The Sutdy Of Influence Factors Of NO_x Conversion Effiency And Temperature Character Of Catalyst Of Diesel Engine SCR System

The selective catalytic reduction(SCR) system is one of the most widely used after-treatment technology in the diesel engine. It is necessary to optimize the SCR system to improve the NOx removal efficiency to meet the increasingly strict emission standards. The exhaust temperature is one of the most important influence factors, which has influences on the inner temperature characteristics of the SCR converter and the catalyst temperature directly. Further, for the SCR control strategy, the catalyst temperature affects the calculation and correction of the urea injection amount. Therefore, it is very important to understand the characteristics of the internal catalytic monolith and accurately calculate the catalyst temperature. It is helpful for the urea control strategy to calculate a more accurate urea injection amount, and finally to improve the NOx removal efficiency and meet the strict mission standards.This paper mainly studies the influence factors of NOx removal efficiency and the outlet monolith temperature time delay.Build the test bench SCR system of diesel engine and test the NOx removal efficiency and the catalyst temperature characteristic under differenct conditions on this test bench. The test results show that: under steady state condition, the inlet and outlet catalyst converter temperature have little difference. But the outlet temperature has a time delay of 25~40s relative to the inlet temperature.According to the main chemical reaction, heat transfer and chemical reaction kinetics, analyze the energy changing process of the catalyst converter. The physical processes include the forced convection heat transfer between exhaust gas and the converter inner wall, the forced convection heat transfer between exhaust gas and the monolith, the thermal conduction among the monolith, the converter insulation and the outer wall of the converter and the radiation and convection between the outer wall of converter and the environment. Chemical heat transfer processes include the heat transfer of the pyrolysis and hydrolysis reaction of urea, the main catalytic conduction reaction and the adsorption and desorption process of NH3. Among them, the forced convection heat transfer between the exhaust and the monolith and the inner wall of the converter have a great influence on the temperature of the catalyst.A 1-dimision simulation model of the SCR system is built according the whole energy transfer processes. The simulation results show that: exhaust gas temperature, oxygen concentration, urea concentration, urea injection amount, NO/NOx ratio and exhaust flow rate on the NOx removal efficiency and leakage of NH3 affect the NOx removal efficiency and NH3 emission seriously. At the same time, study the influence factor of the time delay of the center point temperature of monolith rear face and found that: monolith diameter, length and cell number have the relative apparent influence and the effect of monolith diameter is the most significant, then comes the monolith length, and the monolith cell number is the last one.