Data Sources

The data in this paper is selected from the Web of Science (WOS) Core Collection database, which is the most authoritative database for tracking high-quality research. It holds significant academic value across various fields, including natural sciences, engineering technology, social sciences, arts, and humanities. The search terms used were: “TS = (oxidation and mercury and flue gas) OR TS = (adsorption and mercury and flue gas) OR TS = (catalytic and mercury and flue gas) OR TS = (mercury removal and mercury and flue gas)”, with a period from January 1, 1998, to December 31, 2024. The article types selected were “Article” and “Review Article”. The selected papers were exported in “plain text file” format, with the content option set to “Full Record and Cited References”, and then imported into Histcite Pro and CiteSpace software. The papers were also exported in “Tab-delimited file” format, with the content option set to “Full Record”, and imported into VOSviewer software. A total of 2,489 papers were retrieved, involving 4,496 authors, 290 journals, and 46,153 references.

Research Methods

Statistical analysis of literature related to flue gas mercury removal was performed using the WOS database’s built-in analysis tools, Histcite citation analysis software, VOSviewer, CiteSpace literature visualization software, Excel 2019, and Origin 2021. The research outcomes in the field of flue gas mercury removal were summarized, including publication volume and annual trends, major research institutions, leading journals, research hotspots, and frontiers. Citation analysis was conducted using Histcite Pro 2.1 software to gather statistics on publication volume, total citations, and local citations. Total global citation score (TGCS) refers to the total number of citations of articles within different categories in the WOS database, where the citing papers may be unrelated to the original research direction. Local total citation score (TLCS) refers to the number of citations of articles within different categories in a local dataset (i.e., all literature imported into Histcite Pro 2.1 software after entering search terms into the WOS database). Since articles in the local dataset are related to the search terms, TLCS is often used to reflect the influence within the research field. VOSviewer 1.6.20 literature visualization software was used to map author collaboration and keyword co-occurrence networks, reflecting the core academic groups and research hotspots in the field. CiteSpace 6.3.1 was used for scientific mapping analysis of institutional collaborations and research hotspots.

Fig. 1.

Trend of published literature on flue gas mercury removal research from 1998 to 2024.

Number of Publications and Annual Trends

The publication volume reflects the temporal variation in the number of papers published within a research field, thereby providing readers with insights into the developmental status of the field and enabling predictions of future trends [14]. Analysis of publication volumes over distinct periods in a specific domain can intuitively reveal research dynamics. Analysis of the annual publication volume of flue gas mercury removal research from 1998 to 2024 (Fig. 1) demonstrates an overall increasing trend in publications. The growth trend can be divided into three distinct stages:

(1) 1998-2004: A stage characterized by low publication output and slow growth, with a total of 90 publications and an annual average of approximately 13 papers. It indicates that during this period, research in flue gas purification predominantly focused on conventional pollutants such as sulfur oxides (SO_x) and nitrogen oxides (NO_x), with limited attention to mercury removal.

(2) 2005-2011: A stage marked by accelerated growth, with 307 publications and an annual average of approximately 44 papers. This signifies that mercury removal gradually emerged as a cutting-edge and highly prioritized topic in flue gas purification, attracting global research attention.

(3) 2012-2024: A stage of rapid growth, with 2,092 publications and an annual average of approximately 174 papers. The publication volume peaked at 241 papers in 2021, accounting for 84.05% of the total publications within the statistical period. During this phase, global attention to mercury pollution intensified. In January 2013, the United Nations Environment Programme adopted the Minamata Convention on Mercury to mitigate global mercury emissions and pollution. In China, a series of laws and regulations addressing mercury pollution were enacted starting in 2011. Notably, the Emission Standard of Air Pollutants for Thermal Power Plants (GB13223-2011), implemented on January 1, 2015, for the first time classified mercury as a controlled pollutant in coal-fired power plants and stipulated a stringent emission limit of 0.03 mg/m³. The enforcement of these conventions, laws, and standards has significantly heightened academic interest in mercury removal research, driven by increased research investments and, consequently, accelerated publication growth.

Analysis of Research Countries

The publication volume in a particular research field serves as an indicator of a nation's academic impact and scientific proficiency within that domain [15]. According to the SCI database in the Web of Science core collection, researchers from 64 countries and regions have conducted studies on mercury removal. Table 1 lists the top 10 countries and regions based on publication volume. China holds the leading position with 1,892 papers, representing 76.01% of the total publications, establishing overwhelming superiority in this field. The United States follows with 374 papers (15.03%). China's dominance is further evidenced by exceptional citation metrics: a Total Local Citation Score (TLCS) of 27,437 and a Total Global Citation Score (TGCS) of 62,964, both substantially higher than those of other nations. This reflects substantial research investment and the presence of numerous research teams in China, resulting in a high volume of scientific output. It also highlights China’s strong commitment to mitigating mercury pollution from flue gas. In the United States, scholars have made notable advancements in optimizing Activated Carbon Injection (ACI) technology and elucidating the synergistic mechanisms of mercury removal via SCR catalysts [16, 17]. Canadian researchers have focused on developing low-temperature catalytic systems; notably, the University of Waterloo’s CuO/TiO₂ catalyst achieved Hg⁰ oxidation rates exceeding 85% at 120 °C [18, 19]. In South Korea, significant progress has been made in novel catalytic materials. The Korea Advanced Institute of Science and Technology (KAIST) developed a Ce-doped MnO_x/TiO₂ catalyst that enables highly efficient oxidation at 100 °C [20]. At the same time, Yonsei University investigated synergistic mercury removal by integrating photocatalysis with wet desulfurization processes [21, 22]. In Spain, the Royal Academy of Sciences reported that ionic liquid scrubbing technology achieved a mercury removal efficiency of 98%, and the Technical University of Madrid developed nitrogen-doped activated carbons with enhanced adsorption capacity [23]. Collectively, these efforts underscore the global nature of research on mercury removal technologies and the diverse contributions made by different countries.

Analysis of Key Research Institutions

According to the SCI database in the Web of Science core collection, a total of 1,245 research institutions have conducted studies on mercury removal. Table 2 reveals that all the top 10 institutions in publication output are Chinese. Huazhong University of Science and Technology leads with 295 publications, followed by North China Electric Power University (183) and Southeast University (150). In citation metrics, Huazhong University of Science and Technology maintains its leadership with a TLCS of 6,238 and a TGCS of 12,262. Shanghai Jiao Tong University achieves the highest TLCSPR (about 29 citations per paper), demonstrating the significant impact of both institutions in this field. These findings confirm Chinese research institutions as primary contributors to mercury removal research with remarkable achievements.

Fig. 2.

Co-occurrence network map of publishing organizations.

This study employed CiteSpace 6.3.1 to construct an institutional co-occurrence network (Fig. 2), visually illustrating publication contributions and collaborative relationships among institutions. Each node represents a research institution, with concentric rings of varying colors denoting publication timelines (inner rings correspond to earlier years). The width of each colored ring reflects annual publication volume, while connecting lines indicate collaborations, with line thickness proportional to collaboration frequency [24]. The analysis yielded a network with N = 296 nodes (institutions) and E = 278 links (collaborations), with a network density of 0.0064. The relatively high number of inter-institutional connections suggests active collaborations among leading contributors in this field. Analysis indicates that research institutions in China are at the forefront of this field, possessing notable developmental advantages and occupying a central role in global research networks. Their publication output and frequency of collaboration considerably exceed those of institutions in other countries. International collaborations are primarily concentrated with entities such as the University of Waterloo in Canada and laboratories affiliated with the U.S. Department of Energy. However, these partnerships remain relatively limited, often taking the form of sporadic, single-institution collaborations—for example, joint research on catalyst mechanisms between Huazhong University of Science and Technology and the U.S. Argonne National Laboratory. Overall, international cooperation in this domain remains insufficient and warrants further strengthening. Looking ahead, it is essential to promote more robust technical exchange through transnational research projects, the establishment of joint laboratories, and other collaborative platforms to accelerate the global development of mercury removal technologies for flue gas treatment.

Main Published Journals

Bibliometric analysis enables researchers to identify core journals within a specific research domain. Analysis using the HistCite Pro 2.1 software revealed a total of 290 journals published research papers on flue gas mercury removal during the statistical period. Table 3 shows the top 10 journals by publication volume, predominantly in energy, environmental, and chemical engineering fields, mainly comprising high-impact journals in their respective fields.

In terms of publication volume, Fuel ranked first with 336 publications, demonstrating strong thematic alignment with mercury removal research, as well as indicating its strong appeal to researchers and high submission rates. Chemical Engineering Journal and Energy & Fuels follow closely with 252 and 236 publications, respectively. Subsequent journals include Environmental Science & Technology, Journal of Hazardous Materials, and Fuel Processing Technology, with publications ranging from 108 to 138. These six journals, each exceeding 100 publications, represent the most influential platforms for flue gas mercury removal research.

From the perspective of Total Local Citation Score (TLCS), Environmental Science & Technology (EST) dominates with 6,543 citations, significantly outperforming other journals. As an authoritative and highly influential journal in environmental sciences, it consistently leads research frontiers and hotspots in this field. Chemical Engineering Journal and Fuel rank second and third with TLCS values exceeding 4,500 each, followed by Energy & Fuels at 3,793 citations. These three top journals in chemical engineering and energy demonstrate broad academic impact.

Regarding the Total Global Citation Score (TGCS), Environmental Science & Technology, Fuel, and Chemical Engineering Journal all surpass 10,000 citations with narrow margins between journals, reflecting their global recognition and exceptional influence in the field. Journal of Hazardous Materials, Energy & Fuels, and Fuel Processing Technology follow with TGCS ranging from 5,802 to 6,959, showing notable gaps compared to the top three.

Synthesizing publication and citation metrics, Environmental Science & Technology emerges as the most impactful and guiding journal in flue gas mercury removal. Despite not having the highest publication volume, its TLCS and TGCS demonstrate overwhelming advantages, providing researchers worldwide with cutting-edge breakthroughs and making outstanding contributions to field development. Chemical Engineering Journal and Fuel also exhibit significant strengths in publication volume, TLCS, and TGCS, playing vital roles in advancing this research area. Close attention to these journals enables researchers to efficiently track global trends and hotspot issues in flue gas mercury removal studies.

Distribution of Authors

Table 4 summarizes the top 10 most prolific authors in the field of flue gas mercury removal research from 1998 to 2024, all of whom are affiliated with Chinese institutions. Notably, some authors belong to the same research teams, further highlighting the significant contributions of Chinese researchers in this field.

In terms of publication output, Duan Yufeng from Southeast University, Wu Jiang from Shanghai University of Electric Power, and Liu Yangxian from Jiangsu University ranked as the top three individual contributors. Among the top 10 authors, Liu Yangxian and Wang Yan from Jiangsu University formed the most productive team, achieving a total of 186 publications. Yang Jianping and Li Hailong from Central South University ranked second with 178 collaborative publications. It demonstrates these researchers' sustained engagement in mercury pollution control from flue gases, reflecting their profound expertise, consistent academic output, and substantial influence in this field.

Regarding citation metrics, Li Hailong attained the highest total local citation score (TLCS) of 3,079, followed closely by Qu Zan (2,869 TLCS) and Zhang Junying (2,799 TLCS). In total global citations (TGCS), Liu Yangxian led with 5,346 citations, followed by Li Hailong (4,894 TGCS) and Qu Zan (4,824 TGCS), with Yan Naiqiang and Zhang Junying both receiving approximately 4,500 TGCS. When considering average local citations per paper (TLCSPR), Zhang Junying, Yan Naiqiang, and Qu Zan (along with Li Hailong) demonstrated particularly high performance. These citation metrics confirm these scholars' substantial influence and recognition in mercury removal research, with Li Hailong, Qu Zan, Yan Naiqiang, and Zhang Junying emerging as representative figures whose work exemplifies China's research capabilities and plays a pioneering role in advancing this field.

Systematic analysis of authors and their collaborative networks facilitates identification of core academic groups and prolific contributors in flue gas mercury removal research [25]. Key research teams are led by Liu Yangxian, Duan Yufeng, Li Hailong, Yang Jianping, Wu Jiang, and Qu Zan (Fig. 3). The team led by Liu Yangxian focuses on photocatalytic oxidation and radical-based advanced oxidation methods for multi-pollutant removal [26]. Duan Yufeng's group investigates mercury and heavy metal removal from coal combustion, sustainable renewable adsorbent technologies and equipment for mercury removal in coal-fired power plants, online monitoring systems for mercury in flue gas, and high-efficiency mercury removal technologies and devices [27]. Li Hailong and Yang Jianping's team investigates catalytic oxidation mechanisms of elemental mercury (Hg⁰) and adsorption-fixation mechanisms and stability of mercury in coal-fired flue gas [28]. Wu Jiang's group explores combustion and pollutant control, solar photovoltaic/thermal/photochemical applications, and novel catalytic/energy storage materials, achieving breakthroughs in emerging advanced oxidation technologies and photocatalytic removal of heavy metals from flue gas [29]. Qu Zan's team pioneers innovative methods and theories for mercury pollution control, with notable advancements in Hg⁰ speciation regulation, interfacial reaction mechanisms, and functional materials development [30].

Fig. 3.

Author cooperative network density visualization.

Cluster Analysis of Research Hotspots

“Keywords” refer to the high-level generalization and refinement of an author’s academic ideas, research themes, and research content within a particular field [31]. By analyzing the keywords and their frequency within a specific field, one can grasp the current research hotspots and assess the pace of updates in the field’s research and the vitality of the discipline [32].

Keyword analysis was performed on the literature retrieved for this study using VOSviewer 1.6.20 software, generating a keyword clustering analysis map (Fig. 4). The map uses different colors to represent distinct clusters, and the size of the circles indicates the frequency of keyword occurrences. The keyword co-occurrence network is divided into three clusters: Cluster A focuses on elemental mercury in coal-fired flue gas, addressing mercury forms, emissions, removal, and removal efficiency, primarily via oxidation, catalytic oxidation, adsorption, and other methods for removing elemental mercury. It includes research on adsorbents, catalysts, oxidants, photocatalysis, and selective catalytic reduction. Cluster B centers on the removal of elemental mercury, focusing on low-temperature removal, photocatalytic oxidation of elemental mercury, reaction mechanisms, and reaction kinetics. Cluster C focuses on the simultaneous removal of SO₂, NO_x, and Hg⁰ from flue gas.

Clusters A and B indicate that over the past few decades, many techniques for controlling elemental mercury have been developed, including adsorption removal, catalytic oxidation, advanced oxidation, and traditional chemical oxidation [33-36]. Adsorption removal technology can firstly convert Hg⁰ into Hg^p, which is then captured by electrostatic precipitators (ESP) or bag filters, effectively removing Hg⁰ from flue gas. The main adsorbents used for removing Hg⁰ from flue gas include activated carbon, fly ash, and other carbon-based adsorbents, calcium-based adsorbents, metal oxides, and natural mineral materials [37, 38]. Currently, exploring and developing high-performance and low-cost adsorbents is a key research focus and direction in this field. In recent years, catalytic oxidation removal technology for Hg⁰ in flue gas has also gained widespread attention because it can fully utilize selective catalytic reduction (SCR) denitrification catalysts to oxidize Hg⁰ to Hg²⁺ in a simplified and low-cost process [39, 40]. Currently, the focus in this field is on developing cost-effective and highly reliable catalysts. Both adsorption and catalytic oxidation technologies show promising results in laboratory studies, but still require further improvement for industrial applications due to their low stability, reliability, lack of effective regeneration methods, and high costs. Additionally, traditional chemical oxidation methods, such as KMnO₄, NaClO₂, H₂O₂, O₃, ClO₂, KClO, and ferrates, have been used in various reactor types, employing ionic liquids, air, halides, and membrane separation for the removal of gaseous Hg⁰ [41-44]. However, these technologies remain commercially unviable due to their high costs, secondary pollution, and low removal efficiency. Therefore, more research is needed to develop more cost-effective flue gas mercury removal technologies for large-scale applications. Advanced oxidation technology refers to methods that generate hydroxyl radicals (·OH) through various physical or chemical processes. ·OH is a highly potent oxidant in nature, enabling nearly complete oxidation of most environmental pollutants. Additionally, ·OH-induced oxidation reactions produce almost no secondary pollutants, as the final decomposition products are O₂ and H₂O [45]. Currently, there are four major types of advanced oxidation technologies with great potential for removing Hg⁰ from flue gas: (1) plasma advanced oxidation technology; (2) TiO₂ photocatalytic advanced oxidation technology; (3) photochemical advanced oxidation technology; and (4) active oxidant advanced oxidation technology [46-49].

Cluster C focuses on research related to the simultaneous removal of SO₂, NO_x, and Hg⁰, with the most mature technologies for removing SO₂, NO_x, and Hg⁰ from flue gas being selective catalytic reduction (SCR) [50], wet flue gas desulfurization (WFGD) [51], and activated carbon injection (ACI) [52]. Although this multi-stage treatment strategy can achieve deep removal of SO₂, NO_x, and Hg⁰, it still faces drawbacks such as high construction and operating costs, large land requirements, complexity, and low stability. Therefore, developing an economical, efficient, and integrated method for the simultaneous removal of SO₂, NO_x, and Hg⁰ is one of the main research directions in atmospheric pollution control. Efficient and rapid removal of NO and Hg⁰ is the key step in the simultaneous removal of SO₂, NO_x, and Hg⁰ [53]. Traditional adsorption techniques exhibit limited efficacy in the removal of nitric oxide (NO) and elemental mercury (Hg⁰). Catalytic oxidation has emerged as a superior approach for synergistic control of multiple flue gas contaminants, notably sulfur dioxide (SO₂), nitrogen oxides (NO_x), and Hg⁰. This methodology enhances subsequent capture processes as oxidized species (including NO₂, NO₃, N₂O₅, and Hg²⁺) demonstrate significantly improved solubility, enabling efficient removal through downstream pollution control devices such as wet flue gas desulfurization systems or electrostatic precipitation units. These simultaneous removal methods are classified based on the reaction medium as solid-gas phase, liquid-phase oxidation, gas-phase oxidation, and combined-phase oxidation [54- 57]. Another technological approach is to develop more efficient SCR catalysts that can oxidize Hg⁰ to Hg²⁺ efficiently without reducing the NO reduction rate [58].

Fig. 4.

Keyword co-occurrence cluster visualization.

Keyword Emergence Analysis

Emergent keywords refer to terms that suddenly appear and have high citation frequencies, which, to some extent, represent the research hotspots and the spatiotemporal evolution patterns of a research field [59]. In terms of time, frontier keywords evolve continuously, showing a phased development pattern. This paper uses CiteSpace 6.3.1 software to extract the top 25 keywords with the strongest emergence intensity in the field of flue gas mercury removal from 1998 to 2024 (Fig. 5). These keywords were chronologically arranged based on their initial burst years for burst analysis. As shown in Fig. 5, 1998-2006 marked the early stage, featuring keywords with the strongest emergence intensity, including "sorption (18.88)", "sorbents (16.79)", "fired power plants (21.23)", and "activated carbon (12.67)". Initial research primarily focused on Hg⁰ removal through adsorption methods, particularly using activated carbon as a sorbent. The intermediate stage (2007- 2015) was characterized by prominent keywords such as "combustion flue gas (20.35)", "emissions (16.99)", "low temperature (12.57)", and "catalysts (11.23)". Research during this period emphasized mercury emission issues in coal-fired flue gas and employed catalytic oxidation/photocatalytic methods for elemental mercury removal. Recent developments (2016-2023) featured high-emergence keywords including "efficient removal (11.39)", "simultaneous absorption (9.75)", and "integrative process (9.56)". Current research hotspots focus on synergistic removal of multiple pollutants to enhance mercury elimination efficiency through integrated approaches. In summary, existing mercury removal technologies still face challenges, including high investment and operational costs, low removal efficiency, and secondary pollution risks. Therefore, developing more efficient and cost-effective mercury removal technologies and equipment remains crucial and represents the primary direction for future research.

Fig. 5.

Keyword emergence analysis.