Simulation Of SCR System For The NO_x Emission Control Of Coal-Based Oxygenated Blended Fuel

Oil shortage and environmental pollution are increasingly serious.Diesel engine is not only one of the main consumers of petroleum resources,but also the emission pollutants such as Nitrogen Oxide（NO_x）and Particular Matter（PM）in its exhaust components will cause great harm to the ecological environment and human health.Based on the abundant coal production and the increasingly stringent emission regulations in China,the development of alternative coal fuels and the combination of effective post-treatment technology to reduce exhaust pollutants is an effective measure to alleviate the energy and environmental problems.This article uses two coal-based oxygenated fuels,Polyoxymethylene Dimethyl Ethers（PODE）and methanol,as basic fuels,formulate PODE/methanol blended fuel with methanol volume fraction of 0,10%,20%and 30%,combustion the prepared blended fuel on a high-pressure common rail diesel engine,and use the combustion analyzer and exhaust gas analysis system to explore the combustion and emission characteristics of these four fuels.The results show that the addition of methanol will cause the peak of the combustion pressure in the cylinder to drop under various working conditions,and the flame retardation period will gradually extend.With the proportion of methanol in the PODE/methanol blended fuel increases,when the diesel engine runs at low load,the peak heat release rate decreases,the combustion duration first shortens and then extends,the combustion temperature in the cylinder gradually decreases,but at high load,the peak heat release rate rises,and combustion period gradually shortens,and the combustion temperature in the cylinder gradually rises.In addition,the addition of methanol has an inhibitory effect on the NO_x emission of the blended fuel and increases the NO₂/NO_x ratio.Compared with MP0,when MP30 is used,NO_x emissions at 20%load and 100%load are reduced by 64.9%and 17.7%,and the NO₂/NO_x ratio has increased by 34%and 35.8%respectively.In order to further reduce the NO_x emissions of the PODE/methanol blended fuel,the SCR system was arranged in the exhaust pipe of the diesel engine to explore the influence of the SCR system on the NO_x emissions of the PODE/methanol blended fuel.The results showed that with the increase of urea injection,the NO_x conversion efficiency of the SCR system increased first and then decreased.Under the condition of 100%engine load and MP30 fuel,the NO_x conversion efficiency reaches the highest 69.3%when n（NH₃）/n（NO_x）is 1.1.When n（NH₃）/n（NO_x）exceeds 1.1,NH₃ leakage amount has increased significantly.The SCR system simulation model is constructed based on Coverage software,to explore the effects of n（NH₃）/n（NO_x）,exhaust temperature and exhaust flow rate on the performance of the SCR system.The results show that the increase of n（NH₃）/n（NO_x）can improve the NO_xconversion efficiency of the SCR system,but too much urea injection will cause insufficient evaporation and cracking of the urea aqueous solution,resulting in a decrease in NO_x conversion efficiency and an increase in NH₃ leakage.The increase in exhaust temperature can promote the urea evaporation and pyrolysis reaction,improve the catalyst activity,which is conducive to the catalytic reduction of NO_x in the SCR system,but an excessively high temperature will promote the oxidation reaction of NH₃ and inhibit the progress of the NO_x reduction reaction;The increase of exhaust flow rate will not only reduce the evaporation and pyrolysis time of urea,but also reduce the contact time between the catalyst and the exhaust gas,resulting in a decrease in the conversion efficiency of NO_x.The SCR mixer is an important component to improve the performance of the SCR system by improving the concentration uniformity and flow speed uniformity of NH₃.In this paper,the method of numerical simulation is used to explore the influence of mixer parameters on the performance of the SCR system with the test data when the engine is fueled with MP30 under the condition of 100%load as the boundary condition.The results show that,with the increase of the number of blades and blade pitch angle,the uniformity of NH₃ and flow speed uniformity can be improved,but when the number of blades exceeds 16,and blade pitch angle is greater than 30°,the pressure drop inside the SCR system increases,exhaust fluid movement is retained.NH₃ uniformity and speed uniformity gradually decrease,resulting in a decrease in NO_xconversion efficiency and an increase in NH₃ leakage.Taking the test data of MP30 under the engine 100%load condition as the boundary conditions,orthogonal optimization was carried out,and the optimal design of SCR mixer was formulated:the number of mixer blades is 16,the inclination angle of mixer blades is 30°,and the width of mixer blades is 16mm.The optimized SCR system has higher NH₃ distribution uniformity and speed distribution uniformity,NO_x conversion efficiency is 75.4%,which is an increase of 6.2%,and NH₃ leakage is 0.96×10^-6,which is a reduction of 31.2%.