Energy & Fuels 1992,6, 709-715
709
Fouling of Convection Heat Exchangers by Lignitic Coal Ash Peter M. Walsh’ Fuel Science Program, Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802
Adel F. Sarofim and J h o s M. Beer Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received December 16, 1991. Revised Manuscript Received May 27, 1992
Ash deposita formed at moderate temperatures (1100-1300K)in the convective sections of utility boilers firing coals containinglignitic ash are associatedwith the formation of sticky particles following condensation of sodium sulfate from the vapor phase at 1200-1300 K. The deposita are enriched in calcium, due to the abundance of calcium among the particles impacting and sticking and the low melting temperature of mixtures of calcium, magnesium, and sodium sulfates. Because the formation of these deposita occur8 only in a limited range of gas temperatures, approximately from the dew point to the melting temperature of sodium sulfate, they are not observed during experiments in which deposition is accelerated by increasing the temperature to values typical of furnace exit gas or higher. The principal features of the process have only become evident from observations in the field. On startup of a boiler after cleaning, deposita are formed by ash particles made sticky by a layer of condensed sodium sulfate. Although sodium sulfate is most concentrated in the smallest particles, the deposit is formed by particles large enough to impact and small enough to stick. These particles may contain less than the average sodium content of the coal ash. At early times, growth is most rapid at downstream locations, where the thickness of the sticky layer of sodium sulfate on suspended particles is greatest. At later times, the rate of deposition increases when deposita become thick enough that sticky sulfates are present on their surfaces. Growth is then more rapid upstream, where the concentration of sodium sulfate in deposita is greatest, due to direct condensation of vapor. Because deposit accumulation is the difference between the rates of deposition and erosion, the amount and size distribution of erosive particles, such as quartz, are important factors in deposit formation.
Introduction Three types of slagging and fouling may be observed as one moves downstream from the radiant superheaters into the convection section of a utility boiler firing coal containing lignitic ash.l-l9 1. Slag deposita on the radiant superheaters caused by the combined effects of the radiative heating of the deposit
* Author to whomcorrespondenceshouldbe addressed 206B Academic Projecta Building, Pennsylvania State University, University Park, PA 16802-2303. (1) Boll, R. H.; Patel, H. C. Trans. ASME (J.Eng. Power) 1961,83A, 451. (2) Gronhovd, G.H.; Wagner, R. J.; Wittmaier, A. J. Comparison of
Ash Fouling Tendencies of High- and Low-Sodium Lignite from a North Dakota Mine. American Power Conference, Chicago, IL, April 26-28,
1966. (3) Gronhovd, G. H.; Tufts, P. H.; Selle, S. J. Some Studies on Stack
Emissiona from Lignite-Fired Powerplanta. Proceedings of the Symposium on Technology and Use of Lignite, Grand Forks, ND, May 9-10, 1973; Gronhovd, G. H., Kube, W. R., Eds.; Bureau of Mines Information
Circular 8650,1974; pp 83-102. (4) Winehip, A. D.; Bender, F. Ash Deposition Research on Canadian Lignites. Proceedings of the Symposium on Technology and Use of Lignite, Grand Forks, ND, May 1-2,1969; Elder, J. L., Kube, W. R., Eda.; Bureau of Mines Information Circular 8471, 1970; pp 103-112. (5) Reid, W.T. External Corrosion and Deposits: Boilers and Gas Turbines; American Elsevier: New York, 1971. (6)Gray, R. J.; Moore, G. F. Burning the Sub-Bituminous Coals of Montana and Wyoming in Large Utility Boilers. ASME 74-WA/Fu-l, 1974. (7) Tufte, P. H.; Beckering, W. Trans. ASME (J.Eng. Power) 1975, 97A, 407.
surface and local gas temperature near the softening temperature of the whole ash.12 Although the initial deposit on a clean tube is dry, ita surface becomes sticky (8) Tufte, P. H.; Gronhovd, G. H.; Sondreal, E. A.; Selle, S. J. Ash Fouling Potentials of Western Subbituminous Coals as Determined in a Pilot Plant Test Furnace. American Power Conference, Chicago, IL, April 22, 1976. (9) Hein, K. Trans. ASME (J.Eng. Power) 1977,99A, 679. (10) Sondreal,E. A.;T&,P. H.; Beckering,W. Combust.Sci. Technol.
1977, 16, 95.
(11) Sondreal, E. A.; Gronhovd, G. H.; Tufte, P. H.; Beckering,W. Ash Fouling Studies of Low-Rank Western U.S. Coals. Ash Deposits and Corrosion Due to Impurities in Combustion Gases; Bryers, R. W., Ed.; Hemisphere: Bristol, PA, 1978; pp 85-111. (12) Hein, K. Formation of Fireside Deposita during Brown Coal Combustion. Fouling and Slugging Resulting from Impurities in Combustion Gases; Bryers, R. W., Ed.; The Engineering Foundation: New York, 1983; pp 69-83. (13) Hupa, M.; Backman, R. Slagging and Fouling during Combined Burning of Bark with Oil, Coal, Gas, or Peat. Foulingof Heat Ezchanger Foundation: New York, Surfaces;Bryers, R. W., Ed.; The Engineering 1983; pp 416432. (14) Rindt, D. K.; Jones, M. L.; Schobert, H. H. Investigations of the
Mechanism of Ash Fouling in Low-Rank Coal Combustion. Fouling and Slugging Resulting from Impurities in Combustion Gases;Bryers, R. W., Ed.; The Engineering Foundation: New York, 1983; pp 17-35. (15) Raask, E. Mineral Impurities in Coal Combustion: Behavior, Problem, and Remedial Measures; Hemisphere: Bristol, PA, 1985. (16)Smith. M. Y.: Beck, W. H.: Hein, K. Combust. Sci. Technol. 1986, 42,.115. (17) Bryers, R. W. An Overviewof Slagging/FoulingDue to Impurities
in Coal. Proceedings: Effectsof Coal Quality on Power Plants; Mehta, A., Dooley, R. B., Eds.; EPRI CS-5936-SR, Electric Power Research Institute: Palo Alto, CA, 1988; pp 1-1to 1-23.
0 1992 American Chemical Society
Walsh et a1.
710 Energy & Fuels, Vol. 6, No. 6, 1992
NoOH
CHAR PARTICLE CONTAIN1 NG ORGANIC Co, Mq, Ne. S
No
’
Ne2S04
f
/
c TUBE WALL
INERTIAL IMPACTION
Si02 - A l p 0 3 CLAY INCLUSIONS DROPLETS OF CP, Mg, AI, S i OXIDES
SODIUM SULFATE -COATED COS04 CeO- MgSOg MgOS i O p - Alp03
-
-
BONDED DEPOSIT
Figure 1. Schematic diagram of the processes leading to deposition of sodium sulfate-coated particles from flue gas in the temperature range from approximately 1160 to 1250 K.
after some deposit has accumulatedand thereafter collects almost everything which hits it. 2. Sintered deposita on convective tubes just downstream from the furnace exit: The particles responsible for these deposits are sodium silicate-coated particl&WJ617W% and &dum alumjnc&~a~,5,7J*1%15-19S whose softening temperatures are lower than the local gas temperature. The differences between these deposits and type 1 are the absence of flame radiation and the fact that the local gas temperature is now too low to melt the whole ash, but still high enough to melt individual particles having particular ranges of bulk or surface composition. Even when these deposita become very thick, they remain dry, although usually sintered. Deposit types 1 and 2 are enriched in silica, indicating that the sodium silicate ~ ~important ~ mechaniem of Wall and c o - ~ o r k e r s * ~ %is2an contributor to their formation. 3. Deposita farther back in the convective section are enriched in calcium, and calcium sulfate is their most abundant c o m p ~ n e n t . ~ -Deposita l ~ ~ ~ $ of ~ this type occur at temperatures in the range over which mixtures of calcium, magnesium, and sodium sulfates are sticky, approximately from the dew point of sodium sulfate to the freezing point of mixtures of calcium-magnesium(18) Jones, M. L.; Beneon, S.A. An Overview of FoulinglSlaggingwith Western C d . Proceedings: Effect8 of Coal Quality on Power Plants; Mehta, A.,Dooley, R.B., Eds.; EPRICS593&SR,ElectricPowerResearch Inetitute: Pal0 Alto, CA, 1988, pp 1-25 to 1-47. (19) Ziwmer, B.; Hatt, R.The Role of Calcium in Fouling of Westem Coals. Proceedings: Effects of Coal Quality on Power Plants; Mehta, A., Dooley, R. B., Eds.; EPRI CS-5936-SR, Electric Power Research Imtitutm Palo Alto,CA, 1988, pp 1-49 to 1-61. (20)Wibberly, L.J. Alkali Ash Raactiona and Deposit Formation in Pulverized Coal Fired Boilers. Ph.D. Dissertation, University of Newcastle, Australia, 1980. (21) Wibberly, L. J.; Wall, T.F. Fuel 1982, 61, 93. (22) Wibber1v.L. J.: Wall,T. F. DemitFormationandStickvParticles from ‘Alkali-Ash&ctio& Fouling and Slogging Resulting from Impurities in Combustion Cases; Bryers, R. W., Ed.; The Engineering Foundation: New York, 1983; pp 493-513. (23) h e r , D. E.;Nagarajan, R. Toward a Mechanistic Theory of Net Deposit Growth from Ash-Laden Flowing Combustion Gaeee: SelfRegulatbd Sticking of Impacting Particlee and Deposit Erasion in the Prewnce of VaporDeponitd-or SubmicronMist-“Glue”. Lyczkoweki, R. W., Ed.; AIChE Symp. Ser. 1987,83 (257), 289-296. (24) Lindner, E. R. A Study of Sodium-Ash Reactions during the Comburtion of Pulveriaed Coal. Ph.D. Dissertation, University of Newwtle, Awtralia, 1988. (25) Lmdner, E. R.; Wall, T.F. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1991; pp 1313-1321. (26) Loehden, D.;Walsh, P. M.; Sayre, A. N.; Be&, J. M.; Sarofim, A. F. J. Inst. Energy 1989,62, 119. (27) Bryere,R.W. Examination of Foulingof ConvectiveHeat Transfer Surface by Cdcium and sodium Using Micro-Analytical Techniques. ASME W-JPGC-FACT-5,1986. (28) Huffman, G. P.; Huggins, F. E.; Levasaeur, A. A.; Durant, J. F.; Lytle, F. W.; Greegor, R. B.; Mehta, A. Fuel 1989,68, 238.
sodium sulfates, near 1200 K.13J6723p2”33These deposita are the subject of the present paper. Sulfate deposita have been observed downstream from regions in which little or no deposition was observed, showing that a mechanism was operating which was distinctly different from that responsible for the accumulation of ash near the furnace exit. Our objective is to show how observations in the field are related to the behavior of sodium sulfate and to identify the coal properties and convective section conditions which control deposit growth. Model for Fouling by Calcium Sulfate Mechanism. The processes contributingtothe growth of sulfate deposita are shown in Figure 1,beginning with the release of ash from burning char, at the left. During combustion of the char, organically bound calcium and magnesium accumddte on the receding char surface, along with aluminosilicate droplets formed by quartz and clay mineral inclusion^.^*^^ The more volatile sodium leaves the burning p a r t i ~ l e ~ and ~ p ~reacts ~ p ~with gaseous species to form Na0H.1J6~22~29~37-39 Droplets of calcium and magnesium aluminosilicates are shed from the surface or released by fragmentation of the ~ h a r . 3 ~ ~ ~ ~ In the temperature range 1300-1850 K, NaOH reacts with quartz and aluminosilicates, forming a sticky layer which contributes to deposition near the furnace exit.1J6*2+25~~~3g This process, not shown in Figure 1, is not a major contributor to deposit formation at 1250 K and below, but it does determine the concentration of sodium remaining in flue gas to react with sulfur oxides.35~39 Sodium sulfate forms at 1300-1400 K.1J69223’939 At gas temperatures in the range 1200-1300 K, sodium sulfate begins to condense on suspended particle~.~J~J6.9~-~ Calcium and magnesium oxides react with sulfur oxides (29) Roener, D. E.; Chen, B.-K.; Fryburg, G. C.; Kohl,F. J. Combust. Sei. Technol. 1979, 20, 87.
(30)Roee, J. S.;Rameum, M.; Anderson,R. J.; Nag-an, R. Prediction and In Situ Measurement of the Thermal Conductivity of Multiphaee Fouling Deposita Formed in Direct Coal-Fired Combustors. ASME 87WA/HT-6, 1987. (31) Nagarajan, R.;Anderson, R.J. Effect of Coal Constituents on the Liquid-Assisted Capture of Impacting Ash Particles in Direct Coal-Fired Gas Turbines. ASME BB-GT-192,1988. (32) Rosner, D. E. PhysicoChem. Hydrodyn. 1988,10,663. (33) Ross,J. S.; Anderson, R. J.; Nagarajan, €2. Energy Fuels 1988,2, 282. (34) Quann, R.J.; Sarofii,A. F.Fuel 1986,65,40. (35) Erickson, T. A,; Ludlow, D. K.; Benaon, S . A. Energy Fuels 1991, 5 , 539. (36) Neville, M.; Sarofii,A. F. Fuel 1986,64,384. (37) Halatead, W. D. J. Inst. Fuel 1969,42, 419. (38) McNallan, M. J.; Yurek, G. J.;Elliott, J. F. Combuat. Flame 1981, 42, 45. (39) Wibberly, L. J.; Wall, T. F. Fuel 1982, 61, 87.
Energy & Fuels, Vol. 6, No. 6, 1992 711
Fouling of Heat Exchangers by Lignitic Coal Ash
to form their sulfates.16 Sodium sulfate also condenses from the vapor phase onto cool ~ u r f a c e s . ~Homogeneous 3~~ nucleation of sodium sulfate in the free stream is inhibited by the high concentration and smallsize of particles already present.% At gas temperatures between the melting temperature of sodium sulfate (1157K)and the dew point (1250f 50 K),particles coated with a layer of liquid sodium sulfate stick on collision with a tube or deposit surface.23,303133 The mixture of calcium,magnesium, and sodium sulfates melta near the melting temperature of pure Na2S04.40p41 On an exposed tube or deposit surface below about 1157 K, the CaSO4-MgSOrNa2SOr mixture freezes; between 1157 and about 1250K the sulfate mixture is molten, and at deposit temperatures above 1250 K, little NazSO4 is expected to be present. Deposita grow most rapidly when their surfaces are sticky, at surface temperatures between 1157 and 1250 Sulfation of unreacted calcium and magnesium oxidesremaining in the deposited particles continues on the deposit surface. Erosion of a dry surface by dry particles, e.g., quartz, a process not included in Figure 1,competes with deposit g r ~ w t h . ~ The J ~ . balance ~~ can be tipped from control by erosion (tube stays clean) to aggressive fouling, by small changes in coal ash composition or gas temperature.lg Quantitative descriptions of the controlling processes are presented below. Sodium Sulfate Condensation on Particles. Condensation of sodium sulfate on suspended particles begins when the gas has cooled to the sodium sulfate dew point, assumed here to be 1250K (averageof values from Raaskls and Halstead3'). Some sodium sulfate will have been condensing directly onto the available cold tube and deposit surfaces upstream from this p0int,2~"~ but the amount removed from the flue gas prior to ita reaching the dew point is expected to be small. Below the dew point, more sodium sulfate condenses on particles than on tubes, because although the surface area of the tubes is greater, the mass-transfer coefficient for transport to particles is much larger than that to tubes. The partial pressure of sodium sulfate in the gas, assuming negligible backpressure of ita condensate on the particle surfaces, decays exponentially with time following the onset of condensation. The time constant for this process is governed by the specific surface area-weighted mean size of the particles and the particle concentration: K.1112,13,16123,31,33
For 4-pm particles the characteristic time is 0.03 8, shorter than the 0.1 s required for flue gas to cool from 1250 to 1157 K. Because there is little time for diffusion into the substrate, the volume of sticky material on a particle is approximately equal to the volume of sodium sulfate condensed. The development of the sulfate layer is described by (40)M i l k , H.Neues Jahrb. Mineral. Geol. 1910,30,36. Levin, E. M.; Robbina, C. R.;McMurdie, H. F. Phase Diagram for Ceramists; American Ceramic Society: Columbus, OH, 1964; p 345. (41) Mikimov, S.M.;Filippova,2.I. Akad. Nauk Uzb.SSR,Tcrshkent Znst. Khim. Tr. 1949,2,131. Levin, E. M.; Robbim, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists (1969 Supplement);Reser, M. K., Ed.; American Ceramic Society: Columbus, OH, 1969; p 261.
Table I. Conditions Chosen for the Calculation of Deporit Growth parameter furnace exit gas temperature, K temperature gradient in flue gas in tube banks, K/m gas velocity, m/s tube metal temperature, K tube diameter, m ' ash in coal, as received, w t % fraction of ash retained in the furnace fly ash composition (mass fractions, % ,503free)a calcium and magnesium aluminosilicates q -
sodium oxide from extraneous pyrite, apatite, etc. mean size of quartz,b fim mean size of calcium aluminosilicates,b pm geometric standard deviation of the particle size diatributionsb density of particles: kg/ma Sherwood number for suspended particlesd fraction of NazO in ash forming NazSOle molecular diffusion coefficient for NazSOlf m2/s Sherwood and Nusselt numbers for tub& d i u m sulfate dew point) K melting temperature of sticky sulfates,' K surface tension of sticky sulfates: N/m voidage of deposit thermal conductivity of deposit: W/mK erosivity of quartz toward dry deposit (mass removed/maaa impacting)'
value 1533 60 14
900
0.070 5
0.3 83 9 2
6 20 5
2 3300 2
1 1 X l P
40 1250
50
1157 0.2 0.5
0.5 0.06
a Wyoming subbituminous C; Loehden et al.,= Sayre." Quann and Sarofim," Loehden et ale,= Sayre." Calcium oxide. Particle velocity equal to gas velocity. e Worat caae. The fraction reacting with silica and aluminosilicates varies with coal and combustion f Rindt, Jones, and S ~ h o b e r t8. Nuseelt ~~ number conditions.26@*96 on upstream stagnation line. McAdams, W. H. Heat Tran."eion, 3rd ed.; McGraw-Hill: New York, 1954. Raaak,'6 Haletaad.s7 Sodium sulfate. j Raaak.I6 Creek, R. C. Thermal Properties of Boiler Aah Deposits. State Electricity Commission of Victoria, Research and Development Department, Brown Coal Research Division, Report No. ND/85/014,1985. I Adjusted.
For the coal and conditions specified in Table I, the ultimate thickness of the sticky layer on a 10-pm particle is 0.015 pm. Sticking Probability of the Sulfate-Coated Particles. The derivation of the sticking probability was based on the analysis developed by Wibberly20 and Wibberly and Wall22to explain deposition of particles made stickyby a layer of sodium silicates. Their argument was that, if the interfacial energy of the area over which contact is established between dry and sticky surfaces is greater than the kinetic energy of the incoming particle, then the particle will stick, otherwise it will not. The assumption made here was that the sticking probability of a sticky particle on a dry surface was proportional to the ratio of interfacial energy to kinetic energy: (3)
Substitution of eq 2 and the relation between the thicknew of the layer and the area over which contact is established with a flat surface leads to
The particle velocity is ita value on arrival at the surface,
Wabh et al.
712 Energy &Fuels, Vol. 6,No. 6,1992
/'
t
size (Table I, The fraction of impacting particles of a particular size incorporated in a dry deposit is
sT1cK4 fdep,dry
PROBABILITY
[Pd&CASYCAS
- ke,$QYQ][l
+ RNa$OI]
(7)
The rate of deposit growth was obtained by integrating the product of this expression and the impaction efficiency over the range of impacting particle sizes:
A>m+ U-
mJ
o m .4
aa
Fdep,dry
n m
0
a
a .2
0
I 1 .3
/< I
/
/
I 3 IO 30 PARTICLE SIZE ( r m )
I 100
Figure 2. Comparison of particle size distribution, impaction efficiency, and sticking probability for sodium sulfate-coated calcium aluminosilicate particles. According to the present model, a narrow range of sizes, near 15 pm, is expected to make the largest contribution to deposit growth on dry surface.
from the correlation of Wessel and Righi.42 The sticking probability has a very strong dependence on particle size, which is compared with the fly ash particle size distribution and impaction efficiency in Figure 2. The segregation of particles into two classes, having sticking probabilities of 0 and 1,as was done by Wibberly and is therefore a good approximation. The combined effects of sticking probability and impaction efficiency suggest that deposit is formed by particles from a narrow portion of the size distribution. Mass Fraction of Sodium Sulfate in the Particles. Each particle incorporated in a deposit carries with it the sodium sulfate which has condensed on its surface. The contribution of this material to the deposit must be known in order to calculate the total deposit mass and thickness and to estimate the fraction of stickysurfaceon the deposit when it becomes thick enough for sulfates on its surface to melt. The ratio of the mass of condensed sulfate on a particle to the original particle mass is
Direct Condensation of Sodium Sulfate on Deposits. Sodium sulfate also condenses directly on deposit surfaces. The diffusion-controlledrate of sodium sulfate deposition per unit of projected area, neglecting the backpressure at the condensate surface, on a tube located t seconds downstream from the point at which the gas temperature reaches the dew point is
Neither homogeneous nucleation nor condensation of sodium sulfate on existing particles in the thermal boundary layer of the d e p o ~ i twas ~ ~considered. ?~~ Growth of Deposit When All of Its Surface Is Dry. When no part of the deposit surfaceis sticky,the deposition rate for ash particlesof agiven size is the difference between sticking and erosion.23 The most erosive particles were thought to be extraneousquartz, whose stickingprobability on a dry surface, from eq 4, is small because of their large (42)
Wessel, R. A.; Righi, J. Aerosol Sci. Technol. 1988, 9, 29.
= FeshJidp-fdep.dry(dp)
f i m ~ ( ~ pd(ln ) dp)
Growth of Deposit When Its Surface Is Sticky. When the deposit surface is between the dew point and the melting temperature, the presence of sticky sulfates on the surface increases the sticking probability for all particles having less than unit sticking probability on dry s ~ r f a c e . 2 3For ~ ~ particles ~ ~ ~ ~ ~colliding ~~ with a sticky surface, the incremental increase in sticking probability was assumed to be proportional to the volume fraction of sodium sulfate at the surface. The total sticking probability is Paticky - P d r y ( l - 'Na.$O,) + 'NGO, (9) where P d r y is the average sticking probability for particles on a dry surface. The volume fraction of sticky sulfates at the surface of a deposit is approximately 'N%SO4
F
FNa$04
+ Pstickflim$Na+304
Na2S04 + PstickPimp@N+SO,
+ pha$O,/p$h)
(10)
where R N a 2 S 0 4 is the average ratio of sodium sulfate to ash in impacting particles:
The particle deposition rate is now Fdep,sticky
= P s t i c k Y i m p ( ' + RNa$O,)
(12)
The change in shape of the target was disregarded in the calculations of impaction, sticking, erosion,and direct condensation; therefore, deposit growth is linear. However, the shape of the deposits was acknowledged in calculatingtheir thickness. The maximum thickness,from the tube to the apex of the wedge, after a period of operation at steady load, without deposit sloughing off and without sootblowing,is
The factor of 2 accounts for the deposita' triangular cross section, and e is their voidage. Sulfation of calcium and magnesium oxides following deposition increases the volume of solid material and decreases voidage. The net increase in volume of deposit due to this reaction was neglected. The calculationof deposit surfacetemperature was based on the maximum thickness, and the Nusselt number on the upstream stagnation line of a clean tube. A calculation of deposit growth is shown in Figure 3 for conditions, specified in Table I, chosen to simulate the convectivesection of a utility boiler. Growth is slowest at short times when the deposit is dry, and an abrupt increase (43) Sayre, A. N. Properties of Ash Particles Generated during Pulverized Coal Combustion. MIT-EL 88-013WP,The Energy Laboratory, Massachusetts Institute of Technology: Cambridge, MA, 1988.
Energy & Fuels, Vol. 6, No. 6,1992 713
Fouling of H e a t Exchangers by Lignitic Coal Ash .81
1
2
-
1
I
I
/l
V
i .4
DEW POINT
I
I
G A S TEMP. 1 2 0 0 K
I
MELTING POINT
I-
-
I
/
iu,,l] i
/
0 0
2
4
6 8 TIME (months)
10
12
Figure 3. Growth of deposit from sodium sulfate-coated calcium aluminosilicate particles impacting a tube having metal temperature of 900 K in gas at 1200 K. The thickness shown on the ordinate is the maximum from the tube surface to the tip of a wedge-shaped deposit. Conditions a t the deposit surface were estimated using the heat- and mass-transfer coefficients on the upstream stagnation l i e of a clean tube. The lower rate of growth a t short times is characteristic of dry deposit. The increase in rate occurs when the deposit becomes thick enough that the mixture of calcium, magnesium, and sodium sulfates is sticky on the deposit surface, at about 1157K. In the present model, growth is linear in both regimes.
in rate occurs when the temperature of the surface reaches the melting point of sodium sulfate. In the present model, growth is linear even on a sticky surface,since no allowance was made for the temperature dependence of sodium sulfate vapor pressure. Rosner and Nagarajan23consider a finite vapor pressure of sodium sulfate at the deposit surface and ita temperature dependence. Deposition in the presence of an evolving thermal conductivity of deposita was treated by Rosner and Nagarajm~~~ and by Ross et Effects of the deposition mechanism on deposit microstructure were analyzed by Tassopoulos et al.44
Results and Discussion The regimes of growth are shown in Figure 4. The figure is arranged with the flow from left to right, so that temperature decreases in that direction. At gas temperatures above 1250 K, particles are dry and any particles on the tubes are also dry, sincethe tube metal temperature is below the sulfate melting temperature. The small amount of sodium sulfate which condensesdirectly on the tubes solidifies and is removed by erosion. No deposit is expected to form under these conditions. When the gas temperature drops to 1250 K, sodium sulfate begins to condense on the suspended particles, but no deposit is observed until enough sodium sulfate has accumulated to make the sticky surface layer on some impacting particles thick enough that deposition overcomes erosion. The erosion coefficient was adjusted to permit deposition after a short delay. As one moves downstream, sticking probabilities increase, approaching a constant value as sodium sulfate is transferred from the vapor phase to the particles. After about 2 months of continuous full load operation, without sootblowing, the profile of deposit thicknesses is expected to look like the dashed curve in Figure 4, increasing with increasing distance from the point in the convective section at which ~~
~
(44)Taesopoulos,M.; OBrien, J. A.; Rosner, D. E. AIChE J . 1989,35,
967.
-
1350
4
1250 1200 1150 GAS TEMPERATURE ( K )
1300
6
8
1100
IO
1050
12
DISTANCE FROM FURNACE E X I T (m)
Figure 4. Regimes of deposit formation resulting from the influences of the sodium sulfate dew point, the calciummagnesium-sodium sulfates melting temperature, and the increase in deposit surface temperature with increasing thickness. Sulfate deposita occur in the region where sticky particles are present, where gas temperatures are between the sodium sulfate dew point and melting temperature. The variation of deposit thickness with local gas temperature (axial position) 2 months after a thorough cleaning of the tubes is shown by the dashed curve. The deposit surface has just become sticky a t the point where this curve is tangent to the curve separating the sticky surface and dry surface regions. The delay between the onset of condensation, a t 1250 K, and the onset of deposition, a t 1235 K, is observed because erosion is more rapid than deposition on the cold tube surface in this region.
deposition becomes greater than erosion. At this time, the deposit at the axial position where gas temperature is 1210 K is just thick enough that ita surface is sticky (point at which the dashed curve is tangent to the c w e separating the “sticky surface” and “dry surface” regions). The rate of growth of deposit at this location increases,followed by similar increases at adjacent points upstream and downstream. The position of most rapid growth gradually shifts upstream as the upstream deposita become thick enough to have stickysurfaces,but the depositacan move upstream no farther than the point at which deposition began, since at this point and beyond, erosion continues to keep the tubes clean. When deposits at all locations are thick enough to be sticky, the deposition rate decreases with increasing distance in the flow direction, because the amount of sodium sulfate on a deposit depends upon the amount contributed by direct condensation from the vapor, which is greatest where the vapor concentration is highest. Deposition stops abruptly once the gas temperature drops to 1157 K, the assumed melting temperature of the sulfates, when the particles solidify. Lower melting materials, e.g., sodium pyr~sulfate,~*~ have been proposed to explain the formation of deposita at even lower temperatures. The thickness of deposits after 3,6,9,and 12 months of continuous full load operation without sootblowing or sloughing of deposita is shown in Figure 5. After long periods during which deposition has occurred in the presence of the sticky surface, the thickness of deposita decreases with increasing distance downstream from the initiation point. Direct observations of deposit growth in boilers, in conjunction with measurementsof particle properties,flow
714 Energy & Fuels, Vol. 6, No.6, 1992 I
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Walsh et al.
deposition process. The instantaneous deposition rate is a function of coal ash content, the proportions of sodium oxide, calcium aluminosilicates,and quartz in fly ash, the fraction of sodium vaporized during char combustion, the size distributions of calcium aluminosilicates and quartz, and gas and deposit surface temperatures. The deposit surface temperature itself depends upon all of these conditions through their influence on deposit shape, density, thermal conductivity,and emissivity. Laboratory, pilot-scale, and field measurementsare needed to test this model and develop it into a tool for evaluation and comparison of coals and combustion equipment.
I
MONTHS AT ~ L L L O A D
.6
I
01
I
Y
1240
I
I
I
I
1200 1180 1160 GAS TEMPERATURE ( K ) 1220 I
I
I
0
9 IO DISTANCE FROM FURNACE E X I T (m)
Figure 5. Thickness of deposita expected in regions of the convective section having gas temperatures between approximately1160and 1250K,after3,6,9,and12monthsofcontinuous full load operation without sloughing of deposits or sootblowing. Direct condensation of sodium sulfate from the vapor onto the deposit increases the stickiness of the deposit surface, but only after the deposit becomes thick enough for its surface to be sticky (surface temperature 11157 K). At long times, the deposit is therefore thickest where the concentration of sodium sulfate vapor is highest, just following the onset of deposit formation a t 1235 K. As in Figures 3 and 4,the Conditions chosen for the calculations are those on the upstream stagnation line, and the thickness shown on the ordinate is the distance from the tube surface to the tip of a wedge-shaped deposit.
conditions, and deposit composition, are needed to test and refine this analysis. The evolution of ash particle size and composition distributions during combustion, the distribution of sodium, the composition dependence of sticking coefficients, and erosion are among the controlling processes whose understanding could be advanced by additional laboratory and pilot-scale studies. Extension of the model to account for effects of sootblowing and thermal shock associated with load change will require a more detailed description of deposit structure and properties,Nt4 including the developmentof deposit strength. Conclusion The formation of a sticky layer of sodium sulfate on suspendedparticles, and the presence of a molten mixture of calcium-magnesium-sodium sulfates on deposits, is consistent with fouling by calcium sulfate from lignitic ash at temperatures near 1200K. The portion of a boiler's convective section in which these deposits are formed is defined approximatelyby the dew point of sodium sulfate and the melting temperature of the sulfate mixture. Erosion of deposit by dry particles (e.g., large quartz particles on which the condensed sulfate layer is too thin to give high sticking efficiency) is an important factor in deposit growth. At early times, following boiler cleaning, deposita are formed by a narrow range of particle sizes (large enough to impact, small enough to stick), but larger particles contribute when the deposit becomes thick enough to have a sticky surface. Because condensed sodium sulfate is most concentrated in small particles having low impaction efficiencies, the concentration of sodium in a deposit is not a useful indicator of the importance, or lack of importance, of sodium to the
Acknowledgment. This work was begun while the first author was a member of the research staff in the Energy Laboratoryat the MassachusettsInstitute of Technology. The research at MIT was supported by AMAX Coal Sales Co., American Electric Power Service Corp., Canadian Electrical Association, Combustion Engineering, Detroit Edison Co., Electric Power Research Institute, Empire State Electric Energy Research Corp., New England Power Service Co., Northeast Utilities Service Co., Nova Scotia Power Corp., Public Service Electric and Gas Co., and Shell Development Co. The work has benefitted from valuable contributions by many workers in the area of lignitic ash behavior, especially C. Richard Pelley (AMAX Coal Sales),Alan N. Sayre (InternationalFlame Research Foundation),David Loehden (CH2MHill), David B. Kehoe (CQ Inc.), Dennis J. Miller (Detroit Edison), John Guillaumin (Detroit Edison), Michael B. Doherty (American Electric Power), Drucilla G. Davis (Shell Development Co.), and Bruce G. Miller (Penn State). Glossary contact area between a sticky particle and the surface of a tube or deposit, m2 particle size, m minimum size of particle impacting a tube or deposit, m external geometric surface area-weighted mean particle size, m tube diameter, m molecular diffusion coefficient for species i, m2/s mass fraction of impacting particles deposited on dry surface, dimensionless mass fraction impacting, dimensionless ash flux, kg/m2.s flux of material depositing, kg/m2.s flux of particles impacting, kg/m2.s flux of sodium sulfate condensing directly on deposit, kg/m2.s log normal probability density function for sizes of particles of species i, dimensionless mass of deposit removed by erosion, per unit mass of quartz particles impacting, dimensionless deposit thickness, m mass of particle, kg sticking probability on dry deposit surface, dimensionless average sticking probability on dry deposit surface, dimensionless sticking probability on sticky deposit surface, dimensionless ratio of the mass of sodium sulfate condensed on a particle to the original particle mass, dimensionless average ratio of condensed sodium sulfate mass to original particle mass for impacting particles, dimensionless
Energy & Fuels, Vol. 6, No.6,1992 716
Fouling of H e a t Exchangers by Lignitic Coal Ash particle Sherwood number, dimensionless tube or deposit Sherwood number, dimensionless residence time in flue gas stream, s time for deposit growth, s particle velocity on impact, m/s volume fraction of sodium sulfate at deposit surface, dimensionless molecular weight of species i, kg/kmol mass fraction of species i in ash, dimensionless surface energy per unit area, surface tension, N/m thickness of sticky layer on a particle, m voidage of deposit, dimensionless concentration of species i in flue gas, kg/m3
apparent density of species i, kg/ms characteristic time for condensation of sodium sulfate on particles, s
P; 7
Subscripts CAS calcium aluminosilicates dry surface
?
sticky
q-
sticky surface
Registry NO. N&O4,7767-82-6; C&01,7778-18-9; MgSO4, 7487-aa-9.
