Development Of Multi-parameter Laser Spectral Sensor For Monitoring Selective Catalytic Reduction Process Of Diesel Engines

Nitrogen oxides(NOX)are several of the most harmful gases during combustion process of diesel engines.They also contribute to the formation of photochemical smog and acid rain,which are extremely harmful to environment.NOX emission can be reduced by an aftertreatment technique,such as selective catalytic reduction(SCR).By using ammonia(NH3)and catalyst under required reaction temperature,NOX can be reduced to be N2 and H2 O.In SCR process,water(H2O)concentration also concerned the SCR efficiency,for it could be competitively adsorbed on the surface of the catalyst to replace the adsorption of NH3.So,there is a great need for in-situ,multi-parameter monitoring of the SCR process,such as the unreacted,excessive NH3 slip,H2 O concentration,and the reaction temperature.The main works of the paper are summarized as follow:Firstly,method of multi-parameter measurement based on a single NIR laser was developed.By spectral investigation,the absorption bands of NH3 and H2 O were determined;wavelength modulation spectroscopy(WMS)with varied modulation amplitude for multispectral line detection was proposed,and a model for optimal modulation amplitude selection was reconstructed according to the spectral characteristics,the selected optimal modulation amplitudes not only enabled the SNR of the measured signal,but also greatly reduced the overlapping interference between adjacent spectral lines.In order to simultaneously measure the multi-parameter without calibration in SCR process,an optimized multispectral fitting algorithm was also presented which not only greatly reduced the calculation time of the algorithm,but also eliminated the concentration measurement error of NH3 and H2 O due to temperature and pressure changes.Secondly,a multi-parameter laser spectral sensor was developed and verified.According to requirements of the SCR monitoring,a low cost,miniaturized and multi-parameter laser spectral sensor was demonstrated,and the mechanical interface of the sensor was also designed for in-situ detection.The parameters of the sensor then were determined,and the laser characteristics were accurately measured as well.In addition,the wavenumber nonlinearity of the laser were corrected.Finally,the accuracy and reliability of the sensor were verified under laboratory conditions.Thirdly,the SCR process of a diesel engine was monitorred by the developed sensor.The SCR system of a diesel engine was constructed.The NH3 slip concentration,H2 O concentration and temperature of the SCR process under different working condition(varied urea injection volume and varied engine power)were monitored simultaneously and in-situ.And the measurement results were analyzed and discussed in detail.In conclusion,the developed sensor can be well used for monitoring the multi-parameter in-situ with high sensitivity during the SCR process,which is also quite robust in harsh environment.The in-situ relative measurement uncertainty of NH3 slip concentration,H2 O concentration and temperature were 8.4 %,8.0 % and 5.3 % respectively.Methods of the varied and optimized modulation amplitude and multi-parameter analysis can also be used for the diagnosis of combustion and chemical reaction process.