Investigation On The Mixing Performance Optimization Of Urea Aqueous Solution In Marine SCR System

With the promulgation and implementation of Tier ? regulations,ship exhaust NOx control is facing severe challenges,and higher requirements are placed on the improvement of the catalytic conversion efficiency of marine SCR systems.As a key component of the SCR system,the static mixer has an obvious role in improving the conversion efficiency of urea and the uniformity of the spatial distribution of ammonia gas,and is widely used.However,under the circumstances of small installation space of marine engines,near-shore maneuvering and low exhaust temperature under low-load operating conditions,the single static mixer marine SCR system cannot complete the efficient conversion of NOx.Therefore,this article will focus on the problems caused by strict emission regulations and various restrictions on marine SCR systems,with the goal of improving the urea decomposition rate and the uniformity of ammonia concentration,starting from improving the mixing quality of urea aqueous solution and diesel exhaust Research on the performance optimization of the marine SCR system of the static mixer,and through the collaborative optimization with the structural parameters of the marine SCR system,the working performance of the original marine SCR system is effectively improved.In this paper,Converge software is used to establish a numerical model of the process of urea aqueous solution droplets hitting the wall,evaporation,decomposition and liquid film formation.Based on the full-scale marine SCR model,the number of mixers and the structural parameters of dual static mixers for marine.The influence rule of parameters such as internal flow field of SCR system,urea liquid film thickness,urea decomposition rate and uniformity of ammonia concentration distribution.The study found that after the use of dual static mixers,the SCR system urea aqueous solution evaporation decomposition rate,catalyst inlet crosssectional velocity field and ammonia concentration distribution uniformity were improved significantly.The urea decomposition rate was increased by 25% compared with the single mixer,and the ammonia concentration uniformity reached 95%;the increase in the number of mixer blades is conducive to the further improvement of the uniformity of the spatial distribution of ammonia concentration;and the narrowing of the distance L of the double mixer can enhance the turbulence intensity in the mixing tube and strengthen the mixing of urea droplets and exhaust gas to improve urea decomposition However,at the same time,the axial velocity of the central flow field of the mixing tube rises,which shortens the residence time of small-sized urea droplets in the mixing tube and negatively affects the conversion and decomposition of urea.As the number of static mixer blades increases,the effect of turbulence intensity on the final urea conversion efficiency is more significant.Then,the effect of the geometric structure parameters(mixing tube length and expansion tube angle)of the dual static mixer marine SCR system on the urea decomposition rate and ammonia distribution uniformity was studied.The research shows that in the dual static mixer marine SCR system,the effect of the mixing tube length on the uniformity of ammonia distribution is weak,but since the mixing tube length is the main factor affecting the residence time of urea droplets in front of the reaction chamber,its The effect on urea decomposition rate is still obvious.In contrast,the expansion angle of the expansion tube has a significant effect on the uniformity of the ammonia concentration distribution.An excessively large expansion angle deteriorates the adherence of the exhaust gas,making the ammonia distribution uniformity coefficient significantly lower,but its effect on the urea decomposition rate is limited.Finally,using a combination of orthogonal design and simulation,the dual static mixer parameters and the SCR system geometric structure parameters(mixer spacing,number of blades,mixing tube length,expansion tube angle)on urea decomposition rate and ammonia distribution uniformity were studied.The degree of influence.Based on this,the ship's SCR system was optimized and designed.At the same time,the pressure drop of the SCR system,the evaporative decomposition rate of the urea aqueous solution,and the uniformity of the ammonia concentration distribution were compared and analyzed.The study found that: compared with the original single mixer,The urea decomposition rate,velocity field and ammonia concentration distribution uniformity of the dual static mixer marine SCR system are optimized than the original single mixer SCR system,but the flow resistance caused by the double static mixer has increased,and as the load increases,The trend becomes more obvious.But even under 100% load,the pressure drop of the dual static mixer SCR system is less than 3kPa,which meets the SCR system design requirements.