Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)1

Practical Design of Marine SOx Scrubber for
Mega-Container Ships

RYOZO SASAKI*1

TATSUTO NAGAYASU*2TOSHIKI SHINGU*3

YUSUKE WATANABE*4

TAKAAKI MORI*5

HIDEAKI SAKURAI*6

From January 2020, the International Maritime Organization (IMO) will implement astricter limit for sulfur in fuel oil of ships operating in all sea areas based on their regulations. This
report presents the practical design of a SOx scrubber that can stay within this limit with atreatment capability for large-output engines of ultra-large container ships. By marinizing therectangular flue gas desulfurization system for onshore boilers, which Mitsubishi Hitachi Power
Systems, Ltd. (MHPS) has improved over the years, we have given the SOx scrubber advantages interms of the installation layout, especially in large container ships, also enabling it to be installed
without reducing the cargo capacity. The system we offer has a high sulfur removal efficiency andis compliant with the regulations of all sea areas including Emission Control Areas (ECAs) which
are subject to the particularly strict emission regulations.

|1. Introduction
Regulations on SOx emissions into the air environment have conventionally focused on
fixed, onshore emission sources such as thermal power plants.
Regarding ships operating offshore, the IMO regulations, which came into effect on January
1, 2015, mandate the use of fuel oil with a sulfur content not exceeding 0.1% in pollutant ECAs
such as the North Sea in Europe, as well as the Baltic Sea and coastal sea areas in North America.
For ships operating in global sea areas excluding ECAs, the use of fuel oil with a sulfur content notexceeding 0.5% will become compulsory from January 1, 2020 (Figure 1).

Figure 1

Regulatory limits for sulfur in marine fuel oil*1

AQCS Plant Engineering Department, Air Quality Control Systems Technology Division Mitsubishi Hitachi Power Systems,Ltd.

*2

Director, Air Quality Control Systems Technology Division Mitsubishi Hitachi Power Systems,Ltd.

*3

Chief Staff Manager, Plant Construction Division Civil Department, Mitsubishi Hitachi Power Systems,Ltd.

*4

Chief Staff Manager, Ship & Ocean Engineering Department, Marine Engineering Center, Mitsubishi Shipbuilding Co.,Ltd.

*5

Nagasaki Designing Department, Marine Engineering Center, Mitsubishi Shipbuilding Co.,Ltd.

*6

Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries,Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)2

Meanwhile, an approved alternative method for using low-sulfur fuel oil is the combustion of
conventional heavy oil in the diesel engines used for propulsion and power generation, followed by
the use of exhaust gas cleaning systems (EGCS) by which the level of sulfur oxides in the
combustion flue gas is reduced to or below the regulatory limit. As SOx regulation guidelines for
similar measures, MEPC.259(68): 2015 Guidelines for Exhaust Gas Cleaning Systems (hereafterreferred to as the IMO EGCS Guidelines) have been adopted.1) Given the circumstances in which
low-sulfur fuel is more costly than conventional heavy oil and its supply capacity is unclear in
anticipation of the demand that will grow in the coming years, there is increasing demand for
EGCS enabling the continuous use of conventional heavy oil with a sufficient supply capacity atlower prices. Many marine equipment manufacturers and flue gas desulfurization unit
manufacturers have entered this market.

|2. Seawater desulfurization system for thermal power plants
In Japan, stricter emission standards for air pollutants have been gradually implemented sincethe 1960s as countermeasures against aggravating environmental issues. Flue gas desulfurization
system manufacturers have taken up the challenge to conserve the environment by designing andsupplying environmental equipment mainly for domestic/overseas thermal power plants.
The major technologies used to desulfurize flue gas from thermal power plants are the wet
process, dry process, and semi-dry process (Table 1).
Table 1 Overview of major desulfurization technologies in thermal power plants
Desulfurization
methodWet process

Process overview
Alkali absorption method(typical examples)○ Limestone-Gypsum method○ Seawater method○ Caustic soda method○ Magnesium hydroxide method
Spray-dry method

Semi-dry/dry process
Activated carbon method(1) Absorb SO2 with an alkaline aqueous solution or slurry(2) Oxidize the produced sulfites to sulfates, as needed(3) Collect the products for reuse or disposal(4) There are several different processes depending on thealkali in use(5) The major processes are the limestone-gypsum method
and the seawater method(1) Spray slaked lime slurry into the reaction tower to turnSO2 into powdery substances such as calcium sulfite(2) Collect using the precipitator and dispose of thecollected matter(1) Adsorb SO2 with activated carbon(2) Thermally desorb the adsorbed SO2 to collecthighly-concentrated SO2 and turn it into gypsum,
sulfuric acid, etc.

Among these technologies, we will address wet flue gas desulfurization with seawater, which
is suitable for flue gas treatment for ships.
Figure 2 is a system flow diagram for seawater desulfurization. In a desulfurization systemusing seawater, the alkaline components (e.g., HCO3-) naturally contained in seawater are utilized
for desulfurization, instead of using chemicals like limestone and magnesium hydroxide asabsorbents for sulfur in flue gas.

Figure 2

System flow diagram for seawater desulfurization

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)3

In recent years, because of its simple configuration and desulfurization being possible evenwhen there is difficulty in preparing absorbents or handling by-products, this system has been
increasingly adopted by power plants in emerging countries such as India and those in Southeast
Asia and the Middle East. The principle of seawater desulfurization is the absorption of SO2 in fluegas into seawater inside the absorption tower, followed by the oxidation treatment of the formed
sulfite ions (HSO3-) through contact with large quantities of air in the oxidation tank to produce
harmless sulfate ions (SO42-). As sulfate ions are contained abundantly in seawater, there is little
impact on the marine environment. In the oxidation tank, pH is adjusted by means of neutralization
and aeration and the reduced level of dissolved oxygen is recovered by oxidation, before the treatedseawater is ultimately discharged into the sea. The reactions are as follows:
Absorption: SO2 + H2O → H+ + HSO3Oxidation: HSO3- + (1/2)O2 → H+ + SO42Neutralization: HCO3- + H+ → H2O + CO2↑
MHPS’s deliveries of seawater desulfurization systems includes one installed at a heavy
oil-fired plant that treats flue gas with one of the highest SO2 levels by seawater desulfurization(Saudi Arabia), and another installed at the world’s largest class 1000 MW coal-fired plant(Malaysia). Making use of their proven normal operational records, we tailor the design of seawater
desulfurization systems that can satisfy the diverse needs of our customers. The actual value of therate of desulfurization by our seawater desulfurization systems for thermal power plants exceeds
99%, which is significantly higher than that required in ECAs (97.1%).

|3. Application to marine SOx scrubber
Using seawater as a desulfurization absorbing liquid, our marine SOx scrubber employs aseawater desulfurization system that has a simple configuration and can be installed in a limited
space. Figure 3 is a system flow diagram of the marine SOx scrubber. The basic principle is thesame as the desulfurization system for thermal power generation and the seawater taken from thesea is supplied to the absorption tower, directly spraying into flue gas. The system utilizes alkaline
components naturally contained in seawater to remove sulfur. What differentiates this system is thepost-desulfurization seawater treatment method. In the case of thermal power plants, the seawaterused for desulfurization undergoes treatments such as oxidation and pH adjustment in the oxidation
tank according to the requirements of each country or region, before being discharged into the sea
near the power plants. On the other hand, the marine SOx scrubber complies with the IMO EGCS
Guidelines and makes sure that the effluent pH, polycyclic aromatic hydrocarbons (PAHs),
turbidity, etc., are within the respective limits before being discharged overboard. Each unit of themarine SOx scrubbers needs to be approved by a classification society regarding its compliancewith the Guidelines.3)

Figure 3

System flow diagram of marine SOx scrubber

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)
4

Table 2 lists the differences of desulfurization system design conditions between thermal
power plants and large ships. When it comes to the system size, those for onshore thermal power
plants are larger and require higher desulfurization performance.
Table 2 Differences of desulfurization system design conditions between thermal power plants
and large ships
Flue gas treatment capacity(Nm3/h)Fuel
SO2 level at the inlet ofdesulfurization unit (ppm-d)
SO2 level at the outlet ofdesulfurization unit (ppm-d)

3.1

For thermal power plants

For large ships

600,000 to 4,000,000

23,000 to 540,000

Heavy oil/coal, etc.

Heavy oil

100 to 1800

700

10 to 220

20

SO2 removal efficiency (%)

75 to 98

97.1 (3.5%S → 0.1%S): ECAs
85.7 (3.5%S → 0.5%S): global sea areas
excluding ECAs

Regulatory items for seawater
discharge

pH, dissolved oxygen (DO),temperature, etc.

pH, PAH, turbidity, nitrates

Summary of IMO EGCS Guidelines1),3)

Here, we will describe the regulatory items for seawater discharge, which are stipulated as
the requirements particular to marine SOx scrubbers.
3.1.1 Effluent pH criteria for discharge
These criteria set the limits for the acidity of washwater effluent. The effluent at 4 mfrom the overboard discharge point with the ship being stationary should have a pH of no lessthan 6.5. Continuous monitoring is obligatory while EGCS is in operation.
3.1.2 Effluent turbidity criteria for discharge
These criteria set the limits for the turbidity of washwater effluent. The maximum
turbidity difference between inlet water and discharge water should be no greater than 25 FNUs
(formazin nephlometric units) or 25 NTUs (nephlometric turbidity units). Continuous
monitoring is obligatory while EGCS is in operation.
3.1.3 Effluent PAH criteria for discharge
These criteria set the limits for the PAH concentration in washwater effluent. For
example, the maximum difference between the PAH concentration in inlet water and that indischarge water should be no greater than 50 μg/L PAHphe (phenanthrene equivalence) at awashwater flow rate of 45 t/MWh. The stipulated limits are normalized according to thewashwater flow rate. Continuous monitoring is obligatory while EGCS is in operation.
3.1.4 Effluent nitrate criteria for discharge
These criteria set the limits for the nitrate concentration in washwater effluent. The
nitrate concentration should not exceed the following, whichever is greater.
(1) 12% of the NOx emissions from flue gas(2) Nitrate concentration of 60 mg/L at a washwater flow rate of 45 t/MWh

3.2

Points to note in design of marine SOx scrubber

3.2.1 Countermeasures against scrubber vibrations caused by external vibrations
When installing the SOx scrubber in a ship, we have to consider countermeasures against
the vibrations particular to ships. Every instrument or structure has its own frequency at which
it tends to oscillate (i.e., natural frequency). When an external exciting force, which works toshake an instrument or structure, has a frequency close to the natural frequency, a phenomenon
called sympathetic resonance occurs. As a result, the vibration of the instrument or structure
intensifies, which may damage the structure. Therefore, it is important to understand natural
frequencies by means such as numerical simulation and design a structure that can prevent theoccurrence of sympathetic resonance with the exciting frequencies.
Table 3 gives some examples of the vibratory forces on board ships and their consequent
ship vibration directions. As shown in the table, the forces on board ships that cause the SOx
scrubber to oscillate include the inertial force of the main engine involving the reciprocating
motion of the engine piston and the rotational motion of the crankshaft, as well as the propeller
vibratory force originating from the rotating screw propeller that results in surface force on theMitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)5stern outer shell through seawater.
Figure 4 shows a vibration response analysis model of the entire ship. Regarding theSOx scrubber, to simultaneously assess vibration response and fatigue strength, modeling wasdone by allowing the shell elements to include shaped steel parts such as stanchions andreinforced parts such as plating. The beam elements also included steel tubes such as piping,internal structural members and support structures.
Table 3 Examples of vibratory forces on board ships and consequent ship vibration directions
Vibratory force
Inertial force of the main engine

Degree
1N

2N

●

●

Vertically transmitted force

3N

4N

●

●

5N

6N

9N

●

10 cylinder ship●

11 cylinder ship

Figure 4

8N

10N

11N

●

Propeller vibratory force

Major vibration direction

7N

Up/down andbow/sternUp/
Bow/stern andTorsional
down
up/down●
Torsional andport/starboard●●―

Port/starboard

Vibration response analysis model of entire ship

The vibration response analysis results using this model show that the entire structure of
the SOx scrubber, ducts, internal structural members, surrounding wall panels, and piping
successfully ward off sympathetic resonance with the frequencies of external vibratory forces.
It has also been indicated that the stress concentrated areas in the SOx scrubber structure havesufficient fatigue strength.
3.2.2 Material selection method
As the seawater desulfurization unit needs to have resistance to the acids absorbed fromseawater and flue gas, the materials to be used should be highly resistant to corrosion. In a
desulfurization unit for onshore thermal power plants, materials with high acid resistance suchas resin lining materials and fiber reinforced plastic (FRP) have often been used. On the other
hand, for ships with a shorter maintenance period than thermal power plants, the use of resinmaterials is avoided in many cases, because it takes longer for maintenance. Metals are
therefore a preferable choice in most cases, and two-phase stainless steel is mainly used for the
scrubber tower. As the flue gas inlet where hot flue gas comes into contact with seawater for
the first time is subject to the most severe corrosive environment, the use of stainless steel withhigh corrosion resistance is essential. This environment is so severe that corrosion will occur
within a short period of time, if the seawater spray method and structure of the flue gas inlet are
not adequately considered at the same time.

|4. Marine SOx scrubber features
4.1

Operable in open loop mode in ECAs

The marine SOx scrubber operates in one of the following modes: open loop in which
seawater is sent as an absorbing liquid to the absorption tower in a single pass system, closed loop
in which a circulating liquid containing chemicals such as caustic soda is used, and hybrid in which
the other two modes are used in combination. Together with Mitsubishi Shipbuilding Co., Ltd.,MHPS has employed the open loop mode for desulfurization treatment of flue gas from 30-75 MWMitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)
6class large-output engines, without the need for the use of chemicals such as caustic soda. The
developed marine SOx scrubber also complies with the regulatory limits in ECAs.

4.2

Rectangular absorption tower with high volumetric efficiency

Figure 5 is the external view of the marine SOx scrubber. As the height/width ratio of therectangular absorption tower can be changed relatively freely, the optimal layout in the limited
space of a ship becomes possible, achieving a higher volumetric efficiency than the existing
cylindrical scrubbers. Especially for large container ships, the scrubber can be installed without
reducing the container burden, which indicates that the rectangular shape has a considerable layout
advantage. Figure 6 depicts an image of the layout of the scrubber on board a large container ship.
This is an example of a large container ship with a two-island design with living quarters separate
from the engine casing in which the scrubber unit is accommodated, thus enabling the installation
to be carried out without reducing the number of containers on board.

Figure 5 External view of marine SOx
scrubberFigure 6 Conceptual image of scrubber locatedon board large container ship

|5. Engineering for installation
Mitsubishi Shipbuilding Co., Ltd. provides the engineering service for installation tocustomers who want the SOx scrubber to be installed on board their planned ships more quickly,
easily and securely. The company assists in their plans through the provision of service by taking
advantage of its strength and shipyard experience together with the knowledge it possesses as aSOx scrubber manufacturer. While this service is also available to customers who want to retrofit
the SOx scrubber to their in-service ships, an increasing number of shipbuilders are using thisservice for ships that they plan to construct. The company can propose an installation plan
harmonious with the ship in question, whether it is newly built or already in service or whether webuilt the ship or it was built by another company.
With regard to the retrofitting of in-service ships, the engineering service for installation isprovided according to the following steps:(1) Reverse engineering (3D laser measurement)(2) Basic plan
(3) Detailed engineering(1) Reverse engineering
To do the engineering work for installation in relation to the retrofitting of in-service
ships, the drawing information on the ship in which the scrubber is to be installed is required.
The detailed piping drawing in particular is needed when designing the layout arrangement.
However, as it is usually a working drawing used at shipbuilding dockyards, it may be
unavailable in some cases. Even when the working drawings are available, the drawing
information may be different from the actual ship because of post-launch modifications, etc. If
this is the case, 3D laser scanning is conducted to design the layout arrangement based on thecreated 3D model (called a 3D as-built model).

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)7

With the 3D laser scanner, a hemisphere with a radius of several dozen meters (thescanner is located at the center of the hemisphere) is continuously irradiated by a laser pulse
and the distance from each object for measurement in the hemisphere (e.g., ship shell, piping
and auxiliary equipment) is calculated by comparing the phase difference between the incident
and reflected waves. The point of reflection is determined based on the angle of where the laser
originally came from and data of a group of several thousand points can be collected with asingle measurement. The measurement is repeated until the whole area for retrofitting iscovered. Combining these data together enables the positioning information to becomplementary synthesized to three-dimensionally visualize these objects. The data of groupsof points are converted so as to be handled as 3D CAD data.
(2) Basic plan
The basic plan starts on the basis of the information on the scrubber unit and the ship in
which the scrubber is to be installed. In the retrofitting of in-service ships, the value added by
the engineering work for installation is to propose a retrofit plan with minimized work over ashorter time span.
Based on the 3D as-built model of the planned ship, we plan how to lay out thecomponents such as the scrubber tower, flue gas damper, seawater pump, scrubber control
panel and sensors and decide the large-diameter piping routes such as seawater pipes and fluegas pipes. At the same time, a retrofit plan is made regarding the upper hull structure in which
the scrubber tower is to be fitted, as well as for the structure of the sea chest at the bottom of theship through which seawater is taken from the sea. Moreover, the impact of scrubber
installation is examined in terms of ship stability (metacentric stability), hull strength, cargoload, onboard power consumption, etc. We proceed with the basic plan as described above andpropose the optimal retrofit plan that provides a clear understanding of the impact on shipsafety and performance to the customer.
(3) Detailed engineering
The details of retrofitting in relation to scrubber installation are reflected in the detailed
engineering drawings such as the engine chamber layout diagram, piping system diagram,detailed piping diagram, table of electricity, electrical power system diagram, electronics layout
diagram, and ship shell structure diagram. The amount of retrofitting work is thus estimated. At
the same time, we proceed with certification by classification societies and the relevant
authorities. The amount of retrofitting work will be used as reference information to estimate
the cost at the customer’s repair dockyard, while the detailed engineering drawings are used to
draw up a work schedule at the repair dockyard.

|6. Conclusion
As a response to IMO environmental regulations, the following choices are available: the useof low-sulfur fuel compliant with the regulations, the installation of a SOx scrubber, and the use ofliquefied natural gas fuel. The installation of a scrubber enables the continuous use of conventionalfuel, resulting in lower fuel cost than in the case of using expensive low-sulfur fuel. The periodrequired to recover the investment in scrubber installation is estimated to be 2 to 5 years in manycases, and demand is increasing.
Given the stricter regulations to reduce the impact of marine flue gas on the air environment,
we have developed a marine SOx scrubber by expanding the application of flue gas desulfurization
technology for thermal power plants. This scrubber unit has specifications that satisfy theregulatory limits regardless of the sailing sea area, thereby enabling us to meet the diverse needs ofour customers.
Toward the implementation of stricter regulations in 2020, we want to contribute to solutions
to global issues of energy and the environment (which is one of our Vision Statements) through theprovision of marine scrubbers to our customers worldwide.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 3 (September 2019)8

References
(1)
(2)
(3)

(General Incorporated Foundations) Nippon Kaiji Kyokai, EGCS Guideline (Ver.2), (2017)
Ryozo S,,et.al. Industrial Machinery，No.812，p23（2018）
Mitsubishi Shipbuilding Co., Ltd.，Marine Exhaust Gas Cleaning System DIA-SOx.
http://www.msb.mhi.co.jp/en/products/AF/