| Selective catalytic reduction(SCR)technology can effectively remove NOx from the marine exhaust gas to meet the requirements of IMO Tier III regulations.According to the layout,the marine SCR system can be divided into high pressure SCR system(before turbine)and low pressure SCR system(after turbine).In this paper,a diesel simulation model matching with SCR system simulation model are established and coupled for a certain type of marine low-speed two-stroke diesel engine and its matching SCR system.Temperature,space velocity and NH3/NOx,the influential factors of SCR reaction were studied.The minimum temperature required for the SCR system to operate at different sulfur levels was analyzed.The measures to improve the inlet temperature of SCR reactor are put forward,and the performance of its low working conditions is optimized.The diesel engine matching high-pressure SCR system is simulated by the influence of bypass modulation on the diesel engine.First of all,based on a high-pressure SCR system matched with a diesel engine,the simulation model of the SCR system is established and validated.The effects of reaction conditions on the denitration efficiency and ammonia slip of SCR system were analyzed from the aspects of exhaust gas temperature,space velocity,NH3/NOx and NO2/NOx.The simulation results show that with the increase of exhaust gas temperature,the denitrification efficiency of the SCR system shows a slight decrease at first and then decreases.The denitrification efficiency is the highest at 350?,while the ammonia slip decreases with increasing temperature.The larger the exhaust gas velocity of the SCR system,the lower the denitrification efficiency is,and the greater the ammonia slip is.The higher the NH3/NOx ratio in the SCR system,the higher the denitrification efficiency and the greater the ammonia slip.When the temperature is below 250?,the effect of NH3/NOx on denitrification efficiency is almost negligible.Secondly,the influence factors of formation and decomposition of nitrates and sulfates in SCR system at low temperature were studied by using SCR system simulation model.The effects of two types of sediments on SCR system were analyzed.The results show that when the ratio of NO2/NOx is in the range of 0?0.25,the larger the NO2/NOx ratio is,the more NH4NO3 is generated,and the lower the temperature is,the more NH4NO3 is generated.The denitrification efficiency also increases with the increase of NO2/NOx.The formation and decomposition of sulfate occur mainly in the range of 200?350?.Within this interval,the higher the concentration of SO2 and O2,the lower the denitrification efficiency,the less ammonia escape.When the temperature is below 300?,if the exhaust gas contains SO2,the running time is longer and the SO2 concentration is higher,the denitrification efficiency is lower.When the temperature is above 325 ?,the formation amount of sulfate and nitrate are very small,and SO2 has a limited impact on the denitration efficiency of the SCR system.Therefore,when the diesel engine uses high-sulfur oil as its fuel,the exhaust temperature of the SCR system should reach above 325 ?.Finally,a low-speed marine two-stroke marine diesel engine is taken as the research object.The diesel engine model is established by numerical simulation and the model is verified.The model was coupled with the SCR model,and the exhaust gas bypass modulation and the clinder bypass modulation were carried out on the overall model of diesel matching SCR system.The results are as follows:After the diesel engine matches the SCR system,its specific emissions can not meet IMO Tier III regulations when it burns high sulfur oil.The main reason is that the diesel engine's NOx emission is too high under 25%operating conditions.Therefore,The diesel engine is modulated under the condition of 25%.Under 25%conditions,when the valve opening of the EGB valve is 0.3 or the valve opening of the CBV valve is 0.4,the IMO Tier III regulations can be met,and the impact on the power and economy of the diesel engine is small. |
PDF Full Text Request