An open-loop scrubber system uses seawater to remove SOx from the engine exhaust gas. The system mainly uses the natural alkalinity of seawater, so no additional chemicals are normally required.

Operation

• Exhaust gas enters the bottom of the scrubber tower.
• Seawater is pumped into the top of the scrubber and sprayed through several stages of spray nozzles.
• The nozzles produce an evenly distributed spray pattern throughout the scrubber.
• As the exhaust gas passes through the water spray, the sulphur oxides (SOx) are absorbed by the water droplets under the required temperature and process conditions.
• SOx reacts with water and forms sulphuric acid.
• The natural alkalinity of seawater neutralises the acid, thereby removing SOx from the exhaust gas.
• The cleaned exhaust gas leaves through the top of the scrubber tower.
• The used seawater/wash water flows to the bottom of the scrubber and is discharged overboard.
• The wash water is treated and continuously monitored at the inlet and outlet to ensure that it meets the required discharge criteria.
• pH, turbidity and PAH (Polycyclic Aromatic Hydrocarbons) are continuously monitored in accordance with IMO/MARPOL Annex VI requirements and MEPC.184(59).
• When operated in compliance with the applicable discharge requirements, the treated wash water can be discharged to sea.

The open-loop scrubber reduces the SOx emissions from the exhaust gas to an equivalent level corresponding to 0.1% sulphur content in fuel. It is particularly suitable for vessels operating mainly in open waters, where compliant overboard discharge is permitted.

Example: LINEA MESSINA had an open-loop scrubber system installed and was the first vessel reported to operate commercially with a scrubber system. This enabled the vessel to meet the 0.1% sulphur emission requirement in EU ports and provided preparation for the 0.1% Emission Control Area (ECA) limit introduced in 2015.

Simple Sketch

Detailed Sketch:

A closed-loop scrubber system circulates the same scrubbing water continuously within the system. Fresh water and sodium hydroxide (NaOH/caustic soda) are added to maintain the required pH and SOx removal efficiency.

It is mainly used when a vessel is operating in ports, coastal areas or other areas where overboard discharge of scrubber wash water is prohibited or must be avoided.

Operation

• Exhaust gas enters through the bottom of the scrubber tower.
• Scrubbing water is pumped from the wet sump, through a cooler, to the top of the scrubber.
• The water is sprayed into the exhaust gas through spray nozzles.
• Water is also supplied to the middle section of the scrubber to further improve SOx removal efficiency.
• The scrubbing water passes through the packing bed and is collected at the bottom.
• The water absorbs SOx, heat and other components from the exhaust gas.
• The pH of the circulating scrubbing water is automatically monitored and controlled by alkali dosing.
• Caustic soda (NaOH) is automatically added to neutralise the acidity and maintain the required process pH and SOx removal efficiency.
• The cleaned process water is pumped back to the top of the scrubber and recirculated.
• Only a small quantity of bleed-off is continuously extracted from the circulating water to remove accumulated impurities.
• The bleed-off contains traces of oil and combustion products, and its pH is typically close to neutral.
• The bleed-off is sent to an emulsion-breaking water treatment unit/separator.
• Solids and oil are separated from the polluted water and form sludge.
• The sludge is pumped to the vessel's sludge storage tank.
• Clean effluent from the treatment unit may be discharged overboard when permitted, or transferred to an effluent holding tank when overboard discharge is to be avoided.
• The effluent quality is monitored before discharge.
• A bleed-off buffer tank may be provided before the treatment unit. This gives operational flexibility and allows the scrubber to continue operating even when the treatment unit is temporarily out of operation.

Wärtsilä Closed-Loop System

In the Wärtsilä closed-loop system, the wash water is continuously circulated within the scrubber. Only a small bleed-off is removed from the loop, while fresh water and alkali are added.

The SOx reduction efficiency can be approximately 97.15%, corresponding to reducing the fuel sulphur content from 3.5% to 0.1%.

Fresh-water consumption is case-dependent, but an estimated value of approximately 0.2 m³/MWh may be used. Fresh water is required to compensate for:

• Scrubbing-water evaporation losses.
• Extracted bleed-off.
• Periodic rinsing of the droplet separator at the top of the scrubber.

The fresh-water supply can be connected to the scrubbing-water wet sump or pump module.

The main components of the alkali feed system are:

• Alkali pump.
• Alkali feed control system.
• Alkali storage tank.

Caustic soda consumption by weight is approximately 6–15% of the diesel engine fuel-oil consumption, depending on the fuel sulphur content and the required cleaning efficiency.

Important Points

• Open-loop: Uses seawater, relies on its natural alkalinity, and the treated wash water is normally discharged overboard when permitted. It is mainly suited to open-water operation.
• Closed-loop: Uses fresh water + NaOH/caustic soda, continuously recirculates the scrubbing water, and removes a small bleed-off for treatment. It is used where overboard discharge is prohibited or needs to be avoided.
• Both systems remove SOx from the engine exhaust so that the resulting emissions are equivalent to operation on fuel with approximately 0.1% sulphur content.