A Comprehensive Review of Secondary Carbon Bio-Carriers for Application in Metallurgical Processes: Utilization of Torrefied Biomass in Steel Production

Abstract

This review aims to show the significance of the use of secondary carbon bio-carriers for iron and steel production. The term ‘secondary carbon bio-carriers’ in this review paper refers to biomass, torrefied biomass, biochar, charcoal, or biocoke. The main focus is on torrefied biomass, which can act as a carbon source for partial or complete replacement of fossil fuel in various metallurgical processes. The material requirements for the use of secondary carbon bio-carriers in different metallurgical processes are systematized, and pathways for the use of secondary carbon bio-carriers in four main routes of steel production are described; namely, blast furnace/basic oxygen furnace (BF/BOF), melting of scrap in electric arc furnace (scrap/EAF), direct reduced iron/electric arc furnace (DRI/EAF), and smelting reduction/basic oxygen furnace (SR/BOF). In addition, there is also a focus on the use of secondary carbon bio-carriers in a submerged arc furnace (SAF) for ferroalloy production. The issue of using secondary carbon bio-carriers is specific and individual, depending on the chosen process. However, the most promising ways to use secondary carbon bio-carriers are determined in scrap/EAF, DRI/EAF, SR/BOF, and SAF. Finally, the main priority of future research is the establishment of optimal parameters, material quantities, and qualities for using secondary carbon bio-carriers in metallurgical processes.

Full text

Citation: Kieush, L.; Rieger, J.;
Schenk, J.; Brondi, C.; Rovelli, D.;
Echterhof, T.; Cirilli, F.; Thaler, C.;
Jaeger, N.; Snaet, D.; et al. A
Comprehensive Review of Secondary
Carbon Bio-Carriers for Application
in Metallurgical Processes:
Utilization of Torrefied Biomass in
Steel Production. Metals 2022,12,
2005. https://doi.org/10.3390/
met12122005
Academic Editor: Houshang
Alamdari
Received: 20 October 2022
Accepted: 22 November 2022
Published: 23 November 2022
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4.0/).
metals
Review
A Comprehensive Review of Secondary Carbon Bio-Carriers for
Application in Metallurgical Processes: Utilization of Torrefied
Biomass in Steel Production
Lina Kieush 1, * , Johannes Rieger 2, Johannes Schenk 1,2 , Carlo Brondi 3, Davide Rovelli 3,
Thomas Echterhof 4, Filippo Cirilli 5, Christoph Thaler 6, Nils Jaeger 7, Delphine Snaet 8, Klaus Peters 8
and Valentina Colla 9
1Montanuniversitaet Leoben, Chair of Ferrous Metallurgy, 8700 Leoben, Austria
2K1-MET GmbH, 4020 Linz, Austria
3
Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, National Research
Council of Italy, 20133 Milan, Italy
4Department for Industrial Furnaces and Heat Engineering, RWTH Aachen University,
52074 Aachen, Germany
5Rina Consulting-Centro Sviluppo Materiali S.p.A., 00128 Rome, Italy
6Voestalpine Stahl, 4020 Linz, Austria
7Thyssenkrupp Steel Europe, 47166 Duisburg, Germany
8European Steel Technology Platform ASBL, 1000 Brussels, Belgium
9
Scuola Superiore Sant’Anna Pisa—Center ICT for Complex Industrial Systems and Processes, 56127 Pisa, Italy
*Correspondence: lina.kieush@stud.unileoben.ac.at
Abstract:
This review aims to show the significance of the use of secondary carbon bio-carriers for iron
and steel production. The term ‘secondary carbon bio-carriers’ in this review paper refers to biomass,
torrefied biomass, biochar, charcoal, or biocoke. The main focus is on torrefied biomass, which can
act as a carbon source for partial or complete replacement of fossil fuel in various metallurgical
processes. The material requirements for the use of secondary carbon bio-carriers in different
metallurgical processes are systematized, and pathways for the use of secondary carbon bio-carriers
in four main routes of steel production are described; namely, blast furnace/basic oxygen furnace
(BF/BOF), melting of scrap in electric arc furnace (scrap/EAF), direct reduced iron/electric arc
furnace (DRI/EAF), and smelting reduction/basic oxygen furnace (SR/BOF). In addition, there
is also a focus on the use of secondary carbon bio-carriers in a submerged arc furnace (SAF) for
ferroalloy production. The issue of using secondary carbon bio-carriers is specific and individual,
depending on the chosen process. However, the most promising ways to use secondary carbon
bio-carriers are determined in scrap/EAF, DRI/EAF, SR/BOF, and SAF. Finally, the main priority
of future research is the establishment of optimal parameters, material quantities, and qualities for
using secondary carbon bio-carriers in metallurgical processes.
Keywords:
secondary carbon bio-carriers; biomass; torrefaction; biocoke; iron and steel industry; ferroalloys
1. Introduction
Biomass is considered a valuable renewable energy source [
1
]. Given that the iron
and steel industry is one of the most energy- and emission-intensive industrial sectors,
the use of substitutes for conventional fossil fuels is an extremely attractive option. The
iron and steel industry is responsible for approximately 1.83 tons of CO
2
per ton of crude
steel (for BF/BOF route), with major amounts from the blast furnace (BFs), cokemaking
ovens, sintering, pelletizing, iron alloy furnaces, and other processes [
2
–
4
]. To reduce the
negative impact on the environment [
5
–
7
], to reduce CO
2
emissions by 50% until 2030,
and reach climate neutrality (zero net CO
2
emissions by 2050) [
8
], as well as to reduce the
use of critical materials, such as coal [
9
–
11
], a considerable amount of research has been
Metals 2022,12, 2005. https://doi.org/10.3390/met12122005 https://www.mdpi.com/journal/metals

Metals 2022,12, 2005 2 of 38
carried out on the use of carbon-bearing substitute materials. For instance, biomass has
promising potential as a fuel and reductant, but its application in the metallurgical industry
has limitations. Numerous studies have concluded that biomass utilization has good
potential for the partial replacement of fossil fuels and reducing agents in metallurgical
coke production [
12
–
20
], as carbonaceous fuel in iron ore sintering [
21
–
24
], in iron ore pellet
production [
25
], pulverized injection into the BF [
26
–
28
], in EAF [
29
,
30
], in the reduction of
iron [31–33], and in ferroalloy production in a submerged arc furnace (SAF) [34–36].
It is worth noting that raw biomass can have several disadvantages [
37
], such as
high moisture content, low calorific value, hygroscopic nature, low bulk density, and high
content of oxygenated volatile matters (VM). According to [
38
], the FC content of raw
biomass typically ranges from 9–25 wt.%, while VM can range from 63–88 wt.%.
Therefore, in most cases, pre-treatment is inevitable in order to obtain bio-substitutes
with properties allowing a partial or complete replacement of fossil fuels in metallurgical
processes. Improving the properties of raw biomass can be achieved via thermal treatment
(e.g., torrefaction [
38
], hydrothermal processing [
39
], gasification [
40
], combustion [
41
],
pyrolysis [
42
], etc.). The advantages of thermal biomass treatment include reduced moisture
and oxygen content, while increasing the carbon fraction [
43
,
44
] and the calorific value of
the solid product [
45
], making it more suitable for use in various metallurgical processes.
However, after heat treatment, there is an increase in ash content and a decrease in the
mechanical strength of the biochar [
46
–
48
]. Many studies have focused on using biochars
for metallurgical purposes after pyrolysis from approximately 350
◦
C to 1100
◦
C [
25
,
49
,
50
].
The disadvantage of pyrolysis at high temperatures is a reduction of the solid product,
which is the most valuable for its further use in metallurgical processes. In this regard, the
most attractive way to improve the properties of biomass is torrefaction at 200–300
◦
C [
51
],
which is also referred to as a mild pyrolysis process. The biomass torrefaction process
can be carried out at temperatures of 200–235
◦
C (light torrefaction), 235–275
◦
C (medium
torrefaction), or 275–300
◦
C (severe torrefaction) [
37
] under an inert atmosphere for minutes
to hours. Compared to pyrolysis, it is a less violent heat treatment process that improves
the properties of biomass without affecting the high molecular compounds.
Additionally, torrefied biomass has less ash than biochar after pyrolysis. Moreover, the
biomass weight can be reduced by 30–70%, and torrefied biomass can store 90% of its energy
after the torrefaction [
52
]. Furthermore, torrefaction can reduce CO
2
emissions by 50 kg/t
steel [
53
]. It is worth noting that the torrefaction process changes the distribution of biomass
components. Decomposition of cellulose and hemicelluloses occurs at temperatures from
200–400
◦
C [
54
], while lignin decomposes slowly at higher temperatures of 200–900
◦
C.
Torrefaction reduces the hemicellulose content from 22% to 4.6% at 300
◦
C, while the
cellulose slowly decomposes and the lignin content increases [
55
]. During torrefaction,
the biomass dries up, slowly decomposes, and simultaneously releases about 20% of CO
and about 80% of CO
2
[
56
], H
2
O, and a small amount of volatile organic compounds. As
a result, the carbon content of the torrefied biomass increases, while the hydrogen and,
especially, the oxygen content decreases. Torrefied biomass has the following properties:
-
Fixed carbon (FC) varies from 10–50 wt.% [
57
], the VM varies from 34–85 wt.%, and
ash varies from 9.20 wt.% to 15.04 wt.% [58];
- Higher calorific value from 16–29 MJ kg−1or energy density [37];
- Lower O/C and H/C atomic ratios [59,60].
After torrefaction, the content of FC can reach 50 wt.%. The greater the severity of the
torrefaction process, the greater the value of FC will be obtained. The effect of torrefaction
can greatly vary, depending on the type of biomass subjected to heat treatment. Torrefaction
is the only heat treatment method in which the yield of solid residue is a maximum and
is in the range of 75–90% [
61
] at a heating rate of 1–10
◦
C/min and a residence time of
10–60 min [62].
Moreover, torrgas can be used for drying the biomass by reusing the waste heat,
according to [63], but before reusing, it should be normally de-dusted using a cyclone.

Metals 2022,12, 2005 3 of 38
Torrefied biomass is a good option for partial or complete replacement in processes
that require coal [
64
]. However, the properties of torrefied biomass are limiting in some
metallurgical processes that use only coke as a source for carburizing or as a fuel and
reducing agent [
65
]. They should be improved by compaction [
65
,
66
] or by adding them to
the coal blend to obtain biocoke.
As for the industrial use of torrefied biomass in practice, there is already an active
project, TORERO (Horizon 2020 Project, 2017–2024, Grant Agreement No. 745810), coordi-
nated by Arcelor Mittal Belgium NV [
67
]. This project considers using torrefied biomass
for injection into the BF to replace fossil coal. In addition, the POLTORR system has been
developed by ThyssenKrupp for drying and torrefying biomass to replace fossil fuels [
68
].
Many researchers have published several reviews on the application of biomass in
selected iron- and steelmaking processes. Additionally, all the papers have focused on
using biomass, either in its original state or after pyrolysis. This review paper aims to
suggest and discuss the pathways for the integrated use of secondary carbon bio-carriers in
four steelmaking methods, as well as ferroalloy production. The paper is mainly focused on
the use of torrefied biomass. However, it is worth noting that this paper is not focused on a
specific type of biomass, but biomass is instead considered in general as it is a promising
carbon source. The systematization of requirements for fuel and reducing agents for
different metallurgical consumers is the focus of this review.
ESTEP and Clean Steel Partnership to Decarbonize the EU Steel Sector
The steel industry is an important engine for sustainable growth, adding value and
high-quality employment within the European Union. As mentioned above, it is com-
mitted to reducing CO
2
emissions by developing and upscaling technologies. The Clean
Steel Partnership (CSP, a public-private partnership fostering the decarbonization of the
European steel sector), which is led by the European Steel Association (EUROFER) and
the European Steel Technology Platform (ESTEP), is focused on research and development
to accelerate the implementation of technological CO
2
mitigation pathways comprising
Carbon Direct Avoidance (CDA), Smart Carbon Usage (SCU), and a circular economy (CE).
The targets of CSP are integrated into a balanced set of Key Performance Indicators (KPI),
which contribute to reaching a technological readiness level (TRL) 8 (system or process
complete and qualified) to reduce CO
2
emissions stemming from EU steel production by
80–95% compared to 1990s levels, ultimately leading to climate neutrality [69].
ESTEP, the main coordinator of CSP, is a non-profit organization according to Belgian
law (international ASBL) [
70
]. The mission of ESTEP is to engage in collaborative EU
actions and projects using technologies that tackle EU challenges (notably on renewable
energy, climate change, i.e., low-carbon emissions, and CE) to create a sustainable EU steel
industry. Figure 1shows the organizational structure of ESTEP [70].
A Board of Directors governs ESTEP. The steering group pilots the overall ESTEP
research program. Additionally, the implementation group deals with all issues regarding
the CSP. It reviews the activities of the seven focus groups (FG). Two of these FGs, which
are low carbon and energy efficiency and circular economy, deal with steel production.
The former works on developing safe, clean, energy-efficient, and innovative technologies,
while the latter deals with innovative solutions to increase the circularity of steel. The
major concerns here are reducing CO
2
emissions, conserving resources, and boosting waste
recovery. Three FGs cover steel applications: steel solutions for transport and mobility,
steel solutions for construction and infrastructure, and steel solutions for energy markets,
including engineering. The FG People deals with activities for attracting people to the steel
industry, skills development, education and training programs, and occupational safety.
The FG Smart Factory covers issues for intelligent and integrated manufacturing, applying
developments in the field of information and communication techniques [
70
]. The CSP and
its main strategic foci are explained in further detail in the next subsection.

Metals 2022,12, 2005 4 of 38
Metals 2022, 12, x FOR PEER REVIEW 4 of 41
Figure 1. Organizational structure of ESTEP [70].
A Board of Directors governs ESTEP. The steering group pilots the overall ESTEP
research program. Additionally, the implementation group deals with all issues regarding
the CSP. It reviews the activities of the seven focus groups (FG). Two of these FGs, which
are low carbon and energy efficiency and circular economy, deal with steel production.
The former works on developing safe, clean, energy-efficient, and innovative
technologies, while the latter deals with innovative solutions to increase the circularity of
steel. The major concerns here are reducing CO2 emissions, conserving resources, and
boosting waste recovery. Three FGs cover steel applications: steel solutions for transport
and mobility, steel solutions for construction and infrastructure, and steel solutions for
energy markets, including engineering. The FG People deals with activities for attracting
people to the steel industry, skills development, education and training programs, and
occupational safety. The FG Smart Factory covers issues for intelligent and integrated
manufacturing, applying developments in the field of information and communication
techniques [70]. The CSP and its main strategic foci are explained in further detail in the
next subsection.
All R&D&I activities supporting the achievement of the CSP’s objectives are
classified in the Strategic Research and Innovation Agenda (SRIA, also called roadmap)
according to six areas of intervention (AoIs) (see Figure 2 [69]).
Figure 1. Organizational structure of ESTEP [70].
All R&D&I activities supporting the achievement of the CSP’s objectives are classified
in the Strategic Research and Innovation Agenda (SRIA, also called roadmap) according to
six areas of intervention (AoIs) (see Figure 2[69]).
The aforementioned technological pathways comprising CDA, SCU, and CE represent
four of the six AoIs. CDA means steel production using hydrogen or renewable “green”
electricity. SCU is separated into two sub-parts: CCUS (carbon capture, utilization, storage)
and PI (process integration). CCU (carbon capture, utilization) encompasses technologies
that use CO and CO
2
in steel plant gasses or fumes as raw materials for production or
integration into valuable products. PI allows for the reduction of fossil fuels (coal, natural
gas, etc.) that are used in both BF-BOF and EAF steel production to reduce the CO
2
emissions generated by the steel industry. In addition to the CE, enablers are another
AoI. This field includes integrating technologies, such as artificial intelligence and digital
solutions, into industrial production. The development of new measurement techniques
and digital tools for monitoring and control in the new steel production processes, new
predictive and dynamic models, and strategic scheduling tools are examples of enablers
that will ensure the planning, assessment, and optimization of the industrial transition
process toward a climate-neutral steel sector [
71
,
72
]. The last AoI (denoted as “combination”
in Figure 2above) defines research initiatives in which the different pathways interact with
each other [69].
The use of secondary carbon bio-carriers for iron and steelmaking processes clearly ad-
dresses the CSP roadmap justified by the following facts, also mentioned in the roadmap ([
69
]):
-
The decarbonization pathway SCU-PI is addressed by (i) the integration of carboniza-
tion, pyrolysis, and gasification processes for using secondary carbon bio-carriers
as a substitute for fossil sources in existing iron and steelmaking process chains;
(ii) the adaptation of grinding, drying, and pneumatic injection technologies to tor-
refied/carbonized bio-based sources in the BF and EAF; (iii) the design of new solid
raw material injectors to use alternative material (i.e., the substitution of coal); (iv) use
of auxiliary reducing agents and slag foaming materials (e.g., polymers from waste
plastics, rubber form tires, biochar from agricultural/food residues).
-
The decarbonization pathway CDA is addressed by (i) replacing traditional carbons
and hydrocarbons with secondary carbon bio-carriers in existing melting processes;

Metals 2022,12, 2005 5 of 38
(ii) preheating processes implementing multi-fuel burners for primary and secondary
metallurgy with the use of secondary carbon bio-carriers as fuels.
Metals 2022, 12, x FOR PEER REVIEW 5 of 41
Figure 2. Areas of interventions and their interactions.
The aforementioned technological pathways comprising CDA, SCU, and CE
represent four of the six AoIs. CDA means steel production using hydrogen or renewable
“green” electricity. SCU is separated into two sub-parts: CCUS (carbon capture,
utilization, storage) and PI (process integration). CCU (carbon capture, utilization)
encompasses technologies that use CO and CO2 in steel plant gasses or fumes as raw
materials for production or integration into valuable products. PI allows for the reduction
of fossil fuels (coal, natural gas, etc.) that are used in both BF-BOF and EAF steel
production to reduce the CO2 emissions generated by the steel industry. In addition to the
CE, enablers are another AoI. This field includes integrating technologies, such as artificial
intelligence and digital solutions, into industrial production. The development of new
measurement techniques and digital tools for monitoring and control in the new steel
production processes, new predictive and dynamic models, and strategic scheduling tools
are examples of enablers that will ensure the planning, assessment, and optimization of
the industrial transition process toward a climate-neutral steel sector [71,72]. The last AoI
(denoted as “combination” in Figure 2 above) defines research initiatives in which the
different pathways interact with each other [69].
The use of secondary carbon bio-carriers for iron and steelmaking processes clearly
addresses the CSP roadmap justified by the following facts, also mentioned in the
roadmap ([69]):
- The decarbonization pathway SCU-PI is addressed by (i) the integration of
carbonization, pyrolysis, and gasification processes for using secondary carbon bio-
carriers as a substitute for fossil sources in existing iron and steelmaking process
Figure 2. Areas of interventions and their interactions.
2. Pathways for the Use of Secondary Carbon Bio-Carriers in Metallurgical Processes
Iron and steelmaking can be principally conducted by four routes: BF/BOF, scrap/EAF,
DRI/EAF, and SR/BOF [
69
,
73
]. The BF/BOF route includes cokemaking, iron ore sintering,
iron ore pelletizing, BF-based ironmaking, casting, rolling, and power stations [
74
]. Current
BFs operate with 70–80% sinter, 20–30% pellets, and 10–20% lump iron ore [
75
]. Figure 3
shows the four main iron and steelmaking routes using coal or coke that can be considered
for the integrated use of secondary carbon bio-carriers.
As shown in Figure 3, torrefaction (light, medium, or severe, depending on the desired
final properties of the solid product) can be used to improve the properties of the raw
biomass. The torrefied biomass can either be directly used in cokemaking, sintering, and
carbon composite agglomerates (CCAs) production, or it can be injected into the BF. It can
be subjected to a further modification of properties through carbonization and compaction
beneficiation by removing ash or adding minerals. Once the properties have been modified,
secondary carbon bio-carriers can be directed to biocoke production. During cokemaking,
the torrefied biomass can be used in a wide range from 3 to 50 wt.% of the feed mixture,
depending on the requirements for the carbon-bearing material in a particular process. The
biocoke obtained can be directed to:
- Sintering of iron ores to act as fuel;
-
BF to carry out functions as a fuel and reducing agent (delivers chemical energy to melt
the burden and contributes to the reduction of iron oxide to metallic iron), as a filter

Metals 2022,12, 2005 6 of 38
for entrained particles from the raceway, and provides the carbon for the carburization
(saturate hot metal with carbon);
- EAF to enable carburizing and slag foaming;
-
Melter gasifier to generate heat, to act as a reducing agent, to produce a reducing gas,
to ensure the permeability of the burden, and to carburize the hot metal.
Metals 2022, 12, x FOR PEER REVIEW 6 of 41
chains; (ii) the adaptation of grinding, drying, and pneumatic injection technologies
to torrefied/carbonized bio-based sources in the BF and EAF; (iii) the design of new
solid raw material injectors to use alternative material (i.e., the substitution of coal);
(iv) use of auxiliary reducing agents and slag foaming materials (e.g., polymers from
waste plastics, rubber form tires, biochar from agricultural/food residues).
- The decarbonization pathway CDA is addressed by (i) replacing traditional carbons
and hydrocarbons with secondary carbon bio-carriers in existing melting processes;
(ii) preheating processes implementing multi-fuel burners for primary and
secondary metallurgy with the use of secondary carbon bio-carriers as fuels.
2. Pathways for the Use of Secondary Carbon Bio-Carriers in Metallurgical Processes
Iron and steelmaking can be principally conducted by four routes: BF/BOF,
scrap/EAF, DRI/EAF, and SR/BOF [69,73]. The BF/BOF route includes cokemaking, iron
ore sintering, iron ore pelletizing, BF-based ironmaking, casting, rolling, and power
stations [74]. Current BFs operate with 70–80% sinter, 20–30% pellets, and 10–20% lump
iron ore [75]. Figure 3 shows the four main iron and steelmaking routes using coal or coke
that can be considered for the integrated use of secondary carbon bio-carriers.
Figure 3. Main pathways for using secondary carbon bio-carriers in iron and steelmaking units.
As shown in Figure 3, torrefaction (light, medium, or severe, depending on the
desired final properties of the solid product) can be used to improve the properties of the
raw biomass. The torrefied biomass can either be directly used in cokemaking, sintering,
and carbon composite agglomerates (CCAs) production, or it can be injected into the BF.
It can be subjected to a further modification of properties through carbonization and
compaction beneficiation by removing ash or adding minerals. Once the properties have
been modified, secondary carbon bio-carriers can be directed to biocoke production.
During cokemaking, the torrefied biomass can be used in a wide range from 3 to 50 wt.%
Figure 3. Main pathways for using secondary carbon bio-carriers in iron and steelmaking units.
For each metallurgical route, the carbon-bearing material requirements and the possi-
bility of using secondary carbon bio-carriers will be discussed in detail.
2.1. Cokemaking
2.1.1. Features of the Process and Requirements for the Carbon-Bearing Material
During the cokemaking process, coking coal undergoes several chemical and physical
changes, including softening, swelling, shrinkage, and re-solidification, which are require-
ments for forming a strong coke structure. The coke quality should meet strict requirements
for the application in the BF because it is the main consumer. Biocoke, in turn, should
also meet these requirements, but they are more challenging to achieve, as part of the
coking coal has been replaced by secondary carbon bio-carriers. Table 1shows the main
characteristics of conventional coke.

Metals 2022,12, 2005 7 of 38
Table 1. Main properties required for conventional coke.
Parameters Conventional Coke References
Ash, wt.% <11 [76]
Volatile matters, wt.% <1.1 [76,77]
Moisture by dry quenching, wt.% <0.7 [76]
Moisture by wet quenching, wt.% <5 [76]
C, wt.% 96.5–97.5 [76]
Fixed carbon, wt.% 88.8 [77]
H, wt.% <0.8 [76]
O, wt.% <0.4 [76]
N, wt.% <1.3 [76]
S, wt.% 0.5–1.2 [76]
Coke reactivity index, wt.% <30 [76]
Coke strength after reaction with CO2, wt.% <65 [76]
Structural strength, wt.% <80 [78]
Size distribution, mm 40–80 [76]
Bulk density, kg/m3430–500 [76]
Porosity, % 45.0–55.0 [76]
Electric resistivity, mΩ·m 10–12 [78]
Calorific value (MJ/kg) app. 29.0 [79]
Table 2compares the properties of torrefied biomass and biocoke. Biocoke is a coke
in which part of the coal in the coal blend is replaced by biomass (biomass can be used
in its original state or after heat treatment) and obtained at a temperature of 1100
◦
C like
conventional coke. The values for biocoke (mentioned in Table 2) can be within a very
wide range, as they depend on the type of biomass used, the amount of coal substituted,
and the conditions under which the biocoke is produced. The use of torrefied biomass
in cokemaking plants is limited because it adversely affects the coke quality. After all,
biocoke increases the porosity, the coke reactivity index (CRI), and reduces strength after
a reaction with CO
2
(CSR). Increased CRI and porosity can be advantageous for some
metallurgical processes, such as for injection in EAF. The low abrasion resistance and the
chemical composition of the ash, which can accelerate its reactivity with CO
2
in the BF, are
limiting factors for the use of torrefied biomass [80].
Table 2. Comparison of the properties of torrefied biomass and biocoke [16,78,79,81–84].
Parameters Torrefied Biomass Biocoke
Moisture, wt.% 4.8 0.65 or 1.35
Volatile matters, wt.% 34–85 1.4–2.7
Ash, wt.% 0.4 5.8–10.8
Fixed carbon, wt.% 13–45 87.8–92.4
C, wt.% 45–68 86.38–91.65
S, wt.% traces 0.22–0.23
Coke reactivity index, wt.% n/a app. 25–50
Strength after reaction with CO2, wt.% n/a app. 65–20
Calorific value, (MJ/kg) 16–29 app. 18–32
n/a is not available.

Metals 2022,12, 2005 8 of 38
Usually, biochar is characterized by a lower amount of ash and sulfur, which, compared
to biocoke, has an advantage for use in metallurgical processes. However, biocoke has a
sufficiently high FC and a low VM. It is worth noting that, compared to conventional coke,
biocoke has a lower value of ash and sulfur due to the replacement of coal within the blend.
It should be noted that the pre-treatment of secondary carbon bio-carriers before
adding them to the coal blend contributes to an increase in its amount. Thus, raw biomass
can be added to the coal blend only in a small amount of up to 3 wt.%, which does not
adversely affect the properties of the coke [
85
]. However, after preliminary heat treatment
of the biomass, its use can be increased by up to 10% [
86
]. Further increase in torrefied
biomass does not allow biocoke with the requirements necessary for use in a BF to be
obtained. The torrefied biomass in the coal blend acts as an inert material, reducing the
caking ability of the blend [
87
]. In addition, the size of the torrefied biomass is important
for the process of coal caking. Thus, a smaller particle size reduces fluidity to a greater
extent than larger particles [
75
]. Therefore, using the process of compaction of torrefied
biomass can improve its properties and minimize the impact on the quality of the final
product [
78
,
82
]. Comparing the results of various studies on CRI and CSR, it can be
concluded that biocoke with the addition of charcoal is more reactive compared to biocoke
from bio-briquettes. This is because, in the case of charcoal, there is a more active biomass
structure within the coke.
Kudo et al. [
88
] carried out briquetting of solid biomass (bamboo, larch, and ground
wood) at a temperature and high mechanical pressure of 130–200
◦
C and 114 MPa, respec-
tively, with subsequent carbonization at 900
◦
C. As a result of the research, the authors
obtained coke with a tensile strength (TS) of 5–19 MPa.
Castro-Díaz et al. [
89
] performed carbonization tests with hydrochars obtained after
the hydrous pyrolysis at 350
◦
C for 6 h using pine kraft lignin, torrefied lignin, and a
mixture of initial and torrefied lignins with a ratio of 50:50 wt.%/wt.%. The amount of ash
was less than that of good coking coal. However, the reactivity of the obtained biocoke was
high compared to coke from good coking coal, and the mechanical strength of biocoke was
significantly lower than that of coke.
Kim et al. [
73
] studied the TS of cokes and their reactivity using ash-free coal (AFC)
as a binder and added torrefied biomass. The TS of the coke containing the torrefied fuel
slightly decreased. The reactivity of the coke containing AFC and torrefied fuel was higher
than that of the coke containing only AFC.
Castro-Díaz et al. [
80
] found that blends containing 70 wt.% low-rank coal, 24 wt.%
torrefied lignin (before or after demineralization), and 6 wt.% phenolic resin produced
biocokes with a suitable mechanical strength. However, reactivity was higher compared
to coke.
Table 3provides a comparative analysis of the required amount of raw biomass and
torrefied biomass to replace conventional coke in a blast furnace under the conditions
of Lorraine, Saint-Gobain PAM plant [
90
]. It can be concluded that the torrefaction pro-
cess allows for improving the properties of biomass and reducing its quantity to replace
conventional coke.
Table 3.
Amount of raw biomass and torrefied biomass required to substitute conventional coke in
BF on the example of Lorraine, Saint-Gobain PAM plant [90].
Source Amount of Required Biomass (kt/Year)
20 wt.% 50 wt.%
Raw biomass 192.5 481.3
Torrefied biomass 77.0 192.5
The use of torrefied biomass in the BF can be the most environmentally friendly
option, with 14.7 % CO
2
-equivalent reduction, followed closely by pulverized biomass

Metals 2022,12, 2005 9 of 38
char injection and charcoal lumps loaded at the top of the furnace, with 14.5 and 14.4 %
CO2-equivalent reduction, respectively [90].
2.1.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
It can be concluded that, according to some research groups, secondary carbon bio-
carriers can be considered an alternative to conventional metallurgical coke when finding
the optimal conditions for obtaining biocoke. To mitigate the negative impact of torrefied
biomass additives, the best way is to use biomass after torrefaction at the maximum
possible temperature of 300
◦
C and in a compressed form, which will facilitate their local
arrangement within the blend volume, and often not as uniform as when using the initial
biomass. However, the use of compressed torrefied biomass in the coal blend may allow
the use of a higher amount to replace coal without drastically degrading the properties
of the biocoke. Compared to the existing conventional coke, the advantage of biocoke
is reduced ash and sulfur. The amount of torrefied biomass may vary depending on the
further purposes of using the biocoke and may reach 50 wt.%.
2.2. Iron Ore Sintering
2.2.1. Features of the Process and Requirements for the Carbon-Bearing Material
Sintering is the most economical and widely used process for preparing fine iron ore
for use in a BF. Compared to pellets, sinter production is cheaper, and compared to lump
ore, fluxed sinter is often more reducible with better softening characteristics [
91
]. At the
same time, the sintering process accounts for about 10% of CO
2
emissions from the entire
metallurgical industry [92].
Sintering occurs at temperatures of 1200–1400
◦
C, during which a mixture of iron ore
fines and other materials (e.g., sinter return fines, limestone) is used [
93
]. Coke breeze or
coal with low volatiles are used as fuel for sintering in the amount of 3–5 wt.% [
94
]. Table 4
shows the main properties of coke breeze for iron ore sintering. For iron ore sintering,
the fuel should have a low VM of <3 wt.%, a high density of >700 kg/m
3
, a small size of
<0.3–3.0 mm, and an FC content of more than 76 wt.%.
Table 4. Main properties of coke breeze for iron ore sintering.
Parameters Values References
Moisture, wt.% <15 [50,95]
Volatile matters, wt.% <3 [96]
Ash, wt.% <12 [50,95]
Density, kg/m3>700 [96]
Size, mm 0.3–3 [96]
Total sulfur, wt.% <2 [50,95]
Fixed carbon, wt.% <76 [97]
There are few studies in the literature [
61
,
98
,
99
] that have analyzed the substitution
of coke breeze with 10–25% of raw biomass in the process of iron ore sintering. It has
been found that adding biomass can lead to some adverse effects, such as a decrease in
productivity, an increase in the total fuel consumption (coke breeze and biomass), which
negatively affects the economics and environment, and a decrease in the strength of the
sinter. The use of highly reactive charcoal can also increase sintering velocity.
When using renewable fuel in iron ore sintering, it is essential to find the optimal
ratio [
100
] because substitution can deteriorate the strength of the iron ore sinter and re-
ducibility index, and heat treatment of the biomass is also essential to improve its properties
as a fuel. This allows fuel requirements for use in iron ore sintering to be met.
Many studies [
101
–
105
] have focused on biomass pyrolysis and charcoal after pyrolysis
for iron ore sintering. Generally, all research results were based on using biochars after

Metals 2022,12, 2005 10 of 38
pyrolysis at 400–1000
◦
C. These studies found that, when using biochar with a relatively
high FC content in the sintering process, a similar sinter yield and productivity to those
obtained by using coke breeze can be achieved. Additionally, the application of biomass
char in a sintering plant allows for a reduction of 6.7 % CO
2
-equivalent compared to the
use of anthracite [90].
2.2.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
Biomass, after severe torrefaction, does not gain the necessary properties to completely
replace the coke breeze. Biomass torrefaction may allow partial replacement of coke breeze
and/or anthracite at 20–25% [
99
,
106
] without degrading the properties of the iron ore sinter.
Moreover, the use of torrefied biomass in a compressed form can be considered.
The use of biocoke produced using torrefied biomass up to 50 wt.% may allow the
50 wt.% replacement of conventional coke breeze. Generally, the advantage of using biocoke
compared to coke breeze is that the ash content is much lower, which guarantees a smaller
particulate matter content in the flue gas.
2.3. Bio-Based Carbon Composite Agglomerates (CCAs)
2.3.1. Features of the Process and Requirements for the Carbon-Bearing Material
CCAs are mainly used in the BF and the direct reduction process [
107
]. Conventional
CCAs are produced as pellets by cold bonding with or without a binder or briquettes by
hot or cold pressing. Special studies are needed on the production aspects of these pellets,
especially since their use in a BF requires higher qualities in terms of strength. CCAs can
be referred to as new raw materials for iron production when consisting of carbon-bearing
biomaterial powder, iron ore powder, and a small amount of binder materials. Carbon-
bearing biomaterials used in pellets can be used in the raw state or after heat treatment.
However, there are several requirements for this type of CCA. For example, top-loaded
CCAs should meet the minimum mechanical strength requirements for a BF. Otherwise,
they can worsen the efficiency of the process [
108
]. In addition, carbon-bearing biomaterials
directly affect the mechanisms of mass and heat transfer, temperature profile, and gas
distribution inside the BF.
According to Ahmed et al. [
109
], the ash content should not increase when using new
carbon-bearing components. In this regard, the use of torrefied biomass is a promising
approach. Furthermore, agglomerates should have an FC content in a range to ensure
the iron carburization process. The reactivity requirements for carbonaceous materials
are not very stringent. Nevertheless, it is important that the carbon in the CCAs does not
participate in any chemical reactions below the set temperature of the heat reserve zone, as
this can reduce the efficiency of the process. One of the most important quality parameters
is the strength of composite pellets. Chemical reactions lead to the formation of gasses
within the pellet, increasing its porosity [
109
], and it has been reported by Mousa et al. [
108
]
that the main disadvantage of CCAs is their low crushing strength.
According to Khanna et al. and Ueki et al. [
96
,
110
], up to 46% of biochar can be used
in agglomerates in the direct reduction (DR) process. It has been pointed out that the
reduction rate of iron oxide is higher in biochar-based CCAs due to its greater reactivity
than in conventional coal or coke-based CCAs. For instance, Hu et al. [
111
] focused on
using only biochar-iron ore composites. It was concluded that pellets with 60 wt.% iron
ore content and a temperature higher than 800
◦
C promoted carbon conversion and iron
ore reduction. Praes et al. [
112
] carried out tests on iron ore pellets firing, which consumed
natural gas, coal (anthracite fines), and two different eucalyptus charcoal (partially replaced)
with two diverse ranges of VM. The first charcoal had a VM range of 20.3–25.98%, and the
second varied from 9.4% to 11.1%. It was concluded that replacing 7.5% anthracite fines
with the two eucalyptus charcoal is possible; 10% anthracite fines replacement is possible
with the charcoal with the lower VM.
In another research work [
113
], the effect of using palm kernel shells as a reducing
agent was studied. Iron oxide in iron ore can be completely reduced to magnetite and

Metals 2022,12, 2005 11 of 38
partially reduced to wustite when up to 30 wt.% palm kernel shells are present in the blend.
In addition, the degree of reduction increases with increasing temperature up to 900
◦
C, as
evidenced by the mass loss of the composite pellet and the mass of oxygen removal.
The levelofmetallizationofthe reducedpelletsisalso animportant factor.
Srivastava et al.
[
114
]
studied the effect of 20 wt.% fine wood on the quality of the resulting pellets. Pellets were
fired at different temperatures, and residence times were also studied. In most cases, the
total iron loss in the slag from the original pellets was less than 1% by weight. Pellets that
contained 97% Fe on average were obtained, and at the highest firing temperature, the
pellets contained 98.10% Fe.
Han et al. [
31
] studied the application of bamboo char, charcoal, and straw fiber to
produce DRI. The carbon content was 87.5%, 68.5%, and 20.89% for bamboo char, charcoal,
and straw fiber, respectively. The research results showed that the metallization level
increased with increasing temperature and, as a result, reached 91%. Furthermore, one
of the interesting results was that, despite the low FC content in straw fiber, the rate of
metallization of the pellets was higher than that of charcoal and bamboo charcoal pellets
due to the increased amount of carbohydrates. Regarding the compressive strength of the
pellets, it is possible to achieve production requirements, namely >1800 N, only when using
a high firing temperature.
2.3.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
Based on the above and the properties of torrefied biomass, torrefied biomass can
be used after the maximum possible torrefaction temperature. This allows the maximum
possible values of FC to be reached, which is important for ensuring the carburization
process. As the VM of the fuel increases, the possibility of thermal decomposition becomes
more likely. This means that secondary carbon bio-carriers with a relatively low VM are
less likely to decompose at lower temperatures. It can be concluded that torrefied biomass
has the potential to completely replace coal in the production of CCAs and to achieve the
necessary production requirements, but finding the optimal technological parameters is
also required.
2.4. Injection of Pulverized Secondary Carbon Bio-Carriers
In conventional pulverized coal injection (PCI) technology, non-coking or weakly
coking coals are injected into the raceways of BFs to partially replace the coke [
115
]. This
technology is the most efficient method of replacing non-renewable fuels in the BF with
different secondary carbon carriers. For example, there is a practice of injecting about
20 kg/tHM of waste plastics at Voestalpine Stahl BF A [116].
Table 5shows the selected properties of conventional pulverized coal.
Table 5. Selected parameters of conventional pulverized coal [26,117].
Component (wt.%) C H O N S Ash Volatile
Matters Moisture
80.6 4.35 5.35 1.65 0.45 10.89 19.50 0.01
References [26] [26] [26] [26] [26] [26] [117] [117]
The use of biomass in its raw state after torrefaction, as well as after pyrolysis, in
PCI has been well studied [
28
,
75
,
118
–
122
], and the use of secondary carbon bio-carriers in
this technology is proven. The papers mentioned above revealed the possibility of using
20–40% biomass injection, or even up to 100% replacement of injected coal. However, it
should be noted that for the purposes of PCI, the secondary carbon bio-carriers should
be evaluated through several properties, such as the fuel ratio, ignition temperature, and
burnout [123].

Metals 2022,12, 2005 12 of 38
Phanphanich et al. [
124
] reported that proximate and elemental compositions of
torrefied biomass could be improved after torrefaction at temperatures ranging from 225
◦
C
to 300 ◦C, and were comparable to coal.
Chen et al. [
125
] studied the effect of torrefaction on improving the physical and
chemical properties of pulverized biomass for use in a BF. The authors concluded that the
calorific value could be improved by subsequent compaction.
In another paper by Chen et al. [
126
], the torrefaction and burning characteristics
of bamboo, oil palm, rice husk, bagasse, and Madagascar almonds were studied and
compared to high-volatile bituminous coal. As a result, the authors emphasized that a
torrefaction temperature of 300
◦
C is suitable for converting the initial biomass into biochar,
which can subsequently be used for injection into a BF. These results are consistent with
the results of studies by Du et al. [
123
], who reported that biomass torrefied at 300
◦
C or
carbonized at temperatures below 500
◦
C could be applied with coals for PCI. However,
torrefied biomass can only partially replace coal to keep a good burnout in raceways.
Recommendations for the Use of Secondary Carbon Bio-Carriers
The main limitation of the widespread use of torrefied biomass is its high yield of
VM. Therefore, the partial replacement of coal from 20–50% with biomass after severe
torrefaction can be considered. For a complete replacement of coal, it is possible to consider
the option of using a blend of torrefied biomass at 300
◦
C and biochar obtained after
carbonization of torrefied biomass at a ratio of 50:50.
2.5. Blast Furnace Process
2.5.1. Features of the Process and Requirements for the Carbon-Bearing Material
The BF is currently the most predominant technology to produce iron for steelmaking.
The principle of the process is the conversion of iron oxides to iron using carbon-based
reducing agents. The BF is a large countercurrent metallurgical shaft furnace in which
iron oxides and coke fed into the furnace from above move downwards, and the reducing
gasses move upwards [
127
]. The reducing conditions in the furnace are created by top-
charged coke and tuyere-injected reducing agents; for instance, pulverized coal (PC). Using
carbon-bearing materials is essential to operate a BF, and the requirements are strict [128].
Two of the most important parameters for using metallurgical coke in a BF are the CRI
and CSR. According to Alvarez et al. [
129
], the industrial quality requirements for coke
are a CRI under 30% and a CSR above 55%; according to Babich et al. [
130
], CRI and CSR
requirements in European BFs are 23% and 65%, respectively. In addition to these two
parameters, the coke should have good mechanical strength; M40 should be >88%, M25
should be >90%, and M10 should be <6% [
50
]. These indices represent the percentage of
material grain sizes remaining > 40 mm, >25 mm, and >10 mm after mechanical treatment
(100 revolutions in a drum) according to the Micum drum test [
50
]. All of this limits the
use of secondary carbon bio-carriers for producing coke for BF purposes.
There is a current practice of using charcoal or a mixture of charcoal and PC in mini
BFs with a production capacity of 40,000–350,000 t/year [
131
]. The advantages of the mini
BF technology are low emissions, low sulfur content in the iron, and low slag volumes.
The carbonization temperature for charcoal can range from 300–700
◦
C, depending on the
quality requirements. For example, a relatively high VM can be useful in PCI technology.
Another study on the mini BF technology was carried out by de Castro et al. [
132
]. In
this work, various scenarios for using charcoal and hydrogen-rich fuel gas were simulated.
Some scenarios have shown that it is possible to increase productivity and, at the same
time, reduce carbon consumption.
The use of charcoal has also demonstrated the potential to reduce CO
2
emissions in
steel production. However, according to Hanrot et al. [
133
], the successful use of charcoal
can be implemented if local conditions and quality criteria permit, such as the availability
of biomass cultivation and the production of charcoal in a sustainable manner.

Metals 2022,12, 2005 13 of 38
2.5.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
The main pathways for secondary carbon bio-carriers in the BF/BOF route are shown
in Figure 4.
Metals 2022, 12, x FOR PEER REVIEW 13 of 41
in which iron oxides and coke fed into the furnace from above move downwards, and the
reducing gasses move upwards [127]. The reducing conditions in the furnace are created
by top-charged coke and tuyere-injected reducing agents; for instance, pulverized coal
(PC). Using carbon-bearing materials is essential to operate a BF, and the requirements
are strict [128].
Two of the most important parameters for using metallurgical coke in a BF are the
CRI and CSR. According to Alvarez et al. [129], the industrial quality requirements for
coke are a CRI under 30% and a CSR above 55%; according to Babich et al. [130], CRI and
CSR requirements in European BFs are 23% and 65%, respectively. In addition to these
two parameters, the coke should have good mechanical strength; M40 should be >88%,
M25 should be >90%, and M10 should be <6% [50]. These indices represent the percentage
of material grain sizes remaining >40 mm, >25 mm, and > 10 mm after mechanical
treatment (100 revolutions in a drum) according to the Micum drum test [50]. All of this
limits the use of secondary carbon bio-carriers for producing coke for BF purposes.
There is a current practice of using charcoal or a mixture of charcoal and PC in mini
BFs with a production capacity of 40,000–350,000 t/year [131]. The advantages of the mini
BF technology are low emissions, low sulfur content in the iron, and low slag volumes.
The carbonization temperature for charcoal can range from 300–700 °C, depending on the
quality requirements. For example, a relatively high VM can be useful in PCI technology.
Another study on the mini BF technology was carried out by de Castro et al. [132]. In
this work, various scenarios for using charcoal and hydrogen-rich fuel gas were
simulated. Some scenarios have shown that it is possible to increase productivity and, at
the same time, reduce carbon consumption.
The use of charcoal has also demonstrated the potential to reduce CO
2
emissions in
steel production. However, according to Hanrot et al. [133], the successful use of charcoal
can be implemented if local conditions and quality criteria permit, such as the availability
of biomass cultivation and the production of charcoal in a sustainable manner.
2.5.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
The main pathways for secondary carbon bio-carriers in the BF/BOF route are shown
in Figure 4.
Figure 4. Main pathways for using secondary carbon bio-carriers in the BF/BOF route.
Figure 4. Main pathways for using secondary carbon bio-carriers in the BF/BOF route.
In a BF, the following ways of using secondary carbon bio-carriers can be considered:
-
The use of biocoke produced with the addition of torrefied biomass, as discussed
earlier in Section 2.1 Cokemaking;
-
The use of torrefied biomass or biocoke to produce sinter, which is afterwards used in
the BF;
-
The use of bio-pulverized coal injection technology with partial replacement of coal
with up to 50% torrefied biomass or full replacement of coal with a mixture of torrefied
biomass and torrefied biomass after carbonization;
- The use of torrefied biomass to produce CCAs, which are afterward used in the BF.
2.6. Electric Arc Furnace
2.6.1. Features of the Process and Requirements for the Carbon-Bearing Material
EAF-based steel production accounts for 28% of global output (~42% in the EU), ac-
cording to the World Steel Association [
134
]. EAFs mainly use electricity with a small
amount of carbon-bearing material [
135
]. Carbon-bearing material is added to the EAF
route to perform the following functions: (a) charge carbon, with the main aim of adding
chemical energy and creating a reducing atmosphere during smelting that minimizes the
oxidation of alloys and metals; (b) injected carbon, also known as slag foaming carbon,
where slag foaming technology in the EAF is used to increase energy efficiency and pro-
ductivity, reduce operating costs, and improve the quality of steel produced [
119
]; and (c)
afterward as a carburizer carbon in the ladle furnace for the carburizing process.
Norgate et al. [
119
] have shown that replacing a conventional carbon source from
50% to 100% is possible with charcoal after pyrolysis; also under life cycle assessment
was charcoal production from Mallee eucalypt biomass. Yunos et al. [
136
] investigated
the possibility of using biomass in the EAF; using palm shell char to partially replace
coke in a laboratory-scale reactor at 1550
◦
C using the sessile-drop approach in an argon
atmosphere. The test results showed an improved interaction with EAF slag compared to
conventional coke.

Metals 2022,12, 2005 14 of 38
In contrast, Huang et al. [
137
] concluded that the interaction between biochars and slag
was weak compared to other carbon-bearing materials. The authors studied five carbon-
bearing samples to replace conventional fuels: biochar obtained from wood biomass by
slow pyrolysis at 900
◦
C, biochar obtained from wood biomass by fast pyrolysis at 400
◦
C,
technical graphite, metallurgical coke, and semicoke obtained from waste tire pyrolysis
at 700 ◦C.
Another study was carried out as part of the GreenEAF project (funded by the frame-
work of the Research Fund for Coal and Steel, RFCS, 2009–2012, RFSR-CT-2009-00004).
Bianco et al. [
30
] suggested a 1:1 substitution of coal (anthracite) and charcoal used in
the EAF on an energy basis, assuming that charcoal is similar to or higher quality than
coal and charcoal. The tests showed that charcoal could be used for both charge and slag
foaming. Additionally, to achieve good foaming, the authors suggested several process
approaches, such as improving the wettability of charcoal and slag, and the charcoal should
be injected under the slag. Additionally, Fidalgo et al. [
138
] studied two biochars obtained
from agricultural residues, grape seed and pumpkin seed chars, for EAF steelmaking. Hard
coal and three types of anthracites were also used to compare the results. Biochars were
obtained during the GreenEAF project for test runs. The temperatures used to produce
biochar were 500
◦
C and 600
◦
C. A lower pyrolysis temperature of 500
◦
C was applied
to obtain injected carbon with a higher VM to positively affect the foaming behavior. A
pyrolysis temperature of 600 ◦C was applied to obtain charge carbon, for which it is more
important to have a lower VM. The authors found that the biochar used in the project
could replace coal with regard to reactivity. Furthermore, it was noted that the high VM of
biochar is an adequate stimulation for slag foaming.
As a follow-up to the project mentioned above, Meier et al. [
139
] performed test runs
with different carbon carriers in a dynamic process simulation model to investigate their
application in an EAF within the framework of the GreenEAF2 project (RFCS, 2014–2017,
RFSP-CT-2014-00003). Within this project, charcoal obtained from pyrolysis and torrefaction
and virgin ligneous biomass (palm kernel shells) were used. The results using biomass
showed a different behavior of the two materials studied; namely, anthracite and palm
kernel shells. The mass fraction of waste gasses increased in the case of palm kernel shells,
while melting the first portion of scrap because they have a higher VM content and are
a more reactive material. It was concluded that it is possible to increase the use of post-
combustion oxygen early in the process for palm kernel shells. It may subsequently lead to
higher energy releases and increased melting rates.
Nwachukwu et al. [
140
] presented a model for the application of biofuels in steel
production. According to the scheme, the amount of conventional fuel substitution ranged
from 0% to 100%. However, it is worth noting that the model focused on using charcoal
and bio-gas products.
Furthermore, Echterhof [
29
] presented a review of the utilization of alternative carbon
sources by the EAF steel production route. The review showed that the use of alternative
carbon sources in the EAF is required to produce fully environmentally friendly and carbon-
neutral steel. Many research results have shown that it is fundamentally possible to use
such substitutes.
2.6.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
Figures 5and 6show the pathways for secondary carbon bio-carriers in the EAF,
where scrap is used as the main raw material, in the subsequent ladle furnace, and by
DRI-EAF routes.
There are several possible ways to apply secondary carbon bio-carriers (biocoke):
- a charge carbon in an EAF;
- an injected carbon in an EAF;
- a carburizer source in a ladle furnace.

Metals 2022,12, 2005 15 of 38
Metals 2022, 12, x FOR PEER REVIEW 15 of 41
As a follow-up to the project mentioned above, Meier et al. [139] performed test runs
with different carbon carriers in a dynamic process simulation model to investigate their
application in an EAF within the framework of the GreenEAF2 project (RFCS, 2014–2017,
RFSP-CT-2014-00003). Within this project, charcoal obtained from pyrolysis and
torrefaction and virgin ligneous biomass (palm kernel shells) were used. The results using
biomass showed a different behavior of the two materials studied; namely, anthracite and
palm kernel shells. The mass fraction of waste gasses increased in the case of palm kernel
shells, while melting the first portion of scrap because they have a higher VM content and
are a more reactive material. It was concluded that it is possible to increase the use of post-
combustion oxygen early in the process for palm kernel shells. It may subsequently lead
to higher energy releases and increased melting rates.
Nwachukwu et al. [140] presented a model for the application of biofuels in steel
production. According to the scheme, the amount of conventional fuel substitution ranged
from 0% to 100%. However, it is worth noting that the model focused on using charcoal
and bio-gas products.
Furthermore, Echterhof [29] presented a review of the utilization of alternative
carbon sources by the EAF steel production route. The review showed that the use of
alternative carbon sources in the EAF is required to produce fully environmentally
friendly and carbon-neutral steel. Many research results have shown that it is
fundamentally possible to use such substitutes.
2.6.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
Figures 5 and 6 show the pathways for secondary carbon bio-carriers in the EAF,
where scrap is used as the main raw material, in the subsequent ladle furnace, and by
DRI-EAF routes.
Figure 5. Main pathways for using secondary carbon bio-carriers in the EAF, where scrap is used as
the main raw material.
There are several possible ways to apply secondary carbon bio-carriers (biocoke):
- a charge carbon in an EAF;
- an injected carbon in an EAF;
- a carburizer source in a ladle furnace.
Figure 5.
Main pathways for using secondary carbon bio-carriers in the EAF, where scrap is used as
the main raw material.
Metals 2022, 12, x FOR PEER REVIEW 16 of 41
Figure 6. Main pathways for using secondary carbon bio-carriers in the DRI/EAF route.
It should be noted that, in the DRI/EAF route, it is also possible to consider using
CCAs obtained using torrefied biomass.
Based on the above requirements, it can be concluded that the use of torrefied
biomass is practically limited. However, the use of biocoke up to 100% as a carbon source
is possible even with a high additive amount of torrefied biomass up to 50% within coal
blend because it can meet the requirements for this process in terms of the VM and has a
sufficient amount of FC > 85% for carburizing the steel or creating foaming slag to improve
the energy efficiency of the melting process. Additionally, it is important to mention that
there are no strict requirements for the strength of the carbon source for use in an EAF
because of the features of the furnace [119]. Therefore, using biocoke with a high amount
of torrefied biomass is a promising pathway for future research.
2.7. Smelting Reduction Processes
There are mainly two smelting reduction processes that are commercially proven,
COREX and FINEX. Iron ores are heated and pre-reduced to DRI within these processes
by the off-gas coming from the melter gasifier. The pre-reduction step could be
implemented in a reduction shaft (COREX) or a fluidized bed reactor (FINEX). Pre-
reduced iron ores are then melted in the melter gasifier. The melter gasifier uses oxygen
and coal as a reducing agent. Subsequently, the hot metal is fed to the BOF for steelmaking
[141–143]. These processes generally use non-coking coal as a fuel source, with the main
requirements of FC > 40% and VM < 34%. Considering these requirements, secondary
carbon bio-carriers can be used after torrefaction to the highest possible temperature, thus
obtaining biochar with the highest possible values for FC and the lowest VM. Another
option is to use biocoke, whereby it is possible to consider using biocoke obtained with a
high amount of torrefied biomass of up to 50% in this route. An additional benefit of
biocoke is that sulfur-containing compounds and ash formation are minimized.
2.7.1. COREX Process
COREX is a process that produces hot metal out of lumpy iron carriers (mainly
pellets, but also lump ore and sinter). The main reducing agent is briquetted coal. The
process mainly consists of two reactors; i.e., a reduction shaft and a melting gasifier [144].
For the COREX process, the most suitable conventional carbon source is non-coking coal
with 55–70 wt.% FC content. There are also a number of requirements for the ash content
and moisture content. The ash content should be lower than 12 wt.%, as a higher ash
content increases the slag volume, resulting in high specific fuel consumption, poor
Figure 6. Main pathways for using secondary carbon bio-carriers in the DRI/EAF route.
It should be noted that, in the DRI/EAF route, it is also possible to consider using
CCAs obtained using torrefied biomass.
Based on the above requirements, it can be concluded that the use of torrefied biomass
is practically limited. However, the use of biocoke up to 100% as a carbon source is possible
even with a high additive amount of torrefied biomass up to 50% within coal blend because
it can meet the requirements for this process in terms of the VM and has a sufficient amount
of FC > 85% for carburizing the steel or creating foaming slag to improve the energy
efficiency of the melting process. Additionally, it is important to mention that there are
no strict requirements for the strength of the carbon source for use in an EAF because of
the features of the furnace [
119
]. Therefore, using biocoke with a high amount of torrefied
biomass is a promising pathway for future research.
2.7. Smelting Reduction Processes
There are mainly two smelting reduction processes that are commercially proven,
COREX and FINEX. Iron ores are heated and pre-reduced to DRI within these processes by
the off-gas coming from the melter gasifier. The pre-reduction step could be implemented
in a reduction shaft (COREX) or a fluidized bed reactor (FINEX). Pre-reduced iron ores are
then melted in the melter gasifier. The melter gasifier uses oxygen and coal as a reducing
agent. Subsequently, the hot metal is fed to the BOF for steelmaking [
141
–
143
]. These
processes generally use non-coking coal as a fuel source, with the main requirements of
FC > 40%
and VM < 34%. Considering these requirements, secondary carbon bio-carriers
can be used after torrefaction to the highest possible temperature, thus obtaining biochar
with the highest possible values for FC and the lowest VM. Another option is to use biocoke,

Metals 2022,12, 2005 16 of 38
whereby it is possible to consider using biocoke obtained with a high amount of torrefied
biomass of up to 50% in this route. An additional benefit of biocoke is that sulfur-containing
compounds and ash formation are minimized.
2.7.1. COREX Process
COREX is a process that produces hot metal out of lumpy iron carriers (mainly pellets,
but also lump ore and sinter). The main reducing agent is briquetted coal. The process
mainly consists of two reactors; i.e., a reduction shaft and a melting gasifier [
144
]. For
the COREX process, the most suitable conventional carbon source is non-coking coal with
55–70 wt.% FC content. There are also a number of requirements for the ash content and
moisture content. The ash content should be lower than 12 wt.%, as a higher ash content
increases the slag volume, resulting in high specific fuel consumption, poor drainage
through the coal bed, and reduced productivity [
145
]. The moisture content should be as
low as possible. The main requirements for the properties of the carbon-bearing material
are given in Table 6.
Table 6. Main requirements for non-coking coal for COREX [145].
Parameters Values
Moisture, wt.% <4
Ash, wt.% <12
Volatile matters, wt.% 25–27
Sulfur, wt.% <0.6
Fixed carbon, wt.% 55–70
Calorific value, kJ kg−1>27,000
Although a significant amount of coal is used in the COREX process, 10–20% of metal-
lurgical coke is required for heat generation, reducing gas production, and maintaining char
bed permeability. The coke quality required for the COREX process is shown in Table 7.
Table 7. Main requirements for coke for COREX [141,146].
Parameters Values
Coke reactivity index, wt.% <35
Coke strength after reaction, wt.% >55
Volatile matters, wt.% app. 25
Ash, wt.% <15
Sulfur, wt.% <1
Grain size, mm 10–15
Several studies have investigated the use of torrefied biomass for the COREX ironmak-
ing route. Adeleke et al. [
147
] investigated using coal briquettes and pre-treated biomass
for use in COREX ironmaking processes. The initial biomass was ground to <2 mm and
subjected to a torrefaction process at a temperature of 260
◦
C and a residence time of 60 min.
Carbon fines of 95 wt.%, torrefied biomass of 5 wt.%, and binder were homogeneously
mixed, followed by the addition of water and proper mixing to activate the binder for
agglomeration. The authors concluded that the coal fines-torrefied biomass briquettes
satisfactorily met the required physical properties for the COREX ironmaking process.
Moreover, according to the scheme of mass, iron, and carbon balance for the Bio-
COREX/BOF case by Yang et al. [
148
], the biochar substitution rate can be as high as
45 %LHV.

Metals 2022,12, 2005 17 of 38
Figure 7shows the main pathway for using secondary carbon bio-carriers in the
COREX/BOF route. This layout can consider using the following:
- Biomass torrefied at the highest possible temperature can partially replace coal;
-
Biocoke for complete replacement of the conventional coke. At the same time, it
is possible to consider using biocoke with a high amount of torrefied biomass as a
substitute for coal.
Metals 2022, 12, x FOR PEER REVIEW 18 of 41
Figure 7. Main pathways for using secondary carbon bio-carriers in the COREX/BOF route.
In addition, the use of bio-pulverized injection and CCAs should be considered
possible options for this route.
2.7.2. FINEX Process
The FINEX smelting-reduction process is based on the direct use of non-coking coal
and fine ore. The major difference between the COREX and FINEX processes is that the
FINEX process can directly use sinter feed iron ore without an agglomeration [146]. The
main FINEX process consists of a melter-gasifier and a series of fluidized bed reactors,
forming a countercurrent system in which fine ore is reduced to DRI in three or four
stages. The fine DRI is then compacted and loaded as hot compacted iron (HCI) into a
gasifier melting unit. The HCl is then reduced and melted. The heat required for reduction
and melting is provided by coal gasification. The reducing gas, also from coal gasification,
passes through fluidized-bed reactors [146]. From the point of view of the fuel route, non-
coking coals and coal briquettes are directly loaded into the melter-gasifier unit. The main
fuel quality requirements for the FINEX process are shown in Table 8.
Table 8. Main requirements for fuel for FINEX.
Fixed Carbon,
wt.%
Ash,
wt.%
Volatile Matters,
wt.%
Sulfur,
wt.% Reference
min. 55 up to 25 <35 <1 [149]
Figure 8 shows the use of secondary carbon bio-carriers in the FINEX/BOF route.
Figure 7. Main pathways for using secondary carbon bio-carriers in the COREX/BOF route.
In addition, the use of bio-pulverized injection and CCAs should be considered
possible options for this route.
2.7.2. FINEX Process
The FINEX smelting-reduction process is based on the direct use of non-coking coal
and fine ore. The major difference between the COREX and FINEX processes is that the
FINEX process can directly use sinter feed iron ore without an agglomeration [
146
]. The
main FINEX process consists of a melter-gasifier and a series of fluidized bed reactors,
forming a countercurrent system in which fine ore is reduced to DRI in three or four stages.
The fine DRI is then compacted and loaded as hot compacted iron (HCI) into a gasifier
melting unit. The HCl is then reduced and melted. The heat required for reduction and
melting is provided by coal gasification. The reducing gas, also from coal gasification,
passes through fluidized-bed reactors [
146
]. From the point of view of the fuel route,
non-coking coals and coal briquettes are directly loaded into the melter-gasifier unit. The
main fuel quality requirements for the FINEX process are shown in Table 8.
Table 8. Main requirements for fuel for FINEX.
Fixed Carbon,
wt.%
Ash,
wt.%
Volatile Matters,
wt.%
Sulfur,
wt.% Reference
min. 55 up to 25 <35 <1 [149]
Figure 8shows the use of secondary carbon bio-carriers in the FINEX/BOF route.
It is important to note that the use of secondary carbon bio-carriers in this scheme
does not differ much from the COREX/BOF route. It is possible to partially replace coal
briquettes using biomass torrefied at the highest possible temperature and subsequent
compaction. The use of lumpy biocoke can be considered a complete replacement for coal.
At the same time, it is also possible to consider the use of biocoke obtained with a high
amount of torrefied biomass. Bio-pulverized injection and CCAs can be considered for
using secondary carbon bio-carriers.

Metals 2022,12, 2005 18 of 38
Metals 2022, 12, x FOR PEER REVIEW 19 of 41
Figure 8. Main pathways for using secondary carbon bio-carriers in the FINEX/BOF route.
It is important to note that the use of secondary carbon bio-carriers in this scheme
does not differ much from the COREX/BOF route. It is possible to partially replace coal
briquettes using biomass torrefied at the highest possible temperature and subsequent
compaction. The use of lumpy biocoke can be considered a complete replacement for coal.
At the same time, it is also possible to consider the use of biocoke obtained with a high
amount of torrefied biomass. Bio-pulverized injection and CCAs can be considered for
using secondary carbon bio-carriers.
2.7.3. HIsarna and Hismelt Processes
HIsarna is a melt-in-bath technology that combines coal preheating and partial
pyrolysis in a reactor [150]. This technology uses a melting vessel for the final reduction
of ore and a melting cyclone for ore melting. Reduced CO
2
emissions are enabled due to
the absence of sintering and coking processes. The HIsarna technology reduces CO
2
emissions by almost 70% by using biomass or natural gas instead of coal, flue gas
treatment, CO
2
storage, and thermal energy reuse [151]. Pulverized coal [152] and biomass
[151] can be used as fuel in this process. Khasraw et al. [153] studied the properties of two
samples of charcoal (birch wood and grass) compared to thermal coal. It was observed
that chars showed a faster reaction compared to thermal coal. According to another paper
by Khasraw et al. [150], torrefied grass contains a large amount of water and CO
2
that is
released at a very low temperature. Therefore, pre-treatment to approximately 400 °C is
essential to obtain biochar with properties similar to the coal introduced into HIsarna.
Htet et al. [154] studied the properties of charcoal, thermal coal, and carbon black as
reducing agents for the HIsarna process, and also studied their properties to understand
how these materials can affect the process. In the case of using substitutes, the important
parameters are the alkalinity index, VM content, and its amount, as well as the amount of
ash.
Figure 9 shows the use of secondary carbon bio-carriers in the HIsarna/BOF route.
Figure 8. Main pathways for using secondary carbon bio-carriers in the FINEX/BOF route.
2.7.3. HIsarna and Hismelt Processes
HIsarna is a melt-in-bath technology that combines coal preheating and partial py-
rolysis in a reactor [
150
]. This technology uses a melting vessel for the final reduction
of ore and a melting cyclone for ore melting. Reduced CO
2
emissions are enabled due
to the absence of sintering and coking processes. The HIsarna technology reduces CO
2
emissions by almost 70% by using biomass or natural gas instead of coal, flue gas treatment,
CO
2
storage, and thermal energy reuse [
151
]. Pulverized coal [
152
] and biomass [
151
]
can be used as fuel in this process. Khasraw et al. [
153
] studied the properties of two
samples of charcoal (birch wood and grass) compared to thermal coal. It was observed
that chars showed a faster reaction compared to thermal coal. According to another paper
by
Khasraw et al
. [
150
], torrefied grass contains a large amount of water and CO
2
that is
released at a very low temperature. Therefore, pre-treatment to approximately 400
◦
C is
essential to obtain biochar with properties similar to the coal introduced into HIsarna.
Htet et al. [
154
] studied the properties of charcoal, thermal coal, and carbon black as
reducing agents for the HIsarna process, and also studied their properties to understand
how these materials can affect the process. In the case of using substitutes, the important
parameters are the alkalinity index, VM content, and its amount, as well as the amount
of ash.
Figure 9shows the use of secondary carbon bio-carriers in the HIsarna/BOF route.
Metals 2022, 12, x FOR PEER REVIEW 20 of 41
Figure 9. Main pathways for using secondary carbon bio-carriers in the HIsarna/BOF route.
Based on the analysis of the results, it is challenging to use torrefied biomass even at
the highest possible temperature due to limitations in its properties. However, the first
option of using torrefied biomass after carbonization can be considered for improving the
properties. Secondly, a blend of coal with torrefied biomass (up to the highest possible
torrefaction temperature) can be used. For further research, it could be of interest to
replace coal with up to 50 wt.%. Additionally, the third option of using biocoke obtained
using torrefied biomass is viable. At the same time, the amount of torrefied biomass in
biocoke can reach up to 50 wt.% because the VM and FC will be at the required level even
with such an amount of a substitute.
A similar recommendation for using biomass to replace pulverized coal with a
particle size of <3 mm can be applied to the Hismelt process, which uses a two-stage rotary
kiln process to preheat and pre-reduce iron-bearing raw materials (particle size < 6 mm)
at 750 °C.
2.8. Ferroalloy Industry
2.8.1. Submerged Arc Furnace
The ferroalloy industry refers to iron alloys with a high proportion of additional
elements, such as chromium, manganese, silicon, aluminum, and other elements. They are
generally produced in a SAF at a temperature of >1500 °C. A three-phase electrode (AC
power supply) is inserted into a mixture of ferroalloys and carbon-bearing reductants
[155]. Carbon-bearing materials (coke or anthracite) are used to produce ferroalloys
through a carbothermic reduction, which acts as a reducing agent. Biobased reductants
can potentially replace fossil-based ones, mitigating anthropogenic CO
2
and GHG
emissions and increasing the efficiency of the smelting process. This leads to a search for
new ways to produce and use alternative bioreductants to produce ferroalloys. The most
important properties required for carbonaceous reducing agents are high reactivity,
electrical resistance, high conversion rates, low sulfur content, high bulk density, and
specific energy. Table 9 gives some of the important properties of conventional
carbonaceous materials for use in ferroalloy production.
Table 9. Required properties for conventional coke in ferroalloy production [35,156].
Parameters Conventional Coke
Fixed carbon, wt.% 86–88
Volatile matter, wt.% ≤1
Ash, wt.% 10–12
Reactivity with CO
2
at 1060 °C
,
%C/s (0.2–0.5)10
−2
Thermal cohesion strength, % 93–97
Thermal abrasion strength, % 82–89
Electrical resistance for carbon material with size, mm: 10–20
Figure 9. Main pathways for using secondary carbon bio-carriers in the HIsarna/BOF route.
Based on the analysis of the results, it is challenging to use torrefied biomass even at
the highest possible temperature due to limitations in its properties. However, the first
option of using torrefied biomass after carbonization can be considered for improving the
properties. Secondly, a blend of coal with torrefied biomass (up to the highest possible
torrefaction temperature) can be used. For further research, it could be of interest to replace
coal with up to 50 wt.%. Additionally, the third option of using biocoke obtained using
torrefied biomass is viable. At the same time, the amount of torrefied biomass in biocoke

Metals 2022,12, 2005 19 of 38
can reach up to 50 wt.% because the VM and FC will be at the required level even with
such an amount of a substitute.
A similar recommendation for using biomass to replace pulverized coal with a particle
size of <3 mm can be applied to the Hismelt process, which uses a two-stage rotary
kiln process to preheat and pre-reduce iron-bearing raw materials (particle size < 6 mm)
at 750 ◦C.
2.8. Ferroalloy Industry
2.8.1. Submerged Arc Furnace
The ferroalloy industry refers to iron alloys with a high proportion of additional
elements, such as chromium, manganese, silicon, aluminum, and other elements. They are
generally produced in a SAF at a temperature of >1500
◦
C. A three-phase electrode (AC
power supply) is inserted into a mixture of ferroalloys and carbon-bearing reductants [
155
].
Carbon-bearing materials (coke or anthracite) are used to produce ferroalloys through a
carbothermic reduction, which acts as a reducing agent. Biobased reductants can potentially
replace fossil-based ones, mitigating anthropogenic CO
2
and GHG emissions and increasing
the efficiency of the smelting process. This leads to a search for new ways to produce
and use alternative bioreductants to produce ferroalloys. The most important properties
required for carbonaceous reducing agents are high reactivity, electrical resistance, high
conversion rates, low sulfur content, high bulk density, and specific energy. Table 9
gives some of the important properties of conventional carbonaceous materials for use in
ferroalloy production.
Table 9. Required properties for conventional coke in ferroalloy production [35,156].
Parameters Conventional Coke
Fixed carbon, wt.% 86–88
Volatile matter, wt.% ≤1
Ash, wt.% 10–12
Reactivity with CO2at 1060 ◦C, %C/s (0.2–0.5)10−2
Thermal cohesion strength, % 93–97
Thermal abrasion strength, % 82–89
Electrical resistance for carbon material with size, mm: 10–20
Electrical resistance at 1000 ◦C, U·m 0.003–0.008
Electrical resistance at 1400 ◦C, U·m 0.003–0.009
Surup et al. [
157
] investigated the pyrolysis treatment of various types of biomass
at high temperatures to obtain biochars, which can be subsequently used to produce
ferroalloys. It was concluded that with a heat treatment > 2400
◦
C, it is possible to obtain
biochar from renewable sources with a reactivity close to conventional coke. In addition,
the co-pyrolysis of biomass with bio-oil was studied, and the prospect of obtaining biochars
with a reactivity comparable to metallurgical coke was shown.
Electrical resistance is also an important indicator for carbon-bearing materials. Ac-
cording to another paper by Surup et al. [
35
], conventional coke used in a SAF is expected to
have an electrical resistivity of 7–10 m
Ω
m at room temperature and 1–10 m
Ω
m at 1400
◦
C
for a particle size of 5–20 mm. Coke substitutes, i.e., charcoal, have an electrical resistance
of more than 106 m
Ω
at room temperature, which decreases with increasing temperature.
The electrical resistance of charcoal obtained after carbonization at 950
◦
C was similar to
that of coal char, varying from 1.7 mΩm to 3.4 mΩm [35].
Bazaluk et al. [
78
] investigated the properties of biocoke obtained at different car-
bonization temperatures and with varying amounts of biomass pellets for use in BFs and
non-BFs. It was found that the electrical resistance of carbon-bearing materials decreases

Metals 2022,12, 2005 20 of 38
with an increase in the carbonization temperature, which is associated with the forma-
tion and/or increase of carbon crystallites. The electrical resistance was in the range of
12.0–15.9 mΩm for 950 ◦C and 10.3–13.8 mΩm for 1100 ◦C.
The selected obtained results [
12
,
34
] are the basis for the approach to use biocoke as a
reductant to produce ferroalloys, and from the point of view of reducing the dependence on
fossil fuel and mitigating emissions associated with the metallurgical industry, in particular
the production of ferroalloys. Biocoke is also suitable for use in terms of such indicators as
the amount of FC of more than 85% and the VM content.
According to Surup et al. [
158
], charcoal can replace conventional carbon-bearing
reductants by more than 40% in the SAF, while complete replacement requires an additional
heat treatment of charcoal.
2.8.2. Recommendations for the Use of Secondary Carbon Bio-Carriers
In ferroalloy production, the application of torrefied biomass is challenging because it
does not achieve the required properties due to the low torrefaction temperature. However,
biocoke has good potential for use in a SAF. Figure 10 is a layout of the main pathway for
the use of secondary carbon bio-carriers in ferroalloy production.
Metals 2022, 12, x FOR PEER REVIEW 22 of 41
Figure 10. Main pathway for using secondary carbon bio-carriers in ferroalloy production.
There are several ways to use secondary carbon bio-carriers:
- Torrefied biomass to produce biocoke and further use of biocoke to produce
ferroalloys. At the same time, it is of interest to replace up to 50% of coal within the
coal blend;
- Subject torrefied biomass to carbonization to increase FC, reduce VM yield, and
achieve the required electrical resistance values close to that of conventional fuel.
Furthermore, after carbonization, secondary carbon bio-carriers can be used in the
SAF;
- Torrefied biomass, also after carbonization, can be subjected to subsequent
compaction, and then the resulting briquettes can be used to produce biocoke.
3. Modification of the Properties of Secondary Carbon Bio-Carriers
The following section gives an overview of treatment methods to enhance the
properties of secondary carbon bio-carriers for subsequent use in metallurgical processes.
3.1. Thermal Carbonization after Torrefaction
Carbonization is a slow pyrolysis process that occurs under an inert atmosphere with
temperatures > 400 °C, heating rates < 80 °C/min, and residence times of hours to days.
The carbonization of biomass after its torrefaction is less intense, making this process less
violent. This is explained by the fact that the main thermochemical conversation of
biomass occurs during torrefaction. As the temperature of biomass torrefaction increases
and after its subsequent carbonization, the yield of bio-oil and tar decreases. However,
this does not significantly affect the yield of the torrgas [159,160]. Carbonization of
torrefied biomass at a higher temperature is the most useful option. It achieves the best
values for the conversion process, efficiency, and quality of the final products. It should
be noted that despite the improvement in the properties of the final products, with
Figure 10. Main pathway for using secondary carbon bio-carriers in ferroalloy production.
There are several ways to use secondary carbon bio-carriers:
-
Torrefied biomass to produce biocoke and further use of biocoke to produce ferroalloys.
At the same time, it is of interest to replace up to 50% of coal within the coal blend;
-
Subject torrefied biomass to carbonization to increase FC, reduce VM yield, and
achieve the required electrical resistance values close to that of conventional fuel. Fur-
thermore, after carbonization, secondary carbon bio-carriers can be used in the SAF;
-
Torrefied biomass, also after carbonization, can be subjected to subsequent compaction,
and then the resulting briquettes can be used to produce biocoke.

Metals 2022,12, 2005 21 of 38
3. Modification of the Properties of Secondary Carbon Bio-Carriers
The following section gives an overview of treatment methods to enhance the proper-
ties of secondary carbon bio-carriers for subsequent use in metallurgical processes.
3.1. Thermal Carbonization after Torrefaction
Carbonization is a slow pyrolysis process that occurs under an inert atmosphere with
temperatures > 400
◦
C, heating rates < 80
◦
C/min, and residence times of hours to days.
The carbonization of biomass after its torrefaction is less intense, making this process less
violent. This is explained by the fact that the main thermochemical conversation of biomass
occurs during torrefaction. As the temperature of biomass torrefaction increases and after
its subsequent carbonization, the yield of bio-oil and tar decreases. However, this does not
significantly affect the yield of the torrgas [
159
,
160
]. Carbonization of torrefied biomass at a
higher temperature is the most useful option. It achieves the best values for the conversion
process, efficiency, and quality of the final products. It should be noted that despite the
improvement in the properties of the final products, with increasing temperature, the yield
of solids decreases, and the yield of bio-oil from both non-torrefied and torrefied biomass
increases [160].
Li et al. [
161
] studied the effect of different atmospheres, such as N
2
, CO
2
, 14% O
2
, or
NH
3
, during torrefaction on the elemental distribution, as well as the subsequent impact
on the pyrolysis process. The research showed that using CO
2
as a torrefaction atmosphere
has several advantages; for example, increasing the carbon content and reusing CO
2
, which
is a GHG.
Louwes et al. [
162
] investigated the differences between the fast pyrolysis of raw and
torrefied biomass. One of the findings was that torrefaction before fast pyrolysis could
be beneficial from an energy point of view, as well as having a higher solids yield and a
reduced bio-oil yield. The two-stage heat treatment proposed by the authors can be of
interest when the targeted product is solid.
3.2. Compaction
The compaction of biomass (also called granulation) involves the application of a
mechanical force to compact residues or waste biomass (sawdust, shavings) into uniformly
sized solid particles, such as pellets and briquettes. The goals of biomass compaction are to:
increase the volumetric energy density and the amount of carbon per unit volume, facilitate
storage and handling, reduce transport costs, and give the material a particular shape and
porosity [
37
,
163
]. Increasing the total energy density of torrefied biomass by compaction is
an important process for modifying its properties, because torrefaction of the raw biomass
results in mass loss and void formation.
There are two options for applying the compaction process [
164
] in industry; namely,
torrefaction before compaction (TOP) [
165
,
166
] and torrefaction after compaction (TAP) [
167
].
The compaction temperature and compaction pressure are important when pressing
torrefied biomass because the compaction of torrefied biomass is a more complex process
than the compaction of raw biomass. The choice of the binder for compaction is no less
important. There have been many studies about the influence of binder systems on the
quality of the resulting torrefied pellets. Regarding the use of the binder, organic binders
are of more interest compared to inorganic binders [168].
As for the study of the influence of parameters such as compaction temperature
or compaction pressure, Matsumura et al. [
169
] reported that, compared to untreated
wood, wood biomass under pressure to a size of at least 10 mm allows an increase in the
addition rate of up to 1.5% while preventing a decrease in the coke strength. Moreover, hot
compaction at a temperature of 200–350
◦
C, at which pyrolysis of wood biomass occurs,
prevents a drop in the coke strength. As a result of hot compaction at 200
◦
C, the density
of the pellets that were obtained was more than 60% higher than that of the pellets at
room temperature.

Metals 2022,12, 2005 22 of 38
Peng et al. [
165
] studied the effect of temperature and pressure on the quality of the
obtained torrefied pellets. The research results showed that a die temperature of 260
◦
C
with a compaction pressure of 187 MPa makes the torrefied pellets 1.24 times stronger than
the reference pellets.
Li et al. [
170
] studied the influence of the degree of torrefaction and compaction
temperature on the energy intensity and properties of the pellets. To obtain strong pellets,
the die temperature was raised to 170
◦
C with a residence time of 30 s. The research results
showed that it is necessary to use a high compaction pressure or high die temperature to
obtain a good density of pellets.
Chen et al. [
37
] comprehensively reviewed torrefaction and compaction processes.
Moreover, it was noted that torrefied pellets have good potential to replace coal in the
energy and metallurgical sectors.
Manouchehrinejad et al. [
171
] performed a technical and economic analysis of the
production of torrefied pellets using integrated torrefaction and compaction systems. This
paper considered both TOP and TAP options, and a comparison with conventional wood
pellets was made. The authors concluded that torrefied wood pellets could partially achieve
the necessary properties to replace coal.
3.3. Beneficiation of Secondary Carbon Bio-Carriers by Ash Removal
In addition to the carbonization or compaction of torrefied biomass, its modification by
ash removal is also beneficial for improving its properties. Charcoal is the most abundant
raw material for use in metallurgy. However, its highly porous structure, as well as its
content of alkali and alkaline earth metals, leads to an increase in reactivity, which makes
its use challenging.
There are several ways to achieve ash removal; for example, physical beneficiation
methods can be efficient, but to a limited extent [
172
]. Another way is to use acids,
alkalis, leachates, oxidants, and various alkalis and acid combinations. According to
Dhawan et al. [173] 80–90% demineralization and desulfurization can be achieved.
Iniesta et al. [
174
] studied different processing options for almond shells with acid and
basic solutions at room temperature at various times. It should be noted that the lowest ash
content and relatively small weight loss were achieved after treatment with H
2
SO
4
for 24 h
and 3 h, as well as after treatment with 3 h with sodium hydroxide (NaOH) and 3 h with
sulfuric acid (H2SO4).
Das et al. [
175
] conducted three different pre-treatment processes of bagasse (fibrous
material that remains after crushing sugarcane or sorghum stalks) used for deashing water
leaching, mild acid treatment with hydrochloric acid (HCl), and mild acid treatment with
hydrofluoric acid (HF). It was observed that the pre-treatment of bagasse led to changes in
the distribution of pyrolysis products due to a combination of changing organic components
and selective removal of inorganic ash elements. Mild acid treatment with HF effectively
reduces the ash content of biomass to minor levels.
Hussein et al. [
176
] studied the effect of heat treatment and acid washing on the
reactivity of charcoal. Charcoal samples were washed using various concentrations of HCl
to determine the optimal leaching conditions. As a result of the treatment, a significant
reduction in the reactivity was achieved due to a decrease in the content of inorganic
minerals, as well as an improvement in the carbon structure after subsequent calcination.
Oudenhoven et al. [
177
] investigated the removal of alkali and alkaline earth metals
from biomass by leaching with pyrolytic acids. The purpose of the research was to assess
the conditions of acid leaching of pine wood, bagasse, and straw that improve the technical
and economic indicators of the pyrolysis process. The ash removal process affects the yield
of the end products.
In addition, several studies have been dedicated to removing harmful metals by
washing them with water. For instance, Vamvuka and Sfakiotakis [
178
] investigated the
effect of washing with water on the thermal decomposition characteristics, reactivity, and

Metals 2022,12, 2005 23 of 38
kinetics of the three energy crops. Afterward, the potassium, phosphorus, sulfur, and
chlorine content in the ash decreased.
Deng et al. [
179
] washed six biomass types (wheat straw, rice straw, corn stalk, cotton
stalk, candlenut wood, and rice hull) with deionized water at different temperatures.
The results showed that potassium, sulfur, and chlorine contained in the biomass could
be effectively removed by washing. As the water temperature increases, the removal
efficiencies of potassium, amorphous silica, and ash increase for all six biomass types.
3.4. Utilization of Mineral Additives for Secondary Carbon Bio-Carriers
Compared to ash removal, the opposite method is the addition of mineral additives
during the compaction of torrefied biomass.
Because mineral substances have an important influence on the activation energy of
chemical reactions, they can change the reactivity of the secondary carbon bio-carriers
and the kinetic and thermodynamic parameters of metallurgical processes. For example,
with an increase in the content of positively charged catalytic oxides or salts (Fe, Ca, Mg,
Cu, Ba, Mn), the initial reaction temperature and the reaction activation energy decrease.
With an increase in the content of negatively charged catalytic oxides or salts (Si, Al, Ti),
the initial temperature of the reaction and the activation energy of the reaction increase.
Zinc chloride (ZnCl
2
) has been used to increase the porosity, specific surface area, and
adsorption capacity of biochar [
180
]. However, Mu et al. [
181
] investigated the effect of Zn
on the reactivity and strength of coke by means of Zn vapor adsorption. The results showed
that Zn could increase the CRI and decrease the CSR. These results are also consistent
with the conclusions made by Li et al. [
182
], that the use of certain mineral additives, i.e.,
compounds with Zn, has harmful effects on the many operations of metallurgical furnaces
and, therefore, cannot be recommended for application.
For some metallurgical processes, an increased CRI is not a disadvantage. In this case,
applying catalytic gasification can lower the initial gasification temperature and increase
the CRI, but the CSR is often reduced [
183
]. CRI can be increased, the activation energy
can be reduced, and the gasification rate can be increased by using alkali vapors due to the
catalytic effect [
184
]. At a low CRI, the catalytic effect is a strong and slight effect when
using highly reactive coke, as well as with increasing temperature.
Fe and Ca are promising catalysts for increasing the CRI in the conditions of the
thermal reserve zone of a BF [
185
]. There are several ways to process coke [
186
–
188
].
The choice of a suitable processing method can change the reaction regime to a more
homogeneous one at a lower reaction temperature, increasing the CSR.
The use of metal salts or metal oxides to modify the properties of biochar is a relevant
issue for research. With this modification method, the key biochar properties can be
changed, and as a result, its characteristics, including the adsorption capacity, catalysis
strength, and magnetism, can be improved. Consequently, it is possible to directly influence
the kinetic and thermodynamic parameters of reactions occurring in metallurgical processes
by using mineral additives. In this approach, the mineral catalytic index [
189
] can be
used as an indicator to measure and characterize the effect of mineral additives on the
reactivity of biochar briquettes, as well as on the kinetic and thermodynamic parameters of
metallurgical processes when using biochar briquettes in an EAF, SAF, iron ore sintering,
and SR. Industrial wastes are of particular interest as mineral additives.
Table 10 summarizes some effects of different modification techniques on the example
of different types of carbon sources and demonstrates the possibility of using this approach
to modify the properties of secondary carbon bio-carriers. It can be concluded that further
carbonization of the torrefied biomass improves its properties, making it more suitable
for use in certain metallurgical processes. The last three techniques allow changing the
properties depending on the requirements of the metallurgical process in which secondary
carbon bio-carriers can be used.

Metals 2022,12, 2005 24 of 38
Table 10.
Effect of different modification techniques on the example of different types of
carbon sources.
Parameters
Sources
CWT CWC TCR (Before
Compaction)
TCR (After
Compaction)
Bch285
(WAT),
[174]
Bch285
(AAT),
[174]
Switchgrass
(Unwashed),
[178]
Switchgrass
(Washed),
[178]
Coke CRC10 CRC20
Carbonization [190] Compaction [43] Beneficiation Mineral additives [186]
C (wt.%)
72.85 84.74
48.3 42.8 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
H (wt.%) 4.56 3.18 5.5 5.9 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
N (wt.%) 0.27 0.39 2.3 2.2 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
S (wt.%) 0.14 0.1 1.0 0.8 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
O (wt.%)
18.95
9.45 33.0 38.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
Ash (wt.%)
0.4 0.7 9.9 9.6 n.a. n.a. 8.5 2.9 n.a. n.a. n.a.
HHV
(MJ/kg)
28.83 32.32
n.a. 19.3 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
R n.a. n.a. n.a. n.a. 4.9 2.0 n.a. n.a. n.a. n.a. n.a.
DI150 15 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 84.5 84.0 81.8
CRI, % n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 23.3 24.8 33.2
CSR, % n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 66.8 68.0 58.6
JIS ReI n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 6.6 39.0 41.8
Inorganics (%):
K2O n.a. n.a. n.a. n.a. n.a. n.a. 11.9 0.86 n.a. n.a. n.a.
P2O5n.a. n.a. n.a. n.a. n.a. n.a. 4.5 1.0 n.a. n.a. n.a.
SO3n.a. n.a. n.a. n.a. n.a. n.a. 0.17 0.11 n.a. n.a. n.a.
Cl (ppm) n.a. n.a. n.a. n.a. n.a. n.a. 70 18 n.a. n.a. n.a.
n.a. is not available; CWT is cedar wood torrefied at 300
◦
C; CWC is cedar wood carbonized at 450
◦
C; TCR is
torrefied Canola residue; Bch285 (WAT) is biochar obtained after heat treatment at 285
◦
C (without acid treatment);
Bch285 (AAT) is biochar obtained after heat treatment at 285
◦
C (after acid treatment); CRC10 is calcium-rich coke
(10 wt.%); CRC20 is calcium-rich coke (20 wt.%); R is reactivity (normalized weight loss percentage min
−1
); C is
carbon; H is hydrogen; N is nitrogen; S is sulfur; O is oxygen.
4. Research Activities within EU Projects for the Use of Secondary Carbon Bio-Carriers
in Iron and Steelmaking
Some funded research projects within the EU, such as TORERO, GreenEAF, and
GreenEAF2, and their results have already been mentioned in this review. However, some
projects that are still being finished or currently being carried out should also be addressed,
as they focus on using secondary carbon bio-carriers in metallurgical processes. Their use
is considered only for selected processes within EU-funded projects. Nevertheless, these
projects are of great importance, as they lead not only to the development of breakthrough
technologies that can mitigate the impact on the environment, but also show the need to
find new ways to use secondary carbon bio-carriers.
As part of the project “Ultra-low CO
2
steelmaking” (ULCOS, 2004–2010, Grant Agree-
ment No. 515960) [
191
], the development of several technologies was planned; namely,
(a) blast furnace with top gas recycling; (b) new smelting reduction process (HIsarna);
(c) advanced direct reduction; (d) electrolysis of iron ores; (e) carbon capture and stor-
age; and (f) H
2
reduction. These technologies should reduce CO
2
emissions by at least
50% per ton of steel. Figure 11 provides a layout of the technologies within ULCOS.
Some developed technologies, such as top gas recycling and direct reduction, have been
commercially introduced.
Within the ULCOS project, the possibility of using sustainable biomass was also
considered. According to its properties, charcoal is a promising reducing agent with a
lower amount of sulfur and ash than conventional agents. One of the tasks was to find
ways to optimize the work of new technologies within ULCOS using charcoal. According

Metals 2022,12, 2005 25 of 38
to Quader et al. [
151
], the use of sustainable biomass has not been developed within the
framework of this project and is of interest for further research on the use of secondary
carbon bio-carriers in steelmaking. However, in [
121
], it was noted that 15% of theoretical
biomass potential is desirable to meet the biomass demand of only 30 plants across Europe,
which will have a negative effect. Therefore, the issue of using sustainable biomass requires
further work to solve the problem of the strategic use of biomass in Europe.
Metals 2022, 12, x FOR PEER REVIEW 27 of 41
Figure 11. Layout of ULCOS breakthrough technologies for reducing CO2 emissions [192].
Within the ULCOS project, the possibility of using sustainable biomass was also
considered. According to its properties, charcoal is a promising reducing agent with a
lower amount of sulfur and ash than conventional agents. One of the tasks was to find
ways to optimize the work of new technologies within ULCOS using charcoal. According
to Quader et al. [151], the use of sustainable biomass has not been developed within the
framework of this project and is of interest for further research on the use of secondary
carbon bio-carriers in steelmaking. However, in [121], it was noted that 15% of theoretical
biomass potential is desirable to meet the biomass demand of only 30 plants across
Europe, which will have a negative effect. Therefore, the issue of using sustainable
biomass requires further work to solve the problem of the strategic use of biomass in
Europe.
Another project entitled “Development of a Low CO2 Iron and Steelmaking
Integrated Process Route for a Sustainable European Steel Industry” (LoCO2Fe, RFCS
project, 2015–2018, grant agreement No. 654013) [193] coordinated by Tata Steel
Netherland Technology was focused on achieving a decrease in CO2 emissions by at least
35%. The object of this project was the HIsarna ironmaking technology, which can achieve
the necessary reductions in emissions, allowing partial replacement of coal with biomass.
Additionally, this technology minimizes the use of coal due to the maximum use of energy
in the reactor by balancing the energy between different parts of the reactor. Moreover,
Figure 11. Layout of ULCOS breakthrough technologies for reducing CO2emissions [192].
Another project entitled “Development of a Low CO
2
Iron and Steelmaking Inte-
grated Process Route for a Sustainable European Steel Industry” (LoCO2Fe, RFCS project,
2015–2018
, grant agreement No. 654013) [
193
] coordinated by Tata Steel Netherland Tech-
nology was focused on achieving a decrease in CO
2
emissions by at least 35%. The object
of this project was the HIsarna ironmaking technology, which can achieve the necessary
reductions in emissions, allowing partial replacement of coal with biomass. Additionally,
this technology minimizes the use of coal due to the maximum use of energy in the reactor
by balancing the energy between different parts of the reactor. Moreover, when carbon
capture and storage (CCS) are used along with HIsarna, CO
2
emissions can be reduced by
up to 80%.
Another EU-funded project called SECTOR (production of solid sustainable energy
carriers, 2012–2015, grant agreement No. 282826 [
194
,
195
]) was coordinated by the German
DBFZ (Deutsches Biomasseforschungszentrum gemeinnützige GmbH). It was focused
on studying torrefied wood-based, agricultural biomass, or energy crops as sustainable
energy carriers. The project included the development of standardized analysis and test-

Metals 2022,12, 2005 26 of 38
ing methods to assess the efficiency of transportation, storage, handling, and end-use of
torrefied biomass. Compaction options such as pelleting and briquetting were studied.
Opportunities for optimization by integrating the torrefaction process into existing pro-
duction facilities were considered. However, in this project, torrefied biomass was only
considered for use in the energy sector in terms of cofiring in coal plants, gasification, and
small-scale combustion. One of the findings of this project was that it is feasible for the
torrefaction technology to be commercially deployed. However, implementation limits
should be overcome to utilize torrefied biomass.
The ongoing project “TORrefying wood with Ethanol as a Renewable Output: large-
scale demonstration” (TORERO) (grant agreement No 745810) [
67
], has already been briefly
discussed in the review. It is worth adding that the project’s most important impact is
expected to be when the torrefaction plant is launched in July 2022. The production of
80 million liters of biofuel to replace fossil fuels is planned (which allows the reduction of
CO
2
emissions by 150,000 tons per year), as well as 50,000 tons of biochar to replace coal
for PCI operation at the BF (which allows the reduction of CO
2
emissions by 100,000 tons
per year) of the ArcelorMittal site in Ghent (Belgium).
Regarding the PCI technology for BF operation and the replacement of coal with more
sustainable carbon carriers, an EU-funded project that was run within the RFCS entitled
“Improved coal combustion under variable BF conditions” (IMPCO, 2012–2015 RFSR-CT-
2012-00002 [
196
]) coordinated by the Swedish research center SWERIM (formerly known as
SWEREA MEFOS) was focused on this issue. Generally, the IMPCO project aimed to reduce
the amount of coke consumption in the BF by increasing the PCI rate by using alternative
carbon carriers. Two types of coals, lignite coke, lignite coal, petroleum coke, and torrefied
biomass were studied as carbon carriers. It was found that the optimal parameters of the
BF operation were maintained when using a blend of 20% torrefied biomass and 80% coal.
Within the framework of the RFCS project “Alternate carbon sources for sintering of
iron ore” (ACASOS, 2007–2010, RFSR-CT-2007-00003, [
197
]) coordinated by the German
VDEh Betriebsforschungsinstitut (BFI), attention was focused on the iron ore sintering
process and the use of alternative carbon carriers to replace coke breeze. Olive pits, wood
pellets, sunflower husks, BF dust and sludge, anthracite, and petroleum coke were chosen,
pre-treated, and tested by iron ore sintering during pilot iron ore sinter plant trials. The use
of 10% wood pellets and 5–15% crushed olive pits deteriorated the process productivity
and quality indicators. The use of up to 30% petroleum coke and up to 20% anthracite had
a low impact on the quality indicators of the sinter.
Another project addressing the iron ore sintering process and the use of alternative
carbon carriers is an ongoing project entitled “Towards a zero CO
2
Sintering” (TACOS,
2019–2022, RFCS Project No. 843722 [
198
]) coordinated by the Belgium research center CRM
(Centre de Recherches Métallurgiques). The research aims to evaluate solutions that provide
significant reductions in CO
2
emissions, as well as reductions in other key pollutants during
the sintering process. In this project, one of the methods used to achieve this goal is the use
of alternative carbon carriers, namely, biomass with or without pre-treatment. Processes
such as gasification, hydrothermal conversion, torrefaction, and pyrolysis are provided
as pre-treatment.
The aim of the RFCS project “Recycling of industrial and municipal waste as slag
foaming agent in EAF” (RIMFOAM, 2014–2017, RFSR-CT-2014-00008 [
199
]) coordinated
by SWERIM was to evaluate and utilize waste mixtures containing both metal oxides and
hydrocarbons as slag foaming agents in an EAF. This project investigated the possibility
of replacing conventional carbon sources with 20% substitutes. The biomaterials used in
the study were olive kernels and sawdust pellets, which were injected with co-injection at
Arcelor Mittal Maizieres Research in France. The best foaming results were obtained with
rubber powder and petroleum coke (the other substitutes tested within this project), and
good foaming was also observed with sawdust pellets.
In another ongoing project, “Implementation of a smart RETROfitting framework in
the process industry towards its operation with variable, biobased and circular FEEDstock”

Metals 2022,12, 2005 27 of 38
(RETROFEED, 2019–2023, grant agreement No. 869939 [
200
]), the main focus is to enable
the use of an increasingly variable, bio-based, and circular feedstock in process industries.
In the specific case study for EAF steel production, a tailored injection system for biochar
has been developed and installed in the steel factory of Ferriere Nord in Italy. In the future,
it is planned to evaluate the capability of biochar to promote foaming and to contribute to
the energy input of the process.
One project regarding ferroalloy production (specifically Mn-alloys) is concentrated
on the use of loose biomass to obtain biocoke. This ongoing Norwegian-funded project is
called “BioCoke4FAI-Bio-Coke for Ferroalloys Industry Production” (2021–2023 [
201
]). The
idea of the project is based on the production of a non-conventional reductant using blends
of coal and biomass (partial replacement) and testing its suitability on a pilot scale.
5. Aspects Being Considered for Evaluation of the Environmental Impacts Due to
Substitution by Biomass
Biomass-based fuels are not always carbon neutral, and they may even show higher
greenhouse gas emissions across their life cycle than with respect to fossil fuels [
202
].
Several studies have documented the potential for a lower carbon footprint associated with
biomass-based fuels [
64
,
75
]; yet to guarantee an actual reduction of environmental damage
across different impact categories, several aspects must be taken into account.
-
Firstly, a lifecycle perspective of the assessment should be ensured. This is generally
accomplished by using the Life Cycle Assessment (LCA) methodology. Carbon foot-
prints of biomass-based fuels are strongly sensitive to upstream processes: cultivation
practices, transports, and further treatments (e.g., compaction, torrefaction, and the
addition of mineral additives) [
64
]. These activities must be optimized as well to
ensure a significant abatement of greenhouse gas emissions. In particular, cultivation
practices are associated with the consumption of energy and materials (e.g., fertilizers
may lead to an additional emission of 0.14–0.55 kg CO
2
-eq per kg of biofuel), and
they may be associated with land use and carbon stock changes (direct and indirect)
which may lead to a drastic increase of greenhouse gas emissions: 0.6–4.0 kg CO2-eq
per kg of biofuel [
203
]. The production and processing of biomass feedstocks are
associated with the variability of LCA results, both considering the only carbon foot-
print [
75
,
195
] and other impact categories [
204
]. In addition, the final applications
of biomass-based fuels in steelmaking plants must be optimized as well, which has
been the focus of several studies documented in the present paper. A comprehensive
environmental analysis must account for possible adverse effects due to the intro-
duction of biomass-based fuels: productivity decreases and an increase in total fuel
consumption, as outlined in Section 2.2.1. These effects mean that a higher quantity
of biofuel is necessary to replace 1 kg of fossil fuel. Therefore, a 1:1 substitution ratio
should be adjusted according to the actual plant configuration.
-
Biomass may be obtained by either agricultural wastes or by cultivations.
Norgate et al.
[
205
]
clarified that biochar is considered to be renewable due to the much shorter carbon
cycle (5–10 years) with respect to fossil fuels (around 100 million years). Still, the
biomass carbon cycle is not null; therefore, the availability of biomass needed for
steelmaking plants should be considered for a correct evaluation. In addition, to
minimize transport, biomass availability should be evaluated for specific regions.
Kamal Baharin et al. [
14
] showed that Malaysia disposes of most of its biomass waste
without using it as fuels, fertilizers, or animal feedstock; therefore, there is a potential
to convert it into biocoke. National analyses for Sweden [
28
] and Finland [
118
] showed
sufficient and available resources to cover the national demand for biomass use in
steelmaking. Piketty et al. [
206
] reported that Brazil could be a remarkable actor in
feeding the global supply chain for charcoal. Instead, Norgate et al. [
119
] performed an
assessment and concluded that 50–150 Mha of land is required as biomass plantation
areas to substitute 47% of the fossil carbon used to produce steel by the integrated
route, through the use of charcoal. Comparing this value with the global productive

Metals 2022,12, 2005 28 of 38
forest plantation area (264 Mha), it emerges that an appreciable amount of land can
be used for biomass cultivation. Finally, Mandova et al. [
121
] further led this kind of
analysis by accounting for both resources and supportive policies to identify where
a significant potential to harvest biomass sourced in a sustainable way is present.
They found that Canada, Sweden, China, the USA, and France were the most suitable
countries. It should be kept in mind that higher demand for crops may lead to indirect
land use changes, e.g., deforestation, and intensification, which is linked to additional
greenhouse gas emissions [207].
-
LCAs should also evaluate different potential impact categories to avoid burden-
shifting risk, i.e., decreasing carbon footprint while increasing impacts in other cat-
egories. Bio-based processes are generally found to increase eutrophication and
acidification impacts due to the agricultural phase [
208
]. Potential concerns are also
related to water use for biomass cultivation [209].
-
Finally, linked to the matter of biomass availability, further environmental and societal
aspects should be accounted for. If biomass is collected from dedicated cultivations,
conflicts on land use for other uses (e.g., food and renewable energy production)
should be accounted for [
119
]. If biomass is collected from agricultural wastes, the
alternative fate of the waste is important to determine the net environmental impacts:
e.g., if wastes would otherwise have been incinerated with energy recovery or used
for landfill with large methane emissions. Consequential LCA approaches can account
for counterfactual uses of different inputs from attributional models [210].
A comprehensive LCA of bio-based carbon carriers’ integration can outline differences
in potential environmental impacts with respect to other improvement measures suggested
by the literature; e.g., hydrogen-based options [211].
6. The Role of Digital Tools in Intensifying Use of Secondary Carbon Bio-Carriers
Most processes involved in the metallurgical production chains are very complex and
challenging, e.g., in terms of required material volumes, energy demands, environmental
conditions, and constraints. Therefore, they come equipped with sophisticated monitoring
and control systems, which are the results of a continuous evolution mostly driven by
targets related to product quality, energy efficiency, emissions control, and health and safety
requirements for personnel. These systems are generally tuned to cope with “standard”
input materials with no or minimal usage of secondary carbon bio-carriers; they are surely
suitable to ensure safety requirements, as well as emissions control when secondary carbon
bio-carriers are used, but further efforts might be required when their use is intensified
to pave the way to their optimal exploitation. For instance, a recent study concerning
the iron ore sinter process [
212
] addressed, among other topics, the issues concerning
process control implementation when alternative fuels, such as biomass, are adopted
with the aim of maximizing sintering performance and minimizing energy consumption
and environmental impacts. Advanced process monitoring solutions, such as the ones
based on soft sensors enabled by artificial intelligence approaches [
213
], are envisaged to
support process capability to dynamically adapt to the gradual and increased introduction
of alternative carbon carriers.
Further support for the intensified use of alternative carbon carriers can be provided
by advanced modeling and simulation tools, and approaches that can be used for prelimi-
nary techno-economic assessment and selection of the most promising use scenarios. For
instance, process models based on mass and energy balances, thermodynamic equilibrium,
and involved reactions (e.g., flowsheeting-based models) are already generally used to
study the effects and environmental/energy impacts on processes and related products
of different operating conditions or process modifications, via the EAF route [
214
]. How-
ever, so far, they have not been applied to investigations of the use of non-fossil fuels and
renewable C-sources via the EAF route. Moreover, the unstoppable digitalization process
of the steel industry provides a large volume of available process data, which can also be
exploited for process modeling and simulation for the abovementioned purposes. In this

Metals 2022,12, 2005 29 of 38
sense, promising opportunities are provided by the hybrid Physics-Guided Machine Learn-
ing (ML) techniques, which integrate physical knowledge in ML-based models design [
215
]
by merging available process knowledge and purely data-driven ML methods [216].
7. Conclusions and Outlook
This review provides an overview of the use of secondary carbon bio-carriers, i.e.,
bio-based sustainable resources in metallurgical processes for iron and steelmaking, as
well as for ferroalloy production. Despite the many studies on biomass torrefaction, the
integration of the use of biomass for many processes in iron and steelmaking remains an
open issue. Therefore, this review discusses the application of torrefied biomass as the
main carbon source or to produce other secondary carbon bio-carriers. Because torrefied
biomass has limitations due to its properties, one of the promising options is to use it in
cokemaking for biocoke production. The amount of torrefied biomass used in the coal
blend may vary depending on the requirements of the particular route. The production
and application of secondary carbon bio-carriers as a partial replacement or the complete
replacement of conventional fossil fuels are technologically achievable and can be based on
known studies and practices. The use of secondary carbon bio-carriers has been considered
for four main pathways: BF/BOF, scrap/EAF, DRI/EAF, and SR/BOF. Despite the most
stringent requirements for carbon carriers for BFs, the use of biocoke may have significant
future use in scrap/EAF, DRI/EAF, and SR/BOF routes. Furthermore, the use of biocoke is
promising in a SAF to produce ferroalloys.
Some methods to modify the properties of secondary carbon bio-carriers, such as car-
bonization, compaction, beneficiation by ash removal, and utilization of mineral additives
for secondary carbon bio-carriers, have also been studied by several research groups. The
last two methods are opposites of each other; however, they may impact the kinetic and
thermodynamic parameters of reactions occurring in metallurgical processes.
An analysis of fulfilled or ongoing projects within the EU showed that secondary
carbon bio-carriers are not only promising substitutes for conventional fuels and reductants
in the future, but are have already been proven in some metallurgical processes. Moreover,
the discussion of these projects revealed that the integrated use of secondary carbon bio-
carriers, namely, torrefied biomass and biocoke, has not been generally suggested and
studied for the four steelmaking routes.
In terms of process, the integrated use of secondary carbon bio-carriers is an innova-
tive approach that requires adapting already existing technologies used in metallurgical
production. Within the EU steel sector, the use of secondary carbon bio-carriers represents
one technological pathway to push the decarbonization of the iron and steelmaking pro-
cesses to reach the climate goal of zero net emission steelmaking processes by 2050. The
efforts to implement secondary carbon bio-carriers are strongly promoted by the Clean
Steel Partnership (CSP), a public-private partnership comprising all relevant stakeholders
from the EU steel sector and scientific experts related to the field. The CSP is coordinated
by the European Steel Technology Platform (ESTEP) and supported by the European Steel
Association EUROFER. A roadmap (Strategic Research and Innovation Agenda, SRIA)
developed by the CSP considers secondary carbon bio-carriers as one important energy
source on the way to a sustainable and decarbonized future steel industry. Based on the
SRIA of the CSP, research initiatives (funded and non-funded) will be necessary in the
future to increase the quantity and quality of secondary carbon bio-carriers for enhanced
use in metallurgical processes.
This review provides a basis for scientific development and future research on inte-
grating biomass in different metallurgical processes. It should be noted that biomass is
well researched for use in biocoke production, sintering, CCAs, PCI, and BFs. However,
applications in SR processes, EAFs, and SAFs should be further studied to achieve new and
optimal results. Additionally, the development and implementation of the use of biomass
in steel production are highly dependent on preconditions, such as sufficient, sustainable
domestic biomass resources, supply chain features, and supportive national policies.

Metals 2022,12, 2005 30 of 38
Author Contributions:
Conceptualization, L.K., J.R. and J.S.; writing—original draft preparation,
L.K. and J.R.; writing—review and editing, J.S., C.B., D.R., T.E., F.C., C.T., N.J., D.S., K.P. and V.C.;
visualization, L.K. and J.R.; supervision, J.S. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
The authors gratefully acknowledge the funding support of K1-MET GmbH,
a metallurgical competence center. The research programme of the K1-MET competence center is
supported by COMET (Competence Center for Excellent Technologies), the Austrian programme for
competence centers. COMET is funded by the Federal Ministry for Climate Action, Environment,
Energy, Mobility, Innovation and Technology, the Federal Ministry for Labour and Economy, the
Federal States of Upper Austria, Tyrol, and Styria, as well as the Styrian Business Promotion Agency
(SFG) and the Standortagentur Tyrol. Besides the public funding from COMET, the research is
partially financed by scientific and industrial partners. Furthermore, Upper Austrian Research GmbH
supports K1-MET. The authors are also grateful to the Reviewers for their insightful comments and
efforts in improving the manuscript’s text.
Conflicts of Interest: The authors declare no conflict of interest.
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