Fig.5: Effects on NOx emission and fuel consumption of combinations of primary emission reduction methods on a 5S70MCE engine
In the case of diversified emission limits, the IE concept allows a special emission-optimised injection pattern to be used for the most stringent limits. Moreover, the fuel injection pumps of the IE concept can act as 'small' pumps as well as 'large' pumps by means of controlling the speed of the plunger. So, if a large amount of water is to be used (to obtain a large reduction of the NOx), the injection pumps may act as large pumps (high plunger speed) while smaller amounts of water may be injected with a correspondingly 'smaller' pump (at lower plunger speed).
To be optimal, the large water amount requires a larger fuel valve nozzle area than the smaller water amount, i.e. a different set of fuel nozzles. This option can be used if, initially, the engine is to operate with a small amount of water and, later on (to comply with stricter NOx limits) with a larger amount of water. If the emulsified fuel is used to comply with different limits during the same voyage, changing fuel nozzles is not practical, whereas the flexible fuel pump function will provide distinct advantages.
2.3 Emission reduction by combining primary methods
In general, it is difficult to evaluate a combination of emission reduction methods. Especially if some of the methods have the same effect on the combustion process, the effects on NOx may not be cumulative. In order to see the potential for combining some of the more promising methods, experiments were carried out on a 5S70MC engine with slide-type fuel valves, reduced firing pressure, water-emulsified fuel and exhaust gas recirculation (EGR). The results are illustrated in Fig.5, which outlines the emission reductions obtained and the corresponding effects on specific fuel oil consumption.
As can be seen, the slide valve gave a reduction of some 20% in NOx (and even a slight reduction in SFOC). A significant further reduction was obtained by combining this with emulsified fuel (50% water addition) and reduced firing pressure, but at a rather high fuel penalty of around 6 g/kWh. By further adding 20% EGR, a total NOx reduction of around 80% was obtained at a fuel penalty of some 7 g/kWh. However, when the firing pressure was re-adjusted to its normal value, almost the same NOx reduction was obtained at a fuel penalty of around 1.5 g/kWh.
EGR thus has a very good trade-off for NOx/SFOC and a large potential for reducing NOx emissions, but this method is not ripe for commercial applications at present. A number of problems must be solved first - such as cooling and cleaning the exhaust gas before it is recirculated, prevention of sulphur-compounds entering the turbocharger or charge air cooling system (corrosion!) and, not least, the logistic problems associated with the dirty, oily waste product from cleaning the exhaust gas.
3. SECONDARY METHODS FOR NOx EMISSION REDUCTION
Very large NOx reductions can be obtained by means of exhaust gas after-treatment using a Selective Catalytic Reactor (SCR), and this method has been used commercially on MAN B&W two-stroke marine diesel engines since 1989.
