Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
1Development of Liquefied CO2 Carriers andOnboard CO2 Capture Systemsfor Realization of Carbon Neutral Society
TAICHI TANAKA*1

SHIN TERADA*2

HIROSHI TANAKA*2

SHINICHI KAWAMATA*3KAZUYA ABE*3

MICHIHISA WATANABE*3

Mitsubishi Shipbuilding Co., Ltd. is developing liquefied CO2 carriers as a means oftransporting CO2 for establishment of a Carbon dioxide Capture, Utilization and Storage (CCUS)
value chain, and onboard CO2 capture systems to reduce CO2 emissions from ships. In thedevelopment of liquefied CO2 carriers, we select materials for the tank system, design thestructures and create the operation process from the perspective of optimizing the entire CCUS
value chain, and in the future, we will establish the technology by building a demonstration ship,
which will be the world's first liquefied CO2 carrier for CCUS. As for onboard CO2 capturesystems, we will continue further development toward commercialization based on the results of thedemonstration of a capture system conducted in a demonstration plant in fiscal 2021. Throughthese activities, we will provide the optimal total solutions for business operators that aim torealize a carbon neutral society.

|1. Introduction
Mitsubishi Heavy Industries Group (MHI Group) is making various efforts toward "energytransition" for the realization of a carbon neutral society. In addition to the low carbonization ofexisting technologies and the utilization of new energy such as hydrogen and ammonia, we areworking on the development of technologies related to the CCUS value chain, which not only
captures CO2 but also stores or converts it to other usage, with the aim of building a CO2ecosystem. It is expected that the CO2 capture amount required to realize a carbon neutral society
will reach 4.3 to 13 billion tons per year in the world in 2050. We anticipate growing demand forliquefied CO2 carriers as an economical means of transporting this enormous amount of CO2 tolocations where it will be stored or converted to other usage. This report presents, with a particularfocus on the liquefied CO2 carriers, the state of the technological development so far and future
prospects.

|2. Characteristics of CO2 and technological requirements forcarriers
CO2 is a colorless and odorless gas that is slightly heavier than air at room temperature andatmospheric pressure. For ocean transportation of CO2, it is desirable to liquefy it to reduce thevolume in the same way as LNG (Liquefied Natural Gas) and LPG (Liquefied Petroleum Gas).
When liquefied, the volume of CO2 becomes about 1/500 of gaseous CO2. As shown in the phase
diagram of CO2 shown in Figure 1, to liquefy CO2, it needs to be refrigerated and pressurized to0.52 MPaA or higher. Therefore, for ocean transportation of liquefied CO2, it needs generally to bekept in a temperature range of -55°C to -20°C and a pressure range of a little less than 1.0 MPaA to2.0 MPaA. For this reason, materials with high strength and suitable low temperature
characteristics are required for a liquefied CO2 storage cargo tank. Generally, the Type-C tanksystem that conforms to the pressure vessel standard among the independent tank systems specified
in the IGC code (formal name: The International Code for the Construction and Equipment of*1

Head of Group, Strategic Planning & Operation Office, Mitsubishi Shipbuilding Co., Ltd.

*2Manager，Strategic Planning & Operation Office, Mitsubishi Shipbuilding Co., Ltd.

*3Strategic Planning & Operation Office, Mitsubishi Shipbuilding Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
2Ships Carrying Liquefied Gases in Bulk) is applied.
If the pressure and temperature of liquefied CO2 fall to the triple point shown in Figure 1
during operations such as transport and loading/unloading, dry ice may be generated, which may
cause operational problems such as blockage of piping. Therefore, within the pressure andtemperature condition range of the transported liquefied CO2 described above, it is necessary toselect actual design condition in consideration of various factors.
For example, to reduce the risk of dry ice generation during operations, it is generally
desirable to transport liquefied CO2 with the pressure kept as high as possible to provide asufficient margin against the triple point. However, in such cases, the cargo tank needs to be able towithstand relatively high pressure and its size is limited from a structural strength standpoint. On
the other hand, to improve the transportation efficiency, it is desirable to keep the liquefied CO2
pressure as low as possible to increase relatively the volume of the cargo tank. However, in suchcases, the liquefied CO2 during transportation and loading/unloading will be under conditions
closer to the triple point, so careful attention is required during operations to prevent the generationof dry ice caused by the conditions falling to the triple point. In addition, since the triple point isgenerally affected by the properties (impurity composition) of CO2, it is necessary to consider thedifference in properties between pure food-grade CO2 and CO2 captured from plants for CCUS.
Furthermore, the density of liquefied CO2 is larger than that of other liquefied gases, about
1.1 ton/m3 to 1.2 ton/m3, and the draft of a CO2 carrier tends to be deeper, so it is necessary todevelop a ship dimension that takes into consideration the compatibility with existing terminals.
In this way, in the development of liquefied CO2 carriers, it is necessary to select theconditions for liquefied CO2 to be transported, as well as the optimum ship and cargo conditions
throughout the CCUS value chain, consider the appropriate cargo tank material and structure, anddevelop process designs and operational processes to avoid or mitigate the risk of dry icegeneration.

Figure 1CO2 phase diagram

In order to be liquid state , CO2 needs to be kept in a low-temperatureand high-pressure state.

|3. Technological development status of liquefied CO2 carrier
We have been studying liquefied CO2 carriers in anticipation of the times so far, utilizing its
knowledge of advanced structural analysis technology, material evaluation ability, and gas handlingtechnology cultivated through the building of liquefied gas carriers (LNG carriers and LPG
carriers), and also participated in the IEA (International Energy Agency) Greenhouse R&D
program in 2004 to study sea transportation of CO2. In particular, in response to the growing
expectations of the CCUS value chain for the realization of a carbon neutral society in recent years,
we are accelerating the development of various technologies in order to bring liquefied CO2

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
3carriers to the market soon. The following sections describe the development status of a cargocontainment system and cargo handling system as examples.

3.1

Development of cargo containment system

As described above, since it is necessary for the liquefied CO2 carrier to keep the cargo, CO2,
in a low-temperature and high-pressure state, materials having high strength and suitable lowtemperature characteristics are used for the cargo containment system. Possible material options
include heat-treated steel, low-temperature steel, and Ni steel, and the most suitable material fromthe viewpoint of safety and economy varies depending on the liquefied CO2 transported and theconditions required for the ship. We have acquired abundant and extensive knowledge of steel
materials for ships, including knowledge of materials applicable for low-temperature conditions
obtained through the development of liquefied gas carriers and knowledge of high-strength
materials obtained through the development of large containerships. Also in the development ofliquefied CO2 carriers, we are selecting the most suitable materials for the cargo containment
system quickly and accurately to meet various demands that differ depending on the customer andthe market.
Although the Type-C independent tank system specified in the IGC code, which is generally
adopted for cargo tanks of liquefied CO2 carriers, is well proven for small LPG carriers, etc., theadoption of this tank type to liquefied CO2 carriers requires appropriate consideration of the impactof greater specific gravity of the liquid cargo (more than twice compared to LPG). For this reason,
we ensure the reliability of the entire cargo containment system by conducting detailed numericalanalysis of structural strength including the tank support structure (Figure 2).

Figure 2 Example of structural strength analysis ofliquefied CO2 cargo tank
Detailed analysis including the tank support structure is carried out toensure reliability.

Furthermore, one of the issues for onboard transportation of liquid cargo that needs to be
fully considered and requires measures is sloshing, which is a phenomenon where the fluid inside atank oscillates greatly when the tank is fluctuated by external forces. Especially in the case of liquidcargo carriers, the fluid load due to sloshing increases when the period of the ship motion and thenatural period of the free surface in the tank are synchronized, which may damage the structure andother fittings inside the tank. Since the specific gravity of the liquid also affects the fluid load(pressure) generated when this sloshing occurs, we conduct detailed numerical simulation by using
Computational Fluid Dynamics (CFD) calculation (Figure 3). First, the ship motions are estimatedunder various sea conditions, and then the sloshing behavior in the cargo tank excited by the ship
motion is predicted by CFD calculation. Based on the result thereof, the pressure of liquefied CO2
acting on the structure and other fittings inside the tank is appropriately evaluated and necessary
measures are taken.
In addition to the examples shown above, we conducted comprehensive technicalverifications such as appropriate evaluation of the effects of thermal stress caused bylow-temperature cargo. As a result, our cargo containment system for liquefied CO2 carrier has
obtained Approval in Principle (AIP) from Bureau Veritas.

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
4Figure 3

CFD sloshing simulation of liquefied CO2 cargo tankPredicting sloshing behavior due to ship motion and evaluating the generatedpressure

3.2

Development of cargo handling system

As explained in chapter 2, the onboard transportation of liquefied CO2 needs to maintainconditions that can provide a certain margin against the triple point in the phase diagram. Sinceliquefied CO2 during transportation and loading/unloading is subjected to various changes inconditions (pressure and temperature) in the cargo tank and piping, it is necessary in various
operation processes to properly anticipate changes in the conditions and give consideration so thattransportation and loading/unloading can still be carried out without causing the conditions to fallto the triple point.
Therefore, we have conducted dynamic process simulations in various operations to identify
risks that the pressure or temperature of the liquefied CO2 falls to the triple point, and consider
measures against the risk. For instance, Figure 4 shows examples of data variations at unloading
operation obtained by a simulation using a process simulator ASPEN PLUS DYNAMICS. This
simulation model consists of elements such as tanks, pipes, valves, cargo pumps and vaporizer, andthe characteristics of each element are properly applied.
In this way, now we are continuously studying to predict liquefied CO2 behavior in detail
generated in each part of the handling system in various operations and to incorporate them intoprocess design and handling operations of liquefied CO2.

Figure 4

Examples of data variations at unloading liquefied CO2

Predicting changes in the pressure, etc., during operation and avoiding the risk of dry ice generation|4. Initiatives in liquefied CO2 transportation demonstration projectfor NEDO
We have signed a contract with Sanyu Kisen K.K., which performs ship management fordomestic and ocean-going ships and other business, to build a demonstration ship for liquefied CO2transportation to be used in Research, Development and Demonstration of CCUS Technology,Large-scale CO2 Capture and Storage (CCS) Demonstration Testing at Tomakomai, and CCUS
R&D and Demonstration Related Project / Large-scale CCUS Demonstration Project inMitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
5Tomakomai / Demonstration Project on CO2 Transportation by New Energy and Industrial
Technology Development Organization (NEDO). Table 1 shows the principal particulars of thedemonstration ship, and Figure 5 shows the schematic external view.
This NEDO project aims to establish liquefied CO2 onboard transportation technology forrealization of large-capacity and long-distance CO2 transportation. Engineering Advancement
Association of Japan, one of the contractors of the NEDO project, will charter the demonstration
ship from Sanyu Kisen K.K.,and transport liquefied CO2 for the project.
The demonstration ship will be delivered in the second half of fiscal 2023 prior to a liquefied
CO2 carrier built for the Norwegian "Northern Lights Project", one of the world's leading CCS(CO2 capture and storage) projects, and is expected to be the world's first liquefied CO2 carrier forCCUS.
We are in charge of the entire design and construction of the demonstration ship, including
the cargo tank system installed thereon, utilizing the knowledge cultivated so far in the constructionof liquefied gas carriers.
In the development of the demonstration ship, we will develop a cargo management systemthat can deal with the technical issues mentioned in chapter 2, especially the CO2 component
unique to CCS, which has properties different from those of food, for avoiding and responding
immediately to the generation of dry ice, and install it on the ship. Then we will verify the effect ofthe development during the subsequent demonstration period to establish the world's firstlarge-capacity and long-distance CO2 transportation technology.
Table 1 Principal particulars of liquefied CO2
demonstration test ship for NEDO
Registration
Length overall
BeamDraft
Tank capacityJapan
72.0 m12.5 m
4.55 m
1,450 m3

Figure 5 Conceptual image of the liquefied CO2 demonstration ship forNEDO
Establishing the world's first large-capacity and long-distance CO2 transportation
technology through the demonstration project|5. Development of onboard CO2 capture system
As part of the above-mentioned MHI Group's aim of building a CO2 ecosystem, we are also
working on the development of onboard CO2 capture systems to reduce CO2 emitted by ships. This
chapter introduces our latest initiative related thereto, an onboard CO2 capture systemdemonstration project “CC-Ocean (Carbon Capture on the Ocean)”*1, which was jointly
implemented with Kawasaki Kisen Kaisha, Ltd. and Nippon Kaiji Kyokai.
In this project, we conducted a demonstration test of a small-scale CO2 capture
demonstration plant (hereinafter, demo-plant) under commercial operating conditions, which wasthe first time in the world, by installing the demo-plant on a coal carrier "CORONA UTILITY"
operated by Kawasaki Kisen, Ltd. in order to verify CO2 capture technology offshore and deal with

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6

requirements for onboard use of the technology.
The project was conducted over a two-year period. A HAZID evaluation of the demo-plant
and a safety evaluation of the equipment and system were conducted by Nippon Kaiji Kyokai, andthen the demonstration plant was fabricated, installed on the coal carrier, and operated in an
offshore environment for approximately six months for measurement and checking of theperformance. The demo-plant installed on the coal carrier is a CO2 capture system that is originallyfor onshore plant exhaust gas treatment employing the chemical absorption method and has been
converted for onboard installation. Figure 6 shows a schematic illustration of the demo-plant.
In this project, the demo-plant achieved higher performance values than planned for CO2
capture amount, CO2 capture rate and captured CO2 purity, and the equipment could be operatedand maintained by the crew of the coal carrier without any problem. We also verified the effects ofengine load fluctuations and ship motion on the CO2 capture performance and the effects of exhaustgas on the CO2 absorbent. We summarized the guidelines for safety measures associated with shipoperations, such as measures against leakage of the CO2 absorbent, ventilation concepts forequipment installation areas, etc., as requirements for operating CO2 capture systems in an offshore
environment.
In the future, based on the knowledge and technical issues obtained and found in thisdemonstration, we will promote efforts toward the commercialization of the onboard CO2 capturesystem through establishing the concept of the entire system including liquefaction and storage
onboard, examining the optimization of the system as an offshore system, considering theunloading of CO2 onboard to land, etc.
*1 This project is supported by Assistance Project for Research and Development of Technological
Advancements in Marine Resource Development, a project subsidized by the Maritime Bureau of theMinistry of Land, Infrastructure, Transport and Tourism.

Figure 6 Illustration of installed small demonstration
plant of onboard CO2 capture system
We were the first in the world to demonstrate CO2 capture onboard
and confirm the performance.

|6. Conclusion and future prospect
This report described liquefied CO2 carriers' role in the realization of a carbon neutral society
and their future demand, and presented the required technical elements, our development thereof
and our efforts on the technology for CO2 capture from ships.
In the future, we will develop and hold CO2 handling technology in a world-leading way,taking advantage of the opportunity to design and build the world's first liquefied CO2 carrier forCCUS. And by selecting the optimum transportation conditions from a bird's-eye view of the entire
CCUS value chain through collaboration with the MHI Group's products, we will provide optimal
total solutions for business operators.
Mitsubishi Shipbuilding co., Ltd, which plays a part in the energy transition strategy
promoted by the MHI Group, established the “Marine Decarbonization Business Development
Group” on February 1, 2022. By integrating the functions of technology development, market
research, and business strategy planning to execution in this organization to acceleratetechnological development and new business creation in the decarbonization field with use of theMitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
7

advanced technology cultivated in our long-established shipbuilding experience, we aim to respond
immediately to global trends and market needs toward the realization of a carbon neutral society inthe maritime industry.