Design And Research Of Marine Diesel Exhaust Emission Purification Device Based On Hypergravity Technology

With the implementation of the second stage of emission limit of marine engines in China,the research on emission control technology of marine diesel engines has attracted more and more attention.SCR,one of the traditional diesel engine post-treatment technologies,can effectively remove NO_x in exhaust gas,but the cost of SCR is high and the catalyst is prone to poisoning for heavy oil diesel engines.High gravity,as an efficient mass transfer technology,can simultaneously remove various components in exhaust gas,which has become a research hotspot.In this paper,a high gravity tail gas purification device was designed for a six-cylinder in-line marine diesel engine,and a set of tail gas post-treatment denitrification system was built.NO_x purification research was carried out by simulation and experiment.Firstly,the design of high gravity tail gas purification device and the analysis of vibration characteristics of the device during operation were carried out.the design requirements of high gravity tail gas purification device were proposed based on the engine emission characteristics of a six-cylinder diesel engine for a fishing vessel.According to the design requirements,the structure of the high gravity tail gas purification device was preliminarily completed,and the structural strength of the main parts was calculated and analyzed using the finite element simulation software.The calculation results show that the structural strength of the spindle and the turntable is in line with the actual operation requirements.The vibration characteristics of the actual operation of the purification device were tested.The vibration displacement,vibration velocity and vibration acceleration of the vertical and horizontal directions of the device at different speeds and the axial direction of the spindle were measured.The test results show that the maximum vibration displacement of the device is in the horizontal direction at the spindle speed of 600r/min.The maximum vibration velocity is in the vertical direction of the device at the spindle speed of 2100r/min,while the maximum vibration acceleration is in the vertical direction of the device at the spindle speed of 2400r/min.Secondly,the numerical imulation of gas-liquid two phase flow in the purification device was carried out.The three-dimensional simulation model of the reaction chamber of the purification device was established.The flow characteristics of the gas in the reaction chamber of the rotor structure of the purification device at different velocities were simulated,and the gas flow and pressure distribution fileds in the reaction chamber were obtained.In addition,the flow velocity distribution of liquid on the surface of rotating disc was simulated.The calculation results show that the rotor speed of the purification device has little effect on the gas flow velocity distribution in the reaction chamber.The pressure distribution in the reaction chamber is uniform,and the flow velocity is not uniform.The calculation results show that the liquid flow velocities has no effect on the flow distribution of the liquid on the surface of the disc.The flow of the liquid on the surface of the disc increases with the increase of the rotational velocity of the disc,and the flow increases gradually along the radius expansion direction.Then,a post-treatment purification system with a DOC and the device for the marine diesel engine was built,and the emission test of the base engine was carried out to analyze the influence of the DOC and engine working conditions on exhaust temperature and NO_x exhaust composition.It is found that the NO_x concentration increases first and then decreases with the engine load increase.The maximum NO_x is 1791ppm at the engine power of 145 kW.The DOC has obvious oxidation effect on NO in NO_x composition,but the oxidation ability at low and high temperatures of tail gas is slightly worse than that at intermediate temperature.Finally,the effects of the concentration of urea aqueous solution,engine operation conditions,and the purification device conditions on NO_x purification efficency were experimentally studied.The result shows that,the NO_x purification efficency of the diesel engine under different working conditions is improved with the increase of the concentration of urea solution and the rotational speed of the device.The tail gas temperature of the diesel engine is relatively high under the conditions of290 kW and 217.5 kW,which is conducive to the hydrolysis and pyrolysis of the urea aqueous solution.The proportion of NO₂ in the tail gas at the engine power of 217.5 kW is relatively high,and the purification efficiency is up to 69.1%.Although the proportion of NO₂ in the tail gas at 145 kW and 72.5 kW is high,the the NOx purification effect is slightly worse due to lower gas temperature.It can be seen that the NO_x purification efficiency is closely related to the temperature and composition of diesel engine tail gas.