Study On Removal Of Particles And NO_x From Diesel Engines By Application Of POC And SCR Technologies

POC and SCR are the widely used after-treatment technology for removement of PM and NO* in China now, which can make the emissions of diesel engine meet CN-IV and more strict emission regulations. Combining POC and SCR technology in diesel emission after-treatment application can remove PM and NO* simultaneously without purification in engine, which possesses well economic as well as little relies on engine modification, and is a novel after-treatment route having large development capacity. Due to the little engine modification be needed, this combined after-treatment technology is suitable for employing in non-electronic control diesel engine to help accomplish the emission upgrade, that is can greatly promote old car’s reconstruction project and purify the atmosphere environment inland. But there is rare research report on the above combined system, especially on the key factors which influence SCR catalyst and POC convertor performance. This work will conduct a systematic research on the POC/SCR combined system, optimizing overall performance of all the functional units in the system, including SCR catalyst, POC convertor and POC/SCR combined simulation design. An engine bench test will be processed at last on a typical non-electronic control diesel engine to obtain the best arrangement plan, which can make the exhaust emission meet CN-V regulation.The experiment is first focused on the key element of SCR system-SCR catalyst, the aim is to improve the total performance of the catalyst. The influence of different TiO2 materials (with WO3 or SiO2 doped), the content of active component-V2O5, the amount of binding materials and coating amount on the vanadium-based catalyst preparation process are investigated, and the optimization values of above factors are acquired through the sample simulative catalytic property evaluation and firmness test. And then the formulation of home-made SCR catalyst is determined, which uses 10%WO3 doped TiO2 powder as carrier material, the content of V2O5 and SiO2 is 3% and 10%, respectively, and the coating amount is 200 g/L. Then the catalytic performance evaluation of fresh, hydrothermal-aged and sulfur-aged state catalyst is carried out on the simulation evaluation device, and the foreign SCR catalyst is used as comparison. The experiment results show that home-made and foreign catalyst have the similar catalytic activity; the former possesses a little low-temperature advantage and the latter is a little better in high-temperature range. The hydrothermal aging has a large negative effect on both home-made and foreign catalysts, but the sulfur aging almost does not. Engine bench test results indicate that home-made catalyst show lower NO* conversion rate than foreign one in ESC cycle, but higher NOx conversion rate in ETC cycle. This can be attributed to the average temperature of ESC is higher than ETC. The test results are also confirmed in single-point experiment. After fast aging, the deteriorate rate of home-made catalyst is higher than foreign sample, but both can also make the engine meet CN-IV requirements. The research also builds a structure-activity relationship between the catalytic performance of the catalyst and the chemical structure based on the catalytic chemistry. With regard to the NOx conversion-catalyst structure relationship, the content of V2O5 can influence the temperature window of the catalyst, which is high V2O5 content favors low-temperature activity; and low V2O5 content favors high-temperature activity. Because the home-made catalyst has higher V2O5 content than foreign one, so the former shows better low-temperature activity but worse high-temperature activity than the latter, which are consistent with the experimental results. With regard to the aging performance-catalyst structure relationship, the major reason for catalytic activity reduction in hydrothermal aging is the loss of BET, and this is close connected to the V2O5 and SiO2 content in the catalyst. Higher V2O5 content can lead to the increase of BET loss ratio during the aging process, thus resulting in the increase of catalytic performance deterioration; But low-temperature catalytic activity can be enhanced by the high V2O5 content. With the add of SiO2, the fresh BET value can be improved, and thus the deterioration tolerance can be strengthened. The excessive SiO2 adding amount can bring about the loss of catalytic active site and performance. So in the catalyst preparation, the adding amount of V2O5 and SiO2 must be balanced based on the actual operating condition of the target engine.The investigations on POC include the optimization of PM removal rate for metal and ceramic-based POC convertor, and the comparison of the above different material-based samples. All the tests are conducted on engine bench, and a typical domestic 2.4 L CN-IV electronic control diesel is selected as test engine. As for metal-based POC optimization, the work focuses on the improvement of wire mesh twine process and confirming the optimal corrugation mesh height. The optimal preparation method is achieved based on the ESC and ETC test results, which is using "two plain with one corrugation mesh" twine process and the height of corrugation mesh is 1.3 mm. As for ceramic-based POC optimization, the research concentrates on the CPSI and arrangement of the POC substrates. The optimal preparation scheme is got also based on the ESC and ETC test results, which is using 300 CPSI substrate,2 parts arrangement(diarneter*height:143.8*76.2,2 parts). Then the comprehensive comparison of optimal metal and ceramic-based POC is carried out. The test results show that two type of POC can both make the engine emission meet CN-IV requirements. The PM emission for metal-based POC is lower than ceramic-based one, and for the case of the average exhaust back-pressure and maximum particle trapping amount, metal-based POC exhibits higher. The ceramic-based POC presented lower burning damage than metal-based, so the former shows more stability than the latter. As in the POC/SCR combined system, the stability is the most critical consideration, therefore, the research chooses ceramic-based POC used in the combined system. In the end, the optimal coating amount of ceramic-based POC(10 g/L) is determined based on the emission performance and exhaust back-pressure.The packaging for "POC/SCR" aftertreatment style has been designed, in other words, the catalytic converter has been designed. The catalytic converter is divided into two sections:the front has two carriers, DOC and POC; the back has two SCR carriers. Based on the evaluation index of air flow uniformity coefficient before carrier, the accept standard for the distribution of air flow and reducing agent has been put forward which value are 0.95 and 0.90 respectively; the overall back pressure for the catalytic converter is restricted to 25 kPa. Using CFD simulation analysis, the structure of the catalytic converter has been optimized. For the front-end uniformity of catalytic converter before DOC carrier, the variable density mixer has been designed, making the air flow uniformity coefficient to achieve acceptance criteria; for the back-end uniformity of catalytic converter before first SCR carrier, the hole diameter and distribution has been optimized, making the air flow uniformity coefficient to achieve acceptance criteria. The difference of the result of CFD simulation analysis for front/back catalytic converter respectively and connectively is very small; the CFD simulation analysis verification for "一" type product structure can be executed partly, the computing time can be shortened. Finally, the transient analysis of spray has been carried out, the catalytic converter which has been optimal designed can meet the design requirements in uniformity and back pressure.At last, this research performs a engine bench emission test on 493 non-electronic control diesel engine employing POC/SCR combined system. The experiments results show that either PM or NOx but not in the case of both can meet CN-V standard with only POC or SCR system, however, both PM and NOx emission in ESC and ETC can reach CN-V standard by employing POC/SCR combined system. The research on regeneration of POC shows that he passive regeneration of POC can be fully achieved by placing the POC ahead of the SCR system due to the sufficient amount of NO2 in the exhaust which can be utilized for regeneration requirement. The optimal arrangement mode is then determined, which is POC+SCR. The WHTC test results indicate that with the POC+SCR combined system, the NOx and PM emission can reach stage V requirements of HJ689-2014. After 200 h fast aging on the bench, the emission test results of ESC and ETC can still meet CN-V standard, but not for WHTC, which NOx emission is out of limits. Nevertheless, this combined after-treatment system can be adopted on non-urban diesel vehicles in CN-V stage. The backpressure of the combined system can be less than 25 kPa during the whole aging process, which has little influence on the engine performance.