Experimental Study on High-Efficiency Cyclic CO₂ Capture from Marine Exhaust by Transition-Metal-Modified CaO/Y₂O₃ Adsorbent

Abstract

CaO-based adsorbent cycling carbon capture technology is an effective way to reduce CO₂ emissions from marine exhaust gases. Metal-modified CaO-based adsorbents represent one of the important ways to improve the cyclic CO₂ capture capacity. In order to obtain economical and efficient CaO-based adsorbents, transition metal (Cu, Fe, Co, Cr, Ni)-modified CaO/Y₂O₃ adsorbents were prepared using the sol–gel method. CO₂ cyclic adsorption capacity tests were carried out in a fixed bed. The microstructure of the adsorbents was analyzed using XRD, SEM, and BET. The adsorption performance and cycle stability of the modified CaO/Y₂O₃ adsorbents were investigated in depth. The results show that the Fe-CaY adsorbent had the best adsorption performance. The initial adsorption capacity of Fe-CaY was 0.62 g/g at 650 °C, and the adsorption capacity was 0.59 g/g at the 25th cycle. Fe-CaY-doped samples with the largest pore size and specific surface area showed the best adsorption performance due to the contribution of macropores in the prevention of sintering. Fe doping can greatly improve the CO₂ adsorption capacity and cycle stability of an adsorbent and also reduce the CaO-based adsorbent cycle temperature. In addition, the Fe-Ni-CaY adsorbent had the best adsorption performance among the bimetallic (Cu-Ni, Fe-Ni, Co-Ni, Cr-Ni)-modified CaO/Y₂O₃ adsorbents. However, compared with Fe-CaY, the adsorption capacity decreased. The reason for this might have been that the addition of Ni destroyed the rich pore structure between Fe-Ca-Y and the stability of the adsorbent particle structure, which led to the aggregation of CaO crystals and reduced the CO₂ adsorption capacity. Therefore, the Fe-CaY developed in this study has excellent adsorption capacity and cyclic stability, which makes it a promising adsorbent for CO₂ capture in marine exhaust gases.

1. Introduction

With the increasingly serious problem of global climate change, CO₂ emissions have become one of the core issues of global concern. The development of maritime transportation has greatly increased the emission of carbon dioxide (CO₂), which is a major greenhouse gas [1]. CO₂ emissions from maritime transportation represent around 3% of the total annual anthropogenic greenhouse gas emissions [2]. CO₂ capture, utilization, and storage technology (CCUS) collects and stores CO₂ released from production activities via appropriate methods to prevent it from being discharged into the atmosphere, which can temporarily solve the current carbon emission problem [3]. CCUS technology is mainly divided into three modes: pre-combustion capture, oxygen-enriched combustion, and post-combustion capture. Post-combustion capture is the most mature method for carbon capture at present. If it is used on ships, it can be modified on an existing waste gas treatment device, which causes little modification to the ship system and less investment. It is the most likely CO₂ capture method to be widely used on ships.

The main problems currently faced in capturing carbon dioxide from ship exhaust gases are limited ship space, as well as safety and economy. At present, the aqueous alkanolamine process and membrane separation technology are applied to marine carbon capture systems. However, an absorption capture system using the aqueous alkanolamine solution requires the addition of absorption towers, regeneration towers, coolers, and pumps. This takes up space and poses security concerns. In addition, the selectivity of membrane materials for CO₂ is not high enough. The separation of marine exhaust gases via membrane materials can cause mixing with other gases, resulting in poor carbon capture. At the same time, the production cost of membrane materials is high.

For CaO-based adsorbents, CO₂ can be stored in the stable compound CaCO₃ and does not require compression and liquefaction. A diagram of the post-combustion CO₂ capture of CaO-based adsorbents for marine applications is shown in Figure 1. The use of CaO-based adsorbents to capture CO₂ can not only save space, but also solve the safety and economic problems mentioned above. By storing CO₂ in CaCO₃, the resulting CaCO₃ can be sold as a building material, and at the same time, the revenue from the sale of CaCO₃ can also subsidize the cost of carbon capture.

Zhou et al. [4] proposed a method that uses NaOH and CaO as absorbent materials to capture CO₂. Through two case studies of bulk carriers and container ships, the economic feasibility data and practical installation guidelines for marine carbon capture were provided. Akker and Feenstra et al. [5,6] proposed an absorption process that uses monoethanolamine (MEA) and piperazine (PZ) as absorbents to capture CO₂. A case study of an 8000 dwt general cargo ship fueled by LNG and diesel was carried out to simulate the CO₂ capture and liquefaction process and to evaluate the economic benefits. Luo and Wang [7] presented the design results for the CO₂ absorption process using ethanolamine on a 35,000 gross ton cargo ship and simulated and evaluated the CO₂ capture situation. From the above research, it can be seen that marine CCUS technology has received extensive attention. However, at present, marine CCUS technology is still in its infancy worldwide and mainly based on theoretical research and laboratory verification.

CaO-based adsorbents, as one of the adsorption materials for post-combustion CO₂ capture, have become a very promising adsorbent for CO₂ capture in ship exhausts [8]. The advantage is that the calcium material has a large theoretical adsorption capacity (0.786 g CO₂/g CaO), wide sources, and low prices, which ensures the good economic value of the calcium circulation system [9]. In addition, less waste is generated in the recycling process, and the adsorption materials used are easy to handle, which causes relatively little harm to the environment. For post-combustion carbon capture in marine systems, solid CaO-based adsorbents have certain advantages in terms of saving energy and space [10]. Currently, the most efficient diesel engines still waste about 50% of their energy. Although the temperature range of the waste heat of a marine engine is lower than the adsorption temperature of a CaO-based adsorbent, the waste heat of an exhaust gas can be used to improve the overall energy efficiency of marine systems [11]. Therefore, reducing the CaO-based adsorbent cycle temperature to improve the economy is also one of the concerns of many scholars. However, with the increase in carbonation and calcination times, calcium-based adsorbents are prone to sintering, which leads to an increase in CaO particle sizes, decrease in specific surface areas, and decrease in adsorption capacities [12,13]. Therefore, the modification of CaO-based adsorbents is particularly important [14]. Metal doping modification can effectively solve the problem of sintering and the inactivation of CaO-based adsorbents [15].

Grasa et al. [16] found that the carbonation conversion of conventional CaO-based adsorbents decreased rapidly with increasing cycle times and the attenuation rate reached 92.0%. Therefore, the improvement of the cycle stability of calcium-based adsorbents has become a research hotspot and the focus of many researchers. Guo et al. [17] doped Zr into calcium-based adsorbents using the sol–gel method and studied the influence of Zr doping on the properties and structure of CaO-based adsorbents. The results of X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the CaZrO₃ phase formed in the adsorbent can not only inhibit the growth and aggregation of CaO microcrystals, but also act as an inert barrier to delay the sintering of CaO. Kuang et al. [18] found that the circulation performance of a Y-modified CaO-based adsorbent is obviously better than that of ordinary limestone [19]; the specific surface area of the adsorbent is obviously increased, and the mesoporous ratio < 20 nm increases, which obviously improves the circulation performance of the adsorbent. However, the price of Y is high, and a large amount of Y₂O₃ doping cannot meet the requirements of high efficiency and economy.

At present, the methods for preparing modified CaO-based adsorbents by doping other elements include the sol–gel combustion method, the dry mixing method, the wet mixing method and the coprecipitation method [20]. Among the CaO-modified adsorbents prepared by the sol–gel combustion method, the additives and CaO are best mixed. Because the preparation process is accompanied by intense citric acid combustion, which produces a very fluffy structure, the pore structure of the adsorbent is developed, and the performance of the modified CaO-based adsorbent prepared is much superior to that of the modified CaO-based adsorbent prepared by other methods [21].

In this study, four transition metals, Cu, Fe, Co, and Cr, were added with monometallic doping on the basis of Y-modified CaO-based adsorbents, and metal Ni was added for bimetallic doping. The effects of different types of transition metals, doping ratios, and carbonation temperature on the adsorption cycle performance were studied by cyclic CO₂ capture experiments. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and N₂ adsorption–desorption were used to study the effects of monometallic and bimetallic doping on the physical and chemical properties and particle structure of the CaO-Y₂O₃ adsorbent. This study aims to provide important and balanced guidance for the selection of transition-metal-modified calcium-based adsorbents, in order to design energy-saving and stable CO₂ adsorbents.

2. Experiment

2.1. Preparation of Adsorbent

The modified CaO-based adsorbent was prepared by the sol–gel method. The principle of the sol–gel method is that the metal salt is uniformly dispersed in the liquid phase by continuous stirring, citric acid is used as a complexing agent to form a complex with metal ions, and the complex gel is formed by the sol–gel process and placed in a high temperature muffle furnace. Citric acid as fuel and metal salt as oxidant undergo a redox reaction, releasing a large amount of gas and heat, causing the gel to burn violently and expand, and finally obtaining a fine powder sample [22].

Y(NO₃)₃·6H₂O, Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O, Cr (NO₃)₃·9H₂O, and Ni(NO₃)₂·6H₂O were used as precursors, and Ca (CH₃COO)₂ was used as the Ca-based precursor. The metal nitrate was dissolved in a beaker with an appropriate amount of deionized water. The mole ratios of total metal: citric acid were controlled at 1:1. The precursor liquid was stirred for 3 h at 80 °C in the water bath and dried at 100 °C for 12 h after the gel appeared. After drying, it became a porous fluffy substance. The fluffy porous substance was calcined in the air atmosphere at 200 °C for 2 h and then calcined in an air atmosphere of 700 °C for 5 h. Finally, a powder was obtained after grinding and stored for characterization experiments in a clean test tube.

Table 1. Synthesized Ca-based adsorbents.

Serial Number	Sample Named	Mass Ratio
1	Cu-CaY	Cu/Fe/Co/Cr:CaO:Y2O3 = 7:85:8
2	Fe-CaY
3	Co-CaY
4	Cr-CaY
5	Cu-Ni-CaY	Cu/Fe/Co/Cr:Ni:CaO:Y2O3 = 5:5:85:5
6	Fe-Ni-CaY
7	Co-Ni-CaY
8	Cr-Ni-CaY

shows the synthesized CaO-based adsorbents [23].

2.2. CO₂ Cyclic Adsorption Experiment

A diagram of the fixed bed experimental setup is shown in Figure 2. The red circled part indicates that this is zoomed in to show its structure. Adsorption experiment: Approximately 400 mg of adsorbent sample was weighed and placed in the middle of the tubular furnace. Before the cyclic adsorption test, the sample was reduced at 650 °C for 1 h in an atmosphere of 20 mL/min of H₂ and 180 mL/min of N₂. In this process, the metal oxides are reduced to metal elements, except for CaO and Y₂O₃, which are not reduced due to the higher reduction temperature required. After reduction, 150 mL/min of CO₂ was introduced for adsorption at 650 °C for 30 min. For the calcination step, the sample was calcined at 700 °C for 20 min in an atmosphere of 180 mL/min of N₂. The above adsorption and calcination processes were repeated to complete the cycle experiment [24].

The above carbonation/decarbonation process was repeated for 25 cycles. The carbonation capacity of the sample was calculated according to the following equation:

$$\mathrm{carbonation} \mathrm{capacity} (\mathrm{g}/\mathrm{g})=\frac{{\mathrm{m}}_1{ - \mathrm{m}}_0}{{\mathrm{m}}_0}$$

(1)

where m₁ is the mass of adsorbent after a certain cycle adsorption. m₀ is the mass of adsorbent before adsorption in this cycle.

2.3. Methods of Characterization of Adsorbents

The Ultima IV X-ray diffractometer made by Rigaku Company of Japan was used for the phase determination and crystal phase analysis of the adsorbent. The sample was laid on a glass slide with a copper target and placed in the X-ray diffractometer. The scanning range was 10~90°, and the scanning speed was 5°/min. Highscore software was used to analyze the test data.

The SUPRA 55 SAPPHIRE field emission scanning electron microscope produced by ZEISS Company of Germany was used to analyze the appearance and distribution of the elements in the samples. Scanning electron microscopy (SEM) was imaged by a backscattered electron beam or secondary electrons. Gold was sprayed under a vacuum for 150 s before the test. The amplification factor was 10~100 K, and the acceleration voltage was 5~20 kV.

The adsorbents were adsorbed and desorbed using an ASAP 2460 automatic specific surface area and porosity analyzer produced by Micromeritics Company of USA (Norcross, GA, USA). The specific surface area and pore characteristics of the adsorbents were analyzed by multimolecular layer adsorption theory and the pore size distribution calculation model.

3. Results and Discussion

3.1. XRD Analysis

Cu-CaY, Cu-Ni-CaY, Fe-CaY, Fe-Ni-CaY, Co-CaY, and Co-Ni-CaY were characterized by XRD, and their crystal structures were analyzed. The XRD patterns of the monometallic and bimetallic doped CaO-based adsorbents are shown in Figure 3. The characteristic peak of CaO can be clearly observed, indicating the formation of the CaO crystal phase. The characteristic peaks of Y₂O₃, Ni, Cu, Fe and Co can also be observed, indicating the formation of the corresponding crystalline phases. However, there are less obvious characteristic peaks of CaCO₃, which may have been caused by the reaction between CaO and CO₂/H₂O from the atmosphere. Meanwhile, the diffraction peaks of doped elements of all samples are not obvious, which indicates that the crystals of these doped elements are highly dispersed on the surface of CaO. Additionally, these crystals prevent the growth and agglomeration of CaO crystals, which is beneficial for the capture of CO₂ in multiple cyclic adsorption processes [25].

3.2. SEM and EDS Analysis

Figure 4 shows the SEM images of monometallic doped adsorbents. The larger bulk particles are CaO crystals, whereas the small granular crystals attached to them are dopants. Because of the different doped metals, there are some differences in the microscopic morphology of the adsorbents. It can be seen from Figure 4a that Cu-CaY has an uneven crystal size and few pores, and the dispersion of the dopant is also uneven. This will make CO₂ difficult to diffuse, and the CO₂ capture performance was low in the CO₂ adsorption experiment [26]. In Figure 4c,d, Co-CaY and Cr-CaY have smaller particle sizes, but the dopants are partially clustered together. The aggregation of dopants will reduce the sintering resistance ability of the adsorbents, which will lead to a decrease in the adsorption capacity of CO₂. Compared to the other three samples, Fe-CaY in Figure 4b is smaller and more uniform, and there are more pores in the adsorbent, while the dopants attached to them are highly dispersed. Fe-Ca-Y maintains a rich pore structure, thus forming channels between particles, and the pore volume and specific surface area should be larger than those of other metal-doped adsorbent materials. The loose and porous structure is conducive to more CO₂ entering the adsorbent particles to react with active CaO, thus improving the CO₂ capture performance of the adsorbent [27].

Figure 5 shows the SEM images of bimetallic doped CaO-based adsorbents. Figure 5a shows a Cu-Ni-CaY adsorbent, and the dopants are more evenly dispersed than those in Figure 4a. This may be due to the addition of Ni and the decrease in Cu content, thus reducing the high-temperature aggregation of the adsorbent due to Cu. Figure 5c,d shows Co-Ni-CaY and Cr-Ni-CaY adsorbents, and the CaO particles are more dispersed than in Co-CaY and Cr-CaY in Figure 4c,d. The addition of Ni plays a spacing role between CaO crystals, which can reduce the sintering phenomenon [28]. Figure 5b shows a Fe-Ni-CaY adsorbent. Compared with Fe-CaY in Figure 4b, the CaO particles are enlarged. It is speculated that the reason may be that the addition of Ni destroys the rich pore structure maintained by Fe-CaY, leading to the aggregation of CaO crystals and reducing the CO₂ adsorption capacity of samples [29].

Figure 6 and Figure 7 show the EDS diagrams for monometallic and bimetallic doped CaO-based adsorbents. Each element is uniformly distributed and highly dispersed in the adsorbent. The uniform distribution of doped metals is beneficial to inhibit the sintering phenomenon between CaO particles, thus maintaining the pore structure of the adsorbent and enhancing the cycle stability of the adsorbent. At the same time, uniformly dispersed CaO particles have more active sites to react with CO₂, which can effectively improve the carbonation effect of the adsorbent [30]. Compared with Figure 6a,c,d, it can be seen from Figure 7a,c,d that the adsorbent particles are more dispersed after doping Ni metal, which is consistent with the results of SEM analysis.

3.3. BET Analysis

To further study the CO₂ capture performance of modified CaO-based adsorbents, the following samples (Cu-CaY, Fe-CaY, Co-CaY, Cu-Ni-CaY, Fe-Ni-CaY, Co-Ni-CaY) with good adsorption performance were chosen as candidates for BET characterization. The results are shown in Figure 8 and

Table 2. Pore size, Pore volume and specific surface area of the adsorbents.

Adsorbent	Pore Size (nm)	Pore Volume (cm3/g)	SBET (m2/g)
Cu-Ni-CaY	19.9588	0.0166	11.0622
Cu-CaY	19.6457	0.0129	7.3152
Fe-Ni-CaY	21.6570	0.0251	25.9717
Fe-CaY	22.9176	0.0317	27.1188
Co-Ni-CaY	7.0618	0.0030	4.1180
Co-CaY	5.9285	0.0025	4.0900

. Table 2 shows the data for the pore size, the pore volume, and the specific surface area of the adsorbents. It can be seen from Table 2 that Cu-Ni-CaY and Co-Ni-CaY have larger pore size, pore volume, and specific surface area than Cu-CaY and Co-CaY. The increases in the specific surface area and pore size of Cu-Ni-CaY and Co-Ni-CaY are obvious. Among all of the adsorbents, Fe-CaY with the largest pore size and specific surface area shows the best adsorption performance because of the contribution of macropores in preventing sintering. Therefore, a sufficiently large pore in the macroscopic range can effectively prevent CaCO₃ from filling and plugging the pores during CO₂ adsorption. The above analysis is consistent with the results observed by scanning electron microscope.

The pore size distribution curves and BET isotherms of various samples are shown in Figure 8. It can be observed from Figure 8a that the ratio of the larger pores of the two samples doped with Fe is significantly higher than that of the samples doped with other metals. In particular, the Fe-CaY adsorbent has the formation of a macroporous structure. It can also be seen from Figure 8b that the adsorption capacity of the Fe-CaY adsorbent is obviously higher than that of other adsorbents. It shows mixed characteristics of type II and type IV (IUPAC classification), indicating that mesoporous and macroporous structures coexist, which is consistent with the above analysis. Isotherms with narrow H3 hysteresis loops reveal the existence of mesopores [25].

4. CO₂ Adsorption Performance

4.1. Monometallic Doped CaO-Based Adsorbent

The high-temperature adsorption properties of CaO-based adsorbents modified by Cu, Fe, Co, and Cr were compared and analyzed. Comparison of the CO₂ adsorption capacity of the monometallic doped CaO-based adsorbent at different temperatures is shown in Figure 9. It is clear that the adsorption capacity of Cu-CaY and Cr-CaY at 600 °C and 650 °C is similar, and the adsorption capacity of Co-CaY at 650 °C is 0.067 g/g higher than that at 600 °C. It shows that the CO₂ adsorption capacity of Co-CaY is greatly affected by the adsorption temperature. The adsorption capacity of Fe-CaY at 600 °C and 650 °C is the highest, which is 0.64 g/g and 0.62 g/g, respectively. Combined with the above characterization results, the specific surface area and micropore characteristics of Fe-CaY are the best among the adsorbents. Therefore, the Fe-CaY developed in this study had the best adsorption performance among the monometallic adsorbents. Zamboni et al. [27] also demonstrated that the addition of Fe can enhance the adsorption properties of CaO-based adsorbents.

4.2. Bimetallic Doped CaO-Based Adsorbents

The high-temperature adsorption properties of Cu-Ni, Fe-Ni, Co-Ni, and Cr-Ni bimetallic modified CaO-based adsorbents were compared and analyzed. A comparison of the CO₂ adsorption capacity of the bimetallic-doped CaO-based adsorbents at different temperatures is shown in Figure 10. It is obvious that the adsorption capacity of Co-Ni-CaY was the highest at 600 °C, reaching 0.57 g/g. At 650 °C, the adsorption capacity of Fe-Ni-CaY was the highest, reaching 0.61 g/g. The CO₂ adsorption capacity of Cr-Ni-CaY samples at different temperatures was the lowest, and the adsorption performance was the least ideal. Among the four bimetallic doped CaO-based adsorbents, only Fe-Ni-CaY samples had a CO₂ adsorption capacity exceeding 0.6 g/g at 650 °C. Among all of the monometallic and bimetallic doped CaO-based adsorbents, the adsorption capacity was greatly improved by Fe doping, which is consistent with the previous analysis.

In this part, in order to investigate whether the addition of Ni promoted or inhibited the adsorption capacity of the adsorbents, a comparison of Ni addition to the CO₂ adsorption capacity of the modified CaO-based adsorbents was investigated at different temperatures, and the results are shown in Figure 11. It illustrated that at 600 °C, only Co-Ni-CaY had a higher adsorption capacity than Co-CaY among the bimetallic-doped CaO-based adsorbents, and the addition of Ni to the other three adsorbents reduced the adsorption performance. This shows that the addition of Ni mainly inhibited the adsorption performance of samples at 600 °C. At 650 °C, the adsorption capacity of Cu-Ni-CaY and Cr-Ni-CaY was higher than that of Cu-CaY and Cr-CaY, which indicates that the addition of Ni could promote the adsorption performance of the above two samples at this temperature. For Fe-modified CaO-based adsorbents, the adsorption capacity of Fe-CaY was higher than that of Fe-Ni-CaY at any reaction temperature. According to the analysis of the microstructure and experimental data in the previous part, the possible reason is that Fe-CaY maintains more pore structure and improves the stability of the structure. The addition of Ni destroyed the channel formed between the Fe-Ca-Y adsorbent particles, which aggregated CaO crystals and reduced the CO₂ adsorption capacity of the samples. Wang et al. [29] also mentioned that Ni disperses on the surface of CaO, blocking the pore structure and ultimately leading to a decrease in the capture capacity of CO₂. Generally, the higher the adsorption temperature, the higher the adsorption capacity of CO₂. However, the adsorption performance of Fe-CaY at 600 °C was better than that at 650 °C. Thus, the next section will study the cycling performance of Fe-CaY adsorbent at different temperatures and delve into their optimal reaction temperature and cycling performance.

4.3. Effect of Temperature on Cyclic Adsorption Performance of Fe-CaY Adsorbent

Figure 12 displays the CO₂ cyclic adsorption performance of Fe-CaY at different temperatures. It shows that the initial adsorption capacity of the Fe-CaY adsorbent was 0.60 g/g at 550 °C, and the adsorption capacity dropped sharply to 0.37 g/g at the fourth cycle. The initial adsorption capacity of Fe-CaY was 0.64 g/g at 600 °C, but the adsorption capacity decreased to 0.48 g/g in the 10th cycle. At 650 °C, the initial adsorption capacity of the Fe-CaY sample was 0.62 g/g. Although the initial adsorption capacity was not as good as that at 600 °C, the cyclic stability of the Fe-CaY adsorbent was optimal at this temperature. The adsorption capacity obviously did not decrease in the first 25 cycles, and it was still relatively stable.

It can be seen in

Table 3. Adsorption properties of different adsorbents.

Adsorbent Property	Ca-Y2O3	Fe-CaY
Reaction temperature (°C)	700	650
Desorption temperature (°C)	850	750
Initial adsorption capacity (g/g)	0.58	0.62
Adsorption capacity after 15 cycles (g/g)	0.52	0.61
Adsorption capacity after 25 cycles (g/g)	--	0.59
References	[18]	In this study

that compared to the Kuang study [18], the Fe doping in this study greatly improved the CO₂ adsorption capacity and cyclic stability, as well as reducing the CaO-based cyclic temperature. The initial adsorption capacity of Fe-CaY was 0.62 g/g at 650 °C; the adsorption capacity was 0.59 g/g at the 25th cycle, and the experimental calcination temperature was 750 °C. According to the microstructure analysis and experimental results in the previous part, the possible reason is that Fe-Ca-Y maintains an abundant pore structure and improves the stability of the structure. The loose and porous structure of the adsorbent is conducive to more CO₂ entering the adsorbent particles to react with the active CaO in the adsorbent particles, thus improving the CO₂ capture performance of the adsorbent.

5. Conclusions

In this study, a series of transition metal (Cu, Fe, Co, Cr, Ni)-modified CaO/Y₂O₃ adsorbents were prepared and applied for CO₂ capture. CO₂ cyclic adsorption capacity tests were carried out in a fixed-bed reactor. The physical and chemical properties of the adsorbents were characterized by XRD, SEM, and BET. The effects of monometallic and bimetallic doping on the CaO-based adsorbents were investigated in depth. The main conclusions obtained were as follows.

(1)

Among monometallic doped CaO-based adsorbents, the adsorption capacity of the Fe-CaY adsorbent is the highest. Compared to the CaO-Y₂O₃ adsorbent, Fe doping optimizes the adsorption performance of the adsorbent. The initial adsorption capacity of Fe-CaY was 0.62 g/g at 650 °C; the adsorption capacity was 0.59 g/g at the 25th cycle, and the experimental calcination temperature was 750 °C. The Fe-CaY adsorbent with the largest pore size and specific surface area of 22.91 nm and 27.11m²/g, respectively, showed the best adsorption performance due to the contribution of macropores to prevent sintering. The loose and porous structure of the adsorbent is conducive to more CO₂ entering the adsorbent particles to react with the active CaO in the adsorbent particles; thus, Fe doping can greatly improve the CO₂ adsorption capacity and cycle stability of the adsorbent, and also reduce the CaO-based adsorbent cycle temperature.

(2)

The results show that Ni addition had a different impact on the CO₂ adsorption capacity of the adsorbent. At 600 °C, the addition of Ni mainly inhibited the adsorption properties of the samples. On the contrary, at 650 °C, the adsorption ability of CaO-based adsorbents modified with Cu and Cr was enhanced by adding Ni. Among the Fe-modified CaO-based adsorbents, the adsorption capacity of Fe-CaY was higher than that of Fe-Ni-CaY. The addition of Ni destroyed the rich pore structure between Fe-Ca-Y and the stability of the adsorbent particle structure, leading to the aggregation of CaO crystals and reducing the CO₂ adsorption capacity of the adsorbent.

Due to limitations in the depth of the research, the carbon capture technology in this study was mainly based on theoretical research and laboratory validation, and more performance tests are needed to evaluate the developed adsorbent for practical application in the capture of CO₂ from marine exhaust. This will also provide enlightenment for the design of low-temperature and high-efficiency circulation absorption and adsorption materials for marine exhaust CO₂. In the future, work will be further carried out to amplify and apply this carbon capture technology to actual carbon emission capture for marine applications.