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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.
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energies
Review
Thermal Treatment of Biomass: A Bibliometric Analysis—
The Torrefaction Case
Adrian Knapczyk 1, * , Sławomir Francik 1, Marcin Jewiarz 1, Agnieszka Zawi´slak 2and Renata Francik 3


Citation: Knapczyk, A.; Francik, S.;
Jewiarz, M.; Zawi´slak, A.; Francik, R.
Thermal Treatment of Biomass: A
Bibliometric Analysis—The
Torrefaction Case. Energies 2021,14,
162. https://doi.org/10.3390/
en14010162
Received: 25 November 2020
Accepted: 24 December 2020
Published: 30 December 2020
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional clai-
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Copyright: © 2020 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Department of Mechanical Engineering and Agrophysics, University of Agriculture in Krakow, Balicka 120,
31-120 Kraków, Poland; slawomir.francik@urk.edu.pl (S.F.); marcin.jewiarz@urk.edu.pl (M.J.)
2Department of Biotechnology and General Food Technology, Faculty of Food Technology, University of
Agriculture in Krakow, 30-149 Kraków, Poland; agnieszka.zawislak@urk.edu.pl
3Institute of Health, State Higher Vocational School, Staszica 1, 33-300 Nowy S ˛acz, Poland;
rfrancik@pwsz-ns.edu.pl
*Correspondence: adrian.knapczyk@urk.edu.pl
Abstract:
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.
Keywords:
torrefaction; hydrothermal carbonization; pyrolysis; gasification; bibliometric analysis;
research trends; research topic; scientometric; solid biomass
1. 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
Energies 2021,14, 162. https://doi.org/10.3390/en14010162 https://www.mdpi.com/journal/energies
Energies 2021,14, 162 2 of 31
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).
Energies 2020, 13, x FOR PEER REVIEW 2 of 38
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 regard-
ing 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., miscan-
thus, willow, poplar, acacia or paulownia as a source is particularly interesting [7,11,13].
Figure 1. Classification of biofuels.
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 biofuelse.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 logis-
tics 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 materialse.g., determining the calorific value, chemical composition of elements
and main energetic parameters in wood and bark of fast growing trees [21-26] and herba-
ceous etc. [27], (4) agglomeration processe.g., assessment of physic-mechanical proper-
ties of agglomerate and the effect of added biochar and bio-oil etc. [10,28-32], (5) the tor-
refaction processbiomass torrefaction [33-38], 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.
BIOMASS
Solid Biofuels
Liquid Fuels
Gaseous Fuels
METHODS FOR PROCESSING BIOFUELS
biological conversion
physical conversion
chemical conversion
thermo-chemical conversion
Figure 1. Classification of biofuels.
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., miscant-
hus, 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
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
32
], (5) the torrefaction
process—biomass torrefaction [
33
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.
Energies 2021,14, 162 3 of 31
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
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. Depend-
ing 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 en-
ergy, 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].
Energies 2020, 13, x FOR PEER REVIEW 3 of 38
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 result-
ing products have various applications, such as biofuels, agglomeration, fertilization, soil
remediation and many more [41-44]. 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. De-
pending on the temperature used the thermal processes can be divided into drying, tor-
refaction, pyrolysis and gasification (Figure 2). Each of these processes has different prod-
ucts 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 individ-
ual 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 (150300 °C) is char-
acteristic 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 [47-49].
Figure 2. Biomass thermal treatmentprocesses and range of temperatures.
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 [12,51-55],
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.
DRYING
25150 °C
TORREFACTION
150300 °C
PYROLYSIS
300600 °C
GASIFICATION
>600 °C
EXPENDITURE
IMPROVING ENERGY
PARAMETERS
Figure 2. Biomass thermal treatment—processes and range of temperatures.
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 [4749].
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
[12,5155]
,
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 param-
eters [
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.
Energies 2021,14, 162 4 of 31
2. Materials and Methods
The research was carried out using the authors’ methodology [
12
,
55
,
59
,
60
] using
bibliometric techniques (Figure 3).
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.
Energies 2020, 13, x FOR PEER REVIEW 4 of 38
2. Materials and Methods
The research was carried out using the authors methodology [12,55,59,60] using bib-
liometric techniques (Figure 3).
The analyzes were divided into two stages:
Iquantitative analysis,
IIqualitativethematic analyses.
The first stage involved searching for indexed documents in the Web of Science
Core Collection (WoSCC) and Scopus database. The search period covered the years
19452019 in English. TOPIC documents were searched: torrefaction *, type: article.
Figure 3. Diagram of research methodology.
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 con-
cerned 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: En-
ergy. Open Access documents are widely available. This is especially important for indus-
try. It makes it possible to use research results and implement selected technologies pro-
cesses and process parameters in production Titles, keywords, abstracts as well as full
publication texts were analyzed. The goal was to indicate process parameters, materials
Search in the WoS-CC and Scopus database
Quantitative analysis
Number of publications, citations, major countries,
major authors, major research area
Term maps
Qualitative analysiscontent of the publication
Torrefaction
Creation of a bibliometric database
Pyrolysis
Hydrothermal carbonization
Gasification and co-combustion
Figure 3. Diagram of research methodology.
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 con-
cerned 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
Energies 2021,14, 162 5 of 31
divided into three categories according to the technology used: (1) Torrefaction, (2) Hy-
drothermal 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.
3. Results
3.1. 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 (Sco-
pus) 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:
Bridgeman T.G. et al.—Total Citations: 474 WoS-CC/545 Scopus [62],
Phanphanich M. and Mani S.—Total Citations: 432 WoS-CC/512 Scopus [63],
Arias B. et al.—Total Citations: 427 WoS-CC/504 Scopus [64].
Figure 4shows the distribution of the number of all publications indexed in WoS-CC
and Scopus database. The years 2005–2019 were chosen to increase readability. A clear
upward trend has been visible since 2010.
Energies 2020, 13, x FOR PEER REVIEW 5 of 38
used for torrefaction and applications of this process. The analyzed publications were di-
vided into three categories according to the technology used: 1) Torrefaction, 2) Hydro-
thermal 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 spec-
ified: application, process parameters and raw materials used in the biomass thermal
treatment process.
3. Results
3.1. Quantitative Analysis Results
The search resulted in 1564 scientific articles published in journals in WoS-CC data-
base 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 torrefac-
tion [61].
The most cited publications in WoS-CC and Scopus databases include:
Bridgeman T.G. et al.Total Citations: 474 WoS-CC/545 Scopus [62],
Phanphanich M. and Mani S.Total Citations: 432 WoS-CC/512 Scopus [63],
Arias B. et al.Total Citations: 427 WoS-CC/504 Scopus [64].
Figure 4 shows the distribution of the number of all publications indexed in WoS-CC
and Scopus database. The years 20052019 were chosen to increase readability. A clear
upward trend has been visible since 2010.
Figure 4. Number of articles in years indexed in WoS-CC and Scopus20052019.
Figure 5 shows the distribution of the number of publications with Open Access li-
cense indexed in WoS-CC and Scopus database. The years 20052019 were chosen to in-
crease readability. A clear upward trend has been visible since 2010.
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
Number of publications in years
WoS-CC
Scopus
Figure 4. Number of articles in years indexed in WoS-CC and Scopus—2005–2019.
Figure 5shows the distribution of the number of publications with Open Access
license indexed in WoS-CC and Scopus database. The years 2005–2019 were chosen to
increase readability. A clear upward trend has been visible since 2010.
Energies 2021,14, 162 6 of 31
Energies 2020, 13, x FOR PEER REVIEW 6 of 38
Figure 5. Number of articles in years indexed on WoS-CC and Scopus with Open Access license
20052019.
Table 1 shows the number of publications indexed in the WoS-CC and Scopus data-
bases. It can be seen that in both cases most publications were published in Asia (China,
South Korea, Republic of China), North America (USA, Canada), Europe (France, Poland).
Table 1. Number of articles in years indexed in WoS-CC and Scopus in countries.
WoS-CC
Number of Articles
Scopus
Number of Articles
China
318
China
221
USA
289
USA
207
Canada
116
South Korea
84
Republic of China
95
Canada
82
France
72
Republic of China
75
South Korea
71
Poland
62
Poland
69
France
57
Sweden
63
Malaysia
55
Japan
62
Brazil
50
Malaysia
62
Italy
50
Figure 6 shows the distribution of publications in the years for the 5 most relevant
categories of Web of Science for the analyzed subject. Selected categories are: Energy Fuels
(1059 doc.), Engineering Chemical (611 doc.), Agricultural Engineering (276 doc.), Biotech-
nology Applied Microbiology (267 doc.) and Thermodynamics (188 doc.). It can be seen
that after 2009 there is an upward trend for all WoS-CC categories. The largest increase
and also the largest total number of publications is in the Energy Fuels category.
0
20
40
60
80
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
Number of publications in years
WoS-CC - OPEN ACCESS
Scopus - OPEN ACCESS
Figure 5. Number of articles in years indexed on WoS-CC and Scopus with Open Access license—2005–2019.
Table 1shows the number of publications indexed in the WoS-CC and Scopus databases.
It can be seen that in both cases most publications were published in Asia (China, South
Korea, Republic of China), North America (USA, Canada), Europe (France, Poland).
Table 1. Number of articles in years indexed in WoS-CC and Scopus in countries.
WoS-CC Number of Articles Scopus Number of Articles
China 318 China 221
USA 289 USA 207
Canada 116 South Korea 84
Republic of China 95 Canada 82
France 72 Republic of China 75
South Korea 71 Poland 62
Poland 69 France 57
Sweden 63 Malaysia 55
Japan 62 Brazil 50
Malaysia 62 Italy 50
Figure 6shows the distribution of publications in the years for the 5 most relevant
categories of Web of Science for the analyzed subject. Selected categories are: Energy
Fuels (1059 doc.), Engineering Chemical (611 doc.), Agricultural Engineering (276 doc.),
Biotechnology Applied Microbiology (267 doc.) and Thermodynamics (188 doc.). It can
be seen that after 2009 there is an upward trend for all WoS-CC categories. The largest
increase and also the largest total number of publications is in the Energy Fuels category.
Energies 2021,14, 162 7 of 31
Energies 2020, 13, x FOR PEER REVIEW 7 of 38
Figure 6. Number of articles in years indexed in WoS-CC in relation to the Web of Science cate-
gory20052019.
Figure 7 shows the distribution of publications in the years for the 5 most relevant
Subject area in Scopus. Selected categories are: Energy (763 doc.), Chemical Engineering
(528 doc.), Environmental Science (500 doc.), Engineering (301 doc.) and Chemistry (236
doc.). It can be seen that after 2010 there is an upward trend for all Subject area. The largest
increase and also the largest total number of publications is in Energy Subject area.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
Number of publications in years
Energy Fuels
Engineering Chemical
Agricultural Engineering
Biotechnology Applied Microbiology
Thermodynamics
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
Number of publications in years
Energy
Chemical Engineering
Environmental Science
Engineering
Chemistry
Figure 6. Number of articles in years indexed in WoS-CC in relation to the Web of Science category—2005–2019.
Figure 7shows the distribution of publications in the years for the 5 most relevant Sub-
ject area in Scopus. Selected categories are: Energy (763 doc.), Chemical Engineering (528
doc.), Environmental Science (500 doc.), Engineering (301 doc.) and Chemistry (236 doc.).
It can be seen that after 2010 there is an upward trend for all Subject area. The largest
increase and also the largest total number of publications is in Energy Subject area.
Energies 2020, 13, x FOR PEER REVIEW 7 of 38
Figure 6. Number of articles in years indexed in WoS-CC in relation to the Web of Science cate-
gory20052019.
Figure 7 shows the distribution of publications in the years for the 5 most relevant
Subject area in Scopus. Selected categories are: Energy (763 doc.), Chemical Engineering
(528 doc.), Environmental Science (500 doc.), Engineering (301 doc.) and Chemistry (236
doc.). It can be seen that after 2010 there is an upward trend for all Subject area. The largest
increase and also the largest total number of publications is in Energy Subject area.
0
10
20
30
40
50
60
70
80
90
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2006
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Number of publications in years
Energy Fuels
Engineering Chemical
Agricultural Engineering
Biotechnology Applied Microbiology
Thermodynamics
0
10
20
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40
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Number of publications in years
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Chemical Engineering
Environmental Science
Engineering
Chemistry
Figure 7. Number of articles in years indexed in Scopus in relation to the Subject area—2005–2019.
Energies 2021,14, 162 8 of 31
The next stage was the keyword analysis for 2015–2019. Such analysis allows visualiz-
ing 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.
Energies 2020, 13, x FOR PEER REVIEW 8 of 38
Figure 7. Number of articles in years indexed in Scopus in relation to the Subject area20052019.
The next stage was the keyword analysis for 20152019. Such analysis allows visual-
izing 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. 20152016 (dark blue/blue/light blue) ‘torrefaction’, ‘bio-
mass’, ‘biochar’, ‘pyrolysis’, ‘combustion’, ‘biochar’, ‘pretreatment’, ‘kinetics’. 2017
(green)—‘hydrothermal carbonization’, ‘wet torrefaction’, ‘fast pyrolysis’, ‘carbonization’,
‘co-combustion’, ‘slow pyrolysis’, ‘bioenergy’, ‘wood pellets’, ‘gasification’. 20182019
(yellow/orange/red)—‘fuel properties’, ‘risk husk’, ‘sewage sludge’, ‘torrefied biomass’,
‘biocoal’, ‘bio-oil’. One can notice the transition from torrefaction, pyrolysis, biomass com-
bustion, 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.
Figure 8. Map of term words for individual years20152019.
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 (20152019), current research topics are new torrefac-
tion technologies, such as HTC, research on improving the physico-mechanical and en-
ergy properties of produced fuel and the use of torrefied biomass in the pyrolysis and co-
combustion process.
Figure 8. Map of term words for individual years—2015–2019.
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 co-
combustion process.
Energies 2021,14, 162 9 of 31
Energies 2020, 13, x FOR PEER REVIEW 9 of 38
Figure 9. Map of term words with reference to citations20152019.
3.2. The Results of QualitativeThematic 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 En-
ergy were analyzed. Groups such as torrefaction, hydrothermal carbonization (HTC), py-
rolysis, 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.
Figure 9. Map of term words with reference to citations—2015–2019.
3.2. 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.
Energies 2021,14, 162 10 of 31
Energies 2020, 13, x FOR PEER REVIEW 10 of 38
Figure 10. Main research topics and process parameterstorrefaction process.
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.
MATERIAL:
TEMPERATURE: 150400 ° C
APPLICATION:
# study of selected physical, mechanical parameters
(true density, grindability, hydrophobicity), selected
chemical parameters, selected energy parameters
# waste from: a) food
processing (apple
pomace, currant pomace,
orange peel, walnut shell,
mushroom spent
compost, Brewer’s spent
gain, grape seed cake,
sunflower seed shells, rice
husk, empty fruit bunch),
b) agricultural
production (corn cobs,
corn stover, wheat straw,
rapeseed straw, palm
kernel shell), c) animal
(elephant dung)
# the impact of raw materials and mixed raw materials
on fuel properties (biomass, sewage sludge, waste
from food processing and agricultural production),
biochar addition on the biogas production kinetics
# the impact of selected process parameters
(temperature, torrefaction time, material fragmentation)
on fuel properties, kinetics of torrefaction process,
optimization of torrefaction process parameters and
comparison of different torrefaction process
# processed wood (beech,
poplar, pine, birch,
spruce, cedar wood, ash,
aspen, sawdust, chips,
bark, stump, forest
residues)
# sewage sludge and
municipal solid waste
# microalgae
Figure 10. Main research topics and process parameters—torrefaction process.
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.
Energies 2021,14, 162 11 of 31
Energies 2020, 13, x FOR PEER REVIEW 11 of 38
Figure 11. Main research topics and process parametersthe hydrothermal carbonization (HTC).
The third group concerned the use of torrefied biomass in the pyrolysis process (Fig-
ure 12, Appendix A: Table A3). There are 19 publications in this group.
MATERIAL:
TEMPERATURE: 120500 ° C
APPLICATION:
# impact of raw material / mixtures of raw materials,
process parameters on improving fuel properties
# examination of selected physical, mechanical,
chemical and energetic properties of HTC product
# impact of HTC on the char reactivity of biomass
# evaluation of hydrochar from lignin hydrous
pyrolysis to produce biocokes after carbonization
# process design, modeling, energy efficiency and cost
analysis hydrothermal carbonization of waste biomass
# waste from: a) food
processing
(rotten apple,
apple pomace, apple juice
pomace, grape pomace,
olive cake), b)
agricultural
production
(corn stover,
off-specification compost)
# processed wood (wood,
beech wood, loblolly pine,
slash pine, spruce, birch)
# sewage sludge
Figure 11. Main research topics and process parameters—the hydrothermal carbonization (HTC).
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.
Energies 2020, 13, x FOR PEER REVIEW 12 of 38
Figure 12. Main research topics and process parameterspyrolysis group.
The fourth group concerned the use of torrefied biomass in the gasification and co-
combustion process (Figure 13, Appendix A: Table A4). There were 25 publications in this
group.
MATERIAL:
TEMPERATURE: 200650 ° C
APPLICATION:
# studies of selected physico-mechanical, chemical
and energy parameters
# waste from: a) food
processing (food waste,
citrus waste),
b) agricultural
production (compost,
feed)
# influence of pretreatment, kinetic study on the
pyrolysis process
# influence of pyrolysis process parameters, raw
material / mixtures of raw materials, comparison of
different pyrolysis technologies, modeling and
optimization of pyrolysis process parameterson fuel
properties
# biomass: a) processed
wood (pine, softwood
bark, softwood lumber
waste, cherry wood, ash,
spruce, mixed waste
wood, red oak, pine,
sawdust, chips, aspen,
lumber, debarked logs,
bark, foliage, douglas-fir,
birch, oak), b) energy
crops
(willow, rye grass,
switchgrass),
c) processed
wood from exotic plants
(rubber tree)
# sewage sludge
Figure 12. Main research topics and process parameters—pyrolysis group.
Energies 2021,14, 162 12 of 31
The fourth group concerned the use of torrefied biomass in the gasification and co-
combustion 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 parametersgasification 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.
MATERIAL:
TEMPERATURE: 3501600 ° C
APPLICATION:
# impact of pretreatment, raw materials / raw material
mixtures / coal, torrefaction on the process
performance on pyrolysis, gasification and
combustion
# waste from: a) food
processing (palm kernel
shell, avocado pit, olive
stone, grape, olive
pomaces, cocoa shell,
sugarcane bagasse, oil
palm, chestnut, shell),
b) agricultural
production
(wheat straw,
olive tree pruning,
Virginia mallow, rice
straw, cassava stalks, corn
cob)
# comparison of different gasification technologies
# influence of gasification process parameters, kinetic
study, modeling and optimization on fuel properties
# design, optimization and energetic efficiency of
producing hydrogen-rich gas from biomass steam
gasification
# characterizes the oxidation properties biomass char
and compare with that of raw biomass char
# biomass: a) processed
wood (spruce bark,
spruce, ash, pine wood,
pine sawdust, birch,
healthy pine, beetle kill
pine), b) energy crops
(miscanthus, willow,
napier grass),
c) processed wood from
exotic plants
(eucalyptus,
bamboo), d) other
biomass (road side grass)
# sewage sludge
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.
Energies 2021,14, 162 13 of 31
Energies 2020, 13, x FOR PEER REVIEW 14 of 38
Figure 14. Main research topicsother topics.
APPLICATION:
# process modeling and forecasting
integrated biomass torrefaction and
pelletization (iBTP)
# carbon efficiency of the biomass to
liquid process
# the analysis integrated systems of
electricity, heat, road transport, aviation
and chemicals in chosen countries
# analysis of domestic and international
bioenergy supply chains for co-firing
plants
# use of biomass in integrated
steelmaking
# coupling of an acoustic emissions
system to a laboratory torrefaction
reactor
# technical assessment of the Biomass
Integrated Gasification / Gas Turbine
Combined Cycle incorporation
# an LCA-based evaluation of biomass
to transportation fuel production and
utilization
# the role of bioenergy and
biochemicals in CO2 mitigation through
the energy system
# a whole-systems analysis of the value
chain associated with cultivation,
harvesting, transport and conversion in
dedicated biomass power stations
# economic impact of combined
torrefaction and pelletization processes
on forestry biomass supply
# the climate contribution of biomass
co-combustion in a coal-fired power
plant
# influence of mill type on densified
biomass comminution
# techno-economic and carbon
emissions analysis of biomass
torrefaction downstream in
international bioenergy supply chains
for co-firing
# an energy analysis comparing
biomass torrefaction in depots to wind
with natural gas combustion for
electricity generation
# an assessment of the torrefaction of
North American pine and life cycle
greenhouse gas emission
# optimal production scheduling for
energy efficiency improvement in
biofuel feedstock preprocessing
# optimization the minimum
production cost for the production of
woody biofuels
# prediction of high-temperature rapid
combustion behaviour of woody
biomass particles
# environmental and energy
performance of the biomass to synthetic
natural gas supply chain
# modeling of biofuel pellets
torrefaction in a realistic geometry
# investigation into the applicability of
Bond Work Index (BWI) and Hardgrove
Grindability Index (HGI) tests for
several biomasses
# Explosion characteristics of pulverised
torrefied and raw biomass
Figure 14. Main research topics—other topics.
Energies 2021,14, 162 14 of 31
4. 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 4and 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 dis-
cussed 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 torrefac-
tion 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
Energies 2021,14, 162 15 of 31
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 op-
timization. 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 au-
thors 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 var-
ious 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 opti-
mization. Process temperatures were in the range: 200650 °C. Materials used in tor-
refaction 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 au-
thors mainly discussed topics such as analysis of the kinetic process, process effi-
ciency (proliferation, gasification, combustion), analysis of the co-combustion pro-
cess 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, en-
ergy and physico-mechanical properties of fuels, process optimization. Process tem-
peratures were in the range: 3501600 °C. Materials used in torrefaction were biomass
of various origins. The main sources of biomass were energy crops, wood with vary-
ing 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.
Figure 15. Number of articles in years in individual thematic groups.
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 bio-
fuels supply chain, analysis of integrated energy generation systems (biomass gasifica-
tion, 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 documentsWoS-CC, 57 documentsScopus). These articles are published in
leading journals, and their usefulness is demonstrated by a large number of citations. Ex-
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Number of publications in years
Years
Torrefaction
HTC
Pyrolysis
Gasification and co-
combustion
Other
Figure 15. Number of articles in years in individual thematic groups.
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.
Energies 2021,14, 162 16 of 31
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 pro-
cess, 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 6and 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.
5. 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 Microbiol-
ogy 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 the-
matic 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.
Author Contributions:
Conceptualization, A.K. and S.F.; Methodology, A.K. and S.F.; Formal analy-
sis, A.K. and S.F. and M.J.; Investigation, A.K. and S.F.; Resources, A.K. and S.F.; Data curation, A.K.;
Writing—original draft preparation, A.K., S.F., A.Z. and R.F.; Writing—review and editing, A.K., S.F.,
M.J., A.Z. and R.F.; Visualization, A.K.; Supervision, S.F., M.J., A.Z. and R.F. All authors have read
and agreed to the published version of the manuscript.
Energies 2021,14, 162 17 of 31
Funding:
Article processing charges were financed from the subsidy of the Ministry of Science and
Higher Education for the Agricultural University of Hugo Kołłataj in Krakow for the year 2020.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Table A1. Torrefaction process—detailed thematic analysis.
Ref. Year Application Process Temp. (C) Material
[70] 2019
- influence of the temperature of the
torrefaction on the hydrophobic properties of
waste biomass
200, 220, 240, 260, 280,
300
apple pomace, currant pomace,
orange peel, walnut shell,
pumpkin seeds
[71] 2019
- fuel characteristics of biochars from
torrefaction (a.k.a., roasting or
low-temperature pyrolysis) of elephant dung
(manure)
200, 220, 240, 260, 280,
300 elephant dung (manure)
[72] 2019
- fuel characteristics of biochars from torrefied
wood sawdust in normal and vacuum
environments
200, 220, 240, 260, 280,
300 wood sawdust
[73] 2019 - production of wood pellets mixed with
torrefied rice straw 220, 280 wood pellets mixed with torrefied
rice straw
[74] 2019
- production of hybrid sewage sludge fuel for
the effective management of sewage sludge 250 sewage sludge
[75] 2019
- concept of spent mushrooms compost
torrefaction-studying the process kinetics and
the influence of temperature and duration on
the calorific value
200–300 mushroom spent compost
[76] 2019
- physical and chemical properties, true
density, grindability and hydrophobicity of
Thar coal along with raw and torrefied corn
cob were investigated
200, 225, 250, 275, 300 corn cobs
[77] 2019
- effects of automatic temperature control in
torrefaction and the use of additives in
pelletization
250–320 wood chips from Japanese cedar
[78] 2019
- the effect of biochar addition on the biogas
production kinetics from the anaerobic
digestion of brewers’ spent grain
200–300 Brewer’s spent grain
[79] 2019
- kinetics of torrefaction and determine the
effects of process temperature on fuel
properties of torrefied products (biochars)
200–300 Sewage sludge
[80] 2019 - a fundamental research on synchronized
torrefaction and pelleting of biomass 200 Corn stover, big bluestem
[81] 2019
- the concept of carbonized refuse-derived
fuel (CRDF) by refuse-derived fuel (RDF)
torrefaction
200–300 RDF
[82] 2018 - impact fo biomass diversity on torrefaction
process 200–300
ash-wood, beech, poplar, willow,
pine, pine forest residues, scot
pine bark, miscanthus, reed canary
grass, corn cob, grape seed cake,
sunflower seed shells, wheat straw
(French), wheat straw (Swedish)
[83] 2018 - ultrasonic pelleting of torrefied biomass for
bioenergy production 200–300 wheat straw
[84] 2018 - energetic properties of torrefied and raw
wheat straw, rapeseed, and willow 220, 260, 300 willow, rapeseed straw, wheat
straw
[85] 2018
- densification of torrefied refuse-derived fuel
260 municipal solid waste
[86] 2018
- correlations to predict elemental
compositions and heating value of torrefied
biomass
200–300
birch, spruce, willlow, beech wood,
lauan, wood mixture, black locust,
pine, eucalyptus, poplar,
leaucaena, sawdust, cedar wood,
ash, aspen
Energies 2021,14, 162 18 of 31
Table A1. Cont.
Ref. Year Application Process Temp. (C) Material
[87] 2018
- torrefaction of manually pressed and liquid
nitrogen treated of microalgae for bioenergy
utilisation
200, 300 microalgae
[88] 2018 - production upgraded wood fuel by
torrefaction 200–300 raw Japanese cedar chips
[89] 2018
- properties of product biomass torrefaction
based on three major components:
hemicellulose, cellulose, lignin
210, 240, 270, 300
microcrystalline cellulose,
beechwood xylan (representative
of hemicellulose), alkali lignin
[90] 2018 - properties of torrefied waste blends 300 paper fiber, plastic waste
[91] 2018
- investigate the optimal temperature range
for waste Wood and the effect torrefaction
residence time had on torrefied biomass
feedstock
200–400 wood waste
[92] 2018 - analyses of torrefied biomass of tropical
plantation species 200, 225, 250
cupressus lusitanica, dipteryx
panamensis, gmelina arborea,
tectona grandis and vochysia
ferruginea
[93] 2018 - concept an installation for sustainable
thermal utilization of sewage sludge 300 sewage sludge
[94] 2018
- the impact of residence time, temperature,
and particle size on torrefied rice husk, using
a bench-scale batch reactor
240–295 rice husk
[95] 2018
- the effect of different parameters were
investigated on two abundant sources of
biomass in South Africa
200–300 marula seeds, blue gum wood
[96] 2018 - Solid fuel characterization of torrefied
coconut shells in an oxidative environment 250–300 Local coconut Shell chips
[97] 2018 - effects of torrefaction on fuel properties of
solid and condensate products 200–300 Cogon grass
[98] 2017
- comparative study on the thermal behavior
of raw and torrefied bark, stem wood, stump
of Norway spruce.
225, 275, 300 Norway spruce (stem wood, bark,
stump)
[99] 2017
- physical and compression properties of
pellets manufactured with the torrefied
biomass of woody tropical species
200, 225, 250
cupressus lusitanica, dipterix
panamensis, gmelina arborea,
tectona grandis, vochysia
ferruginea
[100] 2017 - Preliminary production test of torrefied
woody biomass fuel in a small scale plant. 215 Japanese cedar
[101] 2017 - production of torrefied solid biofuel from
pulp industry waste 260, 280, 300, 320 wood waste with pulp sludge
[102] 2017
- fuel properties of torrefied sorghum biomass
250, 275, 300 sorghum, sweet sorghum bagasse
[103] 2017 - energy densification of animal waste, corn
cob and pine wood 200, 250, 300 Cow dung, corn cob, pine wood
[104] 2016 - production of solid fuel from torrefied
coconut leaves 245–295 coconut leaves
[105] 2016
- comparing grindability of different torrefied
biomass pellets in different laboratory mills 260, 308 forest residues, willow, pine,
poplar, spruce, beech, straw
[106] 2016
- compositional study of torrefied wood and
herbaceous materials by chemical analysis
and thermoanalytical methods
200, 225, 250, 270, 300
black locust wood, wheat and rape
straw
[107] 2016 - thermal desorption of wood railroad ties 250, 275, 300, 325, 350 creosote-treated wood
[108] 2016
- detailed mapping of the mass and energy
balance of a continuous biomass torrefaction
plant
250–265 spruce, ash, willow
[109] 2016
- thermochemical and structural changes in
Jatropha curcas seed cake during torrefaction
for its use as coal co-firing feedstock
200–300 jatropha curcas
[110] 2016
- biochemical conversion of torrefied norway
spruce after pretreatment with acid or ionic
liquid
260–310 Norway spruce
Energies 2021,14, 162 19 of 31
Table A1. Cont.
Ref. Year Application Process Temp. (C) Material
[111] 2015
- identification and quantification of the
condensable species released during
torrefaction of lignocellulosic biomass
250, 280, 300 pine, ash wood, miscanthus,
wheat straw
[112] 2015 - evaluation of solvent for pressurized liquid
extraction in torrefied woody biomass 270, 300 eucalyptus wood chips
[113] 2015
- study on dry torrefaction of beech wood and
miscanthus 240, 260, 280, 300
beech wood, miscnathus (sinensis)
[114] 2015 - composition, utilization and economic
assessment of torrefaction condensates 200–300 spruce, bamboo
[115] 2015 - analysis on storage off-gas emissions from
woody, herbaceous and torrefied biomass 250 Switchgrass (Panicum virgatum)
[116] 2015 - qualitative and kinetic analysis of
torrefaction of lignocellulosic biomass 200, 275, 300 miscanthus, wheat straw
[117] 2015
- comparison of chemical composition and
energy Property of Torrefied switchgrass and
corn stover
180–270 switchgrass, corn stover
[118] 2014 - the effects of torrefaction on the basic
characteristics of corn stalks 150–400 corn stalks
[119] 2014 - decomposition kinetics of torrefaction of
some nigerial lignocellulosic biomass 240, 270, 300
albizia pedicellaris, tectona
grandis, terminalia ivorensis,
sorghum bicolour glume, sorghum
bicolour stalk
[120] 2014 - process simulation of co-firing torrefied
biomas in a 220 Mwe coal-fired power plant 200, 250, 270, 300 palm kernel shell
[121] 2014 - process evaluation for torrefaction of empty
fruit bunch from palm oil mill 300 empty fruit bunch (EFB) from
Malaysian palm oil mill
[122] 2014
- investigates the product yields and the solid
product characteristics from corncob waste
torrefaction
250, 300 corncob waste
[123] 2013 - analysis of efficiency simultaneous
torrefaction and grinding of biomass. 240–330 Danish wheat straw, Danish
spruce chips, Spanish pine chips
[124] 2013
- the influence of the chemical properties
(lignocellulose composition and alkali
content) on the torrefaction behavior with
respect to mass loss and grindability
270, 300
Danish wheat straw, miscanthus,
spruce wood chips, beech wood
chips, pine wood chips, spruce
bark
[125] 2013 - thermal decomposition kinetics of woods 200, 225, 250, 275, 300 Norwegian spruce, birch wood
[126] 2013
- comparison of energy properties torrefaction
by microwave and conventional slow
pyrolysis
200, 230, 250, 300, 350 willow
[127] 2013 - kinetic behavior of torrefied biomass in
oxidative environment 225, 275 birch, spruce
[128] 2012
- chemical compositional changes during
torrefaction miscanthus and white oak
sawdust
220–350 miscanthus, white oak
[129] 2012
- impact biomass torrefaction under different
oxygen concetration on composition of the
solid by-product
240. 280 eucalyptus grandis
[130] 2012
- the effects of particle size, different corn
stover components, and gas residence time on
torrefaction of corn stover
250, 280 corn stover (Zea mays)
[131] 2012
- effect of torrefaction on water vapor
adsorption properties and resistance to
microbial degradation of corn stover
200, 250, 300 corn stover
Energies 2021,14, 162 20 of 31
Table A2. Hydrothermal carbonization (HTC)—detailed thematic analysis.
Ref. Year Application Process Temp. (C) Material
[132] 2019 - improvement of corn stover fuel properties
via hydrothermal carbonization 120–280 corn stover
[39] 2018 - hydrothermal carbonization of peat moss
and herbaceous biomass (miscanthus) 240 peat moss; miscanthus
[133] 2018 - hydrothermal carbonization of biosolids
from waste water treatment plant 180, 200, 220 sewage sludge
[134] 2018 - hydrothermal carbonization of fruit wastes 190, 225, 260 rotten apple, apple chip pomace,
apple juice pomace, grape pomace
[135] 2018
- the impact of hydrothermal carbonisation on
the char reactivity of biomass 200, 225 wood, olive cake
[136] 2018 - impact feedstock, reaction conditions and
post-treatment on properties of hydrochar 180, 220, 250 wheat straw, beech wood
[137] 2017
- evaluation of hydrochars from lignin
hydrous pyrolysis to produce biocokes after
carbonization
250, 300, 330–500 the pine kraft lignin
[138] 2017
- hydrothermal carbonization of loblolly pine
using a continuous, reactive twin-screw
extruder
200, 215, 235, 255, 260,
275, 290, 295 loblolly pine, slash pine
[139] 2017
- process design, modeling, energy efficiency
and cost analysis hydrothermal carbonization
of waste biomass
180, 220, 250 off-specification compost, grape
marc
[140] 2014
- effects of wet torrefaction on reactivity and
kinetics of wood under air combustion
conditions
175, 200, 225 Norway spruce, birch
Table A3. The use of torrefied biomass in the pyrolysis process—detailed thematic analysis.
Ref. Year Application Process Temp. (C) Material
[141] 2019
- effects of pyrolysis temperature and retention
time on fuel characteristics of food waste feedstuff
and compost for co-firing in coal power plants
300–500 food waste, compost, feed
[142] 2019 - combined organic acid leaching and torrefaction
as pine wood pretreatment before fast pyrolysis 530 pine wood
[143] 2018 - expedient prediction of the fuel properties of
carbonized woody biomass based on hue angle 300–410 rubber tree, softwood bark,
softwood lumber waste
[144] 2018 - energy and exergy analyses of sewage sludge
thermochemical treatment 250, 275, 480, 530 sewage sludge
[145] 2018
- impact of thermal pretreatment temperatures on
woody biomass chemical composition, physical
properties and microstructure
220, 260, 300, 350, 450,
550 cherry wood
[146] 2017 - compared fast pyrolysis experiments of raw and
torreried woody biomass feedstocks. 250–300, 500
ash, spruce, mixed waste wood
[147] 2017 - staged thermal fractionation for segregation of
lignin and cellulose pyrolysis products
250. 275, 300–400,
500–600 red oak
[148] 2017
- thermal decomposition kinetics of wood and bark
and their torrefied products 225–450 Norway spruce
[149] 2017 - combined heat and power from the intermediate
pyrolysis of biomass materials 450–550 wood feedstock—pine
sawdust or ground pine chips
[150] 2017 - evolution of chars during slow pyrolysis of citrus
waste 200–650 citrus waste
[151] 2016 - the effect of torrefaction temperature and time on
pyrolysis of centimeter-scale pine wood particles 225, 250, 275, 300, 520 pine wood cuboid
[152] 2016
- mild hydrothermmal conditioning prior to
torrefaction and slow pyrolysis of low-value
biomass
300, 600 willow, rye grass
[153] 2016 - thermal desorption of creosote remaining in used
railroad ties 200, 250, 280, 300, 450 red oak, quercus rubra
Energies 2021,14, 162 21 of 31
Table A3. Cont.
Ref. Year Application Process Temp. (C) Material
[154] 2016
- Effect of torrefaction temperature on lignin
macromolecule and product distribution from
catalytic pyrolysis
500
The southern pine, switchgrass
[155] 2015 - unified kinetic model for torrefaction-pyrolysis
260, 280, 300, 315, 330,
375, 400, 425 aspen wood
[156] 2015
- production and characterization of bio-oil and
biochar from the pyrolysis of fesidual bacterial
biomass from a polyhydroxyalkanoate production
process
550 residual bacterial biomass
[157] 2014 - characterization of biochar from switchgrass
carbonization 300, 350, 400 switchgrass
[158] 2013
- a comparison of producer gas, biochar, and
activated carbon from two distributed scale
thermochemical conversion systems used to
process forest biomass
350–750
lumber, debarked logs, bark,
foliage, douglas-fir, lodgepole
pine
[159] 2009
- kinetic study on thermal decomposition of woods
in oxidative environment 220–590 aspens, birch, oak, pine
Table A4. The use of torrefied biomass in the gasification and co-combustion process—detailed thematic analysis.
Ref. Year Application Process Temp. (C) Material
[160] 2019 - combustion improvements of upgraded biomass
by washing and torrefaction 1400–1600 road side grass, miscanthus,
wheat straw, spruce bark
[161] 2019
- thermal analysis of olive tree pruning and the
by-products obtained by its gasification and
pyrolysis
550, 900 olive tree pruning
[162] 2019
- theoretical and experimental analysis on
co-gasification of sewage sludge with energetic
crops
950 sewage sludge, virginia
mallow
[163] 2019
- torrefaction as a valorization method used prior to
the gasification of sewage sludge 350–900 sewage sludge
[164] 2019 - Influence of microwave pre-treated Palm Kernel
Shell and Mukah Balingian coal on co-gasification 50.2–470.4 palm kernel shell
[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
Energies 2021,14, 162 22 of 31
Table A4. Cont.
Ref. Year Application Process Temp. (C) Material
[176] 2016 - torrefied pine as a gasification fuel using a
bubbling fluidized bed gasifier 790, 935, 1000 pine
[177] 2015
- the commbustion characteristics of
high-heating-rate chars from untreated and
torrefied biomass fuels
1100 willow, eucalyptus
[178] 2015 - characterizes the oxidation properties biomass
char and compare with that of raw biomass char 900, 1200 palm kernel shell
[179] 2015
- design, optimization and energetic efficiency of
producing hydrogen-rich gas from biomass steam
gasification
700 oil palm
[180] 2014 - gasification of torrefied wood: a kinetic study 750 birch, spruce
[181] 2014 - lab-scale co-firing of virgin and torrefied bamboo
as a fuel substitute in coal fired power plants 1400–1600 bamboo species guadua
angustifolia kunth, willow
[182] 2014
- gasification of torrefied Miscanthus x giganteus in
an air-blown bubbling fluidized bed gasifier 600, 700, 750, 800, 850 miscanthus x giganteus
[183] 2014 - high-temperature rapid devolatilization of
biomasses with varying degrees of torrefaction 500, 700, 900, 1200 palm kernel shell
[184] 2013
- flame characteristics of pulverized
torrefied-biomass combusted with
high-temperature air
1150 palm kernel shells
Table A5. Other issues regarding the use of torrefied biomass—detailed thematic analysis.
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
Energies 2021,14, 162 23 of 31
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
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... Depending on the temperature used and the operating conditions, thermal processes can be divided into drying, torrefaction, pyrolysis, and gasification. Thermal treatment of biomass can improve energetic, physico-mechanical, and chemical properties [7,8]. ...
... Torrefaction is one of the methods of thermolysis, which is defined as the chemical decomposition of substances caused by elevated temperature [6,9,10]. The torrefaction process involves the slow roasting of biomass at a temperature of 220-350 • C without air access, at a pressure close to atmospheric [6,8,11,12]. If the preset temperature range of the process is maintained, there is a greater loss of oxygen and hydrogen as compared to the loss of carbon. ...
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This paper presents the CFD modeling results of the torrefied maize straw co-firing with sub-bituminous coal in various mass ratios in the industrial scale boiler, to recognize possible application issues of the coal substitution with upgraded biomass. The steam torrefaction of biomass took place in a pilot in a counter-flow torrefaction reactor fed with superheated steam from the OP-230 (Rafako, Poland) boiler. Using a TGA, it was possible to analyze the combustion indexes and synergy effects after burning the torrefied biomass-coal mixtures. Additionally, a kinetic model of pyrolysis devolatilization was established and used in the modeling along with Ansys Fluent kinetics of the coal and gas-phase combustion models. Due to the numerical modeling, it was possible to determine the temperature distribution in the boiler's furnace chamber, the heat flux densities, the simulated distribution of carbon monoxides and carbon dioxide concentration, and the decomposition of nitrogen oxides resulting from co-combustion. Steam torrefied biomass indicates higher combustion activity compared to coal, ignites easier, and burns more intensely with better combustion stability. A synergistic effect between the coal-torrefied blend was observed. According to numerical analysis, it was found that with the increase of the share of torrefaction in the fuel mixture, the share of unburned fuel in fly ash increases. Additionally, an increased share of the torrefied biomass in the fuel blend from 30% to 40% results in a slight increase in the molar NO concentration in the furnace chamber. The authors strongly recommend the continuation of work on further investigation of the co-firing of the coal with torrefied biomass in the pre-mixed blends injected through all burners.
... Microwaves, combustion and incineration sum up an approximate share of 4%. Torrefaction has been extensively studied, being reported as a growing trend in the thermal treatment for biomass, new torrefaction technologies and aims being recently described [11,173]. Acharya et al. [174] review publications on biomass dry and wet torrefaction technologies, dry torrefaction outnumbering the wet version. ...
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This review aims to compile and discuss the pre-processing procedures utilized previously to biomass thermal conversion techniques, in order to promote the sustainable production of biofuels and other commodities from biomass. These will ultimately replace fossil-based alternatives under an environment-friendly framework, and contribute to the attainment of circular economy goals. Pre-processing methods account for a significantly improved efficiency, as a more homogeneous, dry, suitable and consistent feedstock is achieved, supporting cleaner and proficient conversion methods. This may prevent the complete depletion of non-renewable resources , alleviating the effects of their overexploitation. Findings concerning the main constrains when using biomass as a feedstock for thermal conversion reveal that size, format, moisture content and heterogeneity are the main encountered issues. Whilst mechanical processes, drying, torrefaction and pelletisation have shown enhanced results for physical aspects as well as moisture; hydrolysis, hydrothermal and microwave-based techniques are seen as the most utilized to solve problems related to morphology, degradability and di-gestibility. It was also found that the most significant thermal conversion techniques are torrefaction, hydro-thermal processing, gasification, combustion, pyrolysis and plasma gasification. Relative to the major types of biomass applied for the production of biofuels, these are wood and woody biomass, followed by herbaceous and agricultural streams.
... D'autres proposent un prisme plus conceptuel (Mougenot et Doussoulin, 2022;Yaremova et al., 2021;Tassinari et al., 2021) qui leur permet d'évaluer les facteurs clés du développement dans ce domaine. D'autres encore s'intéressent aux filières connexes comme les bioraffineries (Sganzerla et al., 2021), la forêt (Paletto et al., 2020;Holmgren et al., 2020;Jankovský et al., 2021), les bioénergies (Dal Poz et al., 2017;Mao et al., 2018) ou la biomasse (Knapczyk et al., 2021;Elgarahy et al., 2021). L'ensemble de ces études montrent que la bioéconomie est un domaine très hétérogène et inclusif tandis que d'autres sont plus ciblées, avec une approche méthodologique plus restrictive dans la délimitation du domaine. ...