Onboard with marine scrubbers - resilient solutions for lower emissions

Ever-tightening sulphur oxide (SOx) emission regulations are increasing the use of marine scrubbers globally. Scrubbers operate in a highly corrosive environment and require the resilience of nickel-containing alloys to prevent failure.

Because of known environmental and human health impacts associated with sulphur and diesel particulates, Sulphur Emission Control Areas (SECAs) already exist in the Baltic Sea, North Sea, a North American area (covering designated coastal areas off the United States and Canada) and the United States Caribbean Sea area (around Puerto Rico and the United States Virgin Islands). The fuel sulphur cap limit in SECAs is 0.10% since January 1, 2015.

Mandated by the International Maritime Organization (IMO), new limits for SOx emissions outside of SECAs came into effect on January 1, 2020. The sulphur cap limit in bunker (dense, high sulphur) fuels is reducing from 3.50% to 0.50%. In lieu of using the 0.50% low sulphur fuel, ships can install an exhaust gas cleaning system (scrubber) to limit SOx emissions.

Why high sulphur?

Some crude oil is naturally low in sulphur ‘sweet’ but much is high in sulphur ‘sour’ and needs to be treated differently. High sulphur fuel oil ‘bunker’ is plentiful and relatively cheap. Marine engines tolerate it well and use approximately four million barrels (550,000 tonnes) per day. Its availability and low cost are factored into the current cost structure of marine freight tariffs. The new IMO regulations are challenging that status quo.

Available options to meet the IMO2020 regulations include switching to significantly more expensive low sulphur fuel or retrofitting with an exhaust gas scrubber system.

For the largest vessels in particular, the lower cost and quicker payback of installing a marine scrubber have container ships lined up to get the job done and avoid penalties and fines imposed by IMO nation states. Currently, the high demand for vessels requiring retrofit is stretching the capacity of material suppliers, equipment suppliers and facilities capable of doing the work.


Image courtesy of Meyer Werft
Image courtesy of Meyer Werft

Open and closed

In response to these regulatory requirements, two types of wet scrubber technology have been developed: open loop and closed loop. In many cases, they are combined into a hybrid system that can employ the most appropriate technology, depending on the alkalinity in the marine environment or designated zero discharge areas in which the vessel will operate.

The inside of a scrubber is an extraordinarily harsh environment. The hot acidic chloride solutions require the use of highly corrosion-resistant nickel alloys, such as Alloy 31 (N08031), Alloy C-276 (N10276) and Alloy 59 (N06059).

The inside of a scrubber is an extraordinarily harsh environment. The hot acidic chloride solutions require the use of highly corrosion-resistant nickel alloys

In wet marine scrubbers, the exhaust gas passes through a water stream, sulphur oxides are removed by reacting with the wash water to form sulphuric acid and the scrubbed gas leaves through the funnel. The sulphuric acid that is produced by the reaction with the wash water is neutralised by the alkalinity of the wash water.

The wash water can usually be discharged into the open sea after being treated in a separator to remove any sludge.

Open loop utilises the natural alkalinity of seawater for neutralisation, while closed loop adds an alkali solution (typically sodium hydroxide) to perform neutralisation.

Closed loop scrubber systems are necessary for marine areas with low natural alkalinity. Once cleaned, effluent can be safely discharged into the water. When operating in a zero discharge region, the effluent must be collected in a holding tank for land-based disposal.

Marine scrubbers of one type or another are part of engine management and critical to the safe operation of the vessel. If they don’t work, the shipowner can cause harm to the environment and human health, as well as risking significant legal consequences and damage to their reputation.

The new regulations will significantly improve air quality in many populated coastal and port areas, preventing pollution-related early deaths and asthma, as well as acid rain in these regions. With the help of nickel-containing alloys, the marine industry will be ‘scrubbing up’.

An open loop system uses seawater as the scrubbing solution. The sulphuric acid formed is neutralised by reacting with carbonates and other salts in the seawater to form sulphates. This scrubbed solution is treated to remove solids and raise the pH before being discharged back to sea, and the removed solids are stored onboard for shoreside disposal. Open loop scrubbers work satisfactorily with the natural alkalinity of seawater, while fresh and brackish water is not effective for this system because of their lack of natural alkalinity. For this reason, an open loop scrubber is not considered suitable for areas such as the Baltic Sea, estuaries and areas close to land, where salinity levels are lower. MARPOL regulations require the wash water to be monitored before discharge to ensure that the pH value is not less than 6.5.

A closed loop system uses an alkali solution (typically sodium hydroxide) as its scrubbing solution, which is required for water with low alkalinity (fresh or brackish water) and areas where no discharge is allowed. This process is less corrosive due to lower chloride levels. However, chlorides will increase with time in the scrubbing liquor until it is changed out. Every system, requires varying amounts of nickel-containing materials depending on the operating environment.

The Nickel Institute's Technical Inquiry Service is available to help with questions on nickel-containing materials in scrubbers and other applications. 

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