Structural Optimization And Transient Performance Simulation Of Engine-integrated SCR Systems For Marine Low Speed Engines

Selective Catalytic Reduction（SCR）technology is currently recognized as the most effective and promising NOx emission control technology in the world.However,the traditional SCR system is large in size and limited in ship space,which greatly limits the application of the SCR system.MAN Company and Win GD Company integrated the SCR reactor with the exhaust manifold,and respectively proposed a compact engine-integrated SCR system design scheme.Based on the design scheme of engine-integrated SCR system proposed by MAN,this paper studies the working process of engine-integrated SCR systems for marine diesel engines under steady-state boundary conditions,parameter fluctuation transient boundary conditions and main engine misfire limit conditions using AVL Fire software.The main work and conclusions of this paper are as follows:（1）The engine-integrated SCR system simulation model was established,and the numerical simulation analysis of the flow field distribution of the initial engine-integrated SCR system was carried out.Combined with its working characteristics and design requirements,the flow field optimization is carried out for the speed uniformity and NH₃ concentration uniformity of the engine-integrated SCR system respectively.For the velocity field distribution,the addition of diversion devices,rectifiers,and split baffles can well control the gas flow rate and linear velocity of the reactors in each layer of the engine-integrated SCR system;for the concentration field distribution,from the urea injection cone angle as well as the static mixing grid,the NH₃ concentration field is optimized.Finally,an optimized solution that meets the design requirements is obtained.The speed uniformity of the optimized engine-integrated SCR system has reached 80%;the gas linear velocity of the upper,middle and lower reactors is about 4m/s,the flow ratio is about 1:1.5:1,and the gas volume distribution is reasonable;The average NH₃ concentration uniformity exceeded 85%;the overall pressure drop of the system was 4096Pa,all meeting the design requirements.（2）The working process of this type of engine-integrated SCR system corresponding to the inlet temperature fluctuation,inlet flow fluctuation and multi-cylinder exhaust cycle fluctuation under three different transient exhaust conditions is studied.The corresponding dynamic boundary conditions are established respectively,and the time is used as the calculation step,and the velocity field changes,temperature field changes,NH₃ concentration changes,and turbulent kinetic energy distribution changes in the internal key areas of this type of engine-integrated SCR system at equal time intervalsΔt are respectively calculated.And slice analysis was performed.The study found that the short-term temperature fluctuation at the inlet of the engine-integrated SCR system,the fluctuation of the intake air flow and the disturbance between the exhaust gas of the multi-cylinder cycle have little effect on the internal linear velocity value and distribution uniformity of the SCR system.The generation amount and concentration uniformity of NH₃ are easily affected by the fluctuation of the inlet parameters of the engine-integrated SCR system and the multi-cylinder intake air disturbance.Affected by the multi-cylinder circulating exhaust,the overall pressure drop of the airborne SCR system exhibits periodic reciprocating fluctuations,with a maximum fluctuation range of 54Pa;the disturbance between the multi-cylinder exhausts leads to the continuous distribution of high turbulent kinetic energy in the gas gathering area.In the event of change,this is conducive to the rapid decomposition of the injected urea solution to generate NH₃.（3）In the low-load operation process,the research on the dilution effect of the chilling effect caused by the single-cylinder misfire of the main engine and the low-temperature cold exhaust gas caused by the multi-cylinder misfire on the high-temperature exhaust gas of the engine-integrated SCR system,as well as the influence on the decomposition and crystallization process of urea,was carried out.The working process of the engine-integrated SCR system when the 1#,4#,and 6#cylinders are misfired is calculated respectively in the single-cylinder misfire process,so as to study the influence of the position of the misfire cylinder relative to the urea injection point on the engine-integrated SCR operation.In the process of multi-cylinder misfire,the working process of the engine-integrated SCR system was calculated when the 1#,2#,and 3#cylinders all misfired,and the 2#,4#,and 6#cylinders all misfired,so as to study the misfire of adjacent multi-cylinders.And the impact of the multi-cylinder misfire on the engine-integrated SCR operation.The study found that the closer the corresponding exhaust port of the corresponding misfire cylinder is to the outlet of the gas collection area（such as 1#cylinder,2#cylinder）,the greater the temperature fluctuation and temperature drop caused.The greater the number of misfire cylinders,the higher the proportion of misfire gas,and the greater the temperature fluctuation and temperature drop caused.Compared with the normal operation of the main engine,the HNCO decomposition rate,NH₃generation amount and generation rate in the engine-integrated SCR system corresponding to the occurrence of the main engine misfire decreased significantly.When the main engine is in normal operation,even under low load operating conditions,the liquid film generation in the matched engine-integrated SCR system is almost zero and can be ignored.When the single-cylinder and multi-cylinder of the main engine misfire,the liquid film of the wet wall will continue to form over time,and the corresponding liquid film thickness will continue to increase;the crystallization of the wet wall mainly occurs in the horizontal exhaust pipe,which corresponds to the urea injection section,NH₃ generating and mixing section,and there is also a little liquid film accumulation on the outermost side wall of the elbow section.