February 15, 2022

What exactly is a ship desulfurization system and how to choose?

Since May 2005, the MARPOL Convention has made strict limits on the emission of main and auxiliary engines of ships. The convention calls for the reduction of emissions such as carbon dioxide, sulfide, and nitrogen. At the 58th meeting of MPEC, exhaust gas treatment systems (desulfurization equipment, etc.) were allowed to be used to reduce sulfide emissions from ships.

The main ways that can be used to reduce sulfide emissions are:

1. Use low sulfur fuel oil,
2. Use natural gas instead of fuel oil,
3. Introduce the waste gas treatment system, which is what we often call the desulfurization tower, which will also be what I will introduce next

What is a desulfurization tower system?

The desulfurization tower is a system designed to clean the exhaust gas of main and auxiliary engines with water to remove sulfides. There are many different designs on the market today, however they are mainly divided into two categories, open loop and closed loop. The power consumption of the desulfurization tower system usually accounts for 1-2% of the host energy.

Working principle

The working principle of the desulfurization tower depends on the type of desulfurization tower. Generally speaking, desulfurization towers can be divided into the following types:

Open loop desulfurization tower

The open-loop desulfurization tower uses seawater to clean the exhaust gas. The natural chemicals in the seawater neutralize the sulfide solution in the exhaust gas to achieve the purpose of desulfurization. The cleaning water is simply treated and then discharged back to the sea. 45m3/MWh system.

Closed loop desulfurization tower

Closed-loop desulfurization towers use closed-loop clean water. This water will be treated with some alkaline water, such as caustic soda neutralizer. The wash water will be recycled, and the lost part will be used to add new fresh water. A small amount of wash water will be sent to a sewage treatment plant for treatment before being discharged into the sea. This system can also be designed with a storage cabinet to achieve true zero emissions.

Hybrid desulfurization tower

Hybrid, as his name suggests, is an aggregate of various systems. At Wärtsilä, the name is defined as a system that is both open-loop and closed-loop, enabling the operator to switch flexibly between low- and high-alkali regions. The hybrid series also includes other products, such as open-loop systems that add a certain amount of caustic soda to the clean water to bring the alkalinity of the discharge liquid to a suitable level.

How to choose a suitable desulfurization tower

Which one is better to choose? In general, no one is completely better than the other, just go to a strict supplier with a definite cargo, it’s just a matter of seeing which system is best for your boat type. In the end they all boil down to issues of alkalinity and operating routes. Alkalinity is a term used to describe how acidic clean water is buffered, and is present in all natural waters to some extent. In washing, alkalinity is used to buffer sulfides and dissolve them into clean water. The base will help neutralize the sulfides and keep the pH at a high level.
In the open sea, the alkali value is generally high, so seawater can be used in the desulfurization tower. Open-loop desulfurization towers do exactly that. However, there are also areas in the world where the alkalinity is too low to be utilized by scrubbers. These areas include, for example, the Great Lakes region of the United States, the Port of St. Petersburg, and the Mississippi River. A closed-loop desulfurization system would be the best option if ships had been operating in these locations, as it used caustic soda to buffer the acidity of the clean water.
Therefore, the alkalinity number in the system is independent of the surrounding seawater. If a vessel frequently operates in high alkalinity waters, but occasionally goes to some low alkalinity areas, a hybrid scrubber can be an option for these vessels as the hybrid system can operate in both open loop and closed loop depending on operational needs conversion between systems.
cost! Of course, it is an important reason to consider the advantages and disadvantages of different systems, when considering capital costs, operating costs and usual operability. There may also be some other reasons to choose some special scrubber, not just because of seawater alkalinity and ship’s route.
For example, if a vessel is operating in high alkalinity waters, but with shallow water depths and high sand content, the owner will consider installing a closed loop system or a hybrid system. This is because with an open-loop system, grit can enter the system and damage equipment, especially water processors. This will add a considerable amount of maintenance costs. Another consideration is the amount of water entering the system, as the water pumps in the scrubber equipment are usually very energy-intensive.
An open-loop system uses significantly more water than a closed-loop system, so its energy consumption is also higher. However, in a closed-loop system, the consumed alkaline reagent is also a cost. The number of devices in the system naturally also affects the investment cost. The open loop system looks better in this regard, as it is the simplest system.

how to match

There are usually two cases. One is to equip all engines and boilers with separate desulfurization units, and the other can combine all engines and boilers together. Wärtsilä decided to combine the engine and boiler into one desulfurization plant. This decision was welcomed by boiler manufacturers. Whether combined or standalone, they each have their pros and cons.
Having a separate desulfurization unit for each engine and boiler would allow considerable operational flexibility and thus lower operating costs. But the required investment costs will increase accordingly. The combined scheme can make two or more main and auxiliary engines and boilers use the same desulfurization unit, so it will save space and cost. As just said, such a scrubber will greatly reduce flexibility, and if part of the engine does not start, the scrubber will run at overload most of the time. The wisest approach is to discuss the best solution with a technical expert.

price and return

As shipowners are often able to pass on the additional cost of bunkers to charterers, the actual price of bunkers is often not their primary concern. It is currently expected that after the IMO sulphur cap comes into effect, the increased demand for low-sulphur fuel will reduce the price of high-sulphur fuel. At the same time, the availability of low-sulfur oil will ensure high premiums for high-sulfur oil and light oil. Tanker owners will also be at odds with the price difference between high- and low-sulfur oil, which will determine whether they will install scrubbers to comply with the upcoming sulphur cap.
In addition to the price difference between low- and high-sulfur oil, the age of the vessel is also a key factor. Because younger ships will have more time to recoup their investment in scrubbers. Newbuildings also have this advantage because not only do they have a longer lifespan, but it is also cheaper to install desulfurization equipment on newbuildings. Tailoring and installing desulfurization equipment specifically for existing ships will also be an additional expense.
For VLCC ships, installing an open-loop scrubber on a newbuilding vessel will cost approximately US$2.5-3 million (according to 2.7 million), while the cost of retrofitting scrubber equipment for existing VLCCs will be as high as US$4-4.5 million (Average is about $4.3 million).

In addition, the spread for low-sulfur oil will gradually decrease from 2020 as supply increases. According to Drewry’s forecast, the spread will drop from $300 in 2020 to $87 in 2023, thus reducing the cost of tanker savings from $5.7 million to $1.6 million. This means that ships that choose to retrofit scrubbers in 2020 will likely be able to pay for themselves in the first year. However, for those who install scrubbers later, their payback period will be longer.


In summary, the first thing to look at is the relevant rules that apply to the area in which the vessel operates. It should be kept in mind that this is always the strictest legislation and needs to be followed. Secondly, it is not only necessary to pay attention to the rules of IMO, but also to the local legislation of the ship operating area. Whether the installation of desulfurization equipment is suitable for this ship, or should another alternative be chosen.
Many shipowners will choose consultants at this point in time. Once it is determined that the installation of a scrubber is a good choice, they will then investigate the route of ship operation. Will the boat go into the low alkalinity zone and for how long? It should also be noted that, before the official implementation of the sulfur cap, the cost of the scrubber is only related to the emission control area (similar local legislation). Finally, it is necessary to look at the machinery and equipment connected to the desulfurization tower system.
By looking at the pipeline network and spatial layout of the engine room, the shipowner needs to evaluate which desulfurization equipment is most suitable. However, this is also challenging and most shipowners are now opting to seek out one or more suppliers for guidance. Of course, suppliers are also very happy to provide related support and products, such as layout drawings, on-board surveys, payback period calculations and power consumption estimates.
In general, it is best to involve the supplier’s technical experts in the early planning stages of installing a scrubber, as the process is often lengthy. This is not an “off the shelf” product and cannot be treated as such. Every vessel is unique, which means so is every scrubber system.

source from: soho

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