Research On The Ammonia Concentration Detection System By Tunable Diode Laser Absorption Spectroscopy

Ammonia is a major part of atmospheric pollutants,accurate measurement of the ammonia concentration is very important.TDLAS?Tunable Diode Laser Absorption Spectroscopy?is based on single-line narrowband spectrum absorption characteristics.It has many advantages such as high sensitivity,high selectivity,to avoid interference to other unrelated components.TDLAS can meet real-time online fast and accurate measurement needs,which other measurements can not.It has gradually become an important measurement of ammonia concentration.Based on the principles of gas absorption,this paper describes the basic theory of TDLAS technology.It analyzes the advantages of wavelength modulation technology.And highlights the wavelength modulation and harmonic detection theory that TDLAS technique employed,gives evidence and principles of ammonia concentration measurement by TDLAS.For subsequent analysis,this paper has completed the various modules of the system modeling and simulation according to the basic principle TDLAS system.We have established TDLAS ammonia concentration detection experimental system and introduced ammonia absorption peaks and selections of every part in system.In this paper,the laser tuning parameters and lock-in amplifier parameters are optimized,and has completed the detection of ammonia concentration.Detection of gas concentration by TDLAS is affected by baseline drift and high-frequency noise.Therefore,how to remove the systematic noises has been a hot spot.This paper analyzes the significance of singular value decomposition?SVD?in TDLAS detection system with two different methods of constructing a matrix,and discusses the differences of processing results for different noises by using simulation modeling systems and TDLAS ammonia concentration detection system.The second harmonic signal is arranged in a matrix and decomposed.We select the appropriate threshold and put those singular values smaller than the threshold into zero,then reconstruct the matrix.Experiments show that SVD method does not require additional system components or pass into the zero gas to subtract background.This method is able to remove noises of TDLAS system quickly and effectively.We found that the method of constructing a hankel matrix is suitable for removing high-frequency noise.While,the method of constructing a continuous-cutoff-signal matrix is suitable for removing baseline drift.For example,we set up a TDLAS system to measure the concentration of NH₃,the noise removal rate of second harmonic curve up to 80% after this method.