Simulation On The Urea Atomization And Catalytic Reaction In SCR System Of Diesel Engine

The emission of NO_x from diesel engine exhaust will seriously pollute the atmosphere environment.With the tightening of emission regulations,the selective catalytic reduction?SCR?technology has become one of the most effective technical means to reduce the emission of NO_x from diesel engine exhaust.However,in practical engineering applications,poor atomization quality of urea water solution may lead to the poor uniformity of NH₃distribution and the formation of deposits,which affects the performance of SCR system.Improving the atomization quality of urea water solution and the conversion efficiency of NO_x has always been the focus of researchers.Based on the experimental data in the literature,the catalytic reaction kinetic model and three-dimensional flow model of SCR were validated and the coupling between them was realized.Based on the coupling model,the nozzle structure parameters were optimized by studying the influence of nozzle installation angle,urea spray angle and atomization cone angle on the urea atomization process.The results show that when the nozzle installation angle is 45 degrees,the urea spray angle is 40 degrees and the atomization cone angle is 40degrees,the better uniformity of NH₃ distribution and lower wall-film formation can be guaranteed,under which the atomization quality of urea water solution is better.Based on the optimized nozzle structure,the effects of space velocity?SV?,ammonia to NO_x ratio?ANR?and exhaust temperature on the performance of SCR system were studied.The results show that the SV has no significant effect on the uniformity of NH₃ distribution at the inlet section,but the NH₃ concentration decreases significantly due to the increase of exhaust flow rate.The evaporation amount of the wall-film,the remaining amount of the wall-film and the total amount of the wall-film formation all decrease with the increase of SV.Both NH₃ concentration and reaction time in SCR catalyst decrease,thus NO_x conversion rate and N₂O generation amount decrease gradually with the increase of SV.With the increase of exhaust temperature,the evaporation amount of urea water solution increased significantly so that the concentration of NH₃ and uniformity index of catalyst inlet section increased.The total amount of wall-film formation decreased gradually and the trend of variation of remaining amount of wall-film and evaporation amount of wall-film was the same as that of total amount of wall-film formation.When the temperature exceeded 350?,the NO_x conversion rate decreased because of competition of NH₃ oxidation reaction.The higher the temperature is,the less NH₄NO₃ is produced so that the less N₂O is produced.The larger the ANR is,the more urea water solution is evaporized and pyrolyzed in a unit time leading to higher NH₃ concentration.At the same time the NO_x conversion rate and N₂O generation amount gradually increase.In addition,the more urea water solution is not converted into NH₃ with the increase of ANR,so the remaining mass of the wall-film gradually increases and the increasing rate is faster and faster.