Energy & Fuels 2003, 17, 1501-1509
1501
Sintering and Structure Development in Alkali Metal Salt Deposits Formed in Kraft Recovery Boilers Wm. James Frederick, Jr.* Department of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia
Esa K. Vakkilainen Jaakko Poyry OY, Vantaa, Finland Received May 27, 2003. Revised Manuscript Received September 10, 2003
The composition and sintering characteristics of deposits of alkali metal salt particles formed in-situ in a kraft recovery boiler were investigated. The deposits were formed on air-cooled probes inserted into the convective gas passages of an operating kraft recovery boiler. Their microstructure was characterized by scanning electron microscopy. The results show that, in regions of the boiler where the alkali metal salt particles are no longer molten, two types of deposits can form. The first is composed almost entirely of submicron aerosol (fume) particles. The second contains a mixture of fume and larger particles, with dimensions typically 5 to 70 µm. Two types of larger particles were identified: spherical particles of solidified ash from combustion of black liquor, and irregular chunks from upstream deposits disintegrated by soot blowing. The structure of deposits formed from fume particles was one of branched dendritic growths. The deposits as formed were relatively low density. Sintering proceeded via growth of nodes and thickening of branches, but with slow densification of the deposits. All of the deposits observed appeared to harden via sintering of the much finer fume particles. The larger size particles, when present, appeared to be bound in the deposits by the matrix of finer particles that was produced as the finer particles sintered.
Introduction Blockage of the gas passages by sodium salt deposits has long been a problem in recovery boilers that burn spent pulping liquors. These deposits reduce heat transfer efficiency and lower overall black liquor burning capacity. In the past fifteen years, considerable progress was made toward eliminating these problems. Boilers built in the 1990s have greater furnace volumes and far superior air delivery systems.1 Because of larger furnace volumes, the volumetric heat release rate is almost half of that of the earlier boilers designed to fire 1000 tons of dry solids per day.2 Problems of gas passage blockage by deposits of entrained black liquor droplet residue (carryover particles) have been greatly reduced or eliminated in these boilers. Despite these advances, fouling and blockage of gas passages in the boiler bank of recovery boilers have persisted. Fine, submicron size alkali metal salt particles can foul and plug the superheater, boiler, and economizer banks of kraft recovery boilers. These deposits are initially soft and easily removed by soot * Corresponding author. Phone: 404-894-9550. Fax: 404-385-0522. E-mail:
[email protected]. (1) Vakkilainen, E. K.; Holm, R.; Simonen, L. Emission performance of large recovery boilers. 2001 Engineering/Finishing & Converting Conference & Trade Fair; Tappi Press: Atlanta; pp 118-127. (2) Haaga, K.; Mantyniemi, J. Operating experience of XL-size recovery boilers. Congresso e Exposicao Anual de Celulose e Papel, 35th, Sao Paulo, Brazil, Oct. 14-17, 2002; pp 474-482.
blowing. However they harden with time, sometimes rapidly, and can become strong enough so that soot blowing is not effective in removing them. Sinteringsa process of thermal densification in porous solidssis responsible for hardening of these deposits. Laboratory measurements with compacted dusts suggest that sintering of deposits in many recovery boilers may occur very rapidly, hardening deposits to a degree where soot blowing is ineffective in removing them. However, Sinquefield et al.3 showed that deposits of submicron sodium salt particles may form with much lower densities than the compacted pellets of dusts used in many laboratory sintering studies. These lower density deposits would densify more slowly than initially denser pellets made from the same dusts, with grain growth but less macroscopic densification and strength development.4,5 No measurements of the morphology and development of density have previously been reported for deposits formed and hardened in recovery boilers. (3) Sinquefield, S. A.; Baxter, L. L.; Frederick, W. J. An experimental study of the mechanisms of fine particle deposition in kraft recovery boilers. Proceedings of the 1998 Inteeernational Chemical Recovery Conference; Tappi Press: Atlanta; pp 443-467. (4) German, R. M. Sintering Theory and Practice; John Wiley & Sons: New York, 1996; pp 226-231. (5) Techakijkajorn, U.; Frederick, W. J.; Tran, H. N. Sintering and Densification of Recovery Boiler Deposits: Laboratory Data and a Rate Model. J. Pulp Paper Sci. 1999, 25 (3), 73-80.
10.1021/ef034012s CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003
1502 Energy & Fuels, Vol. 17, No. 6, 2003
Frederick and Vakkilainen
Figure 1. Fume particles collected at the entrance of the electrostatic precipitator of the recovery boiler in Joutseno, Finland. Table 1. Range of Composition and First Melting Temperatures of Electrostatic Precipitator Catch Samples from 47 Kraft Recovery Boilersa potassium, wt % chloride, wt % carbonate, wt % first melting temperature, °C a
low
typical
high
0.3% 0.0% 0.0% 810
3-9% 2-7% 2-8% 530-560
17% 14% 24% 513
Ref 9.
In this paper, we examine the morphology, composition, and sintering characteristics of deposits of alkali metal salt particles from a kraft recovery boiler operating at two different firing rates. Characteristics of Particles in Recovery Boilers The majority of the particles entrained in the combustion gases in the pendant heat transfer regions of recovery boilers fall into two categories. One is submicron size alkali metal salt particles, or fume. These particles are produced by the vaporization of sodium and potassium during combustion, and subsequent condensation.6,7 These inorganic aerosol particles are spherical and from 0.01 to 1 µm in diameter with mass average diameter close to 0.1 µm.8 Figure 1, an SEM image of particles collected from the flue gas at the entrance of the electrostatic precipitator of the Joutseno recovery boiler, is a typical example of the fume particles produced in kraft recovery boilers. In general, fume particles consist mainly of sodium sulfate, but also contain potassium, chloride, and carbonate. These particles have a median diameter of 0.40 µm. Table 1 shows the range of composition of fume particles collected from 47 different kraft recovery boilers.9 Fume particles melt over a temperature range (6) Mikkanen, P.; Kauppinen, E. I.; Jokiniemi, J. K.; Sinquefield, S. A.; Frederick, W. J.; Ma¨kinen, M. The Particle Size and Chemical Species Distributions of Aerosols Generated in Kraft Black Liquor Pyrolysis and Combustion. AIChE Symp. Ser. 1994, 90 (302), 46-54. (7) Mikkanen, P.; Kauppinen, E. I.; Jokiniemi, J. K.; Sinquefield, S. A.; Frederick, W. J.; Ma¨kinen, M. The Particle Size and Chemical Species Distributions of Aerosols Generated in Kraft Black Liquor Pyrolysis and Combustion. AIChE Symp. Ser. 1994, 90 (302), 46-54. (8) Mikkanen, P.; Jokiniemi, J. K.; Kauppinen, E. I.; Vakkilainen, E. K. Coarse ash particle characteristics in a pulp and paper industry chemical recovery boiler. Fuel 2001, 80 (7), 987-999. (9) Duhamel, M.; Tran, H. N.; Frederick, W. J., Jr. The sintering tendency of recovery boiler precipitator dusts. Proceedings of the 2002 TAPPI Fall Technical Conference and Trade Fair; Tappi Press: Atlanta; pp1333-1345.
that depends on their composition. The first melting temperature decreases with increasing potassium and carbonate content,10 while chloride content has only a minor effect as long as at least some chloride is present. Within this range of composition in Table 1, the first melting points are from 513 °C to nearly 600 °C when the chloride content is nonzero. When both with the carbonate and chloride content are zero, the first melting temperature can be as high as 810 °C. Most of the larger particles are produced by one of two different mechanisms. The first is entrainment of fine burning droplets11 and char.12 These inorganic particles are often referred to as “intermediate size” particles because they fall between the submicron fume particles and the larger residue from the 0.5-5 mm black liquor droplets fired in recovery boilers.13 They are mainly spherical, between 5 and 70 µm in diameter.8,11 They contain sodium, potassium, sulfate, chloride, and carbonate, but the chloride and potassium levels in these particles are significantly less than in fume particles.12,14 The second mechanism by which intermediate size particles are produced is disintegration of deposits by soot blowing.8,11 The particles produced are irregular chunks of moderately sintered fume particles, and have a composition similar to that of fume particles. Deposition Measurements The measurements of deposit characteristics reported here were made at the Andritz recovery boiler at the Metsa Botnia Group pulp mill in Joutseno, Finland. The measurements were made at the locations shown in Figure 2. Samples of dusts were collected using a specially designed deposition probe. Figure 3a contains a sketch of the 3-m long probe. It was constructed so that cooling air flows from the cold end of the probe through an annulus between concentric stainless steel tubes. Part of the air exited through small channels in the tip, and part flowed back through the inner tube, exiting through the cold end. The deposition surface was a pair of stainless steel rings mounted on a stainless steel support that screwed into the hot end of the probe (see Figure 3b). The support piece contained holes that allowed cooling air from the probe to pass through it. A channel had been cut in the support piece to permit the tip of a long, sheathed thermocouple to be inserted into it from the cold end of the probe. In all measurements, the probe tip (deposition surface) was inserted approximately 1.3 m beyond the boiler wall. In most cases, the deposition surface con(10) Tran, H. N.; Gonsko, M.; Mao, X. Effect of Composition on the First Melting Temperature of Fireside Deposits in Recovery Boilers. Proceedings of the 1998 Tappi Engineering Conference; TAPPI Press: Atlanta; pp 181-192. (11) Mikkanen, P. Fly ash particle formation in kraft recovery boilers. Dissertation, VTT Publications: 421, VTT Chemical Technology, Espoo, 2000. (12) Kochesfahani, S.; Tran, H. N.; Jones, A. K.; Grace, T. M.; Lien, S. J.; Schmidl, W. Particulate Formation During Black Liquor Char Bed Burning. Proceedings of the 1998 International Chemical Recovery Conference. TAPPI Press: Atlanta; pp 599-614. (13) Adams, T. N.; Frederick, W. J.; Grace, T. M.; Hupa, M.; Iisa, K.; Jones, A. K.; Tran, H. N. Kraft Recovery Boilers; TAPPI Press: Atlanta, 1997. (14) Lien, S. J. Particulate Formation from Char Bed Burning. MS thesis (A190 report), Institute of Paper Science and Technology, 1999.
Alkali Metal Salt Deposits in Kraft Recovery Boilers
Figure 2. Diagram of the recovery boiler at the Metsa Botnia Group pulp mill in Joutseno, Finland, showing the locations (A-D) where deposition probes were inserted.
Energy & Fuels, Vol. 17, No. 6, 2003 1503
After a deposit was formed, the probe was removed from the boiler. Samples of the deposits were removed immediately from the probe. The samples removed remained intact in cross-section (radial direction). They were placed in polyethylene bags within sealed polyethylene containers to prevent absorption of moisture. These samples were subsequently examined by scanning electron microscopy (SEM) to determine their structure and how it changed from the surface in contact with the deposition probe to the outer surface of the deposit. They were also analyzed for sodium, potassium, chloride, carbonate, and sulfate content by dissolving samples of each deposit in deionized water and measuring the concentrations of the ions using ion chromatography. Mixtures of inorganic salts melt over a range of temperatures. The first melting temperature is the temperature at which liquid first appears when a mixture of solid inorganic salts is heated slowly. As heating continues, the temperature of the mixture increases, and the mixture continues to melt until eventually melting is complete. The first melting temperature of each sample was measured by high-temperature differential scanning calorimetry, using a Perkin-Elmer Pyris 1 differential scanning calorimeter. The measurements were made in nitrogen over the temperature range 50-580 °C with a heating rate of 30 °C/min. The first melting temperature was taken as the temperature at the onset of the endotherm associated with melting of the salt mixtures. To obtain a range of deposit compositions, deposition measurements were made at locations in the superheater, boiler, and economizer banks, and at two different liquor firing rates. Characteristics of the Deposits
Figure 3. Sketch of the deposition probe (a), the deposition probe tip, assembled on the probe (b), and the deposition probe with an accumulated deposit (c).
sisted of heavy aluminum foil wrapped around the two rings of the deposition probe tip. Figure 3c shows the probe tip after a deposition measurement. In each measurement, deposits were collected on the air-cooled sample probe, letting the deposits accumulate to a thickness of at least a few millimeters. The time for deposit formation was from 10 min to 2 h. Some of the samples were obtained over deposition times that were much shorter than the interval between soot blowing. These samples were obtained to evaluate the early changes in structure of deposits as they formed.
The types of particles found in deposits depended on the location within the boiler, and on whether soot blowers upstream of and near the deposition probe had been activated during the time when the probe was in place in the boiler. Figure 4 is an SEM photo of particles scraped from the outer surface of a deposit collected at location A in the superheater region. Three types of particles are evident. One type is partially sintered submicron fume particles such as those shown in Figure 1. A second type is irregular particles, probably from disintegration of upstream deposits by soot blowing. The third is small spheres, 10 to 15 µm in diameter. The ones shown here have a dendritic coating of sintered fume particles. Deposits from this location always contained a large fraction of spherical, 10-30 µm diameter particles. The particles in Figure 4 are from a deposit that was produced when the boiler was fired at 79% of its design thermal load. At a higher firing rate of 122% of design thermal load, particles of this size collected at locations A and B had a very smooth surface, indicative of complete melting at the surface. Figure 5 shows the internal structure of another deposit formed at location A. The total deposition time for this deposit was 10 min. The structure of this deposit is one of interconnected fume particles that have grown together via sintering. Some individual particles are distinguishable, but the necks between them are large, more than half of the particle diameters. In many
1504 Energy & Fuels, Vol. 17, No. 6, 2003
Figure 4. Material from the surface of a deposit from location A, at a thermal input rate of 79% of design; time for deposition was 2 h. The labeled particles are (a) partially sintered submicron fume particles; (b) irregular particles, probably from disintegration of upstream deposits by soot blowing; and (c) spherical particles, 10 to 15 µm in diameter.
Figure 5. Internal structure of a deposit from location A, at a thermal input rate of 79% of design; time for deposition was 10 min.
places, sintering has proceeded beyond the neck growth stage and into the grain growth stage. Many of the grains have grown to several microns across. The structure appears to be very porous. The deposits shown in Figure 6 show how different the particle content can be, depending upon the schedule of soot-blowing upstream. The deposit in Figure 6a was produced with the upstream soot blowers shut off. It contains mainly fume particles and very few larger particles. The deposit in Figure 6b is quite different in appearance. It contains a large number of the larger particles. A soot blower immediately upstream of the deposition probe was fired one minute before the probe with this deposit was removed from the boiler, apparently releasing many of the larger particles that were collected as part of this deposit. The structure of the deposits depended strongly on the location at which the probe was inserted. The deposit in Figure 6a was formed at location B. The deposit was formed over a 30 min period at 122% of the design thermal input rate. The flue gas temperature at this location and firing rate was ∼600 °C. Figure 7
Frederick and Vakkilainen
shows a cross-section of a deposit from location B. This deposit was formed over a 15 min period at 122% of the design thermal input rate. The flue gas temperature at this location and firing rate was ∼770 °C. The size scale of the fine structure in these two deposits is quite different. The one formed at location C shows particles of sizes less than 1 µm, connected to several other particles in a linear configuration. The one produced at location B is much larger, on the order of 10 µm in the longest dimension. The structure of the deposited material is also much coarser, typically 1-3 µm in diameter or thickness. Sintering clearly proceeded much more rapidly at the hotter location. Figure 8 contains SEM photos that show the structure of a deposit produced at location B. This deposit is the same one as shown in Figure 7b, but is not imbedded in epoxy resin. In Figure 8a, which shows the outer surface of the deposit, the structure is one of short, stubby branches, 1-2 µm in diameter, radiating from larger nodes, 4-5 µm wide. This structure is reminiscent of the fine, dendritic structure observed by Sinquefield et al.3 in deposits formed in a laboratory study, but sintered further, so that the original, submicron fume particles have been fused together to form a structure with thicker, shorter branches. A few necks are visible, but most branches are nearly uniform in diameter along their lengths. The volume fraction occupied by the strands is very small, again similar to those that Sinquefield et al. observed. In the midsection of deposit (Figure 8b), the structure is similar to that observed at the outer surface, but the branches are thicker, 5 to 8 µm, the same as the nodes connecting them. This mid-thickness part of the deposit was on the probe longer than the material at the surface (∼5 to 10 min versus
