There are several different methods developed to reduce the formation of nitrogen oxides to date. These methods include water emulsion in the fuel, fuel valves with special “low NOx nozzles”, Exhaust Gas Recirculation (EGR), Scavenging Air Moisturisation (SAM), Wetpac, direct water injection (DWI) and catalytic exhaust cleaning, known as Selective Catalytic Reduction (SCR). Some methods are better suited to two-stroke engines, and others are more suitable to four-stroke engines.
To reduce NOx emissions in the Wallenius fleet we have focused on the following techniques:
Sliding fuel valves
Sliding fuel valves are fuel injection valves which are designed to reduce uncontrolled leakage of the fuel that remains in the fuel injection nozzle when the valve is closed. The slide valves installed are also equipped with a specially designed injection nozzle that optimises the distribution of the fuel throughout the combustion process. High temperature peaks are avoided and thereby the generation of NOx is reduced by as much as 30 per cent.
Due to the improved combustion, cylinder oil consumption has been reduced by 40 per cent and the cleaner engines have resulted in less maintenance work. Furthermore, the emission of particles has been reduced with up to 70 per cent. However, low sulphur fuel is also a part in that equation.
Since 2002, slide valves are standard equipment in all new MAN B&W engines. Between 2001-2004, Wallenius installed slide valves on the rest of the fleet with MAN B&W engines.
Moisturisation of the charge air
Moisturisation of the charge air before it enters the combustion process, results in the water mist reducing the temperature peaks and thereby reducing the formation of nitrogen oxides. Wallenius has tested moisturisation of charge air both on the main engine and on auxilliary engines. Measurements shows a NOx reduction by around 35 per cent but there are challenges with duarbility and corrosion.
LNG
Liqufied Natural Gas, LNG, is a fossil fuel but compared to Heavy Fuel Oil (HFO), the emissions are significantly reduced. Compared to HFO there is an about 80% NOx reduction when running a four stroke engine on LNG.
There are already technical solutions in place that makes it possible to run the vessels on LNG but there are several practical challenges. LNG is typically stored in a cylindric tank at minus 163 degrees Celcius. The energy content is about half compared to Heavy Fuel Oil. Those facts makes it easy to understand the tank arrangements for a LNG powered vessels will be very challenging.
Another challenge that slows down the development of LNG as fuel for shipping is that the infrastructure and market is not yet ready.
Questions that still await answers are for example: Where and when will there be LNG available? How much will it cost? How long time will it take to bunker?
Read about our results and statistics within Nitrogen oxides