Optical H2S and SO2 sensor based on chemical conversion and partition differential optical absorption spectroscopy

Accepted Manuscript Optical H2S and SO2 sensor based on chemical conversion and partition differential optical absorption spectroscopy

Yungang Zhang, Yongda Wang, Yunjie Liu, Xuejia Dong, Hua Xia, Zhiguo Zhang, Jimeng Li PII: DOI: Reference:

S1386-1425(18)31032-1 https://doi.org/10.1016/j.saa.2018.11.035 SAA 16599

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

12 July 2018 3 October 2018 12 November 2018

Please cite this article as: Yungang Zhang, Yongda Wang, Yunjie Liu, Xuejia Dong, Hua Xia, Zhiguo Zhang, Jimeng Li , Optical H2S and SO2 sensor based on chemical conversion and partition differential optical absorption spectroscopy. Saa (2018), https://doi.org/10.1016/j.saa.2018.11.035

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ACCEPTED MANUSCRIPT Optical H2S and SO2 sensor based on chemical conversion and partition differential optical absorption spectroscopy Yungang Zhanga, Yongda Wanga, Yunjie Liua, Xuejia Donga, Hua Xiaa, Zhiguo Zhangb, Jimeng Lia,*

150080, China

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Condensed-Matter Science and Technology Institute, Harbin Institute of Technology, Harbin

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*Corresponding author: [email protected], tel: +86-0335-8387567

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College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China

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ACCEPTED MANUSCRIPT Abstract An optical sensor based on chemical conversion and partition differential optical absorption spectroscopy is developed to detect hydrogen sulfide (H2S) and sulfur dioxide (SO2) gas in sulfur hexafluoride (SF6) decomposition products. Given that the absorption cross sections of SO2 and H2S overlap in 170-230 nm band, the differential lines of H2S are very few, meanwhile the

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corresponding absorption cross sections are small in comparison to that of SO2, thus H2S can be detected by reacting with oxygen to convert to SO2 in the presence of UV light. Through the

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concentration variation of SO2 before and after chemical reaction, the concentration of H2S can be

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obtained. Meanwhile the partition differential optical absorption spectroscopy method deduced from Beer-Lambert’s law is introduced to weaken the influence of electronic noise on the measuring

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result, especially in low concentration. The SO2 detection limit of 12 ppb per meter can be achieved.

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The optical sensor can measure the concentrations of H2S and SO2, so it is suitable for the fault

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diagnosis of gas insulated switchgear (GIS).

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absorption spectroscopy

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Keywords: Hydrogen sulfide; Sulfur dioxide; Chemical conversion; Partition differential optical

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ACCEPTED MANUSCRIPT 1. Introduction

Gas insulated switchgear (GIS), which uses sulfur hexafluoride (SF6) gas as insulating medium, is widely used in power system[1, 2]. SF6 gas has several advantages of chemical stability, high strength insulation and arc extinguishing. However, during the long-term operation of GIS, it is liable to failure due to the influence of sparks, arc and partial discharges. Because the GIS is

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enclosed system, it is difficult to diagnose and forecast the internal fault. Now several fault

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diagnosis methods of GIS, such as the ultra-high frequency (UHF) detection method [3-7],

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electromagnetic wave detection method [8, 9], high frequency grounding current method and ultrasonic method [10], have been studied to detect the fault of GIS. Compared with these above

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measurements, the detection methods based on SF6 decomposition products are less influenced by noise and more reliable [11-13]. Once the insulation fault comes out, SF6 will react with other

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chemicals to produce various decomposition products, such as SO2, CO2, H2S, CO, SF4, SOF2, and so on [14, 15]. And different decomposition products correspond to different insulation faults. The

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existing research results indicate that SO2 and H2S, as two important decomposition components

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from SF6, can characterize the severity of internal insulation faults [16-18]. Furthermore, in China, SO2, H2S and CO have been identified as indicators for the fault diagnosis of GIS [19].

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Many gas analysis techniques that are mainly based on the principles of electrochemistry and optics, have been rapidly developed and widely used to measure the concentration of SO2 and H2S

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[20-26]. However, electrochemical sensors cannot provide long-term and real-time monitoring because they have relatively long response time and a limited lifetime as well as require frequent calibration. SO2 and H2S measurement techniques based on optical principles mainly include Fourier transform infrared spectroscopy (FTIR), non-dispersive infrared absorption (NDIR), quartz-enhanced photoacoustic spectroscopy (QEPAS) and differential optical absorption spectroscopy (DOAS). The FTIR technique has been developed for the measurement of gases, which can obtain high measurement precision and accuracy, but requires complex optical systems 3

ACCEPTED MANUSCRIPT and expensive equipment [27, 28]. The NDIR technique has been used to measure concentrations of SO2 and H2S, but it is easily influenced by the variation of light source intensity [29]. The QEPAS sensor has been developed for SO2 and H2S detection in the infrared spectral region [30, 31], and the limits of detection at 142 ppb for H2S and 63 ppb for SO2 have been obtained. However, these optical methods based on infrared absorption spectrum are not suitable for detecting decomposition

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products of SF6 in the fault diagnosis of GIS, because SF6 has a strong infrared absorption capacity, while its concentration is above 99%. The decomposition products of SF6 can only be measured

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using visible and UV optical techniques. The DOAS technique has been developed for the SO2 and

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H2S measurement [32-35]. SO2 and H2S have strong absorption lines in the UV wavelength region due to the electron transitions. However, there are two distinct differential absorption cross sections

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for SO2, whereas H2S has only one broad band absorption cross section with a width of 60 nm. The

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detection limit of H2S is relatively high due to the differential lines of H2S are very few, meanwhile the corresponding differential absorption cross sections are small. So the ultraviolet DOAS is

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suitable to detect SO2 in SF6 decomposition products instead of H2S. In our previous work, a

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detection limit of 17 ppb per meter of SO2 has been achieved [36]. In this paper, an optical sensor based on chemical conversion and partition differential optical absorption spectroscopy is developed to detect H2S and SO2 in SF6 decomposition products. First, an improved concentration

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evaluation method based on a formula deduced from Beer-Lambert’s law is introduced to weaken

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the influence of electronic noise [37, 38] at low concentrations. The relationship between SO2 concentration and optical parameters is established. The SO2 detection limit is improved by using the partition differential optical absorption spectroscopy. Then the concentration of H2S can be measured by converting H2S to SO2. Thus H2S and SO2 concentrations can be successively obtained through the measured change curve of SO2 concentration. The above experiments verify the feasibility of the measurement method. 2. Theory Many gases such as SO2 and H2S have strong absorption lines in the UV wavelength region 4

ACCEPTED MANUSCRIPT due to the electron transitions, which makes the use of broadband UV spectroscopy suitable to the concentration measurement of these gases. SO2 has two absorption bands at 178 - 232 nm and 270 320 nm, respectively [39, 40]. The absorption cross section of SO2 in the absorption bands is shown in Fig. 1. It can be seen that there are obvious differential absorption structures (narrow-band component) for SO2 within the wavelength ranges 178-232 nm and 270-320 nm. However,

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considering that the absorption cross-section and the differential absorption cross-section near 200 nm are 10 times larger than that near 300 nm, and O2 does not have a differential absorption

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cross-section when the wavelength is bigger than 194 nm, thus the range of wavelength

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195.48-224.11 nm is chosen as the characteristic wavelengths to measure SO2 concentration in this study. Meanwhile the absorption band around 200 nm is adopted to improve the SO2 measuring

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performance. H2S has a slowly varying broad absorption band with a center wavelength of 195 nm

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[41]. The absorption cross sections of SO2 and H2S from 178 to 232 nm are shown in Fig. 2. As can be seen, the differential absorption cross section of H2S is negligible compared with that of SO2,

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thus the differential optical absorption spectroscopy does not apply to measure the concentration of

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H2S. The bandwidth of the absorption cross section of H2S is 60 nm, so it is susceptible to the scattering of water vapor and particles. Furthermore, it can be seen that the absorption cross section of H2S overlaps most with the slow-change absorption cross section of SO2, thus SO2 has influence

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on the measurement of H2S. In addition, most of the absorption spectrum has been observed in the

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vacuum ultraviolet region, so O2 has influence on the measurement of H2S below the wavelength of 194 nm and the light cannot be transmitted in the air. Therefore, we can convert H2S into SO2 with the help of photocatalytic reaction, and then by measuring the concentration change of SO2, obtain the concentration of H2S.

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(a)

9 6 3 0 1.2 180

190

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0.9 0.6

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Absorption cross section -18 2 (10 cm /molecule)

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0.3 0.0 270

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Wavelength (nm)

Fig. 1. (a) Absorption cross section of SO2 from 178 to 232 nm.

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(b) Absorption cross section of SO2 from 270 to 320 nm.

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Fitted curve of SO2 H2S

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SO2

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Absorption cross section -18 2 (10 cm /molecule)

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0 180

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Wavelength (nm)

Fig. 2. The slow-change absorption cross section of SO2 and the absorption cross section of H2S from 172 to 232 nm.

The measurement of SO2 concentration is based on the Beer-Lambert law. When the beam of light passes through a specific concentration of SO2 gas, the intensity ratio between incident and received light can be expressed as I ( ) / I 0 ( )  exp( N SO2 ( ) L   ( ))

(1) 6

ACCEPTED MANUSCRIPT where I 0 ( ) and I ( ) are the incident and received radiation intensities, respectively;  SO2 ( ) ( cm2/ molecule) is the absorption cross section;  (  ) is the scattering and absorption coefficient of other components; N (molecule/cm3) is the concentration of SO2; and L (cm) is the absorption optical path length. In Eq. (1), the SO2 absorption spectrum consists of a slow-change portion and a vibrating

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change portion. The absorption cross section can be decomposed into a broadband absorption cross section  0 ( ) and a differential absorption cross section  0 ( ) . Considering

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 SO ( )   ( )   ( ) , the following can be obtained using the Eq. (1): 2

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P( )   N  0 ( ) L  ln( I ( ) / S ( ))

(2)

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where S ( ) is the slow-change absorption spectrum, as shown in Fig. 3(a), which is obtained by

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polynomial fitting of the absorption spectrum of SO2. P (  ) denotes the vibrating change absorption spectra intensity, as shown in Fig. 3(b), which is obtained by calculating the natural logarithm of the ratio of the slow-change absorption spectrum to the absorption spectrum SO2. S ( )  I0 ( ) exp( N 0 ( ) L   ( ))

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(3)

S ( ) can be obtained using the low-resolution spectrum or polynomial fitted spectrum of received

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light.

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Intensity(a.u.)

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(a)

0.94 0.92

Absorption spectrum of SO2 Fitting curve (S(  ) )

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ACCEPTED MANUSCRIPT Fig. 3. (a) Absorption spectra intensity of SO2 and the slow-change absorption spectra intensity. (b) Vibrating change absorption spectra intensity

In order to reduce the influence of random noise, the vibrating change absorption spectra intensity P (  ) are summed for all the wavelengths. In Ref. [33, 36], the absolute values of the vibrating change absorption spectra intensity P (  ) are summed to obtain the absorption signal

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intensity, but it will increase the noise disturbance in extremely low concentration. Therefore, the partition differential optical absorption spectroscopy method is proposed. First, the vibrating change

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absorption spectra intensity P (  ) is divided into two sections according to the differential

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absorption cross section  0 ( ) . The I ( ) and  0 ( ) are expressed as I (i ) and  0 (i )

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respectively, when the value of  0 ( ) is positive. And the I ( ) and  0 ( ) are denoted as I ( j ) and  0 ( j ) when the value of  0 ( ) is negative. Then Eq. (2) can be rewritten as

OP1 A1

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 NL 

OP2 A2

(5)

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 NL 

(4)

in which, OP1 and OP2 (optical parameter) are the sums of signals obtained from all the measuring points, and A1 and A2 are two constant parameters, (6)

OP2   ln( I ( j ) / S ( j ))

(7)

A1    0 (i )

(8)

A2    0 ( j )

(9)

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OP1   ln( I (i ) / S (i ))

Eqs. (4) and (5) can thus be written as: N

OP2  OP1 OP  ( A1  A2 ) L ( A1  A2 ) L

(10)

Finally, the concentration N is obtained by Eq. (10), which is the expression of the linear 8

ACCEPTED MANUSCRIPT relationship between SO2 concentration and OP . 3. Experiment Fig. 4 illustrates the experimental setup of the measuring system for SO2 and H2S concentrations, which consists of two main components: gas preparation portion and gas concentrations measurement portion. The gas preparation system consists of three 8 L cylinders, three pressure

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reducing valves, three mass flow controllers, a micro air pump, an airbag and a reactor. The test gas is supplied through the gas preparation system. The gas concentrations measurement system

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includes a deuterium lamp, two quartz lenses, a gas cell with quartz windows, a multi-mode optical

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fiber, a fiber optic spectrometer, a light barrier and a computer. Spectral acquisition and gas

Flow meter

Flow meter

Lens

Reactor

Light barrier

Gasbag H2S Computer

Spectrometer

Micro pump

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SF6

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Flow meter

Gas cell

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SO2

Lens

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Deuterium lamp

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concentration analysis are performed by the gas concentrations measurement system.

Fig. 4. Experimental setup of the absorption sensing system for SO2 and H2S

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3.1. Preparation of SO2 and H2S

SO2 gases with different concentrations were prepared by mixing standard gas of 222 ppm SO2 (Nanjing Tianze Co., Ltd) with pure SF6 (Nanjing Tianze Co., Ltd) in different proportions with the help of the mass flow controllers (Beijing Sevenstar Electronics Co., Ltd, D07-19C). The two concentrations of H2S are respectively 49.8 ppm and 94.4 ppm (Nanjing Tianze Co., Ltd). The combined gas flow velocity stabilized at 1 L/min during the course of the experiment. And a gasbag (Dalian Delin Gas Packing Co., Ltd) was used to provide trace amounts of O2. At the same time, 9

ACCEPTED MANUSCRIPT experimental exhaust includes SO2 and unreacted H2S was absorbed into the alkaline solution in the reaction vessel. The temperature was controlled and maintained at about 25 degrees centigrade during the experiment. 3.2. Gas concentrations measurement A high-pressure deuterium lamp (Hamamatsu 5601, Japan) with a broadband emission

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spectrum in ultraviolet band is employed as light source, the light from which is transformed into a parallel beam by a quartz lens with a focal length of 45 mm and a diameter of 25 mm. Then the

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parallel beam transmits through a sample cell with a length of 50 cm and a diameter of 15 mm. The

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sample cell equipped with quartz windows was full of target gas, which had an inlet port and an outlet port near each end to guarantee a good air change consequence. The transmitted beam

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collected by another quartz lens with a focal length of 75 mm was focused into a multi-mode optical

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fiber (Ocean Optics QP600-05-SR), and the light that does not pass through the gas cell is blocked by the light barrier. Finally, the light was coupled into a high-resolution fiber optic spectrometer

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(Ocean Optics Maya2000 Pro) with the spectral range of 165.27-271.08 nm. The data analysis is

4. Results and discussion

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performed automatically through the application software written in C# and Matlab.

To verify the advantages of the partition differential optical absorption spectroscopy method at

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low concentration, an extreme case was considered in which a set of spectral signals was measured

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without SO2 gas injection. The vibrating change absorption spectra intensity P (  ) and its absolute values are shown in Fig. 5 (a) and (b), respectively. The P(i ) and P( j ) are shown in Fig. 5 (c) and (d), which are obtained according to the differential absorption cross section  0 ( ) . In Refs. [33] and [36], the absolute values of P (  ) are summed to obtain the optical parameter OP . In the paper, OP is obtained by the difference between OP2 and OP1 . OP1 and OP2 are the sums of P(i ) and P( j ) respectively. The OP values are listed in Table 1. It is observed that the OP

value obtained using the presented method is much less than that of the Ref. [33].The reason is that 10

ACCEPTED MANUSCRIPT in case of low signal-to-noise ratio, the partition differential optical absorption spectroscopy can counteract some electronic noise via subsection summation of spectral signals, yet the method used in Ref. [33] superimposes electronic noise by summing the absolute values of spectral signal. Moreover, the smaller the OP value caused by the electronic noise, the smaller SO2 concentration

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0 -2

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Wavelength (nm)

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Intensity (10 a.u.) Intensity (10 a.u.)

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Intensity (10 a.u.) Intensity (10-4 a.u.)

deviation, and thus the SO2 detection limit can be further reduced.

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Fig. 5. The vibrating change absorption spectra intensity Table 1 Optical parameters for different processing methods

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OP(Ref. [33]) 0.06

OP1

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OP=OP2-OP1

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0.0008

0.0015

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In order to calibrate the SO2 sensing system, the relationship between OP and SO2 concentration was established, as shown in Fig. 6. Absorption spectra of SO2 with different concentrations were measured, and their corresponding OP values can be obtained using the method described in Section 2. For each concentration, more than 100 OP values were measured at a rate of 0.5 Hz, and then select 32 consecutive values from the middle part to calculate the average value as the final OP value. From Fig. 6, it can be observed that a good linear relationship (R2=0.999) between measured OP and SO2 concentration was achieved and the calibration expression of SO2 concentration can be given by 11

ACCEPTED MANUSCRIPT C  16.74(2)  OP

(11)

where C denotes the SO2 concentration, and its unit is ppm.

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Experimental values Fitted curve

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OP(a.u)

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Concentration (ppm)

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Fig. 6. The relationship between optical parameters and SO2 concentration

In order to test the stability and detection limit of the proposed system, SO2 gas at a nominal

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concentration of 1 ppm was measured. The measurement results of SO2 concentration are shown in

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Fig. 7. As can be seen, the average concentration of SO2 is 0.995 ppm. The standard error is 0.012 ppm. Considering the length of the gas cell used in the experiment is 50 cm, the detection limit is 12

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ppb per meter with a signal to noise ratio of 2, which is obtained by analyzing the statistical fluctuations of the signal and the root mean square noise that is essentially independent of

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concentration. The fluctuation in concentration during the 95-minute measurement is 0.06 ppm, which indicates that the measurement system has good stability.

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Average=0.995  0.012 () ppm

1.02

1.00

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Time (min.)

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Fig. 7. The SO2 measurements with nominal 1 ppm

To measure the concentration of H2S, the H2S of nominal concentration 49.8 ppm is injected

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into the sample cell with 1 L/min flow velocity. Then, about 2 minutes, the valves of the inlet and outlet are closed. The trace O2 is injected into the sample cell by the gasbag to promote the

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transformation from H2S to SO2. But in the airtight sample cell, the mass concentration of the H2S

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is not changed after injecting O2. The absorption spectrum of SO2 is collected after the valves of the inlet and outlet are closed. The spectral signals are obtained by the fiber spectrometer at 0.5 minute,

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1.5 minute, 3.5 minute, 5.5 minute and 7.5 minute respectively, which are shown in the Fig. 8. It can be seen that the absorption characteristics of the SO2 are more and more obvious, because H2S

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reacts with O3 to form SO2 according to the chemical reaction equation (12): H2S+O3→SO2+H2O

(12)

The O3 is generated from O2 under the action of ultraviolet light. Furthermore, the absorption characteristics change dramatically from the 1st minute to the 5th minute. Next, the proposed data processing method is employed to analyze the absorption spectrums of SO2 in order to analyze the relationship between the generated SO2 concentration and the injected H2S concentration. Fig. 9 shows the concentration measurement results of SO2 that change from 0 ppm to 49.2 ppm. The concentration of SO2 trends to be stable after 7.5 minute, and 13

ACCEPTED MANUSCRIPT fluctuates around 49.2 ppm. In the case of complete reaction, the concentration of H2S should be equal to the concentration of SO2. From Fig. 9, the final concentration of SO2 is very close to the concentration of injected H2S. The analysis result indicates that the concentration of H2S can be obtained by measuring the concentration of SO2.

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0.006 0.000

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Fig. 8. The absorption spectrum of SO2 after oxygen injection

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Fig. 9. The concentration of SO2 generated from H2S

In order to verify that the proposed sensor system can measure both the concentrations of SO2 and H2S, another experiment was carried out to measure the concentrations of the mixed gas. The 14

ACCEPTED MANUSCRIPT measurement results of the SO2 concentration are shown in Fig. 10. As can be seen in Fig. 10, first, the SF6 was injected into the sample cell, and then, the injected gas was changed from SF6 to 51.7 ppm SO2 at point a. After reaching point b, the concentration of SO2 is 51.4 ppm, which is close to the injected gas concentration of 51.7 ppm. At point c, 51.7 ppm SO2 and 94.4 ppm H2S were injected into the sample cell at the same flow rate. Due to the dilution of injected H2S, the

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concentration of SO2 tends to be close to 50% of the injected SO2 concentration as it reaches point d. At point e, stop injecting the SO2 and H2S, meanwhile the trace O2 was injected into the sample cell.

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Then the concentration of SO2 gradually increased and finally reached 72.4 ppm at point f. The

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difference of SO2 concentrations between point f and point e is 46.8 ppm, which is approximately equal to half of the injected H2S concentration 94.4 ppm. This indicates that SO2 concentration can

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be obtained directly by partition differential absorption spectroscopy, and H2S concentration can be

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obtained indirectly through changes in the SO2 concentration before and after chemical reaction in

D Injected SO2

and H2S

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f

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SO2 concentration (ppm)

mixed gas.

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Fig. 10. The concentration measurement results alternate with SO2 and H2S

5. Conclusions The feasibility of high-precision measurement of SO2 and H2S gas using chemical transformation and partition differential optical absorption spectroscopy has been demonstrated in the paper. A 15

ACCEPTED MANUSCRIPT compact measuring system is constructed using optical measurement systems and gas control systems. The partition differential optical absorption spectroscopy based on Beer-Lambert’s law has been presented to weaken the influence of noise, which makes the detection limit of SO2 reach 12 ppb per meter. Under the action of UV light, H2S can be converted into SO2 by reacting with O2. Therefore, the proposed method can not only measure the SO2 concentration in mixed gas, but also

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obtain the H2S concentration through the change of SO2 concentration before and after the reaction. The experimental results indicate the validity of this method. The proposed measuring system can

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be applied to detect the concentrations of SO2 and H2S in SF6 decomposition products in the fault

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diagnosis of GIS. Acknowledgements

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This work was supported by the Nation Natural Science Foundation of China (Grant no. 61308065

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and 51505415) and the natural Science Foundation of Hebei Province (Grant no. E2015203014).

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ACCEPTED MANUSCRIPT Graphical abstract H2S reacts with oxygen to convert to SO2 in the presence of UV light. The concentration of H2S can be obtained by the concentration variation of SO2 before and after chemical reaction. Meanwhile, the partition differential absorption spectroscopy is proposed to obtain the concentration of SO2, which can achieve the detection limit of 12 ppb per meter. H H

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ACCEPTED MANUSCRIPT Highlights 

We presented the partition differential optical absorption spectroscopy technique.

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Detection limit of 12 ppb*m for SO2 gas is obtained by analyzing electronic

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noise. The H2S concentration is obtained by detecting the change of SO2.concentration.

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Concentrations of SO2 and H2S are successively measured using an optical

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sensor.

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