Sulfation of Potassium Chloride at Combustion Conditions - Energy

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Energy & Fuels 1999, 13, 1184-1190

Sulfation of Potassium Chloride at Combustion Conditions K. Iisa* and Y. Lu Oregon State University, Department of Chemical Engineering, Corvallis, Oregon 97331

K. Salmenoja Kvaerner Pulping, P.O. Box 109, FIN-33101 Tampere, Finland Received April 6, 1999

The sulfation of KCl was studied in the gas and molten phase in a laminar entrained-flow reactor. The experiments were performed at 900-1100 °C with residence times of 0.24-1.22 s. Small, 65-125 micron particles of KCl were partially vaporized in the reactor and allowed to react with SO2, oxygen, and water vapor. The conversions of KCl to K2SO4 were measured in the products from both the vapor and the molten phase. The sulfation was significantly faster in the vapor phase: up to 100% conversion was obtained in the vapor phase under most conditions but only 0.5-2% conversion in the melt. The sulfation rates both in the vapor and molten phases depend on SO2 and O2 but not on H2O concentration. The results suggest that SO3 is formed in the gas phase, and that the sulfation rate depends on the availability of SO3. In the gas phase, the sulfation proceeds to the extent that SO3 is available. In the molten phase the sulfation rate was finite even in the presence of excess SO3.

Background Increased use of annual crops, rice husk, and biological sludges along with bark and wood waste as a fuel for energy and steam production may lead to increased corrosion problems in boilers. In particular, high concentrations of potassium and chlorine may induce unexpectedly high corrosion rates. In pulp mills, the closing up of the chemical cycles and the demand to burn effluents in power and recovery boilers also increases the potassium and chloride contents in the fuels. Corrosion, fouling, and plugging of boiler heat transfer surfaces are probably the most cumbersome issues in boiler technology. Corrosion is generally related to fouling, but fouling does not always mean corrosion problems. A boiler can operate with thick deposits without corrosion problems. However, clean surfaces usually mean trouble-free operation. This applies to all types of boilers. Components responsible for corrosion and fouling are typically alkali metal chlorides, especially sodium chloride (NaCl) and potassium chloride (KCl). These are known to lower the melting temperature of the deposits and to enhance fouling tendency of the boilers.1-2 Increasing amounts of potassium (K) and chlorine (Cl) in the fuels will increase fouling and corrosion problems in boilers. * Author to whom correspondence should be addressed. Present address: Institute of Paper Science and Technology, 500 10th Street, Atlanta, NW, Georgia 30318. Fax: 404-894-5752. E-mail: [email protected]. (1) Backman, R.; Hupa, M.; Uppstu, E. Tappi J. 1987, 70, 123127. (2) Hupa, M.; Backman, R.; Skrifvars, B.-J.; Hyo¨ty, P. Tappi J. 1990, 73, 153-158.

Corrosion of heat transfer surfaces is caused either by molten salts on the tubes or by acid components in the flue gases. Generally, hydrogen sulfide (H2S) and hydrogen chloride (HCl) are the most harmful gaseous species in the flue gases. However, both H2S and HCl require reducing conditions, high concentrations, or high temperatures to initiate the corrosion process. Whenever molten salts appear on the tubes, corrosion problems hardly ever can be avoided. For instance, in chemical recovery furnaces in pulp mills the melt formed on boiler heat transfer surfaces is so aggressive that the corrosion rate can be kept at an acceptable level only by maintaining the metal temperatures below the first melting temperature (FMT) of the deposits. Alkali metals and chlorine that are present in fuels are vaporized to a large extent as alkali metal chlorides from the fuels during pyrolysis and combustion.3 Upon cooling of the flue gases, the alkali chlorides will condense and may deposit on the heat transfer surfaces. The chlorides may react with sulfur dioxide and become sulfated.

2KCl(g,c) + SO2(g) + 1/2O2(g) + H2O(g) f K2SO4(g,c) + 2HCl(g) (1) 2NaCl(g,c) + SO2(g) + 1/2O2(g) + H2O(g) f Na2SO4(g,c) + 2HCl(g) (2) The sulfation reactions of alkali chlorides are important from the corrosion and fouling point of view,4-5 (3) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (4) Salmenoja, K.; Ma¨kela¨, K.; Hupa, M.; Backman, R. J. Inst. Energy 1996, 69, 155-162.

10.1021/ef990057a CCC: $18.00 © 1999 American Chemical Society Published on Web 09/09/1999

Sulfation of KCl at Combustion Conditions

since a deposit containing only sulfates has a higher FMT than deposits containing alkali chlorides. In that sense, alkali sulfates are preferred to alkali chlorides in the deposits. Christensen and Livbjerg6 and Christensen et al.7 have studied the formation of potassium sulfate (K2SO4) in the flue gases of boilers fired with straw. They measured the concentrations of sulfur dioxide (SO2) and HCl in the flue gases, as well as the amounts of KCl and K2SO4 in fine particles. The fraction of potassium in the fine particles as sulfate varied from 7 to 100%. A theoretical model for aerosol formation was used to predict the fine particle compositions and SO2 and HCl concentrations from the known K, S, and Cl contents of the straw and the temperature profile in the boiler. The model suggested that K2SO4 condensed homogeneously and that the condensed K2SO4 served as condensation nuclei for the KCl condensation. Christensen et al. further suggested that the extent of sulfation could be approximately predicted by assuming that the sulfation reaction had obtained equilibrium at a fixed temperature of 812 °C ( 10 °C, and that there was no further sulfation as the flue gases cooled. To the authors’ knowledge, there are no published rate data for the sulfation of KCl. The corresponding sulfation reaction of NaCl has been studied in the solid phase but not in the vapor or molten phases.8-11 The sulfation rate in the solid phase is low. It is deemed to be of no significance for the sulfation of in-flight NaCl particles in recovery boilers but possibly of importance for sulfation of NaCl in deposits.12 In the same study it was concluded that the sulfation of air-borne NaCl particles had to occur in the molten or vapor phase. The objective of the current work was to study the sulfation of KCl in the molten and vapor phase. The effects of temperature and gas concentrations on the sulfation rates were investigated. Experimental Section The KCl sulfation experiments were made in a laminar-entrained flow reactor (LEFR). Solid particles of KCl as well as SO2, oxygen (O2), and water vapor (H2O) were fed into the reactor. The KCl particles became molten and partially vaporized in the hot reactor. The gas phase and molten KCl reacted with the SO2, O2, and H2O and became sulfated. The products (and unreacted reactants) of the molten- and vaporphase reactions were collected separately and analyzed. A schematic diagram of the laminar entrained-flow reactor is shown in Figure 1. The reactor consists of two (5) Michelsen, H.-P.; Larsen O. H.; Frandsen, F.; Dam-Johansen, K. Engineering Foundation Conference Biomass Usage for Utility and Industrial Power; Snowbird, Utah, February 28-March 3, 1996. (6) Christensen, K. A.: Livbjerg, H. Aerosol Sci. Technol. 1996, 25, 185-199. (7) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci., 1998, 29, 421-444. (8) Boonsongsup, L.; Iisa, K.; Frederick, W. J. Ind. Eng. Chem. Res. 1997, 36, 4212-4216. (9) Henriksson, M.; Warnqvist, B. Ind. Eng. Chem. Progress, Des. Dev. 1979, 8, 249-254. (10) Fielder, W. L.; Stearns, C. A.; Kohl, F. J. J. Electrochem. Soc. 1994, 131, 2414-2417. (11) Anderson, A. B.; Debnath, N. C. J. Phys. Chem. 1983, 87 19381941. (12) Boonsongsup, L.; Iisa, K.; Frederick, W. J.; Hiner, L. A. AIChE Symp. Series 1994, 99, 39-45.

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Figure 1. The laminar entrained-flow reactor (LEFR).

concentric mullite tubes that are placed in a three-zone electric furnace. The inner tube constitutes the reaction chamber in which the reactions take place. Solid particles are entrained in a primary gas flow that enters the reaction chamber from the top. A secondary gas enters the reactor from the bottom, flows upward in the annulus between the two ceramic tubes, and enters the reaction chamber through a flow straightener at the top of the reactor. The particles and the primary gas are rapidly heated by radiation from the hot reactor walls and convection from the hot secondary gas. The particle heating rates are 3000-4000 °C/s initially. The particles flow downward in the reactor. The flow pattern in the reactor is laminar, and the particles fall along the centerline of the reactor. The particles and the gas exit the reactor via a watercooled collector. A quench gas is provided to rapidly cool the particles and the gas. The quench gas flow is equal to or greater than the secondary gas flow, and 80% of it is added at the tip of the collector. The rest of the quench gas enters through the porous walls of the collector along the entire length of the collector. The purpose of this flow is to prevent fine particles from condensing on the collector walls. From the collector, the gases enter a cyclone with a cut-size of 3 µm in which the particle residues are collected. Fine particles (condensation aerosols) are separated from the gas in a filter. The residence time in the reactor can be varied by changing the location of the collector or the primary and secondary gas flow rates. The residence time of the particles is calculated by a computational fluid dynamics (CFD) code developed by Flaxman.13 The combined primary and secondary gas contained SO2, O2, and H2O in N2 in predetermined concentrations. The water vapor was added into the secondary gas and the SO2 into the primary gas. The water vapor was generated by passing the nitrogen and oxygen mixture through a water bath held at a constant temperature to produce the desired H2O concentration in the reactor. The quench gas was nitrogen. The gas flows were measured by mass flow meters. (13) Flaxman, R. J. Flow and Particle Heating in an Entrained-Flow Reactor. M.S. Thesis, University of Ottawa, Ottawa, Canada, 1986.

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Table 1. Experimental Conditions temperature residence time KCl feed particle size SO2 concentration H2O concentration O2 concentration gas flow rate (primary + secondary) KCl feed rate

900 °C, 1000 °C, 1100 °C 0.24-1.22 s 63-125 µm 1%, 2%, 4% 5%, 10%, 15% 2.5%, 5%, 7.5% 10 L/min, 20 L/min, 25 L/min (at 25 °C) 0.24 g/min

Table 2. Properties of KCl and K2SO4 melting point, °C partial pressure at 900 °C partial pressure at 1000 °C partial pressure at 1100 °C

KCl

K2SO4

770 °C 0.0037 bar 0.015 bar 0.051 bar

1069 °C 2.8 × 10-7 bar 3.0 × 10-6 bar 2.6 × 10-5 bar

During the experiments, solid particles of KCl were continuously fed into the reactor and removed via the collector. The feed material was prepared from reagent grade KCl (Mallinckrodt) which was ground and sieved. The fraction 63-125 µm was passed through the reactor at 900 °C in N2. The KCl particles caught by the cyclone were resieved and the fraction of 63-125 µm was used as the feed material in the sulfation runs. This procedure was adopted to eliminate plugging problems in the feeder. In the reactor, solid KCl particles first become molten. Part of the KCl is vaporized in the reactor. The rate of vaporization is governed by heat and mass transfer. Both the vaporized KCl and molten KCl may react with SO2, O2, and H2O to form K2SO4 according to eq 1. Due to the low vapor pressure of K2SO4, part of the vapor-phase K2SO4 condenses in the reactor. The vaporphase KCl and the rest of the K2SO4 condense upon quenching at the collector entrance. Submicron particles are produced as the result of the condensation. Two types of particles are present in the outlet stream of the reactor: (1) particle residues within the particle size range 50-100 µm, and (2) submicron condensation aerosols. With the 3-µm cut-size cyclone a very good separation of the two types of particles can be made. Scanning electron (SEM) micrographs of the cyclone catch particles show that there are very few fine particles attached to the surface of the particle residues. Therefore, there is negligible collection of the fine particles onto the large particles in the cyclone. The duration of a typical experiment was 5 min. The amount of KCl fed during each run and the amounts of the filter and cyclone catches were measured. Both the cyclone and the filter catch particles were analyzed for K+, SO42-, and Cl-. Capillary electrophoresis analysis was used for all the ions. The effects of residence time, reactor temperature, and gas concentrations on the conversion of KCl to K2SO4 in the filter and cyclone catch particles were investigated. Table 1 summarizes the experimental conditions. In all other experiments, except in those to study the effect of gas concentrations, the SO2 concentration was 2%, the O2 concentration 5%, and the H2O concentration 10%. Table 2 gives the melting points and vapor pressures of pure KCl and K2SO4. From the equilibrium point of view, all KCl may become vaporized at all other conditions except at 900 °C with a gas flow of 10 L/min.

However, as shown in the Experimental Section, 5-50% of the KCl fed to the reactor became vaporized in the experiments. Thus, both condensed and vapor phase KCl were present in all experiments. The efficiency of collecting the char and fume particles was checked by the closure of the potassium balance. The amounts of potassium in the cyclone catch and filter catches were compared to those in the feed. The closure in all runs was 87-103%, and 94% on the average. This indicates that a good collection of both the large and fine particles was obtained, and that the results can be used for quantitative purposes. Results and Discussion The conversion of KCl to K2SO4 in the filter and cyclone catches can be determined by three methods based on the analyses for K+, Cl-, and SO42- in the samples:

X)

X)

nSO4/2 nK nSO4/2

nSO4/2 + nCl

X)1-

nCl nK

(3)

(4)

(5)

where nSO4 ) moles of SO42- in the sample, nK ) moles of K+ in the sample, nCl ) moles of Cl- in the sample. The conversions in the filter catch samples were 25100%. In most cases, the conversions by the three different methods were within 8%. In a few filter catch samples, high amounts of SO42- were found. The anionto-cation equivalence ratios [(2 × nSO42- + nCl-)/nK+] far exceeded 1 in these samples. The Cl- contents of these filter catch samples were very low. The high amounts of SO42- are apparently due to sulfuric acid condensing on the filters. Some of the SO2 forms SO3 in the gas phase, and the SO3 condenses as sulfuric acid on the filter papers. We believe that the reason extra SO42was not found in all of the filter samples is that in some experiments all of the SO3 was consumed, whereas in others (the ones with high SO42- in the filter) it was not. This will be discussed more in detail in the Results Section of the paper. For the filter samples with excessive SO42- the conversion is calculated by the method of eq 5 (i.e., on the Cl- and K+ analyses). If no Cl- was detected, the conversion is taken to be 100%. The conversions in the cyclone catches were less than 4%. Because of the low conversions, the last method (eq 5) gave large relative errors. Therefore, the cyclone catch conversions were calculated as an average of the first two methods (eqs 3 and 4). The amount of KCl vaporized was calculated on the basis of the potassium in the fine particles. The K recovery in the fine particles, which is assumed to be equal to the KCl vaporization, is shown in Figure 2. The amount of KCl vaporized increases as residence time and temperature are increased. Initially, the vaporization is slow and very little KCl vapor is formed even at the highest temperature. The initial slow increase in the amount vaporized is due to the finite

Sulfation of KCl at Combustion Conditions

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Figure 2. Potassium recovery in fine particles () potassium vaporization) at 900, 1000, and 1100 °C. Test conditions: KCl feed rate 0.24 g/min, 10-20 L/min total gas flow (at 25 °C), 1-4% SO2, 2.5-7.5% O2, 5-15% H2O.

Figure 3. Equilibrium amounts of K2SO4(l) and KCl(g) for the experimental system: 0.24 g () 3.2 mmol) KCl, 10 or 20 L (at 25 °C) gas with 2% SO2, 5% O2, 10% H2O, 87% N2.

rate of heating of the particles. The particles enter the reactor cold and it takes approximately 0.2-0.3 s for them to reach the temperature of the reactor. After the initial slow vaporization, there is more rapid vaporization. Later the rate slightly decreases as the total amount of KCl vaporized increases. As the particles have reached the reactor temperature, vaporization proceeds at a rate limited only by mass transfer. The mass transfer rate depends on the external surface area of the particles, which decreases with time as more KCl is vaporized. Therefore, the vaporization rate slightly decreases with time. The vaporization depends on temperature as well. As the temperature is increased from 900 to 1100 °C, the KCl vaporization is increased approximately by a factor of 10: at 1.2 s, less than 5% of KCl is vaporized at 900 °C but almost 50% at 1100 °C. This is consistent with the corresponding increase in the KCl vapor pressure. The equilibrium for the experimental system was calculated by minimization of the Gibbs free energy using the software package HSC.14 The gas-phase species included in the calculations were KCl, K2SO4, KOH, K2Cl2, HCl, O2, SO2, SO3, H2O, and N2. The condensed species were KCl and K2SO4. Figure 3 shows the amounts of gas-phase KCl and condensed K2SO4 at the experimental conditions. At all conditions, all KCl is in the vapor phase and more than (14) Roine, A. HSC Chemistry, Ver 2.03; Outokumpu Research Oy, Pori, Finland, 1994.

Figure 4. Equilibrium conversion of vapor-phase KCl to K2SO4 and fraction of K2SO4 in the vapor phase. The condensed phase KCl is assumed to be nonreactive. Conditions: 0.24 g () 3.2 mmol) KCl, 10 L (at 25 °C) gas with 2% SO2, 5% O2, 10% H2O, 87% N2.

95% of the K2SO4 is in the condensed phase. A complete conversion of KCl to K2SO4 cannot be obtained under any circumstances. The equilibrium conversion decreases as the temperature is increased. At 900 °C, the equilibrium conversion to K2SO4 is around 95% but only 23-30% at 1100 °C. The fractions of KCl and K2SO4 are not affected by the gas flow rate. The other gasphase K compounds (KOH, K2Cl2) account for less than 5% of the total K in the system under all conditions. The calculations assume that the whole system is in a chemical equilibrium. However, a significant portion of the KCl is not vaporized during the experiments. Sulfation of condensed KCl is slow5,6 and therefore a large fraction of the KCl in the system will remain as KCl. Separate calculations were made assuming that only vapor-phase KCl is at equilibrium with the other compounds, and that any unvaporized KCl remains as KCl. For these assumptions, Figure 4 shows the conversion of the vapor-phase KCl to K2SO4 as a function of the fraction vaporized in the experiments with 10 L/min total gas flow. The equilibrium conversion of gaseous KCl decreases as temperature is increased, and is almost independent of the fraction of KCl vaporized except at extremely low fractions of KCl vaporized. The figure also shows the fraction of the K2SO4 that is present in the gas phase. At 900 °C, the vapor pressure of K2SO4 is so low that practically all K2SO4 is in the condensed phase beyond 0.1% vaporization. At 1100 °C,

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Figure 5. Conversion of KCl to K2SO4 in the filter catch samples with 10 L/min total gas flow rate. Test conditions: 2% SO2, 5% O2, 10% H2O. KCl feed rate 0.24 g/min.

some of the K2SO4 remains in the gas phase. For instance, 9% of the K2SO4 remains in the vapor phase when 20% of the KCl is vaporized. In the laminar entrained-flow reactor, the effects of temperature and residence time on the sulfation in the molten and gas phase were studied at a constant gas composition. The conversions in the filter catch samples are presented in Figure 5 for experiments with a total gas flow rate of 10 L/min and a solid feed rate of 0.24 g/min. The conversions in the filter catch particles are high: 40-100%. The conversion decreases as temperature is increased. This is consistent with the equilibrium calculations. Assuming that only the vaporized KCl is reactive, the equilibrium conversion was 96% at 900 °C, 72% at 1000 °C, and 30% at 1100 °C. The conversions measured in the fine particles slightly exceed the equilibrium conversions. The difference between the equilibrium conversion and measured conversions may be due to measurement errors or deficiencies in the thermodynamic database. It is also possible that the K2SO4 in the gas phase condenses preferentially by homogeneous nucleation as submicron particles but some of the KCl condenses on the particle residues. This would increase the conversion in the fine particles as compared to the conversion in the gas phase. The conversion also decreased as a function of time. This trend may be a result of the relative rates of vaporization and sulfation. The sulfation rate may be fast initially but become slower with time. The rate of KCl evaporation decreases only slightly with time. Therefore more of the KCl that becomes vaporized may remain unreacted, and the conversion of KCl therefore decreases with time. Figure 5 showed the conversion in the filter catch, i.e., the fraction of the KCl that was vaporized that had reacted to K2SO4 by the vapor-phase reaction. Figure 6 shows the fraction of KCl in the feed that was converted to K2SO4 in the vapor phase. This fraction increases as the residence time is increased even though the conversion in the filter decreased. Thus the decrease in the filter conversion as a function of time, which was seen in Figure 5 and discussed above, is due to the increased vaporization at longer residence times. The trend with temperature is interesting. The 900 °C data is below the 1000-1100 °C data but at 1000 and 1100 °C, the amounts of K2SO4 formed are the same within the uncertainty of the data. At 900 °C, the filter catch conversions were 100% or almost 100%, and the overall

Iisa et al.

Figure 6. Amount of K2SO4 formed in the filter catch with 10 L/min total gas flow. Test conditions: 2% SO2, 5% O2, 10% H2O. KCl feed rate 0.24 g/min.

Figure 7. The influence of gas flow rate on the amount of K2SO4 sulfated. Test conditions: 2% SO2, 5% O2, 10% H2O, 1100 °C. KCl feed rate 0.24 g/min, residence time 0.8 s.

amount of K2SO4 equals the amount of KCl vaporized at this temperature. Therefore, the lower amounts of K2SO4 at 900 °C as compared to 1000-1100 °C are due to the small amount of KCl in the vapor phase. Experiments were made with two different gas flow rates: 10 L/min and 20 L/min. By varying the position of the collector, the same residence time could be obtained with the two different gas flow rates. Figure 7 shows the effect of the gas flow rate on the amount of K2SO4 in the filter catch, when the residence time was kept constant. The amount of KCl sulfated is directly proportional to the gas flow rate. Doubling of the gas flow rate doubles the amount of K2SO4 in the filter catch. The fraction of KCl vaporized is the same at the two gas flow rates since the vaporization depends only on the residence time and temperature. Therefore, the conversion at the higher flow rate was twice the conversion at the low gas flow rate as well. The difference in the conversions between the two gas flow rates cannot be explained by the change in the equilibrium conversion. The equilibrium conversion is not affected by the gas flow rate as seen in Figure 3. One possible explanation for the effect of gas flow rate is that the sulfation of KCl is limited by the availability of one of the reactants in the gas stream. However, KCl was the limiting reactant at all conditions. The input consisted of 3 mmol/min of KCl and 9-34 mmol/min of SO2. 5-50% of the KCl vaporized, which makes the flow of KCl(g) 0.15-1.5 mmol/min. Two moles of KCl are required for one mole of SO2 for the formation of K2SO4. Thus, there was always at least 10 times more SO2 in the gas phase than was needed to completely convert the vapor-phase KCl to K2SO4. Less than 4.1% of the SO2 in the feed was actually converted to K2SO4 (the

Sulfation of KCl at Combustion Conditions

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Figure 8. Conversion of KCl to K2SO4 in the cyclone samples with 10 L/min total gas flow rate. Test conditions: 2% SO2, 5% O2, 10% H2O. KCl feed rate 0.24 g/min.

Figure 9. The influence of gas flow rate on the amount of K2SO4 formed in the cyclone catch. Test conditions: 2% SO2, 5% O2, 10% H2O, 1100 °C. KCl feed rate 0.24 g/min.

rest remains mainly as SO2). O2 and H2O were present even in greater abundance than SO2. Therefore, the sulfation of KCl is not limited by the availability of any of these gases. It is possible, however, that the availability of a gas-phase intermediate limits the sulfation of KCl. The most probable candidate for the limiting gas-phase intermediate is sulfur trioxide (SO3). The sulfur in SO2 needs to be oxidized from S(+IV) to S(+VI), and this can easily take place in the gas phase. The oxidation of SO2 would thus be the first step in the reaction sequence:

SO2(g) + 1/2O2(g) f SO3(g)

Figure 10. The effect of SO2 concentration on filter and cyclone catch conversions. Test conditions: 5% O2, 10% H2O, 10 L/min total gas flow, 1100 °C. KCl feed rate 0.24 g/min.

Figure 11. The effect of O2 concentration on filter and cyclone catch conversions. Test conditions: 2% SO2, 10% H2O, 10 L/min total gas flow, 1100 °C. KCl feed rate 0.24 g/min.

(6)

2KCl(g) + SO3(g) + H2O(g) f K2SO4(g) + 2HCl(g) (7) Some of the cyclone catch particles also became sulfated in the experiments. Figure 8 shows the conversions in the cyclone catch samples at the 10 L/min gas flow rate. The conversions are less than 4% at all conditions. The conversion increases with time and temperature. These conversions correspond to less than 2.5% of the KCl in the feed being converted to K2SO4 in the condensed phase. Figure 9 shows the impact of gas flow rate on the amount of K2SO4 formed in the cyclone catch. As with the filter catch, the amount of K2SO4 formed in the molten phase increases as the gas flow rate is increased. Experiments were made to determine the effect of gas concentrations on the amount of KCl sulfated in both the vapor and molten phases. All the tests were made at such conditions that the vapor-phase conversion of

Figure 12. The effect of H2O concentration on filter and cyclone catch conversion. Test conditions: 2% SO2, 5% O2, 10 L/min total gas flow, 1100 °C. KCl feed rate 0.24 g/min.

KCl was less than 100%. Figures 10-12 show the impacts of SO2, O2, and H2O concentrations on the conversions. Both the filter and cyclone catch conversions increase as the SO2 and O2 concentrations are increased, but the conversions are independent of the H2O vapor concentration within the accuracy of the data. The equilibrium conversion of gas-phase KCl to K2SO4 increases when any of the gas concentrations is increased. The change in the equilibrium conversion is largest when SO2 concentration is increased and smallest when O2 concentration is increased. Hence, the observed trends with the gas concentration cannot be explained with changes in the sulfation equilibrium alone. The trends with the gas concentration, however,

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Figure 13. Consumption of SO2 in experiments. Test conditions: 2% SO2, 5% O2, 10% H2O, KCl feed rate 0.24 g/min.

are in accordance with the assumption that the availability of SO3 limits the sulfation rate. The rate of SO3 formation would be affected by SO2 and O2 concentrations, and not by the H2O concentration. In the laminar entrained-flow reactor, SO2 could be oxidized in the feed lines at low temperatures or inside the reactor at high temperatures. We estimated the rate of gas-phase SO3 formation based on kinetics constants given by Bowman.15 The overall production of SO3 in the absence of KCl was less than the formation of K2SO4 in the experiments. However, the rate of the forward reaction (SO2 + O + M f SO3 + M) was higher than the rate of K2SO4 formation in the experiments. Thus it is possible that the formation of SO3 was an intermediate reaction. The forward reaction of SO3 formation and the competing reactions of SO3 decomposition to SO2 and the conversion of SO3 to K2SO4 would then affect the rate of KCl formation. The SO3 concentration in the exit gas was not measured. Indirect evidence for the formation of SO3 in the equipment is obtained from the condensation of sulfuric acid on some of the filter catch samples. Condensation of sulfuric acid could be seen as high SO42- contents in some of the filter catch samples as well as increased pressure drops over the filter during these runs. In all the runs in which the condensation was a problem, all KCl in the vapor phase had reacted to K2SO4. On the other hand, when there was KCl left in the filter samples this was not a problem. This supports the assumption that the availability of SO3 limited the reaction rate. In the runs with 100% conversion in the vapor phase, there is SO3 left in the gas and correspondingly there is condensation of sulfuric acid on the filter. If the conversion is less than 100%, all of the SO3 is consumed and consequently there is no condensation of SO3. The consumption of SO2 in the experiments was calculated from the amounts of K2SO4 formed and is shown in Figure 13 as a function of the residence time for the different temperatures. If the runs are divided into those with complete and incomplete conversion in the vapor phase, two different trends can be observed. Those experiments, in which the filter catch is completely or almost completely sulfated, have a lower fraction of SO2 consumed than (15) Bowman, C. T. Chemistry of Gaseous Pollutant Formation and Destruction in Fossil Fuel Combustion, A Source Book; Bartok, W., Sarofim, A. F., Eds.; John Wiley & Sons: New York, 1989.

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the other runs. Part of the condensed phase becomes sulfated as well. The condensed phase conversions, however, were low, and there is plenty of unreacted molten KCl remaining. Since the consumption of SO2 is lower in these runs than in the rest of the runs, there is SO3 remaining in the gas phase as well. For the runs with incomplete filter catch conversions the SO2 consumption is independent of the gas flow rate, as the data at 1100 °C with 0.8 s residence time show. There is very little difference between the data at 1000 and 1100 °C. No comparison can be made to the 900 °C results since the filter catch samples are completely converted at all conditions at 900 °C. The SO2 consumption increases with time. The data at different gas flow rates and gas concentrations suggest that the sulfation is influenced by the availability of SO3. The SO2 consumption data further suggest that the vapor phase reaction proceeds until all SO3 is consumed. When the conversion in the filter catch is complete, less than 10% of the total sulfation occurs in the condensed phase. Since the sulfation in the gas phase is by far the dominant, it can be concluded that the rate of sulfation in the gas phase is approximately equal to the overall rate of SO2 oxidation. In the runs with complete conversion in the filter catch particles, the overall SO2 consumption is lower than in the runs with incomplete filter catch conversion. In these runs, therefore, there is SO3 available. The sulfation in the molten phase hence proceeds at a slower rate than the formation of SO3. The sulfation rate, however, depends on the concentration of SO3. Conclusions The sulfation of potassium chloride was studied in the gas and condensed phases. The sulfation rate in the gas phase is fast. The rate is limited by the availability of SO3, and the rate approximately equals the overall rate of oxidation of SO2 to SO3. The rate of sulfation in the condensed phase is considerably slower. The rate is affected by the SO3 formation but the reaction proceeds at a finite rate even in the presence of excess SO3. The rate data cannot be directly compared to rates of KCl sulfation in the solid state, since to the authors’ knowledge there is no rate data for solid KCl. However, there are measurements for the corresponding sulfation of NaCl in the solid state. The rate of KCl sulfation in the molten phase is orders of magnitude faster than that of NaCl in the solid phase. Around 1-2% of KCl is converted in 1.0 s in the molten phase, but less than 1% of NaCl is converted in 1 h in the solid phase. The residence times in deposits are long, and some sulfation of condensed deposits may therefore take place. The results of this study suggest that most of the inflight KCl sulfation in a boiler will take place in the gas phase. There will be limited sulfation of condensed KCl in-flight aerosol particles. Any sulfation of deposits will likely take place in molten deposits. Acknowledgment. Ms. Qun Jing at the Institute of Paper Science and Technology is acknowledged for her aid in the data analysis. EF990057A