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A crude way to clean something is to wash it. This principle remains almost the same in all walks of life. If we talk about in terms of rocket science language can we wash exhaust gas? Sounds funny but we can, as after the implementation of global sulfur cap 2020 many ways were suggested and implemented. one such was using scrubber tower for cleaning purposes. This system allows ships to still use HFO as fuel and still achieving sulfur emission regulations. 

Principle: Using an alkaline medium to wash the acidic mixture and thus forming salt and water.

Scrubbing systems are defined as per their nature of alkaline medium Dry or Wet. As per the mode of operation Open-loop, Close loop, Hybrid Scrubber. The hybrid scrubbers are mostly used by the maritime industry due to their volatility and feasibility of the operation.

Dry Scrubbers

Dry scrubbers do not include the use of water in filtering the exhaust gases coming out of a vessel instead they use pellets of hydrated lime to produce a material (Gypsum) that can be used in various industrial applications. This technology has the advantage that disposal water is not needed and also an efficient filtration of exhaust gases can be achieved. This technology needs a separate space for gypsum to be stored onboard also the particular port authority does not need to allow a vessel to dump out gypsum to port. Dry scrubbing can be used by inland sailing vessels in freshwater. Due to the flash of stricter emission norms in the future, the extent of dry scrubbing is closer to extinction. This system can be used by industries to control their sulfur emissions. 

Wet Scrubbers

This system requires an alkali medium in the form of liquid only. This may be by seawater directly or processed water on board. 

Open Loop Scrubber System

Image about open loop scrubber

Figure-1, Source: Global Sulfur Cap 2020(Scrubber Tower Focused) by DNV-GL 2019 Update(Page 11)

Here seawater acts as a cleaning solvent due to its alkalinity. Exhaust gases from engines and boilers are firstly cooled in a venturi using seawater and then passed through a fine mist of seawater in two stages where water droplets neutralize the exhaust gas and leftover water can be treated along with wastewater and finally discharged into the sea. The major problem faced by this system is because of alkalinity of seawater is variable throughout its stretch. This will lead to violation of emission norms and inviting heavy penalties from authorities. Due to the discharge of treated water, some organizations based on their research have red-flagged discharge as it affects microorganisms in that area. Turkey became the first country to ban the open-loop scrubbing system entirely in its waters.

Closed Loop Scrubber System

Image for Closed Loop Scrubber

Figure-2, Source: Global Sulfur Cap 2020(Scrubber Tower Focused) by DNV-GL 2019 Update(Page 11)

In a closed-loop system, process water is circulated in a closed-loop i.e. it is cooled in a heat exchanger to reduce the evaporation of water. The ship’s technical water is added to the process water to compensate for evaporation and water loss, while alkali is added to maintain the alkalinity. Some process water is drained and treated using chemicals. Treated process water is transferred to a holding tank and discharged when permitted, and sludge is transferred to a sludge tank. The closed-loop system requires less water than an open loop. To meet the alkalinity of process water Sodium hydroxide in a 50% aqueous state is required in a separate tank. This system has less maintenance cost but it requires additional tanks like process tanks, holding tanks, heat exchangers which occupy more space onboard. The closed-loop system can be installed successfully on a newly built vessel as its requirement of increased floor area, piping, storage tanks are difficult to arrange in an older ship. Due to its complexity crew needs to be increased thus increasing the total hotel load.

Hybrid Scrubbers

Image for Hybrid Scrubber

Source: Internet, Courtesy: researchgate

Hybrid scrubbers are more effective than other types of scrubbers. This system has both open and close loop configurations normally it runs on an open-loop in non-ECAs and on a close loop in ECA’s and also can be switched according to need and conditions. Hybrid scrubber systems are getting popular with most of the cruise ships and are now getting a good response with the cargo ships, tankers, Ro-Ro vessels also. Hybrid scrubbers offer high practicality as due to the trends the costs of MGO, MDO, LNG is still very high and not in sync with the requirements of the maritime industry. This system falls under the emission norms in ECA’s and non-ECA’s. limitations.

Limitations of hybrid scrubbers: For newer emission norms that will reduce sulfur content to zero and also acknowledges Carbon, NOx emissions need to be controlled. Due to this, the scrubber systems might be a great option for the present but the future seems to be hazy.

Do we have more ways to become compliant with Sulfur cap 2020? Yes, we do have many options to follow compliance with sulfur emissions. Here are the enlisted options.

  • Using distillate fuels. ( MGO, MDO, ETC.) Read more
  • Using LNG as a fuel for boiler and steam operation or in dual-fuel engines.
  • Going towards alternative fuels and shift from conventional fuels.
  • Using EGR, VGT technologies to reduce emissions levels.


Exhaust gas cleaning systems gives the advantage to continue with the same engine type and same fuel for the ships which seems to be very convenient for all stakeholders but due to variable demands of the industry and stricter emission norms like carbon-neutral, greenhouse reduction, Noxious emissions reduction, etc, requires a very different approach and till now no such option is there to deal with all. So the fundamental principle of maximum optimization is being followed.

About the Author

Hello, I’m Hardik

A Marine Engineer by profession, Hardik loves to read and write about the technical happenings in the maritime industry.