Design and performance evaluation of low-speed marine diesel engine selective catalytic reduction system

Abstract

Selective catalytic reduction (SCR) systems are among the most effective methods used to reduce NOX emissions in internal combustion engines. SCR system performance is directly related to exhaust gas characteristics such as temperature and mass flow rate. Two-stroke engines commonly used in ships have different exhaust gas temperatures compared to four-stroke engines. In low-speed two-stroke diesel engines, the gas temperature drops after the exhaust gas expand in the turbocharger turbine, affecting SCR system performance. The main objective of this study is to determine performance parameters of marine SCR systems using computational fluid dynamics (CFD). The working processes of low-pressure SCR (LP-SCR) and high-pressure SCR (HP-SCR) systems were examined comparatively. The SCR system was modeled to include urea-water solution (UWS) decomposition and catalytic reduction reactions in the study. In the designed model, the amount of injected urea-water solutions (UWS), SCR bypass valve (RBV) opening, thermolysis and hydrolysis reactions, ammonia (NH3) slip and NOX reduction performances were investigated. In addition, pressure drop, velocity distribution and optimal catalyst length were determined on the model. The results showed that an increase in UWS reduces NOX emissions however increases the probability of NH3 slip. Increasing the RBV opening also reduced the pressure loss, which was beneficial to engine performance. However, the increasing RBV opening decreased the NOX reduction performance and increased the NH3 slip. The optimal SCR catalyst length was determined as 1200 mm. Furthermore, in the studies carried out under the same boundary conditions, the HP-SCR system meets the IMO Tier III standard, but it was observed that the LP-SCR system could not meet the standard due to the decreasing temperature in the turbine.