| Methane gas exists widely in nature and in human production and life.At low concentrations,it poses flammable and explosive risks,while at high concentrations,it will cause harm to human body.As a greenhouse gas in the atmosphere,its ability to promote global warming is 25 times higher than that of carbon dioxide.In outdoor environment,industrial production or animal husbandry will emit methane,and natural gas pipeline transportation will produce methane leakage;In the indoor environment,methane will also be released from the collection of coal under the mine and the domestic use of natural gas.In order to protect the atmospheric environment and human daily life,it is necessary to design a high-precision methane detection system to detect methane concentration in real time.Infrared absorption spectroscopy is widely used in air quality detection,on-line gas analysis,medical diagnosis and agricultural and industrial emissions monitoring due to its advantages of no electromagnetic interference,no chemical pollution,easy sampling,high sensitivity and miniaturization.Tunable diode laser absorption spectroscopy(TDLAS)is a kind of infrared detection technology.Because it uses a single frequency light source,it has stronger selectivity for gas molecules,and is more suitable for the occasions with higher requirements for resolution,sensitivity and accuracy.Based on a DFB laser with a center wavelength of 6046.9cm-1,a TDLAS methane detection system is studied in this paper.Lab VIEW program is used to control the voltage output and input by DAQ card,and it can control the system and record the signal.It is not only easy to operate and control,but also can realize the lock-in amplification through software program,which saves hardware resources and facilitates the integration of filtering algorithm.Based on the distributed feedback laser with center wave number of 6049.9cm-1,gas chamber with optical path length of 375cm,In Ga As detector and HITRAN database,the TDLAS methane detection simulation system is established by using Lab VIEW software.Firstly,the influence of modulation,modulation frequency,scanning amplitude,scanning frequency and filter order on the second harmonic is studied,and the harmonic optimization simulation experiment is carried out:for the general TDLAS detection system,the appropriate scanning amplitude and filter order should be optimized first,so that the harmonic line would be complete and undistorted.Furthermore,increasing the amplitude and peak width of the harmonic for the optimal modulation depth.And finally choose the suitable modulation and scanning frequency.Then the internal and external interference factors of TDLAS system are analyzed.In the external environment,the lower the temperature is,the higher the harmonic amplitude,symmetry and peak width are;In the normal pressure range,with the increase of pressure,the lower the harmonic amplitude,the higher the symmetry and peak width.Inside the system,the interference comes from the thermal noise,shot noise and pink noise of the detector,and most of the noise is in the low frequency range below200Hz;At the same time,there is power frequency interference caused by commercial power,whose frequency is a multiple of 50 Hz.In order to suppress the noise interference in the system,the wavelet de-noising method is studied through the simulation system.After several experiments with different parameters,when the wavelet basis is coif5,the threshold selection rule is heursure,and the hard threshold is selected,the wavelet de-noising can suppress the high-frequency noise of the system,and effectively increase the harmonic amplitude,symmetry and peak width,It is verified by actual experiments.Then,in order to filter the low-frequency noise in the system,the high pass and low-pass filtering method is introduced:the scanning frequency of the system is increased,the high-pass filter is added after the wavelet de-noising,the low-frequency signal in the system is filtered while the absorption signal is retained,and then the parameters of the low-pass filter in the lock-in amplifier are changed to extract the harmonic signal from the high-frequency modulation signal.Simulation results show that this method can greatly increase the harmonic amplitude and effectively improve the peak width and symmetry.The results show that the second harmonic amplitude after filtering is 8.33 times of that without processing,the peak width is 1.49 times of that before processing,and the symmetry is increased from 0.598 to 0.996,which greatly improves the detection accuracy.Finally,the filtering algorithm is integrated into TDLAS methane detection program,and the system calibration is completed.Then,the stability,detection limit and measurement accuracy of the system were tested.Through the test,the fluctuation range of the concentration measured by the system was 6.100%,the detection limit was 10.684 ppbv(parts per billion by volume),and the comprehensive average error was about 0.020 ppmv(parts per million by volume). |
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