4.2.2 Direct Water Injection into the cylinder for up to 60% NOx reduction
The best approach is to introduce water directly into the combustion chamber with a separate nozzle. Large amounts of water can be used, enabling a substantially higher NOx reduction level than with the water-in-fuel emulsion method. Optimization of the injection timing and duration for maximum NOx reduction is possible.
The key element in the design concept is the combined injection valve through which both fuel and water are injected. The same valve can be used for operation on water injection or fuel injection alone in both modes. Injection is controlled electronically and indirectly by a control oil and rail valve system. Built-in safety features enable quick water shutoff in the event of excessive water flow, water leakage, disturbance in the exhaust gas temperature balance etc. The water injection timing is 20-70 CA BTDC and duration 20-60 CA, depending on the engine speed and load (Figs 6 and 7).
The NOx reduction potential for the direct water injection method is typically 50-60%. In principle, the fuel choice is free. So far, it has been optimized for heavy and distillate fuels.
The system is small in size thereby also enabling retrofit installation onboard a ship. The estimated investment costs are about 30% of those for a SCR catalyst system.
The first direct water injection system has been in continual operation since December 1993 on board the "Aurora af Helsingborg", which plies between Helsingborg, Sweden, and Helsingoer, Denmark. The system is performing excellently after an accumulation of about 3500 operating hours (Fig. 8).
Figure 6. Basic principle of Direct Water Injection.
Figure 7. The combined fuel and water injection valve for Direct Water Injection.
4.3 Compact SCR for 90-95% NOx reduction
SCR (Selective Catalytic Reduction) is the only viable exhaust aftertreatment method for NOx. The SCR principle is based on a reduction of NOx by catalytic reactions with ammonia or alternatively urea injected into the exhaust gas. Typically, very high NOx reduction levels, i.e. 90-95% (1 g/kWh NOx emissions) are achievable with no impact on the engine performance. The investment costs for a SCR installation are typically 80-100 USD/kW. Typical operating costs are 1-1.5 USD/MWh.
