Ship scrubbers questioned

With open-loop scrubbers hazardous substances such as heavy metals and polycyclic aromatic hydrocarbons end up in the water instead of in the air. Photo: Todd Davis/ CC BY-SA

The ecological risks to the marine environment of using sulphur scrubbers are ignored, while the economic benefits have been overestimated, says German environmental organisation NABU.

As from 1 January 2015, fuel used by vessels operating within Emission Control Areas (ECA) is limited to a maximum sulphur content of 0.10 per cent, down from the previous limit of 1.0 per cent. In practice, this means moving from high-sulphur heavy fuel oil (HFO) to low-sulphur marine gas oil (MGO), or to alternative fuels such as liquefied natural gas (LNG) or methanol. However HFO is still permitted to be used if ships are equipped with exhaust gas cleaning systems, such as scrubbers, that achieve equivalent sulphur emission reductions.

So far, about 80 ships out of a world fleet of 55,000 ships have had scrubbers installed, with some 300 additional scrubber systems on order, according to a new study by the Dutch research institute CE Delft on behalf of German environmental organisation Nature and Biodiversity Conservation Union (NABU).

Currently the scrubber market is dominated by wet open-loop scrubbers, which, unlike closed-loop and dry scrubbers, will discharge wash-water into the sea. The different types of scrubbers are described in the Box.

Although the IMO wash-water criteria are generally met, scrubbers may negatively impact on the marine environment through ocean acidification, eutrophication and accumulation of hazardous substances such as heavy metals and polycyclic aromatic hydrocarbons (PAH).

The long-term impacts of the use of open-loop scrubbers, especially in vulnerable coastal areas with a reported moderate water quality, therefore need to be investigated systematically, the study says. Moreover, it should be evaluated if scrubbers can be used in accordance with the EU’s Water Framework Directive and Marine Strategy Framework Directive, which set maximum concentrations for certain hazardous pollutants, prohibit deterioration of water quality, and aim to achieve ‘good environmental status’ respectively.

National governments and ports can set limits for hazardous substances or prohibit the discharge, and the study notes that Germany has prohibited scrubber wash-water discharges in inland waters, rivers and certain ports, including the Kiel Canal, and that Belgium has prohibited discharging within three nautical miles of its coast.

“Obviously nobody ever systematically investigated the impact of scrubbers on the marine environment. It is clear for everyone that simply discharging harmful substances into the ocean instead of to the air will not result in an improvement for the environment,” said NABU Chief Executive Officer Leif Miller.

In terms of greenhouse gas emissions, it is estimated that the use of scrubbers increases energy consumption by about 1.5–3.5 per cent – seawater scrubbers increase ship fuel consumption more than freshwater scrubbers. Production of MGO for use in ECAs will increase refinery emissions of carbon dioxide, but since refineries in the EU are included under the cap of the EU’s emission trading system (ETS), any such increases would have to be offset by reductions elsewhere in the system.

Regarding the business case for scrubbers, the study says that it is difficult to draw firm conclusions on the profitability of using scrubbers, as this depends on the operational profile of the ship, the price difference between HFO and MGO, and the length of time that ships sail in ECAs. The study noted that with the current low price difference, it is not easy to make a positive business case for scrubbers.

“Scrubbers must also be rejected as they allow ship owners to continue to sail on heavy fuel oil instead of investing in cleaner fuels and eco-friendly drives. Ship owners who opt for scrubbers invest a lot of money in the wrong technology. A switch to low-sulphur fuels like LNG or MGO in combination with particle filters and selective catalytic reduction (SCR) is literally the cleaner solution,” said NABU transport policy officer Daniel Rieger.

Christer Ågren

Source: NABU press release 13 March 2015
The study: Scrubbers – An economic and ecological assessment (March 2015). By CE Delft, the Netherlands. 

Four main types of scrubbers

1. Seawater scrubbers (open-loop) use the natural alkalinity of seawater to neutralise the sulphur from exhaust gases. While these scrubbers have greater energy consumption compared to closed-loop systems, there is no need for chemical additives such as caustic soda.

2. Freshwater scrubbers (closed-loop) use caustic soda added to freshwater in a closed system to neutralise the sulphur from exhaust gases. The circulating water is processed after the scrubber and dosed with caustic soda in order to restore the alkalinity of the wash water.

3. Hybrid scrubbers can be used either as closed-loop or open-loop systems. They are generally used as open-loop systems when operating in the open sea and as closed-loop systems when operating in harbours or estuaries, where wastewater discharge is prohibited.

4. Dry scrubbers do not use any liquids in the process, so there is no discharge to the sea. Instead the exhaust gases are cleaned with hydrated lime-treated granulates, producing gypsum that is used to manufacture wallboard. Dry scrubbers use less energy than wet scrubbers.


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