Evaluation of SO3 Measurement Techniques in Air and Oxy-Fuel

Article pubs.acs.org/EF

Evaluation of SO3 Measurement Techniques in Air and Oxy-Fuel Combustion Daniel Fleig,*,† Emil Vainio,‡ Klas Andersson,† Anders Brink,‡ Filip Johnsson,† and Mikko Hupa‡ †

Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ‡ Process Chemistry Centre, Combustion and Materials Chemistry, Åbo Akademi, FI-205 00 Åbo (Turku), Finland ABSTRACT: SO2 is enriched in oxy-fuel combustion due to flue-gas recycle, and a significant higher SO3 concentration can be expected compared to air-firing. Since SO3 can cause high and low temperature corrosion, it is important to measure the SO3 concentration under oxy-fuel fired conditions. However, measurement of SO3 is not straightforward, since SO3 is a highly reactive gas. This paper presents an experimental study in the Chalmers oxy-fuel test unit, comparing different SO3 measurement techniques applied during oxy-fuel and air combustion. Propane (60 kWth) was used as fuel and SO2 was injected in the oxidizer to generate a controllable amount of SO3. The SO3 concentration was measured with four techniques: the controlled condensation method, the salt method, the isopropanol absorption bottle method, and with the Pentol SO3 monitor (previously: Severn Science analyzer). The controlled condensation method was used as the standard for comparison. Additionally, the acid dew-point temperature was measured with a dew-point meter. The controlled condensation and the salt method gave comparable results, and the repeatability with these methods was good. The SO3 concentrations measured with the Pentol SO3 monitor differed in average less than 20% from the SO3 concentrations obtained with the controlled condensation method. With the isopropanol absorption bottle method, a large amount of the SO2 was absorbed in the isopropanol solution, which gives a positive bias if the SO2 is oxidized to sulfate in the isopropanol solution. This was minimized by reducing the measurement time, bubbling argon through the absorption bottles after the measurement to force the SO2 out, and analyzing the solution immediately after the measurement. No principal differences between measuring the SO3 concentration during oxy-fuel combustion and air-firing were obtained. However, a correction factor for the mass flow meter of the Pentol SO3 monitor has to be used because of the high CO2 concentration during oxy-fuel operation.

1. INTRODUCTION During combustion of sulfur-containing fuels, sulfur dioxide (SO2) and sulfur trioxide (SO3) are formed. The gas-phase SO3 formation is based on the oxidation of SO2 and HOSO2:1 SO2 + O( +M) ⇌ SO3( +M)

(1)

SO2 + OH( +M) ⇌ HOSO2 (+ M)

(2)

HOSO2 + O2 ⇌ SO3 + HO2

(3)

composition of the condensate differs from the composition of the gas phase. According to the phase-diagram published by Land,6 the condensate will typically contain more than 70 mol % H2SO4. Due to the azeotropic nature of the H2SO4−water mixture the H2SO4 strength is limited to 94 mol % (the roomtemperature azeotrope composition). The thermodynamic description of the system is further complicated by the fact that the H2SO4−H2O system is not a true two-component system.7 These considerations have to be taken into account when the SO3 concentration is calculated from the measured acid dew-point temperature (see Bolsaitis and Elliot7). Below the acid dew-point temperature acid mist is formed as well as small-sized corrosive drops (or films) on surfaces with destructive effects on metal surfaces as a consequence. Low temperature corrosion can be avoided by keeping the cold end above the acid dew-point temperature or by using acid-resistant materials. A further challenge with SO3 formation is that it may cause fireside corrosion by, for example, forming alkali iron trisulfates.8−11 An environmental problem related to SO3 is when H2SO4 aerosols leaves the flue-gas stack; called blue plume. This may occur when sulfur-rich fuels are combusted and the flue-gas cleaning is inadequate. The formation of SO3 under oxy-fuel combustion has lately received growing attention, since the

As the gas cools, the formation of SO3 is thermodynamically favored.2,3 However, simultaneously, this formation becomes kinetically controlled, and only a small amount of SO3 is typically formed. SO3 can also be formed via catalytic reactions depending on the fuel composition and boiler characteristics.4 At flue gas temperatures below 500 °C, the reaction SO3(g) + H 2O(g) ⇌ H 2SO4 (g)

(4)

starts to shift to the right and H 2 SO 4 (g) becomes thermodynamically stable. For example, if a flue gas with 8% H2O is cooled down to 200 °C, around 99% of the SO3 is converted into gaseous H2SO4 if reaction 4 reaches equilibrium.5 Thus, when SO3 measurements are discussed, these typically refer to the measurement of gaseous H2SO4, which is also the case in the present work. As the flue gas is further cooled, the sulfuric acid dew-point will be reached, and condensed H2SO4 is formed. The liquid H2SO4 and water mixture forms an azeotrope. This means that the © 2012 American Chemical Society

Received: July 5, 2012 Revised: August 26, 2012 Published: August 27, 2012 5537

dx.doi.org/10.1021/ef301127x | Energy Fuels 2012, 26, 5537−5549

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Institute) has performed a first set of studies in which SO3 concentrations were continuously measured by using Fourier transform infrared (FTIR) spectroscopy.32,33 This was done both by extracting flue gas as well as in situ in a flue-gas channel. The main problem in the studies where extractive measurements were conducted was to obtain samples without losses of SO3/ H2SO4. However, the in situ FTIR measurements provided fairly similar results compared with the controlled condensation method (around 20% higher).32 A main problem is that reference spectra have to be made of gases (SO3 and H2SO4) that are most challenging to handle when using an FTIR. Yet another option is to use a mass spectrometer for SO3 measurements. In this case, the critical issue is to transport the gas sample without condensation or reactions to the ionization cell. For example H2SO4 may be partly converted into SO2 at low pressures and therefore not detectable as SO3.34 Meadowcroft35 applied mass spectrometry successfully to measure SO3 in combustion gases. The mass spectrometer was combined with supersonic molecular beam sampling to avoid condensation or reactions. There are several questions regarding the accuracy and applicability of the different SO3 measurement techniques. However, there are not many experimental studies that compare different SO3 measurement techniques, and the only detailed study is done by Cooper et al.26−28 The results found by Cooper et al.26−28 will be discussed throughout this paper. So far, there exists no comparative study with respect to SO3 measurement applied during oxy-fuel combustion. Therefore, the objective of this experimental work is to compare different SO3 measurement techniques and to investigate possible differences in SO3 measurement during oxy-fuel and air firing. The experiments were carried out in the Chalmers oxy-fuel unit with propane (C3H8) as fuel and injection of SO2 in the feed gas. This was done to provide a controllable amount of SO2 and to exclude interactions between SO3 and fly ash particles. If extractive SO3 measurement methods are applied during coal combustion, the separation of particles and gas upstream the SO3 measurement is required. There is a risk in measuring too low SO3 concentrations due to reactions in the filter cake, especially with high alkalinity of the fly ash. It should be noted that this problem will occur regardless of SO3 measurement method. Measures to reduce this effect are given by the works of Cao et al.22 and Maddalone et al.23 The SO3 measurement techniques compared in the present study are the controlled condensation method, the salt method, the isopropanol absorption bottle method, and the Pentol SO3 monitor. The controlled condensation method was used as the standard for comparison. Additionally, the acid dew-point was measured with a dew-point meter.

combustion atmosphere is different from air-firing and the SO2 concentration is several times higher12−18 than the concentration in air-fired conditions. In general, SO3 measurements performed during oxy-fuel conditions shown higher SO3 concentrations than under air-fired conditions.15−21 Consequently, there are many reasons for boiler operators to quantify the amount of SO3 formed during a combustion process. For a coal-fired plant, SO3 measurements might also be relevant to check if additional SO3 injection before the ESP (dust precipitation) is needed or not. Yet, SO3 is difficult to measure because SO3 is a highly reactive gas. Moreover, the analysis of SO3 in the flue gas can be hindered by (1) comparatively low concentrations of SO3 under typical conditions, (2) high SO2 concentrations constituting an interference factor, (3) surface reactions in the sampling line, (4) SO3 starting to form gaseous H2SO4 at temperatures below 500 °C when H2O is available, (5) losses of gaseous SO3/H2SO4 by surface reactions or by reactions in the filter cake,22,23 and (6) condensation before the measurement. Here, a brief summary of various SO3 measurement techniques is given. The techniques used in this paper will be described further in detail in the next section. There exist several methods based on adsorption and absorption of SO3 followed by sulfate analysis, for example: • Controlled condensation method (British Standard BS 1756-4:1977) • Controlled condensation method (American standard D 3226-73T) • Isopropanol drop method (German standard VDI 2462, withdrawn) • Isopropanol absorption bottle method (based on EPA Method 8) • Salt method24−28 • Pentol SO3 monitor29 (first generation described by Jackson et al.30). The controlled condensation method is based on condensation of H2SO4 above the water dew-point and sulfate analysis afterward. The controlled condensation method is presently the most used SO3 measurement technique. However, the British and the American standards for controlled condensation have been withdrawn.31 The isopropanol methods are based on absorption of H2SO4 in an isopropanol [CH3CH(OH)CH3] solution. The main problem with all isopropanol methods is absorption of SO2 in the solution. The salt method is not a 24 ́ commonly used method. It was first described by Kelman, later 25 used by Roiter et al., and more recently evaluated by Cooper et al.26−28 The method is based on the reaction of H2SO4 with NaCl and subsequent sulfate analysis. Cooper et al.26 tried out two types of the salt method. The first comprised three or four inserial connected NaCl impregnated quartz filter tubes, while the second consisted of a cheaper and simpler glass tube filled with quartz wool and NaCl. The detected SO3 concentration was far too low using the NaCl impregnated filter tubes, and Cooper et al.26 assumed that the amount of NaCl was too low or the residence time too short. However, when the simple glass tube packed with NaCl was used, the determined SO3 concentrations corresponded to the results of the controlled condensation method. In the literature, there are also some SO3 measurement studies that are not based on wet chemical methods. Acid dew-point meters for measuring the acid dew-point temperature in the flue gas (described in detail in the next section) is one example of an alternative method. In addition, EPRI (Electric Power Research

2. APPLIED SO3 MEASUREMENT TECHNIQUES As indicated, the following SO3 measurement techniques were compared: (1) Controlled condensation method (based on the British Standard BS 1756-4: 1977) (2) Salt method24−28 (3) Isopropanol absorption bottle method (based on EPA Method 8) (4) Continuous SO3 monitor from Pentol GmbH29,30 The acid dew-point temperature was measured with the acid dew-point monitor from Land.6,36 The sampling system used for the extractive SO3 measurement methods is shown in Figure 1. The “SO3 sorption unit” in Figure 1 represents the applied measurement technique, for example, the glass cooler used in the 5538


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standard solutions containing different concentrations of sulfates. The samples were diluted when necessary. The controlled condensation method was used as the standard for comparison because the method is a widely accepted method for SO3 measurement. Acceptable measurement results in terms of accuracy and repeatability were reported for example by Maddalone et al.23 They recovered 95% of the injected H2SO4 from a synthetic flue gas with the controlled condensation method with a coefficient of variance of ±6.7%. Salt Method or Common Salt Absorption Method. The principle of the salt method is that flue gas is passed, above the acid dew-point temperature, through a bed of sodium chloride (NaCl). Gaseous H2SO4 reacts with NaCl to form sodium bisulfate (NaHSO4) and sodium sulfate (Na2SO4):

Figure 1. Illustration of the sampling system for the extractive methods.

controlled condensation method. A flue-gas flow of around 1 L (STP) /min was extracted during 30 min with an oil-heated probe from the flue-gas duct. The oil-heated probe was equipped with a quartz glass tube in the center to avoid surface reactions. The temperature of the oil-heated probe was kept at around 200 °C to secure a temperature well above the acid dew-point. Since only gas-fired experiments were performed, no particle filter was used, except in one case, as it will be discussed in the result section. For the SO3 monitor the appropriate electrically heated probe from Pentol GmbH29 was used. Controlled Condensation Method. The principle of the controlled condensation method is to condense all sulfuric acid in the flue gas by cooling down the extracted flue gas between its acid dew-point and the water dew-point temperature. A schematic of the glass cooler applied in the controlled condensation method is shown in Figure 2. After the cooling

NaCl(s) + H 2SO4 (g) → NaHSO4 (s) + HCl(g)

(5)

2NaCl(s) + H 2SO4 (g) → Na 2SO4 (s) + 2HCl(g)

(6)

The favored reaction path depends on the temperature. Afterward, the amount of SO3 is determined by dissolving the salt in distilled deionized water and by analyzing the amount of SO42− with ion chromatography or by titration. About 1 g of ultraclean NaCl was tightly packed in a Teflon tube with an inner diameter of 8 mm. Glass wool was used in each end of the tube to keep the salt in its position. Figure 3 shows the

Figure 3. Measurement setup of the salt method for measuring SO3 and SO2 simultaneously.

measurement setup. SO2 was simultaneously determined by bubbling the flue gas through two impingers with a 3 vol. % hydrogen peroxide (H2O2) solution placed in an ice bath. SO2 is oxidized to sulfate in the hydrogen peroxide solution. The sulfate concentration in the hydrogen peroxide solution was determined by the addition of thorin (C16H11AsN2O10S2) as an indicator and titration with barium perchlorate [Ba(ClO4)2] until the color changes from yellow to light red. An additional bottle containing silica gel was used for gas drying. Afterward, the salt and glass wool filter was placed in distilled deionized water and the sulfate content was determined with ion chromatography, as described (see the Controlled Condensation Method section). Isopropanol Absorption Bottle Method. Figure 4 shows the setup of the isopropanol absorption bottle method used in this study. It is based on the EPA Method 8, but with some modifications. Flue gas was first bubbled through one impinger filled with 100 mL of 80 vol. % isopropanol solution (diluted in water) to absorb SO3/H2SO4, second, through two impingers filled with 100 mL of 3 vol. % H2O2 solution to absorb SO2, and, finally, through a bottle with silica gel for gas drying. All bottles were placed in an ice water bath. A difference of the applied method to the EPA Method 8 is that in the applied method no filter was used between the isopropanol and H2O2 solutions.

Figure 2. Schematic of the glass cooler applied in the controlled condensation method.

spiral the cooler contains a sintered glass filter for catching sulfuric acid aerosols. The cooling coil was kept at a temperature between 80 and 90 °C during the measurements. Sulfuric acid condenses and is adsorbed on the condenser walls and in the capillary glass filter. The cooler was flushed with a 5% isopropanol solution with a pH-value of 4.6 and bromophenol blue as an indicator afterward. The amount of sulfate ions (SO42−) in the isopropanol solution was analyzed with ion chromatography (ICS-90 Ion Chromatography System from DIONEX). The ion chromatograph was calibrated with five 5539


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illustrated in Figure 5. The isopropanol solution drops in the extracted flue gas stream and H2SO4 is absorbed. The SO42−

Figure 4. Measurement setup of the isopropanol absorption bottle method for simultaneous SO3 and SO2 measurement.

After sampling, argon (Ar) was bubbled through all bottles to remove possible SO2 from the isopropanol solution. This is not mentioned in the EPA Method 8 but recommended by Gustavsson et al.,37 who state that the absorbed SO2 can be removed by the subsequent bubbling of air through the isopropanol solution. However, Koebel et al.38 noted that the elimination of all SO2 in the solution is difficult, especially if the pH value is too high, for instance due to low SO3 and NOx concentrations or if ammonia (NH3) is present. In the present study, the amount of sulfate in the isopropanol solution was determined immediately after the measurement in order to minimize the oxidation of SO2 in the solution. The determination of sulfate ions was done by titration with barium perchlorate and using thorin as an indicator. Cooper et al.26 measured the amount of sulfate in the isopropanol solution by ion chromatography, and the resulting SO3 concentration was too high. Despite the fact that dry air was bubbled through the bottles to remove SO2, possibly absorbed by the isopropanol solution, high SO2 absorption rates in the isopropanol solution was observed. The determination of the sulfate content by ion chromatography is therefore not recommended by Koebel et al.,38 because, first, it normally takes too long to get the substance to a laboratory and, second, the isopropanol solution must be further diluted, which may favor sulfite oxidation. As a consequence, too high SO3 concentrations will be obtained when ion chromatography is used to detect the amount of SO3 absorbed in the isopropanol solution. The amount of SO2 absorbed in the H2O2 solution was analyzed by titration, as described above (see the Salt Method or Common Salt Absorption Method section). A laboratory study of the isopropanol method was made before the measurement campaign, which showed the importance of flushing the isopropanol solution with an inert gas after the measurement and that the analysis of sulfate ions must be done as quickly as possible after the measurement. The study also showed that by shortening the measurement time the oxidation of SO2 in the isopropanol solution can be reduced. However, reducing the measurement time may lead to inaccuracies in the quantification due to lower concentrations. Pentol SO3 Monitor. The Pentol SO3 monitor29 (previously: Severn Science reactive gas analyzer) is similar to the isopropanol drop method (German standard VDI 2462), but it was developed for continuous measurement of the SO 3 concentration in flue gas. The first generation of that SO3 analyzer is described by Jackson et al.30 The principle of the absorption equipment for the isopropanol drop method is

Figure 5. Simplified schematic of the absorption equipment of the isopropanol drop method.

concentration in the isopropanol solution is detectable by titration (as described above). This analyzing step is automatized in the Pentol SO3 monitor. During sampling, the isopropanol solution is continuously treated with barium chloranilate and the following reaction occurs. SO24 − + BaC6O4 Cl 2 + H+ → BaSO4 + HC6O4 Cl −2

(7)

The acid chloranilate ion absorbs light at 535 nm (violet). The absorption is measured continuously by a photometer and depends on the amount of chloranilate ions in the solution. The higher the absorption, the more sulfate in the solution. The amount of sulfate in the solution is proportional to the SO3 concentration in the flue gas. A mass flow controller (MFC) measures the extracted dry flue gas flow and the SO 3 concentration is calculated by the instrument. During our measurements, we found that the MFC in the SO3 monitor is calibrated for N2 and, therefore, has to be adapted for oxy-fuel operation because of the high CO2 concentration. This was confirmed by Pentol GmbH.29 The manufacturer of the MFC, Brooks Instrument,39 gives correction factors for different gases. The flue-gas composition of the operated oxy-fuel case results in a correction factor of 0.76. That means the reading (SO3 concentration in ppm) of the Pentol SO3 monitor has to be divided by this factor. It should be pointed out that, in principle, also under air-fired conditions a correction factor could be used since flue-gases contain CO2. However, since the correction factor is not far away from unity, it was not taken into account for the measurements done during air firing. Jackson et al.30 and Blauenstein29 (Pentol GmbH) claim that there occurs no significant sulfate oxidation of possible absorbed SO2 in the isopropanol solution when using the Pentol SO3 monitor. Denne et al.32 also state that this instrument provides credible results, but only with a skilled operator (see Fernando4). However, Cooper27 measured at an Orimulsion fired power station significant higher SO3 concentrations (25 times higher in average) with the Severn Science analyzer compared to the SO3 concentrations obtained with a controlled condensation method. A possible explanation discussed by Cooper27 is that absorption of SO2 in the isopropanol solution occurs together with a partial oxidation of the resulting SO32− to SO42−. It should be mentioned that the SO2 concentrations were relatively high 5540


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Figure 6. Chalmers 100 kWth oxy-fuel test unit. M8, M9, M10, M13, and M15 are the used measurement positions.

3. EXPERIMENTS

(>1000 ppm) while the SO3 concentrations were low (around 2 ppm, obtained with the controlled condensation method). Acid Dew-Point Meter. The principle of an acid dew-point meter is that acid condensation is forced on a temperaturecontrolled surface. The conductivity on the surface is measured and acts as an indicator for acid condensation. The temperature at which a first acid condensation occurs is called the acid dewpoint temperature. If the H2O concentration of the flue gas is known, together with the total pressure, the SO3 concentration can be calculated from the acid dew-point temperature. Often the SO3 concentration is estimated using an empirical relation, for example, by the relation given by Verhoff and Banchero.40 The principle of the acid dew-point meter was already developed by Johnstone41 in the 1920s and further improved by Taylor.42 Nowadays, there are commercially available acid dew-point meters. In the present study, an acid dew-point monitor (ADM 220) from Land Instruments6,36 was used to obtain the acid dew-point temperature of the flue gas. The probe has a sensor in the tip consisting of two platinum electrodes insulated from each other with a pyrex glass. The surface temperature of the sensor is controlled by adjusting the air flow. The probe tip is inserted into the flue-gas channel. After the surface temperature has reached the flue-gas temperature, the temperature has to be slowly decreased. The dew-point meter shows a current of 0 μA as long as no condensation occurs on the sensor surface. After Land,6 the acid dew-point temperature is reached when the rates of evaporation and deposition on the sensor surface are equal and therefore the conductivity is constant. Different recommendations for a stable current reading were found in the literature; for example, Land6 recommend a stable current reading of around 100 μA, and Stuart36 shows a stable current reading of around 50 μA. In the present study, a 50 μA or 100 μA reading seemed to result in too low acid-dew point temperatures. Instead, the acid dew-point temperature at a stable current reading at around 1 μA was taken. The reason for this discrepancy is possibly the low gas velocity (∼2.5 m/s at STP) in the flue-gas channel leading to low deposition rates.

Chalmers Oxy-Fuel Test Unit. The Chalmers 100 kWth oxy-fuel test unit with its cylindrical furnace and the top-mounted burner is shown in Figure 6, with further details provided by Andersson et al.43 Figure 6 gives the flue-gas measurement positions applied in this work: M8, which was used to measure the SO3 outlet concentration, M9, which was used to measure the acid dew-point temperature, M10, for measuring the SO3 concentration after the flue-gas cooler, M13, for measuring the flue-gas composition, and M15, for measuring the feedgas composition. The SO3 concentration is similar in ports M8 and M9, since these ports are close to each other, but both measurement ports were used for practical reasons in terms of physical space. The experiments were performed using C3H8 as fuel and injection of SO2 in the oxidizer (feed gas) downstream of the O2 injection. The fuel was injected in the center of the burner at the furnace top through an annular burner nozzle. The feed gas, containing SO2, was inserted in the primary and secondary registers. The minimum purity of the SO2 is 99.8%, according to the gas supplier with the remaining constituents being H2O and H2SO4. The furnace was vacuum cleaned before running the experiments to exclude errors from sulfate-containing ash particles formed during previously conducted coal-fired experiments. During the SO3 measurement campaign, it was obvious that the SO3 formation increased significantly with an increasing temperature in the furnace similar to what was reported by Crumley and Fletcher.44 It was therefore of interest to quantify the temperature level in the furnace. This was done by calculating an average furnace wall temperature from 14 continuously logged thermocouples placed 2 cm from the inner surface of the furnace wall. Table 1 lists the distance between the furnace top and the thermocouples (each level has two thermocouples, at 180° from each other). The range of variation between thermocouples was

Table 1. Position of Wall Thermocouples

5541

wall thermocouple

distance from furnace top (inner wall) [m]

T1/T8 T2/T9 T3/T10 T4/T11 T5/T12 T6/T13 T7/T14

0.046 0.215 0.384 0.553 0.8 1.0 1.4 dx.doi.org/10.1021/ef301127x | Energy Fuels 2012, 26, 5537−5549

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Measurement Matrix. Table 3 lists all performed SO3 measurements. The measurements are numbered by letters and listed chronologically. In addition to the test case and the applied SO3 measurement technique, the time from start-up and the average furnace wall temperature are given. The time from start-up of the unit is the start time of each measurement. Flue Gas Composition. The O2, CO2, and SO2 concentrations were measured with conventional gas analyzers and with at least two of each connected in series. SO2 was analyzed with nondispersive infrared (NDIR) analyzers (NGA 2000 from Fisher Rosemount). One SO2 analyzer was calibrated at 1540 ppm SO2 and the other at 2730 ppm SO2.

around 170 K for a given average temperature. Figure 7 shows a typical trend for the average furnace wall temperature during a test day from

4. RESULTS AND DISCUSSION First a general overview of the results from the different SO3 measurement techniques is given. Figure 8 shows the measured SO3 concentrations versus the time from start-up for the air-fired case and OF30 case at the furnace outlet obtained with the controlled condensation method, salt method, isopropanol absorption bottle method, and the Pentol SO3 monitor. In addition to the SO3 concentration, the average furnace wall temperature is shown in Figure 8. The furnace wall temperature increases as the furnace heats up during the test occasion. The SO3 formation increases with operation time because of the increased temperature level in the furnace. This makes it more difficult to compare the different methods, and the furnace temperature must be taken into account when comparing the results. However, after around 6 h of operation, the SO3 level was relatively stable in the air-fired case, whereas in the OF30 case, an increasing SO3 trend was observed during the whole operation time. If the measured SO3 concentrations are plotted versus the average furnace wall temperature, as shown in Figure 9, it can be concluded that the average furnace wall temperature is a good indicator of the SO3 formation in our test unit. Surprisingly good results were obtained with all methods. The strongest variations can be observed in the data obtained with the isopropanol absorption bottle method. The controlled condensation method and salt method gave comparable results, and the repeatability for these methods was good. The differences between the different data points from the controlled condensation method and salt method result from variations in the combustion process itself rather than inaccuracies in the measurement technique. In the next sections, the benefits and drawbacks for each SO3 measurement method will be discussed and the results of the different SO3 measurement methods will be compared to the results from the controlled condensation method. Salt Method. Figure 10a shows the measured SO 3 concentrations with the salt method for the air-fired case and Figure 10b for the OF30 case versus the time from start-up of the unit. The measurement data obtained with the controlled

Figure 7. Furnace wall temperature trend (average of 14 wall thermocouples, see Table 1) of the Chalmers oxy-fuel test unit during an operation day (Oct. 5, 2011).

start-up of the unit. The furnace was shut down every evening, but similar temperature profiles were obtained during each test day. Test Cases. Tests were performed during air and oxy-fuel combustion with dry flue-gas recycling. The experimental test conditions and the flue-gas concentrations of O2, SO2, H2O, and CO2 are given in Table 2. Six cases were operated, two air-fired cases and four oxy-fuel cases with dry recycle. The oxy-fuel cases differ by the O2 concentration in the feed gas, and the denotation was made according to the O2 concentration in the feed gas, for example OF30 means 30 vol. % O2 in the feed gas on a dry basis. The excess O2 concentration and the SO2 concentration were kept the same for all oxy-fuel cases. The SO2 concentration of the OF30 case was set to 3000 ppm on dry basis, which is equal to a SO2 concentration of 2438 ppm on wet basis. The other oxyfuel cases were adapted so that the same SO2 concentration on a wet basis was reached as in the OF30 case. Since the injected mass flow of SO2 was limited by the equipment, the maximum achievable SO2 concentration in the air-fired case was 1000 ppm on a dry basis. A second air-case with a low SO2 concentration (500 ppm on a dry basis) was also investigated. During SO3 measurements, the gas composition in the flue gas was analyzed continuously, and the variation in concentration was less than ± 2% from the data given in Table 2. The heat input by the fuel was 60 kWth in all cases based on the lower heating value. The air case with 1000 ppm SO2 and the OF30 case with 3000 ppm SO2 were operated as reference cases; most samples were collected during these conditions to provide an extensive basis for data comparison. Additionally, the OF30 case was operated with different SO2 concentrations, namely 200 ppm, 600 ppm, 1000 ppm, 1500 ppm, 2000 ppm, and 2500 ppm on dry basis after combustion.

Table 2. Experimental Conditions of the Test Cases flue gas concn. (on a wet basis) test case

O2 in feed gas [vol.%, dry]

oxygen−fuel equiv. ratio, λ

air air low SO2 OF30 OF25 OF35 OF40

21 21 30 25 35 40

1.38 1.38 1.25 1.31 1.21 1.18

recycle rate

SO2 in the flue gas on a dry basis, [ppm]

XO2 [%]

XSO2 [ppm]

XH2O [%]

XCO2 [%]

0.829 0.867 0.789 0.747

1000 500 3000 2887 3116 3218

5.39 5.39 5.39 5.39 5.39 5.39

885 443 2438 2438 2438 2438

12 12 18.7 15.6 21.8 24.7

8.6 8.6 ∼71 ∼74 66.9 64.6

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Table 3. Measurement Matrix measurement

test case

port

SO3 measurement technique

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR

OF30 OF30, SO2 variation OF30 air air air air air air air air air air air OF30 OF30 OF30 OF30 OF30 OF30 OF30 air low SO2 air low SO2 air air air air OF25 OF25 OF35 OF35 OF35 OF40 OF40 OF30, SO2 variation OF30 OF30 OF30, SO2 variation OF30 1500 ppm SO2 OF30 OF30 air low SO2 air low SO2 no combustion, only air +1000 ppm SO2 no combustion, only air +1000 ppm SO2 air air

M8 M8 M8 M8 M8 M8 M8 M8 M8 M8 M8 M10 M8 M10 M8 M8 M8 M9 M8 M8 M8 M8 M9 M8 M8 M9 M10 M9 M8 M9 M8 M8 M9 M8 M8 M8 M8 M8 M8 M8 M8 M8 M8 M8

c. condensation c. condensation c. condensation c. condensation c. condensation c. condensation salt method salt method salt method isopropanol isopropanol dew-point monitor c. condensation c. condensation salt method salt method salt method dew-point monitor isopropanol salt method (serial) salt method (serial) salt method dew-point monitor salt method (serial) salt method (serial) dew-point monitor dew-point monitor dew-point monitor c. condensation dew-point monitor c. condensation salt method dew-point monitor salt method Pentol Pentol c. condensation Pentol c. condensation Pentol c. condensation Pentol c. condensation c. condensation

2011/09/12 2011/09/13 2011/09/13 2011/09/14 2011/09/21 2011/09/21 2011/09/27 2011/09/27 2011/09/27 2011/09/27 2011/09/28 2011/09/29 2011/09/29 2011/09/29 2011/10/03 2011/10/03 2011/10/03 2011/10/04 2011/10/04 2011/10/04 2011/10/04 2011/10/05 2011/10/05 2011/10/05 2011/10/05 2011/10/05 2011/10/05 2011/10/06 2011/10/06 2011/10/06 2011/10/06 2011/10/06 2011/10/06 2011/10/06 2011/10/10 2011/10/10 2011/10/10 2011/10/11 2011/10/11 2011/10/11 2011/10/11 2011/10/11 2011/10/11 2011/10/12

M8

Pentol

2011/10/12

M8 M8

Pentol c. condensation

2011/10/12 2011/10/12

AS AT AU

condensation method are also shown for comparative reasons. The average furnace wall temperature is given for each SO3 measurement. If one considers that the measurements with the controlled condensation method were done at different test occasions than the measurements with the salt method and that there is an increase of SO3 formation during each test occasion, the agreement between the two measurement methods is good. For example, 54 ppm SO3 was measured with the salt method in the OF30 case when the average wall temperature was 496 °C,

date

SO3 [ppm] or acid dew-point temp.

time from start-up

avg. furnace wall temp.

07:05

500

56

09:10 08:02 05:35 06:20 03:11 06:39 08:56 10:54 11:06 05:45 08:08 10:11 04:03 04:59 06:48 06:10 07:33 13:46 13:46 02:36 02:58 05:02 05:02 05:28 05:53 02:49 03:18 05:09 05:18 06:25 07:49 08:05

539 508 506 511 461 512 530 541 544 506 529 530 442 464 496 499 520 566 566 457 460 500 500 503 508 446 463 501 508 523 540 543

61 34 36 38 25 33 35 45 34 153 °C 41 27 37 40 54 153−158 °C 49 96 3 17 121−124 °C 35 6 132−136 °C 141−149 °C 123− 133 °C 9 154 °C 25 27 156− 164 °C 79

09:12 09:52

530 535

84 75

07:30 10:05 10:37 12:16 13:22

509 544 548 546 549 292

43 65 87 27 35 1

278

10

377 412

12 17

01:10 01:28

and 56 ppm SO3 was measured with the controlled condensation method when the average wall temperature was 500 °C. The high value (96 ppm) measured with the salt method during OF30 operation can be explained by the high average temperature in the furnace (average wall temperature 566 °C). Also, for the airfired case, the measured SO3 concentrations with the salt method are similar to those concentrations measured with the controlled condensation method for similar average furnace wall temperatures. The SO2 concentrations measured by absorption of SO2 5543


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Figure 8. SO3 concentrations at the furnace outlet obtained with the different measurement techniques and the corresponding average furnace wall temperature for (a) air-fired case and (b) OF30 case.

Figure 9. Measured SO3 concentrations versus average furnace wall temperature for the (a) air-fired case and (b) OF30 case.

Figure 10. Measured SO3 concentrations at the furnace outlet with the salt method and the controlled condensation method: (a) air-fired case and (b) OF30 case.

salt filter without being absorbed or if there is a reaction of SO2 with NaCl. As seen in Figure 10, the amount of sulfate found in the second salt tube, here recalculated as SO3 in the flue gas, is small compared to the amount of sulfate found in the first salt tube. Therefore, it can be concluded that there cannot be a

in the hydrogen peroxide solution were in good agreement with SO2 concentrations obtained from the NDIR analyzers. In the air-fired case as well as in the OF30 case, one measurement was carried out with two salt tubes connected in series to estimate if some SO3/H2SO4 is going through the first 5544


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Table 4. SO3 and SO2 Concentrations Measured with the Isopropanol Absorption Bottle Method and the Calculated SO2 Concentration Captured in the Isopropanol measurement

case

avg. furnace wall temp. [°C]

SO3 [ppm]

sampling time [min]

J K

air air

541 544

45 34

30 20

S

OF30

520

49

20

purging with Ar 10 min 1.5 L/min 15 min 1.5 L/min

SO2 meas. (in H2O2 solution) [ppm]

SO2 captured in isopropanol [ppm]

350 570

540 320

2160

280

and the OF30 case. The results are shown in Table 4. In the first test during the air-fired case, an SO3 concentration of 45 ppm on wet basis was obtained (2 h after the last measurement “I” with the NaCl method, see Table 3). Worth noting is the large amount of SO2 that was captured in the isopropanol solution. A sulfate amount corresponding to an SO2 concentration of only 350 ppm was found in the H2O2 solution, which means that around 540 ppm of SO2 was captured in the isopropanol solution. In the second test during the air-fired case, the sampling time was shortened from 30 to 20 min, to avoid oxidation of SO2 to sulfate in the isopropanol solution, and argon was bubbled through the bottles after the measurement to force the SO2 out from the first impinger. The obtained SO3 concentration in the second test was 25% lower than that in the first test and is somewhat lower as the SO3 concentrations measured with the controlled condensation method for similar furnace temperatures (see Figure 9a), but still, some of the SO2 was trapped in the isopropanol solution. Simultaneously, a loss of H2SO4 aerosols cannot be excluded because no filter was used between the isopropanol and H2O2 solutions. For the OF30 case, the measured SO3 concentration was 49 ppm on wet basis with the isopropanol absorption bottle method, which is of the same order of magnitude as measured with the salt method and controlled condensation method (see Figure 10b). However, some SO2 was trapped in the isopropanol solution despite bubbling argon through the bottles for 15 min. This would have caused a significant error if the SO2 would have had time to oxidize to sulfate, for example, during transportation to a laboratory. In these experiments, the analyses were done immediately after the measurement. However, the error resulting from oxidation of absorbed SO2 might be significant also for cases where the SO3/SO2 ratio is low, despite an immediate analysis. It should be mentioned that this isopropanol method (EPA Method 8) was developed for measuring sulfur oxide emissions from sulfuric acid manufacturing plants where the SO3/SO2 ratio is high. To conclude, the SO3 concentrations obtained with the isopropanol absorption bottle method was roughly similar to those obtained with the controlled condensation method when the sampling time was shortened and argon bubbled through the isopropanol solution. However, it was not possible to measure the SO2 concentration correctly, due to the large amount of SO2 trapped in the isopropanol solution. For other conditions, a significant error in SO3 concentrations may occur due to the interactions with SO2, as discussed. Pentol SO3 Monitor. For investigation of possible interactions of SO2 with isopropanol in the Pentol SO3 monitor, measurements were performed without combustion in the test unit to provide an almost SO3 free gas. A mixture of air and SO2 (1000 ppm) was added to the warmed-up furnace and measurement AR was performed with the controlled condensation method. Less than 1 ppm of SO3 was detected with the controlled condensation method, which is in line with the purity

significant slip of SO3/H2SO4 during the first salt filter and that there cannot be a significant reaction of SO2 with NaCl. Surprisingly, the amount of sulfate found in the second salt tube applied in the air-fired case is higher than the amount of sulfate found in the second salt filter from the OF30 case, despite an almost three times higher SO2 concentration in the OF30 case compared to the air-fired case. This indicates that the sulfate content found in the second salt filter comes mainly from a slip of SO3/H2SO4 during the first salt filter and not from reaction between NaCl and SO2. In a laboratory study by Cooper et al.,26 the capture of SO2 in NaCl was found to be negligible. In their study, the gas flow was 1 L/min (STP), the SO2 concentration was 294 ppm, the sampling time varied between 41 and 79 min, and the temperature in the salt was 200 °C. Future work will investigate possible interactions of SO2 with NaCl within the setup used in this paper. Additional measurements were performed in which the salt method was applied directly after measuring with the controlled condensation method with the aim to minimize the effect of operation time on SO3 formation in the test unit (due to practical reasons, it was not possible to apply both measurement techniques simultaneously). The measured SO3 concentration for the OF35 case was similar for both methods; see Table 3 measurement set AE and AF. The value from the salt method was 9% higher than the measured SO3 concentration with the controlled condensation method, which is probably mainly the result from the increased furnace average temperature (15 K higher), since the salt method was used around 1 h later than the controlled condensation method. It should be mentioned here that potassium chloride (KCl) was used as sorbent. A detailed investigation of the salt method with different salts as absorbent will follow as a separate publication. For the OF40 case (measurement AH), an additional quartz wool filter was used upstream of the salt filter, due to the considerable soot formation. The quartz wool filter was black and entirely covered by soot after the measurement, and a significant amount of sulfate was found in the filter. The SO3 concentration was 79 ppm measured with the salt method (KCl as absorbent), when the amount of sulfate in the additional quartz wool filter was also taken into account. The amount of sulfate found in the filter corresponds to 14% of the total SO3 concentration. The relatively high amount of sulfate found in the filter shows the risk of losing a part of the SO3/H2SO4 in a filter when measuring in a dust-laden flue gas. However, it is unclear if all sulfate found in the filter occurs from SO3/H2SO4. The main conclusion from the tests with the salt method is that it gives comparable results with the controlled condensation method and that both are applicable during air-firing as well as oxy-fuel combustion conditions. The salt method is, compared to the controlled condensation method, a less expensive and simpler method. Isopropanol Absorption Bottle Method. The isopropanol absorption bottle method was used during the air-fired case 5545


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measurements with the controlled condensation method. The strong peaks in SO3 concentration (see Figure 11) obtained from the SO3 monitor were not taken into account when the average concentration was calculated (Figure 12a), because the SO3 monitor was partly out of range. In Figure 12b, the average furnace wall temperature is shown for each measurement. The SO3 concentrations measured with the SO3 monitor were slightly higher than those obtained with the controlled condensation method. However, there is no correlation between this difference and the average furnace wall temperature. After switching off the unit and taking out the probe, it took a long time for the SO3 monitor to come down again to zero ppm in SO3 concentration. For example, one hour later after finishing the measurements in the OF30 case (measurement AJ), the monitor still showed an SO3 concentration of 60 ppm. Since the measurements with the controlled condensation method and the SO3 monitor in Figure 12 were carried out on different days also some measurements were performed in which both methods were applied successively. The results are listed in Table 5. The difference between the two methods was, in

for the SO2 supplied from the gas cylinder (with the given minimum purity of 99.8% for the SO2 the measured SO3 concentration should be below 2 ppm). However, the following measurement AS with the Pentol SO3 monitor resulted in a measured SO3 concentration of almost 10 ppm. This means that SO2 may interfere with the SO3 measurement when using the SO3 monitor. Figure 11 shows the measured SO2 concentrations with the NDIR-analyzer and the measured SO3 concentrations with the

Table 5. Comparison between Pentol SO3 Monitor and Controlled Condensation Method

Figure 11. Continuously measured SO3 concentration with the Pentol SO3 monitor in the OF30 case with different SO2 concentrations (measurement set AI).

measurement

case

AL/AM

OF30 1500 ppm SO2 OF30 OF30

AJ/AK AN/AO

Pentol SO3 monitor for OF30 operation. The measurement started with an SO2 concentration of around 200 ppm and was increased during the experiment up to 3000 ppm SO3 on a dry basis. In Figure 12a the average SO3 concentration measured with the SO3 monitor from Figure 11 and from a second measurement sequence is shown and compared to earlier

SO3 concn. [ppm] from Pentol monitor and avg. furnace wall temp.

SO3 concn. [ppm] from controlled condensation and avg. furnace wall temp.

difference [%]

38 (528 °C)

43 (509 °C)

−13

84 (530 °C) 65 (544 °C)

75 (535 °C) 87 (548 °C)

12 −26

average, below 20%. With the controlled condensation method, a higher SO3 concentration was measured when the average furnace wall temperature was increased (see Table 5), which is in line with the experience from the SO3 measurement campaign (see Figure 9). However, with the SO3 monitor, the measured

Figure 12. Comparison of the Pentol SO3 monitor (average values) with the controlled condensation method in the OF30 case with different SO2 concentrations (measurements B, C, AI, AJ, AL, and AN in Table 3), (a) SO3 concentration versus SO2 concentration and (b) average furnace wall temperature. 5546


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the air-fired case with the low SO2 concentration (here, 530 ppm SO2 on a dry basis) and for the air-fired case with 1000 ppm SO2 in measurement port M9 and M10. Measurement port M10 was used because the flue-gas temperatures were expected to be more suitable for the dew-point monitor than the temperatures in M9, which can be above the specified maximum operational temperature of 400 °C for the acid dew-point probe. The results are summarized in Table 6. The measured acid dew-point is converted to concentrations using the correlation

SO3 concentration was lower when the average furnace wall temperature was increased. A hysteresis behavior of the SO3 monitor could be observed, that is, the measured SO3 concentration depended on the previously recorded SO3 concentration data with a direct influence on the measurement result. Figure 13 shows the SO2 concentration measured with the NDIR-instrument and the SO3 concentration measured with the

Table 6. Measured Acid Dew-Point Temperature for the AirFired Case and the Resulting SO3 Concentrations on a Wet Basis test case and measurement position measurement avg. furnace wall temp. meas. flue-gas temp. meas. acid dewpoint temp.

air low SO2, furnace outlet (M9)

air, furnace outlet (M9)

air, after cooler (M10)

air, after cooler (M10)

W 460 °C

Z 503 °C

AA 508 °C

L 506 °C

400 °C

446 °C

201 °C

199 °C

141, 144, 149, and 140 °C

153 °C

11−27

40

29−75

114

121 and 124 °C 132, 134, and 136 °C Calculated SO3 Concentration [ppm] after Verhoff and ∼2 5−8 Banchero40 after Bolsatis and 3−4 12−18 Elliott7

Figure 13. Measured SO3 concentration with the SO3 monitor and the controlled condensation method for the air-fired case with 500 ppm and 1000 ppm SO2 on a dry basis.

given by Verhoff and Banchero.40 Additionally, a more recent study by Bolsaitis and Elliott7 is used. The model of Bolsaitis and Elliott7 is based on an ideal treatment of the gas phase for the SO3−H2O system. In Figure 14, the acid dew-point temperature

SO3 monitor during air firing. One measurement performed with the controlled condensation is also shown. In the air-fired case with the low SO2 concentration an SO3 concentration of 27 ppm was measured with the SO3 monitor (measurement AP) and somewhat later an SO3 concentration of 35 ppm with the controlled condensation method was measured (measurement AQ). When the SO2 concentration was increased to 1000 ppm, the SO3 concentration measured with the SO3 monitor increased continuously and the SO3 monitor went out of range, and it took several hours for the SO3 concentration to come down again. It was important not to switch off the SO3 monitor too early after the measurements. Otherwise, too high SO3 concentrations would be obtained next time since potentially absorbed SO2 in the tubing would have time to oxidize to sulfate. The air case was repeated on the next day. The SO3 concentration measured with the Pentol SO3 monitor was 12 ppm (average furnace wall temperature 377 °C, measurement AT) and the SO3 concentration measured with the controlled condensation method (average wall temperature 412 °C, measurement AU) was 17 ppm. These values are quite low, compared to previous SO3 measurements during the air fired case, because of a significantly colder furnace. It can be concluded that it was possible to measure the SO3 concentration during air firing and oxy-fuel operation with the Pentol SO 3 monitor. The difference in measured SO 3 concentrations between the controlled condensation method and the Pentol SO3 monitor was in average less than 20%. Acid Dew-Point Temperature Monitor. The acid dewpoint temperature was measured during the two air-fired cases with the acid dew-point monitor from Land Instruments. In addition to the acid dew-point temperature, the flue-gas temperature was measured with the acid dew-point monitor. The measurements were conducted in measurement port M9 for

Figure 14. Sulfuric acid dew-point temperature for the air case (12 vol. % H2O), according to Verhoff and Branchero40 and Bolsaitis and Elliott.7

obtained after Verhoff and Branchero40 and Bolsaitis and Elliott7 are compared for the air case. The measured acid dew-point temperature of 144 °C (measurement AA) is shown as an example. It is obvious that these two correlations predict very different SO3 concentrations: 16 ppm with Verhoff and Branchero40 and 45 ppm with Bolsaitis and Elliott.7 This means also that the prediction of the acid dew-point from the SO3 concentration is dependent on the method used. In general, the SO3 concentration is sensitive to the acid dew-point temperature. For example, an acid dew-point temperature of 153 °C corresponds to a 50% higher SO3 concentration than an acid dew-point temperature of 149 °C for both correlations. 5547


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It is thus relevant to discuss the applicability and reliability of the two dew-point correlations. The work of Bolsaitis and Elliott7 has a wider objective compared to the empirical correlation given by Verhoff and Branchero,40 which is based on measurements relevant for flue-gas conditions. On the one hand, it is possible that the correlation by Bolsaitis and Elliott7 does not perfectly reproduce the relation between the H 2 SO 4 and H 2 O concentration and the acid dew-point temperature in flue gases, due to optimization in the model. On the other hand, the formula given by Verhoff and Branchero40 should not be regarded as a fundamental relation; in addition, the calculation of SO3 concentrations from acid dew-point temperatures is very sensitive, as discussed. As a result, for the acid dew-point temperatures measured in the present study, it can be observed with respect to some cases that the correlation of Verhoff and Branchero40 results in too low SO3 concentrations, whereas the correlation of Bolsaitis and Elliott7 results in too high SO3 concentrations. Higher acid dew-point temperatures were measured in port M10 than in M9, despite a fraction of the H2SO4 being removed by the flue-gas cooler (situated between M9 and M10) due to condensation of H2SO4 on the relatively cold surfaces of the fluegas cooler. For example, a concentration of 41 ppm SO3 was measured in port M8 with the controlled condensation method (measurement M), but only an SO3 concentration of 27 ppm was measured two hours later in port M10 with the controlled condensation method (measurement N). The low acid dewpoint temperatures measured in port M9 can be due to the high flue-gas temperature in the flue-gas channel, leading to a large temperature gradient between the sensor and the flue gas, resulting in a difference between the real temperature of the sensor surface and the measured one. Another reason might be that the conversion of SO3 to H2SO4 is not sufficiently fast at such high temperatures to form sulfuric acid on the sensor surface. It should be mentioned here that in measurement Z the temperature in the flue-gas channel where the acid dew-point probe was inserted was almost 50 °C above the specified maximum operational temperature of 400 °C for the acid dewpoint probe. The acid dew-point temperature monitor was also used during oxy-fuel operation. In Table 7, the results from acid dew-point temperature measurements are shown for all oxy-fuel cases. Table 7 shows further the SO3 concentrations measured with the controlled condensation method and the salt method as well as the average furnace wall temperature and the flue-gas temperature in M9 obtained with the acid dew-point temperature monitor. The acid dew-point temperature was measured in port M9, and the acid dew-point monitor worked somewhat better than in the air-fired case, probably because the flue-gas temperature in the furnace outlet was lower in the oxy-fuel cases compared to the air-fired cases. To conclude, the difference between the calculated SO3 concentration from the measured acid dew-point temperatures and the results from the SO3 measurement methods are large. Reasons for this could be a high temperature difference between the flue-gas temperature in M9 and the acid dew-point temperature, especially for the air-fired case. Koebel and Elsener38 pointed out that there is a difference between the true dew-point temperature and the surface temperature of the probe. Further, the small flue-gas channel (107 mm) combined with a relatively low flue-gas velocity did not yield the best measurement conditions (since the rate of deposition increases with gas velocity6). On the other hand, the relatively high SO3

Table 7. Measured Acid Dew-Point Temperature in Port M9 for the Four Oxy-Fuel Cases and Compared to the Measured SO3 Concentrations with the Controlled Condensation Method and Salt Method oxy-fuel case

OF30

OF25

controlled AC condensation meas. 9 meas. SO3 concn. [ppm] avg. furnace wall 463 °C temp. salt method meas. meas. SO3 concn. [ppm] avg. furnace wall temp. acid dew-point R AB monitor meas. avg. furnace wall 499 °C 446 °C temp. meas. flue gas 369 °C 352 °C temp. measured acid 153− 123−133 °C dew-point 158 °C temp. Calculated SO3 Concentration [ppm] after Verhoff and 27−43 1.4−4.1 Banchero40 after Bolsaitis 70−115 2−10 and Elliott7

OF35

OF40

AE 24.8 508 °C AF

AH

27

79

523 °C

543 °C

AD

AG

501 °C

540 °C

366 °C

377 °C

154 °C

156−164 °C

26

27−61

66

66−142

concentration, the high water concentration, and the particle-free environment should simplify the acid dew-point temperature measurement. Further, large differences in the correlation of acid dew-point and SO3 concentrations exist, which makes the conversion of acid dew-point to SO3 concentration or vice versa challenging.

5. CONCLUSIONS A comparative study of four SO3 measurement techniques applied during air-firing and oxy-fuel combustion is presented. The techniques used are the controlled condensation method, the salt method, the isopropanol absorption bottle method, and the Pentol SO3 monitor. The controlled condensation method was used as a reference method for comparison with the other techniques. Additionally, the acid dew-point temperature was measured with an acid dew-point monitor from Land Instruments. From the comparative study, it can be concluded that the salt method is suitable for SO3 measurement. The Pentol SO3 monitor is recommended if long continuous SO3 measurements are of interest. The SO3 concentrations measured with the Pentol SO3 monitor differed in average less than 20% from the SO3 concentrations obtained with the controlled condensation method. The isopropanol absorption bottle method is not recommended for measuring SO3 in flue gases, since SO2 can give a positive bias to the SO3 concentration. The acid dew-point meter seemed to give more accurate readings at flue-gas temperatures close to the acid dew-point temperature than at higher flue-gas temperatures. However, the estimation of the SO3 concentration based on the acid dew-point measurement is not recommended due to possible uncertainties in the (1) acid dewpoint measurement and (2) calculation of the SO3 concentration. 5548


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IEAGHG Special Workshop on Oxyfuel Combustion , London, Jan. 25− 26, 2011. (22) Cao, Y.; Zhou, H.; Jiang, W.; Chen, C.; Pan, W. Environ. Sci. Technol. 2010, 44, 3429−3434. (23) Maddalone, R. F.; Newton, S. F.; Rhudy, R. G.; Statnick, R. M. J. Air Pollution Control Assoc. 1979, 29, 626−631. ́ (24) Kelman, F. N. Zavodskaya Lab. 1952, 11, 1316−1318 (in Russian). (25) Roiter, V. A.; Stukanovskaya, N. A.; Korneichuk, G. P.; Volikovskaya; Golodets, G. I. Kinet. Katal. 1960, 1, 408−417 (translation). (26) Cooper, D.; Ferm, M. Jämförelse av Mätmetoder för Bestämning av SO3 Koncentrationer i Rökgaser, Värmeforsk report 494; Värmeforsk and Institutet för Vattenoch Luftvårdsforskning (IVL): Göteborg, 1994. Available online: http://www.varmeforsk.se/rapporter?action= show&id=640). (in Swedish) (27) Cooper, D. Optimization of a NaCl Adsorbent Tube Method for SO3 Measurements in Combustion Flue Gases, Report B-1177; Institutet för Vattenoch Luftvårdsforskning (IVL): Göteborg, 1995. (28) Cooper, D.; Andersson, C. Bestämning av SO3 i Rökgaser med NaCl-metodenen Jämförelse av Olika Metoder, Värmeforsk Report 616; Institutet för Vattenoch Luftvårdsforskning (IVL): Göteborg 1997. Available online: http://www.varmeforsk.se/rapporter?action= show&id=1808. (in Swedish) (29) Pentol GmbH, Degussaweg 1, D-79639 Grenzach-Wyhlen, Personal communication with Blauenstein, O. Available online: http:// www.pentol.com/so3_monitoring.htm. (30) Jackson, P. J.; Hilton, D. A.; Buddery, J. H. J. Inst. Energy 1981, 54, 124−135. (31) (a) ASTM. http://www.astm.org/DATABASE.CART/ WITHDRAWN/D3226.htm (b) BSI. http://shop.bsigroup.com/en/ ProductDetail/?pid=000000000000052803 (32) Denne, C.; Himes, R. Continuous Measurement Technologies for SO3 and H2SO4 in Coal-Fired Power Plants, Technical Report; EPRI (Electric Power Research Institute): Palo Alto, 2004. (33) EPRI, IMACC, FTIR monitoring of NOx, SOx, SO3, and H2SO4. Environmental Controls Conference, Pittsburgh, PA, May 16−18, 2006. (34) Villinger, J. Personal communication with respect to determine the SO3 concentration by mass spectroscopy, V&F Analyse und Messtechnik GmbH, Andreas-Hofer-Strasse 15, A-6067 Absam, 2008. (35) Meadowcroft, D. B. Mater. Sci. Eng. 1989, 121, 669−675. (36) Stuart, D. D. Continuous measurements of acid dewpoint and sulfur trioxide in stack gases. 101st Air and Waste Management Association Annual Conference and Exhibition, Portland, OR, June 24− 27, 2008. (37) Gustavsson, L.; Nyquist, G. Värmeforsks Mäthandbok; Värmeforsk Service AB: Stockholm, 2005. (in Swedish) (38) Koebel, M.; Elsener, M. Gefahrstoffe−Reinhaltung der Luft 1997, 57, 193−199 (in German). (39) Installation and Operation Manual of Brooks Smart-Series Digital Mass Flow Meters and Controllers; Brooks Instrument: Hatfield, PA, 2003; Appendix A, p 28. (40) Verhoff, F. H.; Banchero, J. T. Chem. Eng. Prog. 1974, 70, 71−72. (41) Johnstone, H. F. University of Illinois Circular No. 20 1929, 1−22. (42) Taylor, A. A. J. Inst. Fuel 1942, 16, 25−28. (43) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2008, 47, 1835−1845. (44) Crumley, P. H.; Fletcher, A. W. J. Inst. Fuel 1956, 29, 322−327.

There were no differences obtained when measuring the SO3 concentration in an oxy-fuel atmosphere or in an air-fired atmosphere. However, it should be noted here that a high water vapor concentration, which occurs for oxy-fuel combustion with wet flue-gas recycle, might complicate the SO3 measurement because of elevated acid and water dew-point temperatures.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +46-(0)31-772-1453. E-mail: daniel.fl[email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support by Vattenfall AB and the Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged. We would also like to thank our colleague Johannes Ö hlin for the technical assistance during the measurements and unit operation.

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REFERENCES

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Evaluation of SO3 Measurement Techniques in Air and Oxy-Fuel

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