3.2.1. Co-Citation Literature Network
Literature analysis is one of the primary contents of knowledge mapping. The literature references can be analyzed from the perspective of visualization. The network of the literature co-citation in the research area of CCS technology can be developed using CiteSpace, which is shown in
Figure 4. The nodes are the literature topics and the lines illustrated as various colours in the figure represent that the co-citation frequency of lines reached the threshold. What is shown that the important publications were tightly linked, which means that those studies concerned about similar research topics. Some previous studies represent the major research contents in this specific domain, while the core research concepts received a progressive and continuous development. Furthermore, the nodes separated from the core literature represent some new research branches, especially the new technical means and practice of this particular area.
Table 3 lists the Top 20 references in CCS research area according to their frequency of citation. They were selected for analysis by CiteSpace. It can be seen that the literature was the most frequently-cited article and the frequency was 656, in which latest advances and developing trends in CO
2 separation were discussed, and a particular attention was also paid on advances in the technical area of metal-organic frameworks [
27]. Then, the literature ranked 2nd has a frequency of 625, in which CO
2 adsorption behaviors were described while the current available CO
2 adsorbents and their significant features were also presented [
28].
In terms of the source of the highly cited literature in the research area of CCS technology, there were three documents published in
Energy & Environmental Science (Impact factor: 29.518). In addition, there are two documents published in
Science. One of the references was published in
Science, with a frequency value of 603, in which the process and solvent improvements of amine scrubbing to separate CO
2 from coal-fired power plants were investigated [
29]. The other article published in
Science has a frequency value of 272, in which several hurdles, such as technology, commercialization, and politics related to CCS technology were addressed [
2]. There is one paper published in
Nature with frequency value of 180, in which the technological applications about the metal-organic materials (MOMs) were reported [
30]. The only one book listed in the table is edited by Metz B. [
31] and published in 2005. This book has important influences on both academic and practice in CCS domain. It not only described the situation and challenges of carbon dioxide emissions, analyzed the technological contents of CCS, but also discussed the problems related to the technical applications of CCS.
3.2.2. Co-Occurring Keyword Network
The keywords can reflect the important concepts or core contents of a document. The research topics and hotspots can be detected using the keyword co-word analysis of CiteSpace [
32]. The position, collaboration, and connection of the keywords of the references in the research area of CCS technology can be investigated using a keyword co-occurring network map.
Figure 5 shows the co-occurring network using keywords as “node type”.
It can be seen that most of the keywords are closely connected to each other. Therefore, it can be concluded that the main studies in the field of CCS technology were interlinked, the core keywords in this specific domain includes “carbon dioxide”, “CO2 capture”, “adsorption”, “separation”, “performance”, “metal organic framework”, and “activated carbon” as well as “adsorption”, and more. It means that these keywords represent the research topics in the field of CCS technology. In addition, some keywords, such as “hydrogen storage”, “membrane”, “polymer”, “permeability”, “pressure”, and “carbon sequestration”, have become the important research branches in the research area of CCS technology. Although these keywords have a weaker correlation with core keywords, they may also, to some extent, represent novel theories or practical requirements in the field of CCS technology.
In order to investigate the popular research topics in the discipline of CCS technology and their evolution over time, the keywords that appeared more than 300 times were collected and listed based on years in
Table 4. A total of 35 keywords are listed in the table. Because the purpose of this analysis is to investigate the changes in centrality over time, words/phrases with the same meaning were not merged in this study. As seen in the table, in terms of frequency, “carbon dioxide” appeared most frequently of all the keywords, followed by “CO
2 capture”, “adsorption”, “separation”, and “carbon dioxide capture”.
The studies represented by keywords with a highly-cited frequency have been concerned by many institutions/scientists. Therefore, these keywords can represent the research hotspots in the research area of CCS technology. In addition, it also indicates that the academic researches on CCS technology are mainly due to people’s attention to the problems of CO2 and climate change. Some specialized disciplines included in the table, such as “kinetics”, “flue gas”, “adsorbent”, “power plant”, and “metal organic framework”, reflect the continuous development of CCS technology and the expansion of its application areas.
In terms of centrality, “CO2” is the most important keyword with a centrality of 0.17, which is quite a high centrality value among all the keywords listed in the table. It is followed by “Kinetics” with a centrality of 0.12, and then “CO2 capture” and “sequestration”, both of which have a centrality of 0.11. This means that the researches represented by the keywords with high centrality have a greater impact on the research area of CCS technology, therefore, they represent the core contents and important sectors in this specific domain of technology.
The valuable information from the changes of keywords over time also can be analyzed. In order to illustrate the developing trends in CCS research area, the keywords listed in
Table 4 can be divided into two periods, 2000–2005 and 2006–present. It can be seen that there are 28 keywords in the period of 2000–2005, which accounts for 80% of the total keywords and seven keywords in the period of 2006–present. The keywords “Carbon dioxide capture” in both of the two periods were cited frequently. It can be considered that the research contents represented by the keywords have not changed for a long period and became the foundation of the research field of CCS technology. Some keywords, such as “performance”, “metal organic framework”, “technology”, “solubility”, and “monoethanolamine”, did not appear in the period between 2000–2005, but they appeared in the period from 2006–present, which shows that these keywords may represent new research topics related to practical applications in the domain of CCS technology in recent years.
3.2.3. Main Contents of Important Research Topics
(1) Representative significant research contents related to “Carbon dioxide” or “CO2” (2000).
In the study by Hobbie et al. [
33], they believed that although arctic and boreal area is of significance in current carbon circle, carbon sequestration activities within northern soils have not been understood very well, to explore the special properties of soil carbon in the systems of high latitude can improve current knowledge about carbon fluxes and even be helpful for evaluating the impacts of these systems on climate changes in the future. Schimel et al. [
34] used the historical climate data to invest the impacts of CO
2 and climate on the carbon sequestration in the U.S. ecosystems. Treseder et al. [
35] discussed that it is very important for evaluating the effect of world climate change on the circling of mycorrhizal carbon to investigate the decomposition of hyphal under the conditions of elevated carbon dioxide and nitrogen. Ravikovitch et al. [
36] considered that carbonaceous adsorbents can play significant roles in practice application, therefore, they tried to investigate a special method to obtain the feature of pore size of micro-porous carbonaceous substances. Ding and Alpay [
37] carried out an experiment and found that the complexities of carbon dioxide adsorption on hydrotalcite could change the adsorption efficiency of the material.
(2) Representative significant research contents related to “kinetics” (2001).
Sobkowski and Czerwiński [
38] reported the kinetics of carbon dioxide adsorption by using the technical method of radiometry. They found that the adsorption feature of carbon dioxide is because the surface reaction on the electrode, the adsorption rate will raise if the electrode is set under a specific range, the concentration of surface will not be affected by the temperature and carbon dioxide concentration. Nugent et al. [
30] examined how, in order to purify the products (i.e., fresh water and gases) in the industrial field, the expenditure of energy utilization could account for about 15% of energy output worldwide, and the request for the products is expected to increase three times by the year 2050. They described that the metal-organic materials with special technical properties offered an unprecedented carbon dioxide sorption selectivity over nitrogen, hydrogen and methane. Ding and Alpay [
37] reported that, since the 1950s, the kinetics of carbon dioxide adsorption by different kinds of adsorbents had been examined broadly, while carbon dioxide adsorption capacity on hydrotalcite have been present recently. Serna-Guerrero and Sayari [
39] reported that greenhouse gases have serious implications for our environment and ecosystem, lots of initiatives and new technologies were developed to deal with the emissions of carbon dioxide, and therefore, they tried to simulate the CO
2 adsorption kinetic model in their study. Ochoa-Fernandez et al. [
40] described that, how to remove the carbon dioxide from waste gases is increasingly important during the activities of energy utilization, therefore, it is necessary for materials to regenerate a stable sorption capacity and improve the kinetic properties in the steps of both sorption and desorption.
(3) Representative significant research contents related to “CO2 Capture” (2001).
Johnson and Keith [
41] discussed that CCS technology has been regarded as an important technical measure to solve the contradiction between fossil-based electricity production and environmental issues related to climate changes, the adoption of CCS technology can contribute greatly to decreasing the economical expenditure of dealing with carbon dioxide emissions in electric sector. Huang et al. [
42] described that there are two steps for current carbon storage technology, firstly, to use an amine to separate and capture the CO
2, and then, to pressurize the gas to supercritical carbon dioxide liquid. They found that the Dual Alkali Approach technique could be significant to enhance the efficiency in separating and capturing the carbon dioxide.
(4) Representative significant research contents related to “sequestration” (2000).
White et al. [
43] reported that, as one of the most urgent and severe issues in the area of the environment, global warming has become a worldwide issue and need to be confronted and solved by every nation. They described that carbon storage refers to sequestrating the carbon dioxide in a designated location for a long term, therefore, it has become an effective technical means to mitigate the problem of global warming. Nowak and Crane [
44] illustrated that CO
2 is an important part of greenhouse gases, a larger utilization of fossil-based energy production would be responsible for the emissions growth of CO
2, and urban forests can be tremendous useful for the decline of carbon dioxide in atmosphere, however, urban trees would also have negative effects on ecosystem. Stewart and Hessami [
45] described that the sequestration of CO
2 was not yet mature, it is turned out that the techniques—no matter whether geologic or oceanic injectionare—still not sustainable, therefore, they present a possible technical measure to minimize the emissions of GHGs. Chu [
46] reported that the energy production of fossil-based fuel leads to the growth of CO
2 emissions and causes severe climate change problems, and it is important to drastically decrease the CO
2 emissions in order to avoid possible environmental risks in the future. Herzog [
47] examined the technical means related to carbon storage which involve processes of removing the GHGs and storing them in a selected reservoir. He described that, once a large quantity of carbon is captured, it would be ideal to achieve its commercial use, however, it also have some limitations in large-scale applications, therefore, as to a larger amount of captured carbon, it may be better to store it in geology or ocean.
(5) Representative significant research contents related to “solubility” (2007).
Suekane et al. [
48] reported that, as to residual carbon dioxide stored in geology, solubility trapping is significant important in aquifer storage, some of the CO
2 stored in geology will dissolve and mix in the geological formation water. As to carbon storage technology, solubility trapping and residual gas can help improve storage capacity and efficiency. Keppler et al. [
49] described that carbon solubility in olivine will be useful for capturing the dynamics of carbon dioxide exchange. They found that carbon solubility in olivine is unexpected low, which is fundamentally different from previous studies. Mitchell et al. [
50] investigated the potential of microorganisms for improving the capacity and efficiency of CCS technology by using the techniques of solubility trapping and mineral-trapping.
(6) Representative significant research contents related to “performance” (2006).
Rubin and colleagues [
51] reported that technological performance and economic expenditure are significant influencing factors in policy analysis of CCS technology. They found that, the emissions of carbon dioxide produced by power plant would be greatly reduced by CCS technology. Davision [
52] evaluated the performance and costs of three main technologies for carbon capture employed in power plants, and found that, the economical expenditure of electricity would be expected to reduce by 18% under their designed technical conditions, and the advances in carbon dioxide capture technology would contribute to the reduction of economical expenditure. Abu-Zahra et al. [
53] evaluated that to deploy CCS technology in a generating station will increase the expenditure of its electricity production, and the improvement and optimization of process will contribute to reducing the entire expenditure of carbon capture technology.
(7) Representative significant research contents related to “metal organic framework” (2009).
Sumida et al. [
54] reported that CCS technology can efficiently capture carbon dioxide emitted from industrial sources, however, it can play an important role until the energy infrastructure has been modified greatly. They analyzed that, in order to decrease the cost related to CCS technology, it is essential to pay attention to the influencing factors related to the performance of carbon capture technology. And then, they described that metal-organic frameworks can be employed in carbon capture technology as an emerging and ideal materials. Millward and Yaghi [
55] also analyzed several benefits of metal-organic frameworks for carbon storage technology. Li et al. [
56] reported that, taking cost into account, to identify the emerging materials will become a significant issue in the research area of CCS technology. They examined that, because metal-organic frameworks have advantages in the cost and efficiency of synthesis, they can be greatly developed in the area of carbon capture technology as a separation material.
(8) Representative significant research contents related to “Monoethanolamine” (2007).
Freguia and Rochelle [
57] reported that carbon storage technology has become an important technical means to cope with the environmental problem of global warming. They also described that, as an effective solvent for carbon capture, absorption with aqueous monoethanolamine has achieved practical application status. Strazisar and White [
58] described that, in the technical process of CCS, how to separate carbon dioxide is one of the most critical parts, besides, they also analyzed that, as an effective and mature technology, monoethanolamine-based chemical absorption techniques have been widely practiced in the industry.