| Except for the explosion of methane(CH4),the spontaneous combustion of coal poses the greatest threat to the production safety of coal mines.When the spontaneous combustion temperature of the coal seam rises,a small amount of acetylene(C2H2)will be produced.At this time,the oxidation of the coal sample has entered the open flame stage.Therefore,as a landmark gas of coal seam spontaneous combustion,real-time monitoring of the concentration of C2H2 gas is very important for the analysis and judgment of the spontaneous combustion state of coal seams.During the spontaneous combustion period,the concentration of C2H2 is usually in the range of 0-10 ppm.The concentration of CH4 in the mine can be as high as 80%.Therefore,the detection of trace C2H2 gas under the background of high concentration of CH4 has very important value and significance for coal mine safety production and the prediction and prevention of mine disasters.In this paper,a thorough theoretical and experimental research on the nonlinear effects of gas photoacoustic(PA)spectroscopy is carried out,and the amplitude expression of the photoacoustic pressure generated in the non-resonant cylindrical photoacoustic cell in the case of high-concentration gas measurement is derived.The non-linear response in the photoacoustic measurement of high-concentration CH4 gas and its influence on the measurement of trace C2H2 gas is analyzed,and then a correction method is proposed on this basis.In the expereiment,an all-optical photoacoustic system based on a dual distributed feedback(DFB)laser light source is used to realize the detection of trace C2H2 gas in the background of high concentration CH4.The research content of this paper is as follows:First,the non-linear effect mechanism of the photoacoustic spectroscopy technology is deeply studied,and the generation process of the photoacoustic pressure in the non-resonant cylindrical photoacoustic cell under high-concentration gas measurement is analyzed,and the photoacoustic amplitude is expressed in an analytical form.On the basis of theoretical derivation,MATLAB software was used to simulate the photoacoustic-gas concentration response of 0-100 ppm CO,C2H2,C2H4,C2H6 and 0-100% CH4,CO2,and analyzed the influence of specific heat ratio.In the case of concentration,the change of the specific heat ratio of the mixed gas has almost no effect on the photoacoustic response of CO,C2H2,C2H4 and C2H6,while the nonlinear response of CH4 and CO2 is mainly due to the large change in the concentration caused by the mixing in the photoacoustic cell The change of gas specific heat ratio and the exponential decay of optical power in the photoacoustic cell caused by gas absorption.The linear range of the photoacoustic-gas concentration response of 0-100% CO,C2H2,C2H4,C2H6,CH4 and CO2 is analyzed.The relationship between photoacoustic pressure and gas concentration can only be linearly processed when the gas concentration is low.The influence of high-concentration CH4 gas on the photoacoustic measurement of trace C2H2 gas is analyzed.The amplitude of the photoacoustic signal decreases with the increase of CH4 concentration.Second,the all-optical photoacoustic system based on the dual DFB laser light source realizes the detection of trace C2H2 gas in the background of high concentration CH4.First,according to the absorption spectrum data of the HITRAN database and considering the interference between gases,the center wavelengths of the PA excitation light sources of C2H2 and CH4 are selected,which are 1532.8 nm and 1650.96 nm,respectively.Then,the photoacoustic detection system based on optical fiber acoustic wave sensor,non-resonant photoacoustic cell and photoacoustic spectrometer are introduced in detail.Secondly,high-concentration CH4 gas detection and trace C2H2 gas detection are carried out respectively,and the relationship between the photoacoustic signal and gas concentration is obtained,and the correctness of the analytical formula is verified.In order to study the influence of high concentration of CH4 on the determination of trace C2H2,the mixed gas is measured,and the response of the photoacoustic system to 100 ppm C2H2 under different CH4 concentrations is obtained,and an effective correction method is proposed.Finally,the trace C2H2 gas in the background of high concentration of CH4 is continuously and repeatedly measured,and the measurement results are corrected and compensated by the proposed correction method,and the results of 100 ppm,10ppm,1ppm and 500 ppb are obtained under the background of nearly100% CH4 concentration.The relative error of C2H2 gas measurement is less than 2.5%,4%,10% and 12%.When the concentration of C2H2 is 500 ppb,the measurement error is less than60 ppb. |
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