The Research On Diesel Exhaust NOx Purification Technology(SCR,SNCR)

Nitrogen oxides(NOx)and particulate matter(PM)which include in the diesel engine exhaust are becoming the main pollution sources in our city air,and the two pollution sources have a great harm on human health and ecological environment.Along with vehicles' sharp growth in our country,NOx and PM emissions is increasingly serious,the control and management is imminent.National III emission standards can be achieved through the optimization of diesel engine combustion system.National IV and above emissions standards rely solely on inside cleaning measures have been unable to meet.To have implemented National ? and even ? emissions regulation in the future,exhaust after treatment technology is necessary to meet their requirements and the most effective technical means.Urea-Selective Catalytic Reduction(Urea-SCR)after-treatment technology,which is the mainstream technology in the current international governance NOx emission,is studied.Firstly,diesel engine Selective Catalytic Reduction system was numerically modeled and analyzed by the common CFD software FIRE.Then the laboratory imports a YC4F115-40 high pressure and common rail diesel engine,During the engine test bench experiment,in order to get the original machine NOx concentration pulse spectra of the engine,do not install post-processing control module in the first place.Then install the post-processing control module and take the mass fraction of 32.5% urea aqueous solution which is the international general at present as a reducing agent for bench test,to measure concentration of NOx emission of the engine again.Compare the twice before and after NOx emission concentration,the comparison results show that the NOx conversion rate as high as 93% above.However the currently using urea aqueous solution is easy to freeze under low temperature,nozzle is easy to crystallize in using after a period of time and so on,high sulfur fuel in our country at the same time also have poison effects on catalyst,all of the faults are seriously restricting the development and popularization of the SCR technology in our country.In order to solve these problems,the mass fraction of 40% methylamine solution as the reductant of Selective Non-Catalytic Reduction(SNCR)method is researched.Methylamine solution has excellent low temperature properties,this method needs no catalysts and is very suitable for our country's current status of the fuel quality.Firstly,diesel engine selective non-catalytic reduction system(SNCR)was numerically modeled by the common CFD software FIRE,using 40% aqueous methylamine as reducing agent.Based on the feasibility of the computational model,the effects of different adding scheme of aqueous methylamine on the NOx reduction efficiency was numerical analysed.Then the concentration distribution under different reducing agent adding scheme inner the reactor was compared.Then the impact of factors such as injection direction,injection volume and the form of fuel injector on the agent concentration distribution is studied.And it provides theoretical basis for the optimization design of diesel engine SNCR aftertreatment device.Setting the same working condition of the bench test,and the simulation results show that the SNCR system has the highest NOx conversion rate as high as 89%,and this means that the 40% methylamine solution as automotive diesel engine NOx reductant is feasible.Secondly,replaced the 32.5%urea solution as the 40% methylamine solution which is used as reductant in SCR experiment,and measure the NOx emission concentration under the same operating conditions,the measurement results show that the engine bench test results is consistent with the simulation results,the experimental data showed highest NOx conversion rate can reach 81%.Finally,compared the SNCR and SCR systems of NOx conversion of bench test.The comparison results show that the NOx emission concentration of SNCR and SCR system is in the same level,although the NOx conversion of SNCR system is slightly lower than SCR system,but considering the initial conditions of this research,the control part of the study's simplicity and other influencing factors,the NOx conversion result of SNCR system have been able to prove the feasibility of the new reducing agent.As the engine bench test and numerical simulation results indicate: during the NOx control scheme of automobile heavy-duty diesel engine,40% methylamine solution as a new type of reducing agent can be a good alternative to 32.5% urea aqueous solution,while it can meet the emission requirements as well as overcome the poor properties under low temperature,easy to be crystallization,catalyst be vulnerable to sulfur poisoning and othershortcomings of the urea aqueous solution.