Ship Scrubber System

The Maritime Industry, despite being conservative to changes, has recently been looking into greener methods of propulsion for running their ships. This newfound interest can be primarily attributed to the emission standards of harmful gases like Sulphur oxides and nitrogen oxides being progressively tightened by the IMO under Annex VI of MARPOL. One quick and effective solution to this issue is the adoption of ship scrubbers.

What is a Ship Scrubber ?

A Ship Scrubber also known as Exhaust Gas Cleaning System is a device that is used to treat the exhaust gases from the main engine and the boiler. The exhaust gases are treated such that the harmful components like Sulphur Oxides (SOx) and Nitrogen Oxides (NOx) present in them are removed. This cleaning system is fitted in the exhaust gas system of the Main Engine, Auxiliary Engine and Boiler.

Ship Scrubber System
Scrubber on a Ship (Source: https://shipinsight.com)

How Does a Ship Scrubber Work?

The basic operational principle of a ship scrubber system is that the exhaust gases are passed through the scrubber where the harmful particles are removed using various alkaline scrubbing materials like seawater, chemically treated freshwater or dry substances like hydrated lime pellets.

Based on operation, a ship scrubber system can be classified into:

  1. Dry Scrubbers- This type of scrubber does not use any type of liquid to carry out the scrubbing process of the exhaust gases. Instead, the exhaust gases are exposed to hydrated lime pellets which causes a chemical reaction that removes the SOx emission compounds. It is highly effective in removing the SOx and NOx compounds but is expensive.
  2. Wet Scrubbers- In this type of scrubber, the scrubbing process is carried out by spraying a scrubbing liquid into the exhaust gas which causes the gases to react with the liquid media and the SOx compounds are removed. Based on the type of liquid media used Wet Scrubbers can be further classified into-
  3.  Open-Loop System: In this system, the liquid medium used for the scrubbing process is normal seawater. The sodium oxide emission compounds react with the alkaline seawater to form Sulphuric acid which is neutralized by the natural alkalinity of the seawater. This wash water is then treated using a separator or centrifugation to remove the sludge consisting of particulate matter, heavy metals etc. The wash water once in line with the regulations set by the MEPC can be discharged into the environment. Although an extremely effective system, its effectiveness is highly dependent on the alkalinity of the seawater.
  4. Closed-Loop System: This system is similar to the open-loop system in terms of operating principle and chemical reactions. The major difference is that freshwater with chemical additives like sodium hydroxide and calcium carbonate is used as the liquid medium for the scrubbing process. Another difference is that the wash water is continuously reused after the necessary treatments are done. In doing so, the system is independent of the alkalinity of the water in which the ship is sailing. In this system, the alkalinity of the water is directly controlled and caustic soda is automatically added to the water whenever necessary to control the pH value of the process which indirectly controls the efficiency of SOx removal. The wastewater from this system is not discharged into the ocean and is held in a holding tank until it can be discharged via shore reception facilities.
  5. Hybrid System: This is a system that combines the advantages of both the open-loop and closed-loop systems. When the ship is in the open ocean, the scrubber system operates as an open-loop system and when in ECA zones or ports it is switched over to closed-loop mode.
Working of Scrubber
Working of Scrubber (Source: https://www.dnv.com)

What is the Purpose of a Scrubber?

Although maritime transport is a clean and efficient mode of transportation requiring only 2-3 grams of fuel per ton*km, it has been one of the largest contributors to Nitrogen oxide and Sulphur oxide emissions with a share of 13% and 12% of the global emissions respectively. In order to combat this issue, IMO released a set of regulations known as IMO2020 mandating that all ocean-going vessels should only use fuels with a Sulphur content of 0.50% globally and 0.1% in the ECA’s. One method for complying with these regulations is the use of exhaust gas scrubbing systems. These scrubbers remove the Sulphur contents from the exhaust gas and make sure that the amount of Sulphur compound emissions is less than the specified value.

How much does a Ship Scrubber System Cost?

Retrofitting a ship with a scrubber system is highly expensive. This consists of the actual cost of the system, installation cost and in some cases the cost of retrofitting the pipelines required. The cost of just the scrubber system alone depends on the size of the engine and ranges from 2.5 to 3 million euros for a 20 MW engine. But this does not include the cost of installation, cost of training the crew etc. All in all the total capital expenditure can be anywhere around 4 million euros. Another important cost that comes along with the use of a scrubber system is the operational costs. This includes the cost of chemicals, freshwater, wastewater treatment etc. It is impossible to derive an estimate of this as the costs of these materials are extremely volatile and depends on type of ship.

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