Experimental Evaluation and Field Application of a Salt Method for

Article pubs.acs.org/EF

Experimental Evaluation and Field Application of a Salt Method for SO3 Measurement in Flue Gases Emil Vainio,*,† Daniel Fleig,‡ Anders Brink,† Klas Andersson,‡ Filip Johnsson,‡ and Mikko Hupa† †

Process Chemistry Centre, Combustion and Materials Chemistry, Åbo Akademi University, FI-20500 Turku, Finland Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

‡

ABSTRACT: In this study, an SO3 measurement technique was evaluated and developed. In the method, a salt is used to capture gaseous SO3/H2SO4. Various salts were tested to evaluate the suitability to measure SO3/H2SO4 without interference from SO2. Salts tested include NaCl, KCl, K2CO3, and CaCl2. The salts were tightly packed into a Teflon tube, and the gas was fed through the salt tube with subsequent reaction between SO3/H2SO4 and the salt with formation of sulfates of the respective salt. After the measurement, the salt was dissolved in water, and the solution was analyzed for sulfate ions. The SO3/H2SO4 concentration in the flue gas could then be determined because the gas volume flowing through the salt was measured together with the amount of sulfate bound in the salt. The method was tested in laboratory conditions, in a 100 kWth test unit during airfiring and oxy-fuel combustion, and in an industrial boiler. A first attempt to continuously measure SO3/H2SO4 indirectly with an FTIR, by measuring the release of HCl in the sulfation of KCl, was also made. The conversion of SO3 to H2SO4 in flue gas conditions is discussed. It was found that at the measurement conditions almost all SO3 is present as H2SO4. Therefore, the laboratory study was made with gaseous H2SO4 instead of SO3. The laboratory tests showed that all salts captured all H2SO4. The best selectivity toward H2SO4 was shown for NaCl and KCl; no significant amount of SO2 was captured in these salts. An in situ implementation of the salt method using KCl as salt was used during heavy oil combustion in a Kraft recovery boiler. The salt method showed to be an accurate, inexpensive, and easy way to measure SO3/H2SO4 in flue gases. example, Verhoff et al.6 or Bolsaitis et al.7 In addition to LTC, the SO3 level in the furnace may have an impact on the hightemperature corrosion (HTC) in a combustion process.8−10 Several methods for the measurement of SO3 in flue gases exist. The most common SO3 measurement technique is the controlled condensation method that has been recommended by many authors.11−14 This method is based on selective condensation of gaseous sulfuric acid followed by sulfate analysis of the condensate. This method requires a heated probe to draw the gases from the flue gas channel. The gas is led through a cooling coil, which is heated between the acid and water dew point temperature, leading to the condensation of H2SO4 in the cooling coil. In this work, a simple method, the salt method, for the measurement of SO3 is investigated. Promising results were obtained with the salt method during our SO3 measurement campaign where different SO3 measurement techniques were compared.15 A benefit with the salt method as compared to the controlled condensation method is that it can be used in situ,16,17 in such a way that no heaters and heated probe are needed. This minimizes the risk of losing any SO3 due to unwanted condensation in the probe and sample line used in the controlled condensation method.12 18 ́ The salt method was first described by Kelman, who used sodium chloride (NaCl) to capture H2SO4 from flue gases. In the salt method, a flue gas containing H2SO4 is led through a salt plug, for example, NaCl, which is heated above the sulfuric

1. INTRODUCTION During combustion of sulfur (S) containing fuels, sulfur dioxide (SO2) is formed. A small part of the SO2 oxidizes further to form sulfur trioxide (SO3). SO3 can be formed by two mechanisms: homogeneous gas-phase reactions and heterogeneous reactions in the presence of a catalyst. Homogenous oxidation of SO2 takes place by the reaction of SO2 with oxygen radicals1,2 or via HOSO2 as an intermediate.3 The heterogeneous reactions occur in the presence of catalytic surfaces, for example, iron oxides in ashes or deposits in the furnace.4 As the flue gas temperature drops in the convective pass (≤500 °C), SO3 starts to react with water vapor (H2O) to form gaseous sulfuric acid (H2SO4): SO3(g) + H 2O(g) → → H 2SO4 (g)

(1)

The reaction is fast, and thus, often when SO3 measurements are discussed, the discussion refers to the measurement of gaseous H2SO4, which is also the case for the present work. Gaseous sulfuric acid starts to form an acid mist below the acid dew point temperature or condenses on cold surfaces. Severe low-temperature corrosion (LTC) may occur on cold surfaces, for example, economizers, air-preheaters, or flue gas ducts if their surface temperature is below the acid dew point temperature.5 LTC can be avoided by keeping all surfaces above the acid dew point temperature or by using acid-resistant steels. Therefore, it is crucial to determine the acid dew point temperature in the cold end of the flue gas duct. This can be done either by direct measurement of the acid dew point temperature or indirectly by the measurement of SO3 and then estimate the acid dew point by correlations given by, for © 2013 American Chemical Society

Received: February 15, 2013 Revised: April 11, 2013 Published: April 26, 2013 2767

dx.doi.org/10.1021/ef400271t | Energy Fuels 2013, 27, 2767−2775

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calculated for typical flue gas conditions to determine whether SO3 or H2SO4 is present. This was done to determine if a laboratory study using gaseous H2SO4 by evaporating a weak sulfuric acid solution can be justified. An attempt to continuously measure SO3 indirectly, by measuring the release of HCl in the sulfation of KCl, was also made at the 100 kWth oxy-fuel test unit. Finally, the salt method was used in an industrial boiler burning heavy oil, using an in situ implementation of the method.

acid dew point temperature. H2SO4 is captured in the salt by forming sulfate. After the measurement, the salt is dissolved in water and analyzed for sulfate ions. The amount of sulfate found in the solution can easily be converted to H2SO4 concentration, when the volume of flue gas led through the salt and the amount of sulfate in the salt are known. Gaseous H2SO4 reacts with NaCl forming sodium bisulfate (NaHSO4) or sodium sulfate (Na2SO4): NaCl(s) + H 2SO4 (g) → NaHSO4 (s) + HCl(g)

(2)

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

(3)

2. EXPERIMENTAL SECTION 2.1. Experimental Cases and Principles. The salt method was tested in the laboratory conditions, Chalmers 100 kWth oxy-fuel test unit, and in an industrial boiler. In the laboratory study, four salts, NaCl, KCl, K2CO3, and CaCl2, were tested in controlled conditions with known H2SO4 and SO2 concentrations. In the Chalmers oxy-fuel test unit, NaCl, KCl, and K2CO3 were tested during air and oxy-fuel firing in a particle-free environment with propane (C3H8) as fuel and SO2 mixed in the oxidizer to generate SO3. In addition, a first attempt to continuosly measure the H2SO4 concentration indirectly with an FTIR by measuring the HCl formed in the sulfation reaction of KCl was also made. In the field measuremnts, an in situ implementation of the method was used during heavy oil combustion in a Kraft recovery boiler. In all tests, 1.0 g of ultraclean salt, on a dry basis, was packed into a Teflon tube with an inner diameter of 8 mm. Figure 1 shows the salt

NaHSO4 may react further to Na2SO4 by: NaHSO4 (s) + NaCl(s) → Na 2SO4 (s) + HCl(g)

(4)

Hydrogen chloride (HCl) is released in the reaction between NaCl and H2SO4, and the H2SO4 to HCl ratio is 1:1 in reaction 2 and 1:2 in reaction 3. The favored reaction depends on the temperature. The SO3 concentration in a gas can in principle be continuously measured indirectly by measuring the release of HCl in the sulfation of NaCl. The released HCl can be measured, for example, with an FTIR analyzer. In an expired patent, this release of HCl is utilized.19 In the patent, it is stated that to capture all SO3 the temperature must be kept between 300 and 400 °C in the salt. In the patent, the method was tested in the temperature range of 200−400 °C, and at 200 °C about one-half of the SO3 was shown to be converted to HCl. However, it may be that at lower temperatures reaction 2 is favored over reaction 3; that is, the stoichiometry of SO3 captured and HCl released is 1:1 instead of 1:2. Direct measurement of H2SO4 with an FTIR analyzer has been tested; however, sampling and calibration have shown to be difficult.17 Sulfation of NaCl by SO2 (reaction 5) is an unwanted reaction in the salt method because it leads to a positive measurement bias. Sulfation of NaCl is a relatively slow reaction and depends on, for example, the SO2 concentration and temperature.20

Figure 1. Salt tube used in the experiments. tube used in this study. The salt was heated well above the acid dew point temperature to about 200 °C in the measurements. The flow through the salt was 1 L/min, and the measurement time was 30 min in all experiments. After the experiments, the salts were dissolved in deionized distilled water, and the solutions were analyzed with ion chromatography for sulfate ions. The solutions were diluted so as not to overload the chromatography column. Three injections were made per sample in the IC analysis to prove the repeatability. A Metrosep anion Dual 2 column and a 732 IC detector by Metrohm were used to analyze the samples from the laboratory tests and field measurements. The samples obtained during the measurements in the Chalmers oxyfuel unit were analyzed with an ICS-90 ion chromatography system from DIONEX. The detection limit is determined by the sampling time and the degree of salt dilution required to avoid overloading the IC column. The detection limit with the current setup corresponds to an SO3 concentration of approximately 0.1 ppmv in the flue gas. 2.2. Laboratory Study. Four salts, NaCl, KCl, K2CO3, and CaCl2, were tested in controlled conditions using a synthetic flue gas of varying concentrations of H2SO4 and SO2. The SO2 concentration in the synthetic flue gas was verified with an SO2 analyzer (ABB AO2020 analyzer). The concentrations of CO2, O2, H2O, and N2 were kept the same for all cases and represent a typical flue gas composition. The test cases can be seen in Table 1. At least three repetitions were made for every case and salt. In case 1, the ability to capture H2SO4 was tested with a H2SO4 concentration of 50 ppmv in the absence of SO2 in the synthetic flue gas. In these experiments, two serial salt tubes were used to ensure that 1.0 g of salt was sufficient to capture all H2SO4 in the first salt tube when the sampling time was set to 30 min. To check if any SO2 is captured in the salts, 500 ppmv of SO2 was led through the salts in case 2. Finally, in case 3, the selectivity of the salts was tested by the inclusion of both 50 ppmv of H2SO4 and 500 ppmv of SO2 in the synthetic flue gas.

4NaCl(s) + 2SO2 (g) + 2H 2O(g) + O2 (g) → 2Na 2SO4 (s) + 4HCl(g)

(5)

The salt method using NaCl has been evaluated by Cooper et al.12,16,17 and in our previous experimental study.15 The conclusion was that the method has great potential, but the method needs to be studied further. Silver nitrate (AgNO3) impregnated filters have also been used to measure SO3 in sulfuric acid plants.16 Cooper et al.12,17 made an attempt to test the salt method using NaCl in the laboratory with known SO3 concentrations; however, problems with condensation of sulfuric acid in the equipment and verification of the true SO3 concentration were encountered. This study is a continuation of our previous work in which different SO3 measurement techniques were compared.15 The objective of the present study was to evaluate the salt method and to test different salts, which were NaCl, potassium chloride (KCl), potassium carbonate (K2CO3), and calcium chloride (CaCl2). This was done both in laboratory conditions with a known H2SO4 concentration in a synthetic flue gas and in the Chalmers 100 kWth oxy-fuel test unit during air-firing and oxyfuel combustion. The selectivity of the salt method toward H2SO4 was tested in the laboratory with a synthetic flue gas containing both H2SO4 and SO2. The reaction time and the equilibrium distribution in an SO3−H2SO4 system were 2768


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downstream of the furnace. The measurement setup is illustrated in Figure 3. In addition to the SO3 measurements, the concentration of SO2 was determined by bubbling the flue gas through two impingers filled with a 3 vol % hydrogen peroxide (H2O2) solution. After gas sampling, the sulfate concentration in the H2O2 solution was determined by titration. The measured SO3 concentrations with the salt method were compared to results obtained with the controlled condensation method. The controlled condensation method used here is described by Fleig et al.15 During our SO3 measurement campaign,15 it was observed that the temperature level in the furnace strongly affected the formation of SO3. Therefore, the time from start-up of the unit and the average temperature of the furnace wall was taken into account when comparing the measured SO3 concentrations. The average temperature of the furnace wall was defined as an average of 14 continuously logged thermocouples placed 2 cm from the inner surface of the furnace wall. A first attempt to continuously measure the SO3 concentration indirectly with an FTIR analyzer was made. The flue gas was led through a salt tube containing KCl, and the SO3 concentration was determined by the HCl formed in the sulfation of the salt. It should be mentioned that the HCl formed in combustion must be taken into account when determining the SO3 concentration. However, the flue gas in the present experiments did not contain any HCl because propane was used as fuel. 2.4. Field Measurements. The field measurements were performed in the middle of a start-up procedure in a Kraft recovery boiler. During the start-up, heavy oil was combusted, and the amount of SO3 formed during the start-up was measured with an in situ implementation of the salt method. The measurements were conducted at two locations: below the super heater tubes in the furnace and close to the economizer tubes. The measurement locations and the temperature of those locations are schematically shown in Figure 4. The combustion conditions were exceptional during the measurements; the oxygen content in the flue gas was about 19.5 vol %, and the temperature in the furnace was low. The highly diluted flue gas complicated the measurement due to a lowered concentration of SO3. Additionally, the flames of the oil-burners were heavily sooting, and a particle filter in the probe tip had to be used. The concentration of SO2 varied between 36 and 38 ppmv on a wet basis during the measurements. The measurement setup is shown in Figure 5. A 2 m long probe was used in the measurements, and the salt tube was placed in the probe tip. Salt tubes with KCl were prepared in the laboratory before the measurements. A particle filter of quartz wool was used prior to the salt tube. A fresh quartz filter was used in every measurement, to minimize any reaction between SO3 and soot collected in the filter. Both the quartz filter and the salt tube were separately analyzed for sulfate ions to evaluate any SO3 captured in the quartz filter. To minimize particle contamination of the filter, the probe tip was directed in the opposite direction of the flue gas flow. The probe lining was of Teflon to avoid any reactions between SO3 and the probe

Table 1. Gas Composition for the Three Laboratory Cases case 1 H2SO4 (ppmv) SO2 (ppmv) H2O (vol %) CO2 (vol %) O2 (vol %) N2 (vol %)

case 2

case 3

500 15 14 4 67

50 500 15 14 4 67

50 15 14 4 67

NaCl was chosen as one of the salts because it has been used in previous studies.12,16−18 KCl was chosen because when measuring in conditions with high amounts of NaSO4 in the fly ash (e.g., Kraft recovery boilers), any contamination to the salt can be detected by analyzing the amount of sodium found in the salt. K2CO3 was chosen because of the possibility to measure H2SO4 and HCl in a gas simultaneously, by the formation of K2SO4 and KCl. CaCl2 was included in the test matrix because it may enable the indirect measurement of H2SO4 by the detection of HCl in the sulfation of the salt. CaCl2 does not form bisulfates when H2SO4 reacts with the salt, as is the case for NaCl and KCl, and the ratio of captured H2SO4 to formed HCl is always 1:2. The experimental setup is presented in Figure 2. The flows of SO2, CO2, O2, and N2 were controlled by mass flow controllers (Bronkhorst el-flow). These gases were preheated to 200 °C before being transported into a mixing chamber. The H2SO4 and H2O were generated by evaporating a 0.0185 M sulfuric acid solution in the mixing chamber. The solution was pumped into the mixing chamber with a syringe pump (Perfusor compact S by Braun). The mixing chamber was located in a tube furnace, which was heated to 250 °C. The salt tube was heated well above the dew point temperature to 200 °C in a second tube furnace. Only inert materials were used in the experimental setup to avoid losing any H2SO4; the mixing chamber was made of glass, and the rest of the setup was Teflon. An impinger bottle filled with water was located after the salt to collect any H2SO4 that had not reacted with the salt. 2.3. Air-Firing and Oxy-Fuel Tests. The salt method was tested in the Chalmers 100 kWth oxy-fuel test unit as a part of our SO3 measurement campaign15 in which different SO3 measurement techniques were evaluated. The test unit was fired with propane with a heat input of the fuel of 60 kWth based on the lower heating value. SO2 was injected in the oxidizer to generate SO3 in the furnace. The salt method was tested during air-fired and oxy-fuel conditions. A detailed description of the test conditions, the Chalmers oxy-fuel test unit, and the measurement equipment is given in our previous study.15 Table 2 shows the flue gas composition in the air-fired and oxy-fuel cases. During air-firing, the SO2 concentration in the stack was adjusted to 1000 ppmv, on a dry basis. For the oxy-fuel case, the O2 concentration in the feed gas was adjusted to 30 vol %, and the SO2 concentration was set to 3000 ppmv in the stack on a dry basis. Three different salts were tested: NaCl, KCl, and K2CO3. The flue gas was sampled with an oil-heated probe heated to 200 °C,

Figure 2. Experimental setup in the laboratory tests. 2769


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Table 2. Experimental Conditions for the Test Cases in the 100 kWth Test Unit flue gas composition on wet basis test case

O2 in feed gas (vol %, d.b.)

SO2 in the flue gas (ppmv, d.b.)

O2 (vol %)

SO2 (ppmv)

H2O (vol %)

CO2 (vol %)

air oxy-fuel

21 30

1000 3000

5.39 5.39

890 2440

12 18.7

8.6 ∼71

Figure 3. Measurement setup of the salt method in the simultaneous measurement of SO3 and SO2, at the 100 kWth test unit. flue gas temperature. An SO2 measurement and a drying unit were placed after the probe. An airtight pump was used to draw the gases from the flue gas duct. The flow was controlled with a valve and set to about 1 L/min. The flow was measured with a flow meter.

3. RESULTS AND DISCUSSION 3.1. Laboratory Tests. The results from the laboratory experiments will be presented in this section: first the ability of the salts to capture H2SO4 is presented (case 1), then the interference from SO2 is shown (case 2), and last the selectivity of the salts toward H2SO4 in a gas containing both SO2 and H2SO4 is shown (case 3). The reaction between SO3 and H2O in typical flue gas conditions is also discussed. Figure 6 presents the results from the laboratory experiments corresponding to the conditions of case 1 (see Table 1), with 50 ppmv of H2SO4 in the synthetic flue gas. It can be seen that all salts were able to capture all H2SO4. No sulfate was found in the second salt tube or in the impinger bottle. In previous studies by Cooper et al.,12,17 problems with condensation of H2SO4 in the setup and verification of the true H2SO4 concentration have been observed, but no condensation was

Figure 4. Measurement locations and temperature in the measurement locations in the field measurements. material. A thermocouple was placed in the salt tube, and the measurements were started when the salt temperature had reached the

Figure 5. In situ measurement setup in the field measurements. 2770


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in Figure 8. Both NaCl and KCl showed high selectivity toward H2SO4 as expected from the results in cases 1 and 2 (Figures 6

Figure 6. Average H2SO4 concentration measured with the various salts in a synthetic flue gas containing 50 ppmv H2SO4 in the absence of SO2 (case 1). The standard deviation is determined from at least three measurements.

observed in the present setup. Also, in the present setup the measured H2SO4 concentration was stable, which can be seen from the small standard deviation in the measurements. In case 2 with an SO2 concentration of 500 ppmv in the synthetic flue gas, NaCl and KCl showed good qualities with respect to SO2 capture; only 0.2% of the SO2 was captured in both salts equivalent to 1 ppmv SO2 in the feed gas. As can be seen in Figure 7, a considerable amount of SO2 was captured in

Figure 8. “H2SO4” concentration measured in a synthetic flue gas containing 50 ppmv H2SO4 and 500 ppmv SO2 (case 3). The standard deviation is determined from at least three measurements.

and 7). When using K2CO3 the measured concentration of H2SO4 will be significantly overpredicted because a large amount of SO2 also is being captured in the salt. Thus, K2CO3 is not suitable for measuring H2SO4 in flue gases. Surprisingly, SO2 did not seem to interfere with the H2SO4 measurements when using CaCl2, although the experiment with 500 ppmv SO2 and no H2SO4 (Figure 7) showed that a considerable amount of SO2 was captured. However, we have found no explanation to the fact that SO2 did not interfere in these tests. Therefore, only NaCl and KCl can be recommended in the salt method. In the laboratory study, gaseous sulfuric acid was used instead of SO3. This is motivated by the fast reaction between SO3 and H2O (reaction 1), and thus gaseous H2SO4 is present in a flue gas at 200 °C instead of SO3. The reaction between SO3 and H2O has been studied by many authors and has shown to have a strong negative temperature dependence,22,23 and the reaction order with respect to water is two.22−25 A theoretical study by Morokuma and Muguruma24 showed that the reaction between H2O and SO3 is complex, and a direct reaction with one molecule H2O and one molecule SO3 does not occur. Moreover, the reaction has an intermediate step where H2O and SO3 form an adduct (SO3·H2O), which then reacts with a second water molecule forming H2SO4. The strong negative temperature dependence of the reaction is believed to be due to the formation of this adduct.22 The time for the reaction between SO3 and H2O forming H2SO4 to reach equilibrium as well as the equilibrium distribution of SO3 and H2SO4 were calculated with the rate expression from Lovejoy et al.22 In the calculations of the equilibrium distribution of SO3 and H2SO4, the backward reaction in the formation was included. The calculations were done with the CHEMKIN software. An initial SO3 concentration of 50 ppmv was used, and four H2O concentrations were included: 5, 10, 15 and 20 vol %. The temperature was varied between 150 and 250 °C. The equilibrium conversion of SO3 to H2SO4 is shown in Figure 9a in the temperature range of 150−250 °C and with the various H2O concentrations. The time for the reaction between

Figure 7. Average SO2 captured in the salts, presented as ppmv concentration in the synthetic feed gas. SO2 concentration in the synthetic flue gas was 500 ppmv, and no H2SO4 was present (case 2). The standard deviation is determined from at least three measurements.

the K2CO3 salt tube, about 20% equivalent to 100 ppmv SO2 in the feed gas, while in the CaCl2 about 4% was captured equivalent to 20 ppmv. A higher degree of sulfation of CaCl2 by SO2 as compared to sulfation of NaCl and KCl has also been shown by Matsuda et al.21 in the temperature range of 350− 750 °C. Slow reaction rates for sulfation of NaCl by SO2 have been shown by Boonsongsup et al.,20 in the temperature range of 400−600 °C, and the rate of sulfation of NaCl increased by a factor of 2 by increasing the temperature from 400 to 600 °C. The selectivity toward H2SO4 in a gas containing both SO2 and H2SO4 was examined in case 3, and the results are shown 2771


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Figure 9. (a) Equilibrium conversion of SO3 to H2SO4 and (b) the time for the SO3−H2SO4 system to reach equilibrium, as a function of temperature and H2O concentrations of 5, 10, 15, and 20 vol %. Calculations were done with the rate expression of Lovejoy et al.22 and an initial SO3 concentration of 50 ppmv.

SO3 and H2O to reach equilibrium is shown in Figure 9b. As can be seen from the figures, the reaction reaches equilibrium in a fraction of a second at high H2O concentrations, and almost all SO3 is converted to H2SO4 at equilibrium in the temperature range of 150−250 °C. Thus, it can be assumed that SO3 is rapidly converted to H2SO4 in a flue gas duct of a combustion device with temperature conditions similar to those of Figure 9. Accordingly, the laboratory study of the salt method was done with gaseous sulfuric acid in a synthetic flue gas. Thus, in general, when referring to SO3 measurement techniques, it is in fact gaseous H2SO4 that is measured. 3.2. Measurements in the 100 kWth Test Unit. 3.2.1. Air-Firing and Oxy-Fuel Tests. The measured SO3 concentrations in the air-fired case are plotted as a function of time from start-up of the test unit in Figure 10, in analogy to

leading to a false reading of SO3, as discussed in relation to the laboratory results. The results from the oxy-fuel case are shown in Figure 11. Now the increasing SO3 trend is even more apparent; a much

Figure 11. Measured SO3 concentrations, on a wet basis, with the salt method using NaCl, KCl, and K2CO3 as well as with the controlled condensation method for the oxy-fuel case. The average furnace wall temperature is also plotted.

higher conversion of SO2 to SO3 could be observed toward the end of the measurement day going from 37 ppmv after 4 h from start-up to 96 ppmv after about 14 h from start-up. When the increasing trend is taken into account, it can be seen that both NaCl and KCl give similar results. Three experiments with a second salt tube in series were conducted to evaluate the measurement error from SO2 when using NaCl and KCl in the method. On the basis of the laboratory study, it can be assumed that all SO3 is captured in the first salt tube. In the second salt tube, 2−3 ppmv of SO2 was captured, which means about 0.1% of the total amount of SO2. Also, in this case, the measurement with K2CO3 was affected by SO2, and the measured value was significantly higher as compared to those when NaCl or KCl were used. 3.2.2. Indirect Measurement of SO3 via HCl. The results from the indirect measurement of SO3 by measuring the HCl release in the sulfation of KCl are shown in Figure 12. A blank test was first performed without any addition of SO2, which showed an HCl concentration of around 20 ppmv in the flue

Figure 10. Measured SO3 concentrations, on a wet basis, with the salt method using NaCl, KCl, and K2CO3 as well as with the controlled condensation method for the air-fired case. The average temperature of the furnace wall is also plotted.

the data presented in our previous work.15 Similar SO3 values were obtained when using both NaCl and KCl. In both cases, the concentration of SO3 was somewhat higher toward the end of the measurement day. The results from the salt method using either NaCl or KCl correlate well with the results measured with the controlled condensation method. Considerably higher amounts of sulfate were found when using K2CO3. This was attributed to the reaction of SO2 with K2CO3 2772


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and 1:2 when sulfate is formed. An SO3 concentration of 35 ppmv was measured before the indirect measurement (time from start-up 05:02, measurement AB in Table 3), which would fit the bisulfate curve. The oscillation in the HCl concentration was in phase with the temperature oscillation of the salt tube, caused by the regulation of the heating tape used to keep the salt tube at 200 °C. The temperature in the salt tube varied between 180 °C and 220 °C, and the oscillation in the HCl concentration implies that the reaction between SO3 and KCl is shifting between forming potassium bisulfate (KHSO4) and K2SO4, analogously to reactions 2 and 3. After about 10 h and 30 min from start-up, the SO2 was turned off and the HCl concentration dropped. The oscillation in the HCl concentration after SO2 was turned off might be due to the reaction between KHSO4 and KCl leading to the formation of K2SO4 and HCl. The results indicate that it is possible to quantify the SO3 concentration by measuring the concentration of HCl. However, further work is necessary to clarify the uncertainties in this method as well as to clarify the reaction pathways. 3.2.3. Summary. Table 3 shows each SO3 measurement obtained during the measurements at the Chalmers 100 kWth oxy-fuel test unit with the salt method and with the controlled condensation method. The measurements are numbered by letters and are 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.

Figure 12. Measured HCl concentration downstream of the KCl salt tube and calculated SO3 concentration from the HCl measurement.

gas downstream of the salt tube. The HCl concentration of 20 ppmv measured in the absence of SO3 was used as an offset and was subtracted from the total HCl concentration when the concentration of SO3 was determined. In Figure 12, the SO3 concentration is estimated using the analogous reactions for KCl in reactions 2 and 3. The difference in these is the stoichiometry; SO3 to HCl ratio is 1:1 when bisulfate is formed

Table 3. Summary of Results Obtained during Measurements at the Chalmers 100 kWth Oxy-Fuel Test Unita measurement

test case

SO3 measurement technique

date

time from startup

average furnace wall temp (°C)

SO3 conc. (ppmv, w.b.)

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

oxy-fuel oxy-fuel air air air air air air air air air air air air air oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel oxy-fuel air air oxy-fuel oxy-fuel

CCM CCM CCM CCM CCM salt, NaCl salt, NaCl salt, NaCl salt, KCl salt, KCl salt, KCl salt, K2CO3 salt, K2CO3 salt, K2CO3 CCM salt, NaCl salt, NaCl salt, NaCl salt, KCl tube 1 (serial) salt, KCl tube 2 (serial) salt, KCl salt, KCl salt, K2CO3 salt, KCl tube 1 (serial) salt, KCl tube 2 (serial) salt, NaCl tube 1 (serial) salt, NaCl tube 2 (serial) salt, NaCl HCl measurement downstream KCl salt tube CCM CCM

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

07:05 09:10 08:02 05:35 06:20 03:11 06:39 08:56 03:23 05:22 07:42 01:38 03:08 07:03 08:08 04:03 04:59 06:48 08:14 08:14 09:18 10:30 10:29 12:56 12:56 13:46 13:46 05:02 07:43 09:52 10:37

500 539 508 506 511 461 512 530 468 486 524 426 469 521 529 442 464 496 515 515 526 537 548 563 563 566 566 500

56 61 34 36 38 25 33 35 28 34 41 49 64 72 41 37 40 54 65 3 66 74 127 92 2 96 3 35

535 548

75 87

a

CCM = controlled condensation method. 2773


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significant amount of SO2. When SO2 and H2SO4 were both present in a synthetic flue gas, SO2 did not seem to interfere with the measurement when using CaCl2. More work is needed to understand the reason for this effect. Moreover, both NaCl and KCl showed high selectivity toward H2SO4 in our experimental evaluation of the salt method, and these salts can thus be recommended for measuring the SO3 concentration in flue gases. Both NaCl and KCl in the salt method showed satisfactory results in the air-fired and oxy-fuel combustion cases carried out at the Chalmers 100 kWth oxy-fuel test unit. The results were also in good agreement with the SO3 concentrations obtained with the controlled condensation method. Measurements with K2CO3 in the salt method gave too high SO3 values, indicating that significant amounts of SO2 were captured by the K2CO3. A first attempt to continuously measure SO3 indirectly downstream of a KCl salt tube was made. The principle is based on the continuous measurement of HCl formed by the reaction between the salt and H2SO4. The results indicate that it is in principle possible to quantify the SO3 concentration by measuring the released HCl concentration from the salt; however, further work is necessary to clarify the uncertainties related to this approach. An in situ implementation of the salt method was used in field measurements, where KCl was used as salt and was heated by the flue gas. The in situ implementation of the salt method has some advantages as compared to extractive measurement techniques; there is no need for an external heater nor a heated probe, and the risk of losing any H2SO4 due to condensation in the sample line is minimized. The uncertainty from SO3 reacting with particles in the particle filter was evaluated by analyzing the sulfur content in the filter, and no significant amount of sulfur was found in the particle filter. However, further field measurements with the salt method in different conditions, for example, dust-laden flue gases, combined with the controlled condensation method are required for complete validation of this promising technique.

The time from start-up is the time between start-up of the test unit and the respective measurement. 3.3. Field Measurements. The conditions in the furnace were challenging during the field measurements, due to a highly diluted flue gas, excess O2 was 19.5 vol %, and heavily sooting flames in the oil burners. Figure 13 shows the measured sulfur

Figure 13. Measured SO3 concentration in the flue gas and the amount of sulfur found in the particle filter, presented as ppmv SO3 and S, respectively, in wet flue gases. MP1 was located just below the super heaters, and MP2 was located at the economizer tubes; see Figure 4.

found in the quartz filter, expressed as ppmv S in the flue gas, and the measured SO3 concentration measured in the flue gas. The concentration of SO3 was 1.4 ppmv, below the super heater tubes (MP1), which corresponds to a 4% conversion of SO2 to SO3. A similar empirical conversion at high excess oxygen has been shown by Bennett.26 At the economizers (MP2), the measured SO3 concentration was about 0.5 ppmv. No significant amounts of sulfur, less than 0.2 ppmv in the flue gas, were found in the quartz filters. However, the interference from dust in filters in different flue gases should be studied further. Because of the low SO2 concentration in the furnace, 36−38 ppmv, the error from SO2 can be assumed negligible. Nevertheless, when measuring in flue gases with high SO2 concentration, two serial salt tubes can be used to determine the interference from SO2 by measuring the amount of sulfur in the second tube. For example, in the laboratory experiments with 500 ppmv SO2, about 1 ppmv was captured in the KCl salt tube.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +358407391460. E-mail: emil.vainio@abo.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partly carried out within FUSEC (20112014) as part of the activities of the Åbo Akademi Process Chemistry Centre. Other research partners are VTT, Lappeenranta University of Technology, Aalto University, and Tampere University of Technology. Support from the National Technology Agency of Finland (Tekes), Andritz Oy, Metso Power Oy, Foster Wheeler Energia Oy, UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper Inc., Top Analytica Oy Ab, and Finnish Recovery Boiler Committee is gratefully acknowledged. Other financing has been received from the Graduate School in Chemical Engineering (GSCE) and Vattenfall AB and is gratefully acknowledged. Tor Laurén is acknowledged for the help during the field measurements.

4. CONCLUSIONS The salt method for measuring SO3/H2SO4 in flue gases was evaluated under laboratory conditions, in a 100 kWth test unit during air-fired and oxy-fuel combustion conditions, as well as in field measurements in the flue gas duct and upper furnace of a Kraft recovery boiler. Salts tested in this study include NaCl, KCl, K2CO3, and CaCl2. The time for the reaction of SO3 and H2O to reach equilibrium was calculated to occur in a fraction of a second for relevant flue gas conditions, and H2SO4 is mainly present for equilibrium conditions in the temperature range of 150−250 °C. Thus, it can be assumed that SO3 is converted to H2SO4 when measuring in a flue gas duct of a combustor. Accordingly, the laboratory study of the salt method was done with gaseous sulfuric acid in a synthetic flue gas. The laboratory study showed that all salts were able to capture all H2SO4; however, K2CO3 and CaCl2 also captured a

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