Formation mechanisms of “jellyroll” smelt in kraft recovery …...about 770 C to 780 C (1420 F...

In recovery boiler operation, maintaining a steady flow of molten smelt from smelt spouts is critically important

for ensuring a safe operation of the dissolving tank and a steady flow of green liquor. Smelt spout openings need to be periodically cleaned, as they are often clogged by unburned char and frozen smelt if left unattended. Some boilers experience smelt clogging more often than others for no apparent reasons. In some cases, smoothly flowing smelt suddenly becomes sluggish and forms a viscous blob on the spout trough, partially or completely blocking the smelt flow (Fig. 1).

This type of smelt is commonly referred to as “jellyroll” smelt. The name arises presumably from the appearance of the smelt since it resembles the jelly in a jelly roll cake. Jellyroll smelt is lumpy and viscous and may block the spout openings. Spout blockage often allows a large pool of molten smelt to accumulate behind it, making spout cleaning a dangerous task for boiler operators. As the molten smelt pool becomes larger, deeper, and heavier, it can push the clogged smelt away, re-sulting in a massive amount of molten smelt pouring into the dissolving tank. Such heavy smelt run-offs have been known to cause violent smelt-water explosions, damages to the dis-solving tank, and in severe cases, operator fatality [1-3].

How jellyroll smelt forms is not well understood. Poor com-bustion, low furnace temperature, low liquor sulfidity, and deposits fallen from waterwalls/upper furnace tubes have often been claimed to play a role in jellyroll smelt formation, but no good explanations have been offered to support the claims. The creeping motion of jellyroll smelt on spout troughs suggests that jellyroll smelt is much more viscous than fluid smelt and cannot be shattered readily into pieces by a steam jet. Whatever the mechanism by which jellyroll smelt is formed, it must explain what makes a smoothly flowing mol-

ten smelt suddenly become sluggish and stop flowing. In this paper, we will first examine the key factors that

make fluid smelt less fluid, and how the smelt fluidity is af-fected by smelt composition and lower furnace conditions. We will then propose possible mechanisms for jellyroll smelt to form and practical strategies for minimizing the problem.

FLUIDITY OF MOLTEN SMELTSmelt consists of 60–70 wt% sodium carbonate (Na2CO3), 20–30 wt% sodium sulfide (Na2S), and small amounts of sodium sulfate (Na2SO4), sodium chloride (NaCl), potassium

Formation mechanisms of “ jellyroll” smelt in kraft recovery boilers

HONGHI TRAN and ANDREW K. JONES

OCTOBER 2017 | VOL. 16 NO. 10 | TAPPI JOURNAL 597

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ABSTRACT: Molten smelt normally flows smoothly down the smelt spout of a recovery boiler like water, but at times it suddenly becomes sluggish and forms a viscous blob on the spout trough that partially or completely blocks the smelt flow. This form of smelt is commonly referred to as “jellyroll” smelt. How such smelt forms has been a puzzle to boiler operators and mill personnel for years. Numerous mill observations and the results of a recent study performed on both smoothly flowing smelt and jellyroll smelt collected from a recovery boiler suggest that that jellyroll smelt can form through three main mechanisms: i) the freezing of the molten smelt, ii) the melting of fallen deposits, and iii) the inclusion of a large amount of unburned char in the molten smelt. These mechanisms are con-sistent with mill experience that jellyroll smelt tends to form in older recovery boilers burning liquor with low solids and low sulfidity.

Application: This paper discusses three mechanisms through which jellyroll smelt forms and provides operat-ing strategies for minimizing jellyroll smelt formation.

1. A jellyroll smelt blob “sitting” on a spout trough.

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598 TAPPI JOURNAL | VOL. 16 NO. 10 | OCTOBER 2017

salts, and char, and it has a freezing temperature between 740°C and 780°C. At its typical operating temperature of 820°C to 830°C in the boiler, the smelt must be completely molten.

Figure 2 shows the change in viscosity of 5 smelt sam-ples as they cooled slowly from 960°C (1760°F) in a labora-tory setting [4]. In all cases, the viscosity increased only a few centipoises (cP) as the smelt temperature decreased to about 770°C to 780°C (1420°F to 1440°F), and then in-creased drastically as the temperature decreased further. The abrupt increase in viscosity of each smelt occurred at its freezing temperature. This implies that as long as it is still completely molten, molten smelt has a low viscosity be-tween 2 and 5 cP, which is roughly the same as that of milk (3 cP) at room temperature, but it can become very viscous as it starts to freeze.

The fluidity of a liquid is the ability of that liquid to flow.

It is inversely related to the viscosity of the liquid, which in turn, is a function of the liquid composition and temperature. The fluidity drastically decreases when the liquid also con-tains fine solid particles. Coal-water slurry fuel, clay slip, con-crete mix, ice flurry, etc., are all good examples of such solid/liquid mixtures. The higher the solids content, the higher the mixture viscosity is and the poorer the fluidity becomes.

Molten smelt is made of alkali salts. As it cools and reaches its freezing point, one of the salt components precipitates, forming fine solid particles of that component in the melt. As the temperature decreases further, more and more compo-nents precipitate, forming a solid mass that continues to grow. As the temperature is below the first melting tempera-ture of the salt mixture, the whole mass becomes solid. The abrupt increase in molten smelt viscosity shown in Fig. 2 is likely caused by the sudden appearance of the solid phase in the melt as it cools below its freezing temperature.

2. Effect of smelt freezing temperature on viscosity [4].

3. Effects of liquor sulfidity and chloride content on smelt freezing temperatures of softwood smelt (upper) and hardwood smelt (lower).

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SMELT FREEZING TEMPERATURESince freezing plays the key role in the sudden decrease in molten smelt fluidity, it is important to examine how the freezing temperature of molten smelt may be affected by smelt composition. Figure 3 shows the freezing tempera-ture of molten smelt as a function of smelt sulfidity and chlo-ride content for two levels of potassium (K) content: 5 mole% K/(Na+K) for a typical softwood smelt and 10 mole% K/(Na+K) for a typical hardwood smelt. The freezing tempera-ture data was calculated using FactSage (Thermfact/CRCT Montreal, Canada; GTT-Technologies, Aachen, Germany), a commercially available thermodynamic program. In both cases, the freezing temperature decreases as the sulfidity in-creases up to 32% to 36% on total active alkali (TTA). Beyond this point, it increases with an increase in sulfidity. At the same sulfidity and chloride content, hardwood smelt has a lower freezing temperature than softwood smelt due to its higher potassium content.

Chloride also has a small effect on smelt freezing tempera-ture. At a given liquor sulfidity and potassium content, a de-crease in chloride content from 2 to 5 mole% Cl/(Na+K) in smelt, or from 0.6 to 1.5 wt% in black liquor dry solids, would increase the smelt freezing temperature by 18°C.

A decrease in smelt reduction efficiency is expected to in-crease the freezing temperature as shown in Fig. 4. However, the effect is small. The freezing temperature of a typical softwood smelt with 5 mole% Cl/(Na+K) and 5 mole% K/(Na+K), for example, increases only 12°C as the reduction efficiency decreases from 100% to 75%, or about 0.5°C per every 1% decrease in reduction efficiency. Since the smelt reduction efficiency typically varies between 90% to 94% under normal operating conditions, the difference in freezing temperature may be only 2°C.

JELLYROLL SMELT ANALYSISFigure 5 shows a series of images extracted from a video foot-age of a jellyroll smelt formation episode that occurred during normal recovery boiler operation at Mill A. Prior to the episode, the smelt was fluid, flowing freely down on the spout trough into the dissolving tank beneath. At 00:00 (minutes:seconds), a piece of jellyroll smelt suddenly appeared and occupied the first 20% of the trough length, greatly restricting the smelt flow. As the jellyroll smelt blob edged slowly downward, it grew thicker and longer, occupying 60% of the trough length at 00:10. The remaining 40% of the trough length was still filled with fluid smelt. At 00:20 and 00:25, the jellyroll smelt blob covered the entire trough length, but continued to pour slowly down into the dissolving tank. Bright spots of char burning were clearly visible on the surface of the dark-pink jellyroll smelt. At 00:28, fluid smelt emerged from the furnace, pushing the jellyroll smelt off the trough into the dissolving tank. At 00:30, almost all of the remaining jellyroll smelt was expunged from the trough. At 00:40, the entire trough was covered with fluid smelt and the smelt flow returned to normal. The entire episode occurred in less than 40 s.

Numerous episodes of jellyroll smelt formation occurred on that day. Some were short and over in less than a minute as described previously. Others, however, were much longer. The jellyroll smelt blobs were large and viscous and could not be pushed away by the fluid smelt. The boiler operators were busy that day watching and cleaning smelt spout openings and troughs to ensure safe and stable operation of the dissolv-ing tank.

Two smelt samples (one fluid smelt and the other jellyroll smelt) from two adjacent smelt spouts were collected at the same time that day and cooled in two separate galvanized steel buckets. Figure 6 shows photos of these samples after they were cooled and taken out of the buckets. The fluid smelt was pink, porous, and contained some black char, while the jel-lyroll smelt was black, porous, and contains pink dots. The pink color was the typical color of smelt, while the black color was that of unburned char. The large pores in the samples were likely formed as a result of gas release during the char burning stage of black liquor combustion.

Chemical and physical analysesEach smelt sample was dissolved in hot water and the result-ing solution was filtered. The filtrate (the water-soluble part of the smelt) was analyzed for Na, K, sulfur (S), sulfate (SO4), carbonate (CO3), and chlorine (Cl), which were then used to calculate the reduction efficiency and sulfidity of the smelt. The residue on the filter paper was dried at 105°C for 24 h. The dried residue (the water-insoluble part of the smelt) was essentially a mixture of unburned char and insoluble impuri-ties. It was burned at 80°C to remove the char (carbon). The weight difference of the residue before and after burning was considered to be the char content of the smelt. The bulk den-sity of the residue was also calculated from its weight and volume. The dried residue of the fluid smelt was a brownish

4. Effect of smelt reduction efficiency on the freezing temperature of smelts with different combinations of chloride (Cl) and potassium (K) contents.

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fine powder, whereas that of the jellyroll smelt was a black powder, resembling black carbon printer toner (Fig. 7).

Table I summarizes the analysis results. Both fluid smelt and jellyroll smelt had almost the same composition, reduc-tion efficiency, and sulfidity. The main difference was that the jellyroll smelt contained roughly 2 wt% residue, nearly 6 times more than the fluid smelt, and 1.4 wt% char, compared to al-most no char in the fluid smelt. The bulk density of the char from the jellyroll smelt was low, about 0.06 g/cm3, suggesting that char was light and highly porous. The bulk density of the char from fluid smelt char could not be determined since the sample obtained was too small to analyze.

Since there is no significant difference in composition (par-ticularly sulfidity and reduction efficiency) between fluid smelt and jellyroll smelt, this jellyroll smelt formation incident cannot be attributed to either sudden smelt freezing or fallen deposits. The key difference between these two smelts is the char content. The jellyroll smelt contained 1.4 wt% char whereas the fluid smelt had very little or no char. It is therefore

6. Appearance of fluid and jellyroll smelt samples (pink parts are smelt; black parts are char) after being cooled down to room temperature.

5. A jellyroll smelt formation incident in the recovery boiler at Mill A. Numbers are times in minutes:seconds.

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important to examine the property of the black liquor char from Mill A in order to understand how such a small amount of char can possibly turn a fluid smelt into a jellyroll smelt.

CHAR PROPERTYFigure 8 shows the thermogravimetric (TG) profile for a black liquor sample from Mill A collected at the time when the liquor burnability was poor. The test was performed on a 0.3 g black liquor sample with a dry solids content of 70 wt%, while the combustion process was recorded using a video camera. Details of this test method have been described else-where [5,6]. Snapshots of the sample during the test at times corresponding to A, B, C and D on the TG curve are also shown. The change in slope of the TG curve and the absence of yellow flame around the sample suggest that the sample started its char burning stage at time A.

The volume of the combustion residue (char and smelt) at any given time can be estimated from the video image at that time. This volume value and the corresponding sample weight obtained from the TG profile enables the calculation of the approximate char content in the combustion residue, on both a volume basis and a mass basis, as shown in Fig. 9. On a mass basis, the char content steadily decreased as the char burning proceeded, whereas on a volume basis, the char continued to occupy almost 90% of the total vol-ume of combustion residue until the very end when it almost completely burned off. At time between 110 and 115 s, for example, the char content in the combustion res-idue was about 1% to 3% on mass, but was much higher, 10% to 70%, on volume.

The volume and weight data also enable the calculation of the density of the combustion residue (char and smelt) as well as the density of the char only at a specific time. The results are shown in Fig. 10. The char density varied from 0.02 to 0.11 g/cm3, with an average of about 0.05 g/cm3,

7. Appearance of dried residues left on filter papers of fluid smelt and jellyroll smelt solutions.

I. Analysis summary.

Color

Fluid Smelt Jellyroll Smelt

Pink with black char

Black with pink smelt

Composition

Na, wt% 43.44 42.49

K, wt% 3.28 3.29

CO3, wt% 46.11 44.70

S, wt% 6.19 6.07

SO4, wt% 0.83 1.02

Cl, wt% 0.80 0.83

Total 101.0 100.4

Reduction, % 95.7 94.7

Sulfidity, % on TTA 20.1 20.3

Dried residue color Light brown Black

Dried residue, wt% 0.35 1.98

Combustion ash residue, wt%

0.34 0.60

Char content, wt% 0.01 1.38

Char density, g/cm2 * 0.06

Na = sodium, K = potassium, CO3 = carbonate, S = sulfur, SO4 = sulfate, Cl = chlorine. *Amount too small to measure

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which is comparable to the actual measurement result shown in Table I. The density of the combustion residue was also fairly constant at 0.3 g/cm3 due to the presence of char, but increased to 2 g/cm3 when all char burned off, leaving behind only molten smelt. This density value is about the same as the density of a typical molten smelt.

JELLY SMELT FORMATION MECHANISMSMill experience and the evidence presented suggest that jel-lyroll smelt can form through three main mechanisms: i) the freezing of molten smelt, ii) the melting of fallen deposits, and

iii) the inclusion of a large amount of unburned char in the molten smelt. All these mechanisms can well explain why smoothly flowing molten smelt suddenly becomes sluggish and stops flowing.

Freezing of molten smelt

This mechanism prevails when the smelt temperature (TSM) drops locally below the smelt freezing temperature (TFZ). The difference between these two temperatures (i.e., ΔT = TSM – TFZ) in a recovery boiler is the key parameter that deter-mines the likelihood of jellyroll smelt formation in that boiler.

8. Weight change and appearance of black liquor from Mill A during a combustion test. A: beginning of char burning; B and C: smelt formation during char burning; and D: completion of char burning with only molten smelt remaining on the test tray.

9. Char content of the combustion residue of Mill A black liquor during char burning.

10. Densities of combustion residue and char of Mill A black liquor during char burning.

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The smaller the ΔT at which the boiler is operated, the more likely the jellyroll smelt will form.

TFZ is an intrinsic property of molten smelt. As discussed earlier, TFZ is affected mainly by the liquor sulfidity (the sulfide content of the smelt or the sulfur content of the black liquor), and to a much lesser extent, by the chloride and potassium contents and the reduction efficiency of smelt. For a given mill, the liquor sulfidity is often well controlled, and hence, TFZ does not usually change much, except during the first few days of boiler startup after a long mill-wide outage when the sulfidity can be 5% to 10% lower than target. In such a case, TFZ can be 15°C to 30°C higher than normal, resulting in a smaller ΔT and making it easier for jellyroll smelt to form.

The smelt temperature TSM, on the other hand, is strongly related to boiler operation, especially the combustion condition in the lower furnace. Higher thermal load to the boiler (high fir-ing load, high liquor solids content, high liquor heating value, etc.) and better combustion conditions (optimum air flow, better air distribution and temperature, good swelling liquor, etc.) are key to ensuring high TSM. As long as TSM can be kept well above TFZ, there will be little chance for the molten smelt to freeze or for the jellyroll smelt to form by this freezing mechanism.

Low sulfidity mills must therefore operate their boilers at a hotter bed than high sulfidity mills in order to avoid jellyroll smelt formation problems. This can be attained by increasing the heat input to the boiler, and/or ensuring good combustion near the smelt spouts, and/or installing small hearth burners. If it is impractical to raise the smelt temperature by these means, the only course of action would be to increase the smelt sulfidity to lower its freezing temperature so that the boiler can operate at a larger ΔT. One commonly used method for increasing the smelt sulfidity is to add emulsified sulfur to the as-fired black liquor in the saltcake mix tank. The addition of emulsified sulfur, however, must be done carefully, as it can lead to accelerated corrosion in the lower furnace and to near drum corrosion associated with acidic sulfates. A practical guideline is to discontinue emulsified sulfur addition when the pH of a 1% precipitator ash solution drops below 10.5.

Melting of fallen depositsDeposits in the superheater region typically consist of 60 wt% Na2SO4, 35 wt% Na2CO3, and small amounts of Na2S, NaCl, potassium salts, and char, and have a complete melting tem-perature between 780°C and 840°C. The gas temperature in this region varies between 600°C and 850°C while the tube surface temperature varies between 300°C and 500°C. Thus, the deposit temperature can be between 500°C and 700°C, which is much lower than the molten smelt temperature.

After being knocked off from tube surfaces by sootblow-ers, deposits fall on to the char bed where they interact with molten smelt. Figure 11 schematically shows the fate of a piece of deposit after it has fallen from the upper furnace into the molten smelt pool (0). The deposit is immediately heated up by the molten smelt, and in the process, it lowers the tem-perature of the surrounding molten smelt. The heat transfer

between the deposit and the molten smelt allows a partially molten smelt layer to form at the deposit-smelt interface, which temporally increases the deposit mass and size (1). As the deposit temperature rises, the deposit continues to melt and become smaller and smaller with time (2 and 3). Eventu-ally, it becomes a viscous mass or jellyroll smelt (4) that floats and moves along with the molten smelt. If such viscous de-posit/smelt mass does not completely melt by the time it reach-es smelt spouts, it may block the spout opening or move slow-ly on the spout trough, restricting the smelt flow.

Whether or not a piece of fallen deposit can completely melt by the time it reaches the smelt spout depends on its size, temperature, composition and residence time in the boiler, as well as the temperature, composition, and the movement of the molten smelt. Small pieces of deposit can “disappear” and become part of the molten smelt, while large pieces cannot. For boilers that burn low sulfidity black liquor, the smelt freez-ing temperature is high, making it easier for the molten smelt to be frozen by fallen deposits, and at the same time, more difficult for the molten smelt and deposits to mix.

Many more deposits and larger pieces of deposits fall dur-ing thermal shedding events than during normal boiler op-eration, particularly during the time when the boiler is heated up after the event [7]. The formation of jellyroll smelt by this deposit melting mechanism is consistent with mill experience that the problem is much more severe during thermal shed-ding. Since deposits contain very little or no char, one way to verify if fallen deposits are the root cause of the problem is to check if the jellyroll smelt contains char “sparklers” as it emerges from the boiler.

11. Transition from fallen deposit to jellyroll smelt.

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Inclusion of charThe physical appearance and chemical composition of fluid smelt and jellyroll smelt at Mill A (Fig. 5) suggest that this jellyroll smelt formation incident was likely caused by char inclusion, not by smelt freezing or fallen deposits. With a den-sity of 0.05 g/cm3, char is very porous. Like a sponge, it can absorb a large volume of molten smelt and turn it into a viscous jellyroll smelt blob.

Char bed formation in recovery boilers is a dynamic pro-cess with constant changes in bed size, height, and position. If not controlled by boiler operators, char bed will keep build-ing up to a point when it spontaneously collapses and re-builds. Char blocks may break away from the main bed pile when it collapses or is struck by fallen deposits from above. Char blocks can also be partially burned black liquor residues fallen off from the waterwalls. Since they are light, they float and drift along with the molten smelt toward smelt spouts, where they become jellyroll smelt blobs and block the smelt spout openings (Fig. 12). Small jellyroll smelt blobs can flow with fluid molten smelt into the dissolving tank. Large jelly-roll smelt blobs can float freely inside the boiler where the molten smelt level is deep, but once they are on the spout trough, they can no longer float due to the shallow molten smelt level on the trough. As a result, large jellyroll smelt blobs will “sit” on the trough and only move slowly as they are pushed from behind by the fluid smelt or are removed by the operators.

It is important to note that while the char content in the jellyroll smelt examined in this study was only 1.4% on a mass basis, it is about 50% on a volume basis. This means that the char can occupy as much as half of the molten smelt volume, and it can serve as a supporting frame that keeps molten smelt in place.

These formation mechanisms are consistent with mill ex-perience that older recovery boilers burning black liquor with low solids and low sulfidity tend to be susceptible to

jellyroll smelt formation. The mechanisms also suggest that recovery boilers with a decanting bottom are probably less susceptible to jellyroll smelt formation than those with a slant floor. This is because the deeper molten smelt level in decant-ing bottom boilers not only allows the fluid smelt to flow around or under the jellyroll smelt to reach the smelt spout (Fig. 12a), but also prevents the jellyroll smelt blob to “jump” over and sit on the smelt spout trough. In older and smaller decanting bottom boilers, however, conditions for jellyroll smelt to form still prevail since char can fall and accumulate on top of existing jellyroll smelt blobs near smelt spouts. In slant floor boilers, the smelt spout is typically at a level slight-ly higher than the floor, making it easier for the jellyroll smelt blob to roll or slide over onto the spout trough (Fig. 12b).

PREVENTION STRATEGIESWhile the previously described mechanisms for jellyroll smelt formation appear to be different, they share one common re-quirement: the molten smelt must contain a large amount of particles to make jellyroll smelt. Particles can be the precipi-tated salts in the molten smelt as it freezes, or the solids in the partially molten deposit as it continues to melt, or the partial-ly burned black liquor char in the case of char inclusion. Therefore, minimizing the presence of such particles in the molten smelt is key to the prevention of jellyroll smelt forma-tion. The strategies may include:

• Increasing the molten smelt temperature by adopting “hot bed” operation (i.e., high solids black liquor, high heating value black liquor, optimal air flow rate and dis-tribution, high air temperatures, good fuel/air mixing, etc.).

• Lowering the smelt freezing temperature by increasing the liquor sulfidity (this can be effective only up to 35% on TTA). For mills that already operate at a high liquor sulfidity, the smelt freezing temperature can be lowered by decreasing the liquor sulfidity instead.

12. Floating char block near smelt spout opening: a) decanting bottom unit and b) slant floor unit.

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• Optimizing sootblowing strategies to minimize deposit buildup in the superheater region during normal boiler operation.

• Optimizing sootblowing strategy to minimize massive deposit falling at once during thermal shedding events.

• Avoiding operation with large and/or steeply sloping char beds to minimize blackouts and bed collapsing that can generate a large amount of char (a carbon monoxide monitoring system can help identify poor bed condi-tions).

• Using bed cameras with local bed temperature monitor-ing systems to identify problem areas that may require air flow and temperature adjustments, or that may show where a large char lump has fallen or a local black out is taking place.

• Avoiding liquor burning on the furnace walls that forms large lumps of char near smelt spout openings.

• Keeping char away from spouts by increasing primary air flow, pressure, and temperature.

SUMMARYJellyroll smelt is commonly referred to as a type of molten smelt that suddenly becomes sluggish and forms a viscous blob on the spout trough, restricting the smelt flow. Mill ex-perience and the results of a recent study performed on smoothly flowing smelt and jellyroll smelt collected at the same time from a recovery boiler suggest that that jellyroll smelt can form through three main mechanisms: i) the freez-ing of the molten smelt, ii) the melting of fallen deposits, and iii) the inclusion of a large amount of unburned char in the molten smelt. These mechanisms are consistent with mill

experience that older recovery boilers burning black liquor with low solids and low sulfidity tend to be susceptible to jel-lyroll smelt formation and that deposits fallen from the upper furnace can be a contributing factor.

The common requirement for these three mechanisms to prevail is the presence of a large amount of solid phase in the molten smelt. Smelt freezing and fallen deposit melting are related to a net balance between the lower furnace tempera-ture and smelt/deposit thermal properties, while char inclu-sion is related to black liquor burning behavior and the com-bustion environment around spout openings. To prevent jellyroll smelt from forming, it is ideally important to keep smelt completely molten and free of char particles. This may be attained by: i) operating the boiler at hot bed operation; ii) increasing the liquor sulfidity; iii) minimizing the falling of large pieces of deposits; iv) keeping char away from spouts by increasing primary air flow, pressure and temperature; and v) minimizing wall spraying above spouts. TJ

ACKNOWLEDGEMENTSThis work was conducted as part of the research program on “Increasing Energy and Chemical Recovery Efficiency in the Kraft Process - III”, jointly supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and a consortium of the following companies: Andritz, AV Nacka-wic, Babcock & Wilcox, Boise, Canadian Kraft Paper, Carter Holt Harvey, Celulose Nipo-Brasileira, Clyde-Bergemann, DMI Peace River Pulp, Eldorado, ERCO Worldwide, Fibria, FPInnovations, International Paper, Irving Pulp & Paper, Kiln Flame Systems, Klabin, Stora Enso, Suzano, Tembec, Valmet and WestRock.

ABOUT THE AUTHORSWe chose to study this topic because jellyroll smelt formation has been a persistent safety problem in recovery boiler operation. We wanted to see if we could logically explain the phenomenon so that engi-neers and boiler operators can identify the root cause of the problem at their mills and devise viable solutions to them. This research compliments previ-ous work on black liquor combustion and deposit formation in recovery boilers by our research group and others. The difference is that, in this work, we used the knowledge obtained from previous re-search to explain how jellyroll smelt may form.

The most difficult part of this research was to be able to see what jellyroll smelt looks like as it flows out of the boiler, and to obtain samples of jellyroll smelt and normal smelt at the same time so that meaningful comparison can be made. We were lucky to be in a recovery boiler control room when a jelly-roll smelt incident occurred. Our most interesting finding was that while there are several mechanisms by which jellyroll smelt can form, they all share one

common cause: the presence of solid particles in the molten smelt.

This study yielded logical explanations for how jellyroll smelt may form, and mills may use this infor-mation to identi-fy the main cause of their jellyroll smelt problem and devise a viable solution to the problem.

Tran is Frank Dottori Professor of Pulp & Paper Engineering, Department of Chemical Engineering & Applied Chemistry, at the University of Toronto, Toronto, ON, Canada. Jones is senior engineering fellow at International Paper, Loveland, OH, USA. Email Tran at [email protected].

Tran Jones

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LITERATURE CITED1. Tran, H.N., Jones, A.K., and Grace, T.M., TAPPI J. 14(1): 41(2015).

2. Lien, S. and DeMartini, N., “Dissolving Tank Explosions – a review of incidents between 1993 and 2005,” Report to AF&PA Recovery Boiler Committee, AF&PA, Washington, DC, 6 February 2008.

3. Grace, T.M. “Dissolving Tank Explosions – A Review of Incidents Reported to BLRBAC,” Report to AF&PA Recovery Boiler Committee, AF&PA, Washington, DC, 26 November 2013.

4. Tran, H.N., Sunil, A., and Jones, A.K., J. Pulp Pap. Sci. 32(3): 182(2006).

5. Zhao, L. and Tran, H.N., J. Sci. Technol. For. Prod. Processes 4(6): 50(2015).

6. Zhao, L., Tran, H.N., and Maki, K., TAPPI J. 14(7): 451(2015).

7. Tran, H.N., Martinez, M., Reeve, D.W., et al., Pulp Pap. Can. 94(11): 49(1993).

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Formation mechanisms of “jellyroll” smelt in kraft recovery …...about 770 C to 780 C (1420 F...

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Transcript of Formation mechanisms of “jellyroll” smelt in kraft recovery …...about 770 C to 780 C (1420 F...