Design Of Methane Sensor System And Elimination Of Second Harmonic Background

Methane is the second largest greenhouse gas in the world after carbon dioxide.From the point of view of the number of molecules per unit,the greenhouse effect of methane is 25 times greater than that of carbon dioxide.At the same time,it is also a kind of flammable and explosive dangerous gas.When its concentration in the air reaches 5-15%,it will explode.Although methane is basically non-toxic to the human body,if the concentration is too high,the oxygen content in the air will be significantly reduced,which is easy to suffocate.Furthermore,it is colorless,tasteless,and hard to detect.In the process of coal mining and natural gas use,suffocation and explosion accidents often occur due to excessive methane concentration.Therefore,an effective and convenient method is needed to monitor the methane concentration in real time,especially the safety monitoring of industrial methane.In recent years,the use of optical methods to detect methane gas concentration has aroused great interest in applications due to its advantages such as non-contact,fast,and real-time online.Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology has been widely used in the detection of a variety of gas concentrations.In Wavelength Modulation Spectroscopy(WMS)technology,high-sensitivity detection is achieved through wavelength modulation and harmonic detection,and the second harmonic is generally used as the detection signal.Since the detection signal is moved to a higher frequency,an important advantage of this technology is that the random noise in the system can be ignored.The sensitivity of TDLAS-WMS is 1 to 2 orders of magnitude higher than that of direct detection.At present,the research on TDLAS technology has been relatively complete,and scholars at home and abroad have also done a lot of work on the development and optimization and performance improvement of TDLAS system.However,in actual detection systems,there are still many factors that develop into background signals,such as nonlinear effects,interference fringes,light intensity fluctuations,and so on.The nonlinear effect in the system mainly comes from the nonlinearity of the laser and detector's light intensity-current response and the nonlinearity in the optical transmission process;the interference fringes mainly come from the multiple reflections and reflections on the interface of the laser,coupler,collimator and other devices.These factors lead to high harmonic background,which will seriously affect the sensitivity and authenticity of harmonic signals.Therefore,eliminating background interference has become an important issue to be solved urgently.This thesis mainly focuses on the key technologies involved in actual industrial applications of gas sensing systems based on TDLAS technology,and proposes solutions to eliminate the above-mentioned harmonic background interference,which effectively improves the detection limit and accuracy of the system.The main contents of this article are as follows:1.Introduced common gas detection methods,including electrochemical sensors,calorimetric sensors,pyroelectric sensors,semiconductor metal oxide sensors and optical sensors,and the current research status of nonlinear effects and interference fringes in the TDLAS system is also analyzed.2.Introduced in detail the absorption theory of gas infrared spectroscopy,including molecular infrared spectroscopy,Beer Lambert's law and two major branches of TDLAS technology-direct absorption spectroscopy and wavelength modulation spectroscopy,detailed introduction of the harmonic detection technology of wavelength modulation spectroscopy Implementation principle.3.The design scheme,device selection and hardware circuit of the methane detection system were introduced in detail,and the system design was explained in detail from the three major modules of signal generation module,signal detection module and signal processing module with a modular design method.In the signal processing module,the relevant principles and implementation methods of the lock-in amplifier were emphasized.4.Aiming at the harmonic background caused by the nonlinear effect of the system,based on the needs of real-time remote detection in the industry,a methane sensing system that used a variable optical attenuator to suppress the background was designed,and the minimum detection limit is increased from 80.8ppm to 1.7ppm,which greatly improves the detection sensitivity.5.The source of interference fringes,the influence of interference on the signal and the principle of dual-frequency modulation were introduced in detail.The use of dual-frequency modulation technology effectively suppressed the influence of interference fringes on harmonic signals.