Biomass as Renewable Energy: Worldwide Research Trends

The world’s population continues to grow at a high rate, such that today’s population is twice that of 1960, and is projected to increase further to 9 billion by 2050. This situation has brought about a situation in which the percentage of the global energy used in cities is increasing considerably. Biomass is a resource that is present in a variety of different materials: wood, sawdust, straw, seed waste, manure, paper waste, household waste, wastewater, etc. Biomass resources have traditionally been used, and their use is becoming increasingly important due to their economic potential, as there are significant annual volumes of agricultural production, whose by-products can be used as a source of energy and are even being promoted as so-called energy crops, specifically for this purpose. The main objective of this work was to analyze the state of research and trends in biomass for renewable energy from 1978 to 2018 to help the research community understand the current situation and future trends, as well as the situation of countries in the international context, all of which provides basic information to facilitate decision-making by those responsible for scientific policy. The main countries that are investigating the subject of biomass as a renewable energy, as measured by scientific production, are the United States, followed by China, India, Germany and Italy. The most productive institutions in this field are the Chinese Academy of Sciences, followed by the National Renewable Energy Laboratory, Danmarks Tekniske Universitet and the Ministry of Education in China. This study also identifies communities based on the keywords of the publications obtained from a bibliographic search. Six communities or clusters were found. The two most important are focused on obtaining liquid fuels from biomass. Finally, based on the collaboration between countries and biomass research, eight clusters were observed. All this is centered on three countries belonging to different clusters: USA, India and the UK.


Introduction
The world's population continues to grow at a high rate, such that today's population is twice that of 1960, and it is projected to increase further to 9 billion by 2050 [1].Forecasting models suggest that developing countries will account for 99 per cent of this population increase, with population growth of 50 per cent in urban areas [2,3].This situation has brought about a situation in which the percentage of global energy used in cities is increasing considerably.In the early 1990s, cities consumed less than half of the total energy produced, while they currently use two-thirds of the worldwide energy [4].This means that the share of urban energy use in the global energy mix is growing at a higher rate than the global share of the urban population.Although cities continue to use fossil fuels as the main source of energy, energy sustainability is becoming a key political solution to mitigate problems related to climate change [5].Indeed, cities represent 70% of the total emissions of CO 2 caused by humans [6], being one of the largest contributors to climate change.In addition, cities face devastating effects from climate change.Approximately 70% of cities are already coping with the effects of climate change.Since 90% of all urban areas are coastal, the damage caused by rising sea levels is expected to increase, with some cities in developing countries being particularly vulnerable.The increase in urban energy consumption has also led to an increase in urban air pollution.According to the World Health Organization (WHO), 90% of the inhabitants of urban areas are subject to environmental pollution levels that exceed the recommended limits [7].
Biomass refers to all organic matter existing in the biosphere, whether of plant or animal origin, as well as those materials obtained through their natural or artificial transformation [8][9][10].Biofuels derived from biomass include firewood, wood shavings, pellets, some fruit stones such as olives and avocados, as well as nutshells.Of these, cut and chopped firewood is the least processed, and is usually burned directly in domestic appliances such as stoves and boilers.The chips come from the crushing of biomass both agricultural and forest, with their size being variable depending on the manufacturing process from which they are derived, or the transformation process that they have undergone.Finally, pellets are the most elaborate biofuel, and consist of small cylinders 6 to 12 mm in diameter and 10 to 30 mm in length that are obtained by pressing biofuels with binders.Pellets are used especially in fuels with a low energy/volume ratio [11,12].Fruit stones and seeds, as well as fruit husks, though used to a lesser extent than other standardized fuels such as fuelwood, wood chips and pellets, also represent an increasingly used solid biofuel.Indeed, it has been shown that mango stone, peanut shell and sunflower seed husk have a high energy potential, with a Higher Heating Value (HHV) similar to other commercialized biofuels [13][14][15].This fact, together with the increasing worldwide production of these by-products, makes them especially attractive for thermal energy generation, as well as to reduce CO 2 emissions.Biomass is present in a variety of different materials: wood, sawdust, straw, seed waste, manure, paper waste, household waste, wastewater, etc. [16].
Renewable energy for heating comes either from decentralized equipment in buildings or from centralized generation and its further distribution [17].Decentralized biomass boilers are an emerging technology in constant development [18].Biomass is a carbon-neutral energy source, since the biomass during its growth absorbs CO 2 that is then released into the atmosphere during its combustion, with a zero-net balance of CO 2 emissions [19].However, large amounts of thermal energy are wasted in power generation and in many manufacturing processes.Cogeneration is the most widely used technology to reuse lost heat, generating useful heat as well as electrical energy.Combined Heat and Power plants (CHP) simultaneously produce electricity and heat for use in industrial, trade or residential contexts.Industry consumes all the heat and electricity it needs, and the excess electricity is fed into the grid and is consumed mostly in the local environment [20,21].On the other hand, district heating and cooling networks are a highly effective way to integrate natural resources such as industrial and agricultural biomass, while increasing energy efficiency.Distributed energy systems consist of a network of underground insulated pipes, connected to a thermal or cold heat plant, through which hot or cold water is pumped to several buildings within a district [14].
Due to the wide availability of biomass worldwide, mainly because it can be obtained as a by-product of many industrial and agricultural processes, biomass represents a growing renewable energy source with high growth potential [22].One of the main characteristics of biomass that makes it suitable as an energy source is that through direct combustion it can be burned in waste conversion plants to produce electricity [23] or in boilers to produce heat at industrial and residential levels [24].However, it must be borne in mind that direct combustion of biomass is not always feasible in existing facilities, and that in many cases it is necessary to carry out physical-chemical or biological treatments to adapt it to the quality of conventional fuels.Biomass District Heating (BDH) is a very effective system for the integration of natural energy resources within urban environments, achieving on the one hand a 100% reduction in CO 2 emissions compared to fossil fuels, and on the other hand an increase in energy efficiency due to the lower cost of biofuels.Biomass exists in a variety of different materials: wood, sawdust, straw, seed waste, manure, paper waste, household waste, wastewater, etc. [25].The characteristics of some materials allow them to be used as fuels directly; however, others require a series of pretreatments, which require different technologies before they can be used.
Biomass has its flaws, but also its strengths.Among its great benefits is the forest use of the territory, which would also serve to clean the forest and thus prevent forest fires, and the ability to generate jobs.Biomass generates continuous employment such as the extraction of raw materials from the countryside and the bush [14].Nowadays, the use of biomass as biofuel represents a field of great interest to the scientific community.Table 1 presents a synthesis of the ways of approach from the literature depending of the type of biomass.At present, a large part of the research has been focused on environmentally friendly and sustainable energy from biomass to supplement conventional fossil fuels [33].The main objective of this work was to analyze the state of research and trends in biomass for renewable energy from the last 40 years (from 1978 to 2018) to help the research community understand the future trends, as well as the situation of a country in the international context, all of which provides basic information to facilitate decision-making by those responsible for scientific policy.

Research Approach
The analysis of scientific publications constitutes a fundamental step within the research process and has become a tool that allows us to qualify the quality of the process generating the knowledge and the impact of this process on the environment.Bibliometrics is a discipline of scientometrics and provides information on the results of the research process, its volume, evolution, visibility and structure.In this way, it is possible to assess scientific activity and the impact of both research and sources [34].
Bibliometric studies provide an interesting overview of a country's own scientific activity, as well as its situation in the international context, all of which provides basic information to facilitate decision-making by those responsible for scientific policy.
In this study, a complete search of Elsevier's Scopus database was conducted using the subfields (TITLE-ABS-KEY (biomass) AND TITLE-ABS-KEY (renewable AND energy)) to identify publications addressing the subject of publications from 1978 to 2018 that referred to biomass as renewable energy.
Scopus is a bibliographic database of documents from scientific journals.It contains information relating to more than 35,000 titles from all areas.It includes, in addition to journals, monographic series, conference proceedings, books (emptied at book and chapter level) and patents.Patents are vacated from five official offices (WIPO, EPO, United States, Japan and United Kingdom), offering more than 27 million [35].
Temporal coverage dates back to 1996 but can sometimes be traced back to 1823.
In addition to providing bibliographic information, Scopus offers bibliometric tools that measure the performance of publications and authors, based on the count of citations received for each article.
Once the results of the Scopus search related to biomass as a renewable energy were obtained, this study carried out an analysis of the types of documents, language of the documents, scientific production and its trend, main scientific institutions with scientific productions related to the subject, collaborations between countries and main authors in the subject, and the evolution in the use of key words.Another key aspect of this research was the identification of scientific communities using Vosviewer software (http://www.vosviewer.com/).Vosviewer is free software that allows you to build and visualize bibliometric networks from data obtained from bibliometric searches of important databases such as Scopus [36]. Figure 1 shows a scheme of the methodology used.
The methodology used in this study contained the following steps: 1.
Global search for information.The Scopus database was used to search for information using the following search fields: (TITLE-ABS-KEY (biomass) AND TITLE-ABS-KEY (renewable AND energy)).The search result was a .csvfile containing the following information: Authors, Author Ids, Title, Year, Source title, Volume, Issue, Art.No., Page start, Page end, Page count, Cited by, DOI, Link, Document Type, Access Type, Source, EID.

2.
Bibliometric data analysis.Each piece of the above data was analyzed and studied separately.For example, it was analyzed who the main authors were that have published on this subject, or what types of documents have been published on this subject (articles, reviews, etc.).3.
Community detection.The community detection of thematic clusters (community or cluster detection) was analyzed with VOSviewer, obtaining maps of international collaboration between different countries and authors, and the research trends using keywords.

Results and Discussion
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.

Results and Discussion
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.

Type and Language of Publications
17,274 documents were obtained from the search carried out considering various fields and types of documents for the period 1978-2018.According to the different types of publications, most of the research works were Articles (59.0%), followed by Conference Papers (23.2%) and Reviews (9.4%).In contrast, the smallest numbers of documents were obtained for Notes (1.2%), Books (0.9%) and Short Surveys (0.6%). Figure 2 shows the different types of documents related to biomass as renewable energy during the period 1978-2018.

Results and Discussion
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.

Type and Language of Publications
17,274 documents were obtained from the search carried out considering various fields and types of documents for the period 1978-2018.According to the different types of publications, most of the research works were Articles (59.0%), followed by Conference Papers (23.2%) and Reviews (9.4%).In contrast, the smallest numbers of documents were obtained for Notes (1.2%), Books (0.9%) and Short Surveys (0.6%). Figure 2 shows the different types of documents related to biomass as renewable energy during the period 1978-2018.In reference to the type of languages used in the publications obtained from the search, most of the documents were published in English in international journals, followed by Chinese (2.04%), German (0.30%) and Spanish (0.27%). Figure 3 shows the percentage of the different languages in which the different documents obtained from the bibliometric analysis have been published.In reference to the type of languages used in the publications obtained from the search, most of the documents were published in English in international journals, followed by Chinese (2.04%), German (0.30%) and Spanish (0.27%). Figure 3 shows the percentage of the different languages in which the different documents obtained from the bibliometric analysis have been published.

Characteristics of Scientific Production for the Period 1978-2018
Figure 4 shows the trend in scientific production over the last 40 years.As can be seen, during the first 30 years, there is no significant growth in scientific production on the use of biomass as renewable energy.In 2008 there is a very important growth in publications due to oil peaking at over $136 a barrel in June 2008, and it has never been that high since then.Therefore, the reason was the high price of oil followed by a widespread economic downturn.

Characteristics of Scientific Production for the Period 1978-2018
Figure 4 shows the trend in scientific production over the last 40 years.As can be seen, during the first 30 years, there is no significant growth in scientific production on the use of biomass as renewable energy.In 2008 there is a very important growth in publications due to oil peaking at over $136 a barrel in June 2008, and it has never been that high since then.Therefore, the reason was the high price of oil followed by a widespread economic downturn.

Worldwide Distribution of Publications
Figure 5 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.

Worldwide Distribution of Publications
Figure 5 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.Figure 6 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.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 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.

Institutions Distribution of Publications
Table 3 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 CO 2 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.

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.Table 4 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.

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  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.
Figure 10 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.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 highyield 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.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.
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.Figure 12 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 Figure 12 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.Figure 13 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'.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.
Figure 13 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 [44,45] or insects [46], and was found as a keyword for the country of Brazil [47,48].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 [51].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) [52], sewage sludge [53], urban solid waste [54], agricultural waste [55] and industrial organic waste [56].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 [57].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 [58].Cluster 6, turquoise in color, revolves around pyrolysis and India as a leading country [59].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 [44,45] or insects [46], and was found as a keyword for the country of Brazil [47,48].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,

17 ,
274 documents were obtained from the search carried out considering various fields and types of documents for the period 1978-2018.According to the different types of publications, most of the research works were Articles (59.0%), followed by Conference Papers (23.2%) and Reviews

Figure 2 .
Figure 2. Distribution of different types of publications during the period 1978-2018.

Figure 2 .
Figure 2. Distribution of different types of publications during the period 1978-2018.

Figure 3 .
Figure 3. Language of publications for the period 1978-2018.Figure 3. Language of publications for the period 1978-2018.

Figure 3 .
Figure 3. Language of publications for the period 1978-2018.Figure 3. Language of publications for the period 1978-2018.

Figure 4 .
Figure 4. Trend in biomass as renewable energy publications during the period 1978-2018.

Figure 4 .
Figure 4. Trend in biomass as renewable energy publications during the period 1978-2018.

Figure 5 .
Figure 5. World scientific production in biomass as renewable energy by country of origin.

Figure 6
Figure 6  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.

Figure 5 .
Figure 5. World scientific production in biomass as renewable energy by country of origin.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

Figure 5 .
Figure 5. World scientific production in biomass as renewable energy by country of origin.

Figure 6
Figure6represents 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.

Figure 6 .
Figure 6.Trends in publications on renewable energy in urban areas during the last 18 years for the top five countries.

Figure 7
Figure7represents 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 .csvfile.

Figure 6 .
Figure 6.Trends in publications on renewable energy in urban areas during the last 18 years for the top five countries.

Figure 7 21 Figure 7 .
Figure 7 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 .csvfile.Sustainability 2019, 11, 863 9 of 21

Figure 8 .
Figure 8. Thematic areas of publications obtained from the Scopus search.

Figure 8 .
Figure 8. Thematic areas of publications obtained from the Scopus search.

Figure 9 .Table 6 .
Figure 9. Bar chart of the main authors' publications in the last ten years.

Figure 9 .Table 6 .
Figure 9. Bar chart of the main authors' publications in the last ten years.Table 6.Evolution of the main authors' publications over the last ten years.Year Tippayawong, N. (Thailand) Thrän, D. (Germany)

Figure 10 .
Figure 10.Scientific communities of authors related with biomass as renewable energy.

Figure 10 .
Figure 10.Scientific communities of authors related with biomass as renewable energy.

Figure 11 .
Figure 11.Cloud word of main keywords related to biomass as renewable energy.

Figure 11 .
Figure 11.Cloud word of main keywords related to biomass as renewable energy.

Sustainability 2019 ,
11, 863 16 of 21use 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.

Figure 12 .
Figure 12.Evolution of the top five keywords in the period 2001-2018.

Figure 12 .
Figure 12.Evolution of the top five keywords in the period 2001-2018.

Table 1 .
Main researches on biomass as biofuel during the last ten years.

Table 2 .
Country communities detected in the topic biomass as renewable energy.

Table 3 .
Scientific production and keywords used by the ten most important international institutions.

Table 4 .
Main thematic areas according to the number of publications obtained.

Table 4 .
Main thematic areas according to the number of publications obtained.

Table 5 .
SJR, H-Index and JCR impact factor of principal international journals.

Table 7 .
Forty main keywords used in publications.

Table 7 .
Forty main keywords used in publications.

Table 8 .
Main keywords used by the communities detected in the topic biomass as renewable energy.