| Urea-selective catalytic reduction?Urea-SCR?technology is currently an important technology to reduce nitrogen oxide emissions from diesel engines.There is a risk of urea deposits,and urea deposits affect the uniformity of the NH3 at the front of the catalyst inlet and reduce the NOx conversion of the SCR system.effectiveness.In this paper,a new urea decomposition chemical kinetics model including a biuret,cyanuric acid and cyanuric acid sub-model was constructed,containing 15 components and 11-step reactions.Using the molecular contribution group method,the specific heat capacity relation of cyanuric acid monoamide was established,and the thermodynamic parameters of the reaction of cyanuric acid monoamide were obtained.The reaction rate constant was calculated based on the transition state theory to indirectly solve the activation energy and pre-exponential factor of C3H3N3O3+NH3 ?C3H4N4O2+H2O.The calculation results of the urea injection detailed reaction and the total package reaction show that the detailed mechanism model can reflect the actual urea decomposition process.The change law of decomposition components of urea at the exhaust temperature of 150 ??600? was analyzed.The resultsshow that the exhaust temperature plays an important role in the formation and decomposition of sediments.The formation of sediments is evident at low temperatures,and isocyanates are the most important reactants required for the formation of urea decomposition byproducts.Biuret is the most important intermediate in the formation of urea deposits.After the exhaust gas temperature exceeds 175?,the biuret fraction rapidly decreases and remains stable at 225?and has approached zero.The change trends of cyanuric acid and cyanuric acid monoamide with increasing temperature were basically the same:a large amount was formed between 150-200 ?,and the molar fraction increased rapidly;after 200 ?,the molar fraction decreased drastically and remained basically constant after 250?.The calculation of the influence of the mixer and piping structure on the generation of urea deposits and the uniformity of NH3 shows that the addition of the mixer can increase the turbulence intensity in the pipeline and thus increase the uniformity of the NH3 at the outlet of the exhaust pipe.Increasing the exhaust line length and pipe radius will increase the residence time of urea in the pipeline and increase the NH3 uniformity coefficient at the outlet of the exhaust pipe.The NH3 homogeneity coefficient was used as the single optimization goal to set six structural variables,including pipe length,pipe radius,number of mixer blades,mixer blade length,mixer tilt angle,mixer installation position,spray cone angle,injection direction,The injection flow rate is 3 injection variables,and the optimal Latin hypercube method is used for the design of the experiment?DOE?.The relationship diagram of the influence of each design variable on the NH3 uniformity coefficient is obtained.The Kriging approximate model is established and the exhaust pipe is optimized by Evol.Structure and injection parameters are optimized.The results show that the length of the mixer blade and the angle of inclination of the mixer blade have a strong positive effect on the contribution rate of the NH3 uniformity coefficient.The installation position of the mixer,the jet flow,and the radius of the pipe have a strong negative effect on the contribution rate of the NH3 uniformity coefficient;the pipeline length,the number of mixer blades,the spray direction,and the spray cone angle contribute less to the NH3 uniformity coefficient.When the pipe length is 0.8m;Pipe radius is 71mm;Mixer blade number is 4;Mixer blade length is 0.058m;Mixer blade angle is 73.44°;Mixer installation position is 0.2m;Spray cone angle is 61°;Spray direction 45.6°;Jet flow rate 2.92 X 10-4kg/s,NH3 uniformity coefficient increased significantly. |
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