# Development of an FDM-TDLAS Sensor for Long-Term Online Monitoring of H₂S and CO to Predict Water Wall Corrosion Trends

Xuanbing Qiu, Xiaohe Xiong, Long Li, Guqing Guo, Ting Gong, Songjie Guo, Zhijin Shang, Xiaocong Sun, Yali Tian, Gang Xie, et al.

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## To cite this version:

Xuanbing Qiu, Xiaohe Xiong, Long Li, Guqing Guo, Ting Gong, et al.. Development of an FDM-TDLAS Sensor for Long-Term Online Monitoring of H₂S and CO to Predict Water Wall Corrosion Trends. Analytical Chemistry, 2025, 97 (26), pp.14058-14066. 10.1021/acs.analchem.5c02691. hal-05392307

HAL Id: hal-05392307

https://hal.science/hal-05392307v1

Submitted on 2 Dec 2025

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Development of an FDM-TDLAS sensor for long-term online
monitoring of H₂S and CO to predict water wall corrosion
trends

Xuanbing Qiu¹, Xiaohe Xiong², Long Li¹, Guqing Guo¹, Ting Gong¹, Songjie Guo¹, Zhijin
Shang¹, Xiaocong Sun¹, Yali Tian¹, Gang Xie¹, Houzhang Tan², Christa Fittshen³, Béla Fiser⁴,⁵,
Milán Szőri⁴, György Tarczay⁶, Chuanliang Li¹,*

¹ Shanxi Engineering Research Center of Precision Measurement and Online Detection Equipment and School of

Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China

2 MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi' an Jiaotong University, Xi' an 710049, China

³ CNRS, UMR 8522 - PC2A - Physicochimie des Processus de Combustion et de l' Atmosphère, Université Lille, Lille F-59000, France

⁴ Institute of Chemistry, University of Miskolc, Miskolc H-3515, Miskolc-Egyetemváros A/2, Hungary

⁵ Department of Physical Chemistry, Faculty of Chemistry, University of Lodz, 90-236 Lodz, Poland

⁶ ELTE Eötvös University, Institute of Chemistry, Pázmány P. s. 1/a, 1117 Budapest, Hungary

**1. Abstract**

In order to meet the requirements of emission reduction and energy saving, the majority of coal-fired power plants adopt low-NOₓ combustion (LNC) technologies. However, more hydrogen sulfide (H₂S) and carbon monoxide (CO) are generated in the process of LNC in coal-fired boilers. These gases pose a significant risk of corrosion to the furnace fireside water wall and have detrimental environmental effects. A laser sensor system has been proposed for on-line detecting of both H₂S and CO near the furnace water wall. The sensor system uses two distributed feedback (DFB) lasers with wavenumbers of 6336.61 cm⁻¹ for H₂S and 6380.30 cm⁻¹ for CO, respectively. The device is developed based on frequency division multiplexing (FDM) tunable diode laser absorption spectroscopy (TDLAS) coupled with wavelength modulation spectroscopy. A corrosion-resistant Herriott multipass cell is employed, which can withstand temperatures up to 120°C and extends the optical path length to an impressive 18 meters. Additionally, a home-made electronic system is designed, including laser controller and lock-in amplifier, signal acquisition, and signal processing. Rigorous testing, including high-temperature, low-temperature, and vibration tests, was carried out, with the concentrations varying in the range of 1.5% to 2% during the experiments. This robust testing underscores the reliability and resilience of this system. Based on the Allan variance analysis, the system demonstrates impressive minimum detection limits, with readings of 178 ppb
for H₂S and 116 ppb for CO achievable at integration times of 111 seconds and 144 seconds, respectively. Furthermore, oxygen (O₂) measurements were acquired through an electrochemical sensor to investigate combustion reactions in close proximity to the water wall. These systems were deployed during the campaign for detection at three sampling points at different heights of a coal-fired plant. The results obtained affirm that this device serves as a highly effective real-time instrumentation for monitoring and predicting the corrosion status of the furnace water wall.

### Keywords

TDLAS; WMS-2f; FDM; Corrosion of furnace water wall; H₂S; CO;

## 1.Introduction

Low-NOx combustion (LNC) technologies are widely used in coal-fired power plants, primarily for their dual advantages of NOx emission reduction and enhanced CO₂ emission control through more efficient combustion processes [1]. These LNC technologies enhance the fuel/air ratio in the burner, creating a strong reducing atmosphere that ultimately results in substantial O₂ consumption and the production of excess CO near the water-cooled wall. [2, 3]. Simultaneously, a considerable quantity of H₂S gas is generated, further exacerbating the high-temperature corrosion of the water-cooled wall near the furnace, which will significantly intensify the scaling of the water-cooled wall pipes [2, 4, 5]. Therefore, a long-term and on-line H₂S gas monitoring system plays a pivotal role in predicting and controlling the water wall corrosion. This can be achieved either by indirectly monitoring the concentrations of O₂ and CO, or by monitoring the H₂S concentration directly.

The continuous detection of oxygen (O₂) and carbon monoxide (CO) concentrations is typically accomplished through electrochemical sensor and nondispersive infrared sensor, respectively [1]. However, these two sensors are susceptible to interference from other molecules and exhibit limited sensitivity for H₂S [7]. H₂S concentration is conventionally determined via gas chromatography [8]. This method has strong anti-interference ability, but has the disadvantage of slow response time. Therefore, an on-line and long-term sensor system with high selectivity, high sensitivity and rapid response for both H₂S and CO is desirable for investigating the corrosive mechanism and assessing the corrosion hazard. Tunable diode laser absorption spectroscopy (TDLAS) with the merits of high selectivity, high sensitivity and rapid response is widely used in industrial process, combustion analysis, leak detection and semiconductor manufacturing etc [8-11]. Guo et al., for instance, used a DFB laser with a wavelength of 1.7 µm to construct a gas sensor for measuring H₂S in natural gas based on TDLAS. The sensor continuously measured data for 48 hours and exhibited strong stability [12]. Qiu et al. developed a detection system for fire warning using a DFB laser with a wavelength of 2.3 µm and wavelength modulation spectroscopy combined with second harmonic detection (WMS/2f) technology. The sensor detected CO gas under a path length of 27 m and demonstrated high sensitivity and good selectivity [13].

However, due to the limited tunable range of distributed feedback lasers (DFB) commonly used in TDLAS [14], they can typically only cover one gas in practical measurements unless the absorption spectra of two gases are closely spaced [15]. Therefore, in order to achieve simultaneous measurement of two or more gases [16], multiplexing techniques are commonly employed. Notable methods of multiplexing techniques include Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) techniques [17]. Zhang et al. combined wavelength modulation
spectroscopy with TDM techniques to simultaneously detect CH₄ and C₂H₂ gases under an optical path length of 52 m. While the device achieved detection limits of 0.1 ppm and 0.2 ppm, respectively, it exhibited a relatively long response time of about 44 seconds [18]. Moreover, the timing in this method is not entirely synchronous, which may introduce errors in practical applications. Raza et al. combined WMS with FDM to simultaneously detect CH₄ and C₂H₂ gases under an optical path length of 36 m. The sensor achieved a minimum detection limit of 0.92 ppm for CH₄ and 0.64 ppm for C₂H₂ with an integration time of 0.02 s. While this sensor excelled in sensitivity and time resolution, it was vulnerable to environmental interference and unsuitable for high-temperature operations, posing challenges in achieving on-site in-situ measurements [17]. Furthermore, these sensors are typically tested in a laboratory environment. If used in industrial settings, their reflective mirrors may come into direct contact with the tested gases and be easily contaminated, making long-term online monitoring impractical and lacking in practicality.

In this work, we introduce an in-situ online non-contact monitoring device for H₂S and CO that can be installed in high particulate and high-temperature environments of power plants. Taking into account the influence of gas temperature and contamination of optical windows by H₂S adsorption, a Herriott multipass cell capable of withstanding high-temperature gases is utilized. It is combined with a self-made compact electronic system that integrates FDM with WMS-2f [19]. The device enables continuous online monitoring of H₂S and CO concentrations in coal-fired boilers. The monitoring results obtained align with the combustion characteristics are consistent with the corrosion status of the power plant's water walls. The equipment has been successfully operated for over one and a half years in industrial environments characterized by high dust and temperature, demonstrating its reliability and ability to cope with various complex environments.

## 2. Theory

### 2.1. Selection of absorption lines

The laser sources in the communication wavelength range are lower cost and more reliable in comparison with mid-infrared ones where species present stronger absorption features [20]. Here, a DFB laser is selected as the laser source. Considering the FDM technology, the use of only one photodetector is required. In order to meet the responsiveness of the photodetector, it is best if the absorption lines of two gases are not too far apart. In addition, these lines should not be disturbed by each other or interfered by other molecules. As plotted in Fig. 1, the absorbance of the candidate lines for H₂S and CO have been simulated according to the HITRAN database [21]. It is observed that the absorption lines of H₂S and CO do not interfere with each other. Herein, the possible disturbers from the sampling gas close to water wall, such as CO₂ and H₂O, are also considered in the simulations, which are based on the constitution of flue gas close to the boiler tube [7]. The absorption of H₂O in this regions is weak, but cannot completely be neglected. CO₂ presents a challenge because it overlaps within the selected spectral regions. To circumvent this issue, a low pressure of 10 kPa was employed to narrow the spectral line and avoid the spectrum superposition. The temperature was set as high as 120 °C for avoiding the condensation of water vapor and the adsorption of H₂S.
<figure><img src="image_3.png" /><figcaption>Fig. 1. Simulation spectra of H₂S、CO、CO₂ and H₂O based on HITRAN.</figcaption></figure>

## 2.2. Measurement principle

The principle of TDLAS has been described in detail elsewhere [14]. The sample gas absorption follows the Beer-Lambert law [22]:

$$ \tau(v) = \frac{I_t(v)}{I_0(v)} = \exp(-\alpha(v)L) \quad (1) $$

where $I_0$ is the intensity of the incident light; $I_t$ is the intensity of the transmitted light; $\alpha(v)$ is the absorption coefficient per unit length of the gas molecules, and $L$ is the effective absorption path length. WMS is widely used in TDLAS for improving the signal to noise ratio of gas sensors [23]. The laser injection current is modulated by a low-scanning triangle or sawtooth signal (~Hz) superimposed by a high-frequency sine wave (~kHz) with small amplitude. The low-scanning signal tunes the wavelength of the laser across the absorption line. High-frequency modulated signal shifts the detection frequency to a higher region to reduce the white noise [24]. The WMS measurement can be expressed as:

$$ I_0(t) = \bar{I}_0 \left[ 1 + i_{a1} \cos(\omega_1 t_1 + \varphi_{a1}) + i_{a2} \cos(2\omega_1 t_1 + \varphi_{a2}) \right] + \bar{I}_1 \left[ 1 + i_{b1} \cos(\omega_2 t_2 + \varphi_{b1}) + i_{b2} \cos(2\omega_2 t_2 + \varphi_{b2}) \right] \quad (2) $$

herein, $\bar{I}_x$ (x=1,2) is the intensity of the incident light, $i_{ax}$, $i_{bx}$ is the modulation coefficient, $i_{a1}$, $i_{b1}$ is the linear term, and $i_{a2}$, $i_{b2}$ is the nonlinear term. The $\varphi_{ax}$, $\varphi_{bx}$ is the phase of the harmonic signal, where $\varphi_{a1}$, $\varphi_{b1}$ is the phase shift of the first harmonic signal, and $\varphi_{a2}$, $\varphi_{b2}$ is the phase shift of the second harmonic signal, $\omega_x$ is the angular frequency of the sinusoidal modulated signal.

When two lasers share the same modulation frequency, they will overlap in the spectrum, leading to potential interference issues. Therefore, to achieve FDM, it is necessary to provide different modulation frequencies to two lasers with different wavelengths. In this work, the concentration of H₂S is relatively low while the concentration of CO is generally higher. To improve the signal-to-noise ratio (SNR) of the H₂S absorption signal at low concentrations, we chose to superimpose a higher frequency sinusoidal modulation on the output light of the H₂S laser. Conversely, a relatively lower frequency sinusoidal modulation was applied to the output light of the CO laser.

The transmittance of the laser can be expressed using Fourier series expansion [25], when the absorption is weak ($\alpha < 0.05$):

$$ \tau(t) = \sum_{k=0}^{\infty} H_k \cos(k\omega t) \quad (3) $$
where $H_k$ is the kth-order Fourier coefficient, which is expressed as:

$$H_k = \frac{1}{(1 + \delta_{k0})\pi} \int_{-\pi}^{\pi} \tau(\nu + a \cos \theta) \cos(k\theta) d\theta \quad (4)$$

In WMS, the amplitude of the harmonic signal is negatively correlated with the harmonic order [26]. At the center position of the absorption line, odd-order harmonics are symmetric about the origin and have an amplitude of 0, while even-order harmonics are symmetric about the axis and have the maximum amplitude. Therefore, the second harmonic signal is used to detect the gas concentration, and the amplitude of the second harmonic at the center position of the absorption line is expressed as:

$$S_{2f} = \frac{1}{2} G \bar{I}_x \left[ H_2 + \frac{i_{a1(b1)}}{2} (H_3 + H_1) \cos \varphi_{a1(b1)} + \frac{i_{a2(b2)}}{2} (H_4 + 2H_0) \cos \varphi_{a2(b2)} \right] \quad (5)$$

When the absorption is weak, the gas absorption is not sensitive to the concentration of the target gas. At this time, in order to reduce the impact of intensity changes on the measurement, we can normalize it through the WMS-2f/direct sine (DS) signal:

$$C \propto \frac{S_{2f}}{I_0} \quad (6)$$

where C is the concentration of the gas after normalization, it can be seen that this method eliminates the interference of light intensity during the measurement process.

### 3. Experimental

As presented in Fig.2(a), the laser sensor system is constructed by optical part, electronic and controlling section, and sampling and vacuum subsystem. For the optical part, two DFB lasers are employed, emitting at 1578 nm and 1567 nm for H₂S and CO, respectively. These laser outputs are combined via a 2×1 fiber combiner and subsequently coupled into a Herriott multipass cell with the assistance of a GRIN lens. The Herriott multipass cell was set to 120 °C for preventing the adsorption of H₂S on the cell walls [7]. The laser beams are reflected 65 times to enhance the absorption signal. The separation between the two mirrors is 0.28 m, resulting in an effective optical path length of 18 m. Finally, the exiting beam is focused into a photodetector for further analysis.

The homemade electronic circuit includes the DFB laser controllers and lock-in amplifier mode [27]. The temperatures of the H₂S and CO lasers were respectively 29 °C and 26 °C, with the injection currents set at 90.2 mA to ensure comprehensive coverage of their absorption spectra. The triangle wave is generated by the DAC controlled by the STM32F405 chip, and the sine wave is produced by a 5th-order Butterworth filter from the 1f TTL level signal output by the CPLD. For the implementation of FDM, the low-scanning frequency of 1 Hz is employed, with modulated frequencies of 7 kHz and 2 kHz for H₂S and CO. The sine wave is then added to the triangle signal through an adder [20]. The 2f signal from the CPLD is used as the reference signal, and half of the output from the BPF is used as the modulation signal sent to the AD630 for demodulation.

The sampling and vacuum subsystem is considerably complicated due to the harsh environment and complex compositions of flue gas. It includes the sample probe, ceramic filter, blowback, heated sampling line and sample gas cooler. As presented in Fig. 2(c), a pneumatic cylinder with a rod was deployed to clean blockage of the slag and tar filled in the probe tube. To prevent melting inside the boiler, the gas sampling probe tube should withstand high temperatures
for long time. The ceramic filter is utilized to remove the high dust in the flue gas sample and the compressed air is used for purging the fouling matter of filter and probe tube. In sampling process, the heated sampling line was set to 120°C to avoid the condensation of sampling flue gas which could result in blocking the line. The volatile compounds and water vapor were removed by a sampling gas cooler in order to preclude the potential absorption interference and protect the optical components of multipass cell. A condensate vessel was employed to collect the liquid from the cooler. The pressure of the cell was set to 10 kPa and controlled by a PID system based on PLC. The sensor system can achieve auto-calibration via activating the solenoid valves to transport standard gas into the cell.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a)H₂S measurement equipment flowchart; (b)equipment physical diagram; (c)purging system.</figcaption></figure>

4. Results and discussion

The normalized WMS-2f/DS method was adopted for implementation of a calibration-free
measurement [28], primarily due to the significant intensity variations resulting from dust
attenuation and optical contamination within the gas flue sample, which are critical considerations
in the measurement process. The relationships between concentration and normalized WMS-2f/DS
for H₂S and CO are illustrated in Fig.3 (a) and (b). H₂S was calibrated in the range of 10-2500 ppm,
a concentration range that is typically found in flue gas. The error bar was derived by calculating
the standard deviation of the obtained signals. Its concentration could be determined by
y=0.2086x+0.1268. The inserts are the error bar and WMS-2f signal with high SNR. In practice,
the CO concentration close to water wall is as high as several tens of percent. Therefore, it is
calibrated in region of 0.2%-30%, and its SNR can reach more than 23 for 0.2%. Similar to H₂S,
the CO concentration is evaluated by y=922.55x+3.86. The slope and intercept were determined by
fitting, with R² values of 0.9997 and 0.9995 for H₂S and CO, respectively, showing a good linear
relationship for both species.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** The relationship between the WMS-2f/DS-sine and the concentration. (a) H₂S concentrations range of 10-2500 ppm. (b) CO concentrations range of 0.2%-30%. (Inserts are 2f signal and error bar)</figcaption></figure>

For demonstrating the stability and minimum detection limit of the system, Allan variance analysis was carried out for 100 ppm H₂S and 100 ppm CO within 2000 s. As illustrated in Fig.4(a) and Fig.5(a), the minimum detection limit (MDL) of H₂S reaches 111 ppb at 178 s, while the MDL of CO achieves 116 ppb at 144 s. Their histograms of the concentration distribution are shown in Fig.4(b) and Fig.5(b). The concentration distribution of both H₂S and CO resembles a Gaussian distribution, and the red line in the figure represents the Gaussian fitting to the histogram. According to the fitting, the full width at half maximum (FWHM) for H₂S is 3.34, indicating the measurement precision of 3.34 ppm with an R² value of 0.994. Regarding to CO, the measurement precision is 2.52 ppm with an R² value of 0.994.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (a) Allan variance of 100 ppm H₂S as a function of integration time. (b) Histogram of the measurement of 100 ppm H₂S for 2000 s.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Allan variance of 100 ppm CO as a function of integration time. (b) Histogram of the measurement of 100 ppm CO for 2000 s.</figcaption></figure>

Because it would be installed close to the boiler for continuous real-time monitoring, the sensor system should endure the severe environment including high temperature, noise and mechanical vibration. Therefore, vibration and thermal cycling tests were performed to evaluate the stability and reliability of the system. As shown in Fig.6(a), the retrieved H₂S concentration fluctuated by less than 1.5% in the vibration frequency range of 0-400 Hz, produced by an electromagnetic vibration table set to horizontal full-wave vibration. The H₂S concentration variation was less than 2% in the temperature range of 10-50 °C, shown in Fig.6(b). It proves the reliability of the system for the measurement campaign.

<figure><img src="image_8.png" /><figcaption>Fig. 6. (a) Concentration of 100 ppm H₂S at different vibration frequencies; (b) Concentration of 100 ppm H₂S at different temperature.</figcaption></figure>

## 4. Field measurement

Three laser sensor systems have been deployed to a 300 MW coal-fired boiler. They were used for measuring the concentrations of H₂S and CO from the flue gas close to water wall tube. Additionally, a commercial galvanic cell sensor (KE-50-10) was integrated for simultaneously monitoring the O₂ concentration. The concentration of O₂ is crucial for investigating the corrosive mechanism of fire-side water wall tube. It is a key parameter for adjusting the burner preventing corrosion. Herein, it is also a critical indicator for determining the interval and duration for the blowback, because the rapid increase in O₂ concentration during the purging process leads to a rapid decrease in H₂S and CO concentrations. It could be explained by the following overall chemical
reactions:

$$2\mathrm{H}_2\mathrm{S} + 3\mathrm{O}_2 \rightarrow 2\mathrm{SO}_2 + 2\mathrm{H}_2\mathrm{O} \quad (1)$$

$$2\mathrm{CO} + \mathrm{O}_2 \rightarrow 2\mathrm{CO}_2 \quad (2)$$

Fig. 7 shows the change of H₂S, CO and O₂ concentrations within 5 hours during purging. The blowback duration is set 40 minutes, and their concentrations could be recovered around 3 minutes. It guarantees the effectiveness of purging sampling probe and the rapid response of the sensor system.

<figure><img src="image_9.png" /><figcaption>Fig. 7. H₂S (ppm), CO (%) and O₂ (%) concentration data during purging</figcaption></figure>

Under the ultra-low NOₓ combustion, a reducing atmosphere gradually forms near the water wall, with the major reducing ambient gases being H₂ and CO [29]. The average sulfide amount of coal typically exceeds 1.5% in China [7, 29]. Corrosive gases such as H₂S and SO₂ are primarily released during coal pyrolysis due to the influence of reducing gases [7]. The main paths for generating H₂S are as follows [7, 29]:

$$\mathrm{CO} + \mathrm{H}_2\mathrm{O} = \mathrm{H}_2 + \mathrm{CO}_2 \quad (3)$$

$$\mathrm{SO}_2 + 3\mathrm{H}_2 = \mathrm{H}_2\mathrm{S} + 2\mathrm{H}_2\mathrm{O} \quad (4)$$

$$\mathrm{H}_2 + \mathrm{COS} = \mathrm{H}_2\mathrm{S} + \mathrm{CO} \quad (5)$$

$$3\mathrm{CO} + \mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} = \mathrm{H}_2\mathrm{S} + 3\mathrm{CO}_2 \quad (6)$$

Furthermore, H₂S can be transformed into elemental sulfur [S] through the reaction between H₂S and SO₂, combustion of H₂S under anoxic condition, and decompositions of H₂S at high temperature [30, 31].

$$2\mathrm{H}_2\mathrm{S} + \mathrm{SO}_2 = 2\mathrm{H}_2\mathrm{O} + 3\mathrm{[S]} \quad (7)$$

$$2\mathrm{H}_2\mathrm{S} + \mathrm{O}_2 = 2\mathrm{H}_2\mathrm{O} + 2\mathrm{[S]} \quad (8)$$

$$\mathrm{H}_2\mathrm{S} = \mathrm{H}_2 + \mathrm{[S]} \quad (9)$$

Therefore, the concentrations of H₂S, CO and O₂ are vital parameters for investigating the
formation of H₂S and elemental [S]. Based on the metal high temperature oxidation theory [32], the oxidation layer of the water wall tube could be corroded by H₂S and [S] according to the following chemical reactions:

$$H_2S + FeO = FeS + H_2O \quad (10)$$

$$3Fe_2O_3 + 7H_2S = 6FeS + 7H_2O + SO_2 \quad (11)$$

$$Fe_3O_4 + 4H_2S = FeS + Fe_2S_3 + 4H_2O \quad (12)$$

As for the substrate Fe sulphuration, it could be described by:

$$H_2S + Fe = FeS + H_2 \quad (13)$$

$$[S] + Fe = FeS \quad (14)$$

It is a complicated process for both the coal combustion and the water wall corrosion at different positions of the furnace. Therefore, three devices have been installed at heights of 17.9 m, 21 m, and 23.6 m from the rear wall of the boiler, respectively. The sampling probe points are close to the water wall of the primary combustion zone. The concentrations of H₂S, CO and O₂ are presented in Fig 8, but the purging durations are excluded because the compressed air could disturb the measurement. According to half month on-line measurements, it reveals that the O₂ concentration in the three points is at a low level, and the overall concentration is lower than 3%. The H₂S nearly disappears and the CO concentration decreases when the O₂ concentration is more than 5% around the sixth day, as present in Fig.8 (a). It indicates that the concentration of O₂ has a strong influence on the H₂S and CO concentrations. As shown in Fig 8(b), when the O₂ concentration is at an extremely low level, the concentration of H₂S and CO can exceed 1000 ppm and 20%, respectively. It indicates the high risk of corrosion in these regions. As plotted in Fig 8(c), the variation trends of H₂S and CO are highly consistent, and the concentration of H₂S is dependent on the CO at the 23.6 m sampling point. It can also be clearly seen that the concentrations of H₂S and CO are inverse correlated with that of O₂. The concentration relationships among H₂S, CO and O₂ at 23.6 m are similar to the previous research [29]. This also corresponds to the characteristics shown in chemical reaction 6 above: the presence of CO promotes the generation of H₂S.

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. On-site measurements of H₂S and CO and O₂ concentration data over a period of two weeks.</figcaption></figure>

The data from the three measurement points in Figure 8 were averaged, and the results are shown in Table 1. The concentration of O₂ at the three measurement points is relatively low, with the lowest value of only 0.3% at the 21 m sampling point. This is because it is located in the main combustion zone of the furnace, where intense oxygen-deficient combustion occurs. It is also the reason for the relatively high CO concentration at the three sampling points. As for H₂S, the concentrations at the 17.9 m and 23.6 m are not high, at around 200 ppm. Because these two sampling points correspond to the second and third wind layers, respectively, where the coal content is low and the air flow is relatively high [33]. The average H₂S concentration at the 21 m is the highest, reaching around 800 ppm, which is about four times higher than the other two measurement points. This is due to the 21 m measurement point being located near a coal feeder, resulting in a higher coal content and an increased production of H₂S during combustion. It is worth noting that the H₂S and CO concentrations at the 21 m are generally high, which can lead to high-temperature corrosion in the boiler. According to the results of on-site shutdown and maintenance, severe high-temperature corrosion had occurred at 21 m, with a large corrosion area, in good agreement with the measurement results. Therefore, to prevent and control high-temperature corrosion, it is recommended to maintain a low-reducing or oxidizing atmosphere near the boiler wall to reduce H₂S generation and thus decrease the corrosion rate of the water wall.

<table><thead><tr><th>Different height<br>measuring points /<br>m</th><th>Average<br>concentration of<br>H₂S / ppm</th><th>Average<br>concentration of<br>CO / %</th><th>Average<br>concentration of O₂<br>/ %</th></tr></thead></table>
<table><caption>Table 1. The average concentrations of H₂S, CO and O₂ at different boiler measuring points in half a month</caption><tr><td>17.9m</td><td>220ppm</td><td>15.9%</td><td>2.2%</td></tr><tr><td>21m</td><td>800ppm</td><td>13.6%</td><td>0.3%</td></tr><tr><td>23.6m</td><td>240ppm</td><td>10.7%</td><td>0.7%</td></tr></table>

## 6. Conclusion

A fixed monitoring equipment was established for measuring the flue gas in the primary combustion zone of a coal-fired power plant, aiming to analyze the atmosphere of the coal-fired boiler. The equipment combines WMS-2f with FDM and utilizes a homemade modulation-demodulation circuit system. It incorporates a high-temperature resistant Herriott multipass cell with two laser beams at wavelengths of 1.578 µm and 1.567 µm combined into a single beam, enabling simultaneous monitoring of two different gas concentrations [22]. The stability of the system was tested in anti-vibration and high-low temperature experiments, which validated its strong anti-interference capabilities. The detection precision and stability of the sensor was evaluated using the Allan variance method. The detection limits for H₂S and CO were determined as 111 ppb and 116 ppb, respectively, with corresponding integration times of 178 s and 144 s. Three devices were installed for synchronous monitoring of H₂S and CO at a power plant. It was found that the H₂S and CO concentrations were highest at the 21 m measurement point in the main combustion zone, with an average H₂S concentration of 800 ppm and an average CO concentration of 13.6%. This is the main factor causing high-temperature corrosion in the boiler. Furthermore, the study results indicate that there is a positive correlation between H₂S concentration and CO concentration in the boiler furnace, and a low O₂ concentration directly leads to an increase in H₂S concentration. Therefore, real-time synchronous monitoring of H₂S and CO concentrations in the furnace is of great significance for the prevention and control of high-temperature corrosion and the safe operation of the boiler.

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