Introduction

Nowadays, Natural gas (NG) is known as one of the main energy resources and plays an outstanding role in meeting the required energy. In the last years, the utilization of NG as a low-emission fuel has increased. According to the prediction of the International Energy Outlook, the global demand for NG will increase by 70% from up to 2040 [1]. Actually, NG is used as the inlet fuel to drive many different energy systems, such as hydrogen production, hydrogen purification, carbon storage and capture, and fuel cell technology [2].

The pipelines and NG liquefaction are most effective industrial methods to transport NG [3].

The most important problem in LNG plants is improvement of the technical performance of the liquefaction units [4]. Based on the prior studies, the required electricity in most LNG production systems varied between 250 and 800 kWh/ton LNG [5]. The largest contribution to the overall electricity consumption was related to the compressors [6]. The huge amount of energy consumption is one of the most problems in LNG production process, indicating that the selection of a suitable liquefaction unit play an important role in LNG production plants.

Therefore, in recent years, energy researchers have tried to increase system efficiency and diminish the fuel consumption by introducing novel processes.

To evaluate the NG-based systems, the exergy, advanced exergy, exergoeconomic, and exergoenvironmental analyses have been accomplished. So far, many investigations regarding the mentioned evaluations have been conducted, which in this study are presented and summarized. For instance, the techno-financial assessment is applied to an LNG generation configuration assisted by the auto–cascade refrigeration unit [5]. This structure can generate 31.71 kg s−1 LNG. The total exergetic proficiency and total irreversibility rate are computed as 91.68% and 67,688 kW, respectively. Also, the prime cost of product and Net Present Value (NPV) are obtained to be 0.1959 US$.kg−1 LNG and 2829 MMUS$.year−1, respectively. The exergoeconomic and exergoenvironmental assessments are conducted for an LNG production system driven by the diffusion–absorption refrigeration configuration [4]. The cost per unit exergy of LNG stream is calculated as 13.54 $.GJ−1, as well as the greatest total environmental impact belonged to the air coolers. Advanced exergoeconomic investigation is implemented for a single mixed refrigerant LNG process [7]. The results demonstrated that most of the investment and exergy-related costs are endogenous, as well as most of the irreversibilities costs in the compressors are avoidable. The advanced exergoeconomic assessment is performed on an innovative LNG generation structure assisted by the single effect absorption refrigeration unit [8]. The precooling unit can supply the refrigeration at − 30 °C with a Coefficient of Performance (COP) of 0.49. According to the obtained outcomes, share of the endogenous destruction cost for all devices is higher than 55%. Also, it was found that the most minor portion of irreversibility costs is related to the avoidable exogenous destruction. In another investigation, the exergy analysis and Life cycle assessment (LCA) methodologies are applied to a multi-generation configuration employing LNG cold energy [9]. The total exergy efficiency and the destroyed exergy rate are found as 42.95% and 11,382 kW, respectively. Also, the highest overall environmental impact rate (187.03 Pts.h−1) belonged to one of the heat exchangers. An NG-based multi-generation configuration providing potable water, electrical energy, oxygen, and syngas is proposed, and evaluated in terms of energy, financial, and environmental [10]. Thereafter, three scenarios are introduced. The third scenario is recognized as the most affordable system with a Cost of Electricity (COE) of 0.02 US$.kWh−1, as well as the energy and CO2 capture efficiencies of this scenario are accounted for 83.72% and 76.61%, respectively.

Also, in many investigations, the NG-based systems have been integrated with renewable configurations. Namely, a novel solar-driven structure for the generation of methanol and LNG is proposed and techno-economically analyzed [11]. The entire exergetic efficiency is gained to be 68.72%, as well as the greatest contribution to the total irreversibility is related to the photovoltaic panels (81.33%). Regarding the financial assessment, the payback period and the prime cost of product are found as 4.29 years and 0.3967 US$.kg−1 methanol, respectively. A cogeneration configuration of electricity and liquid ammonia utilizing LNG regasification and solar energy is developed and exergetically assessed [12]. The overall exergetic efficiency and the destroyed exergy are computed with the values of 57.36% and 116,763 kW, respectively. An innovative solar-biomass-assisted cogeneration system of freshwater and LNG is developed and evaluated using the exergy and exergoeconomic methods [13]. According to the achieved findings, the overall exergy efficiency and product cost rate of the system are 11.2% and 15.16 $.h−1, respectively. The techno-economic-environmental study is carried out on a solar-based cogeneration system for providing 520.2 kW electricity and 2 kg.s−1 LNG [14]. The overall exergetic efficiency, total product cost rate, and total environmental impact rate are computed as 6.48%, 80.82 $/h, and 78.23 Pts/h, respectively.

Also, several investigations on CO2 capture systems of natural gas have been conducted, some of which are presented below:

Techno-financial evaluation and optimization of NG-based combined power system with CO2 capture are performed by A. Kazem et al. [15]. An innovative configuration for the generation of electrical energy and synthetic NG from captured CO2 is developed, and Multi-objective optimization, parametric analysis, and financial assessment are accomplished [16]. The values of net electricity produced and payback period are computed as 95,700 kW and 4 years, respectively. Thermodynamic and financial analyses are applied to the chemical looping reforming of NG and CO2 capture unit [17]. It was determined that the net electrical proficiency varied between 40 and 43.4%, as well as the values of Levelized Cost of Electricity (LCOE) were variable between 75.3 and 144.8 $.MWh−1. In another research, the technical assessment of industrial CO2 Capture from NG employing the Diglycolamine is implemented [18].

Due to the high growth rate of NG-based article, the bibliometric assessment tool to track the research trends and scientific production is necessary [19]. Conventional bibliometric tools often assess the research trend by recognizing productive countries, active institutes, high-impact journals, and outstanding researchers [20]. Moreover, citation analysis can be a powerful tool for a more in-depth understanding of the universal research trends [21]. The bibliometric methodology permits scholars to evaluate and develop indexes on the progress of scientific data in a specific field [22]. This method evaluates the specifications of the existing literature on a topic to determine future research paths and to improve the decision-making process by decreasing the border of error [23]. In fact, to indicate a comprehensive NG-based energy systems investigation image in the whole scientific world, the multi-assessments of publications performance must be conducted [24]. Highly cited articles are helpful indexes for specifying “world-class” investigation.

Due to the vital and important role of NG in supplying energy to drive the hybrid energy systems, in this investigation, for the first time, a comprehensive bibliometric evaluation of the NG-based investigations is conducted within a period of 1991 to 2020. The bibliometrics could be a powerful tool for drawing the latest progress around a research field. By evaluating the yearly citations, leading countries, outstanding researchers, successful journals, and distribution pattern of subjects, the publication performance of NG-based systems could be properly analyzed. The document types according to the WOS collection are characterized. In the next step, the yearly number of NG papers and their citations per year is investigated. Thereafter, publication performance regarding the countries and institutions is investigated, as well as the top ten NG papers in terms of the number of citations have been recognized. Finally, the development of the 7 hot and important research foci in NG-based research is investigated. The bibliometric investigation of NG-based systems can help a more in-depth understanding the universal research trends and hotspots.

Methodology

The information of the current investigation has been adapted from the WOS Core Collection and SCI-EXPANDED database of the Clarivate Analytics. The data has been updated on 31 August 2020. Based on the impact factor’s definition, the published documents in 2020 from SCI-EXPANDED are presented after IF2020. The SCI-EXPANDED indicates an overall of 9531 journals across 178 WOS classes, and 114 of them are categorized in the class of energy and fuels in 2020. Search keywords: "natural gas" and "natural gases" were utilized to explore the documents from 1991 to 2020 [25]. The outcomes are refined using the WOS category of the energy and fuels. Microsoft Excel 2016 is utilized to collect and check the number of citations and the full record of SCI-EXPANDED of each document in each year. The extra coding was manually conducted [26, 27]. The Journal Citation Reports (JCR) is employed to gain the journal impact factors (IF2020).

The ‘reprint’ author is considered as the corresponding author. "Corresponding author" remains the primary term rather than reprint author [28]. In multiple corresponding-authors papers, only the affiliation of the last corresponding author has been specified [29]. For single-author papers where authorship is not specified, the single-author can be considered both the first author and the corresponding author.

Affiliations in the countries of Northern Ireland, Scotland, and Wales have been turned to the United Kingdom (UK). The papers published from Hong Kong before 1997 were turned to the China category. Affiliations in Yugoslavia have been turned to the Serbia.

Outcomes and discussion

Specifications of document kinds

Table 1 represents the thirteen document kinds indexed by the WOS. The most popular type belonged to the articles, with an overall of 11,687 articles (91% of 12,837 documents) and an APP of 3.7. The document type of the reviews had the greatest CPP2020 of 65 which was 3.2 times than the CPP2020 of the articles. An overall of 994 articles have been published in 57 journals mainly in International Journal of Hydrogen Energy (143 proceedings papers; 14% of 994 proceedings papers) and Energy (133; 13%). An overall of 729 reviews have been published in the 72 journals mainly in Renewable & Sustainable Energy Reviews (344 reviews; 47% of 729 reviews) with a CPP2020 of 56 and an APP of 3.3. 8/10 of the top most frequently cited documents were review. Using the WOS Core Collection, documents can be classified into two document kinds, resulting in the sum of more than 100% percentages in Table 1 [30].

Table 1 The results of citation evaluation based on the document kind
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The document type, article usually includes all of the investigation ideas, methodologies, outcomes, and discussion. Accordingly, the articles are usually employed for bibliometric assessment [31]. 99% of the papers have been published in English, followed by German (70 articles), Polish (44), French (22), Russian (14), and Japanese (5) were also used. English papers had much more CPP2020 of 21 than non-English papers with a CPP2020 of 1.9. Also, the highest value of APP belonged to the English articles (3.7).

Specifications of the publication outputs

Figure 1 illustrates the variations of TP and CPP2020 from 1991 to 2020, which was named as the TC2020/TP [32]. The number of papers has grown from 1991 to 2003, despite slight fluctuations. Followed a sharper increase was found from 2003 to 2012. The articles have sharply increased natural gas research after 2012 and reached 1289 articles in 2020. It always takes time to gather citations for a paper. According to Fig. 1, it takes CPP2020 about a decade to reach a plateau. Similarly, artificial intelligence [33], as well as environmental related topics: bioaccumulation [34] and wind tunnel [21]. A total of 7804 NG-based papers (67% of 11,687 papers) were without citations in the publication year (C0=0). It was reported that articles have higher C0 in recent years [35]. The top seven natural gas articles in C0 were all published in 2020. Moreover, among the top 100 C0 papers, only 6.0% and 16% of them were among the top 100 TC2020 and C2020 papers, respectively. In 2002 with 89 articles, had the greatest value of CPP2020 (45). The most cited natural gas paper was published in 2002 with a TP2020 of 875.

Fig. 1
figure 1

Changes of TP and CPP2020 from 1991 to 2020

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Journals

In total, 11,687 NG-based papers have been published in 144 journals in the WOS category of energy and fuels. Among them, 40 journals (28% of 144 journals) have not been categorized in the group in 2020 without IF2020. A total of 10,943 natural gas articles were published in 104 of 114 journals in the WOS category of energy and fuels in 2020. Table 2 presents the top ten high-yielding journals with publication indicator (TP), citation indicator (CPP2020), journal impact factor in 2020 (IF2020), and number of authors per publication (APP). The Energy with IF2020 of 7.147 ranked 22nd in the WOS category of energy and fuels, published the most 1077 natural gas articles (9.2% of 11,687 articles). According to the top ten productive journals, a total of 830 natural gas articles were published in the International Journal of Hydrogen Energy (IF2020 = 5.816, ranked 37th in the Web of Science category of energy and fuels) with the highest CPP2020 of 31 while 496 articles published in the Energies (IF2020 = 3.004, ranked 70th) had lower CPP2020 of 6.9. Ten articles were published in the Nature Energy (IF2020 = 60.858, ranked 1st) with an APP of 6.9 and a CPP2020 of 18. The journal ranked not only top in WOS category of energy and fuels but also 2nd in the class of multidisciplinary materials science.

Table 2 The most high-yielding journals
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Performance assessment of publication in terms of institutions and countries

A total of 399 articles (3.4% of 11,687 papers) were without author affiliation data in SCI-EXPANDED. Of the 11,288 natural gas articles with author affiliations from 106 countries, 8946 papers (79% of 11,288 articles) were single-country papers from 84 countries while 2342 (21%) papers were internationally collaborative papers from 99 countries. The top 10 productive countries with 6 publication indexes are presented in Table 3 [36]. These useful indicators can be expressed as follows:

  • TP: Total number of papers

  • IP: Single-country papers

  • CP: Internationally collaborative articles

  • FP: First-author papers

  • RP: Corresponding-author papers

  • SP: Single-author articles [29]

Table 3 Top ten high-yielding countries with six main indicators
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China ranked top in four of the six mentioned indexes with a TP of 2411 papers (21% of 11,288 articles), an IP of 1778 papers (16% of 8,946 single-country articles), an FP of 2,255 articles (20% of 11,288 first-author articles), and an RP of 2158 articles (19% of 11,256 corresponding-author articles) while USA ranked top with a CP of 773 articles (33% of 2342 internationally collaborative articles) and an SP of 230 articles (23% of 987 single-author articles). The USA dominated in the citation indicator with a CPP2020 of 27 for total articles, a CPP2020 of 29 for first-author articles, a CPP2020 of 29 for corresponding-author articles while South Korea ranked top with CPP2020 of 41 for single-author articles. Articles published by authors from the USA as first author and corresponding author had higher CPP2020 but a lower CPP2020 was found in Germany. China had a much higher APP with 5.1 that was higher than APP in natural gas articles with an APP of 3.7. Figure 2 indicates the development trends of the top four high-yielding countries. China (2,411 articles) and the USA (2242 articles) published a similar number of natural gas articles from 1991 to 2020. However, their development trends were very different. The USA published the most articles from 1991 to 2013 but 2010. China had a sharp development trend from 2011 and ranked top in an annual number of articles from 2014. China is the dominant country in natural gas research.

Fig. 2
figure 2

The performance evaluation of the top four high-yielding countries

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With regard to institutes, 5577 articles (49% of 11,288 articles) were published in a single institution while 5711 articles (51%) were inter-institutionally collaborative. The properties of the top 10 high-yielding institutes with the six main publication indicators [36], as well as their citation indicator (CPP2020) and number of authors per publication (APP) are tabulated in Table 4. 8/10 of the top high-yielding institutes were located in China and two in Iran. The China University of Petroleum in China took the first rank for four of the six publication indexes with a TP of 266 articles, an IP of 72 articles, an FP of 210 articles, and an RP of 199 articles while the Chinese Academy of Sciences in China had the first rank with a CP of 225 articles (3.9% of 5,711 inter-institutionally collaborative articles). In addition, the Sila Science in Turkey ranked 20th and published 172 articles including the most single-author articles with an SP of 18 articles (1.3% of 987 single-author articles). Eight and seven of the 18 single-author articles were published by M. Balat and A. Demirbas, respectively. Seven of the top ten productive institutes had a low CPP2020 that is lower than average of CPP2020 in the natural gas articles with 21. Compare the ten in states in Table 4, the Xi An Jiao Tong University in China dominated in the citation indicator with a CPP2020 of 34 for their all articles, a CPP2020 of 38 for their first-author articles, and a CPP2020 of 38 for their corresponding-author articles. The PetroChina in China had the largest value of CPP2020 for their single-author articles with 11.

Table 4 Top 10 high-yielding institutions with 6 main indexes
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Top ten most cited NG-based papers

Based on the WOS Collection, the overall number of citations of a document can be updated. Ho’s group introduced TCyear and Cyear citation indexes [28, 37]. The mentioned indicators are invariant and repeatable compared with the other indicators of WOS Core Collection [25]. Some argue that the number of citations to an article can indicate the impact of scientific publications, although not necessarily the quality [38]. The top articles can be defined as articles that most scholars can read and cite in high-impact journals [39]. By using the title of the article, reasonable details about the topic of the paper can be specified [40]. Author keywords are presented by authors to introduce more information about the main investigations focused on papers. Articles that included search keywords in their abstract only might not relate to the search subject directly [19]. Search keywords: “natural gas” and “natural gases” can be found in 4027 article titles (34% of 11,687 articles), 10,578 abstracts (93% of 11,327 articles with abstract information in SCI-EXPANDED), and 3228 author keywords (34% of 9547 articles with author keywords information in SCI-EXPANDED). 5/8 of the top articles on TC2019 included search keywords in their abstract only. For instance, articles by Dunn [41], Zhang et al. [42], Serrano-Ruiz and Dumesic [43], Rubin et al. [44], and Davison [45] ranked 1st with TC2019 of 875, 2nd with TC2019 of 569, 3rd with TC2019 of 558, 5th with TC2019 of 485, and 6th with TC2019 of 476, respectively. Table 5 lists the 10 most frequently cited NG-based articles with two citation indicators [28]. Three of the 10 articles were published in the International Journal of Hydrogen Energy (IF2020 = 5.816). The USA published the most top ten cited articles of the six articles; followed by Australia, China, Greece, Russia, Turkey, and the UK, one each. No author and institute published more than one the ten most frequently cited articles.

Table 5 The top 10 most cited papers
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Figure 3 exhibits the citation assessment of the top ten most frequently cited papers. Five of the articles by Elvidge et al. [46], Makogon et al. [47], Laubach et al. [48], Huang et al. [49], and Rufford et al. [50] were ranked in both top 31st of TC2020 and C2020 (Table 5). It was pointed that highly cited publications might not be always in high-impact positions [35]. A typical example was the article entitled “A comparative study of fuels for on-board hydrogen production for fuel-cell-powered automobiles” by Brown [51] had a TC2020 of 494 (ranked 4th) but a C2020 of 11 (ranked 795th). Furthermore, highly cited articles might not be the most impactful in the most recent year [27]. Some recent publications, which did not get enough time to accumulate numbers of citations, would be omitted if only TCyear was considered for evaluation of the most frequently cited articles, for example, an article published in 2018 by Keith et al. [52] ranked 76th with a TC2020 of 198 but ranked top in C2020 with 109. The article had a high citation in 2020 shows the related research is popular in the WOS category of biomedical engineering in recent years. Similarly, article by Comesaña-Gándara et al. [53] had a C2020 of 64 (ranked 8th) and a TC2020 of 196 (ranked 668th). Only using the TCyear was not enough to identify some excellent articles in the most recent years. The top three most frequently cited articles by Dunn (2002) [41], Zhang et al. (2010), and Serrano-Ruiz and Dumesic [43] contained search keywords only in their abstract. They might not relate to natural gas directly.

Fig. 3
figure 3

The citation analysis of the top ten most frequently cited papers

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Outstanding subjects in NG-based systems

In this section, with the use of distribution of words in article titles, abstracts, and author keywords, the hot topics in NG-based systems are investigated and their development trends are determined [58]. The 6715 NG-based articles were investigated using their title, abstract, and author keywords from 1991 to 2020. A total of 4,027 articles (34% of 11,687 articles); 10,578 articles (93% of 11,327 articles with abstract); and 3,228 articles (34% of 9547 articles with author keywords) included search keywords in their title, abstract, and author keywords, respectively. Table 6 presents the twenty hottest subjects in four sub-periods (1990s, 2000s, and 2010s). The most frequently used author keywords, except for the searching words such as “optimization,” “renewable energy,” “CO2 capture,” “life cycle assessment,” and “exergy analysis” were getting popular to be used in the recent decade. The findings of keyword evaluation indicate information about the hot and possible research foci in the recent years as each word cluster comprised several supporting words. Therefore, the development of the seven hot and popular topics in NG-based research was investigated. Figure 4 illustrates the development trends of the seven hot subjects. After 2015, the number of articles published in the mentioned fields has grown significantly.

Table 6 The 20 hot topics in NG-based with publication indicators
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Fig. 4
figure 4

Development of the seven main topics

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Topic: renewable energy

In this section, some recent articles related to the integration of NG units and renewable energy configurations are summarized (see Table 7). It can be shown that renewable resources, especially wind and solar energies can be coupled with the NG stream to drive the single and multi-generation systems to generate the main products, such as methanol, LNG, electricity, and potable water. Also, in most investigations, the technical and financial analyses have been implemented, as well as in some articles, the optimization process has been employed. In order to find the related papers, the following support keywords can be used: renewable energy, renewable energy sources, renewable energy storage, renewable energy integration, renewable energy policy, and renewable energy resources.

Table 7 The summary of the last investigations on the integration of NG units and renewable energy configurations
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Topic: capture

The next hot and popular topic is CO2 capture. As mentioned earlier, the NG-based combined power plants play an important role in meeting the required energy, which have the highest share of greenhouse gas emissions (GHG) [65]. So, the exploration of an alternative energy generation source is essential, such as increasing renewable resources. One of the effective solutions to diminish GHG is the utilization of CO2 capture technology [66]. For implementing CO2 capture in each plant, there are three common methods including oxy-fuel combustion, pre-combustion, and post-combustion [67]. So, many investigations regarding the CO2 capture techniques of NG-based systems have been conducted, and some of the outstanding investigations are summarized in Table 8. As can be noticed, in most projects, the hydrogen and electricity production systems have been investigated, as well as technical–economic optimization has been conducted to determine the optimal operating conditions. Multi-generation systems have been used as power plants. In order to find the related articles, the following support keywords can be utilized: capture, captures, CO2 capture, carbon capture, carbon dioxide capture, post-combustion CO2 capture, and dioxide capture.

Table 8 The summary of the last investigations on the CO2 capture of NG-based plants
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Topic: energy efficiency

The energy assessment is the most common technique to evaluate the technical aspect of each energy system [75, 76]. By implementing the first law of thermodynamics, the quantity of energy is investigated, as well as the quality of energy can be evaluated using exergy analysis (topic 5) [77, 78]. Based on the energy analysis outputs, such as electrical efficiency, thermal efficiency, specific electricity consumption, COP, and overall energy efficiency, the NG-based systems can be compared together technically. Table 9 presents the details of energy analysis of some recent NG-based energy systems. As can be seen, in most LNG production systems that are integrated with a refrigeration unit, COP and specific electricity consumption as the two important parameters have been computed. In order to find the related articles, the following support keywords can be utilized: energy efficiency, building energy efficiency, energy efficiency programs, energy efficiency enhancement, energy efficiency improvement, energy efficiency index, energy efficiency resource standards, energy efficient, and primary energy efficiency.

Table 9 Details of energy analysis of some recent NG-based energy systems
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Topic: hydrogen production

Hydrogen is the most abundant element on earth that can be considered alternative clean energy and sustainable source [80]. Hydrogen can be produced using several techniques, such as steam reforming, gasification, dry reforming, and electrolysis [81]. Among the mentioned methods, NG steam reforming is the prominent technique in which hydrocarbons are converted to hydrogen. This section presents some outstanding investigations on hydrogen production from natural gas (see Table 10). As can be seen, in some projects, the renewable resources have been integrated with hydrogen production unit, as well as technical–economic optimization has been conducted to specify the optimal operating conditions. For a better search on this topic, the following keywords can be helpful: hydrogen production, biohydrogen production, distributed hydrogen production, and hydrogen production from hydrogen sulfide.

Table 10 Some outstanding investigations on hydrogen production from natural gas
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Topic: exergy analysis

In this section, some outstanding investigations on exergy evaluation of the NG-based energy units are summarized (see Table 11). In many NG-based systems, for evaluating the technical aspect, the exergetic assessment is implemented. With the use of exergetic outputs, such as overall exergy efficiency and overall exergy destruction rate, the NG-based researches can be compared together [90]. Additionally, to recognize the origin of the irreversibilities in each device, the advanced exergy methodology is employed. Also, to generalize the results to the other operating conditions, parametric analyses on exergetic parameters are performed [91]. To find these articles, the following support keywords can be used: exergy analysis, exergy analyses, advanced exergy analysis, energy and exergy analysis, and advanced exergy analysis.

Table 11 The summary of the last investigations on exergetic analysis of the NG-based energy systems
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Topic: life cycle assessment (LCA)

This section presents the outstanding investigations on implementing the LCA method for the NG-based energy units (see Table 12). In order to evaluate the environmental footprint of a product, the LCA concept is employed [102]. This methodology deeply scans the system and characterizes the environmental effect on the system's performance. Actually, the LCA method investigates the environmental impacts of service over the three phases of construction, operation, and decommissioning. The amounts of environmental effects resulting from the depletion and emissions of a natural resource can be determined employing various LCA methods. Heretofore, many methods, such as CML 2001, ReCiPe Endpoint, and Eco—indicator 99 have been employed for implementation of the LCA in various energy systems [47].

Table 12 The summary of the last investigations on the implementation of the LCA approach on the NG-based energy configurations
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Also, in many projects, the LCA method has been integrated into the exergy analysis to form the exergoenvironmental evaluation. The exergoenvironmental analysis is applied to each device by combining the findings of exergetic analysis and LCA method [103]. In most exergoenvironmental assessment projects, the Eco—indicator 99 is utilized, and this index utilizes the mean European data. To find these articles, the following support keywords can be utilized: life cycle assessment, LCA, exergoenvironmental analysis, and life cycle assessments. Based on the outcomes of LCA, the eco-friendly indicator of NG-based systems can be compared together. As can be seen, in most exergo-environmental investigations, the environmental impacts are reported as the Pts or mPts, as well as to generalize the results and find the effective parameters, sensitivity analyses are conducted.

Topic: shale gas

The NG that is found trapped within shale formations is called shale gas. In fact, Shale gas is trapped within shale formations with low permeability [112]. Shale gas is recognized as a key fuel in the universal energy basket [113]. In last years, many investigations have been implemented on shale gas production. Also, in several studies, the technical, financial and environmental investigations have been conducted on shale gas reservoirs. Some outstanding investigations on shale gas development can be found in Table 13. For a better search on this topic, the following keywords can be useful: shale gas, shale gases, gas shale, gas shales, shale gas reservoir, shale gas development, shale gas exploitation, shale gas revolution, unconventional shale gas, and wet shale gas.

Table 13 Some outstanding investigations on shale gas development
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Concluding remarks

In this investigation, a bibliometric investigation on research trends and performances of NG in the WOS category of energy and fuels is conducted. The current bibliometric assessment has brought together the knowledge of publications on the NG-based energy systems accessible from the WOS database. According to the findings of this investigation, the most outstanding researchers, publishers, countries, and research categories are recognized. The SCI-EXPANDED indexes an overall of 9531 journals across 178 WOS categories, and 114 of them are classified in the category of energy and fuels in 2020.

The most noticeable outputs of the current study can be listed as follows:

  • The highest value of CPP2020 (65) belonged to the reviews. Also, an overall of 729 reviews have been published in 72 journals mainly in Renewable & Sustainable Energy Reviews (344 reviews; 47% of 729 reviews). Additionally, the values of CPP2020 for reprint, discussion, and retraction types were equal to zero.

  • Ninety-nine percent of the papers have been published in English, followed by German (70), Polish (44), French (22), Russian (14), and Japanese (5). Also, English papers had the greatest APP of 3.7 than non-English papers with CPP2020 of 1.8.

  • The articles have sharply increased NG-based researches after 2012 and reached 1,289 articles in 2020. A total of 7804 NG-based papers (67% of 11,687 articles) were without citations in the publication year (C0 = 0).

  • With regard to the articles, The Energy journal (IF2020 = 7.147) with 1077 NG-based papers is recognized as the most productive journal, followed by the International Journal of Hydrogen Energy (IF2020 = 5.816) and Applied Energy (IF2020 = 9.746) stood for the second and third ranks, respectively.

  • The publication performance evaluation demonstrated that 3.4% of the total articles (399 papers) were without author affiliation data in the SCI-EXPANDED. China had the highest amount of article (2,411), followed by USA (2242) and Iran (730). The greatest values of CP (773) and SP (230) are related to the USA.

  • Articles published by authors from the USA as first author and corresponding author had higher CPP2020 but a lower CPP2020 was found in Germany. China had a sharp development trend from 2011 and ranked top in an annual number of articles from 2014. China is the dominant country in NG-based investigations.

  • It was found that 5,577 articles (49% of 11,288 articles) were published in a single institution while 5,711 articles (51%) were inter-institutionally collaborative. 8/10 of the top high-yielding institutes were located in China and two in Iran. The highest value of TP belonged to the China University of Petroleum (266), followed by Chinese Academy of Sciences (264), Tsinghua University (145), and University of Tehran (132). Academy of Sciences in China ranked top with a CP of 225 articles (3.9% of 5,711 inter-institutionally collaborative articles).

  • According to the citation analysis, the USA published the most top ten cited articles of the six articles; followed by Australia, China, Greece, Russia, Turkey, and the UK, one each. The 5/8 of the top papers on TC2019 included exploring keywords in their abstract only.

  • A total of 4,027 articles (34% of 11,687 articles); 10,578 articles (93% of 11,327 articles with abstract); and 3,228 articles (34% of 9,547 articles with author keywords) included exploring keywords in their title, abstract, and author keywords, respectively. The most frequently utilized author keywords, excluding for the exploring words, such as “optimization,” “renewable energy,” “CO2 capture,” “life cycle assessment,” and “exergy analysis” were getting popular to be used in the recent decade.