Uncertainty around the sustainability of open-loop scrubbers continues to escalate in the shipping industry, ahead of the implementation of the International Maritime Organisation’s (IMO) sulphur cap in 2020.
GlobalData’s ship technology writer Varsha Saraogi investigates the controversial system.
Saraogi says: “In October 2008, the IMO adopted the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI that regulates the prevention of air pollution from ships and prohibits deliberate emissions of ozone-depleting substances such as sulphur oxides and nitrous oxides.
“Under the regulation, the amount of sulphur in fuel oil used on board ships – which was 3.5% m/m (mass/mass) will be limited to 0.50% m/m from 1 January 2020.
“From switching to low-sulphur fuels to installing an exhaust gas cleaning system (EGCS) – also known as a scrubber – on board, the entire shipping community is looking at all technologies and procedures to achieve full compliance.
“Exhaust scrubbers have become one of the most practical ways of reducing sulphur exhaust for carriers and container lines, and are used to literally ‘scrub’ pollutants out of emissions. Scrubbers can be used in many ways. While closed-loop scrubbers retain the sulphur emissions for safer disposal at port, open-loop scrubbers release pollutants back in the sea after turning the sulphur dioxide into sulphuric acid.
“There are also hybrid scrubbers, which switch between open and closed-loop depending on situations such as local rules which may or may not prohibit the discharge of water.
“So far, open-loop systems have witnessed more uptake in the industry compared to closed units. According to DNV GL, there are currently 3,756 vessels with scrubbers installed – a huge increase from the 767 in 2018 – and only 65 have closed-loop.
“The IMO outlined that it approves the use of open-loop scrubbers, deeming them as an ‘equivalent’ – which is defined as ‘any fitting, appliance or apparatus to be fitted in a ship or other procedures, alternative fuel oils, or compliance methods used as an alternative’. However, it has strict guidelines for discharge of washwater from exhaust gas cleaning systems.”
A spokesperson from the IMO told GlobalData: “The washwater must meet strict criteria, so that discharge washwater should have a pH of no less than 6.5. A Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) task team has been established to assess the available evidence relating to the environmental impact of discharges of exhaust gas cleaning system effluent – the next session of the sub-committee will be held in February 2020.”