Investigation On The Formation Mechanism Of Deposit In The Exhaust Pipe Of SCR System For Diesel Engine

Under the background of the fourth phase of heavy-duty diesel engine emission regulations has been fully implemented in our country,the control technology to reduce NO_x emissions of diesel engines has become a new research hotspot.At present,selective catalytic reduction?SCR?technology which urea is used as reductant,is widely considered to be the most effective way to reduce diesel engine NO_x emissions.In further study on SCR system,deposit was found to be formed easily on the exhaust pipe wall of diesel engines.Deposit formation in exhaust pipe indicates the decrease of urea utilization rate and NH₃ concentration participating in the SCR reaction,and results in the drop of SCR system conversion efficiency.More severe deposition can also lead to the partially or even completely blocking of exhaust pipe,a sharp increase of exhaust back pressure,and adverse effects on the engine performance.In order to guarantee the normal work of the diesel engine and SCR system,solve the problem of deposit generation in exhaust pipe,based on the nature of the deposit in this paper,the simulation model of deposit production was built based on the detailed mechanism,the general formation rules of deposit and the key influencing factors were revealed by combining the means of simulation and experiments,measures to avoid deposit was put forward,and hidden troubles of the diesel engines coupled with SCR system were eliminated.Main research contents are as follows:1)The components of deposit samples from the vehicle road test was studied and a rational analysis process to determine the organic composition was proposed,including many kinds of chemical analysis methods such as chromatographic analysis,spectral analysis,spectrum analysis and mass spectrometric analysis.The deposit was mainly made up of urea and the by-products that not decomposed completely.As a result,the thermal decomposition processes of deposit sample,urea,biuret and cyanuric acid were researched by thermo-gravimetric mass spectrometry technology.And the decomposition temperature and reaction path of deposit were determined according to the weight loss curves and mass spectrographic analysis result.At the meantime,thermo-gravimetric experiments that urea was heat continuously at nine different constant temperatures were carried out.The formation process of deposit at different temperatures was described in detailed reactions.Considering that the exhaust flow field of diesel engine was complicated and urea injection mass differed,urea pyrolysis processes on the condition of different sample quantity and different heating rates were described.In order to explore the reaction rate and reveal the further reaction mechanism,the thermal decomposition reaction activation energy of urea and cyanuric acid were solved by using three classic thermo-gravimetric kinetic methods-Ozawa,Friedman and Kissinger method.Meanwhile,error analysis was discussed,and advantages,disadvantages and scope of application of each method were evaluated.2)Based on the detailed deposit reaction mechanism,the simulation model of deposit production was built with the help of MATLAB/simulink platform.The comparison between thermo-gravimetric test result,bench test result and simulation result showed that this model can effectively descript deposit production in exhaust pipe quantitatively.Through single factor analysis of simulation,the influence rule of exhaust temperature,space velocity,exhaust pipe materials,the thickness of the pipe,urea temperature and urea injection rate on the deposit production was made clear.The result showed that exhaust temperature had the greatest influence on deposit production,and then followed by the factors of space velocity and urea injection rate;the influence of other factors was negligible.In consequence,in this article,the influence degree of key factors on the deposit production was revealed through the bench test,and the weight coefficients of key factors were determined by orthogonal test design:urea injection rate?0.39?>space velocity?0.37?>exhaust temperature?0.24?.And measures to avoid deposit were presented,which can provide a reference for optimal design of the SCR system,and thereby the hidden dangers of diesel engines coupled with SCR system were eliminated.