WÄRTSILÄ TECHNICAL JOURNAL 02.2007



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# SO

x 

# 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 SO X 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 SO X 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 SO X 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 SO X emissions of ships affects ship design and operation. If the vessel operates in areas where SO X emissions are controlled, compliance can be achieved by using low sulphur fuel, or by cleaning exhaust gases using SO X 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 SO X -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 CO 2 -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.

SO X 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 SO X -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 NO X regulation requirements for engines, the scrubber (Exhaust Gas Cleaning System-SO X , or EGCS) shall be delivered with a technical document, the EGCS-SO X 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 Compliance56 

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100%

50%

0% pH 14 SO 2 reduction

Fig 1. – Scrubber SO X -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 maintenanceshould be recorded in a dedicated “EGCSSO X 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 SO
2 - reduction, is the water alkalinity.

Such alkalinity is to some extent available in sea water, but it can also be added artifi cially in theWÄRTSILÄ 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 SO X . 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.

SO X scrubbing technologies for marine applications Wärtsilä has an impressive reference list of delivered SO X -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/m 3 and a pH of 14. It solidifi es at 12°C, and is typically transported warm.

The caustic soda can be bunkered58 

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Fig 4. – Wärtsilä scrubbers in a RoPax (longitudinal section). 60 50 40 60 50 40 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, SO X -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