In recent years, the International Maritime Organization (IMO), Europe, and the United States and other countries have set up different emission control areas (ECA) for ship exhaust pollutants to enforce more stringent pollutant emission regulations. In order to meet the current IMO Tier III emission regulations, an after-treatment device must be installed in the exhaust system of the ship power plant to reduce the ship NOx emissions. At present, selective catalytic reduction technology (SCR) is one of the main technical routes to resolve excess NOx emissions of marine diesel engines, and is the only NOx emission reduction technology recognized by the IMO that can be used for various ship engines. Compared with the conventional low-pressure SCR system, the high-pressure SCR system can be applied to low-speed marine diesel engines that burn inferior fuels, but its working conditions are relatively harsh, and it can be susceptible to operational problems such as sulfuric acid corrosion, salt blockage, and switching delay during the actual ship tests and ship applications. Therefore, it is necessary to improve the design method and matching strategy of the high-pressure SCR system to achieve a more efficient and reliable operation. This article summarizes the technical characteristics and application problems of marine diesel engine SCR systems in detail, tracks the development trend of the catalytic reaction mechanism, engine tuning, and control strategy under high sulfur exhaust gas conditions. Results showed that low temperature is an important reason for the formation of ammonium nitrate, ammonium sulfate, and other deposits. Additionally, the formed deposits will directly affect the working performance of the SCR systems. The development of SCR technology for marine low-speed engines should be the compromise solution under the requirements of high sulfur fuel, high thermal efficiency, and low pollution emissions. Under the dual restrictions of high sulfur fuel and low exhaust temperature, the low-speed diesel engine SCR systems will inevitably sacrifice part of the engine economy to obtain higher denitrification efficiency and operational reliability.

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Due to the low temperature and complex composition of the exhaust gas of the marine diesel engine, the working requirements of the selective catalytic reduction (SCR) catalyst cannot be met directly. Moreover, ammonium sulfate, ammonium nitrate, and other ammonium deposits are formed at low temperatures, which block the surface or the pore channels of the SCR catalyst, thereby resulting in its reduction or even its loss of activity. Considering the difficulty of the marine diesel engine bench test and the limitation of the catalyst sample test, a one-dimensional simulation model of the SCR system was built in this paper. In addition, the deactivation reaction process of the ammonium salt in the SCR system and its influencing factors were studied. Based on the gas phase and the surface reaction kinetics, the models of the urea decomposition, the surface denitrification, the nitrate deactivation, and the sulfate deactivation were both constructed and verified in terms of accuracy. Moreover, the formation/decomposition reaction pathway and the catalytic deactivation of ammonium nitrate and ammonium bisulfate, as well as the composition concentration and the exhaust gas temperature range were correspondingly clarified. The results showed that within a certain range, the increase of the NO2/NOx ratio was conducive to the fast SCR reaction and the NH4NO3 formation’s reaction. Increasing the exhaust gas temperature also raised the NO2/NOx ratio, which was beneficial to both the fast SCR reaction and the NH4NO3 decomposition reaction, respectively. Furthermore, the influence of the SO2 concentration on the denitrification efficiency decreased with the increase of the exhaust gas temperature because of increasing SCR reaction rate and reversibility of ammonia sulfate formation, and when the temperature of the exhaust gas was higher than 350 °C, the activity of the catalyst was almost unaffected by ammonia sulfate.