| Ethylene is a colorless inflammable gas and will form explosive mixtures when mixed with air. It is used as the index gas for the early detection of spontaneous combustion of coal in coal mine safety production. Ethylene acts as a hormone in higher plants and plays a significant role in the growth of plants. Meanwhile, ethylene is a biomarker for lipid peroxydation in human cells. Additionally, it also has an effect on atmospheric chemistry and ozone generation. So, the research of trace ethylene analysis has significance in the fields of environment monitoring, industrial production, biology and medical treatment.Tunable diode laser absorption spectroscopy (TDLAS) is used for trace ethylene detection and analysis. In near infrared (NIR) area, ethylene spectrum is severely interfered with each other nearby and has no pure single-peak. The key issue of the paper is to establish a calibration method with high sensitive and low detection limit in order to solve the problem of spectra interferences and background signal fluctuation. The gas to be measured is mixed with different species, so how to rapidly separate them and recognize the ethylene spectra is another problem to be solved。Besides, the intensities and line widths of gas absorption spectra will be changed under different temperature and pressure. Effectively modifying the influence of gas temperature and pressure vibration on measurement results is also the topic of the research. According to the problems above, the main research works carried out are as follows:Firstly, the ethylene trace gas analysis system using TDLAS was developed. By analyzing the characteristic of ethylene spectra, considering the detection sensitivity, the spectral cross interference and system cost, the absorption lines in NIR near1626nm were selected for ethylene trace analysis. A custom tunable diode laser in corresponding spectral range was used, a long path absorption cell was designed and the ethylene trace gas analysis system was developed.Secondly, trace ethylene concentration calibration method was studied. In order to solve the problem of the severely spectral overlap of the selected ethylene spectrum, methods of wavelength modulation, absorption coefficient and sparse decomposition were used for ethylene concentration calibration. Wavelength modulation method combined with harmonic detection is free of background signal fluctuation and has strong anti-jamming capacity. However, when the spectra are severely overlapped, the result of calibration with second harmonic wave is not satisfactory. The ethylene detection limit of this method is about34ppm (parts per million). Absorption coefficient method uses the measured absorption coefficient for nonlinear fit of the scanning absorption signal. A detection limit of5ppm can be reached. Sparse decomposition method constructs a over-complete atoms dictionary in accordance with the signal characteristics, and separate the absorption signal by matching pursuit algorithm and express it as the combination of multi-atoms. These atoms contain the background signal as well as the individual absorption lines. Using the strongest line for concentration calibration, this method can reach a detection limit of4ppm.Thirdly, the multi-component gas absorption line separation and detection method was proposed. In order to solve the spectral cross interference in multi-component gas, two line separation methods of multi-peak measurement and multi-Lorentz function decomposition were proposed. Multi-peak method picks out different absorption peaks from the spectra of ethylene and other gas in a wide spectral range and forms linear equations to get the concentration of ethylene and other gas. A mixture of ethylene and methane were measured in the spectral range from1625.8nm to1627.4nm. The result shows that both concentration errors are less than5%. This method has a relative high detection limit and need a relative wide spectral scanning range. Multi–Lorentz function decomposition method builds a triple Lorentz model to describe the ethylene spectra, according to the result of sparse decomposition. The parameters of line widths and central wavelength are decided by sparse decomposition method. As the multi-component gas tests show, the ethylene concentration error is less than3ppm.Finally, the influences of temperature and pressure on concentration measurement were tested and analyzed. The variety of intensities and line widths of absorption line were tested under different temperature and pressure. According to result, compensation methods for temperature and pressure were established, and the gas concentration error descend to2ppm after compensation. The concentration tests for the entire system are made. The results show the system can steadily detect4ppm ethylene, with an average of4.9ppm in30s. The ethylene concentration measurement results of the system is compared with that of gas chromatograph. The deviations of them are less than3ppm. The uncertainties of ethylene concentration measurement of the system were analyzed and the combined standard uncertainty is calculated to be3.1ppm. |
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