Simulation And Analysis Of SCR Catalyst Converter For Heavy-Duty Diesel Engine

Diesel engine is widely used due to its good power, economy and durability in recent years. Meanwhile, legislative restrictions of diesel engine exhaust emissions have shown a steady trend towards more stringent limits. In the nation "the12th five years" plan, two binding targets are newly added which are nitrogen oxide and ammonia nitrogen. According to the conditions in China, optimized combustion+SCR is preferred for realizing Euro IV for HD vehicle diesel engines. In the SCR catalyst work process, the distribution of its internal flow field, pressure field, temperature field and component concentration field has a great effect on the diesel engine’s performance. Because of the complexity of the flow and the chemical reaction process in the catalyst and the limit of the test technology, the research on the velocity field and the component concentration field in the denitrification process remains macroscopic property measurements, the data is empirical and limited. Using3D numerical simulation not only to obtain macroscopic data, but also to observe the concrete course of the internal flow more direct viewing, which is good to study on the influencing factors for SCR conversion efficiency deeply. This paper takes an existing SCR catalyst product as the object of study, establishes a3D simulated model of SCR catalyst by AVL FIRE, and studies on the influencing factors of urea water solution spray characteristic and the component concentration field in the SCR box. The conclusion can guide SCR system to optimally design the structure and provide the theory basis for the urea injecting strategies.First, this paper uses AVL BOOST to establish the ID simulated model of the SCR catalyst to calibrate the chemical reaction parameters of the model combined with the experimental data and analyzes the characteristic of the SCR catalyst with the model and obtains the influence rule of the temperature, GSHV and NH3/NOx on the NOx conversion efficiency. Second, verifies the validity of the spray model by the experimental result of the urea spray impinging on the wall and analyzes the influence of the different working condition, injection pressure, injection angle and the temperature of the urea on the spray characteristics. The conclusion is as follows:The incoming gas flow has a great effect on the spray shape, with the smaller exhaust flow, the higher temperature, the evaporation and thermal decomposition of the urea spray is faster; As the higher injection pressure, increasing velocity of spray, the residence time is shorten and the amount of NH3is decreased. The temperature of the urea water solution has little effect on the conversion of NH3. It is better for evaporation and thermal decomposition of urea solution to enlarge the nozzle injection angle.On the basis of researching the spray characteristics, the influence of the temperature, GSHV, NH3/NOX and NO/NO2on the components concentration field in the SCR box has been studied. The result indicated:1. The higher the temperature is, the larger the amount of NH3and HCNO is, the high concentrated area appears more early, the distribution of the concentration is more uniform. The existence of the flow swirl is advantageous to the evaporation and thermal decomposition of the spray.2. GSHV has big effect on the residence time. GSHV is smaller, the residence time is shorter, the amount of NH3is decreased and the NO conversion is decreased.3. When NH3/NOX is smaller than1, the conversion of NO elevates along with the increasing NH3/NOX. As NH3/NOX is greater than0.85, the increasing NH3/NOX has little effect on the NO conversion effiency, and it is easy to produce NH3slip.4. With NO/NO2increased, the NO conversion is decreased, and it influenced more on the distribution of NO2concentration.