The Research Based On TDLAS To Diesel Engine SCR Ammonia Pollution Detection System

With the continuous improvement of motor vehicle ownership and the enhancement of the people environmental protection consciousness, the use of selective catalytic reduction(SCR) system, the method of reduce NOx emissions also got more and more applications,but because as a reducing agent to the absolute amount of urea aqueous solution are difficult to control so excess NH3 emissions into the air. Because of NH3 not only cause a harm to human health, and it is the main factors of PM2.5 high, which causes diesel engine tail gas treatment of secondary pollution.Tunable Diode Laser Absorption Spectroscopy (Tunable Diode Laser Absorption Spectroscopy, TDLAS) with its advantages of high selectivity, high sensitivity, the overflow of a small amount of NH3 in diesel exhaust treatment will be accurate detection, therefore,based on the TDLAS as a detection method of NH3.This paper in view of the theory is applied to the wavelength modulation spectroscopy,using the theory of gas molecular absorption analyses the reasons of ammonia has specific absorption lines, lines of spectral line, line width, line analysis, this paper introduces the Lambert Beer's law, the direct absorption detection wavelength scanning method are used to obtain accurate ammonia absorption peak, and comparison and analysis of harmonic detection, and the choice of the harmonic detection wavelength modulation method to remove low frequency noise, select second harmonic signals as the concentration of signal acquisition.TDLAS ammonia concentration detecting system is designed, in using the HITRAN database analysis of the ammonia absorption peak and diesel exhaust of H2O, CO2, CO,C2H2, NO and N2O gas composition of absorption peak position, the absorption lines of 1512.22 nm is chosen as the final paper of ammonia absorption lines. To choose the main device in the system model, analyzes reasons for function and the LabVIEW data acquisition program and MATLAB data preprocessing to carry on the design. Automobile exhaust pretreatment systems to design filter out particles of impurities, filter use 3 oxidation 2 aluminium and silicon dioxide of sintered materials.Implements the TDLAS ammonia concentration detecting system of diesel engine, the output temperature of the laser driver and the choice of drive current test analysis, and scanning signals generated by signal generator and frequency and amplitude modulation signal experiment has been done, and ultimately choose to scan signal amplitude is 600 mV,frequency of 5Hz; Modulation signal amplitude is 150 mV, frequency of 50 KHz; Finally to calculate the time constant of phase-locked amplifier, sensitivity, the influence of the phase to the second harmonic and concluded that the experiment analyzed the TDLAS ammonia detection system stability is 98%.Second harmonic of collected data preprocessing, experiments using gas separator gas concentrations to 10000 ppm, 600 ppm, 40 ppm of ammonia were divided into 10%10-100% of the concentration of gas, and gives each concentration ammonia 10 groups of the least-square fitting results of the gas, and then using the 500 ppm standard gas module and processed under test data using least squares fitting algorithm, i ammonia detection limit is 2.4 ppm.