulp and Paper Science, 25 [12]:418-424 (1999).

5. Shenassa, R., Tran, H. N., Kuhn, D.C.S., “Dynamic Study of Carryover Deposition using anEntrained Flow Reactor”, Pulp & Paper Canada, Vol.100, No.10 p. 56-62 (1999).

6. Backman, R., Hupa, M., and Uppstu, E., “Fouling and Corrosion Mechanisms in theRecovery Superheater Area”, Tappi Journal, Vol.70 (6), 123-127 (1987).

7. Tran, H. N., Gonsko, M., Mao, X., “Effect of composition on the first melting temperature offireside deposits in recovery boilers”, Tappi Journal, Vol. 82 (9), p. 93-100 (1999).

Received: December 8, 2000Accepted: January 4, 2001

This paper was accepted for abstracting and publication in the June 2001 issue of TAPPIJOURNAL.

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effects of chemical composition on the removability of ......various amounts of potassium, probe...

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