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In the operation of a high-pressure selective catalytic reduction (HP-SCR) system, variations in the internal exhaust gas flow speed result in non-uniform pressure and temperature distribution within the reactor. These fluctuations, which are neither constant nor linear, can affect the safe and reliable operation of the high-pressure selective catalytic reduction (HP-SCR) system, so the strength simulation analysis is necessary. Based on the high-pressure selective catalytic reduction system of a thermo-fluid-solid coupling marine diesel engine as the research object, this study constructs a calculation model using Space Claim and utilizes computational fluid dynamics (CFD) and computer-aided engineering (CAE) numerical simulation methods to analyze the strength of the high-pressure selective catalytic reduction (HP-SCR) reactor. The results show that the overall pressure drop of the selective catalytic reduction system is 5500 Pa, and the overall temperature rise of the reactor is 24 °C, which mainly occurs in the first layer catalyst, accounting for 62.5%. The pressure and temperature load of the reactor change along the axial direction, and the axial deformation gradient of the cylinder is more. The maximum deformation of the reactor under thermal load is 15 times that under mechanical load, and 97% of the deformation is axial. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As one of the most effective NOx emission removing technologies to meet the Tier III limitation by International Maritime Organization, urea-selective catalytic reduction (SCR) technology is starting to be used in two-stroke marine diesel engines. Based on the two-cycle catalytic mechanism proposed by Topsoe, in combination with the exhaust characteristics of the marine diesel, expansion studies on detailed SCR reaction model were carried out in this paper. According to the temperature dependence of reaction pathway, SCR reaction model was divided into three parts: low temperature reaction pathway, standard SCR reaction pathway, and high temperature oxidation pathways, and an expanded NH3-NO/NO2-SCR reaction model for V2O5 catalyst was proposed in the paper. In order to verify the accuracy of the expanded SCR reaction model, simulating and testing studies of SCR reaction under marine diesel conditions were carried out with a commercial extruded V2O5/TiO2 catalyst. The simulation values are agreed well with experimental values at 150–500 ℃, and kinetics characteristics of SCR reaction process under V2O5/TiO2 catalyst can be predicted accurately with the expanded NH3-NO/NO2-SCR reaction model. [ABSTRACT FROM AUTHOR]

In order to avoid the production of sulfates and nitrates in marine diesel engines that burn sulfur-containing fuels, the operating temperature of their high-pressure selective catalytic reduction (HP-SCR) systems should be higher than 320 °C. For marine low-speed diesel engines, only the pre-turbine exhaust gas temperature can meet this requirement under specific conditions, with the main engine modulation method helping to increase the exhaust gas temperature. However, the main engine modulation method brings down the power output and fuel economy of the main engine and causes the matching problem of the turbine and the other devices with the main engine. The original engine model of the marine low-speed diesel engine and the high-pressure SCR system configuration model have been constructed using one-dimensional simulation software. In addition, the performance of the high-pressure SCR system under the conditions of low-sulfur and high-sulfur exhaust gas was thoroughly analyzed. Moreover, the two main engine modulation schemes of the scavenging bypass and the turbine exhaust bypass of the original engine matching with the high-pressure SCR system were studied. The study found that the weighted average value of the NOx under the condition of low-sulfur exhaust gas met with the requirement of the IMO Tier III regulations when the low-speed diesel engine was matched with the high-pressure SCR system. However, the weighted average value of the NOx under the condition of high-sulfur exhaust gas was slightly higher than that required by the IMO Tier III regulation. In addition, the optimal main engine modulation scheme for this low-speed diesel engine was clarified by comparing the effects of the scavenging bypass and the turbine exhaust bypass modulation on the exhaust performance, and the working performance of the original engine. With an opening of 0.4 of the CBV valve under 25% engine load, the weighted average NOx of the original exhaust gas was 3.38 g/(kW·h), the power had decreased by 0.7%, and the fuel consumption had increased by 1.0%. Furthermore, when the EGB valve opening was 0.3, the weighted average value of NOx was 3.31 g/(kW·h), the power had reduced by 2.4% and the fuel consumption had increased by 2.5%. Both modulation scheme methods made the exhaust performance of the original engine meet the requirements of the IMO Tier III emission regulations, but the scavenging bypass modulation scheme had less impact on the original engine's performance. [ABSTRACT FROM AUTHOR]

The working process of high-pressure SCR (HP-SCR) system is a kind of catalytic reaction process in the high-pressure exhaust flow, which involves urea decomposition reaction, catalytic reduction reaction of NOx and oxidizing reaction of NH3. Based on existing exhaust conditions of marine low-speed diesel engines, computational fluid dynamics (CFD) coupled with the chemical reaction kinetics, the catalytic process, and the working performance of the HP-SCR system were correspondingly studied in this paper. With the optimized scheme of the HP-SCR system, concentration uniformity, linear velocity, and total pressure loss were determined. Likewise, the concentration distribution of NH3, NO, and NO2 were obtained, and the catalytic kinetics of the NOx reduction reaction was analyzed. For the optimized scheme, the concentration of NH3 and the flow uniformity of the catalyst upwind section was met with the design requirements of the HP-SCR system, and the pressure loss of the SCR catalyst layer occupied about 40% of the total pressure loss. With NO, NO2, and NH3 existing together in the exhaust, both the standard SCR reaction and fast SCR reaction are competitive, and they mostly react along the first layer of catalyst. Finally, the weighted average value of the NOx emission behind the HP-SCR system was determined using the ISO8717 test cycle E3, and the value was 2.67 g/kWh, which met with the NOx limited requirement of the IMO Tier III. [ABSTRACT FROM AUTHOR]

[Display omitted] • Characteristics of urea deposits in the marine two-stroke and four-stroke engine SCR system. • Newly developed detailed urea decomposition mechanism. • Kinetic model of urea decomposition. • HCNO and by-products' homologues are important species in the process of urea crystallization. • Urea deposits contain non-urea decomposition by-products such as PM and sulfate. The problem of urea crystallization in selective catalytic reduction (SCR) systems has been a concern for a long time, especially in vehicle engine after-treatment. Compared with the vehicle engine SCR system, the layout space of the marine engine SCR system is more spacious, but the impingement of urea-water solution (UWS) on the wall as well as the formation of urea deposits still cannot be avoided in many cases. In this paper, the urea deposits in the SCR systems of 10 MW marine two-stroke and 140 kW marine four-stroke engines are taken as the research objects. The research results showed that the composition of urea deposits is affected by the composition of exhaust gas. Besides biuret, ammelide, cyanuric acid (cya) and its homologues, the urea deposits contain non-urea decomposition by-products such as particulate matter (PM) and sulfate. In addition, most of the existing urea decomposition mechanisms are obtained by inferring the intermediate reaction pathways through thermogravimetric analysis (TGA), which has the disadvantage of kinetic parameters being inaccurate calculation due to oversimplified reactions, and cannot explain the coexistence phenomenon of cya and its homologues in the urea deposits of marine SCR systems. Therefore, based on density functional theory (DFT), a new detailed urea decomposition mechanism is proposed in this paper, including 15 species and 23 reactions. This mechanism introduces homologues of urea, biuret, and triuret, complementing the reaction pathways for forming cya and its homologue, and is more reliable in theory. On this basis, the correctness of the proposed urea decomposition mechanism is validated by using the TGA experimental data of urea deposits and the urea decomposition kinetic model, and machine learning. [ABSTRACT FROM AUTHOR]