DieselShip

Uptake Emission control

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.

Typical emissions from a MAN B&W low speed engine

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.

Maximum allowable NOx emissions for marine diesel engines

There are two main approaches to reducing NOx

1. Primary methods, aimed at reducing the amount of NOx formed during the combustion process
2. 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
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.

Schematic of exhaust gas recirculation system and water emulsified fuel system

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 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

Stratified fuel-water injection (SWFI)

• The effects of water addition on diesel spray combustion include a thermal effect due to the large latent heat of evaporation and the specific heat of water and a chemical effect due to water gas reaction with free carbon. It is believed that the lowering of the combustion temperature by these effects in the region of combustion contributes to the suppression of NOx generation.
• The aim of SFWI is to add a large quantity of water to the fuel spray after ignitability has been ensured by injecting completely pure fuel oil at the start of injection. Water and fuel are injected separately through the same valve.
• The hydraulically actuated piston delivers water via the solenoid at a time when fuel oil injection is not taking place.
• Delivered water is at a greater pressure then the oil delivery, it pushes back fuel in the passage between the injection pump and the injection valve. By this process, fuel and water are injected into the cylinder during the next injection cycle while retaining stratification in the sequence: Fuel - water- fuel.
• During this cycle the rack becomes higher by an amount corresponding to the amount of water injected.

The following points of note came from an in service trial

• The SWFI system worked in a stable condition throughout in service testing
• Protective devices worked well in the case of abnormalities
• NOx reduced in proportion to water injected
• Engine components did not suffer

Selective catalytic reduction (SCR)

SCR can reduce NOx levels by at least 95%. Exhaust gas is mixed with ammonia before passing through a layer of a special catalyst at a temperature between 300 to 400oC. The lower limit is mainly determined by the sulphur content of the fuel: at temperatures below 270^oC ammonia and SOx will react and deposit as ammonium sulphate; and at excessively high temperatures the catalyst will be degraded (the limit is around 400-450^oC).

NOx is reduced to harmless waste products nitrogen and water vapour. In addition some soot and hydrocarbons in the exhaust are removed by oxidation in the SCR reactor.

Ammonia is stored as a liquid gas under pressure of 5-10bar in a deck mounted storage tank protected to prevent overheating. A computer controlled quantity of evaporated gas is led to the engineroom via a double skinned pipe. A bypass arrangement allows the SCR to be redundant when away from controlled areas.

A flow of air is taken from the scavenge and used to dilute the ammonia in a static mixer. The ammonia concentration is thus below the L.E.L. before it enters the exhaust pipe. The minimum engine load for NOx control with SCR is 20-30% unless more comprehensive temperature control systems are installed. At lower loads the catalyst is by passed.

Ammonia fed to the SCR reactor can be liquid, water free ammonia under pressure, an aqueous ammonia solution at atmospheric pressures or in the form of urea carried as a dry product and dissolved in water before use.