Study On Key Technologies Based On Photoacoustic Spectroscopy For Methane Concentration Detection

Methane gas concentration detection is of great significance for coal mine production safety,gas pipeline safety operation detection,environmental protection and so on.The gas sensing detection technology based on photoacoustic spectroscopy has the advantages of real-time,high sensitivity,long life and good selectivity.It has important theoretical and practical value.On the basis of the photoacoustic spectrum gas detection principle,the design and optimization of the photoacoustic cell and the optical fiber acoustic sensor,and the research of signal detection technology,the methane concentration detection system based on the photoacoustic spectrum is built,and the methane concentration detection is realized.The main research contents are as follows:Firstly,the research significance and purpose of methane concentration detection were introduced,and the current research status of methane concentration detection was summarized.The mechanism of photoacoustic spectroscopy gas detection technology is described,and the gas infrared molecular absorption spectrum,photoacoustic signal generation mechanism,wavelength modulation and second harmonic detection theory are emphatically introduced,which provides a theoretical basis for the establishment of methane concentration detection system based on photoacoustic spectroscopy technology.Secondly,the theoretical model of the photoacoustic cell is established.The influence of the length and radius of the resonator and buffer chamber on the characteristic frequency and the intensity of the photoacoustic signal is studied by using the finite element method.A photoacoustic cell with a resonant frequency of 1400Hz（±5Hz）was fabricated,and its response curve was obtained.The experimental results are in agreement with the simulation results.At the same time,a fiber optic acoustic sensor with resonance frequency of 1400Hz based on cantilever beam is designed,and theoretical analysis and simulation are carried out,which provides a theoretical basis for the manufacture of fiber optic acoustic sensor.Finally,a methane concentration detection system based on photoacoustic spectroscopy was established.The system mainly includes:distributed feedback（DFB）laser with central wave number of 6047 cm^-1,photoacoustic cell,microphone,laser driving circuit,temperature control circuit and phase-locked amplifier.The system is debugged and the resonant frequency of the photoacoustic cell is tested.The methane gas concentration response characteristics of the system were studied experimentally,and the methane gas concentration sensing was realized in the range of 1%-30%.The experimental results show that when the methane concentration is2%-10%,the amplitude of the photoacoustic signal increases rapidly,and the system has high sensitivity and good response.When the concentration is greater than 10%,the amplitude of the photoacoustic signal increases slowly and the sensitivity of the system decreases.When the concentration exceeds 25%,the photoacoustic signal becomes saturated and its amplitude decreases.The methane concentration detection system designed in this paper realizes the detection of methane concentration and provides a way for the early warning of methane concentration in the mine environment.