Article pubs.acs.org/EF
Development and Experimental Evaluation of a Continuous Monitor for SO3 Measurement Chenghang Zheng,† Xiang Li,† Zhengda Yang,† You Zhang,† Weihong Wu,† Xuecheng Wu,† Xunhai Wu,‡ and Xiang Gao*,† †
State Key Laboratory of Clean Energy Utilization, State Environmental Protection Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou 310027, Zhejiang, China ‡ Shenzhen Ruijing Environment Science and Technology Company, Shenzhen 518000, Guangdong, China ABSTRACT: The additional use of selective catalytic reduction has increased the amount of SO3 generated by coal-fired power plants. Accurate measurement of SO3 is of great significance for SO3 removal and boiler operation. This work focuses on development and evaluation of an automated continuous monitor designed for SO3 measurement in coal-fired flue gases. This instrument is based on selective absorption of SO3 in isopropanol and the spectrophotometry determination method with data acquired in real time. Considering that flue gas from a typical power plant could contain high levels of particulate matter (PM), moisture, and SO2, which could interfere with the accuracy of SO3 measurement, targeted modifications were made in the design of sampling, determination, and liquid circulation equipment. The accuracy and transient response of the monitor were evaluated in a pilot-scale experimental system with SO3 concentrations from 0 to 60 ppm. The interference of SO2 in the accuracy of this monitor was also evaluated, and it was shown that the error caused by SO2 absorption and oxidization was 5 ppm, and unremoved SO3 becomes an important source of acid rain and condensable fine particulate emissions and causes damage to forests and buildings.5,7,8 When sulfur-containing coal is burned in a coal-fired boiler, the vast majority of sulfur is oxidized into SO2, of which a small fraction of approximately 0.5−1.5% is further oxidized to form SO3.8 The concentration of SO3 in the flue gas of a typical power plant is typically in the range of 1−25 ppm, but in a unit that burns high-sulfur coal, SO3 emissions can exceed 50 ppm.9 Recently, under stricter nitrogen oxide emission policies, the additional use of a selective catalytic reduction (SCR) system could substantially promote SO3 to higher concentration levels © XXXX American Chemical Society
of approximately 0.25−1.5% and SO2 could be further oxidized to SO3 because of the catalytic effect of SCR.5,10,11 Measurement of SO3 is not a straightforward process because of its low concentration levels and reactive nature, and the results might be biased because of wall losses and SO2 oxidization,16 especially because tests in coal-fired power plants are always performed under dust-laden, high-moisture content, and high-SO2 atmospheric conditions. Certain methods of SO3 measurement are summarized in Table 1. The most commonly used methods are the controlled condensation method and isopropanol (IPA) absorption bottle method, which are both extractive techniques. The controlled condensation method is currently believed to be one of the best methods for accurate measurement of sulfuric acid in flue gas and is commonly used as the standard for comparison. Maddalone et al.17 evaluated the controlled condensation method and obtained acceptable results. Spörl et al.18 investigated SO3 emissions and removal by fly ash in coal-fired oxy-fuel combustion using the controlled condensation method and found that higher ash contents were beneficial for capturing SO3 in a baghouse filter system. The controlled condensation method was also adopted by Belo for SO3 measurement in research on high-temperature SO2/SO3 conversion and the catalytic effect of fly ash on SO 3 formation.19 However, poor repeatability was also reported between different operators, which required strict attention to sampling and analysis techniques. The IPA method is another Received: April 25, 2017 Revised: July 5, 2017 Published: July 11, 2017 A
DOI: 10.1021/acs.energyfuels.7b01181 Energy Fuels XXXX, XXX, XXX−XXX
Article
widely used method for SO3 measurement and is recommended as the standard by the U.S. Environmental Protection Agency (U.S. EPA) and selected by some other countries. Certain researchers noted that oxidized SO2 can be absorbed in the IPA bottle method and might introduce non-negligible errors.8,20 Other methods, such as the salt method and acid dew point method, have also been proposed for measurement of SO3. Vainio et al.21,22 evaluated the salt method in both laboratory experiments and field applications using an oxy-fuel combustion boiler and obtained satisfactory results. Fleig et al.23 compared the dew point meter method with the condensation method and the IPA absorption bottle method during air-fired and oxyfuel combustion, and the results showed that the accuracy of the dew point method decreased when the temperature was not close to the acid dew point because of possible uncertainties in the acid point measurement and calculation of the SO3 concentration. Continuous measurement might aid in instructing SO3 removal and reducing operational costs in certain cases.9,24,25 In recent years, continuous SO3 measurement has attracted an increasing amount of attention. Potentially more rapid optically based instrumental methods have been reported. The EPRI has conducted selected first studies of Fourier transform infrared spectroscopy (FTIR) and differential optical absorption spectroscopy (DOAS) for continuous SO3 measurement. However, only gaseous components could be detected with these optically based methods, and the accuracy could also be affected by PM and other gases. This, the complex system requirements and complicated operation procedures limit these techniques from being more widely used.26 A continuous SO3 monitor was invented by Pentol GmbH according to the prototype described by Jackson and Hilton.27 The monitor was used by certain researchers and corporations for continuous SO3 measurement, but the accuracy of the Pentol monitor was reported to be relatively low for high-SO2 and saturated atmospheres.19 In this work, a continuous monitor for SO3 measurement of flue gases in coal-fired utility boilers was developed on the basis of the selective absorption of SO3 in an IPA/water solution. To further increase the accuracy and adaptability of the monitor, targeted designs were introduced in the design of the sampling, determination, and liquid circulation equipment to minimize the interference of PM, SO2, and moisture. The performance of this monitor was evaluated using both laboratory experiments and field applications. A comparison experiment for the monitor and the controlled condensation method was conducted during a field evaluation study to verify the reliability and accuracy of the developed monitor.
detection of the ultraviolet to visible-light molecular absorption spectra of the measured gas
extraction of the flue gas through a probe (200 °C); absorbance of the SO3 IPA solution without cooling and using the reaction of SO42− with barium salts, determination of the reaction products by spectrophotometry 2 min
in situ real time
Fourier transform infrared spectroscopy (FTIR) differential optical absorption spectroscopy (DOAS) Pentol monitor
in situ real time
fast response, non-intrusive measurement, continuous measurement, complicated operation, complex system, interference by PM, SO2, and droplets fast response, non-intrusive measurement, high sensitivity, complicated operation, complex system, interference by PM, SO2, and droplets continuous measurement, convenient operation and control, prone to SO2 oxidation
0.5−1 h salt method
introducing the gas sample to a salt plug heated to a temperature higher than its acid dew point; employing the reaction of salt and gaseous H2SO4 [e.g., NaCl(s) + H2SO4(g) → NaHSO4(s) + HCl(g)]; dissolving the salt and determining the SO42− concentration by titration with barium salts or ion chromatography analysis of the infrared spectrum of a given gas sample by calculating the ratio of the signal obtained from scanning air to the signal obtained from scanning the sample gas
0.5−1 h dew point meter
determination of the acid dew point with a dew point meter and conversion of the dew point to SO3 concentration with a known H2O concentration
1−2 h IPA absorption bottle method
separation of SO3 from flue gas based on selective absorption of SO3 by bubbling through an IPA solution, which also inhibits oxidation of SO2; determination of the SO42− concentration by titration with barium salts or ion chromatography
fast response; lower accuracy; gas purity requirement; interference by HF, HCl, and charged particles; dust build-up problems easy for field measurements, low detection limit
U.S. EPA method 8;12 Standard BS 17564:197713 U.S. EPA method 8a;14 German standard VDI 246215
comments
high reliability and adaptability, free from SO2 and PM interference, low detection limit, precise operation requirement large measuring range, low detection limit, high repeatability, prone to SO2 oxidation
principle
cooling of the extracted flue gas through a snake-collecting pipe at temperatures between its acid dew point and water dew point; determination of the SO42− concentration by titration with barium salts or ion chromatography 1−2 h controlled condensation method
time required method
Table 1. Comparison of SO3 Measurement Methods
standard
Energy & Fuels
2. METHODS 2.1. Description of the Continuous Monitor. As shown in Figure 1, the designed SO3 monitor primarily consists of four components: the sampling unit, the absorption unit, the reaction unit, and the determination unit. Flue gas was extracted from the gas duct through a heated sampling probe to avoid condensation of gaseous H2SO4 on the inner surfaces. The quartz probe liner was held at 280 °C to ensure complete conversion of sulfuric acid droplets to the gaseous phase even in saturated atmospheres [e.g., outlet of wet flue gas desulfurization (WFGD)]. The inner diameter of the probe liner was 4 mm with a normal length of 2 m. A silylated wool filter was placed in a quartz thimble holder, which was connected to the outlet end of the probe liner to remove particulate matter because PM might promote the oxidization of SO2 and cause positive biases if introduced B
DOI: 10.1021/acs.energyfuels.7b01181 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the continuous SO3 monitor. into the IPA solution. Silylation treatment can greatly reduce the amount of hydroxyl on the wool filter surfaces, and thus, plugging at the filter can be prevented and the lifespan of the filter prolonged, which could prevent frequent replacement of the filter for dust-laden gases [e.g., inlet of the electrostatic precipitator (ESP)]. The filter was also heated to prevent SO3 condensation. The sampled gas flow was circulated through the wool filter to the absorption unit, where gas was mixed with the absorption solution. The absorption unit was also fixed in the quartz thimble holder, and the absorption solution was transferred to the absorption unit from a container through a hose. The absorption unit was made from acidresistant Teflon. The inner chamber was carved into spiral grooves to promote gas−liquid mixing. An 80% IPA solution (in water) was used as the absorption liquid because IPA can inhibit the oxidization of SO2, whereas the solubility of SO3 in pure IPA is not sufficiently high, which could limit the measured range.12,14 The gas−liquid mixture circulated into the gas−liquid separator, where the exhaust gas was separated. An optical detector was set for quantification of the absorption solution volume, and once the liquid reached the set volume, the vacuum valve was automatically triggered to begin the liquid transport to the reaction unit. The gas was circulated to the cooling unit from the separator, where the temperature of the flue gas containing water and IPA vapor was reduced, and the condensate was collected and mixed with the liquid in the gas−liquid separator. This unit was installed to collect the volatile IPA and to maintain the IPA concentration at a constant level in the absorption chamber. The cooling unit consisted of two connected spiral tubes immersed in the freezing medium and held at 2 °C with the refrigeration slice. IPA is a type of volatile compound, and its evaporation rate is much higher than that of water. The saturation limit of IPA and water at different temperatures was calculated and compared. The saturation vapor pressure (Ps) of IPA and H2O is dependent on the gas temperature, and the saturation humidity content (H) refers to the saturated partial density per unit flue gas without water or IPA.
H=χ
Figure 2. HIPA and HH2O at different temperatures. dissolve as SO32−, which could easily oxidize into SO42− and induce positive errors. The cooled gas further circulated to the mass flow controller (MFC), and two connected drying tubes were installed between the MFC and cooling unit in which silica gel is used to remove water (Figure 3). The MFC can measure the flow rates and amend the influence of the temperature change automatically. After flowing through the absorption chamber and the gas−liquid separator, the liquid IPA solution containing SO42− circulated to the reaction bed in which the main component is barium perchlorate, where the following reaction occurred.
SO4 2 − + BaC6O4 Cl 2 + H+ → BaSO4 ↓ + HC6O4 Cl 2−
φPs P − φPs
where χ is the ratio of gas molar mass to flue gas molar mass, which is 0.59 for water and 1.97 for IPA, φ is the relative humidity and is 100% for saturated gas, and P is the total pressure of the flue gas, estimated as the standard atmospheric pressure in this work as 101000 Pa. The relationship between the saturated pressure and temperature can be described by the Antoine equation.28,29 For water, ln Ps = 18.3036 − 3816.44/(t + 227.02), and similarly, the saturated pressure for IPA is log Ps = 8.1182 − 1580.92/(t + 219.62), where Ps is in millimeters of Hg and t is in degrees Celsius. As shown in Figure 2, as the temperature decreases, HIPA decreases more sharply than HH2O does. Cooling the gases to 2 °C and collecting the condensate prevent further dilution of the IPA concentration; otherwise, a moisture fraction that is too high could promote SO2 to
Figure 3. Photograph of the continuous SO3 monitor. C
DOI: 10.1021/acs.energyfuels.7b01181 Energy Fuels XXXX, XXX, XXX−XXX
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thus, conversion rate α of SO2 could be calculated. As shown in Figure 5, the conversion rate was dominated by the catalyst temperature. The
The generated BaSO4 precipitated and was retained in the porous reaction bed. The reacted effluent turned purple as a result of the generated acidic chloranilate ions (HC6O4Cl2−). The reaction bed module was designed for periodical replacement, depending on the SO3 concentration. A spectrophotometer detects the light intensity change through the effluent, and the generated voltages were output by the photometer when the solution containing acidic chloranilate ions passed through the optical cell, thus determining the amount of acidic chloranilate ions, which preferentially absorb light at 535 nm (violet). The spectrophotometer was calibrated with a sulfuric acid solution with a precisely known concentration in IPA solutions. Therefore, the concentration of the sulfate ions in the IPA solution could be calculated, and the concentration of SO3 in the flue gas could be determined with the sampled flue gas rate, absorption solution flow rate, and SO42− concentration in the IPA solution. A programmable logic controller with a control panel processed the original data and output the concentration values. Selected parameters of the monitor are listed in Table 2.
Table 2. Main Parameters of the Continuous SO3 Monitor sampling method determination method probe temperature concentration range detection limit IPA flow rate
IPA absorption spectrophotometry 280 °C 0−200 ppm 0.1 ppm 1 mL/min
Figure 5. Rate of conversion of SO2 in the SO3 generator. conversion rate increased sharply as the temperature increased from 200 to 400 °C. In the 400 min pretest, no SO2 was detected in the gases after the catalyst temperature reached 400 °C, and thus, we can reasonably assume that all SO2 was converted to SO3; accurate generated SO3 concentrations could be acquired.
2.2. Laboratory Evaluation and Experimental Procedure. A laboratory experimental system was built for evaluation of this monitor, as shown in Figure 4. The device consists of a gas simulation system, a humidifier, a duct, and a monitor. An air fan is used to supply a flow rate of ≤2300 m3/h. The diameter of the gas duct is 200 mm, and SO3 is introduced into the humidified air flow. A SO3 generator was designed and assembled for SO3 production, and it was designed to oxidize SO2 to SO3 using catalysts. A controlled amount of SO2 (99% pure) is mixed with air in the gas mixer, and the gases subsequently flow through a reaction chamber maintained at 440−450 °C for the highest SO2 conversation rate, according to the catalyst performance. Generated SO3 is fed into the gas duct and mixed with the humidified air. Simulated gas containing SO3 from 0 to 60 ppm was measured for evaluation of this monitor, and the transient response was tested at a series of concentration levels. Moreover, to investigate the interference of SO2 in the accuracy of the developed monitor, a test was designed with air, 500 ppm SO2, and 1000 ppm SO2. Pretests were conducted to check the SO2/SO3 conversion rate of the SO3 generator. The SO2 concentration was measured with an MRU portable infrared multigas analyzer (Germany, 0−200 ppm), and
⎛ C SO2 ⎞ α = ⎜⎜1 − cSO ⎟⎟ × 100% Cg 2 ⎠ ⎝ SO
where α is the conversion rate, Cg 2 is the generated SO2 molar 2 is the collected SO2 molar concentration. concentration, and CSO c 2.3. Field Evaluation and Application. Field measurements were performed to test the accuracy and reliability of this monitor under real boiler flue gas conditions. The measurements were conducted at different locations in two coal-fired power plants in which the flue gas treatment equipment included SCR, ESP, wet flue gas desulfurization (WFGD), and WESP, as depicted in Figure 6. In site 1, measurements were conducted at the inlet and outlet of the SCR, the inlet and outlet of the ESP, and the inlet and outlet of the WESP, while results were compared with those from the manually controlled condensation method performed by expert operators at the inlet and outlet of the ESP. In site 2, which burned coal with a higher sulfur content, field applications were conducted at the SCR inlet, the SCR outlet, and the stack inlet. For the controlled condensation method, operations were performed under the standard instructions. The water bath temper-
Figure 4. Schematic of the laboratory experimental setup. D
DOI: 10.1021/acs.energyfuels.7b01181 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 6. Measurement locations in field measurements in a coal-fired power plant: (1) SCR inlet, (2) SCR outlet, (3) ESP inlet, (4) ESP outlet, (5) WESP inlet, and (6) WESP outlet. ature of the heated snake-collecting pipe was 70 °C. The sampling flow rate was set to 2 L/min, and a total amount of 80 L of flue gas was extracted in 40 min for each run. Condensed sulfur acid was collected by flushing with an IPA solution, and the amount of sulfate was measured with an ion chromatograph (ICS-900, Thermo Fisher) to obtain the SO3 concentration; tests were performed with an Ionpac ASII-HC analytical column (250 mm × 4 mm inside diameter, Thermo Fisher) and an Ionpac AGII-HC guard column (50 mm × 4 mm inside diameter, Thermo Fisher).
time might be slightly longer when the SO3 concentration is lower. 3.1.2. Influence of the SO3 Concentration on the Performance of the Monitor. To investigate the reliability of this monitor at different SO3 concentrations, a large number of calibration tests were conducted on a wide variety of concentration levels to evaluate the accuracy of this monitor. As shown in Figure 8, an obvious dependence relationship was
3. RESULTS AND DISCUSSION 3.1. Laboratory Evaluation. 3.1.1. System Response in the Transient Regime. The objective of this section is to reflect the response performance of the monitor. As shown in Figure 7, after initialization, the measured concentration increased
Figure 8. Comparison of generated SO3 and measured SO3.
found between the generated SO3 concentration (Cg) and the measured concentration (Cm), and for most tests, the relative errors were acceptable. When the generated concentration was relatively high, the measured concentrations for most cases were lower than the generated values. Apart from wall losses in the flue duct, when the SO3 concentration was higher in this experiment, the absorption liquid was relatively insufficient and did not absorb all the SO3 in the flue gases. For the lowerconcentration runs, the differences were
