CO₂ Marine Transportation from a Techno-Energetic Perspective ^†

Abstract

CCUS (Carbon Capture, Utilization, and Storage) is a cornerstone of most proposed carbon dioxide (CO₂) emissions strategies, as it is necessary to keep atmospheric CO₂ concentrations below 450 parts per million by the year 2100 and, as such, prevent global warming. The Intergovernmental Panel on Climate Change (IPCC) predicts a removal capacity of 12 GtCO₂/yr by 2050, whereas the present capability is 41 MtCO₂/yr. Decarbonization may not be able to proceed quickly enough to reach net-zero emissions without CCUS technologies. In the maritime sector, CCUS serves a dual purpose: capturing CO₂ from fossil fuel combustion and transporting the captured CO₂ for its storage or utilization. This paper examines the importance of vessels as liquid CO₂ carriers, emphasizing the transportation conditions associated with CO₂. A techno-energetic analysis is carried out by studying various combinations of temperature and pressure. From a transport viewpoint, the findings indicate that reducing CO₂ pressure is more cost-effective. In terms of pre-processing, higher CO₂ pressures may lead to energy and, potentially, cost savings. However, the optimal pressure in the entire logistical chain remains uncertain. Further research is advised to broaden the scope of the chain to be analyzed.

1. Introduction

Carbon dioxide (CO₂) plays a crucial role in the process of photosynthesis, which is essential for the growth of plants and, consequently, vital for the existence of animal life on Earth [1]. Additionally, CO₂ serves as the primary greenhouse gas (GHG). GHGs absorb and emit infrared radiation from the Sun, warming the Earth’s surface and the lower levels of the atmosphere [2]. Naturally occurring, it historically constituted roughly 300 parts per million (ppm) or 0.03% of the Earth’s atmosphere. During ice ages, concentrations remained at approximately 200 ppm, while in interglacial periods, they declined slightly below 300 ppm. Scientists largely attribute this surge in CO₂ concentration to human activities, and it is recognized as the primary contributor to global warming [3].

The influence of human activities is believed to have led to an increase of about 1.0 °C in global temperatures above pre-industrial levels, with a probable range of 0.8 °C to 1.2 °C. If the current trend persists, the Intergovernmental Panel on Climate Change (IPCC) predicts that global warming is likely to reach 1.5 °C sometime between 2030 and 2052 [4].

The majority of CO₂ emission reduction models endorsed by the IPCC necessitate substantial reliance on CCUS. As per the IPCC, the adoption of Carbon Capture, Utilization, and Storage (CCUS) is imperative to keep the atmospheric CO₂ concentration below 450 ppm by the year 2100 [5].

Det Norske Veritas (DNV) reports that existing CCUS facilities worldwide can capture 41 MtCO₂/yr, just 0.1% of total CO₂ emissions [6]. The global project pipeline now represents over 400 MtCO₂/yr capture capacity expected to be online by 2030 [7]. However, the average of IPCC’s global net CO₂ emissions scenarios anticipates that the energy sector alone must hold a sequestration capacity of 12 GtCO₂/yr by 2050. Consequently, carbon capture technologies are crucial for the decarbonization process, and achieving zero net emissions quickly might be unattainable without their contribution.

It is important to acknowledge that while CO₂ is a tradable commodity, it lacks an established market. Additionally, Carbon Capture and Utilization (CCU) serves as a supplementary approach rather than a substitute for Carbon Capture and Storage (CCS), according to the IEA [8]. The impact of CCU on international CO₂ emissions reduction is expected to be minimal, with approximately 0.2 GtCO₂/yr by 2050, and it would not rival CCS due to its significantly higher CO₂ capture capacity, which is expected to reach 7.8 GtCO₂/yr by 2050 [9]. In the Net Zero Scenario published by the IEA [10], almost 90% of CO₂ sourcing from Bio-Energy with Carbon Capture (BECC) and Direct Air Capture (DAC) is sequestered, with less than 15% used as feedstock for other products (for example, CO₂-derived fuels).

CO₂ transportation by ship is anticipated to assume a significant role in the initial phases of CCS development, especially for modest capacities and/or for long-distance transportation [11]. The Global CCS Institute [12] states that CCUS technological advances facilitate both the capture and transportation of CO₂ within maritime operations. Firstly, vessels fitted with these technologies have the capability to capture CO₂ emissions generated through the combustion of hydrocarbon-based fuels onboard. This process involves the utilization of scrubbers, which are presently employed for purifying emissions from exhaust gases and can be adapted for CO₂ capture. These technologies would enable shipping companies to extract substantial amounts of CO₂ from their exhaust emissions. Secondly, ships can convey the captured CO₂ to its designated delivery location. Providers of technology have devised safe methods for CO₂ storage during ship transportation, thus ensuring that the pressure and temperature are appropriate, which is comparable to storage solutions for substances like liquid petroleum gas (LPG). These similarities with LPG are also applicable to port infrastructures, including facilities for CO₂ liquefaction, temporary storage, and loading/unloading [13]. As noted by Xing et al. [14], shipowners have various materials to choose from for CO₂ tanks and can enhance cargo hold volume by employing a single sizable tank or multiple smaller tanks. CCS technology integration in maritime applications is at early development stages, and its future sustainability depends on a combination of considerable scientific advances and supportive regulatory measures.

The transportation phase in the carbon value chain acts as the bridge between emission sources and storage locations. In addition to pipelines, CO₂ transportation by ship offers a versatile and expandable CCS platform capable of accommodating upcoming initiatives. Vessels are particularly advantageous for handling CO₂ sources that may not warrant the installation of a dedicated pipeline due to its size or capacity.

Gas transported at pressures near atmospheric levels requires very large facilities due to its expansive volume. However, by compressing the gas, it occupies less space and can be transported through pipelines. Further reduction in volume can be achieved through different processes [15].

CO₂ can exist in either a gaseous or solid state depending on the temperature at atmospheric pressure. Only reducing the temperature at sea level pressure will not result in the liquefaction of CO₂. CO₂ as a liquid can only be achieved through a relatively low temperature combined with a pressure level significantly higher than atmospheric pressure [16], as shown in Figure 1. CO₂ can be liquefied at different pressures within the range spanning from the triple point (5.18 bar, −56.6 °C) to the critical point (83.8 bar, 31.1 °C). Compression of CO₂ can induce a supercritical state characterized by increased density, thereby avoiding bi-phase flow conditions when subjected to pressures beyond its critical pressure and temperature [17].

At this moment, CO₂ can be transported in three distinct forms to either offshore subterranean storage facilities or onshore reception facilities [19]:

• Gaseous transportation: CO₂ is carried via pipeline, using intermediate boosters, after being compressed to 35 bar;
• Liquid transportation: Pipeline or ships are used to convey compressed CO₂;
• Supercritical transportation: Pipelines are used to convey CO₂ that has been compressed to 250 bar.

CO₂ transport by ship is a mature technology founded on the shipping expertise in the alimentary sectors. It has been used on a small scale for more than 30 years, with just 3 MtCO₂ per year. In light of the fact that CO₂ shipping may be more cost-effective than building new extended-range pipelines or adapting gas pipelines at current loading facilities and discharge platforms, CO₂ shipping is now taken into consideration for large-scale CO₂ transport [20].

The most obvious option for ship transportation is liquefied CO₂, although ships that transport compressed, gas phase CO₂ have also been proposed. The transportation of compressed CO₂ is comparable to the transportation of CO₂ via pipelines. As a result, the circumstances of transportation will be similar to those of pipelines, but more flexible and straightforward to inspect. The pressure should be above 75 bar, and the temperature should be about 25 °C as a supercritical liquid. Shipping companies have studied the concept of compressed CO₂ on board, but it has not been proven, and there are no international standards for CO₂ transport of this kind [21].

The majority of the literature suggests CO₂ to be transported as a liquid in conditions close to the triple point for the advantages of increased density and lower storage expenses [22]. Other research, however, points to a higher liquefaction pressure as a means of achieving greater energy efficiency. There is therefore no ideal liquefaction pressure that applies to all circumstances; rather, it should be determined based on the needs of each individual as well as the larger chain and project characteristics [23].

The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) is applicable to new gas carriers that are constructed after 1986. Since this year, all new ships must comply with the IGC Code, as required by the Safety of Life at Sea Convention (SOLAS) revisions [24]. According to Kokubun et al. [25], due to the physical characteristics of CO₂—notably, the vapor liquid equilibrium properties—the design of a liquid CO₂ (LCO₂) tank is remarkably similar to intermediate-pressure LPG onboard storage systems. Different international standards, such as the IGC Code, as well as those of classification societies like Lloyd’s Register (LR), Det Norske Veritas (DNV), and Bureau Veritas (BV), govern the well-established design process for LPG cargo tanks.

As previously stated, the insufficient number of LCO₂ carriers precludes a meaningful comparison. Because of this, it is thought that similar ships might serve as a useful foundation for comparisons pertaining to energy and techno-energetics. Similarities between LCO₂ carriers and LPG carriers (pressurized or semi-pressurized) for LCO₂ transportation can be found. Analogously, similarities can be found between Compressed Natural Gas (CNG) carriers and carriers for CO₂ transportation as a compressed gas. Nevertheless, the world fleet now consists of just one CNG ship [26], and CO₂ transportation as a gas has not yet been developed as a workable alternative.

This article aims to determine the most advantageous pressure range for liquid CO₂ transportation in vessels covering the liquefaction of CO₂ prior to its loading on board and during its transportation by ship. To this end, a techno-energetic study comprising a series of indicators is conducted. To achieve this, two different tank configurations (described in Section 2), a common model ship, and a specific range of pressures (specified in

Table 1. Thermophysical properties of saturated liquid CO₂ for a range of storage pressures from 6 to 45 bar [27].

Case No.	Pressure, bar	Temperature, °C	Density, kg/m3
#1	6	−53.12	1166.00
#2	10	−40.12	1116.90
#3	15	−28.52	1069.50
#4	20	−19.50	1029.40
#5	25	−12.01	993.20
#6	30	−55.52	959.25
#7	35	0.16	926.47
#8	40	5.30	894.05
#9	45	9.98	861.27

) will be considered. The pressure range studied and the CO₂ conditions as a saturated liquid are listed in Table 1.

As it will be explained in subsequent sections, the pressures considered in this table are segmented into two storage arrangements: bilobe tanks and vertical cylinders. The first one, the bilobe tank, as it will be detailed in Section 2.1, is quite common in LPG ships; the second one is proposed for higher-pressure storage of CO₂, as the high pressures would require excessive thickness in the walls of the bilobe tanks for their fabrication. Some of the pressures listed in Table 1 are shared between the two studied storage arrangements.

2. Methods

To conduct the techno-energetic assessment of the different cases defined in Table 1, the boundary conditions related to ship size must be harmonized first. In shipping, it is easy to find economies of scale in ships of larger sizes [28]. To avoid unfair comparisons between the different cases, it is required to set the ship size and the other geometrical constraints before comparing them. Otherwise, it could be easy to fall into misleading results due to the possible economies of scale between alternatives. To solve this problem, the method considered consists of two steps:

• First, choose a representative ship for marine transport of CO₂ in the liquid phase. It is described in Section 2.1;
• Second, some Key Performance Indicators (KPI) are developed to perform the techno-energetic assessment of the different cases defined in Table 1. They can be found in Section 2.2.

2.1. Ship and Alternatives Definition

For harmonization and to avoid economies of scale, a model vessel has been chosen for the comparison of the transportation pressures outlined in Table 1. The model ship chosen after an exhaustive search within the annual publication “Significant Ships” by The Royal Institution of Naval Architects (RINA) [29] is the LPG carrier “Alkaid”.

The vessel was specifically designed for transporting different liquefied gases. It has four cargo holds that can each carry an International Maritime Organization (IMO) type C independent tank, which is bilobe-shaped. The storage pressure is in the range of 5–5.5 bar, and there is a minimum cargo temperature of −40 °C. More technical details and ship drawings can be found in

Table 2. General information and characteristics of “Alkaid” [29,30].

Characteristic	Data
IMO number	9,655,509
Length overall (m)	159.97
Length between perpendiculars (m)	152.20
Breadth (molded) (m)	25.60
Depth (molded) (m)	16.40
Draught (scantling) (m)	10.90
Deadweight (design) (t)	13,650
Deadweight (scantling) (t)	22,700
Cargo capacity (m3)	20,800
Energy Efficiency Design Index (EEDI) (gCO2/(t·NM))	10.7

and Figure 2.

Figure 3 displays the measurements of the LPG tanks (No. 2–4) and cargo holds. The fore tank (No. 1) is assumed to have the same capacity and shape as the cargo tanks 2–4 for computation reasons. A control volume has been established as a restriction for the configurations presented considering the bilobe tank’s maximum dimensions, as defined in Figure 3 and

Table 3. Main dimensions of control volume.

Dimension	Value
Length (m)	22.55
Breadth (m)	22.70
Height (m)	13.50

. The alternative tanks can fully utilize the entire control volume irrespective of their configuration (bilobe, cylindrical), orientation (vertical, horizontal), or thickness, to create a standard basis for comparison.

The new bilobe tanks are placed horizontally, like in the “Alkaid”. Due to the higher case pressures under consideration compared to the pressure for LPG in the model vessel tanks, a greater wall thickness will result in a decrease in the CO₂ cargo net volume. The main parameters of the newly designed bilobe storage tanks are summarized in

Table 4. Main characteristics of bilobe tanks.

Characteristic	Value
Number of tanks per cargo hold	1
Total number of tanks per ship	4
Length (thickness included) (m)	22.55
Breadth (thickness included) (m)	22.70
Height (thickness included) (m)	13.50
Main axis direction	Horizontal

. The range of pressures studied for bilobe tanks includes the pressures covered in Table 1 from 6 bar to 25 bar. The reason why the remaining higher pressures have not been included in the study of bilobe tanks is due to the fact that the higher pressures would require thicker walls that would make the manufacturing difficult.

An alternative configuration for storage that is compatible with higher pressures involves placing the CO₂ storage tanks in smaller-diameter cylindrical tanks within the same control volume. The tanks calculated in this article are arranged inside the cargo holds either horizontally or vertically. There are different variables involved. such as the diameter and number of tanks per cargo hold. Therefore, it is important to carefully evaluate the various possible designs. Considering the management of boil-off, prevention of sloshing (the violent motion of liquids with a free surface), and stability issues, an arrangement of vertical cylinders seems the sensible option, as it reduces the free-surface area. Furthermore, a clearance distance between cylinders has been assumed for isolation purposes and structural supports.

Table 5. Main characteristics of the vertical cylindrical vessels configuration.

Characteristic	Value
Number of tanks per cargo hold	36 (6 × 6)
Total number per ship	144
Length (thickness included) (m)	13.5
Diameter (thickness included) (m)	3.5
Main axis direction	Vertical

summarizes the main characteristics of the vertical cylinder configuration. Considering the limited free-surface area and maintaining a manageable amount of auxiliary equipment, piping, and other components (pumps, valves, and manifolds) for each tank, a tank arrangement of 6 per 6 rows on each cargo hold has been established. The range of pressures studied for cylindrical vessels extends from 10 to 45 bar.

The proposed CO₂ storage alternatives are based on ships that are currently in service or projects that are under development, like the cylindrical tanks from KNCC and the bilobe tanks from Samsung Heavy Industries and Handwa Ocean, which were all given DNV’s Approval in Principle in 2022 and 2023, respectively.

The American Bureau of Shipping (ABS) [31] considers that recognized pressure vessel standards must be met by both bilobe and cylindrical tanks, such as the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), and additional classification societies and statutory regulations. The thicknesses of the domes and body are calculated according to ASME VIII code Div. 1. The main considerations are listed:

• A thin-walled cylinder with a radius of 6.75 m has been used to compute the bilobe tank thickness in order to avoid complicated strength calculations using Finite Elements Analysis (FEA).
• A welded joint efficiency factor of 0.875.
• A corrosion allowance of 1 mm.
• Both types of tanks, bilobe and cylindrical, are built from American Society for Testing and Materials (ASTM) A537 Class 2, a quenched and tempered carbon steel. This material boasts a higher yield and tensile strength, making it ideal for the fabrication of pressurized vessels and steel boilers. Additionally, it is able to withstand unusually low service temperatures down to −60 °C. The mechanical properties of the material are listed in

  Table 6. ASTM A537 class 2 mechanical properties [32].

  Material	Thickness, mm	Yield Strength, MPa	Tensile Strength, MPa
  ASTM A537 cl. 2	Below 65	415	550
  	>65 <100	380	515
  	>100	315	485

  .

2.2. Key Performance Indicators Definition

A set of Key Performance Indicators (KPIs) has been chosen to perform the techno-energetic assessment. The following KPIs are listed and described while taking into consideration the pressures defined in Table 1.

• Liquefaction;

  ○

  KPI 1: Thermomechanical exergy;

• Transport;

  ○

  KPI 2: Mass of CO₂;

  ○

  KPI 3: Mass of CO₂ to mass of tank structure;

  ○

  KPI 4: Volume of tank to volume of cargo hold;

  ○

  KPI 5: Energy Efficiency Design Index (EEDI)-based indicator.

The first indicator studied is the thermomechanical exergy of the CO₂. It represents the minimum work required to change a substance from the restricted dead state to a particular state using the ambient as the only heat source [33,34]. This indicator has been deemed of interest to perform a semi-qualitative assessment of the energy required in the preprocessing of CO₂ from a gaseous state at ambient conditions to a liquid state in the cases listed in Table 1. It is defined by Equation (1), where T₀ is the reference temperature of the surrounding environment (also known as the “restricted dead state”), index 0 denotes the values of the parameters when the system is in thermomechanical equilibrium with the environment, and U, V, and S represent the internal energy, volume, and entropy of a closed system that is not in equilibrium with the ambient. The restricted dead state conditions are described in

Table 7. Conditions of the restricted dead state.

Property	Value
Pressure (kPa)	100
Temperature (K)	288.15

and have been extracted from the NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) [27].

$$Ex=\left(E-U_0\right)+p_0\left(V-V_0\right)-T_0\left(S-S_0\right)$$

(1)

Section 2.1 defines two tank types suitable for transporting liquefied CO₂. This Section estimates their mass capacity for different pressures and temperatures. Considering the size restrictions imposed by the control volume, head and body thicknesses are calculated, as well as volumes, net and gross, mass of CO₂, and mass and volume ratios. To adequately compare the different cases defined in Table 1, the following assumptions are considered:

• The total mass of the tanks and LPG cargo of the model vessel is fixed;
• The calculated mass of CO₂ (to be called “Maximum CO₂”) and the tank structure cannot be higher than the “Alkaid’s” mass, as defined in Equation (2). Hence, the excess CO₂ mass will be subtracted from the CO₂ mass and considered as a cargo loss. The same will apply in the opposite scenario if less CO₂ mass can be transported due to its conditions (pressure, temperature, and density).

$$m_{LPG}+m_{LPG Tanks}=m_{{CO}_2}+m_{{CO}_{2}Tanks},$$

(2)

An indicator based on the Energy Efficiency Design Index (EEDI) is the last KPI considered in this article. The reason to use an indicator based on EEDI instead of the Energy Efficiency Existing Ship Index (EEXI) is due to the fact that even if an existing ship has been used as a reference ship, the ships under study are non-existing. The EEDI gives a precise value measured in grams of CO₂ emitted per ship’s capacity-mile for every individual ship design. This KPI allows for a semi-qualitative comparison of the specific energy consumed per unit cargo and distance, while being at the same time a common indicator in maritime transport. Its computation considers the technical design specifications of a given ship. In accordance with the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI revisions, all ships at Marine Environment Protection Committee No. 62 must comply with EEDI [35].

Equation (3) describes the EEDI calculation. It considers the ship’s speed, deadweight, and emissions [36]. A ship’s energy efficiency and environmental effect are directly correlated with its EEDI score. A ship’s EEDI cannot exceed a certain threshold in order to comply with IMO standards, which mandate that they satisfy a minimum energy efficiency criterion.

$$EEDI=\frac{P\times C\times SFC}{f\times DWT\times V_{ref}}$$

(3)

In Equation (3), P is the individual engine power at 75% of the maximum continuous rating (MCR) in kW, C is the CO₂ emission factor based on the fuel type used by the given engine in g/kWh, SFC is the specific fuel consumed per unit of engine power in g/kWh, $f$ is the non-dimensional factors that were added to the EEDI equation to account for some specific existing conditions, Vref is the ship speed at maximum design load condition in kn, and DWT is the deadweight tonnage of the ship in tons. The deadweight of a ship is the difference between the displacement and the mass of the empty vessel (lightweight). It includes the cargo but also the stores, ballast water, provisions, and crew [37].

The modification of $KPI 5$ with respect to the EEDI is that the DWT is replaced by the CO₂ cargo mass transported instead, because the DWT includes other factors that would smoothen the EEDI variations. With the exception of the mass of the cargo, all other equation-related parameters are assumed to remain identical to the model vessel’s and the cases in Table 1. Consequently, the relationship described in Equations (4) and (5) remains valid.

$${\left(EEDI\times DWT\right)}_{ALKAID}={\left(KPI 5\times Mass of cargo \right)}_{CASE}$$

(4)

$$KPI 5=\frac{{EEDI}_{ALKAID}\times {DWT}_{ALKAID}}{{Mass of cargo}_{CASE}}$$

(5)

where the units of $KPI 5$ are (gCO₂/t_cargo·nm). It can be noted that instead of DWT, it refers to the mass of the cargo only.

3. Results and Discussion

The results obtained in this work are displayed in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9, providing an overview and a detailed breakdown of the findings.

Figure 4 shows the specific thermomechanical exergy for the different alternatives studied, regardless of the packing circumstances. There is more exergy at low pressures and temperatures than there is at higher pressures and temperatures, considering the restricted dead state defined in Section 2. This suggests that as the pressure rises, the expected energy required to move CO₂ from a restricted dead state to the saturated liquid state at the corresponding pressure decreases. Regarding Case No. 1 (6 bar, −53 °C), the exergy is almost 6.5% higher than Case No. 9 (45 bar, 10 °C). Figure 4 shows that from the exergetic point of view, high pressures for liquefaction are more advantageous than the combination of low pressures and temperatures, close to the triple point. The evaluation indicates that the procedures involved in producing liquid CO₂ at 10 bar are anticipated to demand comparatively more energy than the alternatives at higher pressures. An efficient use of energy at this juncture is crucial, given that it carries an associated cost that will have ramifications throughout the entire operational lifespan of the logistical chain, which may extend over a 30-year period easily.

Different characteristic masses are presented in Figure 5 for bilobe tanks and in Figure 6 for cylindrical tanks. The maximum mass for storing CO₂ in both bilobe and cylindrical tanks, based on tank strength calculations, is represented in dark blue and referred to as “maximum CO₂”. The mass of tanks is labeled “Steel”. In all cases considered for bilobe tanks, the maximum CO₂ value is higher than “Alkaid’s” sum of LPG cargo and tanks. As defined in Section 2.2 and Equation (2), to ensure a consistent comparison, the overall mass balance must remain constant relative to “Alkaid”. Hence, any surplus mass compared to the model vessel is identified and discounted and is presented in blue, simply labeled “Excess of CO₂”. Therefore, the transported mass of CO₂ considered in this work’s discussion is labeled “CO₂”.

In contrast to the case of bilobe tanks, the vertical cylinders configuration needs additional ballast when the pressure is below 35 bar so that the balance of mass remains constant. This is because of the reduced storage volume utilization. The loss of CO₂ mass corresponds with the added ballast, as shown in Figure 6.

In all of the cases, the higher the pressure, the lower the mass of CO₂ that can be transported. This is due to the reduction in density with increasing pressure and temperature. Exclusively regarding the amount of CO₂, the best option is bilobe tanks at a pressure of 6 bar. With this option, a mass of 21,318 t can be transported while still meeting the balance of mass limit.

As illustrated in Figure 7, when focusing solely on the mass of the transported CO₂, it becomes apparent that from the 10–15 bar range, cylindrical tanks are preferable due to the rapid decline in CO₂ storage capacity in bilobe tanks at operating pressures beyond this range. However, a detailed calculation of a bilobe tank may result in a higher decisive pressure range, which may be in the range of 15 to 20 bar. Only considering CO₂ storage pressure, lower pressures prove to be more advantageous, as they allow a higher mass of CO₂.

The relation of CO₂ mass to tank mass and the ratio of tank volume to cargo hold volume are represented in Figure 8. These two metrics serve as measures of effectiveness in the realms of mass and volume. For the two KPIs, a higher ratio indicates improved efficiency. On the other hand, a small mass ratio suggests that a greater amount of structural mass is being transported relative to the CO₂. Similarly, a small volume proportion indicates that there is a greater amount of unused space in comparison to the utilized space. In this figure, it is observed that higher pressures lower the efficiency. As anticipated, cylindrical tanks make much less efficient use of cargo space compared to bilobe tanks. Nevertheless, bilobe tanks capitalize on this benefit at low pressures, as structural mass grows quickly. For example, at 25 bar, the mass ratio of the bilobe tank is only 1.4. Focusing on these KPIs, CO₂ transportation at lower pressures will likely result in a reduced cost of acquisition per unit mass of transported CO₂ because the ship’s structural mass significantly influences its final cost. The lower ratios obtained from this approach strongly suggest that the base dimensions of a typical LPG ship may not be optimal for CO₂ transport. Therefore, it can be expected that Figure 2 and Figure 3 would exhibit a different geometry if a specialized bulk CO₂ carrier were designed.

The final KPI is the one based on EEDI, which is shown in Figure 9. It considers the mass of CO₂ for both types of tanks as well as the assumptions listed in Section 2. As the storage pressure rises, the EEDI-based indicator value also increases. As defined in Section 2, all of the parameters not related to the cargo holds are assumed to remain identical between “Alkaid” and the studied LCO₂ carrier. It can be seen that raising the pressure (and temperature) reduces the CO₂ mass transported on board, thus increasing the EEDI-based indicator. There are also differences between tank types. The slope of the curve for the bilobe tank is much steeper than that of the cylindrical tank. This is because with an increase in pressure, the increase in structural mass is higher in the bilobe tank, as can be observed in Figure 8a. This illustrates how the ratio between CO₂ mass and structural mass decreases much more rapidly than in cylindrical tanks. In this scenario, a lower value indicates a more efficient ship. This KPI implies that the ship with a 6 bar pressure and a bilobe configuration will consume less fuel per unit distance and unit mass of transported CO₂, likely resulting in a lower freight rate for the low-pressure ship.

From the point of view of CO₂ marine transportation, the storage pressure on board can be a relevant factor in the total cost of the CCUS value chain. Transporting CO₂ at lower pressures allows for carrying a greater mass of CO₂ for both types of tanks and lighter tank structures, which leads to lower shipbuilding costs and higher gains for each voyage. However, lower pressure demands more energy for the liquefaction of CO₂, as the temperature is much lower than the ambient. In contrast, higher CO₂ pressures lead to the opposite effects. The amount of CO₂ transported decreases, and the usage of the cargo hold is worse than with low pressures, but the energy consumed in liquefying the CO₂ is reduced.

4. Conclusions

This article aims to determine the most advantageous pressure range for liquid CO₂ transportation in vessels by covering the liquefaction of CO₂ prior to shipping and its shipment. To this end, this article performs a techno-energetic study through the use of a series of indicators.

The outcomes from the various analyses conducted lead to conflicting conclusions. From an exergetic perspective, liquefying CO₂ at higher pressures within the range studied demonstrates superior efficiency than at lower pressures. It can be expected that liquefaction plants aiming for higher pressures of liquefied CO₂ might present energy savings compared with their counterparts at lower pressures and temperatures. Furthermore, reducing the CO₂ pressure allows for transporting a greater mass of cargo, which likely results in lowered freight rates. Additionally, the decreased structural weight may lead to lower acquisition costs. Given that the entire CO₂ logistic chain involves preprocessing (liquefaction), transport, and post-processing (re-gasification) costs, determining the optimal transport pressure is not straightforward. Modifying the CO₂ transportation pressure can lead to contrary outcomes for the liquefaction prior to shipping and the shipment itself. Therefore, further investigation is needed to uncover the pressure compensation and the factors that influence it by analyzing the complete CCUS sequence.

It is important to carefully study the design and calculation of the storage tank, as simplified models of bilobe tanks may have excessive uncertainties. There is a CO₂ storage pressure beyond which a bilobe tank may not be the optimal choice due to its weight. For higher pressures, the proposed configuration of cylindrical tanks would be a more suitable option, even though they may have lower volumetric efficiency. Additionally, special attention should be given to the transportation of CO₂ at lower pressures. Within this range, there is an elevated possibility of inadvertent solid formation of CO₂, which has the potential to obstruct pipes or cause damage to pumps.

The authors of this work are currently working on the development of techno-economic models to study the complete logistic chain from CO₂ at ambient pressure and temperature, its liquefaction, and its delivery by ship to a CO₂ terminal, with the aim of determining an estimate cost per unit mass of transported CO₂. The readers should expect a follow-up article from the same authors in the near future.