Space-Saving SOx Scrubber System Designed to Achieve “Zero Cargo Loss” by Combining Shipbuilding and Onshore-Plant Technologies
Case Study

Space-Saving SOx Scrubber System Designed to Achieve “Zero Cargo Loss” by Combining Shipbuilding and Onshore-Plant Technologies


MHI recognizes climate change is one of the most serious concerns of our time.

Radical efforts to protect the environment, including measures against global warming, are also being promoted in maritime logistics. The International Maritime Organization (IMO) has been tightening its environmental regulations year by year and requires a 50% reduction in CO2 emissions by 2050.

As a long-term measure, alternative fuels are being actively discussed. In the short term, the most pressing compliance requirement is meeting the regulations to control sulfur oxide (SOx) emissions in ships’ exhaust gases.

In January 2015, the use of fuel oil with a sulfur content of less than 0.1% became mandatory in Emission Control Areas (ECAs) such as the North Sea, Baltic Sea, and the coastal seas off North America. Since January 2020, the use of fuel oil with a sulfur content of no more than 0.5% is compulsory in general waters outside the ECA.

The existing fuel used in conventional ocean-going vessels, Heavy Fuel Oil (HFO), contains high levels of sulfur and does not comply with the IMO's SOx emission limits. To comply with the regulations, ship-owners and builders have three options:

  1. Switch from HFO to Low/Very low sulfur fuel oil (LSFO/VLSFO), which complies with the regulations
  2. Install an exhaust gas cleaning system (EGCS, or scrubber) to bring emissions within the limits while continuing to use HFO.
  3. Use liquefied natural gas (LNG) as a substitute to HFO.

Each option has its advantages and disadvantages.

Of the above options, the use of LNG (option 3) is currently the best one from an eco-friendly point of view. Not only does it meet the most recent SOx emission standards, but it also reduces CO2 emissions by approximately 25% compared to petroleum-based fuels. It is also cheaper than HFO.

However, the initial investment in building LNG-fueled vessels (CAPEX) is substantial, and there is currently not sufficient onshore LNG supply infrastructure. It is also difficult to find seafarers capable of handling LNG on board. Therefore, the number of ships that can switch to LNG fuel will be limited in future, and the transition period is estimated to take considerable time.

Switching to Low-sulfur fuel oil (option 1) is beneficial because it requires little CAPEX investment. However, prices tend to be more expensive, and operating costs (OPEX) are also higher compared to HFO. Furthermore, there are uncertainties in terms of quality control and supply. Taken together, these issues mean that switching to low-sulfur fuel oil is considered only as a short-term solution.

Fitting a scrubber (option 2) is the most beneficial of the three options in terms of medium-term cost efficiency. Although scrubber installation requires an initial capital investment of several million dollars, it allows for the continued use of inexpensive HFO and reduces fuel costs compared to the use of expensive Low-sulfur fuel.

If the price difference between HFO and low-sulfur fuel oil is around $200 per metric ton, the investment in a scrubber can be paid back in one to two years. Even with a smaller price difference, for example around $50 per metric ton, the payback period is estimated to be three to five years. Thus, scrubbers offer cost efficiency in the medium term.

Desulfurization Is Not the Only Determining Factor for the Performance of Scrubbers

A scrubber is a device that removes the sulfur contained in exhaust gas (desulfurization). The method originated as a way of cleaning sulfur emissions from land-based factories but has been adapted for marine use.

Scrubbers for ships use seawater or freshwater for desulfurization. The scrubber does not work alone and needs to be optimized for the entire engine plant, including the exhaust gas system, water distribution system, and electrical control system.

The installation of a scrubber also has a significant impact on the layout of the ship. Since the purpose of shipping is high-volume maritime transport, it is necessary to design and engineer the ship so that cargo volumes will not be significantly affected by the addition of a scrubber.

Three Methods of Scrubbing

There are three different systems of desulfurization: open-loop, closed-loop, and hybrid.

  1. Open-loop system: this is the lowest-cost scrubbing system. The open-loop system uses seawater as ‘wash water’ for desulfurization and discharges it after a single use. The seawater used for desulfurization inevitably contains impurities and is subject to discharge regulations. Therefore, there are some territorial waters and ports that prohibit the use of the open-loop system.
  2. Closed-loop system: wash water used for desulfurization is reused, cleaned, and disposed of. The closed-loop system can be applied to some territorial waters and ports where the application of the open-loop system is prohibited (Singapore, China, the West Coast of the United States, the Suez Canal, the Panama Canal, etc.)

    However, installing a closed-loop system is 1.5 times to twice the cost of an open-loop system, and the cost of neutralizing and disposing of wastewater residue adds to the running costs compared to the open-loop system.
  3. Hybrid system: a combination of the open-loop and closed-loop systems, it can be used in territorial waters and ports where the open-loop system itself is prohibited.

    Similarly to the closed-loop system, the installation cost is 1.5 times to twice as high as that of the open-loop system. However, by using different operation methods under effluent regulations, running costs can be reduced compared to the closed-loop system alone.

All of these methods must meet the desulfurization performance requirements of the IMO environmental regulations in both general waters and the more restrictive ECA.

Desulfurization performance in compliance with IMO environmental regulations and navigability in all waters
Speed and ease of fitting scrubbers
Minimizing cargo loss due to installation of the scrubber
In addition to providing a high level of desulfurization, the scrubber must be integrated into the ship's operation system and optimized to work smoothly with other on-board systems. It is also crucial to keep the installation of the scrubber straightforward so that installation cost and delivery time can be minimized.
If the ship’s purpose is transportation, the scrubber’s footprint must not significantly reduce the space available for cargo. Saving space is therefore one of the most important factors in installing scrubbers.

Mitsubishi Shipbuilding’s DIA-SOx® scrubber has achieved high desulfurization performance as a shipboard system. The innovative scrubber is a product of Mitsubishi Shipbuilding’s experience and the expertise of its sister MHI Group company, Mitsubishi Power, the global market leader in land-based desulfurization equipment for power plants and industrial sites.

Mitsubishi Power is a manufacturer with numerous developments and the world’s leading market share in supplying land-based desulfurization equipment (boasting a more than 37% share of the global market since 2014).

Utilizing MPS technology, Mitsubishi Shipbuilding, with more than 100 years of experience, has created an innovative scrubber system for ships.

The DIA-SOx® range consists of two types of scrubbers: C-SERIES and R-SERIES.

The cylindrical C-SERIES is designed for general-purpose applications such as bulkers, tankers, and containerships with an engine output of 5 MW to 30 MW.

The rectangular R-SERIES is optimized for ultra-large containerships with an engine output of 30 MW to 75 MW.

C-SERIES and R-SERIES scrubbers can be adapted to both inexpensive open-loop and hybrid systems.

Both types are fully compliant with both general and ECA regulations and capable of operating in all waters, with the hybrid system able to access territorial waters and ports where open-loop systems are prohibited.

Mitsubishi Shipbuilding has established a reliable supply chain with several plants specializing in DIA-SOx® scrubbers throughout East and Southeast Asia. This makes it possible to offer short delivery times and large lot orders, an advantage the company has been able to maintain despite the COVID-19 pandemic.

The company provides engineering services for installing scrubbers on both existing and new ships.


The two-island ultra-large container ship achieves “zero reduction in cargo” by saving space and minimizing the impact on cargo.

The C-SERIES scrubbers are designed as a compact package that can be installed easily, shortening the time it takes to fit them, and which takes up limited space on board. The package can be easily retrofitted to a vessel in service and can also be removed and reused on-board other ships.

The R-SERIES has been developed to optimize installation on large container ships, with a rectangular, thin shape that has a similar footprint to a container. The rectangular scrubbers fit into the existing engine casing space and their length and width can be adjusted, minimizing footprint.

This also enables their use in “dead space” where no cargo can be stored. Particularly in a two-island Ultra Large Container Vessel with separate accommodation structure and engine casings, installing an R-SERIES scrubber does not reduce the number of containers that can be transported, helping customers optimize the tonnage they can transport.

As of June 2020, Mitsubishi Shipbuilding had received 19 orders for its C-SERIES and 39 orders for R-SERIES scrubbers.

Orders for R-SERIES in particular were mainly for retrofits. Although the demand for retrofits has passed its peak, the number of requests for installation on new ships is coming in constantly, and Mitsubishi Shipbuilding expects demand to keep building. The demand for scrubbers is expected to remain high in the future, due to the medium-term cost-effectiveness which scrubbers offer ship-owners and operators.

Mitsubishi Shipbuilding’s broad lineup of scrubber systems is set to evolve as the company continues to develop flexible solutions that fit the changing needs of customers.