Design and scenario comparison study of an amine solvent-based carbon-dioxide-capture process for ships

2.1 Scope of Application

This study aims to evaluate the feasibility of complying with IMO regulations by comparing and analyzing full- and partial-exhaust gas-capture scenarios for a marine carbon-dioxide-capture system applicable to container ships. To achieve this objective, an amine-solvent absorption-based capture process was modeled using Aspen HYSYS V14, a commercial process-simulation tool [5][13].

2.2 Ship and Emissions Information

In this study, a 130,000 DWT container ship was taken as the reference ship. It is equipped with a low-speed, two-stroke dual-fuel main engine, several diesel-driven auxiliary generators, and an auxiliary boiler, and the combined exhaust gas from these units was treated as the main emission source for the OCCS design. The exhaust-gas volume and temperature under each operating condition are presented in Table 1, and the exhaust-gas composition is detailed in Table 2. The representative operating condition was set at a 75% load, reflecting previous reports that the 60%–80% load range is the most frequent in actual commercial ship operations and that the 70%–80% load range accounts for more than 65% of the total voyage, based on the engine-data analysis of large commercial ships [4][8]. Furthermore, the ship considered in this study was assumed to be equipped with an SOx scrubber so that most of the SOx and a substantial portion of the particulate matter and other impurities were removed upstream of the CO₂-capture unit [14][15].

Table 1:

Exhaust gas-flow rate and temperature by load conditions

Table 2:

Flue gas composition(vol %)

In this study, the composition of the gas entering the capture process was simplified into four components: N₂, O₂, H₂O, and CO₂. This simplification was implemented to enhance the safety of the simulations and concentrate on the thermodynamic and reaction analyses of the capture process. However, in actual operational environments, trace amounts of NOx and SOx may persist, potentially leading to solvent degradation and scaling issues during prolonged operation [7][10].

2.3 Reaction Mechanism of Amine Solvent

This study employed an aqueous MEA solution, which is the most extensively used amine solvent. MEA is characterized by a rapid reaction rate and high capture efficiency, and has been widely adopted and validated in existing power and petrochemical plants [10][16]. Previous research has demonstrated that MEA exhibits excellent reactivity with CO₂ because of its low molecular weight and high hydrophilicity, making it the most promising amine solvent for onboard carbon-capture applications [7][12].

The primary reactions between MEA and CO₂ are summarized as follows [17]:

Equation (1) represents the water-dissociation reaction, which supplies the initial OH^- necessary for CO₂ absorption. Equation (2) describes the equilibrium between HCO₃^- (bicarbonate) and CO₃^-2, which determines the buffering capacity of the solution. Equation (3) describes the protonation/deprotonation equilibrium of MEA, which regulates the effective base strength of the solvent. Equation (4) shows the reaction between OH^- and CO₂, a significant pathway during the initial absorption phase. Equation (5) details the bicarbonate dissociation reaction, providing the mechanism for CO₂ re-release, whereas Equation (6) describes the formation of MEACOO^- (carbamate), accounting for a substantial portion of the total CO₂ absorption. Equation (7) describes the decomposition of carbamate, which facilitates CO₂ release during the regeneration stage and is crucial for determining the energy requirements for regeneration. Among these reactions, carbamate formation and decomposition constitute the main regeneration cycle that governs the specific reboiler duty and largely determines the required size of the regenerator and associated heat-exchange equipment in the OCCS. These reaction mechanisms and kinetics were initially introduced in the seminal work of Caplow [17] and were later systematically summarized in a comprehensive review of CCS by Bui et al. [7]. Additionally, a recent study by Said et al. offered an extensive explanation of the various amine-CO₂ reaction pathways, supporting the design feasibility of OCCS processes for maritime applications [18]. Consequently, this reaction is a critical factor that directly affects the economic viability of the OCCS process.

2.4 Simulation Software

The simulations conducted in this study utilized Aspen HYSYS V14, which employs the Acid Gas-Chemical Solvent Property Package. This package is specifically designed for CO₂-removal processes using amine solvents and is capable of accurately predicting the behavior of amine-CO₂ systems. This is achieved by simultaneously considering chemical equilibrium, the reaction rates, and electrolyte equilibrium. The model calculates non-ideal mixing behavior based on the electrolyte non-random two-liquid(eNRTL) model and has been validated in industrial settings, such as power plants and natural-gas-processing facilities [13].

2.5 Process Configuration

Figure 1 depicts the OCCS process modeled in this study, which represents the most commonly employed wet amine absorption-regeneration process. The entire system comprises an absorption tower, regeneration tower, heat exchanger, pump, and make-up line. This configuration has been validated extensively in onshore power and gas plants. When applied to maritime vessels, it is designed to accommodate operational constraints while maintaining the fundamental flow structure [5][6][12].

Figure 1:

Simulation flow sheet

Initially, exhaust gas cooled to approximately 40 ℃ is introduced at the base of the absorption tower. Concurrently, an amine solvent is introduced at the top, facilitating contact between the downward-flowing solvent and upward-flowing exhaust gas, thereby initiating a CO₂-absorption reaction. During this interaction, the primary component, MEA, reacts with CO₂, forming carbamate or bicarbonate. The resulting sweet gas, devoid of CO₂, is expelled from the top of the absorption tower.

Subsequently, the rich amines are extracted from the bottom of the absorption tower. This solution is pressurized to approximately 4 bar using a pump and subsequently heated to approximately 120 ℃ via a heat exchanger. This heat-exchange process is crucial for optimizing the energy efficiency of the absorption-regeneration cycle by increasing the temperature of the rich amine prior to its entry into the regeneration tower.

In the next phase, the rich amine introduced at the top of the regeneration tower undergoes separation from CO₂ upon receiving heat at approximately 140 ℃ from a reboiler. The high-purity CO₂ produced during this process is captured at the top of the tower and conveyed to an onboard CO₂ liquefaction and storage system. Concurrently, the lean amine, from which the CO₂ has been removed, is discharged from the bottom of the regeneration tower.

Subsequently, the discharged lean amine traverses a heat exchanger to transfer heat to the rich amine, followed by passage through a cooler to adjust its temperature to approximately 40 ℃. It is then recycled and reintroduced to the top of the absorption tower, thereby completing the cycle.

Finally, the system incorporates a make-up line. During operation, a portion of the amine solvent is lost due to deterioration, volatilization, and byproduct reactions. Consequently, fresh solvent is replenished through the make-up line to maintain process stability [5][10][13]-[19]. The simulation outputs were analyzed using descriptive statistics, and key performance indicators such as CO₂ capture rate, specific reboiler duty, and required storage volume were compared as absolute vales and percentage differences between the full and partial-exhaust gas capture scenarios.

3.1 Full-Exhaust Gas-Absorption Scenario

A simulation was conducted in which the entire exhaust gas was processed through the absorption tower, and the absorption efficiency was maintained at 90%. The tower height was set at 7.5 m, considering that it is approximately 10 m, which is equivalent to the height of four 40-ft containers. This configuration ensured that the total column height, including the packing, distributor, filler support, and upper and lower clearances, was approximately 10 m. The absorber and regenerator diameters were then determined using the column-sizing function in Aspen HYSYS, assuming a specified packing type and design at approximately 70% of the flooding velocity, while maintaining 90% CO₂-removal efficiency.

As illustrated in Figure 2, the capture efficiency in a process utilizing 30 wt% MEA solvent significantly increased with an increase in the L/G ratio. However, beyond a certain threshold, the rate of efficiency improvement diminished, whereas the energy consumption, such as the reboiler duty of the regeneration tower, continued to increase. This trend has been confirmed by previous studies, indicating a clear tradeoff between capture efficiency and energy consumption [5][7][13]. In this study, the L/G ratio was set at 3.5 to balance the efficiency and energy consumption.

Figure 2:

Variation of capture efficiency and energy consumption with L/G ratio under full-exhaust capture scenario

As shown in Table 3, the total exhaust-gas flow rate was 135,900 kg/h, and the absorption and regeneration towers had diameters of 3.38 m and 2.42 m, respectively. The reboiler duty was approximately 3.4 MJ/kgCO₂, and the CO₂-capture capacity was 7,973 kg/h. Based on these results, assuming a seven-day operational period for a container ship, the vessel would need to store 1,344 tonnes of CO₂, necessitating a storage-tank capacity of at least 1,700 m³ at 14 bar. A conversion to ISO tanks shows that this would require loading 70 20-tonne ISO tank containers, imposing a substantial burden on the ship because of its limited space and payload capacity. Consequently, although total exhaust-gas capture offers the technical advantage of a high capture efficiency, it is unfeasible in maritime operating environments because of spatial and economic constraints [7].

Table 3:

Design output of OCCS simulation (full-capture scenario).

3.2 Partial-Exhaust Gas-Absorption Scenario

In the partial-exhaust gas-absorption scenario, a capture capacity of 2 tonCO₂/h was conceptually set as a practically installable level for this ship, referring to the 2030 IMO CII reduction requirement, rather than a route-specific CII calculation. According to Clarkson Research, this level is deemed sufficient to maintain CII standards until 2030 [8][9]. As depicted in Figure 3, the partial-capture scenario achieved a stable 90% efficiency within the L/G ratio range of 2.5, with a reboiler duty of approximately 2.9 MJ/kgCO₂. This represents a reduction of approximately 15% in the specific reboiler duty per unit of CO₂ captured compared to full-exhaust gas capture, thereby demonstrating favorable economic and feasibility outcomes [7].

Figure 3:

Variation of capture efficiency and energy consumption with the L/G ratio under partial-exhaust capture scenario

The design results for the partial-exhaust gas-absorption scenario are presented in Table 4 as the relative changes with respect to the full-exhaust gas-capture scenario presented in Table 3. Although the same MEA-based process was employed for both full- and partial-exhaust gas capture, significant differences were observed in equipment size, energy consumption, and storage-tank size. The full-capture absorption and regeneration towers were larger, with diameters of 3.38 m and 2.42 m, respectively, complicating their installation on ships. In contrast, the partial-capture towers measured 2.07 m and 1.94 m, respectively, reducing the spatial burden by approximately 30% compared to the full-capture scenario. Furthermore, for a seven-day voyage, the full-capture scenario required approximately 1,700 m³ of LCO₂ storage (equivalent to approximately 70 ISO tanks with a 20-tonne capacity), whereas the partial-capture scenario required only approximately 425 m3 (approximately 18 such ISO tanks), which significantly improved the feasibility under shipboard-space and weight constraints.

Table 4:

Relative changes in key design parameters of the partial-capture scenario relative to the full-capture case

From an energy perspective, although full capture offered a higher capture efficiency, the increased reboiler duty necessitated additional energy from the ship. This, in turn, diminished fuel efficiency and directly affected EEDI and CII indices, potentially hindering regulatory compliance. Partial capture, with a reboiler duty of 2.8 MJ/kgCO₂, provided a relatively higher energy efficiency (approximately 15%) than full capture, thereby demonstrating superior ship economics. This aligned with the conclusion that the partial-capture scenario offered high economic feasibility during the initial application stage [7].

Therefore, although a full-exhaust gas-capture scenario is ideal, it presents significant challenges for practical applications. Conversely, a partial-exhaust gas-capture scenario is considered a realistic and suitable strategy for phased implementation. The results of this study underscore the necessity for a phased technological-development approach that prioritizes partial capture while ultimately aiming for full-exhaust gas capture when designing future marine OCCS.

These numerical trends are broadly consistent with previous studies on MEA-based CO₂ capture for both marine and stationary applications. Luo and Wang [5] and Visoná et al.[6] reported specific reboiler duties of the same order of magnitude as those obtained in this work for high-efficiency MEA based capture and emphasized the strong coupling between L/G ratio, column size, and energy demand. Jung and Seo [10] and Bui et al. [7] likewise showed that moving the full-exhaust to partial-capture configurations, or operating at derated loads, can reduce specific energy consumption while alleviating constraints on column height and Co2 storage capacity. Our full and partial-capture results therefore fall within the ranges reported in the literature and reinforce the conclusion that partial-exhaust gas capture provides a more practical pathway for early-stage deployment of marine OCCS under stringent space and energy limitations.

Author Contributions

Conceptualization, S. T. Jung and Y. S. Choi; Methodology, S. T. Jung; Software, S. T. Jung; Validation, S. T. Jung and T . W. Lim; Formal Analysis, S. T. Jung; Investigation, S. T. Jung; Resources, S. T. Jung; Data Curation, S. T. Jung; Writing—Original Draft Preparation, S. T. Jung; Writing—Review & Editing, T. W. Lim and Y. S. Choi; Visualization, S. T. Jung; Supervision, Y. S. Choi; Project Administration, Y. S. Choi; Funding Acquisition, Y. S. Choi.