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Energy & Fuels 1999, 13, 778-795
Alkali Salt Ash Formation in Four Finnish Industrial Recovery Boilers Pirita Mikkanen,† Esko I. Kauppinen,*,† Jouni Pyyko¨nen,‡ and Jorma K. Jokiniemi‡ VTT Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland
Minna Aurela Finnish Meteorological Institute, Sahaajankatu 22 E, FIN-00810 Helsinki, Finland
Esa K. Vakkilainen Ahlstrom Machinery Corporation, P.O. Box 5, FIN-00441 Helsinki, Finland
Kauko Janka Kvaerner Pulping Oy, P.O. Box 109, FIN-33101 Tampere, Finland Received September 15, 1998
Combustion aerosol measurement methods were introduced and applied for extensive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pressure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 µm and the coarse mode above 3 µm aerodynamic diameter. According to SEM images, fine ash mode consists of individual, almost spherical 0.30.7 µm alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode primary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3-0.7 µm alkali salt primary particles and spherical silica particles. Ash particle main component was sodium sulfate as determined with X-ray diffraction. Sodium-to-sulfur molar ratio of ash particles calculated on the analyses results with an ion chromatography decreased from the upper furnace sampling point to electrostatic precipitator inlet conditions, indicating sulfation of ash particles within the heat exchanger section. Chlorine in ash was bound as sodium chloride, no potassium chloride was detected with X-ray absorption fine structure spectroscopy. Furnace measurements showed that fine mode ash particles are formed already in the furnace via vapor condensation. The extents of release of 12% for Na, 24% for S, and 48% for Cl were determined on the basis of ion concentrations in fine particles and the mass balance calculation on the recovery boiler. Coarse particles observed downstream the heat exchangers are proposed to form mainly via entrainment of large agglomerates of fine ash particles deposited on the heat exchangers. The fine mode particle size was insensitive to the furnace conditions although the particle concentration increased when the furnace gas temperature increased. This and the increase of Na/S molar ratio in the particles indicates that Na volatilization increases with the increasing furnace temperature, whereas S release is less sensitive to the temperature increase.
Introduction Black liquor, the waste sludge from the paper pulping process, is burned in recovery boilers, where the pulping chemicals are recycled and the heat from burning organic matter is utilized to produce steam for electricity and for process powering. Black liquor consists of about 25 wt % (dry) of sodium and potassium. A significant fraction of sodium and potassium in the black liquor is released into the combustion gas in the furnace and * Corresponding author. Fax: +358-9-4567021. E-mail: Esko.
[email protected]. † VTT Chemical Technology. ‡ VTT Energy.
transforms into a high concentration of alkali salt ash particles in the flue gases. Black liquor is sprayed as millimeter-sized droplets into the recovery boiler lower furnace. In the hot furnace the droplets dry and partly pyrolyze in flight. The residue droplets deposit on the char bed at the bottom of the furnace. The char burns in the char bed. The molten salts are recycled for further use. Some of the smaller droplets are entrained with the flue gases and may burn in the upper furnace. These are called carryover particles. Alkali salt ash particles with typical concentrations of 20-30 g/Nm3 in the flue gases can cause severe
10.1021/ef980189o CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999
Alkali Salt Ash Formation in Finnish Recovery Boilers
fouling and corrosion problems in the heat exchangers of the recovery boiler. Even though the gas passages are continuously sootblown, massive deposit accumulation may require an unexpected shutdown of the recovery boiler in order to remove the deposits from the heat exchangers. In case deposits are corrosive, accelerated tube thinning may occur, leading eventually to tube failure. Such shutdowns may interrupt the pulp production, which means significant financial losses. Due to recent concern of the adverse health effects of fine particles, their emission control has received increased attention. The main component in the recovery boiler fly ash, also called fume, is Na2SO4.1,2 In the following, we use the terms fume as well as alkali salt fly ash particles to describe the particles in the combustion gas leaving the furnace. The earliest theory for fume formation was proposed by Lang et al.3 He suggested that the fume formation in the recovery boiler is initiated by the decomposition of Na2CO3 in the char bed to Na2O, which further reacts above the secondary air level with SO3 or CO2 forming sodium sulfate or sodium carbonate, respectively. However, there is water vapor present in the furnace and therefore sodium favors conversion to NaOH instead of Na2O.4,5 NaOH may further react to form sodium sulfate or carbonate. In fact, NaOH has been detected in measurements carried out in the recovery boiler furnace.6,7 Later Cameron 8,9 proposed a reaction enhanced mechanism of fume formation. According to his theory, sodium is released from the char bed as sodium vapor, which reacts with oxygen and carbon dioxide to form Na2CO3. The oxidation of sodium close to the char bed surface reduces the partial pressure of sodium close to the surface, and thus increases sodium volatilization. Cameron suggested that Na2CO3 particles were formed in the lower furnace and further reacted with SO2 to form Na2SO4, which is the main component of the recovery boiler fly ash. In the reducing region of an old furnace burning low solids liquor the measured temperatures range from 960 to 1260 °C.10 However, computer simulations have shown that the maximum gas temperatures in the furnace of modern boilers are higher than 1300 °C.11-13 This indicates that in the modern boilers the furnace gas temperature may exceed the decomposition tem(1) Nguyen, X. T.; Rowbottom, R. S. Pulp Pap. Can. 1979, 80 (10), T318-T324. (2) Tran, H. Tappi J. 1986, 69 (11), 102-106. (3) Lang, C. J.; DeHaas, G. G.; Gommi, J. V.; Nelson, W. Tappi J. 1973, 56 (6), 115-119. (4) Hynes, A. J.; Steinberg, M.; Schofield, K. J. Chem. Phys. 1984, 80 (6), 2585-2597. (5) Srinivasachar, S.; Helble, J. J.; Ham, D. O.; Domazetis, G. A. Prog. Energy Combust. Sci. 1990, 16, 303-309. (6) Borg, A.; Teder A.; Warnqvist, B. Tappi J. 1974, 57 (1), 126129. (7) Tavares, A.; Tran, H.; Barham, D.; Rouilard, P.; Adams, B. 1995 International Chemical Recovery Conference Pre-prints A. 1995, April 24-27, Toronto, Ontario, Canada, A87-A93. (8) Cameron, J. H. PIMA 1986, March, 32-34. (9) Cameron, J. H. Chem. Eng. Commun. 1987, 59, 243-257. (10) Stelling, O.; Vegeby, A. Pulp Pap. Mag. Can. 1969, August 1, 51-77. (11) Jones, A. K.; Chapman, P. J. Tappi J. 1993, 76 (7), 195-202. (12) Jones, A. K.; Chen, K. 1995 International Chemical Recovery Conference Pre-prints A 1995, April 24-27, Toronto, Ontario, Canada, A123-A131. (13) Wessel, R. A.; Parker, K. L.; Verrill, C. L. Technical Paper RDTPA 95-45; Babcock & Wilcox: Ohio, 1995.
Energy & Fuels, Vol. 13, No. 4, 1999 779
perature of Na2CO3. Accordingly, the particles formed by the mechanism proposed by Cameron would decompose. A mechanistic fume formation model was presented by Jokiniemi et al.14 According to this model, volatilized sodium reacts to form NaOH, which further converts to sulfates, if any SO2 is present, or in strongly reducing atmosphere or during combustion of low sulfidity liquor, to carbonates. The equivalent reactions apply to potassium. Fume formation is initiated either by metal oxide seed particle formation in the boundary layer of a burning black liquor char particle and subsequent heterogeneous condensation of Na2SO4 vapor or by homogeneous nucleation of Na2SO4. Alkali sulfates have much lower vapor pressures than hydroxides and in typical recovery boiler conditions they rapidly condense at temperatures lower than 1370 °C. The kinetics of NaOH as well as KOH sulfation are not known.15 Also, it is not known whether Na2SO4 can actually exist in the vapor phase, in which the reactions are usually much faster. However, there is some evidence by Dayton and Frederick16 supporting the assumption that vapor phase sulfate exists. Some papers have been published on fume particle characteristics in the recovery boiler,1,17,18 based on standard inertial impactor measurements of ash size distributions downstream the heat exchangers. However, the earlier studies are not detailed enough to verify the proposed ash formation mechanism. Therefore, we have studied in detail the fume formation mechanisms by carrying out advanced aerosol measurements at operating industrial scale boilers. The fume particles were measured simultaneously with gas composition at the recovery boiler furnace at about 900 °C and at electrostatic precipitator (ESP) inlet at about 150 °C. Furthermore, the boiler operational conditions as well as the black liquor properties were simultaneously recorded. The particles were extracted from the flue gas with a quench probe or a dilution system and collected on quartz filters,19 size classified with a Berner-type lowpressure impactor20 down to 0.01 µm particle diameter and with a three-stage cyclone system.21 Samples were collected on copper grids or on planar filters for electron microscope analyses. The particles were analyzed for their chemical composition with ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). The composition of the particles was further studied with an X-ray diffraction method22 (XRD) and (14) Jokiniemi, J. K.; Pyyko¨nen, J.; Mikkanen, P.; Kauppinen, E. I. Tappi J. 1996, 79 (7), 171-181. (15) Steinberg, M.; Schofield, K. Prog. Energy Combust. Sci. 1990, 16, 311-317. (16) Dayton, D. C.; Frederick, W. J. Energy Fuels 1996, 10, 284292. (17) Bosch, J. C.; Pilat, M. J.; Hrutfiord, B. F. Tappi J. 1971, 54 (11), 1871-1875. (18) Esplin, G. J.; Serenius, S. R.; McIntyre, A. D. Pulp Pap. Mag. Can. 1973, 74 (12), 98-104. (19) Hinds, W. C. Aerosol Technology; John Wiley & Sons: New York, 1982; pp 153-157, ISBN 0-471-08726-2. (20) Kauppinen, E. Experimental studies on aerosol size spectroscopy with multijet low-pressure inertial impactors. Academic Dissertation. 1992. Technical Research Centre of Finland, Publication 86, Espoo, Finland, p 40 + appendix 94 pp, ISBN 951-38-4064-6. (21) Lee, K. W.; Gieseke, J. A.; Piispanen, W. H. Atmos. Environ. 1985, 19 (6), 847-852. (22) Materials Science and Technology, Vol. 2, Characterisation of Materials; Cahn, R. W., Haasen, P., Kramer, E. J., Eds.; Weinheim: New York, 1992; ISBN 0-89573-690-X.
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Mikkanen et al.
Table 1. Operational Conditions at the Recovery Boilers during the Experimentsa boiler code liquor type mixing ratio, hardwood/softwood black liquor flow (kg/s) dry solids content (%) steam flow (kg/s) heat load (kg/s/m2) combustion air (Nm3/s) primary air (%) secondary air (%) tertiary air (%) O2, left/right (%) temperature of flue gas upstream BB, right/left (°C)
#1 1-SW softwood
#1 1-HW hardwood
#2 2-SW-NS softwood
#2 2-SW-HS softwood
31 70.5 70 0.66 65.5 34 53 13 2.5/3.2 545/523
28 66.7 55 0.52 51.5 35 54 11 3.5/3.3 475/480
34 74.2 77 0.52 75 22 46 7 2.9/2.9 553/565
31 85.4 87 0.59 78 23 44 11 2.4/3.0 572/582
#2 2-MW-HS mixed wood 70/30 35 82.7 85 0.57 81 23 45 13 2.7/2.5 578/579
#3 3-HL softwood
#3 3-NL softwood
56 73 120 0.73 120 51 15 34 2.5 535/580
35 73 90 0.57 84 64 8 27 2.1 470/450
#4 4-MW mixed wood 60/40 24 79.4 58 0.58 60 43 n.u.b 14 3.0/2.9 458
a 1-SW ) boiler #1, softwood. 1-HW ) boiler #1, hardwood. 2-SW-NS ) boiler #2, softwood, normal solids. 2-SW-HS ) boiler #2, softwood, high solids. 2-MW-HS ) boiler #2, mixed wood, high solids. 3-HL ) boiler #3, high load. 3-NL ) boiler #3, normal load. 4-MW ) boiler #4, mixed wood. b n.u. ) not in use.
an X-ray absorption fine structure spectroscopy23 (XAFS). A high-resolution scanning electron microscope (SEM) along with an energy dispersive X-ray diffraction spectroscopy (EDX) was applied to determine the morphology and the chemical composition of the individual particles. Similarly, a computer-controlled scanning electron microscopy method23 (CCSEM) was applied to determine the composition and size of the large number of individual ash particles. In this paper, we first describe the boilers chosen for experiments and the operational conditions studied. Then the ash particle sampling, size measurement and analyses methods are introduced. These are followed by reporting of results, starting from particle mass concentration and size distribution results, followed by the discussion on the extents of release for sodium, sulfur, and chlorine. Then we discuss in detail ash particle composition size distributions as determined with IC and ICP-MS analyses of BLPI samples. The forms of occurrence determined with XRD and XAFS are introduced along with the particle morphology and sintering data. Finally, the conclusions concerning the fine and coarse ash particle formation mechanisms as well as mechanistic fume formation model14 validation are presented. Experimental Section Extensive fume characterization experiments were carried out at four operating industrial recovery boilers in Finland (Table 1). Veitsiluoto Oulu mills recovery boiler (boiler #1) periodically fires pure softwood and pure hardwood liquor. Metsa¨-Botnia Kemi mills recovery boiler (boiler #2) was chosen because an advanced evaporator technique called the super combustor was installed just half a year prior to the experiments. With this evaporator technique the dry solids content of the black liquor can be increased about 20%, which means that less water is introduced into the boiler with the fuel. UPM-Kymmene Kaukas mills recovery boiler (boiler #3) currently operates with a partial load. Soon, the boiler load will be increased significantly. The future load was being tested during the experiments. UPMKymmene Kymi mills recovery boiler (boiler #4) is a traditional boiler operating since 1977. (23) Shah, A. D.; Huffman, G. P.; Huggins, F. E.; Shah, N.; Helble, J. J. Fuel Process. Technol. 1995, 44, 105-120.
The operational values in the boilers show wide variety. Some of the values are not comparable, but demonstrate the operational condition in each boiler. However, comparison of the values of the steam flow rate divided by the boiler hearth area, i.e., heat load (Table 1) and the flue gas temperature upstream the boiler bank suggests that the heat load can be used as an indicator of the furnace temperature in each of the boilers, i.e., large boilers with large heat load show higher gas temperatures upstream the boiler bank. The hearth areas, i.e., boiler floor areas for measured boilers are 100 m2 for boiler #1, 148 m2 for boiler #2, 158 m2 for boiler #3, and 100 m2 for boiler #4. The stability of the boiler operation was monitored with gas composition measurements at the electrostatic precipitator (ESP) outlet. Average gas compositions during different operating conditions are shown in Table 2. The values for CO2, CO, SO2, and NOx concentrations are presented as concentrations in dry gases with 3% O2. Sampling and Size Classification. Particles were characterized at two sampling sites. Sampling from the furnace at bullnose level at about 900 °C was carried out in boilers #2 and #3. Sampling at the ESP inlet was carried out at about 150 °C in all four recovery boilers. The schematics of the measurement equipment at the furnace and at the ESP inlet sites are shown in Figure 1, parts a and b, respectively. A quench probe (Figure 2) was designed to collect particles from the recovery boiler furnace. At the tip of the probe the flue gas sample is rapidly diluted and cooled with nitrogen gas. The designed dilution ratio is 1:10 and the designed cooling rate is 105 °C/s. The probe head was designed to separate particles larger than 10 µm by inertial impaction. A detailed schematic of the probe head is shown in Figure 2. Samples extracted from the furnace with the quench probe were collected with a Berner-type low-pressure impactor20 (BLPI) on aluminum foil substrates coated with a thin layer of grease (Apiezon L). Total particle concentration was measured with a quartz fiber filter19 (QF). Individual particles were collected for electron microscopy analyses on planar polycarbonate filters (Nuclepore, thickness 15 µm, pore size 0.4 µm). Due to insulation problems the designed cooling rate was not obtained and the flue gas (Tg ) 300 °C) remained 150 °C warmer than the surface of the
Alkali Salt Ash Formation in Finnish Recovery Boilers
Energy & Fuels, Vol. 13, No. 4, 1999 781
Table 2. Gas Composition Results at ESP Inlet during the Experimentsa-c boiler code liquor type operation
#1 1-SW softwood
#1 1-HW hardwood
#2 2-SW-NS softwood normal solids
#2 2-SW-HS softwood high solids
#2 2-MW-HS mixed wood high solids
#3 3-HL softwood high load
#3 3-NL softwood normal load
#4 4-MW mixed wood
H2O (%) O2 (%)
21 2.5
26 3.5
23 4.1
19 .2
22 3.6
n.a. 3.8
n.a. 3.8
n.a. 2.9
CO (ppm) CO2 (%) SO2 (ppm) NOx (ppm) HCl (ppm)
250 16 40 80 n.a.
150 15 1790 100 n.a.
7 15.2 130 105 80
13 16.2 63 127 8
6 16.1 40 129 75
137 15.6
