Donaji Jiménez-Islas
| Miriam E. Pérez-Romero | José Álvarez García* | Ignacio Ventura-Cruz
© 2023 IIETA. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
OPEN ACCESS
The emission of greenhouse gases emitted into the atmosphere by the burning of fossil fuels has allowed the development of biofuels such as biomethane. The aim of this research was to explore the characteristics of biomethane literature from 1978 to 2020 based on the database of Scopus and its implications using indicators bibliometrics. The information in the database was analyzed through the Gompertz model to determine the specific growth rate over the years. Also, maps were elaborated with the VOSviewer software to show in a visual way the collaboration between authors and keywords related to the topic of study. Documents were examined in a variety of aspects of the publication characteristics such as document type, language, authorship, countries, institutions, journals, high-cited papers. Results showed that the evolution of publications grew exponentially from 2006 to 2020, the specific speed of growth determined with the Gompertz model was 0.4 0.095 years-1 (>0.99). 52% of publications are concentrated in five countries (Italy, India, China, the United States and Spain). Bioresource Technology is the journal with the highest number of publications and citations, the authors publish in quartile 1 and 2 journals. The biomethane topic has a growing number of publications and citations due to the collaboration between researchers from different countries. The present work can be contrasted with the analysis of bibliographic indicators with other databases to determine the level of collaboration with other journals with different index.
biogas, methane, Scopus, biofuels, Gompertz
Global energy demands for conventional fossil fuels have risen as a result of population growth and urbanization [1]. This would raise global temperatures, resulting in more heatwaves, droughts, and floods, posing serious health and economic risks, because each country has its own policies and socioeconomic characteristics [2]. As society, with the introduction of oil the balance was lost due the development of new synthetic products of fossil origin [3] and we need to reduce global greenhouse gas emissions where the use of biofuels can be an option to improving energy security also bring an increase in food production [4-6]. Biofuels can be defined as fuels from biomass, that offer different types of biofuel such as liquid, solid and gaseous.
Liquid biofuels play an important role in the global transition to renewable and sustainable energy sources [6], such as forest residues [7], agricultural material related to food crops [8], no compete with food, such as switchgrass [9], eucalyptus [10] and solid biogenic waste [6]. Also, the liquid biofuels from biomass have become a natural complement for gasoline (as an additive) in the transport sector [11], nowadays, the most common liquids biofuel are bioethanol, biodiesel and biomethanol. Solid plant biofuels, such as wood and agricultural residues, play an important role within the biomass groups while having no negative impact on the food industry; similarly, wood pellets are the only solid biofuels with a global market that is continuously expanding [12]. The gaseous biofuels are biogas, hydrogen and syngas, which could be used as a source of electricity, heating, and transportation fuel [13]. Nevertheless, bio-hydrogen and biogas are the most important gaseous biofuels, it can be generated from lignocellulosic biomass hydrolysates or food waste via anaerobic fermentation [14, 15].
Biogas is an appealing energy source for reducing greenhouse gas emissions, containing approximately 40 to 75 percent CH4 and an unavoidable volume of CO2 depending on the feedstock; waste landfills, agricultural waste treatment plants, sewage end-treatment plants, and animal manure treatment plants are the primary sources of biogas [16, 17].
Biogas technologies (with biomethane upgrade) are clearly capable of generating advanced gaseous fuels from these feedstocks [18]. Also, biomethane development and use will open up new doors for society for reducing greenhouse gas emissions and promote human health [16, 19]. Some authors have reported lignocellulosic biomass pretreatment processes for biomethane production, among them we can mention: the size reduction [20], chemical (acid, alkali and oxidisin agents), thermal (explosion of steam), biological (fungi) and hybrid combinations [21, 22].
In Europe with the economic incentives have fostered that from 6227 in 2009 to 17432 in 2017, the number of biogas production plants has increased, in Germany and Italy the number of biogas plants was 10971 and 1655, respectively [23]. And as a considerable fact in the biogas sector we can mention that in 2017, the global biomethane market was estimated at USD 0.62 billion.
By 2026, $4.96 billion industry is expected to have grown at a rate of 26% per year (8 times the growth compared to 2017), several countries have set ambitious targets for biomethane as natural gas replacement for household consumption, such as France and UK [24].
Biomethane derived from by-products or agricultural residues, as well as biodegradable household waste, are commonly preferred over biomethane derived from energy crops; increased awareness and information will assist consumers in making more informed decisions about biomethane derived products [25].
At present, knowing the development of scientific publications allows to identify the contributions of authors in different areas of knowledge, the need to establish a starting point of the literature of biofuels and specifically biomethane allows to discover the interaction of authors, countries and emerging areas in the subject of study.
1.1 Theoretical framework of bibliometric analysis
Some of the publications relating to biomethane were published using a bibliometric analysis of biomass, thus, several research teams have been studying how to improve biogas for biomethane processing. A study of the 100 to cited papers in the field of biogas was carried out using the Web of Science database, with the majority of documents released between 2006 and 2011 [26].
The biomethane field is receiving a rising number of papers and scientific interest, necessitating the development of a quantitative method to examine scientific output and study trends in this field. The bibliometric analysis is a quantitative analysis to assess the amount of research work in a topic [26-28].
In general, the bibliometric analysis can be used to describe the development of biomethane research, this method is systematic and requires a wide variety of statistical techniques of bibliography counting to evaluate and quantify the growth of global research trends from different perspectives [29-31].
There is no bibliometric analysis of trends on biomethane, and this research uses the Scopus database to perform bibliometric analysis of the key patterns in order to fill this research void. In addition, the study adds to our understanding of biofuel liquid biomethane and its connections to various topics, which will aid researchers in conducting new research on the topic.
From 1978 to 2020, the literature was retrieved using the following search terms in the Scopus database “biomethan*” OR “bio-methan*” AND “digest*” AND “biogas” options, with “title”, “abstract”, and “keywords” to achieve the research production that has been examined on a global scale. The sign (*) in the search box was used to obtain both singular and plural versions of a keyword, and the marks (" ") were used for exact phrases search. The number of documents found under the search criteria was 1477. The data were analyzed using Microsoft Excel. Bibliometric indicators were extracted and analyzed both quantitatively and qualitatively.
The time trend of the publications was analyzed by fitting mathematical models with the solver-function. The Gompertz growth model (1) and normalization equation (2) were used to adjust the profile of documents in the field.
$P(y)=a e^{-b e^{-\mu \ t}}$ (1)
$\left(P_{(y) \text { Scopus }}-P_{(y) \text { model }}\right)^2$ (2)
where, the symbol P(y) represents the cumulative volume of documents by year, “µ” the specific growth rate of publications, “a” the asymptotic publications, “b” an integration constant related to initial publications, “t” is time, P(y) Scopus publications reported on Scopus database and P(y) model estimated publications by model.
The data of Scopus was fitted to determine the value of specific growth rate (µ), the results of Scopus and the generate by model were analyzed using regression curve fitting with statistical significance set at p=0.05 with data analysis of Microsoft Excel and it was estimated the determination coefficient ($R^2$). Also, the data download from Scopus database was imported into Microsoft Excel and VOSViewer software which was used to create network maps. The instructions are the next: Create a map based on bibliographic data, read data from bibliometric database, files, type analysis Co-authorship-countries (unit of analysis), full counting, maximum number of countries per document (25), minimum number of documents of a country (10). To ensure that the results were reliable and correct, the Microsoft Excel program was used to eliminate any possible duplicates [32].
3.1 Document type and publication language
Of the 1477 documents found in the database, 78.19% are articles, 11.87% conference paper, 6.17% review articles and the remaining 3.77% are Book, book chapter, notes and conference review.
As English is a global language, most research papers in our database were published in English [33]. Of 1477 research documents, 97.77% were written in English (n=1444 of the total documents), followed by Chinese (n=11), French (n=11), Portuguese (n=3) and Spanish (n=2). The less frequently used languages were Finnish (n=1), Polish (n=1), Russian (n=1), and Slovenian (n=1). The most researchers will publish the research results in English language types in order to facilitate communication and improve the influence of their research [34].
3.2 Trends of publications
Figure 1 illustrates the trends in the number of scientific publications published from 1978 to 2020. As can be observed, the research trends can be concretely divided into three stages: i) In the first stage from 1978 to 2002, it was a stage which slowly increased and few documents were published on biomethane topics. ii) The second stage from 2005 to 2010, the number of publications on the topic quickly increased. iii) the third stage, the number of biomethane related publications significantly increased from 2010 onwards.
Figure 1. Total number of publications for biomethane research
In this work, we observe that the volume of publications covering grows exponentially and for this reason, the Gompertz model to fit the data. As can be seen in Figure 1, a lag phase was observed during the first 28 years. The Gompertz model fitted the data very well (coefficient of determination R2>0.99) and a rate of growth 0.095 year-1. A recent study examined the rate of publications of biofuels in Latin America, where the logistic model was used with a rate of growth of 0.19 to 0.21 (year-1) [35]. In the same way, it was analyzed quantitatively the growth of publications research with Web of Science (WoS) and Scopus database, the results indicated that WoS and Scopus had a rate of 0.29 y 0.31 year-1 in the topic of bioethanol [36]. The Logistic model can be used to determine the evolution in the next years in the field of biomethane or biofuels in general.
3.3 Most publications countries
The research results show that Italy, India, China, USA, Spain are the five biggest contributing countries on biomethane with more than 100 total publications, Table 1. Of the top 20 countries, two were from America, six from Asia, eleven from Europe, one from Oceania, while no countries were from Africa. Italy had the largest number of total publications (13.8%), followed by India (12.8%), China (10.2%), the United States (8.3%) and Spain (7%), in this case, the top 5 countries concentrate more than 50% of the publications of the biomethane field.
The total number of publications (TP) and total number of citations (TC) are the two basic metrics used to assess the amount and quality of scientific publications in general. The most active authors, organizations, and countries are represented by the indicator TP [37]. The number of citations, on the other hand, is a clear indicator of the content of a paper. As a result, the TC indicator aids in determining the accuracy of scientific articles [37]. Spain and Italy are leading in the TC, however to our TC/TP (number of citations/ number of publications) relationship in Table 1, Belgium with 63.81 citations/documents is the most cited country, followed by Ireland (55.20).
These country level contributions can be visualized using a co-authors network map between countries (Figure 2), the nodes (frames) represent the countries that collaborate on biomethane topics, and their dimensions are proportional to the number of documents they contain. The lines that interconnect these nodes show the strength of the collaboration [38]. A remarkable link can be seen in this field between Italy and Germany, India and South Korea, China and Sweden, United States and Spain, Spain with Chile and Brazil, Figure 2.
Table 1. Top 20 most productive countries on biomethane
|
Ranking of countries |
TP |
% of total publications |
TC |
TC/TP |
|
Italy |
203 |
13.8% |
4832 |
23.80 |
|
India |
189 |
12.8% |
2444 |
12.93 |
|
China |
150 |
10.2% |
2386 |
15.91 |
|
United States |
122 |
8.3% |
2611 |
21.40 |
|
Spain |
103 |
7.0% |
4944 |
48.00 |
|
Germany |
99 |
6.7% |
3285 |
33.18 |
|
United Kingdom |
65 |
4.4% |
2123 |
32.66 |
|
Ireland |
60 |
4.1% |
3312 |
55.20 |
|
Sweden |
53 |
3.6% |
1469 |
27.72 |
|
Denmark |
46 |
3.1% |
2343 |
50.93 |
|
France |
46 |
3.1% |
1359 |
29.54 |
|
Malaysia |
44 |
3.0% |
546 |
12.41 |
|
Belgium |
37 |
2.5% |
2361 |
63.81 |
|
Brazil |
34 |
2.3% |
388 |
11.41 |
|
South Korea |
34 |
2.3% |
506 |
14.88 |
|
Poland |
33 |
2.2% |
365 |
11.06 |
|
Thailand |
33 |
2.2% |
200 |
6.06 |
|
Iran |
29 |
2.0% |
475 |
16.38 |
|
Australia |
28 |
1.9% |
794 |
28.36 |
|
Netherlands |
27 |
1.8% |
1947 |
72.11 |
TP, Total number of publications; TC, Total number of citations
Figure 2. Collaborative network map between countries in biomethane research
There were 11 clusters (54 items) with different colors, the authors in the same cluster almost always indicated that they studied in the same area and collaborated closely [39]. The largest cluster consisted of nine countries (red color); the second cluster consisted of seven countries (green color); see Figure 2. According to network centrality, India, China, Italy, and the United States occupied the core of the international cooperation network of these 20 most active countries.
3.4 Institutions
The affiliation of at least one author was used to estimate the contribution of various organizations to a publication [40]. Table 2 lists the top ten most profitable institutions. First is the University College Cork (47), followed by Danmarks Tekniske Universitet (29), Leibniz-Institut für Agrartechnik und Biookonomie e.V. ATB (23), Beijing University of Chemical Technology (20) and others with less than 20 TP.
The degree of interaction through international collaboration tends to receive more citations than those obtained by individual countries [41].
Table 2. Top 10 most productive institutions/organizations on publications from biomethane
|
Ranking of organization |
TP |
Country |
|
University College Cork |
47 |
Ireland |
|
Danmarks Tekniske Universitet |
29 |
Denmark |
|
Leibniz-Institut für Agrartechnik und Biookonomie e.V. ATB |
23 |
Germany |
|
Beijing University of Chemical Technology |
20 |
China |
|
Isfahan University of Technology |
17 |
Iran |
|
Politecnico di Torino |
16 |
Italy |
|
Universitá degli Studi di Milano |
16 |
Italy |
|
China Agricultural University |
15 |
China |
|
Universidad de Valladolid |
15 |
Spain |
|
Ministry of Agriculture of the People´s Republic of China |
15 |
China |
The global distributions of institutions that published documents for biomethane show a broad geographical coverage classified into seven groups. The first group with three institutions of China (Beijing University of Chemical Technology, China Agricultural University and Ministry of Agriculture of the People´s Republic of China); the second group with two institutions of Italy (Politecnico di Torino, Universitá degli Studi di Milano); and other countries with one institution, Table 2.
3.5 Publications distribution in the biomethane subject areas
The analysis of the documents allows obtaining an overview of the biomethane topic in the various academic fields. On Figure 3, it can be shown that “Environmental Science” is the most highly populated research area, followed by “Energy”, “Chemical Engineering”, “Engineering”, “Agricultural and Biological Sciences” and “Immunology and Microbiology”; the areas described above represent 90.32% of the total found in the Scopus database on the subject of biomethane.
Figure 3. Distribution of publications of biomethane by subject areas
3.6 Journals participation
As show in Table 3, “Bioresource Technology” It has been discovered that has the most publications in this area (167), followed by “Applied Energy” (55); similar result in the rank of journals to the one reported in the field of thermal energy storage using latent heat. In terms of Quart (quartile), of top 20 journals, they are 14 classifieds into Q1, 3 are Q2, 1 is Q3 and two no-determined quartiles, characteristic relevant to determine the quality of journals.
Table 3. Top 10 most productive journal by biomethane topic
|
Journal |
TP |
TC |
TC/TP |
IF |
(Q) |
|
Bioresource Technology |
167 |
7512 |
44.98 |
2.43 |
Q1 |
|
Applied Energy |
55 |
2418 |
43.96 |
3.61 |
Q1 |
|
Journal of Cleaner Production |
46 |
999 |
21.72 |
1.89 |
Q1 |
|
Renewable Energy |
38 |
905 |
23.82 |
2.05 |
Q1 |
|
Renewable and Sustainable Energy Reviews |
31 |
1174 |
37.87 |
3.63 |
Q1 |
|
Energy Procedia |
30 |
229 |
7.63 |
0.55 |
- |
|
Waste Management |
28 |
794 |
28.36 |
1.63 |
Q1 |
|
Biomass and Bioenergy |
26 |
1582 |
60.85 |
1.11 |
Q1 |
|
Waste and Biomass Valorization |
23 |
359 |
15.61 |
0.57 |
Q2 |
|
Energies |
22 |
159 |
7.23 |
0.64 |
Q2 |
|
Science of the Total Environment |
21 |
358 |
17.05 |
1.66 |
Q1 |
|
European Biomass Conference and Exhibition Proceedings |
20 |
5 |
0.25 |
0.16 |
- |
|
Chemical Engineering Transactions |
17 |
108 |
6.35 |
0.32 |
Q3 |
|
Water Research |
17 |
575 |
33.82 |
2.93 |
Q1 |
|
Water Science and Technology |
17 |
1695 |
99.71 |
0.47 |
Q2 |
|
Energy Conversion and Management |
16 |
570 |
35.63 |
2.92 |
Q1 |
|
Energy |
15 |
402 |
26.80 |
2.17 |
Q1 |
|
Fuel |
15 |
564 |
37.60 |
1.8 |
Q1 |
|
Chemical Engineering Journal |
13 |
203 |
15.62 |
2.32 |
Q1 |
|
Biofuels Bioproducts and Biorefining |
12 |
500 |
41.67 |
1.14 |
Q1 |
IF, Impact factor (2019); Q, Quartile
Table 4. Top 10 most cited documents by biomethane topic
|
Title |
Source title (TC) |
University (corresponding author or first author) |
|
Biogas production: Current state and perspectives [42] |
Applied Microbiology and Biotechnology (1556) |
Johann Heinrich von Thünen Institute |
|
Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives [43] |
Bioresource Technology (1261) |
Universitat de Barcelona |
|
Defining the biomethane potential (BMP) of solid organic wastes and energy crops: A proposed protocol for batch assays [44] |
Water Science and Technology (1062) |
Danmarks Tekniske Universitet |
|
Techniques for transformation of biogas to biomethane [45] |
Biomass and Bioenergy (697) |
Katholieke Universiteit Leuven and Science, Engineering and Technology |
|
Evaluation of energy efficiency of various biogas production and utilization pathways [46] |
Applied Energy (509) |
University College Dublin |
|
Methanosarcina: The rediscovered methanogen for heavy duty biomethanation [47] |
Bioresource Technology (413) |
Ghent University |
|
A meta-analysis of the microbial diversity observed in anaerobic digesters [48] |
Bioresource Technology (286) |
The Ohio State University |
|
Towards a standardization of biomethane potential tests [49] |
Water Science and Technology (241) |
Ecole Polytechnique Federale de Lausanne |
|
A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading [50] |
Reviews in Environmental Science and Biotechnology (237) |
University of Valladolid; University of La Frontera |
|
Lignocellulosic materials into biohydrogen and biomethane: Impact of structural features and pretreatment [51] |
Critical Reviews in Environmental Science and Technology (235) |
Laboratoire of Biotechnologie de Lenvironment |
In addition, the journal “Renewable and Sustainable Energy Reviews'' has the highest impact factor (3.63) among all these journals, followed by the journals “Applied Energy” (3.61) and “Energy Conversion and Management” (2.92). In terms of TC/TP ratio, “Water Science and Technology” ranks first (99.71), followed by Biomass and Bioenergy (60.85) and Bioresource Technology (44.98).
3.7 The most frequently cited documents
It is worth noting that the text chosen for citation by an author must be important to the work in which it's cited [35]. Table 4 shows a number of authors have been considered the most influential in the publications of biomethane field (with more than 1000 cites), the first most cited author is Weiland [42] with 1556 TC.
The author describes in the review the current state and prospective of biogas production, including the feedstocks and parameters that determine the efficiency and ratability of the microbial conversion and gas yield [42].
The second most cited author(s) are Mata-Alvarez [43], with 1261 cites to the review “Anaerobic digestion of organic solid wastes. The third document most cites (1062) is a protocol for batch assay to provide important experimental guidelines for the reliable and reproducible assessment of the anaerobic biodegradability of any compound or undefined material to methane and carbon dioxide [45].
The literature output concerning biomethane has shown a strong increase in recent years. With the most publications in the region, Europe is at the forefront of science.
In this study was used the literature on the biomethane published between 1978 to 2020, the main conclusions were as follows:
-The number of publications and rate of growth related to biomethane has significantly increased in the past 15 years by interest in renewable energy and the policy internationally in the countries.
-According to the number of publications on the topic of biomethane, Italy, India, and China were found to be the top three countries.
-In terms of total publications, University College Cork is the leader and Weiland is the author more influential with respect to the number of cites (TC).
-In terms of publications, the top three journals that published on this topic were “Bioresource Technology”, “Applied Energy and Journal of Cleaner Production" with high impact factors that highlights the evident scientific interest in this field.
-The application of the Gompertz model to the biomethane publication data allowed the determination of the specific growth rate with a coefficient of determination close to 1.
This publication has been made possible thanks to funding from the Regional Ministry of Economy, Science and Digital Agenda of the Regional Government of Extremadura and the European Regional Development Fund of the European Union through grant reference GR21161, and the authors are grateful to the Universidad de Extremadura (UNEX) Spain and the Higher Technological Institute of Huichapan, (TecNM) Mexico, for providing the research facilities for this study.
|
$R^2$ |
Determination coefficient |
|
TP |
Number of documents |
|
TC Q |
Total of citations Quartile |
|
IF |
Impact factor |
|
Greek symbols |
|
|
µ |
Specific growth rate, h-1 |
|
$\beta$ |
thermal expansion coefficient, K-1 |
|
Subscripts |
|
|
CO2 |
Carbon dioxide |
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