Consequently, the slide-type fuel valve behaves as if it had no sac-volume from which fuel could seep out during the late part of the cycle. This type of fuel valve has been subjected to long-term service testing on 12K90MC engines. The results have been very good, with very clean combustion chamber components, and the slide type fuel valve is now standard on new large bore engines.
Injection rate shaping, too, has a potential for reducing the NOx formation, by direct influence on the burning rate. This is difficult to obtain in a conventional fuel injection system, whereas the computer controlled mechatronic fuel injection system of the IE concept is able to adapt the fuel injection pattern to such demands. Fig.3 shows that a NOx reduction of some 20% was obtained by this method, utilising pre-injection. This resulted in a reduced peak-value of the rate of heat release and thus reduced peak combustion temperatures. The fuel consumption penalty was a modest 0.6%.
2.2 Larger reductions
For larger reductions of the NOx emission, the addition of fresh water to the fuel and emulsification of the mixture before supplying it to the engine is a very efficient tool, enabling substantial reductions to be obtained. This proven method for reducing NOx emissions is effective in all engine types. The reduction is typically some 10% for each 10% water addition, see Fig.4, and the associated fuel consumption penalty ranges from zero to some 1% increase for each 10% water addition.
Water-emulsified heavy fuel oil (in combination with slightly reduced firing pressure) was first utilised commercially by MAN B&W Diesel in a 20 MW 7L90GSCA engine for a co-generation plant in Puerto Rico, which has been in operation since 1984. This engine operates with 30% water addition to HFO to achieve the required approx. 30% reduction of NOx in order to comply with the US-EPA emission limits. Service experience with this engine is excellent, with low cylinder liner wear and a total absence of problems in the fuel injection system (which was specially adapted to the use of emulsified fuel).
Against this background, MAN B&W Diesel favours the use of emulsified fuels (in combination with Low-NOx fuel nozzle layout) to ensure that engines comply with future stages of the IMO Regulation and with more stringent requirements for land-based applications. An example of the latter is a recent installation on Guam of two 12K80MC-S engines for an 80 MW power station. The engines were designed to use emulsified fuel with up to 50% water addition in combination with mini-sac fuel valves, and had to comply with emission limits on CO, HC, NOx and particulate matter. The engine emissions are well below the contract values, and the NOx values obtained correspond to around 50% of the present IMO limit value for this engine type.
The IE concept provides certain advantages in connection with the use of emulsified fuel, particularly in the case of large water amounts, diversified emission limits or in preparation for a possible future reduction of the NOx emission of a particular engine in a vessel.
The application of a large percentage of water does not, in general, present any combustion problem in the higher load range where ignition is stable despite the cooling effect of the water. At low load, however, the ignition delay increases and sets a practical limit to the maximum water percentage. Thus, a conventional engine may need to operate with a variable water-percentage to avoid ignition problems in the lower load range, and this will limit the NOx reduction, referring to the IMO load cycle. This problem can be counteracted by the IE concept by a small pre-injection during part load operation, enabling a high water-percentage to be used in the full load range.
