Foundmental Research On Reducing Agent Adding Process Of Urea-SCR Systems For Diesel Engines

With increasingly stringent emission regulations, Selective Catalytic Reduction (SCR) is becoming one of the most promising methods to meet the China stage Ⅳ or above regulations. For the present, urea-water-solution (UWS) is the preferred reducing agent provider for SCR applications. It is a very complex process for UWS to convert into ammonia, and the conversion process has significant effect on SCR performance. Therefore, it is very meaningful to investigate the reducing agent adding process of SCR system, helping to improve the conversion of urea to ammonia and reduce the possibility of deposit.This thesis focused on the subject of urea decomposition and deposits formation during reducing agent adding process of Urea-SCR system. The mechanisms of UWS evaporation, thermolysis, spray wall interaction and deposits formation were deeply investigated and modeled. Coupling with exhaust gas flow, a quasi one dimensional model of gas liquid two phase flow was developed, and the gas phase production and deposit formation were simulated. The main work and conclusions of this thesis are as follows:1) Study on evaporation and thermolysis process of a single UWS droplet. A mathematical model of UWS droplet evaporation and thermolysis is developed. An adjustment coefficient depending on ambient temperature was introduced into the evaporation model. A simplified direct decomposition model was proposed to describe the urea thermolysis process in the droplet phase. The simulation results showed that the droplet initial diameter and the ambient temperature had significant effect on evaporation time and the total depletion time of the droplet. The thermolysis process was more sensitive to temperature compared with evaporation process. When temperature was relatively high, evaporation was the dominant factor for droplet depletion process. Because of the compact structure in real SCR applications, the residence time and space for UWS droplet decomposition were limited. As a result, it was difficult for UWS to decompose completely when it reached the catalyst. So it was necessary to take steps to accelerate the decomposition process of the droplets, for examfor, optimization of the injection, appropriate spray wall interaction, adding mixer and strengthen the exhaust turbulence.2) Deposit components analysis and kinetic modeling of urea thermolysis. The deposits experiment was conducted on a hot gas generator which was designed to provide the same conditions of gas flow rate and temperature in diesel engines. Various measurements including TG/DSC, DRIFTS and HPLC were used to analysis the components of the deposit. Results showed that deposit formed at 449K was almost urea, deposits formed at 504K and 540K were composed of biuret and CYA. HPLC analysis incidated that the deposit formed at higher temperature had larger amount of CYA. On this basis, a kinetic model of urea thermolysis was proposed, and the kinetic parameters were optimized based on the PSO algorithm. Finally, the characteristics of urea deposit were simulated. Results incidated that deposit component had a close relation with temperature. As temperature rised, the content of CYA and ammelide would increase. In general, higher temperature and longer reaction time can lead to smaller deposit yield, and no deposits formed if temperature exceeded 650K. The increase of heating rate would produce large amount of CYA. It was preferable to operate at lower heating rate to avoid the formation of heavier by-products.3) Quasi one dimensional modeling of gas liquid two phase flow of SCR system. The process of gas flow, spray motion, droplet evaporation, urea thermolysis, spray wall interaction, heat transfer of the pipe wall, wall film formation and development and deposits formation were modeled in detail. A new concept of the one dimensional wall film was proposed, the different mechanisms of urea thermolysis in droplet and the wall film were considered. Finally, a quasi one dimensional model of two phase flow inside the exhaust pipe was developed, which can calculate not only the generation of reducing agent but also the formation of deposits. The model was comprehensively verified through the aspects of gas phase production, exhaust temperature, deposit mass, position and components. Results indicated that the proposed model had a certain precision, and can reflect the influence of various factors on gas phase production and deposits formation very well.4) Quasi one dimensional simulation and analysis of reducing agent adding process of SCR system. Based on the proposed model, the process of gas phase production and deposits formation were simulated. The simulation results of gas phase production indicated that UWS injection rate had little effect on urea decomposition. Generally a decrease in droplet initial diameter can promote the urea decomposition, however, a high urea decomposition efficiency can be found in a diameter range from 50-70μm. The increase of droplet initial velocity can promote the urea decomposition if the nozzles injected in radial direction. As for injection directions, radial and reverse were better directions for injection. A increase in pipe diameter was an effective approach to improve the performance of SCR system since it not only enhanced the urea decomposition efficiency significantly, but also reduced the pressure loss. The simulation results of deposits formation indicated that temperature was the most important factor for deposits of SCR systems, and it had significant effect on deposits weight and components. The ambient temperature had little effect on deposits weight. Additional heat insulation measures on the exhaust pipe can decrease the heat loss, which was an effective approach to reduce or even prevent deposit formation in SCR systems. The increase of UWS injection rate can significantly increase the deposit weight, but had little effect on deposit component. Deposits in exhaust pipe were mostly caused by large diameter droplets. Therefore, the optimization of UWS injection system which reduced the proportion of large diameter droplets was an effective way to prevent the deposits formation. The effect of droplet initial diameter on deposit formation was very little. An increase in exhaust mass flow can decrease the total weight of deposit and the film region on the pipe wall, but had little effect on deposit component.