New Trends in the Application of Carbon-Bearing Materials in Blast Furnace Iron-Making

Abstract

The iron and steel industry is still dependent on fossil coking coal. About 70% of the total steel production relies directly on fossil coal and coke inputs. Therefore, steel production contributes by ~7% of the global CO2 emission. The reduction of CO2 emission has been given highest priority by the iron- and steel-making sector due to the commitment of governments to mitigate CO2 emission according to Kyoto protocol. Utilization of auxiliary carbonaceous materials in the blast furnace and other iron-making technologies is one of the most efficient options to reduce the coke consumption and, consequently, the CO2 emission. The present review gives an insight of the trends in the applications of auxiliary carbon-bearing material in iron-making processes. Partial substitution of top charged coke by nut coke, lump charcoal, or carbon composite agglomerates were found to not only decrease the dependency on virgin fossil carbon, but also improve the blast furnace performance and increase the productivity. Partial or complete substitution of pulverized coal by waste plastics or renewable carbon-bearing materials like waste plastics or biomass help in mitigating the CO2 emission due to its high H2 content compared to fossil carbon. Injecting such reactive materials results in improved combustion and reduced coke consumption. Moreover, utilization of integrated steel plant fines and gases becomes necessary to achieve profitability to steel mill operation from both economic and environmental aspects. Recycling of such results in recovering the valuable components and thereby decrease the energy consumption and the need of landfills at the steel plants as well as reduce the consumption of virgin materials and reduce CO2 emission. On the other hand, developed technologies for iron-making rather than blast furnace opens a window and provide a good opportunity to utilize auxiliary carbon-bearing materials that are difficult to utilize in conventional blast furnace iron-making.

Full text

minerals
Review
New Trends in the Application of Carbon-Bearing
Materials in Blast Furnace Iron-Making
Hesham Ahmed 1,2
1Minerals and Metallurgical Engineering (MiMeR), Luleå University of Technology, 971 87 Luleå, Sweden;
Hesham.ahmed@ltu.se; Tel.: +46-762-364-686
2Central Metallurgical Research and Development Institute (CMRDI); P.O. Box 87, Helwan,
11421 Cairo, Egypt
Received: 11 October 2018; Accepted: 29 November 2018; Published: 1 December 2018


Abstract:
The iron and steel industry is still dependent on fossil coking coal. About 70% of the total
steel production relies directly on fossil coal and coke inputs. Therefore, steel production contributes
by ~7% of the global CO
2
emission. The reduction of CO
2
emission has been given highest priority by
the iron- and steel-making sector due to the commitment of governments to mitigate CO
2
emission
according to Kyoto protocol. Utilization of auxiliary carbonaceous materials in the blast furnace and
other iron-making technologies is one of the most efficient options to reduce the coke consumption
and, consequently, the CO
2
emission. The present review gives an insight of the trends in the
applications of auxiliary carbon-bearing material in iron-making processes. Partial substitution of top
charged coke by nut coke, lump charcoal, or carbon composite agglomerates were found to not only
decrease the dependency on virgin fossil carbon, but also improve the blast furnace performance
and increase the productivity. Partial or complete substitution of pulverized coal by waste plastics
or renewable carbon-bearing materials like waste plastics or biomass help in mitigating the CO
2
emission due to its high H
2
content compared to fossil carbon. Injecting such reactive materials
results in improved combustion and reduced coke consumption. Moreover, utilization of integrated
steel plant fines and gases becomes necessary to achieve profitability to steel mill operation from
both economic and environmental aspects. Recycling of such results in recovering the valuable
components and thereby decrease the energy consumption and the need of landfills at the steel
plants as well as reduce the consumption of virgin materials and reduce CO
2
emission. On the
other hand, developed technologies for iron-making rather than blast furnace opens a window and
provide a good opportunity to utilize auxiliary carbon-bearing materials that are difficult to utilize in
conventional blast furnace iron-making.
Keywords:
alternative reducing agent; iron-making; blast furnace; biomass; waste plastic; in-plant
fines; recycling; CO2emission
1. Introduction
Iron and steel-making sector is one of the most important sectors due its great impact on the
global growth and economy. The production rate of steel has sharply increased in the recent years [
1
].
However, the iron- and steel-making sector is one of the highest energy and carbon-consuming
sectors [
2
]. About 70% of the total steel production relies directly on coal and coke inputs. Around
1.2 billion tonnes of coal are used globally for steel production, which is around 15% of the total
coal consumption worldwide, which explains the high contribution of the sector in the global CO
2
emission [
3
]. The ore based steel-making units like sintering, coke-making, and blast furnace (BF)
facilities contribute together about 90% of the sector emission [
4
]. Recently, the reduction of CO
2
Minerals 2018,8, 561; doi:10.3390/min8120561 www.mdpi.com/journal/minerals

Minerals 2018,8, 561 2 of 20
emission has been given the highest priority within the iron and steel sector due to the commitment of
governments to mitigate CO2emission according to Kyoto protocol [5].
Therefore, an increasing attention has been recently paid on increasing the replacement rate of
coke by more environmentally friendly alternative sources. Injection of pulverized coal into the BF is
one of the most promising options to reduce the coke consumption. The efficient utilization of in-plant
generated gases and fines as a source of heat and reducing agent can greatly enhance the overall
efficiency of steel industry. The partial substitution of virgin fossil carbon, namely coal and coke,
with H
2
-rich carbon-bearing waste materials like waste plastic and renewable and neutral carbon like
biomass products represents one of few choices which could be in short and middle terms introduced
to reduce the dependency on fossil carbon and reduce the CO2emission [6].
Attempts to decrease dependency of metallurgical coke and consequently reduce the CO
2
emission are for large extent based on the following approaches; (1) substituting coke with H
2
-rich
carbon-bearing materials; (2) producing agglomerates from secondary resources; and (3) shifting the
iron oxide reduction process toward lower carbon utilization
Continuous development connected to reducing coke consumption in, for example, the BF has
been always under investigation. Such development resulted in a decrease in coke consumption by
~60% since 1960 [
7
]. Coke has been partially replaced by other alternative carbon sources (pulverized
coal, natural gas, etc.) through the BF tuyeres over years and such replacement is now practiced in
all modern BFs. Injection of other carbon sources including top gas of different processes, such as
coke-making and steel-making, as well as carbon-bearing wastes has been tried, and even practiced
in some cases [
8
]. Today, coke consumption is in the range of 286–320 kg/tHM and pulverized coal
injection is in the range of 170–220 kg/tHM at the majority of the modern BFs [6].
The possibility to further reduce energy consumption and CO
2
emission in the BF has been showed
through use of reactive coke [
9
–
11
], ferro-coke [
12
,
13
], and coal composite agglomerate with or without
the content of bio-coal [
14
–
16
]. Increased reactivity results in lowering the temperature in the furnace
shaft which consequently leads to reduced coke consumption in the BF [
17
,
18
]. Further development
connected to cutting or at least minimizing the CO
2
emission by means of reducing the dependency on
primary fossil carbon sources is still required due to the pressure set by the governments relevant to
environmental regulations and post-Kyoto requirements. Although, there exists a great deal of research
papers reporting on reducing coke consumption and, consequently, reduce CO
2
emission, there are a
few that summarize the most recent research relevant to the applications of carbon-bearing materials,
including renewable and waste carbon-bearing sources in the iron-making sector. The paper provides
relevant insights into development of new trends in the applications of carbon-bearing materials in
iron- and steel-making.
The strategy adopted in the present review is as follows:
•
The review starts by a brief description of the existing technologies of iron-making, including BF
and alternative technologies.
•
Conventional reducing agents (mainly coke) including production, its role in the BF iron-making
and the most important required properties.
•
Description of materials that have reduction potential (for example; carbon rich in-plant fines,
waste plastic, and bio-based carbon materials)
2. The Making of Iron: An Overview
Iron ore reduction is the conversion of iron oxide minerals to metallic iron. There are several units
in which the iron oxide can be converted into corresponding metallic form. The most common, and the
one with highest production rate, is the BF which is basically dependent on high-quality metallurgical
coke. In countries where coking coal is not available, a great interest was directed toward developing
an iron ore reduction process which is independent of metallurgical coke. Other iron-making processes
are for example rotary hearth furnace (RHF), shaft furnace or fluidized bed. They differ in nature
of iron ore used, their physical and chemical properties, reducing agent, type of fuel used and even

Minerals 2018,8, 561 3 of 20
sometime the process concept. Additionally, the produced sponge iron differs from one to another.
Table 1shows some commercial iron-making processes and their feed requirements [19].
Table 1. Some commercial iron-making processes and their feed requirements.
Furnace Type Process Raw Material Requirement Product
Iron Ore Reductant
Shaft furnace
BF Sinter, pellets Coke and coal Molten iron
MIDREX Pellets, lump Natural gas, syn gas
Solid DRI *
HyL Pellets, lump
Fluidized bed Finex Fines Coal
FINMET Natural gas
Rotary kiln SL/RN Lump, pellets Coal and recycled char
RHF FASTMET Composite pellets coal
* DRI stands for direct reduced iron.
In this section, a brief description of the most common raw materials, iron-making
technologies/routes will be given.
2.1. Raw Materials
The raw materials for ore based steelworks can be classified into four categories: (1) iron ore,
(2) fluxes, (3) reductants and fuels, and (4) reverts. The characteristics of these materials strongly affect
the process performance and the product quality.
2.1.1. Iron Ores
Iron ores include hematite, magnetite and goethite or mixture of them. Based on the shape, particle
size and pretreatment, iron ores could be in form of iron ore lump, concentrate, pellets, or sinter. Iron
ore lumps are roughly in the size range of 6 to 30 mm and can be charged directly to the BF. Iron ore
lump is considered the lowest cost iron bearing material for the BF burden materials. In most cases
and, to improve the iron-making process, the ores are upgraded through a series of crushing, milling,
flotation, and magnetite separation processes. A concentrate with less gangue and higher Fe content is
produced. The concentrate is either sintered or pelletized. Additives can be added during process to
control the product composition for a specific purpose. If the ore concentrate contains Fe
2+
, in addition
to sintering, it is also oxidized to Fe3+ during the induration process [20–22].
2.1.2. Carbon-Bearing Material
Carbon-bearing materials are used as chemical reducing agents and as source of energy in the
iron-making processes. Coke is the major reductant in iron-making BF. In BF, Coke lumps with size
25–80 mm is charged in layers along with the iron burden. Coke is produced by carbonization of a
mixture of coking coals in specific facility called coke oven. In this process, selected coals are crushed
and ground into fine powder. The mix is then charged to the coke oven and the oven is heated to
elevated temperature (approximately 1100
◦
C) in an oxygen deficit atmosphere. The coal is coked
and most of the volatiles are released leaving behind a carbonaceous material with more than 90%
solid carbon. The produced coke is then cooled and screened into the desired size fraction. The highly
mechanical strength produced coke with high energy value provides the required heat, reducing gases,
permeability, and the mechanical support required in the BF [20–22]. Other carbon-bearing materials
that are today used in iron-making processes are coal, oil, natural gas and other hydrocarbons.
2.1.3. Fluxes
Fluxes are materials that are added to the iron burden in minor quantities during processing steps
to adjust its physical and chemical properties to enhance and ensure smooth process performance and

Minerals 2018,8, 561 4 of 20
high-quality product. Additives are added to iron ore during sintering or pelletization to ensure a
sinter/pellets with a good physical properties and high reducibility. Limestone is very common flux
and it is used in iron ore sintering. It has a strong water adhering ability which makes it good for
granulation of the sinter raw mix and, therefore, improves the sinter bed permeability and increases
the productivity. Dolomite which is basically calcium magnesium carbonates provides the MgO for
the BF slag formation. Olivine is also used as a flux in the iron-making processes. Using olivine as the
MgO source shows better sinter strength and productivity than using dolomite. Moreover, to maintain
the BF slag basicity at desired level, silica sand is usually added to the sinter raw mix and brought into
the furnace within the sinter ore [20–22].
2.1.4. Reverts
During the iron and steel-making processes and other consequent processes dusts, sludges, slags,
scales and slurries are produced. These residues in most cases (depending on the process) contain
valuable carbon and iron and are worth to recover. However, their chemical and physical properties
may not be favored by the process. In some cases, they require pretreatment to make their recovery
possible [20–22].
2.2. Methods of Iron and Steel-making
There are four basic routes commercially practiced for iron and steel production [23] (Figure 1):
•
The BF and the basic oxygen furnace (BOF) route; in this route coke and coal are the main carbon
sources. Through this route approximately 70% of the world steel is being produced.
•
Recycling of scrap through melting in electric arc furnace (EAF); through this route about 25% of
the world steel is produced. Therefore, this route is considered the second important route for
steel production.
•
The direct reduction (DR) followed by smelting in EAF; by this route ~5% of the world steel is
being produced and the most common used carbonaceous material in this case is natural gas.
•
The smelting reduction followed by BOF; through this route only ~0.4% of the world steel is being
produced. In this route neither ore preparation nor coking are needed.
Minerals 2018, 8, 561 4 of 20
Fluxes are materials that are added to the iron burden in minor quantities during processing
steps to adjust its physical and chemical properties to enhance and ensure smooth process
performance and high-quality product. Additives are added to iron ore during sintering or
pelletization to ensure a sinter/pellets with a good physical properties and high reducibility.
Limestone is very common flux and it is used in iron ore sintering. It has a strong water adhering
ability which makes it good for granulation of the sinter raw mix and, therefore, improves the sinter
bed permeability and increases the productivity. Dolomite which is basically calcium magnesium
carbonates provides the MgO for the BF slag formation. Olivine is also used as a flux in the iron-
making processes. Using olivine as the MgO source shows better sinter strength and productivity
than using dolomite. Moreover, to maintain the BF slag basicity at desired level, silica sand is usually
added to the sinter raw mix and brought into the furnace within the sinter ore [20–22].
2.2.4. Reverts
During the iron and steel-making processes and other consequent processes dusts, sludges,
slags, scales and slurries are produced. These residues in most cases (depending on the process)
contain valuable carbon and iron and are worth to recover. However, their chemical and physical
properties may not be favored by the process. In some cases, they require pretreatment to make their
recovery possible [20–22].
2.3. Methods of Iron and Steel-making
There are four basic routes commercially practiced for iron and steel production [23] (Figure 1):
• The BF and the basic oxygen furnace (BOF) route; in this route coke and coal are the main
carbon sources. Through this route approximately 70% of the world steel is being produced.
• Recycling of scrap through melting in electric arc furnace (EAF); through this route about
25% of the world steel is produced. Therefore, this route is considered the second important
route for steel production.
• The direct reduction (DR) followed by smelting in EAF; by this route ~5% of the world steel
is being produced and the most common used carbonaceous material in this case is natural
gas.
• The smelting reduction followed by BOF; through this route only ~0.4% of the world steel
is being produced. In this route neither ore preparation nor coking are needed.
Figure 1. Schematic diagram of ore based steel-making routes (adopted with permission from [23]).
Figure 1. Schematic diagram of ore based steel-making routes (adopted with permission from [23]).

Minerals 2018,8, 561 5 of 20
BF is the most common technology to produce iron with a share of ~70% of the total world steel
production. In BF worldwide produced about 1155 million tonnes compared to 75 million tonnes via
the DRI process [
24
]. For the foreseeable future and due to its high efficiency from both heat and mass
exchange points of view, BF will continue to be the main iron-making reactor. The BF is an enormous
vertical steel structure lined with refractory bricks. BF is a counter current heat exchanger and chemical
reactor in which the iron bearing materials, carbon source, and fluxes are charged from the top and
blast (pre-heated oxygen enriched air) is blown from the bottom. It takes 6–8 h for the charge material
to descend through the furnace while it takes only 6–8 s for the blown blast to reach the furnace top.
Until the 18th century charcoal was the only reductant/fuel used in BFs, then coke (after invention
in 1709) gradually replaced charcoal and BFs have grown considerably. The hearth diameter was 4–5 m
with annual production rate of 100,000 t hot metal mostly from lump ore. Nowadays, BFs have hearth
diameter up to 14–15 m with annual production rate of 3–4 million tonnes of hot metal. The largest
known BF nowadays has an inner volume of 5800 m
3
and annual production of 5.65 million tonnes
HM [
21
]. The burden materials have changed from lump ore to more efficient materials, like sinter
and/or pellets. The reductant materials have developed as well from 100% coke based operation to
use other injectant materials through tuyeres. Attempts are also made to charge alternative reducing
agents from the top along with burden materials like carbon composite agglomerates, etc. [
16
]. Modern
BFs favor high Fe content in ore burden. Higher grade of iron ore burden can be realized after physical
beneficiation process. Upwards of 70–80% of the modern BFs all over the world use sinter as the
iron-bearing material while other BFs in Europe apply 100% iron ore pellets.
In a typical modern BF, the furnace is filled with alternating layers of coke and iron ore (sinter
and/or pellets). Hot blast (compressed air) is blown into the BF through tuyeres. The hot blast gasifies
coke and other carbon-bearing materials.
Figure 2shows the major inputs and outputs of a typical modern BF.
Minerals 2018, 8, 561 5 of 20
BF is the most common technology to produce iron with a share of ~70% of the total world steel
production. In BF worldwide produced about 1155 million tonnes compared to 75 million tonnes via
the DRI process [24]. For the foreseeable future and due to its high efficiency from both heat and mass
exchange points of view, BF will continue to be the main iron-making reactor. The BF is an enormous
vertical steel structure lined with refractory bricks. BF is a counter current heat exchanger and
chemical reactor in which the iron bearing materials, carbon source, and fluxes are charged from the
top and blast (pre-heated oxygen enriched air) is blown from the bottom. It takes 6–8 h for the charge
material to descend through the furnace while it takes only 6–8 s for the blown blast to reach the
furnace top.
Until the 18th century charcoal was the only reductant/fuel used in BFs, then coke (after
invention in 1709) gradually replaced charcoal and BFs have grown considerably. The hearth
diameter was 4–5 m with annual production rate of 100,000 t hot metal mostly from lump ore.
Nowadays, BFs have hearth diameter up to 14–15 m with annual production rate of 3–4 million
tonnes of hot metal. The largest known BF nowadays has an inner volume of 5800 m3 and annual
production of 5.65 million tonnes HM [21]. The burden materials have changed from lump ore to
more efficient materials, like sinter and/or pellets. The reductant materials have developed as well
from 100% coke based operation to use other injectant materials through tuyeres. Attempts are also
made to charge alternative reducing agents from the top along with burden materials like carbon
composite agglomerates, etc. [16]. Modern BFs favor high Fe content in ore burden. Higher grade of
iron ore burden can be realized after physical beneficiation process. Upwards of 70–80% of the
modern BFs all over the world use sinter as the iron-bearing material while other BFs in Europe apply
100% iron ore pellets.
In a typical modern BF, the furnace is filled with alternating layers of coke and iron ore (sinter
and/or pellets). Hot blast (compressed air) is blown into the BF through tuyeres. The hot blast gasifies
coke and other carbon-bearing materials.
Figure 2 shows the major inputs and outputs of a typical modern BF.
Figure 2. Input and output of modern BF (adopted with permission from [21]).
The quality demands for the BF burden materials include chemical composition as well as
mechanical durability. The chemical composition must meet the end product properties. The
mechanical durability of the burden is related to the material property in cold, hot, and during
reduction to ensure the furnace permeability and, consequently, good performance and less
Figure 2. Input and output of modern BF (adopted with permission from [21]).
The quality demands for the BF burden materials include chemical composition as well
as mechanical durability. The chemical composition must meet the end product properties.
The mechanical durability of the burden is related to the material property in cold, hot, and during
reduction to ensure the furnace permeability and, consequently, good performance and less operational

Minerals 2018,8, 561 6 of 20
difficulties. The reducibility of the iron ores is for large extent controlled by how easy the reducing
gases can get into the iron oxide particles. The intrinsic reducibility of the burden material become less
important factor if no sufficient gas is transported to the reaction front and the produced gas is moved
away from the reaction site [19].
3. Conventional Carbon-Bearing Materials
Carbon-bearing materials are considered to be the major portion of iron-making cost and their
production causes severe environmental concerns. The major challenge for the iron-making industry
is the emission of greenhouse gases (GHG) from the use of fossil reductant (coke, coal, etc.). Coke is an
inevitable material for the BF iron-making. It is known for its triple role in the BF (mechanical, carbon
source, and energy supplier) [21]:
•
Mechanical role: low reactive and strong coke descending along with the burden materials
ensures good gas permeability and distribution, percolation of liquid iron and adsorption of dust.
Moreover, left unreacted coke provides mechanical support for the descending materials.
•
Source of carbon: coke along with other carbonaceous materials in the BF are responsible for
producing reducing substances and hot metal carburization.
•
Energy supplier: the combustion of carbonaceous materials including coke by hot blast in front of
the tuyeres provides the majority of heat required in the BF.
Coke reactivity and coke strength after reaction are most important properties that determine the
coke quality. Several factors affect the coke properties and consequently the chemical reactivity and
the post-reaction strength. These factors include carbon microstructure, porosity, and pore structure,
ash content, and ash composition and the blending coal ranking [25].
The total reducing agent rate in large BFs is around 460–520 kg/tHM of which 280–320 kg/tHM
is coke. [
26
]. Coke consumption in BF has been a concern for many researchers and steel producers
over the years. A lot of efforts have been made to partially replace coke with other carbon sources.
Examples of these sources are pulverized coal, natural gas, oil [
27
,
28
], plastic, biomass and other
resources derived from wastes [
29
,
30
]. Coal can be considered as a conventional carbon-bearing
material since it has been practiced as injectant material for decades. One of the advantages of injected
coal is its high hydrogen content which significantly helps in reducing the CO
2
emission. However,
injection of PC is limited due to the fact that PC is partially combusted in the raceway. The unburnt
char ascends and accumulates in the cohesive zone which results in impairing the furnace permeability
and consequently the furnace productivity. [31].
4. Alternative Carbon-Bearing Materials
Expected shortcut in the availability of coking coal, continued focus on energy consumption and
GHG emissions as well as the need for best possible raw material utilization will make it necessary to
continue the strive for making use of secondary and renewable resources within the process. The new
carbon-bearing materials should maintain the following properties [21]:
•
Low contents of sulfur, phosphorus, and alkali: sulfur and phosphorus removal in later process
stages increases costs. Higher alkali content results in alkali accumulation and circulation in the
furnace which not only attacks the refractory lining but also results in energy losses.
•Moisture content: moisture content should be kept minimum
•
Volatile content: the volatile content in the carbon source should be controlled as it affects the
gasification process in the raceway. Higher volatiles mean less replacement ratio for coke as well
as low heating value.
•Controlled hardness or grindability
•High solid or fixed carbon content, low ash, and high heating value.

Minerals 2018,8, 561 7 of 20
In the following section a brief description of the most common alternative reducing agents that
are commercially available or still under development will be given.
4.1. Active (Nut) Coke
BF requires special coke size, as well as relatively low reactive coke, to maintain the furnace
permeability in the lower part of the shaft [
32
]. In addition, the size distribution should be narrow
to maintain a stable operation and low coke rate [
33
]. The required size is in the range of 40 to
60 mm which can be achieved by screening the produced coke, the screening results in generation
of under-sieve coke, which is known as nut coke. Due to difficulties and GHG emission to produce
coke, there are several attempts that have been carried out to utilize this under-sieve coke or nut coke
in the BF which, of course, will affect the furnace permeability and operation smoothness, as well
as productivity.
4.2. In-Plant Fines
One more promising reducing agent is carbon rich iron- and steel-making residues. Large
quantities of residues are annually generated during iron and steel production, a significant amount
having potential of being valuable resources of carbon and iron [34,35].
Typical carbon content for the BF dust and sludge from a production site in Sweden is ~43% and
~33%, respectively [
36
,
37
]. When operating the BF on iron ore pellets, all the dry dust may be recycled
through injection in the tuyeres and by cold-bonded agglomeration [
38
]. However, problems arise
when attempting to recycle both the dust and sludge from the gas cleaning system back to the BF.
The main issue is the accumulation of zinc in the furnace which may lead to high zinc loads which,
in turn, disturb the smooth running of the process [
20
]. An additional problem is the cost related to
drying of the sludge prior to recycling.
Upgrading of BF sludge (lowering its zinc content) using a hydrocyclone has been demonstrated
in previous studies [
39
–
41
]. Another way to reduce the zinc content of the BF sludge is by leaching.
This has been realized in different leaching reagents such as sulfuric acid [
37
], hydrochloric acid [
42
],
and carboxylic acids [
36
]. On the other hand, effective utilization of carbon rich integrated steel-making
residues based on pyrometallurgical treatments has been also investigated [43,44].
4.3. Bio-Based Carbon-Bearing Materials
Biomass originating from forest residues, food wastes, etc. is today, to a large extent, used or
intended for use in a number of different applications. Biomass (charcoal) was used to be the main
carbon-bearing material in iron-making process until the 1880s. Later, and in order to protect the forest
trees and plants against massive exploitation, it was illegalized to use such in industrial applications as
a source of energy in many parts of the world. Nowadays, this type of renewable and neutral carbon
has attracted more public and policy attention due its capability of mitigating fossil CO2emission.
The use of charcoal as a reductant in smaller BFs is widely practiced in, e.g., Brazil [
45
]. Utilization
of biomass in metallurgical processes has been studied by many researchers. They have revealed their
high reactivity and high combustion degree [
46
,
47
]. However, pretreatment of biomass (carbonization)
to selectively remove oxygen and improve its grindability, combustibility, and reactivity is required.
A completely carbonized and devolatilized charcoal is, today, in most countries, expensive to be
competitive with fossil carbon sources. A partly devolatilized or torrefied biomass is, however,
a carbon source that might be competitive in the future. The carbonized biomass char is known for its
high reactivity due to its highly porous structure, high specific surface area, and the non-crystalline
nature. It has been reported that reactivity of biomass is couple of dozen times higher than coke which
makes it promising reducing agent for even low-grade iron ores with high efficiency [48].
Biomass can be utilized in the iron-making industry through one or more of following processes:
•Biomass can be mixed with coking coal blend prior to coking in the coke oven [46].

Minerals 2018,8, 561 8 of 20
•
It can partially substitute coke by top charging of lump charcoal or through biomass containing
iron composite agglomerates. A more promising route to introduce biomass in BF could be the
replacement of coal injected through the tuyeres [16,49–51].
•It can also partially replace coke breeze as source of energy in the sintering process [52].
•Synthesis and injection of reducing gas through controlled biomass gasification [18,53]
4.4. Waste Plastic Materials
The demand for plastics has grown significantly over the past decades, and will continue.
Significant amount of these plastics are today landfilled after the end-of-life cycle. Therefore, with an
aim of zero plastic to landfill by 2020 [
54
], it becomes a necessity to develop new recycling technologies
and further increase the chemical utilization of these materials instead of simple incineration. Its high
content of carbon and hydrogen makes it a potential candidate to substitute the conventional
carbonaceous materials used in most of the metallurgical industries [
55
]. It has been reported that
waste plastics have the potential to be a cheap and readily available auxiliary source of carbon. Its high
hydrogen content will directly help in reducing CO
2
emission in iron and steel-making sector. However,
the variation in composition of collected plastics from day to day and the probability of presence of
impurity elements has limited its commercial utilization in many of BFs around the world.
There are three basic ways to use waste plastics in iron-making:
•Synthesis and injection of reducing gas through controlled gasification [18,53];
•
Blending with raw materials (composite agglomerates, coal blend for coke-making and fuel for
sintering) [25];
•Direct use by injection through tuyeres [56].
4.5. Carbon Composite Agglomerates
Composite pellets [
57
] or carbon composite agglomerates (CCA) are agglomerates of carbonaceous
material and iron oxide mixture. The carbonaceous material can be coke fines, coal, charcoal, carbon
rich in-plant fines, biomass, waste plastics, etc., while the iron oxide can be low-grade iron ores, iron
rich in-plant fines, etc. [58].
Utilization of such will not only help in mitigating CO
2
emission but also will help in coke
and energy saving. The close distance between iron and carbon in such agglomerates will improve
the reaction kinetics significantly. The other benefits that can be visualized upon utilization of such
agglomerates are briefly mentioned here [58]:
•Improved reaction kinetics;
•Possibility of using iron and/or carbon rich in-plant fines [59];
•
Lower gasification temperature due to the coupling effect between the gasification reaction and
iron oxide (wustite) reduction [13,57]; and
•Less dependency on CO2and energy intensive ore preparation processes.
Detailed literature survey on utilization of carbon composite agglomerates in different
iron-making technologies using wider range of primary and secondary raw materials has been given
earlier by Ahmed et al. [16].
5. Trends in the Applications of Alternative Carbon-Bearing Materials
Alternative carbon-bearing materials can be introduced to the iron-making processes through
several ways. In the sintering process, biomass or waste plastics can partially substitute coke breeze.
In-plant fines can be used as source of both carbon and iron. In coke-making, attempts were made to
add biomass, as well as plastics to the coking coal blend. Alternative carbon-bearing materials can
either be charged to the BF from the top along with burden materials in form of carbon composites or
lump coal. Carbon rich in-plant fines, waste plastics and/or biomass can be injected to the BF through

Minerals 2018,8, 561 9 of 20
the tuyeres. In DR processes, alternative reducing agents can either be mixed with the iron oxide or
gasified externally and the reduction is executed by the gasification gaseous product. The following
sections are a summary of the recent research conducted in this regard with a focus on biomass.
5.1. Sintering
Sintering of iron ores can be defined as the transformation of iron ore fines into large, hard,
and porous agglomerates. Sinter represents the main feed iron bearing material for the majority of
modern BFs all over the world due the many advantages sinter possesses over the lump iron ore like
efficient utilization of fine iron ore, dryness, less dust generation, and controlled composition [60,61].
The main fuel used in sintering process is the undersize coke or coke breeze which is generated
during coke screening. Sintering contributes to the total iron and steel industry CO
2
emission by
approximately 10% [
52
]. The CO
2
in this case is a result of combustion of coke breeze and the
calcination of limestone for example. Therefore, it becomes a necessity to partially or completely
replace the coke breeze by renewable and neutral carbon. The move from coke breeze to biomass will
affect the process in several ways.
Using higher reactive carbon-bearing materials compared to coke results in decreasing the
maximum temperature and shorten the time of holding at high temperature [
50
,
62
]. Utilization
of biomaterials has been reported to produce a sinter with lower bulk density compared to sinter made
with coke which is attributed to the narrower combustion and sintering zones due to the increased
flame front speed [
63
]. Attempts of utilization of biomass with different levels of carbonization (starting
from raw to highly carbonized biomass) in sintering process instead of coke breeze have been recently
conducted. It has been found that the partial replacement of coke breeze by charcoal leads to improved
off-gas quality with a decrease in SO
x
and NO
x
emissions [
64
,
65
]. A decrease in the net fossil CO
2
emission by 5–15% in such a case is expected [66].
However, the moisture content of the raw biomass and the high affinity of charcoal to absorb
moisture negatively affect the granulation process of sinter feed [
66
]. Mousa et al. [
67
] have studied
the effect of substitution of coke breeze with bio-char with a replacement ratio up to 100%. They
have found that the bed permeability as well as the sintering speed were negatively affected at
replacement rate of coke breeze with biochar higher than 25%. Moreover, the higher reactivity of such
material promotes the increase in CO concentration in the off-gas and decrease the degree of post
combustion. Such decrease in post combustion results in lowering the combustion zone temperature
by 150
◦
C (from ~1390
◦
C in case of coke breeze to ~1240
◦
C in case of bio-char). Other researchers
have demonstrated that up to 40% bio-char can replace coke breeze without negatively affecting the
process [
52
]. The replacement ratio can be further increased through utilization of composites of coke
and bio-char [
50
]. In order to maintain the sinter yield as well productivity larger particles of bio-char
(1–5 mm) with high solid carbon content (fixed carbon > 90%) should be used [68].
Generally, it can be concluded that in order to maximize the substitution ratio of biomass without
adversely affecting the sintering process and the sinter quality, many parameters have to be controlled.
These parameters include particle size, strength, reactivity, and chemical composition. The feasible
share of charcoal is found to be 25–40% and the porous structure of the sinter produced in this case
led to higher reducibility of the sinter without decreasing the sinter strength down to an unaccepted
level [69].
5.2. Coking Coal
As previously discussed, coke cannot be totally replaced by other means. In the meanwhile, coke
is the most expensive and the most CO
2
intensive material to be used in the BF. Additionally, coke
quality is very essential for smooth operation and high quality product. It is obvious that mechanical
strength and reactivity are the very important properties to ensure smooth operation. In modern large
BFs, it is always preferred to have high mechanical strength and low reactivity coke [19].

Minerals 2018,8, 561 10 of 20
Alternative carbon-bearing materials, for example biomass, exhibit different properties compared
to coal. Thus, adding biomass to coal blend affects the following properties which are essential for
producing coke with good metallurgical properties:
•
Fluidity: the addition of alternative carbon-bearing bio-materials is found to invariably decrease
the blend fluidity [
70
,
71
]. The main reason for the deterioration of fluidity when biomaterials,
for example, are added to the coal blend is attributed to their unique physical characteristics [
72
].
•
Reactivity (coke reactivity index, CRI) and strength (coke strength after reaction, CSR) are the
most important measures that are considered for evaluating coke properties. Good quality coke
is always characterized by relatively low CRI (<30) and relatively high CSR (>55) [
73
]. Adding
charcoal to the coal blend increases the reactivity of the produced bio-coke which is attributed
to the calcium and/or alkali content of the charcoal, the higher the alkali index the higher the
reactivity [
70
]. On the other hand, the poor mechanical properties, like strength and low density,
have negative impacts on the mechanical properties of the produced bio-coke [74].
In the last decade, several researchers have tried to produce coke using bio-coal. The produced
bio-coke has showed higher reactivity toward CO
2
and, hence, using such can minimize the total
carbon consumption [
18
]. However, as discussed earlier, adding biomass to coal blend leave behind
bio-coke with poor mechanical properties compared to conventional coke. Some studies showed
that even adding 3% sawdust to the coal blend prior to coking has negatively affected the product
mechanical properties [
72
,
74
] while other studies stated that up to 5% biomass can be added to the
coking coal blend without significant deterioration of the produced coke properties [70,72,75].
Compacting biomass prior to coking was found to reduce the negative effect on both mechanical
and chemical properties [
46
]. Further, the compaction of biomass at 200–350
◦
C was found to suppress
the deterioration of CSR and opens a window to increased addition rate of biomass.
5.3. BF Iron-Making
Partial substitution of virgin fossil carbon-bearing materials in BF with secondary (by products),
or renewable and neutral carbon-bearing materials is one of the promising approaches to mitigate
CO
2
emission. There are two ways to introduce carbon-bearing materials to the BF: top charging or
injection from the bottom through tuyeres. The basis on which the charging way is decided is the
chemical and physical properties of the charging materials and the operating conditions.
5.3.1. Top Charging
In this section the most common carbon-bearing materials that can be charged to the BF from the
top will be discussed except carbon composite agglomerate which is given elsewhere [16].
Nut Coke
Several studies have been conducted to address the effect of charging nut coke with the burden
material on the process efficiency and, hence, the productivity [
8
,
76
–
80
]. The attempts started by
charging the under-sieve coke with different ratios (5–30%) within the coke layer. It resulted in
reducing the productivity by 0.9–6.5%. It also led to non-uniform distribution of ascending gases [
81
].
Later, it has been suggested to charge the nut coke along with the burden material [
82
,
83
]. The idea
was successfully tested and followed by a good improvement in the BF productivity and lowered the
coke rate.
Figure 3depicts how the size as well as its distribution affects the furnace permeability.
This success has inspired researchers to conduct an intensive research to study the effect of
nut coke on different process parameters like permeability, reducibility, total coke consumption,
etc. [
8
,
76
,
77
,
79
,
80
,
84
,
85
]. One of the important issues that should be taken care of is the uniform
mixing of nut coke in the ore layer to maintain proper gas distribution and maintain the required
permeability especially in the cohesive zone [
81
]. Mixing of nut coke within the ore bed was very

Minerals 2018,8, 561 11 of 20
effective in increasing the rate of carbon solution loss [
13
]. This further decreased the gasification
reaction temperature which consequently decreased the lower limit of the thermal reserve zone (TRZ)
temperature. Such a decrease in TRZ temperature shifts the process toward higher utilization of CO
and, consequently, decreases the total coke consumption [
13
]. Figure 4shows the relation between the
thermal reserve zone temperature and coke rate consumption. Mixing nut coke in the ore layer is not
only expected to improve the solution loss reaction but also expected to protect the lump coke from
gasification and consequently abrasion [86].
Minerals 2018, 8, 561 11 of 20
Figure 3. Comparison between conventional charging method and coke mixed charging method
(adopted with permission from [85]).
Figure 4. Reducing agents rate and thermal reserve zone temperature (adopted with permission from
[13]).
The reactivity of small sized coke was further increased by means of surface coating with
compounds that contains iron and calcium. It was found that activated nut coke shows even higher
rates of gasification compared to original ones. The activation was more pronounced for coke with
low CRI values and further reduced the coke rate consumption [87].
In-Plant Fines
Effective utilization of carbon rich integrated steel-making residues in iron-making have been
investigated [43,44,88,89]. Producing agglomerates with a self-reducing property, which is used
further to produce DRI has been proved to be an effective way to recycle the integrated steel plants
generated fines. Heating such agglomerates results in high quality low zinc DRI which can be used
as a replacement for the scrap in the electric arc furnace or can even be charged back to the BF to
decrease the coke consumption [88,89].
Biomass
Nowadays, about 10 Mt of steel is produced using charcoal by mini-BFs with inner volume in
the range of 50–350 m
3
in Brazil [74]. In large modern BFs, partial replacement of top charged coke
by lump charcoal (20 kg/tHM) reduces the coke consumption by approximately 30 kg/tHM without
significant effect on the BF operation [18,70].
Carbon composite agglomerates (CCA) are the other option for top charging of biomass to the
BF. CCA are a mixture of carbonaceous material and iron oxide in form of pellets or briquettes. Quite
often these agglomerates are designed in a way to be self-reducing, C/O molar ratio is ≥1. Due to the
close packing of biomass and iron oxide in the agglomerates and the unique reactivity of biomass
[90], the equilibrium temperature of wustite-iron reaction shifts toward lower temperature. This shift
results in higher efficiency in the BF, improved gas utilization and, consequently, reduced carbon
Figure 3.
Comparison between conventional charging method and coke mixed charging method
(adopted with permission from [85]).
Minerals 2018, 8, 561 11 of 20
Figure 3. Comparison between conventional charging method and coke mixed charging method
(adopted with permission from [85]).
Figure 4. Reducing agents rate and thermal reserve zone temperature (adopted with permission from
[13]).
The reactivity of small sized coke was further increased by means of surface coating with
compounds that contains iron and calcium. It was found that activated nut coke shows even higher
rates of gasification compared to original ones. The activation was more pronounced for coke with
low CRI values and further reduced the coke rate consumption [87].
In-Plant Fines
Effective utilization of carbon rich integrated steel-making residues in iron-making have been
investigated [43,44,88,89]. Producing agglomerates with a self-reducing property, which is used
further to produce DRI has been proved to be an effective way to recycle the integrated steel plants
generated fines. Heating such agglomerates results in high quality low zinc DRI which can be used
as a replacement for the scrap in the electric arc furnace or can even be charged back to the BF to
decrease the coke consumption [88,89].
Biomass
Nowadays, about 10 Mt of steel is produced using charcoal by mini-BFs with inner volume in
the range of 50–350 m
3
in Brazil [74]. In large modern BFs, partial replacement of top charged coke
by lump charcoal (20 kg/tHM) reduces the coke consumption by approximately 30 kg/tHM without
significant effect on the BF operation [18,70].
Carbon composite agglomerates (CCA) are the other option for top charging of biomass to the
BF. CCA are a mixture of carbonaceous material and iron oxide in form of pellets or briquettes. Quite
often these agglomerates are designed in a way to be self-reducing, C/O molar ratio is ≥1. Due to the
close packing of biomass and iron oxide in the agglomerates and the unique reactivity of biomass
[90], the equilibrium temperature of wustite-iron reaction shifts toward lower temperature. This shift
results in higher efficiency in the BF, improved gas utilization and, consequently, reduced carbon
Figure 4.
Reducing agents rate and thermal reserve zone temperature (adopted with permission
from [13]).
The reactivity of small sized coke was further increased by means of surface coating with
compounds that contains iron and calcium. It was found that activated nut coke shows even higher
rates of gasification compared to original ones. The activation was more pronounced for coke with low
CRI values and further reduced the coke rate consumption [87].
In-Plant Fines
Effective utilization of carbon rich integrated steel-making residues in iron-making have been
investigated [
43
,
44
,
88
,
89
]. Producing agglomerates with a self-reducing property, which is used
further to produce DRI has been proved to be an effective way to recycle the integrated steel plants
generated fines. Heating such agglomerates results in high quality low zinc DRI which can be used as
a replacement for the scrap in the electric arc furnace or can even be charged back to the BF to decrease
the coke consumption [88,89].

Minerals 2018,8, 561 12 of 20
Biomass
Nowadays, about 10 Mt of steel is produced using charcoal by mini-BFs with inner volume in
the range of 50–350 m
3
in Brazil [
74
]. In large modern BFs, partial replacement of top charged coke
by lump charcoal (20 kg/tHM) reduces the coke consumption by approximately 30 kg/tHM without
significant effect on the BF operation [18,70].
Carbon composite agglomerates (CCA) are the other option for top charging of biomass to the
BF. CCA are a mixture of carbonaceous material and iron oxide in form of pellets or briquettes. Quite
often these agglomerates are designed in a way to be self-reducing, C/O molar ratio is
≥
1. Due to the
close packing of biomass and iron oxide in the agglomerates and the unique reactivity of biomass [
90
],
the equilibrium temperature of wustite-iron reaction shifts toward lower temperature. This shift results
in higher efficiency in the BF, improved gas utilization and, consequently, reduced carbon consumption.
In a study by Kasai et al. they have found that up to 19 kg/tHM can be saved if the thermal reserve
zone temperature is decreased by 100 ◦C [13].
Plastic Materials
Top charging of plastic materials is practically impossible unless it is in form of CCA due to its
chemical and physical properties. Therefore, all studies on introducing plastic to the BF are either
through carbon composite agglomerates or injection through tuyeres [16,56,91].
5.3.2. Injection
Pulverized Coal Injection (PCI)
PCI is one of essential methods to enhance the BF profitability. Due to the ease of use, oil and
natural gas were popular injectants in 1960s but due to the oil crises in 1970s many companies stopped
the oil injection and turned to coal injection since 1980s. Nowadays, the vast majority of BFs all over the
world apply PCI due to the relatively lower cost of coal compared to other fuels beside the beneficial
effect on the BF efficiency. Injection of PC into the BF provides various economic and operational
benefits [
92
]. These benefits include: (i) lower consumption of expensive coke, (ii) replacing high
rank expensive coal with low grade cheaper coals, (iii) longer life period for coke oven, (iv) higher BF
productivity, (v) higher flexibility in BF operation, (vi) improving the hot metal quality, and (vii) lower
CO2emission.
The raceway can be classified into 3 main zones: (i) PC devolatilization zone, (ii) oxidation or
combustion zone, and (iii) solution loss reaction zone. The concentration of oxygen sharply decreases
in the oxidation zone due to its reaction with carbon of coke and coal to produce CO/CO
2
. Figure 5
shows a schematic representation of the PC injection and the subsequent reactions in the raceway.
For an optimal BF operation using PCI, it is crucial to assure that the whole amount of the injected
coal is gasified as fast as possible [
93
]. The transit time of the injected coal particles in the raceway
could reach 20–30 ms. Within such limited time, the PC particles will not be completely combusted
and a considerable amount of char will escape from the raceway region to reach the active coke zone
as can be seen in Figure 6.
As a result of high injection rate of PC the fine char tend to block the bed voidage and, consequently,
disturb the gas flow and increase the pressure drop, reduce the permeability, reduce the shaft efficiency
and, consequently, the furnace productivity [94].

Minerals 2018,8, 561 13 of 20
Minerals 2018, 8, 561 13 of 20
Figure 5. Schematic diagram for PC reactions in raceway (adopted with permission from [95]).
Figure 6. Schematic of the lower zone of the BF (adopted with permission from [96]).
Injection of Oil and Natural Gas
The injection of oil and natural gas into the BF were firstly practiced before PCI but the energy
crisis during 1970s resulted in more attention to PCI. However, the countries locally produce oil and
natural gas, such as USA, Russia, and some other countries are still injecting these carbon-bearing
materials into the BF to reduce the coke consumption. It has been reported that 1.0 t of oil or natural
gas replaces 1.2 t of coke. The average consumption of natural gas is 70–100 m
3
/tHM (~50–70 kg/tHM)
but often exceeds to 150–170 m
3
/tHM (~105–120 kg/tHM) [97,98].
In-Plant Gases
Utilization of integrated steel plant top-gases becomes necessary to achieve profitability to steel
mill operation from both economic and environmental aspects. The most common gases that are daily
produced in steel works and have further energy potential to be extracted are BF top gas (BFG), coke
oven gas (COG), and basic oxygen furnace gas (BOFG) [97]. The heat value of the COG is greatest
among other generated gases with an average value of 17 MJ/Nm
3
(STP) compared to that of either
BOFG which has a heating value as low as ~8.8 MJ/Nm
3
or BFG which has even much lower heating
value (3.0–3.7 MJ/Nm
3
) [99]. The specific amount of generated coke oven gas is in the range of
Figure 5. Schematic diagram for PC reactions in raceway (adopted with permission from [95]).
Minerals 2018, 8, 561 13 of 20
Figure 5. Schematic diagram for PC reactions in raceway (adopted with permission from [95]).
Figure 6. Schematic of the lower zone of the BF (adopted with permission from [96]).
Injection of Oil and Natural Gas
The injection of oil and natural gas into the BF were firstly practiced before PCI but the energy
crisis during 1970s resulted in more attention to PCI. However, the countries locally produce oil and
natural gas, such as USA, Russia, and some other countries are still injecting these carbon-bearing
materials into the BF to reduce the coke consumption. It has been reported that 1.0 t of oil or natural
gas replaces 1.2 t of coke. The average consumption of natural gas is 70–100 m
3
/tHM (~50–70 kg/tHM)
but often exceeds to 150–170 m
3
/tHM (~105–120 kg/tHM) [97,98].
In-Plant Gases
Utilization of integrated steel plant top-gases becomes necessary to achieve profitability to steel
mill operation from both economic and environmental aspects. The most common gases that are daily
produced in steel works and have further energy potential to be extracted are BF top gas (BFG), coke
oven gas (COG), and basic oxygen furnace gas (BOFG) [97]. The heat value of the COG is greatest
among other generated gases with an average value of 17 MJ/Nm
3
(STP) compared to that of either
BOFG which has a heating value as low as ~8.8 MJ/Nm
3
or BFG which has even much lower heating
value (3.0–3.7 MJ/Nm
3
) [99]. The specific amount of generated coke oven gas is in the range of
Figure 6. Schematic of the lower zone of the BF (adopted with permission from [96]).
Injection of Oil and Natural Gas
The injection of oil and natural gas into the BF were firstly practiced before PCI but the energy
crisis during 1970s resulted in more attention to PCI. However, the countries locally produce oil and
natural gas, such as USA, Russia, and some other countries are still injecting these carbon-bearing
materials into the BF to reduce the coke consumption. It has been reported that 1.0 t of oil or natural
gas replaces 1.2 t of coke. The average consumption of natural gas is 70–100 m
3
/tHM (~50–70 kg/tHM)
but often exceeds to 150–170 m3/tHM (~105–120 kg/tHM) [97,98].
In-Plant Gases
Utilization of integrated steel plant top-gases becomes necessary to achieve profitability to steel
mill operation from both economic and environmental aspects. The most common gases that are daily
produced in steel works and have further energy potential to be extracted are BF top gas (BFG), coke
oven gas (COG), and basic oxygen furnace gas (BOFG) [
97
]. The heat value of the COG is greatest
among other generated gases with an average value of 17 MJ/Nm
3
(STP) compared to that of either
BOFG which has a heating value as low as ~8.8 MJ/Nm
3
or BFG which has even much lower heating

Minerals 2018,8, 561 14 of 20
value (3.0–3.7 MJ/Nm
3
) [
99
]. The specific amount of generated coke oven gas is in the range of
410–560 Nm
3
/t of coke while the amount of BOFG is in the range of 50–100 Nm
3
/t of steel in most
cases. More than 650 million tonnes of coking coal are used to produce 500 million tonnes of coke
annually [
100
]. The coke-making process is therefore accompanied by more than 310 billion Nm
3
of
COG [
101
]. Currently, coke oven gas is used after cleaning in heating stoves (BF), heating furnace
(rolling mills), ignition furnaces (sintering plant), and power generation (power plant) [99].
Waste Plastics Injection
As the plastics contain mainly hydrogen and carbon it can provide additional benefits similar to
oil and natural gas injection into the BF. Injection of “pure” waste plastic has been practiced at several
BFs in Germany, Japan, and Austria with an injection rate up to 60–80kg/tHM [
102
–
105
]. Since the
collected waste plastics are heterogeneous mixture from different types, it is recommended to conduct
heat treatment at 200
◦
C before its injection into the BF. The pre-treatment will generate a homogenous
pulverized waste plastic mixture [
102
]. Moreover, the pre-treatment of waste plastics will perform
de-chlorination for the plastics containing chlorine, such as polyvinyl chloride (PVC) and, hence, avoid
the corrosive effect on the BF tuyeres and the refractory materials in the hearth lining.
Due to the substantial difference in combustion and gasification characteristics between plastic
and coal particles, special precautions have to be taken to insure smooth operation and maintained
productivity. Studies have shown that plastic particles have the capability to gasify completely in the
raceway, which is known to improve the process efficiency and sustain the gas permeability along
the BF cohesive zone. The reaction kinetics of waste plastic materials under the raceway, bird’s nest
and shaft conditions was studied by Babich et al. [
56
]. They found that physical properties of plastic
including grain size, shape, and porosity have strong effects on their reaction kinetics. The unburnt
char in the raceway can be successfully consumed under the conditions of bird’s nest and shaft.
The theoretical limit of waste plastics injection is estimated to be 70 kg/tHM while the higher injection
rate will result in problems similar to that obtained with the relatively high PCI. Every tonne of plastics
used in the BF can replace 750 kg of coke. Injection of waste plastics (polyethylene, polypropylene, etc.)
into the BF can reduce the CO
2
emission by 30% due to the higher H
2
content compared to coke [
29
].
However, due to restrictions mentioned above, the net influence of waste plastics can be small. Waste
plastic collection and pre-treatment still represent the main challenge for its industrial implementation.
An efficient and effective routine has to be developed to attain sustainable and reliable supply of
waste plastics.
In-Plant Fines
BF is the production unit responsible for generating residues rich in carbon. There are two such
residues: namely, the BF dust and the BF sludge. The injection of BF dust into the BF through tuyeres
was actively studied in the 1980s in order to efficiently use the waste materials and increase the
productivity. Further, the injection of BF flue dust and its influence on the coke consumption and hot
metal quality has been tested in LKAB experimental BF (EBF) and it has been applied in a full scale
BF [
106
,
107
]. It was found that the injection of BF flue dust improves the coal combustion efficiency
and the slag formation in the raceway. Injection of such can be a promising solution to increase the
amount of materials recycled in an operation where sintering facility does not exist.
The tuyeres injection of in-plant fines (BF dust) could achieve various advantages to the
iron-making BF represented in lower top-charging of pellets and sinter, more flexibility in controlling
the raceway flame temperature and hot metal quality. On the other hand, the injection rate of in-plant
fines has to be adjusted with the injection rate of PC and the condition of hot blast. The calculation
based on the raceway heat balance indicated that 184–100 kg/tHM of in-plant fines can be co-injected
with 100–200 kg/tHM of PC, respectively [106].

Minerals 2018,8, 561 15 of 20
Biomass Injection
The top charging of biomass into the large BF is still suffering from some problems, which are
related to the low mechanical strength and the high volatile content compared to coke. In order
to overcome these problems, the tuyeres injection provides a possibility for biomass utilization in
modern BFs. Mechanical strength is not a prerequisite in this case. What actually important is the
devolatilization, gasification and combustion characteristics as they affect (along with the heating
value) the raceway adiabatic flame temperature (RAFT). Injection trials using specially-designed
injection rig have demonstrated that biomass possess higher reactivity and higher combustion degree
compared to pulverized coal [46,47].
The replacement rate of coal by biomass materials like charcoal, torrefied (pretreated at relatively
low temperature 200–250 under inert atmosphere for short time 5–15 min) and raw biomass was
theoretically investigated by means of static heat and mass balance model. A total of 155 kg/tHM of
pulverized coal could be replaced by 166.7 kg/tHM charcoal. In the case of torrefied and raw wood
pellets, the replacement ratio decreases significantly. The former can replace 22.8% while the later can
replace only 20% of the injected pulverized coal. Further increase in the injection rate of bio-based
materials requires further oxygen enrichment and it might lead to decreased RAFT temperature and
increased the top gas temperature [108,109].
Theoretically, all injected coal can be replaced by charcoal (200–220 kg/tHM), which corresponds
to a reduction in net CO
2
emission up to ~40% [
51
]). Moreover, a highly efficient process is expected
due to lower slag volume and low sulfur content and consequently increased production rate [
110
].
However, the undesired physico-chemical properties of biomass still represent the major obstacle for
commercial implementation of biomass injection. These obstacles include the high moisture content,
low density, and different grindability characteristics compared to coal and the low heating value [
90
].
6. Conclusions
Although steel is an essential product for everyday use in our life and one of the main drivers for
the global growth and development, its production is considered as one of the most intensive CO
2
emission sources with a share of ~7% of the global CO
2
emission. Approximately 1.8 t of CO
2
are
emitted per every produced tonne of finished steel product. The mean reason for this is that iron and
steel production is mainly dependent on fossil coking coal. Recently, the reduction of CO
2
emission
has put on top priorities of iron and steel producers due to the commitment of governments according
to Kyoto protocol.
In the present review, the partial substitution of virgin fossil carbon namely, coal and coke,
in iron-making processes with either secondary or renewable carbon-bearing materials has been
discussed. It has been revealed that such substitution represents one of vital options to reduce the
dependency on virgin fossil carbon and, consequently, reduce the CO2emission.
Attempts to decrease the dependency on metallurgical coke and consequently reduce the CO
2
emission have been primarily using the following approaches:
•
Efficient utilization of active under size coke and in-plant gases and fines instead of the
virgin metallurgical coke results in lowering the overall energy and carbon consumption and
consequently decreases the CO2emission.
•
Replacing the coke with renewable, neutral and H
2
rich carbon-bearing materials will directly
reduce the CO
2
emission due to the increased share of H
2
as a reducing agent. These materials
can be introduced to the iron-making processes via several means:
(a) Partial replacement of coke breeze in the sintering process;
(b) Blending with coking coal prior to the coke-making process;
(c) Partial replacement of top charged coke by lump charcoal; and
(d)
Replacement of injectant pulverized coal with waste plastics, charcoal, or torrefied biomass.

Minerals 2018,8, 561 16 of 20
•
Producing agglomerates from secondary resources and/or alternative carbonaceous materials
provides an opportunity to utilize wide range of materials, including mechanically unsuitable
materials for direct use.
•
Utilization of highly reactive carbon and/or carbon composite agglomerates will shift the iron
oxide reduction process toward lower carbon consumption.
Funding: This research received no external funding.
Acknowledgments:
The work was carried out within CAMM—Centre of Advanced Mining and Metallurgy at
Luleå University of Technology, Sweden.
Conflicts of Interest: The author declares no conflict of interest.
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