Realization Of Methane Gas Concentration Measurement System Based On TDLAS Technology

Nowadays,the problem of air pollution is getting worse,which has a great impact on people's living environment and health.Methane is the main component of atmospheric pollution gas among them.In the process of coal mining and the use of natural gas,asphyxiation or explosion often occur due to the concentration exceeding the limit or leakage,and methane is also a major greenhouse gas.Its greenhouse effect is far greater than the same concentration of carbon dioxide,so it is urgent to strengthen the detection of methane gas concentration.However,the current detection methods and equipment are still relatively backward,and there are some problems,such as low sensitivity,large size and not portable measurement,so it is of great significance to design a reliable portable methane gas concentration measurement system.TDLAS has the advantages of high spectral resolution,non-invasiveness and fast response,so this system selects the TDLAS-WMS technology to measure the methane gas concentration.At present,the system based on TDLAS-WMS technology mainly inverts the gas concentration through peak value method or parameter fitting method,but both of these methods have problems to be improved.The peak value method is used to invert the concentration of gas through the peak value of the second harmonic wave.However,in the actual measurement process,the peak point of harmonic signal is susceptible to drift and random error,and the measurement accuracy has a large error.The parametric fitting method is to invert the gas concentration by repeated iterative fitting of the simulated and measured normalized second harmonic signals.At present,the digital phase-locking method is mainly used to extract the simulated and measured normalized second harmonic signals.Therefore,digital phase-locking needs to be used many times in the iterative process.As a result,a large amount of computation and a large amount of hardware resources are consumed,which is not conducive to the development of miniaturization and integration of gas measurement system.In this paper,several problems existing in the application of TDLAS-WMS technology in gas concentration measurement system are studied in depth.Based on the inversion of gas concentration by multiple digital phase-lock method,the miniaturization design of methane gas concentration measurement system based on FPGA is carried out and a fast fitting method based on generalized series expansion is proposed.In this method,the concentration value only needs to be substituted into the formula of normalized second harmonic signal derived by generalized series expansion,and then the simulated normalized second harmonic signal R_2f/1f required by concentration inversion can be quickly fitted.By this method,the gas concentration can be inversed by digital phase lock only once in the whole iterative process,which greatly reduces the use of storage space and logical resources compared with the existing methods.Therefore,it is more conducive to the miniaturization and integration of the gas concentration measurement system.Therefore,it is more conducive to the miniaturization and integration of the gas concentration measurement system.Finally,taking methane and carbon dioxide gas of different concentrations as an example,experiments and simulation measurements are carried out in the miniaturization system designed in this paper.The results show that the simulated normalized second harmonic signal R_2f/1f calculated by generalized series expansion method is in good agreement with the measured normalized second harmonic signa S_2f/1ff/1f extracted by digital phase-locked method,and the maximum relative error is 5‰.Therefore,this method can realize the inversion of gas concentration with high accuracy.