Figure 4: EGR process diagram. Bypass matching


temperature of the exhaust gas. When the sulphur content in the fuel is equal or less than 0.1%, a temperature of approximately 310°C would be sufficient. At low exhaust gas pressures, the required minimum temperature will be lower. The minimum temperatures required


Figure 5: EGR process diagram. TC cut-out matching


to avoid the formation of ammonia bisulphate are found in Figure 6, which shows the relation between the fuel sulphur content and the exhaust gas pressure. Figure 6 shows a high pressure curve (4.0bar) and a low pressure curve (1.5bar), which is the approximate pressure at high engine load and at low engine load respectively. On the other hand, the temperature


must not be too high as this will result in an increased SO3 catalyst. SO3


formation in the subsequently reacts with


water creating sulphuric acid, which appears as an undesired white aerosol plume. Another undesired reaction which also limits the upper temperature for SCR operation is the oxidation of NH3


approaches 500°C, ie. more NH3


as the exhaust gas temperature is


needed. Additionally, the catalyst material starts to sinter at temperatures above 500-550°C. In other words, to ensure a robust


as a reducing agent. The catalyst in the reactor consists of blocks with a large number of channels, providing a large surface area in which the catalytic process takes place.


SCR system An essential parameter of the SCR process is the inlet gas temperature. A lower temperature limit is dictated by the sulphur content in the fuel and the subsequent formation of sulphuric acid in the gas. At low temperatures, the sulphuric acid is neutralised by ammonia. This forms a sticky product, ABS (ammonium bisulphate, (NH4


)


Figure 6: Required temperatures for SCR related to sulphur content and exhaust gas pressure


The Naval Architect March 2016 33 HSO4), which may accumulate in the


SCR elements. However, this reaction can be suppressed by keeping a high


SCR operation it is crucial to maintain exhaust gas temperatures within a certain temperature window. The low sulphur SCR systems can be chosen as high-pressure or low-pressure installations.


Feature 3


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