Figure 9. "Silja Symphony" and "Silja Serenade" plying between Stockholm (Sweden) and Helsinki (Finland)
Locating a catalyst before the turbine also increases the risk of turbine wear and failure e.g. due to ammonia slip and catalyst dust. Combined gas turbine/SCR installations require a duct burner for heating the exhaust gas before the SCR catalyst due to low-temperature exhaust gases. The SCR system increases the gas back pressure, thereby reducing the thermal efficiency of the gas turbine.
The size of the catalyst package is a function of the inlet NOx level. Consequently modern low-NOx engines have made it possible to combine the silencer and the SCR catalyst into a small Compact SCR unit representing the most cost-effective solution for tightest possible NOx legislation (Fig. 10).
Figure 10. Compact combined silencer and SCR unit.
