Thermal Treatment of Biomass: A Bibliometric Analysis— The Torrefaction Case

: The aim of the paper was to summarize and discuss current research trends in biomass thermal treatment (torrefaction process). Quantitative analyses were carried out, in which the main countries, research units and scientists were indicated. The analysis showed a clear upward trend in number of publications after 2010. Most scientists on selected topics come from China, USA, Canada, South Korea, Republic of China, Poland (Web od Science—Core Collection (WoS-CC) and Scopus databases). Quantitative analysis also showed that the most relevant WoS-CC categories in the summary are: Energy Fuels, Engineering Chemical, Agricultural Engineering, Biotechnology Applied Microbiology and Thermodynamics and Scopus Subject area: Energy, Chemical Engineering, Environmental Science, Engineering and Chemistry. Thematic analysis included research topics, process parameters and raw materials used. Thematic groups were separated: torrefaction process (temp.: 150–400 ◦ C), hydrothermal carbonization process (HTC) (temp: 120–500 ◦ C), pyrolysis process (temp.: 200–650 ◦ C) and gasiﬁcation and co-combustion process (temp.: 350–1600 ◦ C). In the years 2015–2019, current research topics were: new torrefaction technologies (e.g., HTC), improvement of the physico-mechanical, chemical and energetic properties of produced fuel as well as the use of torreﬁed biomass in the process of pyrolysis, gasiﬁcation and co-combustion. The raw materials used in all types of biomass thermal treatment were: energy crops, wood from fast-growing and exotic trees, waste from the agri-food industry, sewage sludge and microalgae.


Introduction
In recent years, there has been an increase in population around the world. This clearly affects the increase in energy consumption [1,2]. Most energy comes from the burning of fossil fuels, which is associated with the emission of large amounts of greenhouse gases [3]. Therefore, a reduction in the usage of fossil fuels is needed. There should be an increase in participation of renewable energy sources (RES) in general energy balance [4].
In the European Union (EU) policy climate issues play an important role. An element of EU's energy policy is "2020 climate & energy package" so-called "three 20 targets" (20% improvement in energy efficiency reduction greenhouse gas emissions, 20% of EU energy from RES, 20% improvement in energy efficiency), voted through by the European Parliament in 2008. It is a collection of acts aiming to ensure the achievement of EU objectives regarding counteracting the climate change. One of the objectives is increasing the share of RES up to 20% to 2020 [5,6].
The need to reduce the consumption of fossil fuels, greenhouse gas emissions and increase the expenditure on renewable energy has increased the interest of researchers in the subject of RES. Increasing demands regarding the protection of natural environment and the increasing demand for energy caused greater interest in RES which use biomass, sun, wind, water, sea and geothermal energy. This caused an intensive development of scientific research towards RES [7]. In Poland such research mainly regards RES using biomass, solar energy and wind energy. There are also researches regarding the energetic use of waste.
Numerous studies have showed that biomass has the greatest energy potential from all RES sources. Technologies for the processing and energy use of biomass can be applied in developing and developed countries [8,9]. In EU an directive has been taken in regarding biofuels (2015/1513) which promotes the use of renewable biofuels in EU Member States [4]. There is an increase in researchers' interest in issues related to the production of biofuels from biomass, e.g., from energy crops, agricultural waste, etc. [10,11]. In Poland in 2014 76% of energy from RES originated from solid biomass [6]. Biomass can be a raw material for the production of liquid biofuel, gaseous biofuel and solid biofuels [11,12] (Figure 1). the subject of RES. Increasing demands regarding the protection of natural environment and the increasing demand for energy caused greater interest in RES which use biomass, sun, wind, water, sea and geothermal energy. This caused an intensive development of scientific research towards RES [7]. In Poland such research mainly regards RES using biomass, solar energy and wind energy. There are also researches regarding the energetic use of waste.
Numerous studies have showed that biomass has the greatest energy potential from all RES sources. Technologies for the processing and energy use of biomass can be applied in developing and developed countries [8,9]. In EU an directive has been taken in regarding biofuels (2015/1513) which promotes the use of renewable biofuels in EU Member States [4]. There is an increase in researchers' interest in issues related to the production of biofuels from biomass, e.g., from energy crops, agricultural waste, etc. [10,11]. In Poland in 2014 76% of energy from RES originated from solid biomass [6]. Biomass can be a raw material for the production of liquid biofuel, gaseous biofuel and solid biofuels [11,12] ( Figure 1).
In order to meet the demand of the global energy market, research is ongoing on the search for new sources of biomass. These include: wood from eucalyptus and bamboo, energy crops, agricultural waste, agricultural and food industry waste, municipal waste, sewage sludge, etc. [6,8]. The use of lignocellulosic biomass of energy crops, e.g., miscanthus, willow, poplar, acacia or paulownia as a source is particularly interesting [7,11,13]. According to current trends in research on solid biofuels presented by Knapczyk et al. [12] main groups of topics are: (1) searching for new raw materials for the production of solid biofuels-e.g., waste biomass from palm oil [14], sunflower husks [15], peanut shells [16], new species etc. [17,18], (2) optimization of the supply chain, warehouse logistics and biofuels legislation, e.g., development of and identification system to ensure the quality of biomass [19], legal analysis of the international standard classification of solid biofuels etc. [20] (3) optimization of plant cultivation, study of physicochemical properties of raw materials-e.g., determining the calorific value, chemical composition of elements and main energetic parameters in wood and bark of fast growing trees [21][22][23][24][25][26] and herbaceous etc. [27], (4) agglomeration process-e.g., assessment of physic-mechanical properties of agglomerate and the effect of added biochar and bio-oil etc. [10,[28][29][30][31][32], (5) the torrefaction process-biomass torrefaction [33][34][35][36][37][38], hydrothermal waste carbonization, wood mixtures etc. [39,40] and the remaining group, in which the authors raised topics such as composition testing and combustion modeling in household.

Solid Biofuels
Liquid Fuels Gaseous Fuels

METHODS FOR PROCESSING BIOFUELS
• biological conversion • physical conversion • chemical conversion • thermo-chemical conversion In order to meet the demand of the global energy market, research is ongoing on the search for new sources of biomass. These include: wood from eucalyptus and bamboo, energy crops, agricultural waste, agricultural and food industry waste, municipal waste, sewage sludge, etc. [6,8]. The use of lignocellulosic biomass of energy crops, e.g., miscanthus, willow, poplar, acacia or paulownia as a source is particularly interesting [7,11,13].
According to current trends in research on solid biofuels presented by Knapczyk et al. [12] main groups of topics are: (1) searching for new raw materials for the production of solid biofuels-e.g., waste biomass from palm oil [14], sunflower husks [15], peanut shells [16], new species etc. [17,18], (2) optimization of the supply chain, warehouse logistics and biofuels legislation, e.g., development of and identification system to ensure the quality of biomass [19], legal analysis of the international standard classification of solid biofuels etc. [20] (3) optimization of plant cultivation, study of physicochemical properties of raw materials-e.g., determining the calorific value, chemical composition of elements and main energetic parameters in wood and bark of fast growing trees [21][22][23][24][25][26] and herbaceous etc. [27], (4) agglomeration process-e.g., assessment of physic-mechanical properties of agglomerate and the effect of added biochar and bio-oil etc. [10,[28][29][30][31][32], (5) the torrefaction process-biomass torrefaction [33][34][35][36][37][38], hydrothermal waste carbonization, wood mixtures etc. [39,40] and the remaining group, in which the authors raised topics such as composition testing and combustion modeling in household. Methods for processing biofuels can be divided into: biological conversion, physical conversion, chemical conversion and thermo-chemical conversion. These methods are used to improve the physico-mechanical and chemical parameters of biofuels. The resulting products have various applications, such as biofuels, agglomeration, fertilization, soil remediation and many more [41][42][43][44]. As a result of processing, the products can be used directly or serve as a raw material for further processing.
One of the main methods for processing solid biofuels is thermal processing. Depending on the temperature used the thermal processes can be divided into drying, torrefaction, pyrolysis and gasification ( Figure 2). Each of these processes has different products and intermediates. With increasing temperature, energy parameters may increase, but this is associated with increased energy expenditure. Thermal biomass treatment can improve energy, physico-mechanical and chemical properties. The parameters of individual processes may vary depending on the temperature, atmosphere and exposure time. The efficiency of the process is also influenced by the raw material parameters (humidity, degree of comminution, etc.) [45,46]. Methods for processing biofuels can be divided into: biological conversion, physical conversion, chemical conversion and thermo-chemical conversion. These methods are used to improve the physico-mechanical and chemical parameters of biofuels. The resulting products have various applications, such as biofuels, agglomeration, fertilization, soil remediation and many more [41][42][43][44]. As a result of processing, the products can be used directly or serve as a raw material for further processing.
One of the main methods for processing solid biofuels is thermal processing. Depending on the temperature used the thermal processes can be divided into drying, torrefaction, pyrolysis and gasification ( Figure 2). Each of these processes has different products and intermediates. With increasing temperature, energy parameters may increase, but this is associated with increased energy expenditure. Thermal biomass treatment can improve energy, physico-mechanical and chemical properties. The parameters of individual processes may vary depending on the temperature, atmosphere and exposure time. The efficiency of the process is also influenced by the raw material parameters (humidity, degree of comminution, etc.) [45,46].
The thermal process with absence of air, is called pyrolysis. In the literature often the name is related to the process temperature. Low temperature range (150-300 °C) is characteristic of the torrefaction and above 300 we have the right pyrolysis. This processes are both endothermic one, so energy needed to be supplied for the process [47][48][49]. Bibliometrics is a collection of mathematical and statistical tools used for objective assessment of scientific achievements. This term was used for the first time in 1969 by A. Pritchard [50]. Bibliometric analysis allows to highlight current research topics [12,[51][52][53][54][55], observing scientific trends in the world and in the selected country [56,57]. It gives the opportunity to evaluate scientific units, journals, researchers based on selected parameters [58]. Using bibliometric techniques, it is also possible to indicate a network of connections between authors, countries, research topics. Data for bibliometric analysis should come from reputable and widely recognized databases of scientific publications and patents, such as Web of Science, Scopus. The main quantitative indicators include: the number of publications, number of citations, Impact Factor and Hirsch index.
The aim of the research was to determine the main research topics in the area of the use of thermal processing (torrefaction process) of biological materials and to indicate process parameters and raw materials. The thermal process with absence of air, is called pyrolysis. In the literature often the name is related to the process temperature. Low temperature range (150-300 • C) is characteristic of the torrefaction and above 300 we have the right pyrolysis. This processes are both endothermic one, so energy needed to be supplied for the process [47][48][49].

25-150 °C
Bibliometrics is a collection of mathematical and statistical tools used for objective assessment of scientific achievements. This term was used for the first time in 1969 by A. Pritchard [50]. Bibliometric analysis allows to highlight current research topics [12,[51][52][53][54][55], observing scientific trends in the world and in the selected country [56,57]. It gives the opportunity to evaluate scientific units, journals, researchers based on selected parameters [58]. Using bibliometric techniques, it is also possible to indicate a network of connections between authors, countries, research topics. Data for bibliometric analysis should come from reputable and widely recognized databases of scientific publications and patents, such as Web of Science, Scopus. The main quantitative indicators include: the number of publications, number of citations, Impact Factor and Hirsch index.
The aim of the research was to determine the main research topics in the area of the use of thermal processing (torrefaction process) of biological materials and to indicate process parameters and raw materials.
The analyzes were divided into two stages: I-quantitative analysis, II-qualitative-thematic analyses. The first stage involved searching for indexed documents in the Web of Science-Core Collection (WoS-CC) and Scopus database. The search period covered the years 1945-2019 in English. TOPIC documents were searched: "torrefaction *", type: article.
The analyzes were divided into two stages: I-quantitative analysis, II-qualitative-thematic analyses.
The first stage involved searching for indexed documents in the Web of Science-Core Collection (WoS-CC) and Scopus database. The search period covered the years 1945-2019 in English. TOPIC documents were searched: "torrefaction *", type: article. Then bibliometric data (authors, title, year of issue, key words, additional key words, publishing house) was downloaded and quantitative analyses were performed. As part of them, the number of publications in years, number of publications in countries, leading scientists and main research centers dealing with biomass thermal treatment were shown. WoS-CC categories and Scopus Subject area were also indicated, which thematically concerned the publications.
The next stage was the analysis of keywords of the authors of the publication which were analyzed in the VOSviewer program. This program is free and is used to visualize bibliometric networks. The analyses may concern authors, keywords, journals, and others. Generated thematic maps can be created on the basis of the frequency of occurrence of keywords in years, citations, networks and others. All documents analyzed concerned the WoS-CC Energy Fuels category and Scopus Energy. Maps of terms were generated, which were used to analyze the frequency of occurrence of keywords in years and most often cited. Such maps allow to indicate current research topics and trends changes in years.
The last stage consisted of a detailed thematic analysis of the publication in the Open Access license for the entire Wos-CC: Energy Fuels category and Scopus Subject area: Energy. Open Access documents are widely available. This is especially important for industry. It makes it possible to use research results and implement selected technologies processes and process parameters in production Titles, keywords, abstracts as well as full publication texts were analyzed. The goal was to indicate process parameters, materials  Then bibliometric data (authors, title, year of issue, key words, additional key words, publishing house) was downloaded and quantitative analyses were performed. As part of them, the number of publications in years, number of publications in countries, leading scientists and main research centers dealing with biomass thermal treatment were shown. WoS-CC categories and Scopus Subject area were also indicated, which thematically concerned the publications.
The next stage was the analysis of keywords of the authors of the publication which were analyzed in the VOSviewer program. This program is free and is used to visualize bibliometric networks. The analyses may concern authors, keywords, journals, and others. Generated thematic maps can be created on the basis of the frequency of occurrence of keywords in years, citations, networks and others. All documents analyzed concerned the WoS-CC Energy Fuels category and Scopus Energy. Maps of terms were generated, which were used to analyze the frequency of occurrence of keywords in years and most often cited. Such maps allow to indicate current research topics and trends changes in years.
The last stage consisted of a detailed thematic analysis of the publication in the Open Access license for the entire Wos-CC: Energy Fuels category and Scopus Subject area: Energy. Open Access documents are widely available. This is especially important for industry. It makes it possible to use research results and implement selected technologies processes and process parameters in production Titles, keywords, abstracts as well as full publication texts were analyzed. The goal was to indicate process parameters, materials used for torrefaction and applications of this process. The analyzed publications were divided into three categories according to the technology used: (1) Torrefaction, (2) Hydrothermal carbonization (HTC), (3) Pyrolysis, (4) Gasification and co-combustion. The other group contains publications that focused on related topics. In each of the groups it is specified: application, process parameters and raw materials used in the biomass thermal treatment process.

Quantitative Analysis Results
The search resulted in 1564 scientific articles published in journals in WoS-CC database and 1231 documents in Scopus. Among the found publications, 318 (WoS-CC) and 188 (Scopus) were published under the Open Access license. The oldest publication on torrefaction dates from 1919 and is entitled: Article on the theory of drying and torrefaction [61].
The most cited publications in WoS-CC and Scopus databases include:   used for torrefaction and applications of this process. The analyzed publications were divided into three categories according to the technology used: 1) Torrefaction, 2) Hydrothermal carbonization (HTC), 3) Pyrolysis, 4) Gasification and co-combustion. The other group contains publications that focused on related topics. In each of the groups it is specified: application, process parameters and raw materials used in the biomass thermal treatment process.

Quantitative Analysis Results
The search resulted in 1564 scientific articles published in journals in WoS-CC database and 1231 documents in Scopus. Among the found publications, 318 (WoS-CC) and 188 (Scopus) were published under the Open Access license. The oldest publication on torrefaction dates from 1919 and is entitled: Article on the theory of drying and torrefaction [61].
The most cited publications in WoS-CC and Scopus databases include:              The next stage was the keyword analysis for 2015-2019. Such analysis allows visualizing the relationships between keywords as well as analyzing these words over a selected period. In turn, keyword citation analysis allows to identify research topics of greatest interest to researchers The first analysis concerned the incidence of keywords in years (Figure 8.). The colors represent individual years. 2015-2016 (dark blue/blue/light blue)-'torrefaction', 'biomass', 'biochar', 'pyrolysis', 'combustion', 'biochar', 'pretreatment', 'kinetics'. 2017 (green)-'hydrothermal carbonization', 'wet torrefaction', 'fast pyrolysis', 'carbonization', 'co-combustion', 'slow pyrolysis', 'bioenergy', 'wood pellets', 'gasification'. 2018-2019 (yellow/orange/red)-'fuel properties', 'risk husk', 'sewage sludge', 'torrefied biomass', 'biocoal', 'bio-oil'. One can notice the transition from torrefaction, pyrolysis, biomass combustion, analysis of selected parameters of biomass heat treatment processes, through new technologies (HTC), and ending with the search for new materials for torrefaction, e.g., sewage sludge, optimization of process parameters, testing of selected fuel properties. The next stage was the keyword analysis for 2015-2019. Such analysis allows visualizing the relationships between keywords as well as analyzing these words over a selected period. In turn, keyword citation analysis allows to identify research topics of greatest interest to researchers The first analysis concerned the incidence of keywords in years (Figure 8.). The colors represent individual years. 2015-2016 (dark blue/blue/light blue)-'torrefaction', 'biomass', 'biochar', 'pyrolysis', 'combustion', 'biochar', 'pretreatment', 'kinetics'. 2017 (green)-'hydrothermal carbonization', 'wet torrefaction', 'fast pyrolysis', 'carbonization', 'co-combustion', 'slow pyrolysis', 'bioenergy', 'wood pellets', 'gasification'. 2018-2019 (yellow/orange/red)-'fuel properties', 'risk husk', 'sewage sludge', 'torrefied biomass', 'biocoal', 'bio-oil'. One can notice the transition from torrefaction, pyrolysis, biomass combustion, analysis of selected parameters of biomass heat treatment processes, through new technologies (HTC), and ending with the search for new materials for torrefaction, e.g., sewage sludge, optimization of process parameters, testing of selected fuel properties. The second thematic map (Figure 9) concerns the analysis of citations covered by topics/keywords. The closer the keywords are to orange/red, the higher the number of citations. The most recent keywords are: 'bioenergy', 'wood pellets', 'wet torrefaction', 'hydrothermal carbonization', 'hydrophobicity', 'densification', 'co-combustion' It can be stated that in the presented period (2015-2019), current research topics are new torrefaction technologies, such as HTC, research on improving the physico-mechanical and energy properties of produced fuel and the use of torrefied biomass in the pyrolysis and cocombustion process. The second thematic map (Figure 9) concerns the analysis of citations covered by topics/keywords. The closer the keywords are to orange/red, the higher the number of citations. The most recent keywords are: 'bioenergy', 'wood pellets', 'wet torrefaction', 'hydrothermal carbonization', 'hydrophobicity', 'densification', 'co-combustion' It can be stated that in the presented period (2015-2019), current research topics are new torrefaction technologies, such as HTC, research on improving the physico-mechanical and energy properties of produced fuel and the use of torrefied biomass in the pyrolysis and cocombustion process.

The Results of Qualitative-Thematic Analysis
The second stage of research was a detailed thematic analysis of the publications in the Open Access license for the entire Wos-CC category 'Energy Fuels'-134 articles and Scopus Subject Area: 'Energy'-75 articles. Publications which were published mainly in journals such as: Energies, Biomass & Bioenergy, Energy & Fuels, Fuel and Applied Energy were analyzed. Groups such as torrefaction, hydrothermal carbonization (HTC), pyrolysis, gasification and others have been specified. In each of the groups, the torrefaction process was used as one of the components (end product, raw material or intermediate for further processing).
The first group concerned the analysis of the applications of the torrefaction process ( Figure 10, Appendix A: Table A1). There were 62 publications in this group.

The Results of Qualitative-Thematic Analysis
The second stage of research was a detailed thematic analysis of the publications in the Open Access license for the entire Wos-CC category 'Energy Fuels'-134 articles and Scopus Subject Area: 'Energy'-75 articles. Publications which were published mainly in journals such as: Energies, Biomass & Bioenergy, Energy & Fuels, Fuel and Applied Energy were analyzed. Groups such as torrefaction, hydrothermal carbonization (HTC), pyrolysis, gasification and others have been specified. In each of the groups, the torrefaction process was used as one of the components (end product, raw material or intermediate for further processing).
The first group concerned the analysis of the applications of the torrefaction process ( Figure 10, Appendix A: Table A1). There were 62 publications in this group. The second group concerned the hydrothermal carbonization (HTC) process and is presented in Figure 11 and Appendix A: Table A2. There are 10 publications in this group. The second group concerned the hydrothermal carbonization (HTC) process and is presented in Figure 11 and Appendix A: Table A2. There are 10 publications in this group. The third group concerned the use of torrefied biomass in the pyrolysis process (Figure 12, Appendix A: Table A3). There are 19 publications in this group. The third group concerned the use of torrefied biomass in the pyrolysis process ( Figure 12, Appendix A: Table A3). There are 19 publications in this group. The fourth group concerned the use of torrefied biomass in the gasification and cocombustion process (Figure 13, Appendix A: Table A4). There were 25 publications in this group.  The fourth group concerned the use of torrefied biomass in the gasification and cocombustion process (Figure 13, Appendix A: Table A4). There were 25 publications in this group.
Energies 2020, 13, x FOR PEER REVIEW 13 of 38 Figure 13. Main research topics and process parameters-gasification and co-combustion.
The last group concerned the other topics related to the process of thermal biomass processing, which thematically did not fit directly into the previously mentioned ( Figure  14, Appendix A: Table A5). There were 35 publications in this group. The last group concerned the other topics related to the process of thermal biomass processing, which thematically did not fit directly into the previously mentioned ( Figure 14, Appendix A: Table A5). There were 35 publications in this group.

Discussion
Originally, the analysis of the literature was to be carried out within the torrefaction process area. In the course of a thorough analyses, it turned out that the various thermal biomass conversion processes appear together with torrefaction. It is therefore not possible to analyse their content without taking into account other thermal processes.
Quantitative analysis showed a clear upward trend in the number of publications after 2010 (all publications and in the Open Access license), as presented in Figures 4 and 5. The trends of the graphs indicate that the number of publications may be expected to increase significantly in the following years. At the stage of quantitative analysis, it is difficult to determine unambiguously what is the maximum ceiling for this parameter. A detailed qualitative analysis should bring us significantly closer to this answer. At this stage there are no significant leaders in these topics, but there are some regions that seems to most active at this field. Most scientists studying the selected topics come from China, USA, Canada, South Korea, Republic of China, Poland (WoS-CC and Scopus databases) as presented in Table 1. Interestingly, categorization according to WoS-CC shows various publication profiles ( Figure 6). This indicates that the interest is not only in the fields of energy fuels or chemical engineering, but also in other branches, like agriculture, biotechnology, or microbiology. As the Scopus categorization gives similar results (Figure 7), this could be a very promising direction for research into these processes, not just in the fuel context. Importantly, the analysis of the keywords also indicated a change in the context of the use of the term torrefaction. Initially (2015-2017), it tended to appear in the context of pyrolysis and combustion, but in later years (2017 to 2019), it also appears in the context of new technologies such as HTC and also co-firing. The newest papers also cover waste and sewage sludge treatment A significant progress in the number of publications on this subject is directly related to the increased interest of scientists working in the thermal processing field, not only of biomass, but also of other materials, including waste. The properties of biocarbon can also be an important catalyst for the interest of the scientific world. Over the years, the possibilities for using this product have increased significantly, and new possibilities are constantly emerging. This covers with increasing interests on this topics in other branches.
As a result of qualitative (thematic) analysis, a clear upward trend was demonstrated in the thematic groups: torrefaction and gasification and co-combustion ( Figure 15). The main applications of biomass heat treatment in selected groups are: -Torrefaction process ( Figure 10, Appendix A: Table A1)-The authors mainly discussed topics such as the impact of torrefaction on the hydrophobic properties of fuel, analysis of chemical, energy, physico-mechanical parameters of biochar, kinetics of the torrefaction process, influence of torrefaction process parameters on the energy properties of fuel, the impact of biogas on the efficiency of anaerobic digestion, the impact of raw material/mixtures of raw materials on the effectiveness of the torrefaction process, the effect of addition of biochar on the agglomeration process, mass and energy balance analysis in a continuous torrefaction installation, optimization of torrefaction process parameters, comparison of different technologies used in the torrefaction process, comparison of selected properties of biochar to coal. The process temperatures ranged from 200-400 • C. The materials used in torrefying were biomass of various origins. The main sources of biomass were: energy crops, wood from fast growing and exotic trees with varying degrees of processing, mixtures of torrefaction and biomass, agricultural waste, food industry waste, sewage sludge and microalgae. -HTC process (Figure 11, Appendix A: Table A2)-The authors mainly discussed topics such as the impact of HTC on improving fuel energy properties, the impact of raw material and parameters on process efficiency, modeling and optimization of the HTC process, the impact of HTC on reactivity and combustion kinetics. The process temperatures were in the range of 120-500 • C. The materials used in the hydrothermal carbonization process were biomass of various origins. The main sources of biomass were energy crops, wood with various degrees of processing, agricultural waste, food industry waste and sewage sludge. -Pyrolysis process (Figure 12, Appendix A: Table A3)-The authors mainly discussed topics such as the impact of process parameters (temperature, time) on the chemical, energy and physical-mechanical properties of fuels, kinetic analysis of the pyrolysis process, production and characterization of bio-oil and biochar, activated carbon from the pyrolysis, biochar characteristics from biomass carbonization, process optimization. Process temperatures were in the range: 200-650 • C. Materials used in torrefaction were biomass of various origin. The main sources of biomass were energy crops, wood with varying degrees of processing from fast-growing, fruit and exotic trees, agricultural waste, food industry waste and sewage sludge. -Gasification and co-combustion process (Figure 13, Appendix A: Table A4)-The authors mainly discussed topics such as analysis of the kinetic process, process efficiency (proliferation, gasification, combustion), analysis of the co-combustion process of mixtures, e.g., sewage sludge, coal with biomass (biochar, raw), the impact of material changes (torrefaction, mixing of various raw materials) on the chemical, energy and physico-mechanical properties of fuels, process optimization. Process temperatures were in the range: 350-1600 • C. Materials used in torrefaction were biomass of various origins. The main sources of biomass were energy crops, wood with varying degrees of processing from fast-growing, fruit and exotic trees, agricultural waste, food industry waste and sewage sludge. Blends of biomass with coal were also used.
Energies 2020, 13, x FOR PEER REVIEW 16 of 38 -Pyrolysis process (Figure 12, Appendix A: Table A3)-The authors mainly discussed topics such as the impact of process parameters (temperature, time) on the chemical, energy and physical-mechanical properties of fuels, kinetic analysis of the pyrolysis process, production and characterization of bio-oil and biochar, activated carbon from the pyrolysis, biochar characteristics from biomass carbonization, process optimization. Process temperatures were in the range: 200-650 °C . Materials used in torrefaction were biomass of various origin. The main sources of biomass were energy crops, wood with varying degrees of processing from fast-growing, fruit and exotic trees, agricultural waste, food industry waste and sewage sludge. -Gasification and co-combustion process (Figure 13, Appendix A: Table A4)-The authors mainly discussed topics such as analysis of the kinetic process, process efficiency (proliferation, gasification, combustion), analysis of the co-combustion process of mixtures, e.g., sewage sludge, coal with biomass (biochar, raw), the impact of material changes (torrefaction, mixing of various raw materials) on the chemical, energy and physico-mechanical properties of fuels, process optimization. Process temperatures were in the range: 350-1600 °C . Materials used in torrefaction were biomass of various origins. The main sources of biomass were energy crops, wood with varying degrees of processing from fast-growing, fruit and exotic trees, agricultural waste, food industry waste and sewage sludge. Blends of biomass with coal were also used. Publications that did not fit thematically in the groups: torrefaction, HTC, pyrolysis, gasification and co-combustion were placed in the "other" group ( Figure 14, Appendix A: Table A5). The main topics included regional analysis and global effectiveness of the use of torrefaction and biochar in the bioeconomy, analysis and optimization of the solid biofuels supply chain, analysis of integrated energy generation systems (biomass gasification, gas turbine and others) in selected areas of the economy, product life cycle analysis (LCA), technical and economic analyzes of the biomass torrefaction process in selected countries, optimization of production scheduling, forecasting and modeling of biomass thermal treatment processes, investigation into the applicability of the Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses.
To date, many review publications have been created covering a selected thematic area ( Publications that did not fit thematically in the groups: torrefaction, HTC, pyrolysis, gasification and co-combustion were placed in the "other" group ( Figure 14, Appendix A: Table A5). The main topics included regional analysis and global effectiveness of the use of torrefaction and biochar in the bioeconomy, analysis and optimization of the solid biofuels supply chain, analysis of integrated energy generation systems (biomass gasification, gas turbine and others) in selected areas of the economy, product life cycle analysis (LCA), technical and economic analyzes of the biomass torrefaction process in selected countries, optimization of production scheduling, forecasting and modeling of biomass thermal treatment processes, investigation into the applicability of the Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses.
To date, many review publications have been created covering a selected thematic area (67 documents-WoS-CC, 57 documents-Scopus). These articles are published in leading journals, and their usefulness is demonstrated by a large number of citations. Examples of thematic areas discussed: improvement of selected properties through the torrefaction process, analysis of the technologies used, indication of application areas [65,66], application of thermochemical conversion for selected biological materials [67,68], co-combustion of coal with biomass, as well as biomass pretreatment in selected countries [69]. The results of this publication complement the current literature research with a comprehensive review of publications in the Open Access license in the scientific databases WoS-CC (WoS-CC category: Energy Fuels) and Scopus (Subject area: Energy).
All this information shows clearly that thermal processing of biomass, and in particular torrefaction is a promising technology, and in the next few years we can expect to see many papers about this process. Qualitative analysis shows that at this time, we cannot precisely forecast what the trends in the next years will be. For sure papers concerning torrefaction will more concentrated on the other branches than energy and fuels. Microbilology seems to be promising way, as trend analysis (Figures 6 and 7) shows potential to improvement in this category. Also production and biochar analysis seems to be still a very desired direction. Introduction of new materials, like waste biomass, or sewage sludge, seems to be next stage in research list of many scientists. As this topic is still new, there is big potential for good papers, which valuable will be evidenced by high citation scores.
Thanks to the holistic approach and cumulative presentation of results, this publication can be a valuable source of knowledge for researchers dealing with selected topics and industry. Especially in the latter area, trade and scientific magazines in the Open Access license are important because they can be an inspiration to implement innovative solutions and establish scientific cooperation.

Conclusions
(1) Thermal biomass processing is a current research topic. A clear upward trend in the number of publications after 2010 can be noticed. Quantitative analysis also showed that the most important categories of WoS-CC in the selected topic are: Energy Fuels, Engineering Chemical, Agricultural Engineering, Biotechnology Applied Microbiology and Thermodynamics and Scopus Subject area: Energy, Chemical Engineering, Environmental Science, Engineering and Chemistry. (2) In 2015-2019, current research topics were: new torrefaction technologies (e.g., HTC), improvement of the physico-mechanical, chemical and energy properties of the fuel produced and the use of torrefied biomass in the processes of pyrolysis, gasification and co-combustion. (3) The raw materials used in all types of heat treatment processes were energy crops, wood from fast-growing and exotic trees, waste from the agri-food industry, waste from agricultural production, sewage sludge and microalgae.
The thematic scope of the analyzed publications was very diverse. Four main thematic groups were identified: torrefaction process, HTC process, pyrolysis process and gasification and co-combustion process. In addition to research topics related to process analysis and optimization, improvement of chemical, energetic and physical-mechanical properties of fuel, properties of raw materials and their mixtures, the authors also discussed the topics of technical and economic analysis of the torrefaction process, LCA, analysis and optimization of the supply chain and investigation into the applicability of Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses.

Conflicts of Interest:
The authors declare no conflict of interest.
Appendix A Table A1. Torrefaction process-detailed thematic analysis.

Ref.
Year     [165] 2018 -high temperature gasification of high heating-rate chars using a flat-flame reactor. 1300 Norway spruce [166] 2018 -analyzed the possibility of co-firing a series of avocado biomass samples carbonized with coal. 400, 500, 600 avocado pit [167] 2018 -torrefaction of healthy and beetle kill pine and co-combustion with sub-bituminous coal 500 healthy pine, beetle kill pine [168] 2018 -coupled effect of torrefaction and blending on chemical and energy properties for combustion 900 napier grass, rice straw, cassava stalks, corn cob [169] 2018 -co-gasification of pine and oak biochar with sub-bituminous coal in carbon dioxide 833, 900, 975 pine biochar, oak biochar, coal, pine biochar-coal blend, oak biochar-coal blend [170] 2017 -effect of torrefaction on the process performance of gasification of hardwood and softwood 850 spruce, ash [171] 2017 -CFB gasification of commercial torrefied wood pellets 800-850 wood, [172] 2017 -Organic carbon emissions from the co-firing of coal and Wood in a fixe Bed combustor 400 pine wood, pine sawdust [173] 2017 -optimization of a bubbling fluidized bed plant for low-temperature gasification of biomass 900 pine, chestnut, shell, olive stone, grape, olive pomaces, cocoa shell [174] 2017 -The effect of torrefaction on syngas quality metrics from fluidized bed gasification of SRC willow 900 willow [175] 2016 -characterization and the effect of lignocellulosic biomass value addition on gasification efficiency 900 sugarcane bagasse Table A4. Cont.

Ref.
Year Application [185] 2019 -Boosting carbon efficiency of the biomass to liquid process with hydrogen from power [186] 2019 -Influence of structural modification on VOC emission kinetics from stored carbonized refuse-derived fuel [187] 2019 -Process simulation of an integrated biomass torrefaction and pelletization (iBTP) [188] 2019 -Evaluating integration of biomass gasification process with solid oxide fuel cell and torrefaction process [189] 2018 -Improving carbon efficiency and profitability of the biomass to liquid process with hydrogen from renewable power [190] 2018 -Integrated systems analysis of electricity, heat, road transport, aviation, and chemicals: a case study for the Netherlands [191] 2018 -International vs. domestic bioenergy supply chains for co-firing plants: The role of pre-treatment technologies [192] 2018 -Use of biomass in integrated steelmaking-Status quo, future needs and comparison to other low-CO 2 steel production technologies [193] 2018 -climate impact and energy efficiency of internationally traded non-torrefied and torrefied wood pellets from logging residues [194] 2018 -Coupling of an acoustic emissions system to a laboratory torrefaction reactor [195] 2017 -Technical assessment of the Biomass Integrated Gasification/Gas Turbine Combined Cycle incorporation in the sugarcane industry [196] 2017 -An LCA-based evaluation of biomass to transportation fuel production and utilization pathways in a large port's context [197] 2017 -The role of bioenergy and biochemicals in CO 2 mitigation through the energy system-a scenario analysis for the Netherlands [198] 2017 -a whole-systems analysis of the value chain associated with cultivation, harvesting, transport and conversion in dedicated biomass power stations [199] 2017 -Economic impact of combined torrefaction and pelletization processes on forestry biomass supply [200] 2017 -thermoliquefaction of palm oil fiber using supercritical ethanol. [201] 2017 -The climate contribution of biomass co-combustion in a coal-fired power plant [202] 2017 -the influence of pre-treatment of biomass on products distribution and characteristics of torrefaction products [203] 2016 -Influence of mill type on densified biomass comminution. [204] 2016 -Techno-economic and carbon emissions analysis of biomass torrefaction downstream in international bioenergy supply chains for co-firing [205] 2016 -An energy analysis comparing biomass torrefaction in depots to wind with natural gas combustion for electricity generation Table A5. Cont.

Ref.
Year Application [206] 2016 -Processing and sorting forest residues: Cost, productivity and managerial impacts [207] 2016 -Fast hydrothermal liquefaction for production of chemicals and biofuels from wet biomass-The need to develop a plug-flow reactor [208] 2016 -Technical improvements and economic-environmental assessment along the overall torrefaction supply chain through the SECTOR project [209] 2016 -an assessment of the torrefaction of north american pine and life cycle greenhouse gase emission [210] 2016 -Optimal production scheduling for energy efficiency improvement in biofuel feedstock preprocessing considering work-in-process particle separation [211] 2016 -optimization the minimum production cost for the production of woody biofuels [212] 2016 -Prediction of high-temperature rapid combustion behaviour of woody biomass particles [213] 2016 -Environmental and Energy Performance of the Biomass to Synthetic Natural Gas Supply Chain [214] 2016 -Modeling of biofuel pellets torrefaction in a realistic geometry [215] 2015 -regionalized techno-economic assessment and policy analysis for biomass molded fuel in China [216] 2015 -Investigation into the applicability of Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses compared to Colombian La Loma coal [217] 2015 -Explosion characteristics of pulverised torrefied and raw Norway spruce (Picea abies) and Southern pine (Pinus palustris) in comparison to bituminous coal [218] 2015 -high moisture corn stover pelleting in a flat die pellet mill fitted-physical properties and specific energy consumption [219] 2015 -comparative cradle-to-gate life cycle assessment of wood pellet production with torrefaction [220] 2011 -to achieve a first understanding of the possibility to combine torrefaction and hydrolysis for lignocellulosic bioethanol processes, and to evaluate it in terms of sugar and ethanol yields