Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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68 Exhaust Emissions and Control

l Combustion air treatment: Miller supercharging, turbocooling, intake

air humidiﬁcation, exhaust gas recirculation (EGR) and selective

non-catalytic reduction. (Miller supercharging and turbocooling are

discussed in Chapter 7.)

l Change of engine process: compression ratio and boost pressure

The basic aim of most of these measures is to lower the maximum tem-

perature in the cylinder since this result is inherently combined with a lower

NOx emission. A combustion chamber geometry which is designed to optimize

mixing of the fuel and air in the cylinder achieves more complete and homo-

geneous combustion, avoiding the temperature peaks that cause over 90 per

cent of NOx formation. New low-swirl cylinder heads and high-compression

re-entrant pistons contribute to a more favourable gas ﬂow and hence a

decrease in NOx formation.

The temperature of combustion—and hence NOx formation—can also be

decreased by retarding fuel injection, although this measure increases the spe-

ciﬁc fuel consumption (the so-called diesel dilemma). Common rail fuel injec-

tion systems enable precise and ﬂexible control of injection pressure, timing

and duration, helping performance, emissions and fuel consumption to be opti-

mized over the entire engine load and speed ranges.

The Miller cycle involves early closure of the inlet valve, causing the intake

air to expand and cool; the lower intake air temperature reduces the combustion

temperature peaks responsible for most NOx formation. Higher emissions of

PM at part load are suffered, but this PM penalty can be eliminated by adopt-

ing a variable valve timing system with the Miller process. High-efﬁciency

turbochargers with increased pressure ratios compensate for the shorter inlet

valve opening times associated with the Miller cycle, ensuring that the quan-

tity of combustion air entering the cylinder—and thus the engine performance

and efﬁciency—remains unaffected. Two-stage turbocharging systems are also

under development to support Miller cycle applications.

De-NOx technology options are summarized in Figure 3.5.

New generations of medium-speed engines with longer strokes, higher

compression ratios and increased ﬁring pressures addressed the NOx emission

challenge (Figure 3.6). The low NOx combustion system exploited by Wärtsilä

throughout its medium-speed engine programme, for example, is based on an

optimized combination of compression ratio, injection timing and injection

rate. The engine parameters affecting the combustion process are manipulated

to secure a higher cylinder pressure by increasing the compression ratio. The

fuel injection equipment is optimized for late injection with a short and dis-

tinct injection period. NOx reductions of up to 50 per cent are reported without

compromising thermal efﬁciency, achieving an NOx rate of 5–8 g/kW h com-

pared with the 15 g/kW h of a typical conventional engine with virtually no

effect on fuel consumption.

Low NOx combustion is based on

l A higher combustion air temperature at the start of injection, which

signiﬁcantly reduces the ignition delay

l A late start of injection and shorter injection duration to place combus-

tion at the optimum point of the cycle with respect to efﬁciency

l Improved fuel atomization and matching of combustion space with fuel

sprays to facilitate air and fuel mixing.

Some Wärtsilä Vasa 32 engines were equipped with a ‘California button’

to meet the US regional authority’s strict NOx control. A planetary gear device

allows the necessary small injection timing retard to be effected temporarily

while the engine is running when the ship enters the Californian waters.

WATEr-bAsED Nox rEDuCTioN TEChNiquEs

Water–fuel emulsions injected via the fuel valve achieve a signiﬁcant reduc-

tion in NOx production. The inﬂuence varies with the engine type, but generally

1 per cent of water reduces NOx emissions by 1 per cent. Fuel water emulsion

Water-based Nox reduction techniques 69

Pre-treatment Substitute fuel

Post-treatment

Internal

treatment

Technology

Methanol

LNG

Emulsified fuel

Fuel injection timing

retard

Catalytic decomposition

De-oxidized furnace

Lean combustion

Rich combustion

Pre-chamber type

combustion

Combustion

Scavenging

Water injection

into cylinder

Water addition

Cycle

modification

Emission

de-NOx

Selective catalytic

reduction

Exchange gas

recirculation

Water mixture into

suction air

Water mixture

(mixed valve)

Water mixture

(independence valve)

Scavenging air cooling

High pressure of fuel

injection

Fuel valve nozzle spec.

modification

FigurE 3.5 Methods of reducing Nox emissions from marine diesel engines

(Mitsubishi heavy industries)

70 Exhaust Emissions and Control

(FWE) methods mix the fuel with freshwater onboard the ship to form an emul-

sion suitable for injection into the combustion chamber. In the case of heavy

fuel oil, up to 30 per cent water can be emulsiﬁed into the fuel, resulting in a

NOx reduction of approximately 30 per cent.

FWE can be produced continuously online during engine operation (the

preferred method) or ofﬂine in batches. MAN Diesel’s continuous method is

installed in a number of cruise ships and cargo vessels, reportedly demonstrating

very satisfactory long-term performance. Moderate capital costs, compactness

and minimal impact on fuel consumption are cited as merits.

Wärtsilä notes that the proportion of water added for the water–fuel emul-

sion is mainly limited by the maximum delivery capacity of the fuel injection

pumps. In practice, therefore, the engine has either to be derated or the maxi-

mum achievable NOx reduction limited to around 10 or 20 per cent. Obtaining

the maximum NOx reduction at full load also would dictate redesign not only

of the injection system but also of the camshaft and its drive. The proportion of

water added is also limited by the viscosity of the emulsion and the degree

of heating required to reduce the viscosity for injection: this is a property of

the water–fuel emulsion and cannot be addressed by engine or system design.

Another aspect is that the injection nozzle design (such as hole diameter) has

to be adapted to the increased quantity of liquid injected.

Wärtsilä explains that with such a modiﬁed nozzle design, both fuel con-

sumption and component temperatures may be penalized when the engine is

running without water. Although camshaft-controlled engines are restricted

in the timing of injection, electronically controlled designs (such as Wärtsilä

RT-ﬂex engines) enjoy a ﬂexibility that is beneﬁcial when burning water–fuel

emulsions. The electronic control system can be readily adapted to different

optimized injection characteristics for operating on emulsions or fuel alone.

Furthermore, engine settings can be optimized according to engine load and

NOx (g/kW h)

Slow-

speed

reference

Medium-

speed

levels

Applies to medium-speed engines

Low NOx

combustion

Direct water

injection

EGR Diesel

EGR

Gas Diesel

SCR + Low NOx

combustion

FigurE 3.6 Emission control measures and their potentials in reducing Nox output.

Egr  exhaust gas recirculation; sCr  selective catalytic reduction (Wärtsilä Diesel)

speed. The electronic control system would also be extended to provide load-

dependent mixing of water and fuel in the emulsiﬁer.

Water can also be introduced to the combustion space through separate

nozzles or by the stratiﬁed segregated injection of water and fuel from the same

nozzle. Unlike other techniques, direct water injection (DWI) enables the water

to be injected at the right time and place to obtain the greatest NOx reduction.

It thus represents one of the most efﬁcient means of reducing NOx emissions

through internal measures.

DWI is effective in reducing NOx by adding mass and stealing heat from

the combustion process when the water is evaporated. Wärtsilä developed DWI

for its medium-speed engines after initially investigating the merits of inject-

ing ammonia into the cylinder during the expansion stroke. It was found that

injecting water into the combustion chamber during the compression stroke

could achieve the same degree of NOx reduction as ammonia.

Wärtsilä medium-speed engines with DWI systems feature a combined

injection valve and nozzle for injecting water and fuel oil into the cylinder

(Figure 3.7). The nozzle has two needles that are controlled separately, such

that neither mode (water on/water off) will affect operation of the engine. The

engine can be transferred to non-water operational mode at any load, the trans-

fer in alarm situations being automatic and instantaneous. Water injection takes

place before fuel injection, resulting in a cooler combustion space and hence

lower NOx generation; water injection stops before fuel oil is injected into the

cylinder so that the ignition and combustion process is not disturbed.

Clean water is fed to the cylinder at a pressure of 210–400 bar (depend-

ing on the engine type) by a common rail system, the pressure generated in a

high-pressure pump module; a low-pressure pump is also necessary to ensure

a sufﬁciently stable water ﬂow to the high-pressure pump. The water is ﬁltered

Water-based Nox reduction techniques 71

Pressure

transformer

Flow fuse

Control

unit

Solenoid

valve

Water

Water needle Fuel needle

Fuel

Water

tank

FigurE 3.7 Wärtsilä’s DWi system features a combined injection valve and nozzle for

water and fuel oil

72 Exhaust Emissions and Control

before the low-pressure pump to remove all solid particles; the pumps and

ﬁlters are built into modules to ease installation.

A ﬂow fuse installed on the cylinder head side acts as a safety device, shut-

ting off the water ﬂow into the cylinder if the water needle gets stuck; immediate

water shut-off is initiated in the event of excessive water ﬂow or leakage. Water

injection timing and duration are electronically controlled by the control unit,

which receives its input from the engine output; timing and duration can be opti-

mized conveniently via a keyboard for different applications. Space requirements

for the equipment are minimal, facilitating retroﬁt applications.

Water and fuel are injected in a typical water-to-fuel ratio of 0.4–0.7:1,

reducing NOx emissions by 50–60 per cent without adversely affecting power

output or engine components. NOx emissions are typically 4–6 g/kW h when

the engine is running on marine diesel oil, and 5–7 g/kW h when heavy fuel

oil is being burned. NOx reduction is most efﬁcient from 40 per cent load and

higher of nominal engine output.

Numerous applications of the above-mentioned DWI system have ben-

eﬁted Wärtsilä 32 and 46 medium-speed engines powering diverse ship types.

Some W46 ‘EnviroEngines’ (see Particulates, Soot and Smoke, p. 81) combine

electronically controlled common rail fuel injection with DWI, the water then

injected into the combustion chamber separately from the side of the cylinder

head through a dedicated valve.

A DWI system developed and tested by Wärtsilä reportedly conﬁrmed the

potential of the technology for two-stroke engines, the water being handled

by a fully independent, common rail delivery system under electronic control.

The system offers the possibility of injecting very large amounts of water and

fuel with different timings. Water can be injected in parallel with the fuel and/or

during the compression stroke so that optimizing injection timing with respect

to fuel and water consumptions, NOx and other emissions is possible without

undermining engine reliability.

When DWI is applied to a Wärtsilä RT-ﬂex common rail two-stroke engine,

the fuel injection system can be provided with different engine settings so that

it can run with the water injection turned on or off without affecting the fuel

injection behaviour. There is no restriction on the quantity of water injected.

With around 70 per cent water/fuel ratio, DWI has shown the capability to

reduce NOx emissions down to around 8 g/kW h or some 50 per cent below the

IMO Tier I limit.

Wärtsilä notes that whichever system of injecting water is applied, con-

sideration must be given to the logistics of providing sufﬁcient freshwater

onboard. Substantial quantities will be required when 20–50 per cent water

addition is used, and there must be sufﬁcient tank capacity for the water and an

associated handling system.

Mitsubishi Heavy Industries developed a stratiﬁed fuel–water injection

(SFWI) system using a common valve to inject ‘slugs’ of fuel/water/fuel

sequentially into the combustion chamber. The system reportedly worked in a

stable condition throughout extensive trials on a ship powered by a Mitsubishi

UEC 52/105D two-stroke engine, NOx emission reduction being proportional

to the amount of water injected.

Charge Air humidication

Another way of introducing water into the combustion zone is by humidify-

ing the scavenge air: warm water is injected and evaporated in the air intake,

whose absolute humidity is thereby increased. An early drawback was that too

much water in the air can be harmful to the cylinder condition but the introduc-

tion of the anti-polishing ring in the liner (see Chapter 16) has allowed a much

higher humidity to be accepted.

In the combustion air saturation system (CASS), which resulted from co-

operation between Wärtsilä and the Finnish company Marioff Oy, special

Hi-Fog nozzles introduce water directly into the charge air stream after the tur-

bocharger in the form of very small droplets. Only a few micrometres in size,

the droplets evaporate very quickly in an environment of more than 200°C and

75 m/s air velocity. Further heat for evaporation is provided by the air cooler

(but acting now as a heater), resulting in combustion air with a humidity of

around 60 g/kg of air. Wärtsilä reports that with such an amount of water,

experiments demonstrated it was possible to secure NOx levels of 3 g/kW h

(assuming the starting value is 10–15 g/kW h).

Wärtsilä’s latest WetPac H system promises NOx emission reductions of

30–60 per cent from its four-stroke engines, applying 1.5:2 water/fuel ratios,

with no substantial increase in fuel consumption and no rise in material

temperatures.

MAN Diesel has also pursued NOx reduction by increasing the humidity of

the charge air with water vapour. Based on the humid air motor (HAM) system

developed by the German company Munters Euroform (Figure 3.8), the process

can reduce NOx formation by up to 65 per cent. Turbocharged combustion

air is saturated with water vapour produced onboard from raw sea water using

Water-based Nox reduction techniques 73

Turbine

Compressor

Heat exchange

Bleed-off

Catch tank

Water circuit

Water filling

Humidification

tower

Humidified and

cooled air

Hot compressed air

Engine

FigurE 3.8 schematic diagram of hAM system for Nox emission reduction

74 Exhaust Emissions and Control

engine heat sources. Compressed and heated air from the turbocharger is passed

through a cell that humidiﬁes and cools the air with evaporated water, the distil-

lation process making it possible to use sea water rather than freshwater.

Over 90 per cent of NOx formation results from combustion temperature

peaks. The HAM principle is used to humidify the inlet air in order to lower

these peaks. The ability of water to decrease NOx formation is exploited in the

same way as with fuel water emulsiﬁcation. However, the quantity of water

added is much higher, and the heat for vaporization is taken from the com-

pressed air after the turbocharger or from other engine-related heat sources.

When the water vapour is mixed with the compressed charge air, two

mechanisms can be identiﬁed: an increase in the speciﬁc heat capacity of the

mixture and dilution of the charge air (water vapour replaces air). The quantity

of water (in gram per kilogram dry air) which can be injected into the inlet air

depends on the temperature and the pressure of the mixture. When the air tem-

perature rises, so does the quantity of water it is possible to vaporize.

MAN Diesel says that HAM’s outstanding advantage is its use of the heat

of the engine to raise the sea-water temperature; no external energy source

is needed. In addition to the heat of the charge air after the turbocharger, in

many applications heat from the engine coolant and exhaust gases can be intro-

duced into the charge air to increase its capacity to absorb moisture. An NOx

reduction level of 40 per cent is said to be achievable without using additional

heating of the intake air, with a 65 per cent reduction when additional heat is

introduced from the engine coolant or exhaust gases.

Using untreated sea water fosters very low operating costs. Further beneﬁts

cited are very low maintenance costs, decreased lube oil consumption and high

availability, with only minimal impact on fuel oil consumption. The presence

of water vapour in the combustion chamber also yields cleaner combustion,

reducing deposits in the chamber, on the turbine side of the turbocharger and

the rest of the exhaust gas tract.

Test results from an SEMT-Pielstick 3PC2.6 medium-speed engine

equipped with a prototype HAM system showed an NOx reduction from

13.5 g/kW h to 3.5 g/kW h. There was no signiﬁcant inﬂuence on speciﬁc fuel

consumption, no signiﬁcant increase in carbon monoxide and HC emissions,

and no smoke deterioration. These results—along with no trace of water in the

lube oil, no corrosion and cleaner engine internals—were repeated in subse-

quent seagoing installations. In retroﬁt projects, the HAM system replaced the

original intercoolers.

Exhaust gas recirculation

Exhaust gas recirculation (EGR) is a method of modifying the inlet air to

reduce NOx emissions at source, an approach widely and successfully used in

automotive applications. Some of the exhaust gas is cooled and cleaned before

recirculation to the scavenge air side. Its effect on NOx formation is partly due

to a reduction of the oxygen concentration in the combustion zone, and partly

due to the content of water and carbon dioxide in the exhaust gas. The higher

molar heat capacities of water and carbon dioxide lower the peak combustion

temperature, which, in turn, curbs the formation of NOx.

EGR is a very efﬁcient method of reducing NOx emissions (by 50–60 per

cent) without affecting the power output of the engine but has been considered

more practical for engines burning cleaner bunkers such as low sulphur and

low ash fuels, alcohol and gas. Engines operating on high sulphur fuel might

invite corrosion of turbochargers, intercoolers and scavenging pipes.

An EGR system pursued by MAN Diesel for its two-stroke engines is

based on recirculating exhaust gas on the engine side of the turbocharger, part

of the exhaust being recirculated from the exhaust gas receiver to the scavenge

air system downstream of the turbocharger compressor (Figure 3.9). An elec-

trically driven high-pressure blower forces the exhaust gas (at 3.3 bar) through

a wet scrubber to the higher pressure scavenge air receiver (3.7 bar). The scrub-

ber cleans the exhaust gas by removing SOx and particulates, and also cools

it through humidiﬁcation before re-introduction to the combustion chamber.

The resulting NOx-reducing effect is due to part-replacement of the oxygen by

carbon dioxide, which lowers the maximum peak temperatures by decelerating

combustion.

NOx emissions from MAN Diesel’s 4T50ME-X two-stroke research engine

at 75 per cent load were reduced by up to 70 per cent (compared with the econ-

omy engine layout) with 30 per cent recirculation of exhaust gas. At maximum

continuous rating using 24 per cent recirculation, NOx was reduced by 60 per

cent with only a slightly negative impact on speciﬁc fuel consumption.

Cylinder conditions were studied before and after the comprehensive EGR

test programme, no negative effects being evident. Combustion chamber com-

ponent temperatures showed a decreasing trend with higher EGR rates due to a

higher speciﬁc mass ﬂow through the cylinder. The lower material temperature

Water-based Nox reduction techniques 75

Water

treatment

system

Scrubber

EGR

valve

EGR

blower

Sludge

out

Clean

brine out

Scavenge

air cooler

Turbocharger

FigurE 3.9 schematic diagram of Egr system tested by MAN Diesel on its 4T50ME-X

low-speed research engine

76 Exhaust Emissions and Control

is cited as a positive side effect of the EGR process. Controlling the wet scrub-

ber’s water content and keeping water-free droplets out of the scavenge air,

however, is vital for protecting cylinder liners and piston rings. Overall, MAN

Diesel is conﬁdent that EGR is a competitive NOx-reducing technique that will

be applied to large two-stroke engines in the future.

For low-speed engines, especially those with electronically controlled exhaust

valve timing, Wärtsilä suggests that the water-cooled residual gas (WaCoReG)

technique—a combination of DWI and EGR—offers an opportunity for NOx

reduction. In this concept some of the exhaust gas is left in the cylinder, which

in normal circumstances leads to increased thermal loading and inferior combus-

tion. These drawbacks are largely avoided, however, if the residual gas is cooled

by an internal spray of water. As the surrounding combustion space components

are rather hot, there will be no condensation of acid products on the metal sur-

faces. A reduction in NOx emissions of up to 70 per cent below the IMO Tier I

limit (around 5 g/kW h) is anticipated from the WaCoReG system.

Internal recirculation normally increases the thermal load of the engine, so

the water injection is applied to reduce temperature levels, thereby maintain-

ing thermal loads much the same as when running without internal EGR. With

WaCoReG, the water is injected earlier in the compression stroke than with

just DWI.

In contrast to four-stroke engines in which it is common practice to recircu-

late exhaust gases through external manifolds, in two-stroke low-speed engines

Wärtsilä prefers to adapt the engine scavenging process to decrease the purity

of gas in the cylinder at the start of compression. This is achieved by reduc-

ing the height of the scavenge ports to lower the scavenge air quantity ﬂowing

through the cylinder. One beneﬁt is that smaller turbochargers are required for

the reduced gas ﬂows; the lower scavenge ports also allow greater expansion in

the cylinder and thus improve fuel consumption. Reduced gas ﬂows have the

further beneﬁt that exhaust gas temperatures are raised, which is helpful for

heat recovery systems.

Fuel Nozzles

Different fuel nozzle types and designs have a signiﬁcant impact on NOx for-

mation, and the intensity of the fuel injection also has an inﬂuence.

The increased mean effective pressure ratings of modern engines require

increased ﬂow areas throughout the fuel valve, which, in turn, leads to increased

sac volumes in the fuel nozzle itself and a higher risk of after-dripping.

Consequently, more fuel from the sac volume may enter the combustion cham-

ber and contribute to the emission of smoke and unburnt HC as well as to

increased deposits in the combustion chamber. The relatively large sac volume

in a standard design fuel nozzle thus has a negative inﬂuence on the formation

of soot particles and HC.

The so-called ‘mini-sac’ fuel valve introduced by MAN Diesel incorporates

a conventional conical spindle seat as well as a slide inside the fuel nozzle. The

mini-sac leaves the ﬂow conditions in the vicinity of the nozzle holes similar

to the ﬂow conditions in the conventional fuel nozzle. But its much reduced

sac volume—only about 15 per cent that of the conventional fuel valve—has

demonstrated a positive inﬂuence on the cleanliness of the combustion cham-

ber and exhaust gas outlet ducts. Such valves also reduce the formation of NOx

during combustion.

A new type of fuel valve—essentially eliminating the sac volume—was

subsequently developed and introduced by MAN Diesel as standard to its

larger low-speed engines (Figure 3.10). The main advantages of this slide-type

fuel valve are reduced emissions of NOx, CO, smoke and unburned HC as well

as signiﬁcantly fewer deposits inside the engine. A positive effect on the cylin-

der condition in general is reported.

Applying slide fuel valves to a 12K90MC container-ship engine yielded a

40 per cent reduction in smoke (BSN10) compared with the mini-sac valved

engine, while HC and CO were reduced by 33 per cent and 42 per cent, respec-

tively, albeit from a low level. NOx was reduced by 14 per cent, while the fuel

consumption remained virtually unchanged with a slight reduction at part load.

sElECTivE CATAlyTiC rEDuCTioN

Primary methods are generally adequate for the IMO NOx emission limits, but

tougher international and regional controls may dictate the use of secondary

methods—exhaust gas treatment techniques—either alone or in combination

with engine modiﬁcations. Here the focus is on SCR systems, developed from

land-based power station installations for shipboard applications, which can

selective catalytic reduction 77

Conventional fuel valve

Sac volume 1690 mm

Mini-sac valve

Sac volume 520 mm

Slide-type fuel valve

Sac volume 0 mm

FigurE 3.10 Evolution of fuel injection valve design for MAN b&W Diesel MC two-

stroke engines; smoke and Nox emissions were lowered by reducing or eliminating

the sac volume