In this paper, a scheme for on-line monitoring of multi-component decomposition based on TDLAS is presented, which is mainly composed of hydrogen sulfide, carbon monoxide and hydrogen fluoride. Considering that the most important problem of multi-component gas detection is the crossover of components, the characteristic spectra were first analyzed. The characteristic absorption peaks of hydrogen fluoride are very strong, which makes it easy to detect hydrogen fluoride with high sensitivity. However, the absorption peak of hydrogen fluoride covers hydrogen sulfide and carbon monoxide, making it difficult to detect. The interference problem is analyzed and the solution is given. In addition, multi-parameter transmitter in power industry is used to provide data for on-line calibration. The concentration of sulfur hexafluoride is retrieved from SF6 density information provided by multi-parameter transmitter. According to the temperature information, decomposition gas concentration value is calibrated. In this paper, an on-line monitoring device for SF6 decomposition mixture gas based on laser absorption spectroscopy is developed. The results of temperature cycle test show that the influence of environmental parameter changes on the concentration results can be reduced after on-line calibration.
The characteristic absorption lines of carbon monoxide gas cross with that of other gases in near-infrared and midinfrared bands, the detection results are easily disturbed. Especially in some environments such as coal mines and petrochemicals, it is more important to avoid cross interference, in order to achieve accurate measurement of carbon monoxide. In this paper, a DFB laser beam at 2330nm was selected and scanned for absorption line of carbon monoxide. Although interference of other gas components was avoided, the overlap of carbon monoxide and methane spectra still existed. Firstly, the absorption lines of methane and carbon monoxide were studied. According to the theory of molecular spectroscopy, the FWHM of methane and carbon monoxide absorption lines are different. Once, there is methane gas in the background, the harmonic signals will be different due to different line shape. The second harmonic signals with different modulation coefficients are simulated. So the troughs width is selected as the evaluation function of the harmonic signal characteristics for component judgement. Secondly, after component determination, when mixed gas is present, adjacent methane absorption peaks are also scanned for deduction. The influence of temperature is also calculated and the evaluation function is modified again. Finally, the free calibration measurement of carbon monoxide concentration in the mixture is realized. In summary, for the working conditions of complex background gas such as coal mine and petrochemical. This paper presents a calibration-free method for reliable carbon monoxide concentration detection based on a TDLAS technique.
During industrial process of gas monitoring, gas pumps are typically used for extract gas dynamically. In order to improve the response time of system, high flow rate pumps are used to draw gases under detection into the gas cell, and therefore, the airflow during monitoring has an impact on system stability. In this paper, we firstly optimize the design of absorption cell through software simulation, so as to improve the airflow stability at high flow rate. And secondly, the normalization algorithm is used to try to suppress the influence of airflow fluctuation on the stability. Finally, we built a dynamic methane measurement system, and the results of normalized and un-normalized concentrations over time were provided for comparison, respectively. The experiment results show that under the flow rate is 34L/min, the response time is 1.2s, and after normalized treatment, the stability is improved from 2.68ppm to 0.64ppm (1σ).
Tunable diode laser absorption spectroscopy (TDLAS) is often used to detect the industrial processing gas component, but the concentration measurement in the field is easily affected by the surrounding temperature. Firstly, the effect of temperature on absorption spectrum was analyzed. Taking methane as an example, DFB laser with 1654nm was used to collect the concentration under temperature from 253K to 323K (with the interval of 10K for each point) based on WMS method. The results showed that the maximum relative error was 25%. Based on the experimental data, the correction formula of second harmonic full waveform was fitted, and the gas detection system with temperature compensation was constructed at the same time. The experimental results illustrated that the maximum relative error under the same test conditions was reduced to less than 5%, and the detection accuracy were significantly improved under variable temperature environment, which provided the guarantee for TDLAS online monitoring and broadened the use scenario of the technology.
Propane is a key component of liquefied petroleum gas (LPG) and crude oil volatile. This issue summarizes the recent progress of propane detection technology. Meanwhile, base on the development trend, our latest progress is also provided. We demonstrated a mid infrared propane sensor system, which is based on wavelength modulation spectroscopy (WMS) technique with a CW interband cascade laser (ICL) emitting at 3370.4nm. The ICL laser scanned over a sharp feature in the broader spectrum of propane, and harmonic signals are obtained by lock-in amplifier for gas concentration deduction. The surrounding gas is extracted into the fine optical absorption cell through the pump to realize online detection. The absorption cell is designed in mid infrared windows range. An example experimental setup is shown. The second harmonic signals 2f and first harmonic signals1f are obtained. We present the sensor performance test data including dynamic precision and temperature stability. The propane detection sensor system and device is portable can carried on the mobile inspection vehicle platforms or intelligent robot inspection platform to realize the leakage monitoring of whole oil gas tank area.
Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors involves with many desirable features, such as being small and portable, with fast continuous in situ measurements possible. In QEPAS systems, reference cells filled with gas mixtures for wavelength locking and calibration are used to improve the precision and stability of the trace gas concentration measurement. For this study, a 5 m length hollow core photonic bandgap fiber (HC-PBF) splicing with single mode fibers at two ends was manufactured as reference cell, which has long absorption path, low transmission loss and easy connectivity. Hollow cores under high pressure (3.0 × 105 Pa) were filled with a certified mixture of ammonia and nitrogen gas to reach equilibrium rapidly. The experiment results indicated that absorption spectra of reference cell with a low loss <3.5dB maintained stable after 150 days at atmosphere. For trace gas detection, 1.531 μm DFB laser with wavelength modulation technique was demonstrated based on QEPAS. A normalized noise equivalent absorption coefficient (NNEA) of 1.18×10-7 cm-1W/√Hz was obtained at room temperature and pressure of 760 Torr. This results in a minimum detection limit of 3.6ppm for noise equivalent concentration within a 1s lock in integration time.