Research On The Ammonia Escape Detection Of SCR Processing System In Vehicle Based On TDLAS

As one of the contributors to PM2.5in the atmosphere, NH3has great harm tothe environment and human health. NH3emission pollution from diesel vehicle withselective catalytic reduction (SCR) system is getting more and more attention. It isvery important to realize fast and high-precision NH3detection.Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been widely usedin the field of environment and atmosphere detection, because it’s a fast onlinedetection tecnology with high seletectivity, sensitivity and high accuracy. In this paper,the relevant research on NH3detection of the diesel vehicle with selective catalyticreduction (SCR) system is conducted.Based on the the theory of absorption spectroscopy theory and harmonicdetection, a in-depth theoretical research on TDLAS detection is presented. Lorentzand Gauss absorption linetype are analyzed, and the harmonic expression of theabsorption signal based on Fourier series expansion is derived. The harmonicexpression considering the light intensity modulation is also described. The detectionsystem based on Simulink is modeled. The characteristics of the second harmonicchanging with the modulation depth under different absorption linetypes areresearched. The distortion theory of the second harmonic and the generatingmechanism of residual amplitude modulation (RAM) are researched, and someconclusions are given.The NH3detection system is built, and the main components are described. NH3absorption peak is selected, and the design of gas absorbtion cell is completed. Theexperimentation research and optimization of the paramenters of laser controller andlock-in amplifier are conducted. The effects of relevant parameters on secondharmonic are analysed, and the proper parameters that are suitable for this sysetem aregiven.The background signal is analysed, and the weak second harmonic signalsubmerged in the background signal is extracted and detected, which realizes thedetection of NH3of low concentration.The methods of calculating concentration usingthe maximum amlitude and the peak height of the second harmonic signal arecompared, and varieties of smoothing filtering methods are also researched. Peak height fitted with concentraion and the Savitzky-Golay filtering methods arecombined to detect NH3. Detection limit of this system is analysed, and experimentproves that this system has good ammonia concentration detection capability.