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Energy & Fuels 1999, 13, 390-395
Field Study on Ash Behavior during Circulating Fluidized-Bed Combustion of Biomass. 2. Ash Deposition and Alkali Vapor Condensation T. Valmari,† T. M. Lind,‡ and E. I. Kauppinen*,‡ VTT Energy and VTT Chemical Technology, Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland
G. Sfiris and K. Nilsson Vattenfall Utveckling AB, Energy Conversion, S-162 87 Stockholm, Sweden
W. Maenhaut University of Gent, Institute for Nuclear Sciences, Proeftuinstraat 86, B-9000 Gent, Belgium Received April 17, 1998
Fly ash deposition on heat-exchanger surfaces during fluidized-bed combustion of biomass causes operational problems such as reduced heat transfer and corrosion of superheater tubes. Ash deposition and alkali vapor condensation were studied during circulating fluidized-bed combustion of forest residue in a 35 MW co-generation plant. A 70 ( 10% amount of fly ash was deposited on the heat-exchanger surfaces in the convective back pass between soot-blowing periods. Practically all the largest ash particles (d > 10 µm) but only a small fraction of particles d < 3 µm were deposited. The deposition efficiency of particles with a given size was not correlated with their elemental composition. About 50% of alkali chloride vapors (KCl and NaCl) were condensed in the convective back pass on fine-mode particles (d < 0.6 µm) and the other 50% on the coarse-mode particles. Alkali chlorides were not effectively deposited since they were depleted in the largest (d > 10 µm) ash particles.
Introduction Ash deposition during fluidized-bed combustion of biomass occasionally causes severe operational problems. In the convective back pass, ash deposition reduces heat transfer and may contribute to corrosion of superheater tubes leading to the increased need for tube replacements. During fluidized bed combustion, ash-forming compounds may either attach to the bed material or become released from the furnace as fly ash particles and inorganic vapors. Coarse ash particles (d > 1 µm) include predominantly ash-forming constituents that were not volatilized during the combustion process. Fine ash particles in the submicrometer size range (d < 1 µm) are formed from the volatilized ash fraction by nucleation and subsequent condensation.1 A fraction of the volatilized ash species condenses on the coarse ash particles. The concentration of condensed species in ash particles is lower for larger particles, since condensation is favored on small particles.2 During combustion of * Author to whom correspondence should be addressed. † VTT Energy. ‡ VTT Chemical Technology. (1) Flagan, R. C.; Seinfeld, J. H. Fundamentals of air pollution Engineering; Prentice-Hall: Englewood Cliffs, NJ, 1988. (2) Hinds, W. C. Aerosol Technology. Properties, Behavior, and Measurement of Airborne Particles; Wiley-Interscience: New York, 1982.
biomass, the fine mode has been found to consist mainly of alkali chlorides and sulfates.3,4 KCl and K2SO4 are usually the dominant species over NaCl and Na2SO4, since the potassium content in biomass is typically higher than the sodium content. Alternatively, vapors may condense directly on the heat exchangers or duct walls or form condensed compounds via chemical reactions. The deposition mechanisms are different for coarse ash particles, submicrometer ash particles, and vapors.5,6 The deposition velocity of coarse particles due to inertial and turbulent impaction is large, leading to an extensive deposition rate, assuming that all the impacted particles also stick on the surface. Fortunately, the sticking efficiency of coarse particles is limited. On the other hand, submicrometer particles are driven toward heat-exchanger surfaces mainly by thermophoresis and diffusion. However, these mechanisms are not as effective as impaction is for coarse particles. Thus, (3) Christensen, K. A.; Livbjerg H. A Field Study of Submicron Particles From the Combustion of Straw. Aerosol Sci. Technol. 1996, 25, 185-199. (4) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. Fly Ash Formation and Deposition During Fluidized Bed Combustion of Willow. J. Aerosol Sci. 1998, 29, 445-459. (5) Rosner, D. E. Transport Processes in Chemically Reacting Flow Systems; Butterworth-Heinemann: 1986. (6) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corp.: 1985.
10.1021/ef9800866 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/24/1998
Ash Deposition and Alkali Vapor Condensation
the deposition rate of submicrometer particles is expected to be smaller than that of the coarse ash, even if their sticking efficiency is high.7 Deposition of submicrometer alkali-rich particles or condensation of alkali vapors on the heat-exchanger surface may create a sticky layer that promotes the coarse particle retention on the deposit layer. Ash deposit becomes problematic if it is too tenacious to be removed by soot-blowing. The biomass ash tenacity is difficult to predict and is usually determined empirically.8,9 Enrichment of alkali chlorides, and in some cases alkali sulfates, has been observed in superheater deposits.8,10 The deposit was found to be harder, i.e., more difficult to remove, in straw-fired boilers if it was directly condensed on the surface or partially melted. The fraction of hard deposit was higher when the sulfur content in the deposit was high.10 The present paper is one in a series describing our experimental results from ash formation studies during a utility-scale circulating fluidized-bed combustion (CFBC) of two different biomass fuels in 1996-1997. Ash retention in the bed and fly ash characteristics upstream of the convective back pass during combustion of forest residue and willow are presented by Valmari et al.11 In the present paper, the ash deposition on the heat-exchanger surfaces is approached by comparing the fly ash characteristics upstream and downstream of the convective back pass. Information was obtained about condensation of alkali chlorides and ash deposition efficiency as a function of particle size and composition during combustion of forest residue. Results for willow have been presented by Lind et al.12 Experimental Section Measurements were carried out for two consecutive days in a 35 MW circulating fluidized-bed combustion (CFBC) cogeneration plant (Figure 1). The gas temperature in the bed increased from 750 to 830 °C on the first day and was about 780 °C on the second day. Ash-forming constituents in the vapor and particle phases were collected with a filter sampling system upstream of the convective back pass, after the process cyclone (Figure 1). The flue gas temperature at the sampling station was 850 °C on the first day and 830 °C on the second day. A Berner-type low-pressure impactor (BLPI) was used for fly ash particle mass size distribution measurements. A more detailed description of the combustion process, analysis results for forest residue fuel, and the results from the measurements carried out upstream of the convective back pass are given by Valmari et al.11 (7) Jokiniemi, J. K.; Pyyko¨nen, J.; Lyyra¨nen J.; Mikkanen, P.; Kauppinen, E. I. Modelling Ash Deposition During the Combustion of Low Grade Fuels. Applications of Advanced Technology to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996. (8) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Boiler Deposits from Firing Biomass Fuels. Biomass Bioenergy 1996, 10, 125-138. (9) Baxter, L. L. Ash Deposition During Biomass and Coal Combustion: A Mechanistic Approach. Biomass Bioenergy 1993, 4, 85-102. (10) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition Investigation in Straw-Fired Boilers. Energy Fuels 1997, 11, 1048-1055. (11) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. A Field Study on Ash Behaviour During Circulating Fluidized Bed Combustion of Biomass. 1. Ash Formation. Energy Fuels 1999, 13, 379. (12) Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Fly Ash Deposition Onto the Convective Heat Exchangers During Combustion of Willow in a Circulating Fluidized Bed Boiler. Presented at an Engineering Foundation Conference on The Impact of Mineral Impurities in Solid Fuel Combustion, Kona, Hawaii, November, 2-7, 1997.
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Figure 1. Schematic view of the circulating fluidized bed boiler. Fly ash particle sampling locations upstream (1) and downstream (2) of the convective back pass are indicated. Flue gas temperature was decreased to 150 °C in the convective back pass. Ash particles were collected after the convective back pass from the flue gas with an electrostatic precipitator (ESP). Heat-exchanger-tube surfaces in the convective back pass were typically cleaned by soot-blowing every second day. The frequency of the soot-blowing periods was determined by the flue gas temperature at the ESP inlet. However, during our measurements, soot blowing was carried out every morning. Fly ash was collected from the ESP hoppers and weighed several times each day. The collection time of ESP ash was 8 h during day 1 and 12 h during day 2. The ESP-collected ash represents a fraction of fly ash that was not deposited on the heat-exchanger surfaces between two sootblowing sequences. The BLPI was used for fly ash particle size distribution measurements also downstream of the convective back pass. A BLPI with a precyclone was located inside the flue gas duct (Figure 2). There were two important differences between BLPI setups at the two sampling locations. When sampling upstream of the convective back pass (850 °C), the flue gas sample was cooled between the precyclone and the BLPI to about 100 °C by dilution air causing vapor-phase species to condense on ash particles. On the other hand, downstream of the convective back pass, the flue gas temperature was low enough (150 °C) that no cooling was needed. Also, precyclone cut diameters were slightly different (aerodynamic diameter 3 µm at 850 °C and 8 µm at 150 °C). Two measurements at 150 °C with a sampling time of 2 min were carried out during day 1. The first measurement was carried out 1.5 h and the second one 3.5 h after the previous soot blowing in the convective back pass. The precyclone collection at 850 °C was started 30 min after the latter measurement at 150 °C. Two BLPI-samples at 850 °C with sampling times of 9 and 10 min were collected during the precyclone collection that was carried out for 151 min. Two test samples were collected downstream of the convective back pass while the setup was tested under slightly different process conditions (25% higher boiler load). The precyclone contributed 10% of the total mass in one of the two test samples. Precyclone samples appeared to be too small for
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Figure 2. Particle mass size distribution measurement system downstream of the convective back pass. Particles with an aerodynamic diameter >8 µm were collected on a precyclone. Particles 10 µm) were deposited by impaction on the heat-exchanger surfaces. In contrast, only a small fraction of particles smaller than 3 µm was deposited. The deposition efficiency of particles with a given size was not found to be correlated with their elemental composition. The ash species which were volatilized and condensed (alkali chlorides) were not deposited efficiently, because condensation was favored on the fine particles. However, volatilized ash species may also deposit efficiently. Vapors may chemically react with coarse ash particles which are deposited with high efficiency. This was the case with sulfur. There was 80% of the sulfur in fly ash deposited, even though sulfur was volatilized during the process. The volatilization was evidenced by small retention of sulfur in the bed.11 The behavior of sulfur was totally different during combustion of willow at the same plant under process conditions essentially similar to the ones during the forest residue measurements.12
More than 50% of sulfur in the fly ash was present as fine alkali sulfate particles during the willow measurements. Consequently, the deposition efficiency of sulfur in the convective back pass was lower than the average fly ash deposition efficiency. In fact, the behavior of sulfur was the most significant difference observed between the two fuels. At first sight, the small deposition efficiency of potassium and especially of chlorine would seem to be in disagreement with the earlier findings that alkali sulfates and chlorides are enriched in superheater deposits when burning biomass with high alkali content.8 However, the effect of periodic soot blowings have to be considered when studying the long-term behavior of ash deposits. Soot blowing removes most of the deposit but not necessarily the innermost deposit layer, which may be enriched with alkali compounds. In addition, the reactions of gaseous Cl species with the deposit could also lead to enrichment of chlorine on the deposit. Finally, since about 70% of the fly ash was deposited in the convective back pass, it is proposed that the emission taking place during soot blowing is of major interest when studying the fly ash behavior and the performance of gas cleaning devices. Acknowledgment. This study was funded by Finnish Technology Development Center (TEKES), VTT Chemical Technology, and Foster Wheeler Energia via the LIEKKI 2 national combustion research program as well as by the Commission of European Community, Vattenfall Utveckling AB, and VTT Chemical Technology via Joule 3 (Contract No. JOR3-CT95-0001). We acknowledge the discussions with Mr. Jouni Pyyko¨nen, Dr. Jorma Jokiniemi, and Dr. Jouko Latva-Somppi from the VTT Aerosol Technology Group, Dr. David P. Brown for carefully revising the manuscript, as well as the contribution of the power-plant operating staff. W. Maenhaut is indebted to the “Fonds voor Wetenschappelijk Onderzoek-Vlaanderen” for research support. He is also grateful to J. Cafmeyer and K. Beyaert for assistance in the chemical analyses. EF9800866
