WÄRTSILÄ TECHNICAL JOURNAL 02.2007 

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SOx scrubbing of marine exhaust gases

AUTHOR: Torbjörn Henriksson, Propulsion and Applications Expert, Technical Service, Wärtsilä in Finland

When Ship type Area % Act

19.5.2006 All Baltic SECA 1.5 Marpol

11.8.2006 All Baltic SECA 1.5 EU

11.8.2006 Passenger ships All EU 1.5 EU

11.8.2007 All North Sea + 

 English Channel SECA 1.5 EU

22.11.2007 All North Sea + 

 English Channel SECA 1.5 Marpol

1.1.2010 All All EU ports 0.1 EU

1.1.2010 Inland waterway 

 vessels All EU inland waterways 0.1 EU

1.1.2012 16 Greek ferries Greek ports 0.1 EU

Table 1. – A summary of existing SOX emission regulations.

For the commercial implementation of 

marine scrubbers to become viable, there 

are a number of challenges to be faced. 

These include the return on investment 

with fuel prices as they are at present, 

the need to clarify rules relating to the 

discharge of wash water, the certifi cation 

process, and the absence of reference 

installations. Wärtsilä has combined 

it’s long experience of developing and 

delivering SOX scrubbers for stationary 

diesel power plants with it’s specifi c 

expertise in marine system design, and 

has now produced in-depth feasibility 

studies for several selected ship types.

Existing SOX

 emission regulations 

Currently, the most important 

legislation, particularly for ship 

operation in Europe, is covered by the 

Marpol Annex VI Act and the new 

EU directive. Table 1 shows a rough 

summary of these regulations. 

New legislation governing SOX

emissions of ships affects ship design 

and operation. If the vessel operates 

in areas where SOX emissions are 

controlled, compliance can be 

achieved by using low sulphur fuel, 

or by cleaning exhaust gases using SOX

scrubbers, or by a combination of both. 

Additionally, the California Air 

Resources Board (CARB), for example, 

has adopted regulations for auxiliary 

diesel engines and diesel-electric engines 

in ocean-going vessels within regulated 

California waters (24 nautical miles 

from California). The limits are 0.5% 

of sulphur in the fuel from 1.1.2007, 

and 0.1% of sulphur from 1.1.2010. 

Exhaust gas abatement is an alternative.

Locally, ports and local authorities for 

example in Sweden, offer reductions in 

port and fairway fees depending on the 

sulphur content in the fuel being used.

Future regulations under discussion

The clear goal of the IMO and EU is to 

further curb the emission of sulphur oxides. 

The work on revising MARPOL Annex 

VI is actively being pursued at the IMO, 

and an expert group has been nominated. 

Decisions are expected during 2008.

Six alternatives are presently being 

discussed. Four of them include more 

stringent requirements for certain 

regions, while the other two alternatives 

focus on unifi ed SOX-limits that would 

be applicable globally on all seas. 

One option is to use distillate quality 

everywhere. However, while this would 

certainly simplify many things, it has 

met with considerable objections based 

upon its drastic impact on shipping 

and the oil industry, and the increased 

CO2

-emissions at refi neries.

The fi ve remaining alternatives are 

more “goal-setting”, specifying the 

required emission performance, but 

leaving the solutions to the industry. In 

these scenarios, exhaust gas cleaning is 

also considered as being an economically 

feasible alternative to using much more 

expensive fuel. The low-sulphur fuel 

price premium is expected to grow due to 

reduced sulphur limits and to the fact that 

regions with more stringent requirements 

are being geographically extended. 

SOX SCUBBER REGULATIONS

The IMO and the EU are being urged 

to produce a consolidated approach to 

reasonable wash water discharge criteria 

and a unifi ed geographical defi nition.

The development of legislation is 

presently considered to be the most 

important means of ensuring the 

successful introduction of exhaust 

gas cleaning on ships. Here again 

decisions are expected during 2008.

IMO scrubber certifi cation

There are already some regulations 

stipulating performance, verifi cation, and 

certifi cation issues for SOX-scrubbers. 

IMO Resolution MEPC.130(53) 

requires a SECA Compliance Plan 

(SCP) describing methodology for 

compliance by each ship using scrubbers 

rather than low-sulphur fuel. 

Similarly to the NOX regulation 

requirements for engines, the scrubber 

(Exhaust Gas Cleaning System-SOX,

or EGCS) shall be delivered with a 

technical document, the EGCS-SOX

Technical Manual (ETM), outlining the 

nominal capacity, type of combustion 

units, operating limits (gas temperature, 

sea water alkalinity etc.), maintenance, 

and survey procedures to ensure proper 

operation etc. Upon successful approval 

of the ETM and demonstration of 

performance, a SECA Compliance 
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100%

50%

0% pH 14

SO2 reduction

Fig 1. – Scrubber SOX-reduction versus pH of the scrubbing water. 

Fig 2. – Alkalinity of the Baltic Sea. 

Longitude (°)

Latitude (°)

Alkalinity

Open sea alkalinity

Surface data (0...15m)

Data from 2001-2005

Certifi cate (SCC) will be issued. 

During operation, scrubber maintenance 

should be recorded in a dedicated “EGCSSOX record book”, or alternatively 

in a Planned Maintenance System. 

Maintenance and survey procedures of 

monitoring systems are documented in an 

“Onboard Monitoring Manual (OMM)”. 

IMO wash water requirements

Marpol Annex VI stipulates that “waste 

streams shall have no adverse impact 

on ecosystems based on criteria of port 

State”. While this may sound obvious, 

countries have been slow in defi ning their 

criteria. This is soon expected to change, 

as IMO Wash Water Guidelines will be 

available (approval expected in 2008).

EU regulations

EU Marine Fuels Sulphur Directive 

(2005/33/EC) includes provisions 

for the testing and installation of 

emission abatement technologies. 

The European Commission is 

responsible for establishing criteria for 

the use of scrubbers in enclosed areas. 

Similarly, ships utilizing scrubbers are 

required to be fi tted with continuous 

emission monitoring equipment. It is 

hoped that the EU will harmonize this 

requirement with the provisions of IMO 

Resolution MEPC.130(53), which offers 

both unit approval (scheme A) and 

continuous monitoring (scheme B).

National regulations

For authorities it is diffi cult to get a 

full picture of the composition and 

quantity of scrubber wash water, and 

what is “effl uent” and what is “residue”. 

This is fully understandable, as little 

information is publicly available, and 

several scrubber technologies exist with 

considerable variation in this respect. 

Some countries adjacent to SECA 

areas have then defi ned certain coastal 

zones, within which the discharge of 

scrubber wash water is currently not 

permitted (in contrast to the discharge 

from inert gas scrubbers on tankers).

ALKALINITY, PH AND SALINITY

The driving factor for sulphur acid 

neutralization, and therefore SO2-

reduction, is the water alkalinity. 

Such alkalinity is to some extent 

available in sea water, but it can 

also be added artifi cially in the 
WÄRTSILÄ TECHNICAL JOURNAL 02.2007 

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Fig 3. – Sulphur removal from exhaust gas by fresh water scrubbing, 

Wärtsilä scrubber system layout.

Closed loop works 

with freshwater, to 

which NaOH is added 

for the neutralization 

of SOX.

Exhaust 

gas

Fresh water

Water treatment

Cooling

Process tank

Holding tank

Closed loop

=

Zero discharge

in enclosed area

Scrubber

pH pH

Source: Finnish Institute of Marine Research

NaOH unit

Seawater

form of an alkaline chemical. 

Alkalinity does not refer simply to 

pH, but to the ability of water to resist 

changes in pH. Buffering materials 

are primarily bicarbonate, carbonate, 

but also consist of hydroxide, borates, 

silicates, phosphates, ammonium, 

sulphides, and organic compounds. Total 

alkalinity, AT, is the sum of all these.

Salinity describes the salt content of 

water. The salinity of ocean water is 

approximately 3.5%-weight. Water can 

have high alkalinity and zero salinity 

depending on the calcium concentration.

Alkalinity in nature

Ocean alkalinity is usually constant and 

high; approximately 2200-2300 μmol/l.

Alkalinity in coastal areas, ports, rivers 

and estuaries is mainly affected by the 

different drainage areas of the infl owing 

rivers, resulting in large variations in 

the chemical quality. Rivers running 

through soil rich in carbonates will be 

high in alkalinity. For example, the 

northern rivers of the Baltic Sea run 

through granite bedrock resulting in 

low alkalinity, while the southern rivers 

run through calcite bedrock resulting 

in high carbonate concentrations with 

consequently high alkalinity (see Figure 2). 

In general, the alkalinity in the Baltic 

Sea is lower than normally in sea areas 

because of the minimal exchange of 

water through the Danish straits.

At low alkalinity levels the seawater 

scrubber can still operate, but it 

leads to lower cleaning effi ciency 

and to low effl uent pH fi gures. 

SOX

 scrubbing technologies 

for marine applications

Wärtsilä has an impressive reference list 

of delivered SOX-scrubbers for stationary 

plants, and has consequently accumulated 

vast experience of scrubbing technology. 

Based on this background, Wärtsilä has 

concluded that there are two different 

technologies worth further development 

for marine applications, namely sea 

water (“open loop”) scrubbing, and fresh 

water (“closed loop”) scrubbing with an 

added chemical (typically caustic soda).

As ships have sea water available 

in unlimited quantities, sea water 

scrubbing may appear to be the obvious 

choice. There are, however, some 

limitations involved in this concept, 

which will be discussed below.

SEA WATER SCRUBBING

The main benefi t of a seawater scrubber 

is simplicity; it requires neither 

additional chemicals nor fresh water for 

operation. Instead seawater alkalinity, or 

buffering capacity, is used to neutralize 

the exhaust gas sulphur. In order to 

maintain high effi ciency, a seawater 

scrubber needs a high fl ow of seawater 

with an adequate level of alkalinity. 

FRESH WATER SCRUBBING

The fresh water scrubber is a good 

alternative if high effi ciency cleaning is 

needed, or as a means of avoiding seawater 

alkalinity issues. In such scrubbers, a 

caustic soda (NaOH) solution is used 

to neutralize sulphur compound. 

Fresh water scrubber cleaning effi ciency 

is typically higher than 90%. A fi gure as 

high as 97% can be specifi ed for generator 

engines to reach an equivalent of 0.1% 

fuel sulphur, as will be required in, for 

example, EU ports and California (see 

Table 1). Thus, engines will always be 

able to run on conventional HFO. 

The power demand for pumps 

is very low, between 0.5 and 1% 

of the engine in question.

System layout

The fresh water scrubber system 

layout is shown in Figure 3. The 

main principle is as follows: 

The washing solution is pumped 

from the process tank through a 

system cooler to the scrubber. From 

the scrubber the washing solution 

returns to the process tank by gravity.

NaOH is fed to the system via a 

small feed pump. Topping-up of fresh 

water is needed to the extent that the 

evaporated or discharged water is not 

fully compensated by the humidity in the 

exhaust gases (from engine combustion).

A small portion (the “bleed-off”) of 

the scrubbing water fl ow is directed 

to the treatment unit. The treated 

effl uent is either discharged overboard 

or collected in a clean bilge water 

tank or other suitable holding tank. 

This feature is unique, as the system 

can periodically be operated in a 

“Zero Discharge Mode” i. e. with no 

discharge of wash water overboard. 

The captured contaminants 

(sludge) are transferred to the 

existing vessel’s sludge tank.

The process tank can be large enough 

to temporarily hold some bleed-off for 

periods when the scrubber is running but 

the treatment plant is not, or vice versa. 

Caustic soda

The typical commercial solution is a 

liquid with a concentration of 50%. 

It has a density of 1.52 t/m3

 and a 

pH of 14. It solidifi es at 12°C, and 

is typically transported warm. 

The caustic soda can be bunkered 
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Fig 4. – Wärtsilä scrubbers in a RoPax (longitudinal section). 

40 50 60 40 50 60

Original Converted

Scrubber 1

Scrubber 2

Process tank

Scrubbing

water

cooler

Fig 5. – Wärtsilä scrubbers in a RoPax (transverse section). 

Original Scrubber

from trucks via fi lling connections in 

the bunker stations. The storage tank 

can be of normal shipbuilding steel. 

Based on price fl uctuations of caustic 

soda during the last 20 years, the cost is 

between 0.5 and 4% of the fuel costs. 

Exhaust gas

The exhaust gas plume in traditional wet 

scrubbers has a high relative humidity. 

Wärtsilä’s marine scrubber includes a 

feature to minimize the water vapour 

of the plume, as well as the water lost 

to the atmosphere and, therefore, 

the need for topping-up water.

As the scrubber provides noise 

attenuation, it can replace the existing 

silencer. Attenuation of high frequencies 

is inherent in the scrubber design, while 

lower frequencies are taken care of by a 

suitable geometry considering the emission 

spectrum of the engine in question. 

As a separate silencer is not 

needed, the overall back pressure 

of the exhaust gas system can be 

maintained within acceptable limits. 

Retrofi ts and newbuilds 

Scrubbers can be fi tted on newbuildings 

as well as on existing ships. 

On newbuildings there is full freedom 

to locate components. It is also possible 

to specify a “readiness for later retrofi t 

of scrubbers”. Wärtsilä can provide 

information relating to matters such as 

space allocation etc. (see Figures 4 and 5). 

On existing ships the funnel 

may not be large enough. In such 

cases the funnel geometry can be 

modifi ed, but it is also possible to fi t 

the scrubber itself outside the funnel, 

with the other equipment inside. 

As an example, a scrubber for an 

8400 kW engine is 8.0 meters high, 

2.9 meters in diameter, and the 

weight in operation is 13.4 tons. 

CONCLUSION

With more stringent regulations looming 

in the future, SOX-scrubbing is being 

seen as an increasingly attractive way 

of minimizing operational costs by 

using HFO in an environmentally 

sound way. The interest of shipping 

companies is steadily increasing, and 

authorities are working actively to 

develop corresponding regulations. 

NOTE: For more information, please contact 

us at scrubber@wartsila.com