Near-infrared Methane Gas Detection Technology Based on TDLAS with High Sensitivity
In order to achieve the detection of methane gas concentration and monitoring of methane leakage,and to improve the monitoring efficiency and accuracy,a high sensitivity near-infrared methane gas detection technique based on the Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology is proposed.As an absorption spectroscopy technology,TDLAS is based on the Lambert-Beer law.When the emission laser makes contact with gas molecules,the gas molecules will absorb the laser energy if the wavelength of the laser coincides with the absorption line of the gas molecule.And it is known that the absorption rate of gas is directly proportional to the effective absorption distance.A larger attenuation of laser intensity tends to produce a stronger TDLAS sensor signal.Therefore,it is necessary to increase the amount of light intensity attenuation by using a long optical path gas cell.At the same time,in order to improve the measurement sensitivity and detection limit of the system,the system uses a self-developed gas absorption cell.Simulation design of Herriott-type gas absorption cell uses TracePro optical simulation software.Under the consideration of no excessive interference,uniform spot distribution,and reasonable angle between incident and outgoing rays,two concave spherical mirrors with a diameter of 50.8 mm are designed to form a Herriott absorption cell with a cavity length of 220 mm.The system adjusts the current injected into the Distributed Feedback(DFB)laser to make its output central wavelength at 1 653.7 nm and serve as the detection light source of CH4.The thermo electric cooler temperature control circuit adjustes the temperature stability of the laser and is used to operate the laser at 26℃.The lock-in amplifier generates low frequency sawtooth wave signal and high frequency sine wave signal.The two signals are superimposed by the adder.The emitted laser light is absorbed by the measured gas and emitted from the end of the gas chamber to the photodetector.The photodetector converts the optical signal into a corresponding electrical signal.After being collected,the signal is sent back to the lock-in amplifier for data processing.The results are displayed on the oscilloscope.By configuring different concentrations of CH4 gas,the correctness of absorption spectrum selection and the feasibility of system construction are verified by the direct absorption method.The different low concentrations of CH4 gas are studied experimentally,and the second harmonic signal is recorded and linearly fitted.The peak value of the second harmonic signal shows a good linear relationship with concentration,and the linearity is 0.998 52.The concentration of the gas to be measured can be calculated by fitting the linear equation.The experiments have demonstrated that the dependability of the second harmonic signal for concentration detection and,to a certain extent,confirm the stability of the detection system.The lower detection limits of the system can reach 4.82 ppm for methane.Allan variance analysis is conducted within 960 s with CH4 of 390 ppm.As the integration time increases,Allan variance shows a tendency to decrease and then stabilize.When the integration time reaches 112 s,Allan variance is in a stable state,and the sensitivity of the detection system is 4.27×10-7,which realizes the high-precision measurement of CH4 gas.Test findings demonstrate that the precision,accuracy,and detection limits of the system have been improved based on the use of a low-cost light source and a small-sized absorption cell.The proposed system combined with the method can be widely used in gas monitoring and early warning of mine disasters,gas leakage monitoring,and early warning of hazardous chemical field stations and transportation pipeline networks.
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