Conceptualization of CO₂ Terminal for Offshore CCS Using System Engineering Process

Abstract

In this study, the basic configuration and operation concept of a CO₂ terminal were identified by conducting a system engineering process. The performance goal of a CO₂ terminal was determined by requirement analysis. Then, functions and timelines were derived by functional analysis to meet the performance goal. Equipment to perform the functions were defined and finally, a process flow block diagram of the CO₂ terminal was acquired. The CO₂ terminal in this study consisted of three parts. First, the CO₂ loading/unloading part is responsible for liquid CO₂ unloading from the carrier and loading vapor CO₂ onto the carrier. Secondly, the liquid CO₂ transmission part extracts liquid CO₂ from the storage tanks and increases the pressure until it satisfies the offshore pipeline transportation condition. The vapor-treatment part collects boil-off gas, generates vapor CO₂, and charges the storage tanks with vapor CO₂ to control the pressure of the storage tanks that discharge liquid CO₂. Finally, the study results were compared with a liquefied natural gas (LNG) terminal. The biggest difference between the CO₂ terminal in this study and the LNG terminal is that a vaporizer is essential in the CO₂ terminal due to the smaller storage capacity of the CO₂ terminal and, therefore, the lower amount of boil-off gas.

1. Introduction

Many efforts are underway worldwide to reduce atmospheric emissions of carbon dioxide (CO₂), which is a dominant cause of global warming. To limit the global temperature increase to below 2 °C, the annual amount of greenhouse gas emissions should be less than approximately 14 Gt CO₂ by 2050, which is about a 60% reduction compared with the current level [1]. Countries that have a manufacturing-oriented industrial structure or rely heavily on electricity production from fossil fuels may struggle to quickly switch to a low-carbon emission structure using renewable energy. Carbon capture and storage (CCS) can be an attractive greenhouse gas reduction option for these countries [2]. CCS basically consists of three parts: (1) separating and capturing CO₂ from a large-scale emitter such as a power plant; (2) transporting CO₂ through pipelines or carriers from capture stations to storage sites; and (3) injecting and storing CO₂ into geological formations [3]. Many studies have focused on the capture and storage aspects, but few have focused on transportation [4,5]. However, interest is growing in the transportation aspect as it has been shown to be responsible for a high proportion of the costs in CCS projects [6].

Between pipelines and carriers, which are the two most common methods of CO₂ transportation, pipelines have been the most widely used [5]. When transporting CO₂ to a pipe, a variable CO₂ transportation network is required for the flexible operation of the CCS chain, which is technically challenging [7,8]. Considering the flexibility of CO₂ carriers, however, the significance of carriers is likely to increase in the near future [9,10,11,12,13]. A CO₂ terminal is required when multiple sources and storage sinks are linked or when pipelines and carriers are linked in the CCS transportation chain [14,15,16,17]. In the future, once CCS is widely distributed as a CO₂ emission reduction option, the likelihood of CCS projects using multiple sources and storage sites will be high [11]. This means that CO₂ terminals will become important facilities within the whole CCS chain that can simultaneously contribute to the realization of CO₂ carriers as a rational CO₂ transportation option.

Unfortunately, research regarding the configuration, functions, and roles of CO₂ terminals has been limited. Only a few studies [15,18,19,20,21,22,23] addressed CO₂ terminals. It is worth mentioning the Norwegian full-scale CCS project named the Northern light project [20,22,24], which includes the concept of CO₂ terminal. The terminal is located onshore and has a role of receiving and temporary storing of CO₂ transported by a carrier. It is equipped with a vaporizer to maintain vapor and liquid CO₂ balance during injection [24]. Vermeulen [15] suggested a CO₂ liquid logistics shipping concept for a complex CCS chain connecting multiple CO₂ emitters and storage sites. This concept uses complex CO₂ terminals that can receive large amounts of CO₂ through pipeline networks and barges from various emitters and can send CO₂ out to multiple storage sites using carriers and pipelines. However, no study has defined an overall configuration of the terminal and its operational concept. Lee et al. [21] simulated a CO₂ process flow in an intermediate storage facility attached to a capture site on the coast for loading CO₂ to a carrier. The storage facility in their study consisted of a CO₂ input process, a storage tank, a loading process, a recirculation process, and a boil-off gas (BOG) reliquefaction process. However, they did not clearly explain how they chose the components for the CO₂ terminal. In a few studies about CO₂ carrier transportation, CO₂ terminals are considered part of the description of the CCS chain, but the research did not focus on CO₂ terminals [18,19,25,26,27]. Several studies [28,29,30,31] focused on the direct injection of CO₂ into geological formations from carriers did not consider the CO₂ terminal. As such, the configuration of a CO₂ terminal, reflecting its functions and roles, has rarely been studied. Conversely, considerable research has been conducted on liquefied natural gas (LNG) terminals [32,33,34,35,36,37,38,39,40,41,42,43], which can be thought of as similar to CO₂ terminals. LNG terminals can be used in developing CO₂ terminals, but a cautious approach is required as the transport characteristics of CO₂ and LNG differ.

Designing a simple system or a precedent system without systematic design processes is possible. However, for a complex system or an unprecedented system like a CO₂ terminal, it is highly recommended to follow a system engineering process (SEP), as shown in Figure 1.

The SEP consists of (1) requirements analysis, which transforms the stakeholder’s system needs into functional requirements from an engineering point of view; (2) functional analysis and allocation, which defines what functions are to be performed; and (3) design synthesis, which determines how these functions are combined into the design [44]. The SEP is completed when this series of processes is conducted. The strength of SEP during the concept development stage is that necessary functions are reflected, whereas unnecessary functions are removed in advance.

In this study, the conceptual design of a CO₂ terminal was structurally and systematically developed using the SEP. Our conceptual design focuses on the configuration and basic operation concept of a CO₂ terminal. In the requirement analysis (Figure 1), a CCS chain was analyzed to derive the goal and performance objectives of a CO₂ terminal. For requirement analysis, specific conditions of the CCS demonstration project promoted in Korea were applied. Then, the functions were derived to meet the goal and performance objectives of the CO₂ terminal using functional analyses. Several functional analyses were conducted to define the top-level functions and their basic operational concepts. Design synthesis was performed to determine the equipment that corresponds to each function. Based on the SEP, a process flow block diagram of the CO₂ terminal was finally suggested that explicitly depicts the configuration of the CO₂ terminal.

2. Description of CCS Chain

CO₂ terminals have various forms depending on the connection methods between capture and storage sites [15]. Since the objective of this study was to demonstrate the basic configuration and operation concept of a CO₂ terminal, a relatively simple form of a CCS chain was selected as a target system (Figure 2a). The concept of a CCS chain displayed in Figure 2 is identical to the 1-million-ton-scale demonstration project in Korea. This CCS chain liquefies captured CO₂ at a thermal power plant located on the coast, and transports liquid CO₂ (LCO₂) to a CO₂ terminal through CO₂ carriers. CO₂ is transmitted to an offshore platform through an offshore pipeline and injected into offshore geological formations.

The CCS chain can be expressed as a functional flow block diagram, as depicted in Figure 2b. Since we focused on the CO₂ terminal, we omitted the explanation of a functional flow block diagram of the whole CCS chain. Contrary to other CCS chain descriptions, the return of CO₂ carriers is included to consider the vaporized CO₂ (VCO₂) in returning carriers.

The specific conditions of the CCS project in Figure 2 are as follows. The annual transport amount is 1 million tons of CO₂. The distance from the capture source to the CO₂ terminal is 580 km. The temperature and pressure of CO₂ are to be maintained at −27 °C and 16 bar, respectively, during the CO₂ carrier and CO₂ terminal operation. The CO₂ in the terminal is to be pressurized and heated for offshore pipeline transportation.

3. System Engineering Process

In this study, the SEP in Figure 1 was followed to identify the basic configuration and operation concept of the CO₂ terminal. System engineering is a useful method for designing new systems that are complex or did not exist previously. Figure 1 depicts the system engineering process presented by the U.S. Department of Defense, which is widely used in many fields [44]. The system engineering process involves requirement analysis, functional analysis and design synthesis, as shown in Figure 1. Requirement analysis, functional analysis and allocation, and design synthesis are iterative and mutually complementary [18]. A system analysis and control were performed to balance this series of procedures. A more detailed explanation of each procedure is provided below.

3.1. Requirement Analysis

Requirements analysis is the first step in the system engineering process, in which the system requirements definition process converts the stakeholder representation into a technical representation of the product. The requirement analysis can transforms the project’s needs into engineering language, which consequently enables the system designer to conduct the design concept [45]. During requirement analysis, the CCS chain is analyzed to derive the performance requirement of the CO₂ terminal.

3.2. Functional Analysis

After identifying the system requirements, functional analysis is performed to define the logical architecture that can satisfy the identified requirements. Functional analysis is the step of defining the basic functions that the system should perform. This analysis focuses on “what” must be performed, not “how” functions will be performed. Notably, this analysis is function-oriented rather than equipment-oriented. In functional analysis, three functional analysis tools are used: a functional flow block diagram (FFBD), integration definition for function modeling (IDEF0), and timeline analysis (TLA). Through FFBD, the functional flow of the whole CCS transport chain is determined. The sequence and relationships between the functions of the CO₂ terminal are defined by IDEF0. To consider the time durations of functions, TLA is used. The results of TLA support the operation concept of the CO₂ terminal.

3.2.1. Functional Flow Block Diagram (FFBD)

FFBD is graphical tool used to show the sequence of all functions that the system should perform. FFBD focuses the sequence of each function, not the time required or the flow of time between functions [8]. In FFBD, each function represented by a block is identified in terms of inputs and outputs [45]. Each block can be expanded to a series of sub-functions. A function is represented by a rectangular block with the title of the function. The title of the function is composed of an action verb followed by a noun. Some functions may be performed in parallel when necessary.

3.2.2. Integration Definition for Function Modeling (IDEF0)

The IDEF0 diagram integrates the inputs, control, outputs, and mechanisms of functions. IEDF0 allows us to understand the correlation between functions derived from FFBD. In IDEF0, the block represents the function to be performed, and the left and right arrows show the input and output of the process, respectively. The up and down arrows indicate the controls and the mechanism of the function, respectively (Figure 3).

3.2.3. Timeline Analysis (TLA)

Although the FFBD and IDEF0 represent the logical sequence of functions, they cannot show the time duration of or between functions. TLA is a method for identifying specific time-related design or operating requirements, useful for reflecting the durations of time-critical functions, such as reaction time, turnaround time, time limits, etc., and to clarify time-related design constraints. TLA complements FFBD or IDEF0 [46].

3.3. Design Synthesis

Physical equipment that performs the functions of the system is specified in the final stage of the design synthesis. In this study, the requirement allocation sheet (RAS) was used for the design synthesis. In the RAS, the configuration or equipment that will perform the functions are derived by connecting the functions with the allocated performance and the physical system. Usually the functions are listed in the left column in the RAS, the performance or requirements that each function should meet are listed in the middle column, and the corresponding equipment that can perform each function is listed in the right column [47]. This methodology clearly indicates the interrelationships between the main equipment and minimizes the risk of missing key design variables.

4. Results

4.1. CO₂ Carrier and CO₂ Terminal Requirements

To derive the performance requirements of the CO₂ terminal, we clearly defined the logistics concept, including carrier size, number of the carriers, carrier transportation cycle, etc., using the CCS chain conditions of the Korean project described in Section 2. This CCS chain aims to store 1 million tons of CO₂ annually, which can be converted into a daily transport rate of 2740 tons/day.

4.1.1. Time for Carrier Round Trip and the Availability of Transport

The distance from the capture site to the CO₂ terminal is about 580 km, and the carrier speed is usually about 14 knots. In other words, one day is required for one-way transportation. An additional day is required for berthing, purging the pipeline, and loading/unloading CO₂ at the capture site and the CO₂ terminal each. Therefore, a total of four days is required for one carrier to complete a round trip between the capture site and the CO₂ terminal, including loading and unloading of LCO₂. The availability of transport is assumed to be less than 80% considering the possibility of bad weather conditions.

4.1.2. Number of Carriers

In general, large carriers have lower capital expenditure (CAPEX) and operating expenditure (OPEX) per cargo volume than smaller carriers. Therefore, operating a small number of carriers is cost effective. In other words, the cost of one carrier is inexpensive comparing the cost of two half-sized carriers. A more detailed comparison of costs between one and two carriers is explained in Section 5.3.

When operating only one carrier, if an unexpected failure of the carrier occurs, the entire CCS chain will be stopped, making the continuous injection of CO₂ difficult. Intermittent injection of CO₂ lowers the CO₂ injection capacity, reducing the efficiency of the entire CCS chain [48,49]. On the other hand, if two carriers are operated, one will be able to operate even if the other stops due to an unforeseen failure. Therefore, the entire CCS chain can be operated with a reduced injection rate while avoiding injection interruption, even if it is not able to satisfy the target storage amount of 1 million tons/year. Thus, this study determined the number of carriers as two even though it is inexpensive to operate one carrier. In the case of this study, even if one carrier fails to operate, it will be able to transport 75% of the normal transport rate if the maximum availability of the other carrier is maintained during the repair period.

4.1.3. Size of Carrier

With two carriers, four days of shipping time, and less than 80% shipping availability, the minimum amount of LCO₂ carried by single transportation can be derived as follows:

1 × 10⁶ tons/(365 days × 80%/4 days)/2 carriers ≈ 6849 tons/carrier.

(1)

A total of 2.5 days are required to capture 6849 tons of LCO₂ considering a capture rate of 2740 tons/day. However, the CCS transport chain can be both safe and simple if one carrier transports 3 days’ worth of captured CO₂ at one time. Therefore, one carrier transports 8220 tons (2740 tons × 3 days) of LCO₂. In this case, the availability is about 66%. Since one carrier transports 3 days’ worth of captured CO₂, the carrier arrival cycle in the CO₂ terminal is also 3 days.

The same volume of VCO₂ must be loaded into the carrier’s cargo tank when unloading LCO₂ from the carrier to the CO₂ terminal. The carrier’s cargo tank is displaced by the VCO₂ of the storage tanks at the terminal, while LCO₂ fills the storage tanks. There are two reasons for loading VCO₂ into the carrier’s cargo tank. The first is to allow the pressure and temperature of the cargo tank to be controlled during the unloading process. Constant pressure and temperature facilitate the process. The second reason is to prevent the rapid decrease in temperature due to Joule–Thomson cooling. This rapid decrease in temperature could lead to material damage [50]. The VCO₂ and LCO₂ density ratio is about 0.041 under conditions of −27 °C and 16 bar. Therefore, 337 tons of VCO₂, which correspond to 4.1% of the 8220 tons of LCO₂, is returned to the capture site. For the net transport amount of CO₂ to be 8220 tons per carrier, this amount of CO₂ should be additionally carried from the capture site. The total transport rate is, therefore, 8557 tons per carrier. Considering the BOG in the cargo tanks during the ship transportation, the cargo tanks should be approximately 95% filled and then, the total size of the CO₂ cargo tanks would be increased to be ~9000 tons. In conclusion, two 9000-ton carriers that transport 8857 tons of LCO₂ over four days in one shipment are needed in our CCS chain and two unloadings of CO₂ in six days would be completed in the CO₂ terminal.

4.1.4. Size and Number of Storage Tank in Terminal

Since the maximum capacity of a pressurized CO₂ storage tank is 5000 tons, the size of the CO₂ storage tanks in the CO₂ terminal can be easily determined to be half the size of a carrier, 4500 tons. Two empty tanks should be ready before unloading the LCO₂. The CO₂ terminal should maintain an appropriate amount of buffer CO₂ to operate smoothly, although the CO₂ supplied by the carrier may be halted for at least 60 h due to typhoons or other events and conditions. The 60-h buffer corresponds to 6850 tons of CO₂. At this stage, four CO₂ storage tanks are required. The number of storage tanks will be checked again using TLA to determine if four tanks are sufficient for buffering CO₂ in the next section. These requirements will be checked through functional analysis and allocation and design synthesis in the SEP in the following sections.

4.1.5. The Estimation of the Amount of BOG in the CO₂ Storage Tank at Terminal

To maintain the pressure in the storage tank that emits LCO₂ to the pipeline at the CO₂ terminal, it is necessary to inject the same volume of VCO₂ as the LCO₂ is emitted. Since CO₂ is stored at a low temperature in the storage tanks, the generation of BOG is inevitable. And in the CO₂ terminal design, it is very important to compare the required VCO₂ and BOG quantities. In other words, if the amount of BOG generated is less than the required VCO₂, a vaporizer is needed, and if it is large, a reliquefier must be installed. Since this study focuses on the concept design of the CO₂ terminal, BOG is simply estimated by assuming that the storage tanks are a thin-walled spherical tank of metal covered with a thick insulation layer on the outside as shown in Figure 4. The BOG generated in the storage tanks at the CO₂ terminal was determined by assuming that all external heat ingress is converted into latent heat of vaporization of CO₂, as shown in the equation below:

$$\dot{m}=\frac{q}{h_{fg}},$$

(2)

where q is heat ingress from the surrounding atmosphere, $\dot{m}$ is vaporized mass of CO₂, and h_fg is latent heat of vaporization of CO₂. Heat ingress is defined as the difference between the ambient air temperature and the temperature of CO₂ in the tank divided by the sum of the thermal resistances [51], as shown in Equation (3):

$$q=\frac{T_{\infty ,2}-T_{\infty ,1}}{R_{tot}},$$

(3)

where T_∞,1 and T_∞,2 represent the temperature of CO₂ in the storage tank and ambient air temperature, respectively. The ambient air temperature, T_∞,2 is assumed to be 35 °C, which is the highest temperature in summer in Korea. R_tot is the total thermal resistance and consists of the conduction resistances and the convection resistance. The detailed equations are as follows:

$$R_{tot}=R_{cond, 1}+R_{cond, 2}+R_{conv}$$

(4)

$$R_{cond, 1}=\frac{1}{4\mathsf{\pi }{k}_1}\left(\frac{1}{r_1}-\frac{1}{r_2}\right)$$

(5)

$$R_{cond, 2}=\frac{1}{4\mathsf{\pi }{k}_2}\left(\frac{1}{r_2}-\frac{1}{r_3}\right)$$

(6)

$$R_{conv}=\frac{1}{h4\mathsf{\pi }{r}_3^2}$$

(7)

where R_cond,1 and R_cond,2 are the conduction resistances in the thin-walled spherical metallic tank whose material is SA537-cl2 and in the insulation layer, respectively. k₁ and k₂ are the thermal conductivities of SA537-cl2 and insulation material of perlite, whose values are 52 and 0.047 W/mK, respectively. r₁, r₂, and r₃ are inner radius of thin-walled spherical tank, outer radius of thin-walled spherical tank, and radius of insulation outer surface, respectively. The inner radius of the spherical tank, r₁, was calculated as 10.24 m to allow the spherical tank to have a volume of 4500 m³. The thickness was calculated to have a range of 39.9 to 44.4 mm by using the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section VIII Division 2 [52]. This study assumes that the thickness of the thin-walled spherical tank is constant to 40 mm and the insulation material layer has a thickness of 0.2 m. R_conv is the convection resistance in the outer surface of tank and h is a heat transfer coefficient which is assumed as 20 W/m²K. From Equations (5)–(7), R_cond,1, R_cond,2, and R_conv were calculated as 5.05 × 10⁻⁷, 2.73 × 10⁻³, and 3.16 × 10⁻⁵ K/W, respectively. Since both R_cond,1 and R_conv are much smaller than R_cond,2, R_tot is almost the same as R_cond,2, where R_tot calculated by Equation (4) is 2.77 × 10⁻³ K/W.

The amount of BOG calculated by Equation (2) is 241 kg/h. Since the number of tanks in the CO₂ terminal is 4, the total BOG is 965 kg/h. The amount of VCO₂ required is 4.1% of the emitted CO₂, which corresponds to 4680 kg/h. Since the amount of BOG in the storage tanks is about 20% of the required VCO₂, a vaporizer is needed to fully obtain the required VCO₂ [24]. It is noted that BOG generated per day a tank is 6.65 tons/day, which corresponds to about 0.16% of the CO₂ storage capacity. It is also noted that the stratification phenomena inside of the CO₂ tank [53] could increase the BOG generation, but the amount of BOG generated would be still smaller than the required amount.

4.2. Functional Analysis of the CO₂ Terminal

4.2.1. FFBD of CO₂ Transport Chain

To derive the basic functions of the CO₂ terminal, a functional flow block diagram was used. Since the basic functions should be connected with the entire CCS chain, the three CO₂-terminal-related functions—‘6.0 Unload LCO₂ to CO₂ terminal’, ‘7.0 Store LCO₂ intermediately’, and ‘8.0 Pressurize’ (Figure 2b)—were set as the higher-level functions.

For the higher-level function of ‘6.0 Unload LCO₂ to CO₂ terminal’, the lower-level functions of ‘6.1 Unload LCO₂’ and ‘6.2 Load VCO₂’ are developed. The function ‘6.2 Load VCO₂’ involves loading the VCO₂ into the CO₂ cargo tank on the carrier to prevent the rapid pressure decrease of cargo tanks on the carrier caused by unloading LCO₂ from the carrier.

To achieve the function of ‘7.0 Store LCO₂ intermediately’, LCO₂ should be stored in the CO₂ storage tank after unloading LCO₂ from the carrier at adequate temperature and pressure, and then be transmitted for offshore pipeline transport. Therefore, ‘7.1 Receive LCO₂’, ‘7.2 Store LCO₂’, and ‘7.3 Transmit LCO₂’ functions can be derived as lower-level functions. VCO₂, which will be loaded onto the carrier, is supplied from the CO₂ terminal, so the function of ‘7.4 Generate VCO₂’ can also be derived. The temperature and pressure of LCO₂ extracted from the storage tanks needs to be increased to meet the appropriate conditions for injection before transmission through the offshore pipeline. Therefore, the function of ‘8.1 Increase LCO₂ T & P’ is included as the lower-level function of ‘8.0 Pressurize’. To summarize, seven basic functions are defined for the CO₂ terminal: ‘6.1 Unload LCO₂’, ‘6.2 Load VCO₂’, ‘7.1 Receive LCO₂’, ‘7.2 Store LCO₂’, ‘7.3 Transmit LCO₂’, ‘7.4 Generate VCO₂’, and ‘8.1 Increase LCO₂ T & P’, indicated by the blue region in Figure 5.

4.2.2. Integration Definition for Function Modeling (IDEF0) of CO₂ Terminal

Through IDEF0, input and output between functions can be explicitly diagrammed. IDEF0 clearly demonstrates the correlations between the functions to be performed. These IDEF0 relationships are represented in Figure 6.

As the input of the system is LCO₂ moved by a carrier, the function ‘6.1 Unload LCO₂’ is the first function among the seven functions. When the carrier and the CO₂ terminal are connected, LCO₂ is unloaded from the carrier. The function ‘7.1 Receive LCO₂’ is receiving the unloaded LCO₂ at the CO₂ terminal. Therefore, the output of function 6.1 is ‘Unloaded LCO₂’, which becomes the input of function 7.1. Once LCO₂ is received, LCO₂ should be retained until transmitted to the offshore pipeline, and this function corresponds to function ‘7.2 Store LCO₂’. To store the received LCO₂ in storage tanks, the temperature and pressure of LCO₂ should be maintained constantly until LCO₂ is transmitted. In this study, we assumed that the temperature and pressure of LCO₂ are controlled by extraction of BOG. Therefore, the outputs of the function ‘7.2 Store LCO₂’ are ‘Stored LCO₂’ and ‘Removed VCO₂’.

After, the function ‘7.3 Transmit LCO₂’ is completed, which is the transmission of the LCO₂ from the storage tanks to offshore pipeline transportation and then the input of function 7.3 is ‘Stored LCO₂’. When LCO₂ is transmitted from the CO₂ storage tanks, the pressure in the CO₂ storage tanks decreases. To prevent this decrease in pressure, VCO₂ should be supplied to the storage tanks and this function corresponds to ‘7.4 Generate VCO₂’. To minimize the wasted CO₂, we assumed that input of function 7.4 is the above-mentioned BOG. However, this might not provide a sufficient amount of VCO₂. Therefore, we also assumed that the small amount of LCO₂ transmitted from the CO₂ storage tanks is the second input of function 7.4 and this LCO₂ should be vaporized to meet the required VCO₂ supply. Once LCO₂ transmission is finished, the CO₂ storage tank is again ready to receive the LCO₂ that is transported by the next carrier.

When conducting ‘7.1 Receive LCO₂’, the VCO₂ filled in the CO₂ storage tanks should be removed to receive LCO₂. We assumed that the VCO₂ in the CO₂ storage tank is loaded into the carrier cargo for the safe return of the carrier to the capture site for the next LCO₂ transport. This function corresponds to ‘6.2 Load VCO₂’.

Most of the LCO₂ transmitted from the CO₂ storage tank is transported via offshore pipelines for injection. Before offshore pipeline transport, the temperature and pressure of LCO₂ should be increased appropriately and this function corresponds to ‘8.1 Increase LCO₂ T & P’. Then, the output of function 8.1, ‘The temperature and pressure increased LCO₂’, becomes the input of function ‘9.0 Transport LCO₂ through offshore pipeline’.

4.2.3. Timeline Analysis (TLA) of the CO₂ Storage Tank Operation

For this study’s CO₂ terminal, the CO₂ is supplied by a CO₂ carrier and then transmitted to an offshore pipeline. The CO₂ terminal should have enough buffer CO₂ to manage CO₂ supply interruption. The transmission of CO₂ from the CO₂ terminal to the offshore pipeline is continuous, whereas the supply of CO₂ from a carrier is intermittent. The amount of buffer CO₂ should be balanced by both the continuous transmission and the intermittent supply of CO₂; therefore, the simple calculation cannot assure the smooth operation of the CO₂ terminal without any logistics problems. In this study, TLA, which was explained in Section 3, was used to verify the CO₂ logistics supply concept, transmission and buffer amounts, and to determine the storage time of LCO₂ and VCO₂ per storage tank. In TLA, it is assumed that concurrent filling and transmitting of CO₂ in the same tank is forbidden for operational safety reasons.

Figure 7 shows the TLA results. In Figure 7, each cell on the x-axis represents four hours, and the y-axis represents the seven functions derived in Section 4.1. The activating time of each function is specified by filling the correspondent time square in Figure 7. On the y-axis below the functions, rows are additionally inserted to show each LCO₂ storage tank state and the arrows represent the transmission of CO₂ to the pipeline. Each carrier shipping CO₂ is distinguished by filling the time squares with green and blue colors. The yellow represents the initially filled CO₂.

In our TLA, we assumed that the CO₂ unloading work is conducted for four hours starting from 12:00 every three days. The normal operating period is days 1 to 12. As shown in Figure 7, the seven functions are performed successfully. In a normal operating period, immediately before and after unloading the LCO₂, the lowest and highest amounts of buffer CO₂ are 7130 and 15,212 tons, respectively. During a three-day cycle, two storage tanks retain VCO₂ for 12 and 48 h, respectively, while the other two storage tanks store LCO₂. This means that in case of emergency, it is possible to inspect any dysfunctional CO₂ storage tank after evacuation of VCO₂.

The period from days 13 to 15, marked in red in Figure 7, corresponds to an emergency period where the CO₂ carrier arrives at the CO₂ terminal 60 h late, but was originally scheduled to arrive on day 13 at noon. As shown in Figure 7, if the unloading of LCO₂ from the carrier starts at 20:00 on day 15, it is possible to continuously transmit the LCO₂ to the offshore pipeline without any gap. This means four storage tanks enable a buffer of 60 h. For longer buffer hours, additional storage tanks would be required.

4.3. Identification of Physical Equipment of CO₂ Terminal

Identifying the equipment consisting of the system and the performance requirements is important in the system design. In this study, a RAS was used to determine the requirements related to the seven lower-level functions. Firstly, the functions and the related requirements are specified in the left columns in

Table 1. The derived requirements and equipment corresponding to functions of the CO₂ terminal.

No.	Functions		Requirements	Equipment
5.0 and 11.0	Transport LCO2 by carrier and Return carrier to capture site		(1) Number of carriers = 2 (2) One-way transport time ≤ 1 day (3) Availability of carriers < 80% (4) Amount of CO2 per one carrier = 8549 tons	Two 9000-ton CO2 carriers
6.1		Unload LCO2	(1) Amount of unloaded LCO2 = 8549 tons (2) Time required for unloading ≤ 4 h	LCO2 unloading system
6.2		Load VCO2	(1) Amount of loaded VCO2 = 329 tons (2) Time required for loading ≤ 4 h	VCO2 extracting system VCO2 loading system
7.1		Receive LCO2	(1) The size of 1 storage tank = 4500 tons	4500-ton storage tanks
7.2		Store LCO2	(1) Temperature and pressure CO2 in storage tank are maintained at −27 ℃ and 16 bar, respectively, by removing BOG (2) Buffer capacity = 60 h’ injection amount (3) The required number of CO2 tanks = 4	BOG removal system Four 4500-ton storage tanks
7.3		Transmit LCO2	(1) Continuous extraction rate of LCO2 from storage tank ≥ 2740 tons/day	LP Pump
7.4		Generate VCO2	(1) Daily required amount of VCO2 = 109.7 tons (2) Prevent decrease in pressure of storage tanks by charging VCO2	VCO2 generating system VCO2 charging system
8.1		Increase LCO2 T & P	(1) Target temperature = 3–5 ℃ (2) Target pressure ≥ 120 bar	Heat exchanger Booster pump

, which were obtained from Section 2 and the results of Section 4.1. Then, the derived requirements were written in the next column, which were derived from Section 4.2.1 and Section 4.2.2. Finally, the necessary equipment was determined and is listed in the right column in Table 1.

5. Discussion

5.1. Process Flow Block Diagram of CO₂ Terminal

Based on the results obtained from the five analyses, a process flow block diagram of the CO₂ terminal was derived (Figure 8). The solid lines and the dotted lines in Figure 8 represent flows of LCO₂ and VCO₂, respectively. Figure 8 shows that the CO₂ terminal consists of three major parts. The first part is the CO₂ loading/unloading part (Figure 8, yellow), including an LCO₂ unloading system, a VCO₂ loading system, and a VCO₂ extracting system. According to the TLA result, the CO₂ loading/unloading part operates for four hours during a three-day operating cycle. The VCO₂ displaced from the storage tanks in the terminal is loaded into the cargo tanks in the carrier. This means the CO₂ loading/unloading parts do not require any externally supplied VCO₂. The second part is LCO₂ transmission (Figure 8, blue), which controls the temperature and pressure of LCO₂ for transmission to the offshore pipeline. The corresponding equipment systems are a low pressure (LP) pump, a heat exchanger, and a booster pump. It operates all the time for continuous injection of LCO₂. The final part is the vapor-treatment part (Figure 8, red), which is necessary to control the temperature and pressure of the storage tanks for smoothly discharging LCO₂ to the offshore pipeline. The most effective method is to recharge the storage tank with VCO₂ in the same volume as the discharged LCO₂. The vapor-treatment part aims to produce and charge the required VCO₂. The vapor-treatment consists of a BOG removal system, a VCO₂ charging system, and a VCO₂ generating system. In this study, the required VCO₂ can be obtained using two methods. The first method involves obtaining VCO₂ by BOG from other tanks that are not under discharge. The system responsible for this function is a BOG removal system. The second method involves vaporizing a small portion of the discharged LCO₂. This function is handled by a VCO₂ generating system. A VCO₂ charging system is responsible for injecting VCO₂ determined by the above two methods into the storage tanks that discharge LCO₂.

The process flow block diagram depicted in Figure 8 helps with understanding the entire CO₂ flow and equipment groups. However, with the four CO₂ storage tanks and the roles of each tank changing with time, it is difficult to understand the role of each tank using Figure 8 alone. The storage tanks block in Figure 8 is included in all three parts. Then, the storage tanks block is expanded in Figure 8 into four tanks to clarify the different roles of CO₂ storage tanks explicitly, as shown in Figure 9.

Figure 9 shows the extended process block flow diagram when storage tanks 1 and 2 are being filled with LCO₂ from the carrier during normal operation. This time corresponds to 12:00 to 16:00 on days 7 and 10 in Figure 7. These two CO₂ storage tanks belong to the CO₂ loading/unloading part (yellow) in Figure 8. Before the carrier has arrived, the two tanks are filled with VCO₂ subsequent to the transmission of LCO₂ to the offshore pipeline. After filling LCO₂ into CO₂ storage tanks 1 and 2, the function of CO₂ storage tanks 1 and 2 are converted to the function ‘Store LCO₂′ and the corresponding equipment in Figure 9 is ‘storage tank 3′. ‘Storage tank 3′ belongs to the vapor-treatment part in Figure 8 (red). At this time, the pressure and temperature in the CO₂ storage tank are kept constant by extracting the BOG generated in the tank. The equipment responsible for this role is shown in Figure 8 and Figure 9 as ‘BOG removal system’. After a period of time, the LCO₂ in the tank is then transmitted by the ‘LP pump’ and the corresponding tank is ‘storage tank 4′. ‘Storage tank 4′ belongs to the LCO₂ transmission part in Figure 8 (blue). Most of LCO₂ is sent to the ‘offshore pipeline’ via the ‘heat exchanger’ and a ‘booster pump’, and only a small portion of LCO₂ is sent to a ‘vapor generating system’ to generate VCO₂. Here, the ‘vapor generating system’ should also collect the BOG from the ‘BOG removal system’. The generated VCO₂ is moved into ‘storage tank 4′ through the ‘vapor charging system’ to control the pressure of the tank being discharged. VCO₂ is ultimately reloaded to the CO₂ carrier through a ‘VCO₂ extracting system’ and a ‘VCO₂ loading system’.

5.2. Comparison with LNG Terminal

LNG terminals have already been commercialized, with many models available. The concepts CO₂ and LNG terminals are similar for temporarily storing carrier-transported liquid in a storage tank and then sending it out through a pipeline. This similarity leads to the mis-prediction that the differences between the LNG terminal and the CO₂ terminal are limited to size and operational pressure and temperature conditions. The LNG transport chain and CCS transport chain have considerable differences in transportation distance and transportation pressure and temperature conditions. The LNG transport chain typically has a long carrier transport distance of 6500 km on average [54] because the LNG production sites are distributed only in a specific area. However, a CCS chain’s carrier transport routes are relatively short because offshore CO₂ storage sites are widely distributed around the world and the probability of offshore storage sites being near capture sites is high. The temperature and pressure conditions of LNG carriers are usually −162 °C and atmospheric pressure, respectively [55], but the temperature and pressure conditions of CO₂ carriers range between the triple point of −56 °C and 5.1 bar and the critical point of 31 °C and 74 bar. Based on the results of this study, the differences in the transportation conditions between an LNG chain and a CCS chain lead to the following important differences in terminal design and operation:

(1)

Differences in the number of storage tanks due to the size limit: LNG carriers usually travel long distances and convey a large amount of LNG, so this necessitates a large amount of LNG storage. However, relatively less CO₂ storage is required in a CO₂ terminal. Since the LNG tanks are operated in cryogenic and atmospheric pressure conditions except for small-scale satellite terminals, one LNG storage tank can be as large as ~160,000 m³ [56]. However, the pressure of the CO₂ storage tank should be higher than the triple-point pressure of 5.1 bar, meaning the CO₂ storage tanks are pressurized tanks, which are hard to manufacture at capacity bigger than around 5000 tons. This means the CO₂ terminal requires multiple tanks, and in many cases, the carrier-transported CO₂ has to be unloaded into several storage tanks. In this study, the carrier transports ~9000 tons of CO₂, which is unloaded into two tanks that are 4500 tons each. Because overfill is one of the common causes of operational accidents [57], the multiple tanks in the CO₂ terminal pose a high risk of overfill. Therefore, ensuring a high level of safety is necessary.

(2)

Importance of vaporizer: The storage time in an LNG terminal is usually longer than in a CCS terminal due to the longer carrier transport cycle. In this case, the amount of BOG generated in LNG carriers or storage tanks is huge and needs to be re-liquefied at a high cost and requiring considerable amounts of energy [58]. However, the CO₂ carrier transportation cycle and temporary storage period in the terminal are relatively short. In our study, the carrier-transported CO₂ is sent to the offshore pipeline within six days according to the TLA results. The amount of BOG in the CO₂ terminal is less than the required vapor for preventing the decrease in pressure in the CO₂ storage tanks as LCO₂ is discharged to the pipeline. Therefore, the vaporizer in a CO₂ terminal is essential.

5.3. Cost Comparison between One and Two Carriers

This study has determined the number of carriers as two in Section 4.1.2 because operating two carriers is advantageous for continuous injection, although operating one carrier is economically more advantageous. If only one carrier is operated, the design of the terminal is also partially changed, so it is meaningful to check the costs according to the number of carriers operated, including the changes in the cost of terminal induced by the number of carriers.

In the case of operating one carrier, the carrier size and the number/size of storage tanks at the terminal were obtained by following the procedures in Section 4.1.3 and Section 4.1.4. According to Equation (1), the amount of CO₂ transported during one cycle is 13,700 tons, when operating one carrier, which corresponds to the amount CO₂ captured for 5 days. Taking into account the additional VCO₂ required at the terminal and assuming that 95% of the cargo tank is filled, the carrier size can be determined as 15,200 tons. As mentioned in Section 4.1.4, the maximum size of one storage tank at the terminal is 5000 tons. Therefore, the number and size of storage tanks required for unloading of LCO₂ at the CO₂ terminal will be four and 3800 tons, respectively. Considering two buffer tanks and one emitting tank, the total number of required storage tanks is seven.

CAPEX for a CO₂ carrier was calculated as $210,000,000 × (carrier capacity/155,000 tons) ^0.65 by calibrating the price of LNG carrier of 155,000-ton size. OPEX for a CO₂ carrier was calculated as the sum of 5% of CAPEX and fuel costs. Fuel costs for 9000-ton carrier and 15,000-ton carrier were determined as $5500 and $7100, respectively by applying the equation of the daily fuel cost × 365 days × availability. Assuming that only the number and capacity of storage tanks at the terminals are changed depending on the number of carriers, this study only considered storage tank costs at the CO₂ terminal. CAPEX of one storage tank was estimated to be 3.46 times the metal price corresponding to the weight of a thin-wall spherical tank. The weight of the thin-walled spherical tank was obtained from its diameter and thickness, and the metal price was assumed to be $1,666 per ton. OPEX is assumed to be 5% of CAPEX.

To make the comparison between costs easier, CAPEX is converted into the capital recovery cost using the capital recovery factor. The capital recovery cost means the annual equivalent cost of CAPEX. The capital recovery factor is 0.061 with a repayment period of 20 years and an interest rate of 2%.

The calculation results are summarized in

Table 2. Summary of capacity, capital expenditure (CAPEX), and operating expenditure (OPEX) of carrier and CO₂ terminal depending on the numbers of carriers.

No. of Carriers		One Carrier	Two Carriers
Carrier	Size of one carrier (tons)	15,000	9000
	CAPEX ($)	51.7 M	76.2 M
	Capital Recovery Cost ($)	2.8 M	4.0 M
	OPEX ($)	4.4 M	6.0 M
	Annual cost of carrier ($)	7.2 M	10.0 M
Terminal	No. of storage tanks	7	4
	Size of storage tank (tons)	3800	4500
	CAPEX of Storage tanks ($)	14.9 M	9.5 M
	Capital Recovery Cost ($)	1.0 M	0.6 M
	OPEX of storage tanks ($)	0.7 M	0.5 M
	Annual cost of storage tanks ($)	1.7 M	1.1 M
Total	Sum of annual costs of carriers and storage tanks at terminal ($)	8.8 M	11.0 M

. As shown in Table 2, the annual cost of one carrier is $7.2 million, which is 72% of the cost of two carriers. However, if the storage tank cost is included, the total annual cost when operating one carrier increases to 80% of the total annual cost when operating two carriers. This is due to the increased number of storage tanks at the terminal when operating one carrier. The results show that the number of carriers is very closely related to the terminal design. Therefore, to make an accurate economic evaluation, it is necessary to consider the design parameters of carriers and terminal simultaneously.

It is still economical to operate one carrier even if the cost of the terminal increases. If the carrier’s failure rate can be reduced to very low values, operating one carrier would be a good option. Adopting novel technologies such as prognostics and health management which have been emerging recently, can minimize unexpected failure of carriers.

6. Conclusions

In this study, we aimed to derive the configuration and the operational concept of a CO₂ terminal that connects CO₂ carriers and an offshore pipeline using the SEP. This paper helps understanding a basic concept of a CO₂ terminal. In addition it clearly shows how system engineering process is applied. The following points were derived by conducting the SEP:

(1)

FFBD was used to identify the seven basic functions of the CO₂ terminal: ‘Unload LCO₂′, ‘Load VCO₂′, ‘Receive LCO₂′, ‘Store LCO₂′, ‘Transmit LCO₂′, ‘Generate VCO₂′, and ‘Increase LCO₂ T&P’. Then, IDEF0 was used to identify the correlation between these functions.

(2)

The short and repetitive CO₂ carrier transport affects the configuration of the CO₂ terminal. The CO₂ terminal here is operated with a three-day cycle and at least four 4500-ton storage tanks are needed. The four tanks allow for continuous CO₂ transmission to an offshore pipeline even if a CO₂ carrier could not arrive at the CO₂ terminal due to the bad weather for 2.5 days. This operational concept of a CO₂ terminal was verified by the TLA.

(3)

A process flow block diagram was derived from the results of the functional analysis and design synthesis. The configuration of the CO₂ terminal consists of a CO₂ loading/unloading part, an LCO₂ transmission part, and a VCO₂ treatment part. These results were used in the subsequent design phase.

(4)

In this study, VCO₂ is required for two purposes. The first is for filling the carrier cargo tank with VCO₂ when unloading LCO₂ from the CO₂ carrier. In this case, the required VCO₂ is covered by displaced VCO₂ as LCO₂ fills the storage tanks in the terminal. The second purpose is for controlling the pressure in the storage tank that is transmitting LCO₂ to offshore pipelines. The required VCO₂ is supplied from BOG from the other three storage tanks and vaporized CO₂ from the small portion of discharged LCO₂.

(5)

The comparison of our results with an LNG terminal indicated that a vaporizer is important in the CO₂ terminal. As mentioned above, the BOG alone cannot meet the required amount for discharging the VCO₂ in a storage tank. Therefore, it is necessary to pay more attention to vaporization rather than reliquefaction in the CO₂ terminal. The CO₂ terminal requires multiple small-sized storage tanks, unlike the LNG terminal.

(6)

Major design factors, such as the number and capacity of storage tanks at terminal, change depending on the number of carriers. Therefore, in order to minimize costs, the design parameters of the carrier and the terminal must be considered simultaneously.