Exhaust emissions from marine diesel engines largely comprise nitrogen, oxygen, carbon dioxide and water vapour, with smaller quantities of carbon monoxide, oxides of sulphur and nitrogen, partially reacted and non-combusted hydrocarbons and particulate material. SOx and NOx emissions, together with carbon dioxide, are of special concern as threats to human health and the environment.
Dominating influences in the formation of NOx in the combustion chamber are temperature and the longer the residence time in the high temperature, the more thermal NOx will be created.
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SOx generation is a function only of the fuel oil sulphur level and is therefore best addressed by burning lower sulphur fuels. Emissions are considered low for a given sulphur level thanks to the high efficiency of large diesel engines.
Emissions of carbon monoxide (CO) , also low for large diesel engines are a function of the air excess ratio, combustion temperature and air/fuel mixture.
During the combustion process a very small part of the hydrocarbons (HC) in the fuel is left unburned: up to 300ppm in large two-stroke engines, depending on the fuel type.
Particulate emissions (typically 0.8 to 1 g/kWh) originate from partly burned fuel, ash content in the fuel and cylinder lubricated oil/dosage; and deposits peeling off in the combustion chamber and exhaust gas system
Emission factors (g/kWh) for marine engines under steady state.
Low speed engines Medium speed engines
NOx 18.7 13.8
CO 2.1 1.8
HC 0.5 0.6
SO2 >21.0 x Sulphur content of fuel
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There are two main approaches to reducing NOx
· Primary methods, aimed at reducing the amount of NOx formed during the combustion process
· Secondary methods, aimed at removing NOx from the exhaust gas by downstream treatment
Primary methods include: reducing the maximum combustion pressure by delayed fuel injection, recirculating the exhaust gas, reducing the amount of scavenge air, injecting water into the combustion chamber or emulsified fuel. And the use of special fuel nozzles.
Reducing the firing pressure via fuel injection retardation readily lowers the peak temperatures and yields lower NOx but also invariably reduces the maximum temperature and leads to higher fuel consumption.
Different fuel valve and nozzle types have a significant impact on NOx generation, as well as on smoke and hydrocarbon emissions, and the intensity of the fuel injection is also influential. The influence on NOx is due to the control by the fuel injection system of the formation and combustion of the fuel/air mixture, the local temperature level and the oxygen concentration in the fuel area.
MAN B&W cites tests with a K90MC engine at 90% load which yielded the following results (NOx/ 15% oxygen):
Standard fuel nozzle 1594ppm Six hole fuel nozzle 1494ppm
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Slide type fuel nozzle 1232ppm
it was verified years ago that water emulsification of the fuel can achieve a significant reduction in NOx emissions with no detrimental effect on engine maintenance costs, MAN B&W Diesel citing long experience with low speed engines in power stations. The influence of water emulsification varies with low speed engine type but generally 1% of water will reduce NOx by 15
A standard engine design allows the addition of some 15% water at full load, says MAN B&W, thanks to the volumetric efficiency of the fuel injection pumps-but does not represent a limit from the combustion point of view. Larger ratios have been tested - up to 50/50 fuel and water- with the same or similar impact on NOx reduction but this would call for engine modifications.
Emulsification is performed before the circulating loop of the fuel system, in a position in the fuel flow to the engine from which there is no return flow. Thus it is the fuel flow that controls the water flow. The water flow could also be controlled by measuring the NOx in the exhaust, should continuous NOx monitoring be required.
Water can also be added to the combustion space through separate nozzles or by stratified segregated injection of water and fuel from the same nozzle (see SWFI). The results are similar but retrofitting emulsifiers is simpler.
Humidifying the scavenge space id another way of introducing water into the combustion zone though not as appealing since too much water can cause damage to the cylinder conditions.
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Exhaust gas recirculation (EGR) can be applied to modify the inlet air and reduce NOx emissions, a technique widely used in automotive practice. Some of the exhaust gas after the turboblower is led to the blower inlet via a gas cooler, filter and water catcher.
The effect of EGR on NOx formation is partly due to a reduction in the combustion zone and partly due to the content of water and carbon dioxide in the exhaust gas. These constituents have high specific heats, so reducing the peak combustion temperature which, in turn reduces the generation of NOx.
Kawasaki Heavy Industries on a MAN 5S70MC
· Injection timing retard, which can easily be applied in practice, reduced NOx emissions by 10% with a penalty in specific fuel consumption of 3%
· A 20-30% reduction in NOx emissions was yielded by ,modified fuel valves, notably the slide valve. The local increase in heat load has to be taken into account.
· Water emulsified fuel in a 50% ratio achieved a 35% NOx reduction. The method's effectiveness is considered to result mainly form a reduction in the flame temperature caused by a decrease in the flame temperature caused by a decrease in the calorific value. (This is the EGR system )
· Exhaust gas recirculation was confirmed as the most effective method, resulting from a reduced flame temperature brought about by the enlarged theoretical air-fuel ratio. NOx emission was reduced by 69% with an EGR rate of 28%, accompanied by a very small
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rise in smoke and fuel consumption . (An adjustment of Pmax, however, virtually eliminated the fuel economy penalty for a slight decrease in the reduction of NOx emission.)
The scavenging system was slightly fouled, however, and Kawasaki suggests that further investigations and long term service testing are necessary to ensure protection against fouling and corrosion before EGR is applied to production engines burning heavy fuel with a high sulphur content.
· Combining several methods yielded greater reductions in NOx emissions. A combination of the slide-type fuel valve , 49% water emulsified fuel and 20%EGR, for example, lowered emissions by 81%, the best result in the test programme