Experimental And Simulation Research On Influence Of SCR Urea Injection On PN Emission And Optimal Proportion Of NO₂

In order to cope with the current and future increasingly stringent emission regulations,diesel engine integrated aftertreatment systems have become increasingly complex,and emerging scientific issues have emerged.On the one hand,traditional research usually only pays attention to the influence of SCR on NOx instead of particulate matter.With the introduction of regulations on the PN emission limit,the mechanism of the increase of PN emission when the SCR system injecting urea-water-solution has become an emerging issue that needs to be studied urgently.On the other hand,for SCR catalysts,traditional research generally believes that the NOx catalytic conversion efficiency reaches the maximum when the inlet NO₂ ratio is 0.5,while the Cu-based zeolite catalyst has the NH₄NO₃ inhibition effect and the generation of N₂O emission,so the optimal NO₂ ratio is not a fixed value,but the trend of optimal NO₂proportion under different working conditions has not been reported in the current research.In addition,under the optimal NO₂ ratio,the effect of ammonia mixing uniformity on the performance of the SCR catalyst has not been studied yet.This article focuses on three aspects of the problem,using a combination of experiment and simulation to carry out in-depth exploration.Firstly,a coupling model of urea aqueous solution droplet evaporation and pyrolysis was constructed,and its regression coefficient R² compared with the experiment can reach more than 0.94,which verifies the validity of the model.Then the PEMS system has been used to conduct actual road tests on diesel vehicles equipped with the DOC+DPF+SCR integrated aftertreatment system.In the whole road test,the average BSPN increase value reached 1.54E+11#/k Wh.In the area where the SCR inlet temperature is below 325℃,the BSPN increase value is basically below 1E+12#/k Wh;in the area above 325℃,the increase The BSPN of some operating points is greater than 1E+12#/k Wh.As the space velocity increases,the increase in BSPN first increases and then decreases.The peak value of the increase in BSPN is at about 25000h^-1.Using the urea-water-solution droplet evaporation pyrolysis model combined with the measured urea solution spray particle size distribution,it was found that the spray breaking mechanism contributed 12.3%to the increase in PN emissions,and the wall breaking mechanism contributed 23.4%to the increase in PN emissions,About 80%of the particles formed by the wall-impact mechanism are aggregated particles,and the main component of the particles is undecomposed urea.Nitrate mechanism,sulfate mechanism and micro-explosion mechanism together accounted for 64.3%of the contribution to the number of particles.The SMD of the droplet group after the micro-explosion breakage reaches about 6μm,which can make the micro-explosion mechanism particles reach the BSPN increase value in the actual road test.At this time,about 80%of the particles formed by the micro-explosion mechanism are aggregated particles,20%are nuclear particles,and the main components of the particles in the micro-explosion mechanism are cyanuric acid and ammelide.In this paper,a chemical reaction kinetic model is established for the Cu-SSZ-13catalyst.On the basis of the traditional model,the NH₄NO₃ suppression function is introduced to describe the inhibitory effect of NH₄NO₃ blocking the catalyst surface pores on the NOx reduction reaction.The regression coefficient R² compared with the experiment is both reach 0.98 or more.Through simulation,it is found that the optimal NO₂ proportion curve shows a trend of first increasing and then decreasing with the increase of temperature,reaching a peak around 250℃,and above 300℃,the optimal NO₂proportion decreases to zero.Compared with the case where the ratio of NO₂ is0.5,the nominal conversion efficiency of NOx under the optimal NO₂ ratio can increase by 2%-25%,and the N₂O emission can be reduced from the highest value of 12.5 ppm to the highest value of 2.5 ppm.When the ammonia-nitrogen ratio changes from 0.8 to1.4,as the ammonia-nitrogen ratio increases,the peak value of the optimal NO₂ ratio curve rises from 0.2 to 0.4 and then drops to 0.3.When the space velocity changes from20000 h^-1 to 50000 h^-1,the peak value of the NO₂ optimal proportion curve gradually increases from 0.2 to 0.4.When the optimal NO₂ proportion curve drops to the minimum,the corresponding temperature also increases.Moving in the direction of high temperature,it rises from 275°C to 350°C.Under the various space velocity conditions involved in the calculations in this paper,after the optimization of the proportion of NO₂,the N₂O emission value can be maintained within 5 ppm.The NH₃ mixing uniformity test platform was constructed to measure the NH₃concentration distribution at the end face of the SCR catalyst,and the result was used as the inlet condition for simulation.Under the optimal NO₂ distribution condition,when the NH₃ is unevenly distributed,the temperature,exhaust air velocity and The effect of ammonia-nitrogen ratio and other factors on the conversion efficiency of NOx was simulated.When limiting the NH₃ leakage value to no more than 10 ppm,for the nominal conversion efficiency of NOx,the loss of the nominal conversion efficiency of NOx caused by the uneven distribution of NH₃ under the conditions of ANR=1.2and exhaust airspeed of 35000 h^-1 As the temperature first increases and then decreases,reaching a peak at 250°C,the maximum conversion efficiency loss is about 4.5%.At200°C,the nominal conversion efficiency loss value of NOx caused by the uneven mixing of ammonia is basically unchanged with the increase of the ammonia-nitrogen ratio.At 250°C-400°C,the nominal conversion efficiency loss of NOx caused by the uneven mixing of ammonia first increases and then decreases with the increase of the ammonia-nitrogen ratio,and reaches the peak at the ammonia-nitrogen ratio of 1.0.Under the conditions of 200℃and 250℃,the nominal conversion efficiency loss of NOx caused by the uneven mixing of ammonia decreases with the increase of the exhaust space velocity,while under the conditions of 300℃to 400℃,the loss of the ammonia gas is unevenly mixed.The loss of nominal conversion efficiency of NOx caused by it increases with the increase of space velocity.