Monitoring of industrial sulfur dioxide emissions using a
broadband
absorption spectroscopic sensor
XU Feng 1, YU Lili 2
5 (1. Department of Applied Physics, Nanjing University of Aeronautics and Astronautics, NanJing 211106; 2. Department of Applied Physics, Nanjing Forestry University, NanJing 210037)
Foundations: Specialized Research Fund for the Doctoral Program of Higher Education (No.20093218120030) Brief author introduction:F. Xu, (1976- ),male,associate professor,spectroscopy. E-mail: fengxu@https://www.doczj.com/doc/2d4944131.html, Abstract: The monitoring of sulfur dioxide (SO 2) from industrial pollution emissions was studied in the ultraviolet spectral range using a broadband absorption spectroscopic sensor. The sensor has a 10
detection limit of 1 ppm, and was employed for two certified SO 2 concentrations in laboratory. The continuous 24 h measurements of concentrations and temperatures of SO 2 in the flue gas emitted from an industrial coal-fired boiler was performed in the field measurement campaign, which demonstrated the repeatability, sturdiness, and practicability of the sensor by analyzing the concentration variation
with gas temperatures and comparing the measured value with the true value of total release.
15 Keywords: sulfur dioxide monitoring; industrial pollution emissions; broadband absorption spectroscopic sensor
0 Introduction
Today, combustion of fossil fuel from industry is the main anthropogenic source of sulfur
20 dioxide (SO 2) emission. SO 2 released to atmosphere can be deposited on the ground, which causes severe effects on many ecosystems by lowering pH values. The increased acidity of the ground can dissolve both Al as well as nutritive substances such as Ca, Mg and K from the soil [1], which in turn causes undernourishment of the soil. Effective control of industrial pollutant depends on useful monitoring for industrial pollution emission. Therefore, the increased need for the 25 development of powerful, reliable, and affordable techniques for pollutant emission measurement has highlighted over past few decades. At present, many techniques are used for monitoring SO 2 in air, e.g., differential optical absorption spectroscopy (DOAS) is a line-of-sight integration method that acquires the multispecies average concentration measurements [2-5], correlation spectroscopy (COSPEC) is especially suitable for leak detection and remedy against explosion in 30 the petrochemical and gas industry [6-9], differential absorption lidar (DIAL) uses pulsed laser sources and enables three-dimensional mapping [10-12], and tunable diode laser absorption spectroscopy (TDLAS) achieves miniaturization and high sensitivity [13].
Recently, we have reported on a novel concentration evaluation method for SO 2 monitoring using broadband absorption spectroscopy in the ultraviolet (UV) spectral region [14], which 35 employs the direct absorption signals for data evaluation without requiring any reference spectrum and fitted polynomials. The effect of temperature on the concentration evaluation method has been thoroughly analyzed [15]. Then we have carried out the first field measurement campaign, and employed the broadband spectroscopic sensor for real-time monitoring of SO 2 emission from an industrial coal-fired boiler in 2006, China [16]. Our simple scheme is based on differential 40 absorption, and has the advantage of straightforward data evaluation, limited susceptibility for interference from other gases.
In the present paper, we used this portable SO 2 sensor to continuously measure two certified concentrations of 699 ppm and 1460 ppm, respectively, at atmospheric pressure in laboratory to validate the accuracy of the sensor. And the second field measurement campaign was made and 45
described in this paper for an industrial coal-fired boiler in Harbin, China, yielding a 24 h continuous recording of concentrations and temperatures of SO2 in the flue gas. Above experiments and analysis were performed to perfecting the sensor for putting into operation.
1Field Experimental Setup
Field experimental optical setup of the broadband SO2 sensor composed of three independent 50
modular parts benches is shown in Fig. 1. The first breadboard (BB1) includes a deuterium lamp (DL) (Beijing Union Optics-Electronic Co. 5601) operated as a UV light source, and a quartz lens (QL) of 100-mm focal length fixed on a two-dimensional sliding stand was adjusted to collimate the exiting light. The UV light beam passed through two side holes of an exhaust chimney with 55
flowing flue gas, across the exhaust gas, to a second breadboard (BB2), where the transmitted light was carefully focused by a second quartz lens of 100-mm focal length into a multimode optical fiber (MOF) (Ocean Optics OFLV-200-1100). The chosen quartz lenses had relatively short focal lengths for compactness of the sensor, and could match the numerical aperture of the UV light source and the fiber. The UV light output from the multimode optical fiber was sent into
60
a high-resolution spectrometer (resolution of ～0.1 nm), which was mounted on a third breadboard
(BB3). The high-resolution spectrometer was composed of a monochromator and a 2048-element CCD-array detector (Ocean Optics HR2000). The signal from the CCD was transmitted to a personal computer, where the continuous instrument settings, together with real-time spectra acquisition and evaluation (maximum rate 0.5 Hz) were performed automatically using software written in Visual Basic?.
65
MOF
Computer Breadboard
Fig. 1 (Color online) Field experimental setup of the broadband spectroscopic sensor for SO2 monitoring. The
instrument is composed of three modular parts: First breadboard (BB1), second breadboard (BB2) and third
breadboard (BB3). DL: deuterium lamp, QL: quartz lens, FP: fiber port, MOF: multimode optical fiber, MC:
70
monochromator and 2048-element CCD-array detector.
Additionally, the temperature of the SO2 gas was measured by a K-model thermocouple with an accuracy of 0.1 K. All mechanical components were made of stainless steel, which is a corrosion-resistant material, and were designed to enable on-site substitution of damaged optics without need for realignment. Fig. 2 shows the photograph of the portable SO2 sensor composed 75
of the three parts, where BB1, BB2, and BB3 are marked with arrows in the figure.
Fig. 2 (Color online) Photograph of the portable SO 2 sensor is shown.
80 2 Evaluation Principles
The concentration evaluation method used by the SO 2 sensor using direct absorption
spectroscopy was described in Refs. [14] and [15]. An average SO 2 concentration can be deduced from the Beer-Lambert law [17], the ratio between the received radiation intensity ()λt P and the original intensity ()λo P can be written as
85 ()()
()()[](){}λαλσλλλ+?=NL R P P o t exp , (1) where ()λR is the dissipation due to refraction, diffraction and scattering of the gas at the
wavelength λ. N is particle number density of the gas, which is referred to the gas concentration here. ()λσ is the total absorption cross section of the gas, L is the absorption path length, and ()λα is the absorption coefficient of other gas. For two specific wavelengths
90 1λ and 2λ, Eq. (1) can be written as
()()()()[](){}1111
1exp λαλσλλλ+?=NL R P P o t , (2)
()()()()[](){}22222exp λαλσλλλ+?=NL R P P o t . (3) Division by terms of above Eqs (2) and (3) yields a data evaluation formula for the average SO 2 concentration according to
95
()()[]()()[]L
P P N t t 2121/ln λσλσλλ??=. (4) In Eq. (4) the original intensity at the two wavelengths, ()1λo P and ()2λo P cancel due
to the negligible difference of the source intensity within a limited wavelength range (～1 nm). In our experiments, absorption coefficient of other gases ()λα at the two selected wavelengths was neglected due to the small difference in their absorption cross section within selected narrow
100 wavelength range used and because of the low concentration of other gases compared to SO 2. Furthermore, the influence of the dissipation ()λR from other gases and dust is canceled by calculating the intensity ratios.
A thorough analysis of the temperature effect on the proposed concentration evaluation
method was presented in Ref. [15], where we introduce into Eq. (4) a temperature correction 105
coefficient )(T C , which is determined experimentally and condenses the temperature dependence due to the changes of both the volume increase and the absorption line broadening, it can be written as [15] ()T T C 31082.731.1?×+?=. (5)
Thus, after temperature correction, Eq. (4) should be rewritten as
110 ()()()[]()()[]L
P P T C N t t 2121/ln λσλσλλ??=. (6) Eq. (6) is the key expression used in the proposed broadband absorption spectroscopy evaluation method. Pressure can also influence the absorption spectra measurements of the gas. We had measured the absorption spectral features at atmospheric and at low (1.2 mba) pressure for the
115 same SO 2 concentration [14]. As expected from theoretical considerations [18], we found that the high and low pressure spectra overlap for the present detector resolution of 0.1 nm, thus, pressure does not affect the concentration evaluation using this technique.
3 Measurements and Results
3.1 Monitoring of Certified Concentrations
120 The absorption cross section of SO 2 with a Fourier transform spectrometer around 300 nm
changes rapidly [19], as shown in Fig. 3(a). In order to achieve maximum contrast, the two wavelengths 1λ and 2λ are chosen at the on/off-resonance pair 300.02 nm and 301.39 nm, respectively. Using the received radiation intensities ratio of ()1λt P to )(2λt P , inserting the values of the absolute absorption cross section )(1λσ and )(2λσ at the selected two
125 wavelengths [18,20], and the absorption path length L and temperature correction coefficient )(T C were carefully measured and accurately obtained on site, the SO 2 concentration can be acquired directly. Thus, the key to Eq. (6) is the possibility of obtaining the absolute SO 2 concentration directly without the need of any reference spectrum for the concentration evaluation, as in the case of the DOAS technique. 130
Wavelength (nm)T r a n s m i t t e d i n t e n s i t y (a .u .)302010015105A b s o r p t i o n
c r o s s -s e c t i o n
(×10-19 c m 2
)0 Fig. 3 (Color online) (a) The absorption cross section of SO 2 obtained with a Fourier transform spectrometer (Ref. [19]). (b) Absorption spectra recorded with certified SO 2 concentrations of 699 ppm and 1460 ppm under the room temperature at atmospheric pressure. λ1 and λ2 are marked with arrows in the figure.
135 Typical absorption spectra of two certified SO 2 concentrations of 699 ppm and 1460 ppm are shown in Fig. 3(b), using 350-mm-long gas absorption cell equipped with quartz windows and above setup under the room temperature at atmospheric pressure. The ratio of the received radiation intensities of the selected on/off wavelengths marked with arrows in Fig. 3(b), is direct proportion with SO 2 concentration. Comparison can be made with the Fourier transform 140
interferometer spectrum shown in Fig. 3(a) [19]. The sampling time of the CCD detector was automatically adjusted for a good signal-to-noise ratio (SNR) and high resolution integrated spectra. The spectral integration was allowed to continuously adjust until the transmitted intensity was slightly below the CCD threshold value, which ensured a good SNR without overflowing its measurement range. Thus, on top of the transmitted intensity, we found that the two absorption 145
spectra overlap about 320 nm. We investigated the system accuracy and stability by the continuous measurements of the two certified concentrations of 699 ppm and 1460 ppm in dry N 2 gas over one hour, using 4 s integration time, as shown in Fig. 4(a) and 4(b), respectively. From Fig. 4(a), we find an average concentration of SO 2 to be 699 ppm, with a standard deviation of 0.9 ppm, which means a
150 measurement precision of about 0.13%. While Fig. 4(b) gives an average SO 2 concentration of 1460 ppm with an evaluated standard deviation of 3.1 ppm, which implies a measurement precision of about 0.21%.
S O 2 c o n c e n t r a t i o n (p p m )Time (min) Fig. 4 (Color online) Certified SO 2 concentrations of 699 ppm and 1460 ppm recorded over one hour at
155 atmospheric pressure, respectively. Each point was obtained with 4 s integration time.
3.2 The Field Measurement Campaign
It can be presumed from Eq. (6) that for very high SO 2 concentrations, such as strongly
polluted emissions, the absorption path length L should be correspondingly shortened to avoid 160 saturation of the absorbed signal. Oppositely, in the case of very low gas concentrations, the absorption measurement sensitivity can be enhanced by averaging long time (>10 s) or increasing absorption path length (i.e., path length of 1-3 m, which is feasible in single-pass cross-stack monitoring). The extrapolated detection limit of the instrument was previously estimated to be ~1 ppm [14].
165 The field measurement campaign was carried out for an industrial coal-fired boiler in April 2008 in Harbin, China. The sensor was applied for measurement of SO 2, using a chimney flue of an absorption path length of 1.6 m, emitted from an industrial coal-fired boiler. A continuous 24 h recording of concentrations and temperatures of SO 2 is shown in Fig. 5, where every dot represents the value obtained with 1.1 min integration time. In Fig. 5, SO 2 concentrations were 170 synchronously changed with the temperature fluctuation of the flue gas, and we estimated the average SO 2 concentration to be 113±8 ppm. The synchronization between concentrations and temperatures of SO 2 in the results could be explained that lower concentrations meant lower coal consumption in boiler, which implied lower temperatures. Additionally, the coal consumption was estimated to 1.5 ton/h and the average sulfur content of coal in Northeast China is 0.36% [21], 175 yielding a total expected 24 h release of 0.26 ton/day. This value could be verified by actual measurement. Utilizing the obtained SO 2 concentrations and the measured flux of the exhaust gas, which was 50802 m 3/h, we could calculate a 24 h total SO 2 release of 0.23 ton/day. The incomplete combustion of the fuel led to the slight discrepancy of the results. However, the field measurement campaign demonstrated the repeatability, stability, and sturdiness of our broadband 180
spectroscopic sensor in industrial environment.
80100
120140160433413393373S O 2 c o n c e n t r a t i o n (p p m )Time (h)Temperature (K) 353 Fig. 5 (Color online) Twenty-four hours recording of concentrations and temperatures of SO 2 using a chimney flue of an absorption path length of 1.6 m, pollutants emitted from an industrial coal-fired boiler in China. Each
point was obtained with 1.1 min integration time.
185
4 Discussion and Conclusions
The sensor used the absorption spectrum for data evaluation and acquired directly the SO 2 concentration based on our concentration evaluation method without requiring any reference spectrum. Two certified concentrations of 699 ppm and 1460 ppm in dry N 2 gas were 190 continuously monitored using 4 s integration time over one hour at room temperature and atmospheric pressure. The standard deviations of two certified concentrations were 0.9 ppm and
3.1 ppm, respectively, which implied the measurement precision of about 0.13% and 0.21%, and validated the accuracy of the sensor. Ultimately, the field measurement campaign was employed for measurements of SO 2 from an industrial coal-fired boiler in Harbin, China. The field campaign 195 yielded a 24 h continuous recording of concentrations and temperatures of SO 2 in the flue gas. We found that the concentrations and temperatures of SO 2 synchronously changed in the results. By analyzing the concentration variation with gas temperature and comparing the measured value with the true value of total release, we demonstrated the repeatability, stability, and practicability of the sensor. It should also be noted that the design of the portable sensor was found to be 200 suitable for measurement of industrial pollution emissions, for example, it enabled on-site, continuous, and unattended operation over long time in hostile environments with large fluctuations of temperature and pressure. The main purpose of above experiments was a last test for perfecting the sensor and completing the end product in 2008, China.
The proposed data evaluation method using broadband absorption spectroscopy had been 205 employed for measurement of NO 2 [22], thus, the synchronizing measurements of SO 2 and NO 2 will be integrated to one system by choosing appropriate light source and CCD array detector in the future.
References
210
[1] J.O. Reuss, D.W. Johnson, Acid deposition and the acidification of soils and waters, Ecological Studies 59, Springer, Berlin, 1986.
[2] H. Edner, P. Ragnarson, S. Sp?nnare, S. Svanberg, Differential optical absorption spectroscopy (DOAS)
system for urban atmospheric pollution monitoring, Appl.Opt., 1993, 32: 327-333.
215
[3] U. Platt, Differential Optical Absorption Spectroscopy (DOAS), in Air Monitoring by Spectroscopic
Techniques, M. W. Sigrist, ed., Vol. 127 of Chemical Physics Series (Wiley, New York, 1994), pp 27-84.
[4] U. Platt, D. Perner, Direct measurement of atmospheric CH2O, HNO2, O3, and SO2 by differential absorption
in the near UV, J. Geophys. Res., 1980, 85: 7453-7458.
[5] J. Mellqvist, H. Axelsson, A. Rosén, DOAS for flue gas monitoring III. in-situ monitoring of sulfur dioxide, 220
nitrogen monoxide and ammonia, J. Quant. Spectrosc. Transfer., 1996, 56: 225-240.
[6] M.M. Millan R.M. Hoff, Remote Sensing of Air Pollutants by Correlation Spectroscopy Instrumental
Response Characteristics, Atmos. Env., 1978, 12: 853-864.
[7] P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, T. Caltabiano, Monitoring of volcanic
sulphur dioxide emissions using differential absorption lidar (DIAL), differential optical absorption spectroscopy 225
(DOAS), and correlation spectroscopy (COSPEC), Appl. Phys. B., 1998, 67: 419-426.
[8] C. Oppenheimer, P. Francis, J. Stix, Depletion rates of sulfur dioxide in tropospheric volcanic plumes, Geophys.
Res. Lett., 1998, 25: 2671-2674.
[9] S. Svanberg, Environmental and Medical Applications of Photonic Interactions, Physica Scripta., 2004, T110:
39-50.
230
[10] H. Edner, K. Fredriksson, A. Sunesson, S. Svanberg, L. Unéus, W. Wendt, Mobile remote sensing system for
atmospheric monitoring, Appl. Opt., 1987, 26: 4330-4338.
[11] P. Weibring, H. Edner, S. Svanberg, Versatile mobile lidar system for environmental monitoring, Appl. Opt.,
2003, 42: 3583-3594.
[12] P. Weibring, J. Swartling, H. Edner, S. Svanberg, T. Caltabiano, D. Condarelli, G. Cecchi, L. Pantani, Optical 235
monitoring of volcanic sulphur dioxide emissions-
comparison between four different remote-sensing spectroscopic techniques, Opt. Lasers Eng., 2002, 37: 267-284.
[13] G. Somesfalean, Z.G. Zhang, M. Sj?holm, S. Svanberg, All-diode-laser ultraviolet absorption spectroscopy
for sulfur dioxide detection, Appl. Phys. B., 2005, 80: 1021-1025.
[14] F. Xu, Z. Lv, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Concentration evaluation method using broadband 240
absorption spectroscopy for sulfur dioxide monitoring, Appl. Phys. Lett., 2006, 88: 231109.
[15] F. Xu, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, H.S. Wang, S.H. Wu, Temperature-corrected spectroscopic
evaluation method for gas concentration monitoring, Appl. Phys. B, 2007, 86: 361-364.
[16] F. Xu, Y.G. Zhang, G. Somesfalean, H.S. Wang, S.H. Wu, Z.G. Zhang, Broadband spectroscopic sensor
for real-time monitoring of industrial SO2 emissions, Appl. Opt., 2007, 46: 2503-2506.
245
[17] S. Svanberg, In Atomic and Molecular Spectroscopy: Basic Aspects and Practical Applications,
Springer, 2004, pp. 164-165.
[18] D.J. Brassington, Sulfur dioxide absorption cross-section measurements from 290 nm to 317 nm, Appl. Opt.,
1981, 20: 3774-3779.
[19] A.C. Vandaele, P.C. Simon, J.M. Guilmot, M. Carleer, R. Colin, SO2 absorption cross section measurements 250
in the UV using a fourier transform spectrometer, J. Geophys. Res., 1994, 99: 25599-25605.
[20] D.J. Brassington, Measurement of the SO2 absorption spectrum between 297 and 316 nm using a tunable
dye laser, Laboratory Note RD/L/N 184/79 (Central Electricity Research Laboratories, Leatherhead, UK, 1979).
[21] Z.Y. Zhang, W.H. Li, Z.G. Xu, S.J. Qu, in On the appropriate processing and utilization of coal, Chinese U. of
Mineralogy, 2000, pp. 53-55.
255
[22] F. Xu, Z. Lv, X.T. Lou, Y.G. Zhang, Z.G. Zhang, Nitrogen dioxide monitoring using a blue LED, Appl. Opt.,
2008, 47: 5337-5340.
宽带吸收光谱传感器对工业二氧化硫污染气体排放的
监测研究
徐峰1，于莉莉2
260
（1. 南京航空航天大学理学院应用物理系，南京 211106；
2. 南京林业大学理学院应用物理系，南京 210037）
摘要：本文使用宽带吸收光谱传感器对工业二氧化硫（SO2）污染气体排放开展了系统的研究工作。通过对两种标准浓度的SO2气体测量，获得这种SO2气体传感器的探测精度为1ppm。265
使用该传感器对燃煤锅炉排放的SO2污染气体进行了24小时的实时现场测量，通过分析气体排放浓度随温度的变化，并对比传感器的监测结果与实际排放的数据，证明了传感器的可重复性、耐用性和实用性。
关键词：二氧化硫气体监测；工业污染气体排放；宽带吸收光谱传感器
中图分类号：O433.4
270