| SCR technology has become one of the research focuses in diesel engine aftertreatment systems.Especially under most low-temperature conditions,the formation of urea by-products not only reduces the rate of urea transamination,but also hinders the normal flow channel in the aftertreatment tube,which can block or even damage the catalytic converter in severe cases.At present,many phenomena-level studies combine CFD simulation to macro-predict the formation of urea deposits.In order to further clarify the formation of deposits,it is necessary to study thoroughly on the fact from the perspective of chemical reaction process.A product conversion mechanism of SCR was constructed by quantum chemical analysis.Using the M06-2X method in density functional theory and the base group 6-311G(d,p),combined with the transition state theory,it searches for the theoretical transition states of the mutual conversion reactions between low-temperature SCR side reaction components such as urea-based products,s-triazine products,gaseous isocyanic acid,and ammonia.Furthermore,a high-precision IRC analysis was carried out to verify the accuracy of the transition state and the rationality of the reaction path.Finally,a 20-step urea by-product chemistry reaction mechanism containing 10 main components was constructed.In the transition state search,there are multiple polycondensation reaction paths for urea-based products.The polycondensation reaction of these urea-based products and their synthesis with isocyanic acid can generate triuret,which can be further isomerized to form isocyanuric acid(which can be interconverted with cyanuric acid)and ammonia.This reaction path is the key node for the change of deposits to high-temperature resistant tris-triazines.Thermogravimetric experiments were performed at five different heating rates of 5 K/min,8 K/min,10 K/min,15 K/min and 20 K/min to calculate the reaction kinetic parameters,and thermal kinetic analysis was carried out on the chemical reaction mechanism constructed above.The test sample components are mainly high temperature resistant products such as urea,biuret and cyanuric acid and its derivatives.The experiments above start at room temperature and continue to increase temperature until the sample is completely decomposed.The pyrolysis curves of the single components of 6 kinds of low-temperature SCR deposits were obtained,and their pyrolysis characteristics were analyzed in combination with the chemical reaction path.The experimental results show that the urea and biuret in the urea-based product quickly enter the decomposition reaction stage after melting,with the thermal weight loss rate of the two at about500 K almost reaching the maximum,while there may be unsatisfactory side reactions like sample sublimation and sample condensation in the meantime.In addition,when the temperature is higher than 530 K,the remaining urea-based product samples are almost completely converted into s-triazine products,and a very small part of the subsequent s-triazine products are synthesized at high temperature.Melamine is rapidly and completely pyrolyzed at high temperature and its DTG curve has only one peak shape,indicating that there is no side reaction during this period,and it is not one of the main reactants for the formation of high polymers.However,Ammelide and Ammeline show the fact that their decomposition characteristics similar to high temperature resistant products formed by pyrolysis of urea-based products.The thermogravimetric curve of cyanuric acid has complex changes,especially the temperature conditions have a great influence on its pyrolysis process.Due to the existence of two mutually convertible isomers,CYA and AIC,its pyrolysis characteristics are difficult to determine.The thermal analysis method Flynn-Wall-Ozawa(FWO)integration method is used to obtain the reaction kinetic parameters reflected by the thermogravimetric curve in the assumed pyrolysis temperature range of each component.The comparison of the SCR deposit pyrolysis mechanism parameters of SCR deposits and the results of the thermogravimetric experiment suggests that the thermal analysis kinetic method verified the kinetic parameters of the decomposition of urea,biuret and cyanuric acid and urea condensation to form biuret in the SCR product conversion mechanism.The construction of the pyrolysis reaction mechanism of SCR deposits helps to study the technical methods of reducing SCR deposits from the three aspects of reaction path,temperature and catalyst.It is effective to reduce urea-based products through pyrolysis in the low temperature area.However,as the temperature rises,high-temperature resistant cyanuric acid and its homologues will emerge,and the pyrolysis temperature of the latter is close to the catalyst deactivation temperature.A hypothetical scheme of catalytic hydrolysis of s-triazine products is introduced.Besides,the comprehensive requirements of multi-function and high-efficiency stability are put forward for the catalyst. |
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