Experiment And Mechanism Research Of Urea Deposit In SCR System

Urea-SCR（urea-selective catalytic reduction）technology is currently the most effective technology for diesel engines to respond to the upgrade of emission regulations.As the supply source of reducing agent,urea aqueous solution has the characteristics of easy storage and high safety.However,the decomposition of urea is not complete,and various by-products are generated due to side reactions.The by-product accumulates for a long time to form deposit and blocks the exhaust pipe,which makes the operation of diesel engine fail.In order to explore the interaction mechanism of composition,temperature and mechanism of SCR system deposit,thermogravimetry is used to test the thermal decomposition of deposit,urea,biuret,cyanuric acid,ammelide,ammeline and melamine.The experimental results show that biuret is the most important reactant for the formation of cyanuric acid,ammelide and ammeline,and the characteristic of its own condensation reaction is the important reason for the formation of cyanuric acid,ammelide and ammeline;it is considered that2biuret→ammeline+HNCO+2H₂O is the important path for the formation of ammeline.The failure to completely hydrolyze isocyanic acid in time is the root cause of the formation of deposit.Ammelide and ammeline are the main reactants for the formation of polymer melam,melem and melon.During the pyrolysis of melamine,no by-product formation is observed.Using gas chromatograph,derivatization of cyanuric acid and its homologues and deposit using derivatization methods in the food industry,successfully detect the specific components of cyanuric acid homologues in the deposit.Comprehensive gas chromatography test and thermogravimetric test results,the main components of the deposit are urea,biuret,cyanuric acid,ammelide,ammeline.Urea in the deposit mainly decompose at 132°C～184°C,accounting for about 3.5%of the total mass of deposit;biuret mainly decompose at184°C～269°C,accounting for about 18%of the total mass of deposit;cyanuric acid mainly decompose at 269°C～331°C,accounting for about 51.5%of the total mass of deposit;ammelide and ammeline mainly decompose at331°C～414°C,accounting for about 24%of the total mass of deposit.Combining mechanism analysis and experimental research,a chemical reaction kinetic model with 14 steps of 12 components is established.The quantum chemical calculation method is used to estimate the unknown chemical reaction kinetic parameters,and the transition state theory is used to indirectly calculate the activation energy,pre-finger factor and temperature index of the reaction.According to the theory of chemical bond fracture,the transition state structure of the reaction is constructed,and the rationality of the conjecture transition state is verified by Guassian software,and the transition state of ammelide is found to meet the requirements.On the basis of confirming the reasonable conjecture transition state,the kinetic parameters of the reaction of biuret with isocyanic acid and the amination of cyanuric acid to produce ammelide are obtained by the calculation of KISTHELP software.STAR-CCM+software is used to call 14 step chemical reaction kinetic model to monitor the mole fraction of ammonia,urea,biuret,cyanuric acid,ammelide and ammeline at the outlet of exhaust pipe.The simulation results show that in the temperature range of 150°C～190°C,urea decompose incompletely and react with isocyanic acid to form biuret.At the same time,biuret react actively in this temperature range.Its self condensation reaction and reaction with high concentration isocyanic acid are the important reasons for the formation of large amount of cyanuric acid and ammelide.