Tunable laser hydrogen sulfide analysis system in natural gas

2023-01-11 10:42


Hydrogen sulfide is highly toxic and corrosive, and there is a risk of hydrogen sulfide generation and leakage in some industrial production such as mining, sulfur-containing oil extraction and refining, and natural gas extraction and transportation. In order to control the possible pollution threat to human health and the environment caused by hydrogen sulfide gas leakage, it is very important to study a highly sensitive monitoring method. Among the existing standard detection methods, the titrimetric colorimetric method requires sampling and pre-treatment of sample gas, which is a cumbersome process with poor quantitative accuracy, and the data obtained seriously lags behind the gas quality changes in industrial sites; the electrochemical sensor has a long response time, short service life and small monitoring range, therefore, none of the above methods can meet the needs of high-sensitivity hydrogen sulfide online monitoring. The limited number of online analytical instruments imported from abroad is limited to a few gas wells and gas gathering stations due to their high price, and they are also difficult to use and maintain to varying degrees, and are susceptible to interference from the site environment and various pollutants.

Optical detection methods are widely used in the field of online gas monitoring, mainly including differential absorption lidar (DIAL), non-dispersive infrared (NDIR), Fourier transform infrared spectroscopy (FTIR), differential optical absorption chromatography (DOAS) and tunable laser absorption spectroscopy (TDLAS). Since the content of hydrogen sulfide is relatively low and often mixed with other gases, the monitoring methods are required to have high sensitivity and strong specificity. The TDLAS technology has the characteristics of real-time online, simple operation, fast response and accurate stability, which can make up for the shortage of existing measurement methods.

The intrinsic structure of gas molecules determines their unique natural vibration frequency. When the incident beam meets the natural vibration frequency of the molecule under test, the molecule will absorb the energy of the incident beam. When a beam of a certain intensity of the selected frequency passes through the cuvette, the intensity of the beam is attenuated due to the absorption of the gas under test. In order to further improve the signal-to-noise ratio of the concentration measurement, the wavelength modulation technique is used in the actual measurement. The principle is to load a high-frequency sinusoidal signal into the original laser drive model, and the resulting laser signal is absorbed by the gas medium and then demodulated using a lock-in amplifier to obtain its second harmonic signal.

TDLAS technology is based on laser tuning and phase-locked amplification technology, and the light source adopts a tunable laser, using temperature or current tuning to make the laser wavelength sweep through the characteristic absorption peak of the gas to be measured, and using the phase-locked method to bring up the second harmonic component of the absorption signal and calculate the gas concentration according to the intensity of the absorption peak. This method has the advantages of high speed, high sensitivity, large detection range and strong anti-interference ability of signal, which is widely used in various gas online monitoring fields.

The main components of the detection system include the laser tuning circuit and laser unit, the detector and phase-locked amplifier module, and the data processing unit. The laser tuning circuit tunes the output wavelength of the laser by both temperature and current tuning. The temperature tuning range is large enough to ensure that the laser output wavelength range covers the absorption range of hydrogen sulfide gas, and the current tuning rate is fast enough to improve the measurement accuracy and sensitivity. Firstly, the temperature tuning method is used to determine the working wavelength interval, and then the laser is tuned with sinusoidal current driving signal. The phase-locked detection part uses the output sinusoidal signal of the laser tuning circuit as the reference signal, combined with the phase-sensitive detector principle, to extract the second harmonic component from the gas absorption signal measured by the photoelectric detection unit and to fit the absorption line pattern of hydrogen sulfide gas, which is input to the data processing unit and can be modeled and processed to calculate the concentration of hydrogen sulfide gas according to the absorption peak intensity.

Tunable laser absorption spectroscopy method of hydrogen sulfide online analysis system, high measurement accuracy, fast response, and low maintenance costs, easy and simple operation, etc., can make up for the shortcomings of existing measurement methods.

Tunable laser absorption spectrometry of hydrogen sulfide online analysis system has the following product characteristics.

1, the use of a wide range of ranges, high precision, can measure ppmv to percent H2S;

2, covering H2S and total sulfur concentration measurement before and after desulfurization;

3, the measurement process is continuous and uninterrupted, no dead zone;

4、No interference from background gases such as CH4 and CO2, stable and reliable operation in harsh industrial environments;

5、No rotating parts, need special maintenance, simple and easy to use;

6、Automatic periodic calibration of the zero point and reading;

7、Probe life up to 7 to 10 years, no need to replace;

8、Total sulfur analyzer adopts low-temperature hydrolysis technology, high safety;

9, ppmv and mg / m³ units can be set according to user requirements;

10、Stainless steel material, the whole machine anti-sulfur treatment, strong corrosion resistance.

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