The Study Of Methane Concentration Measurement Based On FFT And TDLAS Techniques

With the development of industrial technology,environmental safety issues have been attracted more and more attention.Generally,the content of methane gas in the atmosphere is about 1.8 ppm(parts per million),which is a typical trace gas.With in crease of atmospheric methane concentration of trace,it can obviously increase the greenhouse effect.Reducing methane emissions can effectively reduce the greenhouse effect.Tunable Diode Laser Absorption Spectroscopy(TDLAS),which is widely used in trace gas detection with its advantages of no-contact,high precision and fast response.Based on the basic principles of TDLAS technology to detect trace gases,this thesis proposes a Fast Fourier Transform(FFT)technology under fixed bias based on TDLAS,(TDLAS-FFT)and builds a detection system to obtain corresponding experimental data.Then through the direct absorption spectroscopy(DAS)technology and wavelength modulation spectroscopy(WMS)technology experimental results based on the conventional detection technology of TDLAS.The comparison of the experimental results of the three detection methods proves the feasibility of the TDLAS-FFT technology.The TDLAS-FFT method can not only reduce the cost of the TDLAS trace gas detection system to a certain extent but also has certain practical significance for its further miniaturization and integration.The specific research content of this thesis is as follows:Firstly,use Python to get the methane spectrum data from the HITRAN database,and through a program written in Python to simulate the absorption spectrum of methane to select the laser used in this thesis.This wavelength not only matches the absorption spectrum of methane,but also eliminates the influence of other environmental gases on the accuracy of methane detection for the detection of methane concentration by TDLAS technology.Secondly,designed the important modules and real-time monitoring function.The real-time monitoring function is mainly realized through a graphical programming language(Laboratory Virtual Instrument Engineering Workbench,Lab VIEW),data acquisition card and supporting NI MAX software;Used DAS technology and WMS technology quantitatively detects the concentration of methane and analyzes the respective experimental results.Then analyzed the advantages and disadvantages of the two detection methods obtained.Thirdly,TDLAS-FFT technology to measure methane concentration:a new detection method other than DAS technology and WMS technology is proposed.It is superior to DAS technology and WMS technology in some aspects,and which is a supplement to TDLAS technology.This part includes:starting from the principle of FFT and the basic knowledge of signal frequency domain transformation,introducing the theoretical principle of TDLAS-FFT technology for detecting methane concentration;using TDLAS-FFT technology instead of conventional detection methods to quantitatively measure methane concentration,experimental steps,including the determination of the range of methane absorption point,the frequency domain conversion of the absorption signal and the recovery of the second harmonic signal;by comparing the experimental results of DAS technology,WMS technology and FFT-based TDLAS technology,it analyzes not only the TDLAS-FFT technology and other difference between two conventional technologies also proves the feasibility of TDLAS-FFT technology to detect gas concentration.