Simulation And Analysis Of CH₄ Concentration Measurement Based On Mid-infrared Laser Absorption Spectrum

In recent years,gas detection technology has become a hot research topic.Tunable Diode Laser Absorption Spectroscopy(TDLAS)has become an important research direction due to its simple operation,fast response,high wavelength selectivity,and strong anti-interference ability.Among them,the wavelength modulation method(Wavelength Modulation Spectroscopy,WMS)has effectively improved the measurement accuracy of TDLAS technology,especially suitable for the measurement environment of weak absorption.In recent years,natural gas and gas explosions caused by the leakage of methane(CH₄)are seriously threatening people's lives and production safety.As a gas with extremely high combustion efficiency,methane is very easy to explode in the air,and the explosion limit is 5%?15%under normal temperature and pressure.At the same time,CH₄ is also one of the main gases that cause the greenhouse effect.Considering the danger of leaking gas,in order to ensure the safety of gas detectors,the measurement of methane gas concentration has become an important research direction.The current use of TDLAS technology to measure CH₄ concentration mainly uses a near-infrared 1.6?m distributed tunable semiconductor laser(Distributed Feedback Laser—DFB)as the light source.However,the absorption intensity of CH₄ molecules in the near-infrared band is very weak,and its measurement accuracy is difficult to meet the requirements of CH₄ concentration measurement.Compared with the near-infrared absorption spectrum,the absorption intensity of CH₄ in the mid-infrared band(7-8?m)is very large,which is about several tens of times that at 1.6?m.Therefore,the use of mid-infrared absorption spectrum to measure the CH₄ concentration can greatly improve the measurement accuracy.With the gradual reduction of the cost of Quantum Cascade Laser(QCL),the high-precision gas detection technology based on QCL-TDLAS will have broad application prospects in the fields of air pollution and hazardous gas monitoring in the future.In this paper,MATLAB software is used,the second harmonic theory in wavelength modulation technology is combined,and the CH₄ gas concentration measurement in the QCL-TDLAS system is simulated and analyzed.The working principle of the TDLAS system is studied,and it is based on TDLAS under weak absorption conditions.The simulation model for the measurement of methane concentration by the wavelength modulation method in the technology,including the light source module,the signal detection module and the harmonic extraction module was established;the near-infrared and mid-infrared gas absorption lines of the methane CH₄ molecule were analyzed and determined;The influence and law of the amplitude and width of the harmonic signal have been studied,the selection of the modulation coefficient in the simulation model has been improved,and the optimal modulation coefficient has been determined;the gas leakage and atmospheric tropospheric environment are simulated and simulated respectively,the QCL-TDLAS system Compared with the CH₄ gas concentration measurement process of the DFB-TDLAS system,and compared the sensitivity of the two systems and other related parameters;it is proposed that the sensitivity of the QCL-TDLAS system in the normal temperature and pressure environment and the tropospheric environment is much higher than that of the DFB-Conclusion of the TDLAS system.