Flow Field Simulation And Numerical Optimization Of Marine Denitration System

The marine denitrification system is the dominating equipment to purify the nitrogen oxides in the exhaust from ships,using selective catalytic reduction?SCR?technology for transforming the NOx into environment-friendly nitrogen.It has several advantages such as wide application range,strong practicability and excellent treatment effect.Following the Tier?emission regulations,the numerical method was used in flow field investigation based on a specific marine denitration system,focusing on the injection of reductant;the influence of static mixer;the different design of SCR reactor structures.Integrated into an intact SCR system,the influence of the ash blowing device on the flow field of the system was analyzed.The detailed contents are as follows:?1?The study of the urea solution spray and decomposition revealed the factors that affect the distribution of the decomposition product NH₃,including the flue gas temperature,solution injection rate,average droplet size and solution spray angle.In order to achieve better distribution of NH₃ and conversion efficiency,the optimal conditions were velocity of 45m/s,the average particle size of 85?m and the injection angle of 100°.?2?For the SCR system,the mixer plays a role in accelerating the mixing of NH₃and flue gas.We designed three types of mixers and discussed the influence of central plate and non-central plate on flue gas flow.Using the evaluation indexes of the flow uniformity and pressure drop,a central disc fan blade mixer with 35%porosity was rated as the best design,with which the flow uniformity reached the highest value,while the inlet cone of SCR reactor had a small pressure drop.?3?For the sake of achieving the compact arrangement of marine SCR reactors,asymmetric design and structural optimization were carried out,with a total of 12design schemes.Considering the bending angle?of the expansion section and the coning angle?of the contraction section as the variable parameters,fixing the length of the reactor in the axial direction,two optimal design C_60-45??is 60°,?is 45°?and C_60-30??is 60°,?is 30°?were obtained.In addition to optimal schemes C_60-450-45 and C_60-30,the study investigated the influence of the non-uniform distribution of the inlet flow.The non-uniform distribution of the inlet flow led to the increase of the pressure drop of the system while the uniformity of the flow remained unchanged or significantly decreased.?4?This paper studied the influence of the ash blowing device and analyzed the flow field variations under four working conditions based on the intact SCR system.The emphasis was flow uniformity and pressure drop from inlet to the front side of the catalyst.The results showed that due to the blocking effect of the catalyst,the backflow near the reactor wall formed.The obvious vortex leaded to a decline in the flow uniformity.Meanwhile,high pressure air purging promoted the flow uniformity under different working conditions.