Matching Performance Of High Pressure SCR System For A Marine Low-speed Diesel Engine

Selective catalytic reduction(SCR)technology is the most effective way to clean the NOx emissions of marine diesel to meet the International Maritime Organization(IMO)Tier III standards.The high-sulfur heavy oil fired in marine low-speed engines and low exhaust temperatures has greatly deteriorated the working environment of the SCR system.This article took high-pressure SCR system of 6S46ME-B marine low-speed diesel as the research object,and studied the influencing factors of SCR system working process,urea injection control,distribution scheme and host modulation to solve the problems of high-pressure SCR system theories in ship application.The main work and conclusions of this article are as follows:(1)The first work carried out research on the influencing factors of SCR system working process and urea control strategies.First,based on the theory of surface denitrification reaction,this work built a SCR reaction model,and the influence law of exhaust gas temperature,exhaust flow,reactor size,pore density per unit area,ammonia nitrogen ratio,and exhaust sulfur content on the working process of the SCR system was analyzed.Secondly,based on the SCR reaction model,open and closed urea control models of SCR system were built.The accuracy of the model was verified in terms of NOx conversion rate,ammonia slip(NH3-slip),and urea solution injection amount.(2)The second work carried out research on the diesel matching scheme of the high-pressure SCR system.Using GT-power simulation software,this paper first built a one-dimensional low-speed diesel engine model and verified the accuracy of the model,then built a host transient model based on it,and conducted a comparison verification by loading and unloading.Secondly,based on the low-speed diesel engine model,this article embedded the urea-controlled SCR system into the host exhaust system,and analyzed the influence rule of the turbine/supercharger,the host,and the SCR system itself on the matching and switching process of the high-pressure SCR system.The research found: 1)During the switching process of the high-pressure SCR system,we adopted a modulation scheme of gradually cutting in and out,and the effect is better.2)After stable operation of the high-pressure SCR system matching the engine,the exhaust back pressure of the engine increases significantly,the fuel consumption slightly increases,and the power decreases slightly.For example: at 100% load,the exhaust back pressure of the engine increases by about 7.2k Pa,the fuel consumption increases by about 0.4g/(k W·h),and its increase rate is 0.23%.3)During the gradual loading process,when the ammonia-nitrogen ratio(NSR)of the urea-controlled high-pressure SCR system is adopted with an injection scheme of 0.77-0.85-0.90-0.90,the NOx weighted emission is 2.78g/(k W·h),which meets IMO Tier-III regulations.The NH3-slip of the system was 3.23 ppm,13.48 ppm,33.18 ppm,and 46.09 ppm.4)The thermal inertia of the high-pressure SCR reactor is mainly reflected in the low-load operation of the engine.(3)The third work carried out the engine modulation scheme research combined with the high-voltage SCR system.This paper built a modulation model of the high-pressure SCR system matching diesel engine,analyzed the emission reduction effect of the high-pressure SCR system under the conditions of high sulfur and low sulfur exhaust gas,and specified the engine modulation scheme that combined with the high-pressure SCR system under high sulfur conditions.The research found: 1)Under the low sulfur exhaust,the emission reduction effect of the SCR system can meet the requirements of Tier III regulations,but it is difficult to meet the Tier III regulations under high sulfur exhaust.2)After matching the high-pressure SCR system under high-sulfur exhaust conditions,the engine only needs to increase the exhaust gas temperature to above 320°C under low load,such as the scavenging bypass(CBV)scheme(the valve opening degree is 0.4,and the bleed amount is 11.2% of the total gas amount)and the exhaust gas bypass(EGB)scheme(the opening degree is 0.6,and the amount of exhaust gas is 12.17% of the total amount of exhaust gas)can meet the requirements of Tier-III emission regulations.3)The CBV modulation scheme is slightly better than the EGB modulation scheme in terms of fuel consumption and engine power.