Full Length ArticlePerformance and sensitivity analysis of packed-column absorption process using multi-amine solvents for post-combustion CO2 capture
Introduction
Power generation via fossil fuel-fired power plants is regarded as the largest single source of CO2 emissions [1]. Carbon capture and storage technologies remain critical solutions for rapid CO2 emission reduction. Compared with other carbon capture technologies, the solvent absorption process is the most applicable mature technology for large-scale CO2 capture [1], [2].
The solvent using commercial scale CO2 capture technologies can be divided into two categories; chemical and physical solvent. The chemical solvents achieve CO2 capture by reactions while the physical solvents dissolve CO2 without reactions. Therefore, temperature is one of the key operating variables in chemical absorption processes, but pressure is an important operating variable in physical absorption processes [3]. Alternatively, it was also reported that dry solid sorbents has high potential for CO2 capture [4]. However, the further enhancement of the sorption rate, stability, and/or particle size control for many advanced solid sorbents is required for practical applications [5], [6].
The CO2 capture process using a physical solvent is widely accepted for an integrated gas turbine combined cycle (IGCC) power plant as a pre-combustion CO2 capture [7], [8]. However, in post-combustion CO2 capture, the chemical solvent absorption process is a commercially available competitive technology for fossil fuel-fired power plants [9], [10]. Since a substantial amount of energy is required for chemical solvent regeneration [11], [12], various configurations for absorption process have been developed to reduce high energy demand [13], [14]. The rich-solvent split (RSS) modification for absorption processes could enhance energy efficiency. In addition, since the RSS modification does not need additional devices, the modified configuration of the absorption process is relatively simple and the capital cost is cheap compared with other modifications [13].
Commercially available monoethanolamine (MEA), a representative primary amine, has been widely applied to the absorption process for CO2 capture. However, MEA is not enough to achieve efficient and economical CO2 capture due to high energy consumption in absorption processes. Therefore, considerable efforts have been devoted to the development of advanced amine solvents, which have been reported to have higher CO2 capture capacity than MEA [15], [16].
Primary and secondary amines are known to have fast reaction rates and high heat of absorption. In contrast, tertiary amines are known to have a slow reaction and low heat of absorption [17]. Primary and secondary amines mainly have two amine molecules that can react with one CO2 molecule; hence, a CO2 loading exceeding 0.5 mol CO2/mol amine is difficult to achieve. On the contrary, one tertiary amine molecule can react with one CO2 molecule [18]. Accordingly, many studies have developed solvent blends of primary, secondary, and tertiary amines to supplement their strengths and weaknesses [15], [19]. Different from solvent blends, multi-amines have several amine groups in one molecule. It was reported that some multi-amines could achieve a CO2 loading exceeding 1.0 mol CO2/mol [20]. In addition, multi-amines with primary and secondary amine groups exhibited high reaction rates with CO2.
Many studies have concentrated on measuring and modelling the properties and solubility of CO2 in various advanced and blended absorbents [21], [22]. In addition, the absorption rates and capacities of absorbents have widely been reviewed [20], [23]. On the other hand, only limited absorbents have been evaluated with respect to the efficiency evaluation of absorption processes using newly developed absorbents. Furthermore, few studies for the absorption process using multi-amines is rarely investigated even though the high performance of their absorption capacity and rate were reported. There is a great knowledge gap between the properties of absorbents and the performance of processes using advanced amine solvents because energy consumption mainly occurs in regeneration. Therefore, the feasibility and reliability of multi-amines should be evaluated again in absorption processes regardless of their high absorption rate and capacity.
In this study, the performance and sensitivity of the CO2 absorption process employing multi-amines were evaluated through a rate-based model. The detailed inside phenomena in the absorption process was elucidated to establish the relationship between performance and operating variables. Two multi-amines were selected: 2-(2-aminoethylamino)-ethanol (AEEA) with two amine groups and diethylenetriamine (DETA) with three amine groups as the representative multi-amines because the properties of these two multi-amines were well established as shown in Table 1, compared with other multi-amines. Therefore, the assumptions and uncertainty to simulate the absorption process, resulting from the multi-amines’ properties, could be minimized. The absorption process using MEA was selected as a reference to compare the performance and energy efficiency with the processes using the multi-amines. However, multi-amines with tertiary amine groups were not selected because of their slow reaction rates.
Both of selected multi-amines contain primary and secondary amine groups; hence, the reaction with CO2 can be faster than MEA. However, the information of absorption capacity and rate of absorbents is not sufficient to evaluate the capture efficiency and energy consumption of the absorption process because the cyclic absorption process is operated with an absorber and a stripper. Therefore, the efficiency of the absorption process must be evaluated in detail with respect to the mutual effect of multi-amines on absorption and stripping. Since the comparison was based on the same 30 wt% solvent, the effect of number of amine groups on the performance of the absorption process was also studied.
A rate-based model of the absorption process was use to simulate the capture process, incorporating the equations for vapor–liquid equilibrium (VLE), reaction heat, physical properties, mass transfer, and reaction rate. The rate-based model used in the study was validated with reference to the pilot data of MEA [10], [38], [39]. In this study, the CO2 capture efficiency was defined as the ratio of the flow rate of CO2 produced from a stripper to that supplied from the flue gas. The reboiler duty was determined to achieve 90% of CO2 capture efficiency because many previous studies reported a minimum energy penalty at this capture efficiency [40], [41], [42]. Then, as an important operating parameter, the effect of the liquid-to-gas (L/G) ratio on energy efficiency was investigated based on a 90% CO2 capture efficiency. The energy consumption analysis was categorized based on the latent heat of H2O, CO2 desorption, and sensible heat.
Since the properties of AEEA, DETA and MEA are very different, it was expected that the enhancement by the absorption process modification could be different among the solvents. Therefore, the feasibility of enhancing the energy efficiency of the absorption process using each solvent was also evaluated by modifying the configuration with RSS. Finally, the potential improvements for the absorption process with multi-amines were suggested to increase the energy efficiency of CO2 capture.
Section snippets
Process description
The operational results from a CESAR pilot plant using MEA (Tests 1A-1–1A-4) were selected to validate the simulation and used as comparative reference [10], [38]. The CESAR pilot plant was operated on a slipstream of flue gas from the Dong Esbjerg power station (a 400-MW pulverized bituminous coal power plant). This conventional process using MEA consists of an absorber, a stripper, and a rich/lean solvent heat exchanger, as shown in Fig. 1. The process could capture 90% of the incoming carbon
Validation of a rate-based model
The operational results of the CESAR pilot plant using MEA (Tests 1A-1 to 1A-4) were employed to validate the developed rate-based model. The operating conditions and operating results for Test 1A-1 to 1A-4 were used as the reference values [10], [38]. All the packed-bed column properties and operating conditions are summarized in Table 2. Each operation was conducted at different amine flow rates (different L/G ratio). Therefore, the main difference between the Tests was the circulation flow
Assessment of process modification
It was reported that the RSS configuration, applied to a conventional absorption process using MEA, could enhance the capture efficiency [12]. In this study, the change in reboiler duty is evaluated when the same RSS configuration as that shown in Fig. 1 (red line) is applied to the absorption process using multi-amines. The split ratio in the RSS configuration indicated that a portion of the rich amine flow was directly supplied to the top of the stripper without passing the rich/lean amine
Conclusion
The performance of the absorption process using multi-amines for post-combustion CO2 capture was investigated by developing a rate-based model. The energy efficiency and sensitivity analysis of the absorption process using AEEA and DETA were conducted based on a 90% CO2 capture and 30 wt% amine solution. The results were compared with those obtained using MEA as the reference solvent.
The reboiler duty of the conventional configuration using DETA was 3.242 GJ/tonCO2, which was 8.57% lower than
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2019K1A4A7A03113187).
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