Research And Application Of Infrared Ammonia Detection System Based On TDLAS Technique

In this thesis,we designed two infrared NH₃ detection systems to detect the ammonia（NH₃）leakage in industrial production,namely an infrared NH₃ remote sensing system based on tunable diode laser absorption spectroscopy（TDLAS）technique and an open optical path multi-point periodic NH₃ detection system based on cavity-enhanced technique.The detection theories and structure design schemes of the two systems were introduced in detail.Based on the assembly and debugging of the detection system,the performance of the two systems was tested,such as anti-interference,stability,response time,and other parameters.Outdoor tests were carried out in the application scenarios of the two systems respectively.Firstly,we introduced the important role of NH₃ in industrial production,the main source of NH₃ in the atmosphere,the harm of NH₃ to the environment,and the significance of developing NH₃ detection equipment to our country which is a major chemical industry country around the world.Several common NH₃ detection methods were introduced briefly,including chemical detection,electrochemical sensor detection,meteorological chromatography,and spectral detection.Their advantages and disadvantages were compared.This chapter introduced the detection theories and development status of four optical remote sensing techniques and three infrared absorption spectroscopy detection techniques.Secondly,we introduced the theories of molecular infrared spectra,including the formation theory of molecular rotation spectra and vibrational spectra,line broadening theory of spectra,the function expression and the intensity of absorption line,Lambert-Beer’s law,and its scope of application.Thirdly,we introduced the detection theories of NH₃ remote sensing,including2f/1f signal processing technique and harmonic extraction technique based on the digital lock-in amplifier.And we also introduced the selection criteria for absorption line,the laser output characterization test,the design scheme of system structure,and carried out the performance characterization test based on the calibrated remote sensing system.Finally,we applied the remote sensing system to carry out the outdoor experiments and compared it with the simulation results of the theoretical model.According to the test,the response time of the system was 2.72 s.Allan deviation analysis results showed that the 1σdetection limit was 16.64 ppm·m.When the average time reached 778.41 s,the lowest detection limit of the system was 0.46 ppm·m.Then we introduced the design scheme of the multi-channel gas sampling control module,including the design of the control circuit,the integrated scheme of the sampling module,and the design of the Lab VIEW interface and its workflow.The sampling module used STM32F103C8T6 as the main control chip and operated the gas path switching with the electromagnetic valve.The pipelines were made of Teflon and placed by optimized layout,which minimized the impact of NH₃ attaching to the pipelines on the detection results and shortened the response time of the system.The host computer operated the gas path switching by sending string instructions to the sampling module and had a periodic detection mode.Then we introduced the detection theories of the cavity-enhanced NH₃ periodic detection system,the selection of absorption line,the laser output characterization test,the design scheme of the system structure,and the performance characterization test based on the calibrated detection system.Finally,we applied the open optical path cavity-enhanced multi-point periodic NH₃detection system to carry out the outdoor experiments.According to the test,the effective optical length of the system was 1.177km,the SNR could reach 29.79,the response time of the individual cavity-enhanced part was 1.1 s,and the response time of the assembled multi-point periodic detection system was 7.925 s.Allan deviation analysis results showed that the 1σdetection limit was 0.6 ppm,and when the average time was 438.9 s,the lowest detection limit of the system reached 27.24 ppb.Finally,we summarized the research and application of the two infrared NH₃detection systems.The advantages and disadvantages of the two developed infrared NH₃ detection systems were analyzed,and the ideas for possible improvement and research directions in the future were put forward.The innovations of this thesis are as follows:1.For the characteristics of the NH₃ absorption spectrum,the remote sensing system abandoned the traditional signal modulation scheme of triangle wave adding sine wave,but adopted the scheme of wavelength-locked,which not only avoided the water vapor interference but also improved the refresh rate of the digital system.Combined with 2f/1f signal processing technique the interference caused by varied light intensity and detection distance were suppressed,and the advantages of high detection accuracy of the TDLAS detection system were retained.2.The multi-channel gas sampling module was deployed at different sampling points to carry out periodic sampling and detection which realized the time-division multiplexing.In this way,one cavity-enhanced NH₃ detection system could continuously detect multiple monitoring points,which greatly reduced the deployment cost,and was of great significance to the commercial application of the NH₃ cavity-enhanced detection system.