Energy & Fuels 2006, 20, 61-68
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Effect of Chlorine and Sulfur on Fine Particle Formation in Pilot-Scale CFBC of Biomass Terttaliisa Lind,† Esko I. Kauppinen,*,†,‡ Jouni Hokkinen,† Jorma K. Jokiniemi,†,§ Markku Orjala,† Minna Aurela,| and Risto Hillamo| VTT Processes, P.O. Box 1602, FIN-02044 VTT, Finland, Department of Mathematics and Physics, Helsinki UniVersity of Technology, Espoo, Finland, Department of EnVironmental Sciences, UniVersity of Kuopio, Kuopio, Finland, and Finnish Meteorological Institute, Helsinki, Finland ReceiVed April 26, 2005. ReVised Manuscript ReceiVed October 4, 2005
Behavior of chlorine and sulfur is critical on the formation of fine particles (here particles smaller than one micrometer, PM1.0) during combustion. In this investigation, we studied experimentally fine particle formation in a pilot-scale circulating fluidized bed reactor during combustion of bark and pulp and paper mill sludge. The effect of chlorine and sulfur on fine particle formation was investigated by adding HCl and SO2 into the reactor. Fine fly ash particles were formed from alkali species that were released from the fuel to the gas phase. In low-HCl conditions, alkali species reacted readily with silicates, and therefore, a large fraction of alkalis was retained in the bottom ash. Consequently, fine particle concentrations in the flue gas were relatively low. In this case, fine particles were both alkali metal chlorides and sulfates. HCl addition increased the concentration of fine particles considerably due to gas phase reactions between alkali metal species and HCl. Alkali metal chlorides that were formed condensed producing large mass concentrations of fine alkali metal chloride particles. Due to extensive alkali metal chloride formation, significantly less alkalis reacted with silicates and ended up in the bottom ash than when HCl concentration was low. Further SO2 addition transformed some of the chlorides into sulfates in the fine particle mode. At the same time, the total fine particle concentration decreased, possibly due to formation of coarse mixed K-Ca-sulfate ash particles.
Background Ash is formed during combustion from inorganic, ash-forming components. In fluidized beds, ash can be divided into two fractions: bottom ash is the coarse ash fraction that is removed from the furnace bottom, and fly ash is the fine ash fraction that is transported from the furnace with the flue gases. Fly ash usually consists of two types of particles: fine fly ash particles that are smaller than about 1 µm in diameter and that are formed from gas phase ash-forming compounds, and coarse fly ash fraction that mainly contains compounds that have not been released to the gas phase during combustion. Typically, during biomass combustion, the fine fly ash particles are mainly alkali metal chlorides and sulfates.1,2 Fine particles are important in combustion for several reasons. First, they affect heat transfer in the furnace. Second, they have been identified as possibly initiating deposit formation and consequent corrosion on heat exchanger surfaces.3-6 Third, they are typically emitted through particulate collection devices more efficiently than other particles.7,8 And fourth, once in the * Corresponding author. E-mail:
[email protected]. Tel.: +358 20 722 6165. Fax: +358 20 722 7021. † VTT Processes. ‡ Helsinki University of Technology. § University of Kuopio. | Finnish Meteorological Institute. (1) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 390-395. (2) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421-444. (3) Mikkanen, P. Fly ash particle formation in kraft recovery boilers. Academic Dissertation; VTT Publications 421: Espoo, Finland, 2000. (4) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29-120.
environment, they have adverse health effects and cause increased mortality on humans.9,10 A large fraction of the alkalis potassium and sodium are released from the fuel during biomass combustion. They are released from the fuel mainly as hydroxides and chlorides during char burning.11 Once in the gas phase, they may react with gaseous HCl, SO3, and CO2 to form chlorides, sulfates, and carbonates. These species mainly form new, fine particles either in the furnace or in the flue gas pass. In addition, gas phase alkali metal species may react with bed particles or with coarse fly ash particles. In this case, they end up mainly in the bottom ash and in the coarse fly ash fraction. Chlorine is typically released from the fuel during combustion almost completely.12 Cl is released during relatively early stages of combustion, i.e., the major fraction of it is released during devolatilization. In the flue gases, it is usually present as HCl, gaseous alkali metal chlorides (KCl, NaCl), or as fine alkali metal chloride particles. Chlorine content in the fuel has been (5) Grabke, H. J.; Reese, E.; Spiegel, M. Corros. Sci. 1995, 37, 10231043. (6) Salmenoja, K.; Ma¨kela¨, K.; Hupa, M.; Backman, R. J. Inst. Energy 1996, 69, 155-162. (7) Lind, T.; Hokkinen, J.; Jokiniemi, J. K.; Saarikoski, S.; Hillamo, R. EnViron. Sci. Technol. 2003, 37, 2842-2846. (8) Yla¨talo, S.; Hautanen, J. J. Aerosol Sci. 1998, 29, 17-30. (9) Dockery, D. W.; Pope, C. A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M.; Ferris, B. G.; Speizer, F. E. N. Engl. J. Med. 1993, 329, 17531759. (10) Pope, C. A.; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F. E.; Heath, C. W. Am. J. Respir. Crit. Care Med. 1995, 151, 669-674. (11) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385-1399. (12) Manzoori, A. R.; Agarwal, P. K. Fuel 1992, 71, 513-522.
10.1021/ef050122i CCC: $33.50 © 2006 American Chemical Society Published on Web 10/29/2005
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found to affect volatilization behavior of alkalis and certain heavy metals.13-16 Therefore, increasing chlorine content in the fuel is expected to increase fine particle concentration, as during biomass combustion fine particles are almost entirely alkali metal species. Sulfur is also released readily during biomass combustion.17 The main sulfur compound in the gas phase is SO2, and in condensed phase it is typically found as alkali metal and calcium sulfates. Alkali metal sulfates are formed from gas phase compounds, and consequently, they are mainly present in fine fly ash particles, whereas calcium sulfate is found in coarse fly ash fraction as well as in bottom ash. Sulfur reacts with alkali metal chlorides, and the reaction in the gas phase is limited by SO2 oxidation to SO3.2,18,19 The alkali metal sulfates that are formed contribute to new fine particle formation by nucleation. In the condensed phase the sulfation reaction is slower but is still significant in deposits.19 Sulfur and chlorine appear to be significant in fine particle formation and their characteristics. In this investigation, we studied the influence of chlorine and sulfur on fine particle formation experimentally in a pilot-scale fluidized bed combustion facility. To keep other fuel characteristics than chlorine and sulfur content constant, chlorine and sulfur were added into the reactor as gases, HCl and SO2. This allowed us to vary chlorine and sulfur contents in the reactor independently. In addition, the feed location of the additives into the reactor could be varied. Methods Combustion tests for the investigation of fine particle formation were carried out at a circulating fluidized bed test facility at VTT Processes, Figure 1. The reactor has 50 kW fuel capacity, it is 8 m high, and has an inner diameter of 16.7 cm. The temperature profile in the reactor is regulated with air/water-cooling and electric heaters that are computer-controlled. The reactor walls are ceramic (Hasle D39A), and there are observation ports at five levels in the reactor enabling comprehensive monitoring. The reactor is suitable for the investigation of combustion behavior of a wide range of fuels. Fuel is fed into the reactor from two fuel silos with screw feeders. Combustion air is preheated by an electric heater to 280 °C. It can be fed into the reactor at three levels for desired combustion conditions. Maximum gas velocity in the reactor is 4 m/s, which gives a residence time of 2 s. Particulate matter is separated from the flue gases in the primary and secondary cyclones and, after cooling the flue gases, in a fabric filter. In these tests, the particles separated in the primary cyclone were circulated back into the reactor, and the particles separated in the secondary cyclone were collected for ash analysis. The reactor is equipped with on-line gas sampling for CO2, CO, NO, O2, and SO2. During these tests, an FTIR-analyzer was used for the measurement of HCl, SO2, and several other gases. The FTIR-analyzer was located upstream of the fabric filter in the flue gas temperature of approximately 220 °C. (13) Lindner, E. R.; Wall, T. F. Mineral Matter and Ash Deposition from Coal; Bryers, R. W., Vorres, K. S., Eds.; United Engineering Trustees Inc.: New York, 1990. (14) Gallagher, N. B.; Bool, L. E.; Wendt, J. O. L.; Peterson, T. W. Combust. Sci. Technol. 1990, 74, 211. (15) Jacob, A.; Stucki, S.; Kuhn, P. EnViron. Sci. Technol. 1995, 29, 2429. (16) Jacob, A.; Stucki, S.; Struis, R. P. W. EnViron. Sci. Technol. 1996, 30, 3275. (17) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Horlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189-209. (18) Jokiniemi, J. K.; Lazaridis, M.; Lehtinen, K. E. J.; Kauppinen, E. I. J. Aerosol Sci. 1994, 25, 429-446. (19) Iisa, K.; Lu, Y.; Salmenoja, K. Energy Fuels 1999, 13, 1184-1190.
Figure 1. Schematic picture of the CFBC pilot test facility at VTT Processes in Jyva¨skyla¨, Finland.
Figure 2. Test matrix for CFBC pilot tests.
Description of the Tests. Fine particle formation tests during combustion of a mixture of bark and sludge were carried out in the CFBC reactor during two test periods by feeding fuel collected from an operating full-scale plant.20 Two tests for almost each test condition were carried out to study the repeatability of the results, Figure 2. The sludge content in the fuel mixture was approximately 2 wt %. The fuel had ash content of 3.37%. Ash analysis is given in Table 1. Typical to wood biomass fuels, the fuel was rich in calcium with some Si, Fe, and Al, as well as considerable amounts of K and Na. The fuel was dried and ground to a feed particle size of 4-7 mm. Bed material was natural SiO2-rich sand with particle size 0.1-0.3 mm. Cl-to-alkali metal molar ratio [n(Cl)/{n(Na) + n(K)}] was 0.18, and S/Cl molar ratio was 3.7. The fuel during both test periods was of the same origin, but despite this, it differed slightly in its Cl content. Cl content in the fuel was 0.027% during the first test period and 0.015% during the second test period. Therefore, HCl was sprayed into the fuel during the second test period to match Cl content of the fuel of the first test period. (20) Lind, T.; Hokkinen, J.; Jokiniemi, J. K.; Aurela, M.; Hillamo, R. Abstracts of the European Aerosol Conference 2003. J. Aerosol Sci. 2003, S121-S122 (Supplement).
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Table 1. Fuel Ash Content and Composition as Analyzed with XRFa element
wt %
element
wt %
S (in fuel) Cl (in fuel) ash Na Mg Al Si
0.09 0.027 3.37 1.00 2.38 4.36 7.58
P K Ca Mn Fe Zn
1.00 3.32 24.6 0.86 2.63 0.33
a S and Cl contents are given in fuel, and other elemental concentrations in ash.
The effect of chlorine and sulfur on the fine particle formation was studied by feeding HCl and SO2 as additive gases. The approach of feeding additives as gases was selected instead of fuel mixing to keep the fuel ash characteristics constant while varying chlorine and sulfur concentrations. HCl was fed into the reactor from two locations, one in the bottom of the reactor and the other in the middle of the reactor at 3.2 m from the grate. SO2 was also fed from two locations, one in the middle of the reactor at 3.2 m from the grate and the other at the top of the reactor at 6.2 m from the grate. In the additive tests, HCl feed together with fuel Cl corresponded to fuel Cl content of 0.18%, and SO2 feed together with fuel S corresponded to fuel S content of 0.4%. S/Cl molar ratio was 0.6 for HCl addition tests and 2.5 for HCl and SO2 addition tests. The fuel and air feed as well as reactor temperature were kept constant in the different tests. The fuel feed was 4.3 g/s, and the air feed approximately 0.0167 m3/s (1000 L/min) with 50% of the air as primary air and 50% as secondary air. The bed temperature was 780-830 °C, and the hottest part of the reactor was at approximately 2 m from the grate with temperature of about 900920 °C. Temperature at the top of the reactor was 850-870 °C. Samples were collected and analyzed from the different material streams of the reactor: bed material and circulating matter from the primary cyclone were typically collected five times during each test. Ash from the secondary cyclone was collected four times, and ash from the cooling section, two times. Filter ash from the bag filter was collected after each test. Ash samples were analyzed for the main ash compounds with X-ray fluorescence spectroscopy (XRF). Fine Particle Measurements. During the first test period, fine particles were measured from two locations in the reactor: first measurement location between the primary and secondary cyclone at the temperature of 750-800 °C and second measurement location downstream of the cooling section at the temperature of approximately 220 °C. The particle mass and elemental size distributions were determined with a setup consisting of a porous tube dilutor, a precutter cyclone with cut-size D50 of approximately 5 µm, and a Berner-type low-pressure impactor (BLPI), 21,22 Figure 3. Thin polycarbonate (poreless Nuclepore, NP) films were used as impaction substrates. Prior to the collection, the substrates were greased with Apiezon L vacuum grease to prevent particle bounce and heated in an oven for 6 h at 150 °C to avoid substrate mass loss during collection. The impactor and the cyclone were externally heated to approximately 100 °C to avoid water condensation. Dilution ratio in the porous tube dilutor was set at 1:5, and sampling time at both locations was typically 4 min. Due to space limitations in the reactor, the dilution probe and the cyclone were used out of stack. An elbow-type collection nozzle was designed and used in-stack to collect the sample isokinetically. Despite isokinetic sampling, the sampling losses due to the bend in the sampling nozzle were considerable for the coarse fly ash particles (particle diameter larger than approximately 5 µm),23 (21) Kauppinen, E. I.; Pakkanen, T. A. EnViron. Sci. Technol. 1990, 24, 1811-1818. (22) Kauppinen, E. I. Aerosol Sci. Technol. 1992, 16, 171-197. (23) Brockmann, J. E. Sampling and transport of aerosols. Aerosol Measurement-Principles, Techniques, and Applications; Willeke, K., Baron, P. A., Eds.; John Wiley & Sons: New York, 1993.
Figure 3. Schematic diagram of the aerosol sampling setup consisting of an elbow-type sampling nozzle, porous tube dilutor, precutter cyclone, and a low-pressure impactor.
especially at the sampling location between the cyclones where the coarse particle concentration was high. However, for the fine particle fraction (particles < 1 µm) that was of main interest in this investigation, the losses in the collection nozzle were negligible.23 During the second test period, fine particles were sampled only from the location 2 downstream of the cooling section. The sampling setup was similar to the one used in the first test period. Small differences were that dilution was carried out with an ejectorbased dilutor with a dilution ratio of 1:11, and the impactor used for sampling was a Dekati low-pressure impactor (DLPI). DLPI was not heated. The mass of the impactor-collected particle samples was determined by weighing the samples prior to and after the sampling. In addition, the samples were analyzed for water-soluble ions with ion chromatography (IC) and for the main elements with ICP-MS. As sample preparation for the ICP-MS analysis, the samples were dissolved in hydrofluoric and nitric acids. Chemical Equilibrium Model Calculations. Chemical equilibrium model calculations were carried out with software program Fact-Sage 5.1 to help in interpretation of the results. The program uses Gibb’s free energy minimization method. Thermodynamic data were taken from the software database including all the appropriate data for nonideal solid and liquid solutions. The calculations were carried out in the temperature range 600-1600 K. The following elements were included in the calculations: C, O, H, N, S, Cl, Na, K, Ca, P, Mg, Si, and Al.
Results Particle Mass Concentrations and Size Distributions. Fine particle (particle diameter < 1 µm) concentrations were 27-
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Figure 4. Fine particle mass size distributions from two locations in the reactor during the first test period: location 1 between the primary and secondary cyclone and location 2 downstream of the cooling section. Particles collected in the precutter cyclone are not included in the figure. Table 2. Total Particle Concentrations and the Concentrations of the Fine Particles (mg/nm3) (Dp < 1 µM) in the Pilot-Scale CFBC Reactor at Two Locations As Determined with Low-Pressure Impactors and Cyclones between the cyclones test 1 test 2 test 3 test 4 test 6
downstream cooling section
fine particles
tot. particles
fine particles
tot. particles
27 54 50 82 42 87
5490 2820 644 733 784 1050
33
432
140 105 76 149
334 310 535 706
87 mg/nm3 in the first sampling location between the primary and secondary cyclone and 33-150 mg/nm3 in the second sampling location downstream of the cooling section during the first test period, Table 2. Fine particle concentrations at location 2 were lower during the second test period, 44-110 mg/nm3. This was presumably due to the slightly lower Cl content in the fuel during the second test period. The total particle concentrations were 640-5 500 mg/nm3 in the first sampling location and 310-710 mg/nm3 in the second sampling location. The coarse particle (particle diameter > 1 µm) concentration at the first measurement location was considerably higher during test 1 than in the other tests. Presumably, the reactor reached a steady state in regard to coarse particle formation and deposition to the reactor walls during test 1. The fine particle concentrations were lower and coarse particle concentrations higher at the first sampling location than at the second sampling location. The lower coarse particle concentration at the second measurement location was due to collection of coarse ash particles in the secondary cyclone and deposition of the particles in the cooling section. The fine particle concentration increased in the cooling section, because at the first location between the primary and secondary cyclone at the flue gas temperature of 780-800 °C, a large fraction of the alkali metal chlorides was in the gas phase. All the gas phase compounds did not have enough time to condense in the dilution-cooling in the sampling. However, they condensed in the cooling section of the reactor, mainly on the fine particles increasing their mass concentration. At the same time, deposition velocity of these particles in the cooling section was low. The addition of HCl into the reactor increased the concentration of fine particles considerably. The results were similar in tests 2, 6, and 8, where HCl was fed into the reactor in different locations. Consequently, it did not appear to make a significant difference whether HCl was fed into the reactor in the bottom of the reactor or in the middle of the reactor. When SO2 was
Figure 5. Composition of the fine particles during pilot-scale tests as analyzed with IC.
fed into the reactor in addition to HCl, the fine particle concentrations decreased. However, the fine particle concentrations were still about double the concentration of the reference tests when no additive gases were fed. The particle mass size distributions were clearly bimodal in all the tests at both measurement locations, Figure 4. The average diameter of the fine mode particles was 0.1-0.2 µm. The precutter cyclone collected 73-97% of the total particle mass at the first measurement location and 21-34% at the second measurement location. As the cyclone cut-size was approximately 5 µm, the particles collected in the cyclone were all coarse mode particles. Fine Particle Composition. Major fractions of fly ash bound Na, K, and Cl were found in the fine mode particles at both measurement locations. At the second measurement location, 73-89% Cl, 76-89% Na, and 81-94% K were in the fine mode particles during the first test period. At the same time, only 2-5% Ca and 6-32% SO4 were found in the fine particle mode. The main components in the fine mode particles were K, Na, Cl, and SO42-, Figure 5. Only small amounts of Ca were found in fine particles, even though it was one of the main components in the fly ash. According to ICP-MS analysis, no Al, Fe, Mg, or Mn was found in fine particles. Si content was below the ICP-MS analysis detection limit for all the fine particles. The fine mode size distributions as determined downstream of the cooling section for Na+, K+, Cl-, and SO42- showed significant differences between the different tests. For K, Na, and Cl, the fine mode concentration was lowest in the reference tests 1 and 7. The addition of HCl into the reactor increased the amount of K, Na, and Cl in the fine particles. At the same time, SO42- concentration in the fine particles decreased. When
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Energy & Fuels, Vol. 20, No. 1, 2006 65
Figure 6. Molar ratios (Na + K)/Cl and (Na + K)/(Cl + 2S) as a function of particle size in different tests during the first test period in pilot-scale as determined at aerosol sampling location 2.
SO2 was fed in addition to HCl, Cl concentration in the fine particles decreased and SO42- concentration increased, as was expected. The occurrence of alkali metals as chlorides and sulfates was studied by calculating the molar ratios (Na + K)/Cl and (Na + K)/(Cl + 2S), Figure 6. These ratios are one if all the alkali metals are bound as chlorides or as sulfates and chlorides, respectively, and if all the Cl and S are bound with alkali metals. The (Na + K)/Cl ratio was 1 in tests 2 and 6 and approximately 1.2 in test 3 in the fine particle size range. This indicates that almost all the alkalis were present as chlorides in the fine particles. In test 4, the alkali metal-to-chlorine ratio was 1.5, and in test 1 it increased with increasing particle size from one to over 2. This indicates that, in addition to chlorides, alkali metals were bound as other compounds. The (Na + K)/(2 × S + Cl) ratio was 1 in all the tests in the fine particles. Consequently, in tests 1 and 4, the fraction of alkali metals that was not bound as chlorides was present in the fine particles as sulfates. Bottom Ash Composition. Three samples of circulating matter taken every 3-4 h were analyzed for their elemental content from each test. As circulating matter was the material that was separated in the primary cyclone, and circulated back into the reactor, it represented the bed material. Calcium and sulfur contents in the circulating matter increased steadily during all the tests, Figure 7. This indicates that also their contents in the bottom ash increased as the tests progressed. The increase of calcium in the circulating matter was due to attachment of calcium-rich particles on the bed particle surface forming a coating on the bed particles.24 A total of 20-25% of the calcium that was fed into the reactor in the fuel remained in the bed as bottom ash, as calculated on the basis of bottom ash analysis. Sulfur was presumably attached to the bed particles together with calcium-rich particles due to sulfation of CaO forming CaSO4. Bed material was sand with SiO2 content of approximately 74%. Silicon (and silica) content in the circulating matter decreased during all the tests due to calcium increase, Figure 7. Potassium content increased during test 1 and decreased during tests 2 and 6. In tests 3 and 4, no particular trend with time could be seen. In test 1, almost 50% ((10%) of the potassium that was fed into the reactor with the fuel remained in the bed as bottom ash, whereas in other tests, approximately 20% ((4%) of the fuel-bound potassium remained in the bed. Sodium content in the bottom ash was similar to potassium with more than 80% ((15%) remaining in the bed in test 1 and only (24) Lind, T.; Valmari, T.; Kauppinen, E. I.; Nilsson, K.; Sfiris, G.; Maenhaut, W. Proc. Combust. Inst. 2000, 28, 2287-2295.
Figure 7. Ca, S, Si, and K contents in the circulating matter during tests 1-4 and 6.
30-40% ((8%) in the bed in the other tests. Clearly, HCl feed decreased the tendency of potassium and sodium to remain in the bottom ash. Gas Composition. Chlorine content in the fuel was 0.027 wt % in dry fuel. In the tests when HCl was added, the HCl concentration was adjusted so that it corresponded to Cl content of 0.18 wt % in the fuel. If all the HCl and chlorine in the fuel would have been in the gas phase as HCl, the HCl concentration
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Table 3. Calculated and Measured Concentrations of SO2 and HCl (in Dry 6% O2 Flue Gases)a test
SO2(calcd)
SO2(measd)
HCl(calcd)
HCl(measd)
1 2 3 4 6 7 8 9 10
80 80 439 439 80 63 63 370 367
4 3 25 25 2 2 3 64 76
26 174 174 174 174 26 174 174 174
2 24 49 54 28 12 47 70 81
a Calculated values are based on the assumption that all Cl and S are in the gas phase as HCl and SO2.
would have been 26 ppm (dry flue gas, 6% O2) in reference test 1 and 174 ppm (dry flue gas, 6% O2) in the other tests, Table 3. Sulfur concentration in the fuel was 0.09 wt %. It corresponded to gas-phase SO2 concentration of 80 ppm (dry flue gas, 6% O2) if no retention in the ash would have taken place. In tests 3 and 4 when SO2 was added into the reactor, SO2 feed corresponded to 0.4 wt % sulfur content in the fuel. This would have resulted in 440 ppm (dry flue gas, 6% O2) SO2 in the flue gas if no retention in the ash occurred. All the measured values of HCl and SO2 were significantly lower than calculated values, Table 3. Therefore, significant retention of both Cl and S in the ash took place. In tests 1 and 7 with no additive gases, practically all Cl and S were retained in the ash, which, of course, is also obvious from the particle composition results. When HCl was added in tests 2, 6, and 8, HCl concentration in the flue gas increased, and SO2 concentration remained very low. In these tests, sulfur was still fully retained in the ash, and also the major fraction of Cl was retained. When SO2 was added in addition to HCl, both HCl and SO2 concentrations in the flue gas increased. In this case, sulfur reacted with alkali metal chlorides releasing Cl to the gas phase as HCl. Sulfur content was so high that a fraction of sulfur remained in the gas phase as SO2. Chemical Equilibrium Model Calculations. Three calculations were carried out with the chemical equilibrium model FactSage: case 1, reference test 1 with only fuel; case 2, tests 2 and 6 with HCl addition; case 3, tests 3 and 4 with both HCl and SO2 addition. Potassium. In case 1, the main gas-phase K compound was KOH with some 20% KCl. In the condensed phase, K was found as sulfate in a salt solution, other sulfates, and chloride (KCl). When HCl was added in case 2, the main potassium compound was KCl both in the gas and condensed phase. Condensed-phase KCl occurred at around 900-1000 K as a salt solution containing both liquid-phase KCl and K2SO4. In case 3, SO2 addition caused a shift to condensed-phase sulfate in a liquid solution and K-containing aluminosilicate. Sodium. The main gas-phase compound of Na in case 1 was NaOH with some NaCl. In the condensed phase, approximately one-third of Na was present as Na2O in a liquid slag, the rest being present as sulfate in a salt solution and NaCl. In case 2 when HCl was added, the sodium compounds were dominated by NaCl both in the gas and condensed phase. Condensed-phase chlorides formed at around 900-950 K. In case 3, NaCl remained the main gas-phase compound, and Na2SO4 in a salt solution was the dominant condensed-phase species. Calcium. According to model calculations, calcium had no gas-phase compounds in any of the cases in the temperature range under investigation. In case 1, calcium was mainly present in a slag as oxide CaO and as different silicates. The addition
of HCl in case 2 increased the amount of CaSO4 slightly, but otherwise the speciation remained fairly similar to case 1. In case 3 when SO2 was added into the system, calcium was mainly present as CaSO4 in T < 1200 K, with some Ca silicates occurring in the whole temperature range. Chlorine. Cl was found as gas-phase KCl and NaCl above 1000 K and as condensed KCl and NaCl below 850 K in case 1. Between 850 and 1000 K, Cl was almost entirely as HCl. In case 2, chlorine was again present as KCl and NaCl both in the gas phase and in the condensed phase. HCl was present in the whole temperature range, with higher concentration in 8001100 K. The concentration of HCl at 600 K was 22 ppm (wet flue gas). In case 3, chlorine was present as gas phase KCl and NaCl but also as HCl at high temperatures (T > 1300 K). HCl clearly dominated Cl speciation in the temperature range 8501300 K. Also, in addition to condensed KCl and some NaCl, HCl remained as a significant component in the lower temperatures (T < 850 K) with approximately one-third of Cl as HCl. The HCl concentration at 600 K was calculated to be 54 ppm (wet flue gas). Sulfur. In all the cases, sulfur was mainly present as SO2 above 1300 K. Below that, it occurred as sulfates of K, Ca, and Na in salt and slag solutions and as solid sulfates with the same elements. Calcium sulfate was the dominant species in cases 2 and 3. SO2 concentration at temperatures lower than 900 K was calculated to be less than 1 ppb.
Discussion and Conclusions Fine particle (here particles with diameters smaller than 1 µm, PM1.0) formation and alkali metal behavior were investigated in pilot-scale CFBC using a mixture of bark and sludge from a pulp and paper mill. Bed material was sand with silica content of 74%. HCl and SO2 were added from different locations in the reactor. By addition of chlorine and sulfur as gases, the content of chlorine and sulfur in the reactor could be varied while other fuel characteristics, e.g. ash composition, remained constant. Significant fractions of Na, K, S, and Cl were found to be released to the gas phase during combustion and form fine particles by nucleation and condensation. No Additive Gases. In the reference tests with no additive gas feed, less than 20% of alkali metals K and Na were released from the fuel as alkali metal chlorides since Cl content in the fuel was only 18% of the alkali metal content on a molar basis. This fraction may have been even less because in earlier studies it has been shown that a major fraction of chlorine is released from the fuel in earlier stages of combustion than alkali metals.11,12 Therefore, even in the presence of sufficient chlorine for all the alkali metals to be present as alkali metal chlorides, typically a major fraction of alkali metals is released from fuel as hydroxides and other species. The fine particle concentrations were low in the flue gas. Only a small fraction of the alkali metals reacted with HCl and SO3 forming fine particles that were alkali metal sulfates and chlorides. Practically no HCl or SO2 were in the gas phase at the reactor exit due to the reactions of Cl and S with the ash. The released alkali metals reacted readily with silicates and ash in the bed.25,26 A total of 50% of the potassium and more than 80% of sodium were removed from the reactor as bottom (25) Lindner, E. R.; Wall, T. F. In Mineral Matter and Ash Deposition from Coal; Bryers, R. W., Vorres, K. S., Eds.; United Eng. Trustees, Inc.: New York, 1990. (26) O ¨ hman, M.; Nordin, A. Energy Fuels 2000, 14, 618-624.
Particle Formation in Pilot-Scale CFBC of Biomass
Figure 8. Simplified schematic picture of fine particle formation and potassium transformations during low-chlorine biomass combustion in a circulating fluidized bed when HCl has been added into the reactor (tests 2, 6, and 8).
ash. Potassium and calcium contents in the circulating matter increased steadily during the tests due to retainment in bed particles. Only a small amount of sulfur remained in the bed as CaSO4 since relatively large amount of sulfur was consumed by alkali reactions. Sulfur content in the bed material was low, and it increased only slightly due to limited reaction with CaO. HCl Addition. HCl addition increased fine particle concentrations significantly. Even in this case, a major fraction of alkali metals was released from the fuel as species other than chlorides, presumably mainly as alkali metal hydroxides. This can be concluded because HCl was fed from a separate line and, therefore, HCl could only react with alkali metals once the alkali metals were outside of the burning fuel particles, i.e., already released. Therefore, HCl addition could not affect alkali metal release from the fuel, but it could alter reactions in the gas phase once the alkali metals had been released. The released alkali metal species reacted readily with HCl forming alkali metal chlorides in the gas phase, and a significantly smaller fraction of alkali metals reacted with bed material and ash in the bed than in the reference tests. Fine particles were formed when gas-phase alkali metal species nucleated and condensed. Presumably, even though sulfate concentration in the fine particles was low, particles were formed by alkali metal sulfate nucleation and then grew larger by alkali metal chloride condensation.27 This formed high mass concentrations of fine particles, mainly KCl and NaCl with minor amounts of K2SO4 and Na2SO4. According to chemical equilibrium calculations, the alkali metal sulfates started to condense at temperatures around 800 °C and alkali metal chlorides at around 700 °C. Therefore, alkali metal chlorides were mainly in the gas phase at the primary cyclone temperature of about 800-850 °C, and condensation took mainly place between the primary and secondary cyclones, in the secondary cyclone, and possibly even in the cooling section. Fine particle formation in this case is presented as a simplified schematic diagram in Figure 8. Only about 20% of potassium and 40% of sodium were removed from the reactor with the bottom ash. During the tests, potassium content in the circulating matter decreased indicating only limited retainment in the bed. Ca content in the circulating matter increased steadily. Clearly, HCl feed had almost no effect on the Ca retainment in the bed material. This was in agreement with earlier results where Ca retainment in bed has been suggested to be a mainly physical phenomenon, i.e., attachment (27) Jensen, J. R.; Nielsen, L. B.; Schulz-Moller, C.; Wedel, S.; Livbjerg, H. Aerosol Sci. Technol. 2000, 33, 490-509.
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of Ca-rich particles on the bed particle surface.24,28 Sulfur content in the bed was clearly higher than in the reference tests indicating more extensive reactions with CaO. This was due to the high HCl concentration that transformed gas-phase alkali metals into alkali metal chlorides instead of sulfates. Consequently, there was more available SO3 to react with CaO. SO2 content in the gas phase at the reactor exit was very low due to efficient capture by CaO. Approximately 25 ppm HCl was in the gas phase at the reactor exit which was very close to the calculated equilibrium concentration. HCl and SO2 Addition. The fine particle concentrations decreased when SO2 was fed into the reactor in addition to HCl. The alkali metals were still mainly released from the fuel as species other than chlorides, presumably hydroxides. The released alkali metals formed alkali metal chlorides and sulfates by gas-phase reactions. In principle, SO2 addition and consequent alkali metal sulfation should not have an effect on fine particle mass concentration if the same amount of alkali metals was available in the gas phase.29 However, we observed a significant decrease in the fine particle concentration from the case when only HCl was fed to the case when both HCl and SO2 were fed. There are two possible explanations to this decrease: (1) Less alkali metals were available in the gas phase for fine particle formation, or (2) the same amount of fine particles were formed but they were deposited either on coarse particle surfaces or in cyclones and on the reactor walls before our fine particle measurements. The first explanation would mean that alkali metals reacted with bed material after being released and before SO2 feed. However, we did not observe a significant increase in the alkali metal concentration in the bed material in the tests when both HCl and SO2 were fed. Moreover, potassium content in the circulating matter did not increase with time as in the reference test when potassium reacted extensively with silicates in the bed. In fact, K content did not vary with time in the circulating matter. The second explanation would mean that alkali metals reacted with HCl and the bed material in the same way both when HCl was fed or when HCl and SO2 were fed. After formation of gas-phase alkali metal compounds, SO3 reacted with these compounds. It formed new, fine particles by nucleation. In the SO2 addition conditions, these particles were formed in the reactor, and therefore, they were deposited in the primary and secondary cyclone more efficiently than alkali metal chlorides that condensed mainly downstream of the primary cyclone in the case when only HCl was fed. The alkali metal sulfates could also have formed mixed sulfates with Ca and, therefore, ended up partly in the coarse fly ash fraction. This explanation is supported by the fact that potassium content was actually somewhat higher in the secondary cyclone-collected ash during the tests with HCl and SO2 than during HCl tests. As Ca, K, and S are commonly found in the same particles in wood-based biomass ash and deposits and mixed sulfates of Ca and Na have been observed in fluidized bed combustion of low rank coals,12 it seems likely that such mixed sulfates would be formed. Consequently, the decrease in the fine particle concentration when SO2 was fed into the reactor in addition to HCl could be due to simultaneous deposition of particles in the reactor and the formation of mixed sulfates in the coarse fly ash particles. (28) Brus, E.; O ¨ hman, M.; Nordin, A.; Bostro¨m, D. Energy Fuels 2004, 18, 1187-1193. (29) Pyyko¨nen, J.; Jokiniemi, J. K. Fuel Process. Technol. 2003, 80, 225-262.
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Si content decreased in the circulating matter, and Ca content increased as in the other tests. Sulfur content in the circulating matter was higher than in other tests due to higher SO2 content in the gas and, therefore, more extensive sulfation of CaO. HCl concentration at reactor exit was approximately 50 ppm which was very close to the value calculated with chemical equilibrium model. SO2 concentration was approximately 25 ppm when the value calculated with equilibrium model was less than 1 ppb. The higher measured values for SO2 were due to limited alkali metal chloride and CaO sulfation. They are both limited by SO3
Lind et al.
formation which does not follow equilibrium in the flue gas temperatures.19 Acknowledgment. We acknowledge the European Community and VTT Processes for funding this investigation under the “EESD” Program through Contract ENK5-CT-2001-00532. We also want to thank everybody who participated in the pilot tests in Jyva¨skyla¨, Finland. EF050122I
