The number of publications of an institution, area or country is a useful indicator to quantify the scientific activity of these units. Its greatest usefulness is obtained when making comparisons with the activity of other institutions, areas or countries, since it is necessary to have a frame of reference within which to locate our object of study. It is also interesting to monitor scientific production over time.
3.3. Worldwide Distribution of Publications
shows world scientific production by country of origin. According to the number of publications each country has an assigned color that goes from red, which indicates a greater number of publications, to gray, which indicates the non-existence of publications. The United States is the country with the highest number of publications (3318) in this field, followed by China (1514), India (1165), Germany (1137) and Italy (993). From this, it can be concluded that the use of biomass as renewable energy in industrialized countries is a key element in achieving sustainable development. Within industrialized countries, governments promote energy policies with the aim of reducing greenhouse gases and consequently global warming.
In the United States, a climate change plan called the ‘Clean Power Plan’ stands out. It includes the lines that the USA Environmental Protection Agency (EPA) announced in September 2013, aiminged to reduce emissions by 30% by 2030. This USA Clean Energy Plan set carbon pollution standards for power plants, states and utilities for the first time, with the flexibility that they need to meet their standards.
represents the evolution of the number of documents from the 5 countries with the highest scientific production related to biomass as renewable energy. It is possible to observe how in 2008 these 5 industrialized countries regain their scientific production on this subject due to the sustainability policies carried out by the different governments.
represents the existing network of collaboration between the countries of different authors sharing the same publication. This figure was extracted from the software VOSviewer v.1.6.6., which uses the information obtained from the Scopus search and which can be downloaded as a .csv file.
From the collaboration between countries and biomass research, eight clusters are observed, see Figure 7
and Table 2
. All this is centralized around three countries belonging to different clusters: USA, India, and the UK. The red cluster is the most important, and is led by the UK; as can be seen, it is composed of its traditional area of political and economic influence, to which Japan joins. The green cluster is the second most important and consists mainly of Latin American countries. The blue cluster is led by the USA and is in close relationship with China and other North American countries such as Canada and Mexico. The yellow cluster is led by Germany and is mainly related to Eastern European countries. With less importance would be the Central European clusters (in the color Turquoise), Scandinavia-Russia (in the color Grey), and in Orange that of some African countries. It can therefore be deduced that there are two forms of grouping: the first is based on the influence or economic relations between groups of countries, which in this scenario would be the four most important clusters; and the second would be in terms of the type of biomass they may have, based on their geographical location or climatic conditions, which in this situation would be the last three clusters.
3.4. Institutions Distribution of Publications
shows the ten institutions with the highest scientific production in the field of biomass as renewable energy, as well as the keywords most used by these institutions.
In the first place, the Chinese Academy of Sciences, with 267 documents, stands out, followed by the National Renewable Energy Laboratory with 180, Danmarks Tekniske Universitet with 136, Ministry of Education China with 116, University of Sao Paulo with 112, USDA Agricultural Research Service, Washington DC with 108, OAK Ridge National Laboratory with 107, Wageningen University and Research Centre with 104, Sveriges Lanbruksuniversitet with 97 and Imperial College London with 84. It should be noted that the keyword most used by most of these institutions is Biofuel, ranking first in five of the ten institutions.
With respect to the type of research carried out, it can be seen how Chinese institutions focus on solid fuels, perhaps as an alternative to coal, while US and Brazilian institutions focus on liquid fuels such as ethanol and how it is obtained from crops, especially corn (Zea Mays) in the USA and sugarcane in Brazil. A third line of research is that of CO2 emissions (carbon dioxide), where they focus on reducing emissions and energy policy, in this case we have the Danmarks Tekniske Universitet and the Imperial College London.
3.5. Subject Categories and Journals Obtained from Scopus
The distribution of publications by thematic areas was also obtained from the bibliometric analysis using the Scopus database. Figure 8
shows that the highest percentage of documents were in the area of Energy (23.4%), followed by Environmental sciences (18.0%) and Engineering (12.4%). Areas such as Earth and Planetary Sciences (2.4%), Materials Science (2.4%) or Immunology and Microbiology (2.1%) were found to a lesser extent. The “Others” area includes unspecified subject areas.
shows the thematic areas of publications obtained from the Scopus search.
The assessment of the impact of papers through the citations they receive is not an immediate measure but can only be applied several years after the publication of the documents. To avoid this problem, an alternative method was introduced for the counting of citations received by papers, consisting of attributing a weight to publication journals based on the average number of citations received by their papers. Since journals that publish more articles are more likely to be cited, the so-called journal impact factor was introduced, which normalizes the number of citations according to the size of the journal.
The 11 main journals that have published in the search field performed with the Scopus database are shown in Table 5
First place goes to the journal “Renewable and Sustainable Energy Reviews” with an H-index of 193 and a JCR impact factor of 9.184, followed by “Biomass and Bioenergy” with an H-Index of 143 and a JCR impact factor of 3.358, and third place goes to “Bioresource Technology” with an H-Index of 229 and a JCR impact factor of 5.807.
3.6. Detection of Scientific Communities and Trends in the Use of Keywords
This study also carried out an analysis of the most prominent authors in the fields of renewable energy production with biomass. Figure 9
and Table 6
show the scientific production of the 4 main authors on this subject in the last ten years. Omer, A.M. stands out in this field with 122 publications since 2008. This researcher has an h-index of 15, followed by Pari, L. with an h-index of 14, Kaltschmitt, M. with an h-index of 17 and Tippayawong, N. with an h-index of 15.
A scientific community can be defined as a set of nodes that are more densely connected to each other than to the rest of the network. Scientific communities tend to have a central nucleus cohesive with peripheral spheres, which are the weakest links as it moves away from the nucleus. The central nucleus would be formed by the most significant elements of the community. Scientific communities are usually groups that relate to members of groups from other communities [37
Clustering scientific publications is an important problem in current research. Identifying communities or clusters is a topic of great current scientific interest, making it possible to identify and to quantify the existing relations of collaboration between the authors of diverse institutions and areas of knowledge. The detection of communities has been successfully applied in fields such as medicine [38
], or energy [39
]. The VOS algorithm mapping technique implemented in the software VOSviewer [40
] was used to identify and to quantify the collaboration between authors. VOSviewer’s algorithm aims at locating the items in a low-dimensional space so that the distance between two items is an accurate indicator of their relatedness.
shows the detection of scientific communities of authors using the VOSviewer software. This figure shows the relationships between the main researchers in the field of biomass as renewable energy. Asian researchers with strong research connections between them stand out due to both the language and the proximity of their institutes. Cluster 1 (red) is the biggest in terms of number of members with ten authors, followed by cluster 2 (green) with eight authors. The top high-yield authors formed their own cooperative team. The largest node is Zhang, X., who has 20 publications and 8 neighbors, followed by Wang, Y., who has 19 publications and 7 members.
Another important analysis to carry out is that of the key words used in the publications related to the topic of study. During the last 40 years, 17,254 documents have been found, among which the keyword Biomass, appearing in 9753 items, stands out, followed by Renewable Energy Resources (4900 items), and in third place Renewable Energy (2511 items). Table 7
shows the 40 most significant keywords in the last four decades.
As a preliminary step to carrying out searches, you must know, for example, prominent authors in the subject matter of the search, keywords related to the subject, type of publications dedicated to that subject and institutions, events in which the subject is dealt with, and institutions and organizations related to it, so that you can locate authors, books, journals, proceedings, etc. that may be of interest. Therefore, the analysis of keywords in scientific publications is of great importance to knowing the research trends and their follow-up. From this study, the existence of different versions for a keyword can be observed, depending on the way each author expresses himself or herself. For example, looking at Table 6
, similar concepts can be observed written in different ways, such as: ‘Renewable Energy’, ‘Renewable Resource’, ‘Renewable Energy Resources’, ‘Renewable Energies’. In Figure 11
, a cloud of words is shown where the size of the letters represents the importance of the keyword according to the number of items in which it appears.
shows the evolution over time of the 5 keywords that appear in the largest number of publications related to the use of biomass as renewable energy. It can be observed that in 2008 the use of these keywords exhibited a great increase, possibly due to the entry into force of the Kyoto Protocol in 2008. The objective of the Kyoto Protocol was to reduce global greenhouse gas emissions by 5.2% compared to 1990 levels over the period 2008–2012 [41
]. It is the main international instrument for tackling climate change. To this end, the Protocol contains targets for industrialized countries to reduce emissions greenhouse gases.
Currently, the trend in the growth of these keywords continues due to the sustainability policies carried out by governments and supported by the fight against climate change, which has become a global issue of interest.
shows a network map of the co-occurrence of the main keywords used by authors of publications related to the use of biomass as renewable energy. The VOS algorithm mapping technique implemented in the software VOSviewer was used to obtain Figure 13
. Colors represent the division between keywords according to the field in which they have been used, the size of the circles shows how often a term has been used, and the lines connecting the different circles indicate the link between keywords through articles and other publications. In this study, ‘Biomass’, Renewable Energy Resources’, ‘Renewable Energy’, ‘Renewable Resource’ and ‘Renewable Energies’ are the most commonly used words, although they are not the ones that are most related to other key words such as ‘Carbon’ or ‘Bioenergy’.
Biofuels are used in all three states of matter, i.e., solid, liquid and gaseous. In solid state are basically wood and chips, charcoal and pellets [42
]. In liquid form, bioethanol and biodiesel stand out. In addition, in the gaseous state, there is biogas (methane, hydrogen and nitrogen, above all), normally obtained either by anaerobic fermentation of microbiological origin, or by gasification (partial oxidation of biomass at high temperature, about 1400 °C, without combustion) [43
]. Table 8
shows the principal characteristics of each cluster detected in Figure 13
. As can be seen, 6 communities or clusters were found. The two most important are focused on obtaining liquid fuels from biomass. The red cluster is mainly focused on the production of biodiesel, both in the classic ways of obtaining by transesterification of oils from seeds (especially rapeseed and sunflower) or from vegetable oil residue of industrial processes. This includes new sources of fat or raw materials such as microalgae [44
] or insects [46
], and was found as a keyword for the country of Brazil [47
]. The second cluster, the green one, is more focused on bioethanol, produced either from sugars from crops such as sugar cane [49
] or beet [50
], or from starches from crops such as potatoes, corn or other cereals. The other line of work in this cluster is the production of hydrogen or other hydrocarbons from biomass gasification [23
]. The third cluster is devoted to two fundamental issues, the generation of electricity from biomass and the production of biogas, the latter produced from livestock waste (slurry) [51
], sewage sludge [52
], urban solid waste [53
], agricultural waste [54
] and industrial organic waste [55
]. Given the diversity of origins in the production of biogas it is logical to find key words from countries as different in customs such as: Germany, Pakistan and Spain. The fourth yellow cluster focuses on China and the assessment of its resources for sustainable development [56
]. Finally, we find clusters 5 and 6, which are the smaller ones. The purple cluster 5 focuses on the Power Plant and the alternative to the Coal, as it has topics related to environmental impact [57
]. Cluster 6, turquoise in color, revolves around pyrolysis and India as a leading country [58