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processes
Review
Life Cycle Assessment of Renewable Reductants in the
Ferromanganese Alloy Production: A Review
Gerrit Ralf Surup 1,*, Anna Trubetskaya 2and Merete Tangstad 1
Citation: Surup, G.R.; Trubetskaya,
A.; Tangstad, M. Life Cycle
Assessment of Renewable Reductants
in the Ferromanganese Alloy
Production: A Review. Processes 2021,
9, 185. https://doi.org/10.3390/
pr9010185
Received: 6 December 2020
Accepted: 13 January 2021
Published: 19 January 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional clai-
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Department of Materials Science and Engineering, Norwegian University of Science and Technology,
7491 Trondheim, Norway; merete.tangstad@ntnu.no
2Department of Chemical Sciences, University of Limerick, Limerick V94 T9PX, Ireland;
anna.trubetskaya@ul.ie
*Correspondence: gerrit.r.surup@ntnu.no; Tel.: +47-934-73-184
Abstract: This study examined the literature on life cycle assessment on the ferromanganese alloy
production route. The environmental impacts of raw material acquisition through the production of
carbon reductants to the production of ferromanganese alloys were examined and compared. The
transition from the current fossil fuel-based production to a more sustainable production route was
reviewed. Besides the environmental impact, policy and socioeconomic impacts were considered
due to evaluation course of differences in the production routes. Charcoal has the potential to
substantially replace fossil fuel reductants in the upcoming decades. The environmental impact
from current ferromanganese alloy production can be reduced by
≥
20% by the charcoal produced
in slow pyrolysis kilns, which can be further reduced by
≥
50% for a sustainable production in
high-efficient retorts. Certificated biomass can ensure a sustainable growth to avoid deforestation
and acidification of the environment. Although greenhouse gas emissions from transport are low
for the ferromanganese alloy production, they may increase due to the low bulk density of charcoal
and the decentralized production of biomass. However, centralized charcoal retorts can provide
additional by-products or biofuel and ensure better product quality for the industrial application.
Further upgrading of charcoal can finally result in a CO
2
neutral ferromanganese alloy production
for the renewable power supply.
Keywords: charcoal; life cycle assessment; sustainable biomass growth; mining; metallurgical coke
1. Introduction
Climate change caused by anthropogenic CO
2
emissions is considered one of the most
prominent issues of the present time. About 5–10% of anthropogenic CO
2
emissions are
emitted by metallurgical industries [
1
,
2
]. Most of these emissions are direct emissions
generated in the smelting furnace, such as blast, electric arc, and submerged arc fur-
naces. The indirect emissions originate from coal dust, ore mining, metallurgical coke, and
power production. Ferroalloys, such as ferromanganese (FeMn), silicomanganese (SiMn),
or ferrochromium, are mainly produced in submerged arc furnaces [
3
], where between
40 and 70%
of the required thermal energy is provided by electrical dissipation, emphasiz-
ing the importance of renewable power production [
4
,
5
]. The annual CO
2
emissions for
the production of ferroalloys accumulate to ≈87Mt (million tonnes) [6].
The differences in greenhouse gas (GHG) emissions generated by ferroalloy manufac-
turers vary in dependency on the source of energy and reductant material (biomass, coal,
or metallurgical coke) [
7
,
8
]. The renewable hydropower makes Norway one of the most
environmental friendly ferroalloy producer worldwide. However, the ferromanganese
alloy production still relies on the use of fossil-based fuel reductants, e.g., metallurgical
coke. The transition from fossil fuel-based to renewable reductants is hampered by the
differences in properties, such as the low mechanical strength of renewable reductants and
its high gas reactivity. In addition, the physicochemical properties of classical charcoal
Processes 2021,9, 185. https://doi.org/10.3390/pr9010185 https://www.mdpi.com/journal/processes
Processes 2021,9, 185 2 of 19
can vary from batch to batch because of the undefined process conditions in charcoal
kilns. However, the renewable charcoal-based reductant together with the carbon capture
and storage (CCS) or carbon capture and utilization (CCU) are intended to eliminate the
anthropogenic emissions in Norway by 2050 [9,10].
Life cycle assessment (LCA) is an analytical tool that supports users with the reduction
of CO
2
emissions and sustainability challenges using ISO standards 14,001 and 14,040.
Traditional cost and process optimization can be combined by linking environmental
calculations for each process step within life cycle analysis. The important aspect of LCA
is the system boundary, including processes and production routes which are considered
and where the system begins and ends [
11
]. While a complete LCA evaluates the overall
life time of a product (cradle-to-grave), cradle-to-gate approaches are commonly used
to describe production of raw materials [
8
,
11
–
13
]. The cradle-to-gate approach includes
the transition phases such as feedstock growth, pretreatment, transport, storage, and
postprocessing by metallurgical smelters. However, not all aspects of sustainability can be
considered within LCA [
11
]. Technical issues caused by the different properties and the
economics of renewable reductants can play a significant role in replacing fossil fuel-based
reductants [
14
,
15
]. In addition, renewable reductants may require more energy for handling
and transport. For example, the milling of biomass requires more energy than that of coal
samples [
16
], which will affect the overall energy demand in the pretreatment process.
A systematic effort is needed to overcome technical and economical hurdles to realize
a shift from fossil fuel-based to renewable reductants [
17
]. The low mechanical strength
and high gas reactivity of charcoal hamper the direct replacement of fossil-based reductants
in metallurgical industry [
18
,
19
]. However, charcoal is used as a major carbon source in
mini blast furnaces in Brazil [
17
]. The low burden height of small size blast furnaces can
result in a compaction pressure that makes the mechanical properties of reductants of less
importance [14].
To the knowledge of authors, this review is the first attempt to examine the literature
on life cycle assessment of ferromanganese alloy production route. This work aims to
evaluate possible GHG emission sources and savings for the production routes of fossil
fuel-based and renewable reductants used in ferromanganese alloy production. Post-
treatment processes to adjust properties of charcoal to the fossil fuel-based counterparts
were neglected, as the industrial process route for the replacement of metallurgical coke
with renewable charcoal reductants is unknown. Three cases were reviewed in the present
study: (1) the base case with the metallurgical coke as the reductant, (2) charcoal produced
using classical charcoal kilns, and (3) charcoal formed by a sustainable production.
2. Ferromanganese Alloy Production
The total ferroalloy production has increased from 18Mt in the 1990s to 36Mt in 2008
and to 40 Mt in 2020 [
7
,
20
,
21
]. About one-third of the total ferroalloy production are ferro-
manganese alloys in a form of high-carbon ferromanganese (HC FeMn), silicomanganese,
and refined ferromanganese alloys [
7
,
8
]. Ferroalloys are often used in high-quality steel
production for the improvement of product strength and hardness [
8
,
22
]. The high-quality
steel contains on average 3% of silicon, manganese, aluminum, or chrome as alloying
elements [
6
]. Ferromanganese is mainly produced from manganese-ore by carbothermal
reduction in submerged arc furnaces (SAF), in which metallurgical coke is used as reduc-
tant. About 59 Mt of manganese ore is estimated to be produced in 2020, with more than
50% reduced and refined in China [23].
The fossil fuel-based production route of ferromanganese alloy is shown in
Figure 1
.
When this study was initiated, metallurgical coke has been mainly used as a reductant in
closed hearth SAF to produce HC FeMn and SiMn. Metallurgical coke is produced from
blends of coking coals in coke oven batteries at the temperature range 1100–1400
◦
C [
24
].
Charcoal is mainly produced from wood in kilns and medium sized retorts for the ferroalloy
purposes [
25
,
26
]. The low bulk density of charcoal increases the transport and storage
demand compared to fossil fuels, resulting in more complex logistics, higher emissions,
Processes 2021,9, 185 3 of 19
and overall process cost. The efficiency for reducing manganese-ore to pure manganese is
assumed to depend on the used furnace and is similar for fossil fuel-based and renewable
reductants. Thus, similar emissions from mining during manganese-ore production occur
using the different process routes.
Mining
(coal / ore)
Process stage EmissionsInput
Electricity
Fuels (diesel)
Water
Land Use
Equipment
Material
Earth/soil Coal / ore
Transport
Ship / train
Energy
Vehicles
Coke oven
& ORC
Electricity
ORC:=Organic Rankine Cycle
Metallurgical
coke
Blast furnace gas
Electricity
Steam
Water
Materials
Air emissions
Dust
Waste
Landfill
Aggregates
Coal waste
Acidification
Air emissions
Water emissions
Coke oven gas
Fugitive
Emissions
Waste heat
Water pollution
Other factors
Human health:
Toxicity
Respiratory symptoms
Socioeconomic
Environmental:
Land transformation
Biodiversity
Land occupation
Human health:
Carcinogenic
Emissions
Respiratory symptoms
PAH emissions
Submerged
arc furnace
(a)
Biomass harvest
Process stage EmissionsInput
Water
Land Use
Fertilizer
Equipment
Material
Earth/soil
Electricity
Fuels
Stemwood
Wood chips
Transport
Ship / train
Energy
Vehicles
Charcoal kiln
Retort
Condensates
(bio-oil) Charcoal
Biomass
Charcoal
Pyrolysis gas
Materials
Deforestation
Dust
Biomass waste
Overfertilization
Air emissions
Water emissions
Pyrolysis gas
Dust emissions
Water pollution
Other factors
Land transformation
Biodiversity
Socioeconomic stress
Land occupation
Acidification
Carcinogenic &
PAH emissions
Submerged
arc furnace
(b)
Figure 1. Production route of ferromanganese alloys using (a) metallurgical coke or (b) charcoal as reductant. Each
process step can be accounted as a possible source for greenhouse gas (GHG) emissions. (a) Fossil fuel-based reductant.
(b) Renewable reductant.
Processes 2021,9, 185 4 of 19
The system boundary is mostly set as a cradle-to-gate approach on the basis of 1t final
ferromanganese alloy. The reviewed articles concerning LCA for the ferromanganese alloy
production route are summarized in Table 1. A bottom-up, layered approach should be
chosen to evaluate sustainability for a charcoal use in a ferroalloy reduction process [
17
].
The fixed carbon yield of charcoal can be used to calculate the useable carbon content for
metallurgical applications [
27
]. The physicochemical properties of renewable reductants
are inferior to those of metallurgical coke and must be adjusted to the application in SAF.
Previous studies have shown that charcoal can be post-treated to achieve properties which
approach those of metallurgical coke [
28
,
29
]. However, classical charcoal and sustainably
produced charcoal are considered as possible reductants for ferromanganese production,
in which post-treatment processes, such as acid leaching or a secondary heat treatment,
can close the gap between renewable to fossil fuel-based reductants [
30
,
31
]. In addition,
renewable reductants can improve metal quality and productivity by their low ash content
and ash composition [32,33].
Power generation and manufacturing of reductants are the main parameters which
affect the overall GHG emissions in ferromanganese alloy production [
8
]. As power supply
is considered CO2neutral in Norway, fossil fuel-based reductants are the main sources of
GHG emissions. The overall emissions can be reduced by the recovery of CO off-gas and
the on-site utilization [
8
], which is already performed by smelters in Norway. Between 50
and 85% of the total GHG emissions in ferroalloy production are generated by coal and
coke [
7
]; 25–35% of the GHG emissions can be ascribed to the on-site air emissions [
8
].
GHG emissions for the production of metallurgical coke, mining, and transportion and
possible reduction by renewable reductants are estimated from the steel industry [
7
,
34
].
About two-thirds of particulate matter emissions are associated with mining, handling,
and preparation of the raw feedstocks, whereas only one-third are direct emissions from
the SAF [
35
]. Direct emissions from tapping can be reduced by simple measures, such as
the addition of curtains and ventilation of the tapping area [36].
Previous studies have shown that the emission factor of ferromanganese alloy pro-
duction was between 1.04 and 6.0 kg CO
2
per kg FeMn, respectively, 2.8 kg CO
2
per kg
SiMn and 3.4 kg CO
2
per kg FeSi [
7
,
8
,
37
]. The acidification potential for ferromanganese
alloy production was stated to be 45g SO
2
-eq. per kg FeMn, while the photochemical
ozone creation potential was determined to be 3 g C
2
H
4
-eq. per kg FeMn [
8
]. However, no
complete LCA study has been found for ferromanganese alloy production.
Processes 2021,9, 185 5 of 19
Table 1. LCA studies reviewed to evaluate GHGe for ferromanganese production.
Process Stage Region Economics Human Health Environment Other Factors Source
BM, coal mining US land occupation and regeneration time [38]
BM North America, Europe toxicity GHG land occupation, acidification [39]
BM considered considered considered policy and societal impacts [17]
BM,CC South America gas, water [40]
BM,CC IN, East Africa considered gas [41]
BM (harvest,pyro) US TRACI TRACI activation of charcoal [42]
Mining CN air, water land occupation [43]
Coal mining PL IPCC human health, ecosystems and resources [44]
Coke (Ferro) considered considered [45]
BM,coke FR feedstock, CO2tax GHG transport (regional <100 km)
Coke production CN considered considered air and water emissions [46]
Coke production UK(steel),AU(coal) considered considered GHG,gas,water [47]
Coke production [48]
Coke production TR considered considered by-product utilization [49]
Mn-alloy production AU, CN, FR, IN, ZA, US GHG, SO2e, C2H4e LCIA, energy demand [8]
Mn-alloy production AU GHG [7]
LCIA:= Life cycle impact assessment
Processes 2021,9, 185 6 of 19
2.1. Mining
Mining of both manganese ore and coal contributes to the GHG emissions by ferro-
manganese alloy production. The emissions depend mostly on the country of origin and if
surface mining or underground mining is applied. Coal mining can lead to air pollution
(especially dust formation), surface and ground water pollution, solid waste land occupa-
tion, and destruction of local ecological environment [
38
,
43
,
50
]. This includes biodiversity,
which has attracted more focus in the recent years [
51
]. Underground mining requires
significantly more electrical energy, and thus GHG emissions from power production can
become predominating. Methane emissions from the shaft system and auxiliary vents
are additional air pollutants and GHG emissions for underground mining of coal [
44
,
47
].
Acidification is the main problem for water pollution at long-established mining operations.
An advanced water management can improve the discharged water quality by prevention
and innovative water treatment technologies. Other direct impacts of land use include
landscape transformation, vegetation removal, and soil destruction [38].
Most of the energy required for surface mines is provided by diesel-powered mo-
bile equipment [
8
,
47
], resulting in GHG emissions from combustion of the required fuel.
Underground mines can also require more electricity, but produce less waste rocks than
surface mines [
8
]. Overall, the environmental impact of power supply for surface mining
is small compared to the particulate matter (PM) or dust formation, that has the largest
impact on the environment (
≈
37%), followed by global warming (
≈
29%) or acidification
(
≈
23%) [
8
,
43
]. Other studies have shown that the largest impact on the GHG emissions
from underground mining is related to the energy demand, methane emissions, and pro-
cessing of wastes [
44
]. The effect of methane emissions for a time frame of 20 and 100 a
(85 and 121 kg CO
2
-eq. per kg) represents about two-third of all factors, while electricity
production is the main GHG factor (representing 50%) for the timeframe of 500 years [
44
].
For a cleaner production, the main aim is to improve the environmental performance
of mining, whereas sustainable development requires minimization of environmental
costs [
51
,
52
]. In addition, the negative health effects due to mining, handling, and pro-
cessing should be taken into consideration [
44
]. The treatment of mining water, especially
acidic water or water with high contamination (heavy metals, total suspended solids, total
dissolved solids and oils), will effectively decrease the environmental impact from coal and
manganese ore mining [
53
]. The CO
2
emission factors for underground coal mining per
tonne of coal were ≈15 kg CO2for electricity production, ≈11 kg CO2for coal processing,
≈
9 kg CO
2
for haulage and hoisting coal,
≈
7 kg CO
2
for ventilation work, and
≈
5 kg CO
2
for exploitation [
44
]. However, secondary effects on the biodiversity and land recultivation
after post-mining have not been considered in previous LCA studies [38,54].
2.2. Reductants
Direct GHG emissions in SAF are mainly produced from fossil fuel-based reductants,
e.g., metallurgical coke. This currently used fossil fuel-based reductants can be replaced by
renewable reductants, such as bio-coke and charcoal. However, the inferior mechanical and
chemical properties of bio-coke and charcoal hamper the direct replacement of metallurgical
coke [
55
]. Most likely, metallurgical coke will be partly replaced by bio-coke in the short-
term [
17
] and by tailor-made charcoal in the long-term [
15
]. Other technologies, such
as electrolytic manganese metal production, COREX
®
or FINEX
®
in combination with
renewable reductants may also provide a CO
2
neutral production route as an alternative to
the SAF [56,57].
Charcoal as an alternative reductant was mainly investigated for steel making, includ-
ing pyrolysis by-product utilization [
17
,
58
]. However, renewable reductants have been
also studied in ferroalloy, silicomanganese, and manganese alloy context [
59
–
61
]. The main
obstacles for using charcoal in metallurgical industry are its high costs, inferior properties
of charcoal, and less investigated efficiency of the tailor-made conversion process. Cost
of the feedstock followed by the consecutive economics of renewable reductants and by-
products are the key parameters to evaluate the performance of renewable reductants in
Processes 2021,9, 185 7 of 19
ferromanganese alloy production [
17
]. On LCA basis, production and conversion routes of
fossil fuel-based reductants must be compared to renewable ones. Socioeconomic factors
are especially important for the biomass and charcoal production in developing countries,
in which biomass and charcoal are the main fuels for heating and cooking [
62
–
65
]. While
a larger charcoal demand positively affects the economy of the producers [
65
], low- and
middle-income countries rely on affordable and sustainable cooking fuel supply [64].
Less than 30% of the GHG emissions are related to coal mining, handling, and prepa-
ration, whereas
≈
60% of GHG emissions are emitted by the metallurgical coke produc-
tion [
66
]. The remaining emissions are generated by the combustion of diesel fuel (transport)
and electricity production. A similar ratio is given for charcoal production, where
≈
61% of
GHG emissions are emitted during charcoal production, and the remaining 39% occurred
during its usage [
40
]. Unsustainable biomass growth can decrease savings related to CO
2
emissions.
2.3. Fossil Fuel Reductants
The main GHG emissions from fossil fuel-based reductants beside its usage occur
from mining, transport, and processing of the coal. The energy demand for surface
mining, washing, and transport of coal is stated to be small [
66
], whereas emissions such
as dust formation or acidification can have a significant impact from LCA perspective.
Coal conditioning, such as coking or metallurgical coke formation, can increase the GHG
emissions by the required energy demand and volatile matter release. The coke production
by dry quenching can result in up to 15% less air emissions compared to the traditional
wet quenching [46].
2.3.1. Coal
Similar to ore mining, coal can be obtained by surface and underground mining. For
underground mining, the average power consumption per washed tonne of coal is stated
to
≈
25 kW h [
66
], resulting in
≈
64 kg CO
2
emissions [
67
]. Power consumption for coal
transportation is 1–2 kW h/(t km) [
68
] and is often neglected due to the low impact [
66
].
However, the on-site preparation of coal at the coke production can highly influence the
environmental performance of the coke production process [
46
]. As a result, coal mining
and coke production have the highest impact on the depletion of fossil fuels, whereas the
energy demand for surface mining and coke formation is low [
49
]. Domestic long-range
transport is carried out by railway, where GHG emissions are based on the power supply
of the train grid system. For example, the average CO
2
emission factor for EU railway
transport in 2009 was 370 g CO
2
per kW h [
69
], whereas average CO
2
emissions factor in
China was 627 g CO2per kW h [67].
2.3.2. Metallurgical Coke
Metallurgical coke is produced from blends of several coal types in coke oven batteries.
About two-thirds of the total production are formed in China by classical technologies from
the 1990s [
46
,
70
]. Previous studies have shown that the production of metallurgical coke
is one of the main GHG emission factors for pig iron production [
45
]. The upstream coal
mining in combination with the coke production are the main sources of GHG emissions,
in which CO
2
and CH
4
significantly contribute with fractions up to
≈
61% and
≈
32%,
respectively [46].
About 2% of the energy demand of coke production is covered by electricity (16–
43 kW h/t) [
66
,
71
]. Most of the energy demand for coke production (
>
90%) is covered by
natural gas, blast furnace gas, or coke oven gas [
47
]. The overall emission factor for coke
production was estimated to
≈
0.8 kg CO
2
per kg coke [
47
]. Energy demand for gasification
coke is about 25% larger than for metallurgical coke production due to the lower yield of
reductant [66].
The emission factor is reduced to 0.5 kg CO
2
per kg coke when the metallurgical coke
yield is increased by 10% or by the usage of natural gas if no coke oven gas is used [
47
]. One
Processes 2021,9, 185 8 of 19
possibility to increase the solid yield per tonne of coal is the production of bio-coke, where
charcoal is added to the coal blend. Previous studies have shown that 2–15% of charcoal can
be added to the coal mixture without any negatively impact on the coke properties [
72
–
74
].
A 20% replacement of fossil fuel-based coke with the renewable charcoal can result in a
reduction of 15% GHG emissions [
75
]. As an alternative feedstock, low rank coals may be
utilized in the coking process to improve economics and energy efficiency of the overall
reduction process [66].
Using the coke oven gas for heating and power generation can improve the envi-
ronmental impact of coke production. For example, the organic Rankine Cycle (ORC)
technology for coke oven and blast furnace gas can reduce GHG emissions by
≈
6% [
47
].
Coal charging and pushing can release additional emissions to the air and freshwater [
46
].
The increased coke production results in an increase of PAH and benzo[a]pyrene emis-
sions [
76
], which have an adverse impact on local ecosystems and human health [
77
,
78
].
The post-treatment of airborne emissions and wastewater in combination with strict control
policies can significantly reduce these emissions [46].
Current coke oven batteries have an emission factor of
≈
0.4–1.27 kg CO
2
-eq. per kg of
coke, resulting in
≈
2.3 kg CO
2
-eq. per kg liquid steel [
7
,
45
,
66
]. Other fossil fuel-based re-
ductants, such as gasification coke showed about 17% greater emission factor [
66
]. Overall,
the release of organics and fine dust affecting human health, as well as the consumption
of resources and the environmental impact by emissions have the largest impact from
metallurgical coke production [
49
]. To minimize fossil fuel depletion and anthropogenic
CO
2
emissions, alternative reducing agents can replace coal and metallurgical coke partially
nowadays and fully in the future [
1
]. The replacement of metallurgical coke by anthracite
or charcoal can decrease GHG emissions by
≈
3.1 and 3.2 kg CO
2
per t of metal, as reported
for the steel manufacturing [75].
2.4. Biomass Growth
Sustainable biomass growth is essential to reduce GHG emissions from metallurgy.
Deforestation, soil degradation, as well as air and water pollution are the main challenges
to avoid unsustainable charcoal production [
79
]. To avoid deforestation in developing
countries, only wood from secondary forestry or biomass waste should be used as feedstock
material in charcoal production. The charcoal production in Brazil showed the transition
from primary to secondary forestry in the last decades [
80
,
81
], whereas African countries
rely on the production of charcoal from primary forestry due to the high demand and low
price [
82
,
83
]. Additional measures are carried out in Brazil to improve biomass growth
in a frame of circular economy [
84
]. Biomass certification, such as the Programme for
the Endorsement of Forest Certification (PEFC), Forest Stewardship Council (FSC), or
European Biochar Certificate (EBC), can support the production of charcoal as a renewable
reductant from sustainable source.
Between 2 and 7% of the anthropogenic CO
2
emissions are attributed to the production
and usage of fuelwood and charcoal [
85
]. Most of the GHG emissions are caused by
deforestation and combustion-related pollutants [85]. Poor plantation management, fuels
for harvesting and transport can result in an emission factor of 105–120kg CO
2
per t of
charcoal [
14
]. Thus, the combination of GHG emissions from charcoal production by
kilns in combination with deforestation can result in a net increase of global warming
potential [
86
]. When a sustainable biomass production is ensured, the main sources of
GHG emissions occur from thermochemical conversion and raw feedstock processing [
11
].
Non-sustainable land use and soil degradation are expected to occur mainly in regions
with high poverty and uncontrolled state management, resulting in deforestation and
degradation of the environment [
87
]. Such unsustainable biomass growth can result in
an emission factor of
≈
40 kg CO
2
per t of dry biomass, which can increase to
≈
80 kg CO
2
per t if additional biomass treatment (e.g., chipping) is required [
14
]. A land usage with
maximizing the biomass growth may negatively affect biodiversity and soil properties [
39
].
Thus, a sustainable biomass production has a larger land requirement to maintain the
Processes 2021,9, 185 9 of 19
biodiversity in the region [
38
]. The land requirements can be divided into the size of
transformed land to produce the biomass and the land occupation for the time the land is
used to produce the biomass. Growth rates of biomass mainly depend on biomass species,
climate, irradiation, and soil and are between 10 and 20 t/(ha a) [14,38,88].
Charcoal produced from both sustainable biomass and waste by-products is beneficial
to reduce GHG emissions from metallurgy and fossil fuel depletion [
1
]. Biodiversity is the
basis of ecosystem health [
89
] and has attracted more attention in recent years. The LCA
for wood from secondary forestry should therefore comprise the environmental, social,
and economic impacts of the whole value chain [
90
]. Forest residues and waste streams
from wood industry can support a sustainable biomass production without transforming
natural forests to secondary forestry [
79
]. The GHG emissions from indirect land use can
increase GHG emissions by factor of ≈13 [17].
2.5. Biomass Pretreatment
Wood is mainly used as feedstock in the classical charcoal production and will be the
most reliable feedstock for metallurgy. Stemwood is the best feedstock material based on
the low ash content. Previous studies have investigated the forest management, harvest,
transport, and processing of biomass, as well as the biomass pyrolysis [
42
]. The emissions
from harvest and transport of biomass are low for short distance transportation and can be
neglected if these emissions are biogenic [75].
Most of the carbon losses occur by the thermochemical conversion process, where
about 50% of the carbon is lost as CO
2
and volatile gases [
75
]. Biomass sizing is necessary
to provide a particle size which is required for the pyrolysis process. The processing of
biomass and charcoal can require less energy than coal, resulting in lower GHG emissions
for power production. For example, milling of dry biomass requires up to 50% less energy
than wet biomass [
91
]. On the other hand, bio-oil production can result in GHG emission
factor of up to 84 kg CO2-eq. [75].
2.6. Classical Charcoal Production
Charcoal in Africa and Asia is often produced in earthmound kilns and pits at a low
efficiency (batch processes), whereas charcoal in industrialized countries is produced in
continuous retorts with by-product utilization [
13
–
15
,
79
]. Classical charcoal production
results in the emission of incomplete combusted volatile matter, such as particulate matter,
volatile organic carbon, organic acids, or polycyclic aromatic hydrocarbons (PAHs) [
86
,
92
],
which are considered hazardous to health and environment [
93
,
94
]. One kg of smoldering
wood can pollute
≈
700 m
3
of air [
41
]. Thus, the LCA of charcoal production should
include different environmental impact categories, such as GHG emissions, acidification,
eutrophication, as well as by-products and waste management [13,43].
At least 80% of the unburnt volatiles can be fully combusted by using an after-
burner [
92
], resulting in a 33–40% reduction in the environmental impact of charcoal
production [
13
]. Although CO
2
emissions from biomass and its derivatives are accounted
as CO
2
neutral, toxicity, and land acidification of volatile matter can result in net GHG
emissions. Based on the emissions, the selection and used technique of the carbonization
process is as important as the sustainable biomass production [11]. Previous studies have
shown that GHG savings from charcoal fines in metal production can be similar to lump
charcoal (≈1780 kg CO2-eq. per t hot metal) [75].
Classical charcoal production can result in an emission factor of up to 9kg CO
2
per kg
of charcoal [
83
]. The high emission factor is based on the low charcoal yield (w
<
20%) and
the incomplete combustion of volatiles. The release of volatile matter (e.g., tars and organic
acids) can increase overall GHG emissions, making the replacement of fossil fuel-based
reductants with the renewable materials redundant. For example, the emission factor for
charcoal as an energy carrier can be 10 times larger than that of the direct combustion of
wood due to the excess of methane and CO
2
release in the pyrolysis [
41
]. In worst case,
production of charcoal by earthmound kilns can result in net GHG emissions [11].
Processes 2021,9, 185 10 of 19
The gross GHG emission factor of charcoal production was 1.6–4.7kg CO
2
-eq. per kg
charcoal [
13
,
14
,
86
] with a 100-year global warming potential of up to 5.685 kg CO
2
-eq. [
95
].
The lowest reported emission factor for biochar production was 0.22kg CO
2
per kg of
biochar [
7
]. These emissions can be accounted as CO
2
neutral if the volatiles are completely
combusted in the process. However, the biomass feedstock, transport distance and plant
size must be considered for the LCA and pyrolysis location [
17
,
96
]. The main factors
that have to be considered in LCA for classical charcoal production are climate change,
photochemical oxidant formation, and human toxicity due to the release of unburnt hy-
drocarbons [
13
]. The impact on climate change can be between 2700–4700 kg CO
2
per t of
charcoal for classical charcoal production and can be reduced by 33–40% for the advanced
charcoal production [13].
2.7. Sustainable Charcoal Production
Sustainable charcoal production is assumed as the currently best available case sce-
nario. The charcoal production in industrial retorts results in a twice to three times larger
solid yield compared to classical charcoal production [
25
,
97
,
98
], while the volatile hydrocar-
bons are recirculated and utilized in the process [
13
]. Both benefits improve the economics
of the process, decrease the GHG emissions, and reduce the risk of local pollution [
13
].
However, these processes have high capital expenditures (capex), and thus are not used
in Africa or Asia. In France,
≈
50 kt of charcoal are produced in industrial retorts for
households, barbeque or catering, and thus cannot cover the demand from metallurgical
industry for high quality charcoal [
75
]. However, modern technologies charged with li-
censed biomass from secondary forestry shall avoid deforestation and ensure a defined
income for the biomass producers in developing countries [85].
Industrialized charcoal production requires higher capex than classical charcoal pro-
duction, and thus is economically feasible only for large scale applications [
13
]. Decentral-
ized biomass growth can be combined with a centralized charcoal production to exploit
by-products and use other synergistic effects [
99
]. The amount of by-products depends
on the feedstock quality and process conditions, such as heating rate, gas residence time,
and final temperature [
14
]. The liquid by-products from pyrolysis can be condensed and
post-treated to biofuels or chemical feedstocks [
100
]. New technologies, such as multi-stage
pyrolysis units, can decrease the energy demand for the process and enable the utilization
of the by-products [
101
]. These by-products can replace other fossil fuel-based feedstocks
and further reduce anthropogenic CO2emissions.
In the long-term, sustainable biomass growth and high efficient charcoal production
will have a much lower area demand, stable economics for producer and consumer, and
greater environmental savings than classical charcoal production. The conditioning of
liquid by-products can open new markets for biofuels, chemicals, or pharmaceutics and
should reduce the environmental impact to minimum [
100
,
102
]. Although GHG emissions
from biomass growth and charcoal production are considered CO
2
neutral, handling and
transportation of reductants will rely on fossil fuels for the next decades.
2.8. Transport
The transport of ore, coal, coke, and charcoal is another source of GHG emissions
and land occupation. Previous studies have shown that the GHG emissions are negligible
(
<
5%) for coal and coke transportation in steel industries [
11
,
46
,
103
]. However, charcoal
has a twice lower density than coal and coke that can affect the cost and environmental
impact of charcoal transportation, especially since volume limitations occur for these low
bulk density [
14
]. The transport of whole stem wood is most favorable for the charcoal
production chain, as bulk density is highest and handling and sizing can be carried out at
the charcoal retort [104].
The metallurgical smelters in Norway are located on the coast and international
transport is mostly operated using seaways. Thus, only biomass transport from harvest to
the pyrolysis plant and charcoal transport to the harbor are expected to use road ground
Processes 2021,9, 185 11 of 19
transportation. Previous studies have shown that an increased transport distance from
50 to 100 km has mainly an effect on the economics, whereas the effect on total energy
consumption and GHG emissions was small [
66
]. Loose biomass, such as forest residues or
straw can be mechanically densified to improve transport efficiency [
104
,
105
]. Wood or
forestry residues can also be used as a transportation biofuel to reduce GHG emissions by
up to ≈45–75% [106,107].
3. Environmental Impact
Most LCA studies are carried out for the environmental impact, e.g., GHG emissions,
acidification, and land occupation, whereas some also include human health issues and
economics. Solid waste, eutrophitcation, and dust formation are also environmental
impacts which should be considered due to the feedstock growth [
43
]. While the standard
approach for LCA is “cradle-to-grave”, the “cradle-to-gate” approach is most suitable
for the metallurgical industry. This approach can be used according to ISO 14040 for
silicomanganese or ferromanganese alloys [
8
]. The emission factors of ferroalloy production
vary between 1.04 and 1.15 kg CO
2
per kg ferromanganese [
37
], 1.4 and 6.9 kg CO
2
per kg
silicomanganese, 2.5 and 4.8 kg CO
2
per kg ferrosilicon [
8
,
108
,
109
], and 6.0 kg CO
2
per kg
manganese [8]. The large differences in the emission factors were mostly observed due to
the electric power supply by hydropower or coal.
Mining and biomass growth have the largest impact on the land consumption and
local biodiversity. The direct and indirect GHG emissions from mining expands over time
and are often driven by global factors which are uncontrollable by the local management or
policies [
110
]. The renaturation of mining areas to the full recovery can take several decades
to hundreds of years [
38
] and is concomitant with changes in the landscape, in which the
change in landscape may result in environmental and social impacts [
110
]. Secondary
forestry and other biomass growth scenarios have a large area consumption to provide
sufficient biomass for renewable reductant production. The increased global biomass
demand will open new markets and rise the risks of rapid changes in local landscapes and
ecosystems [87] and the local impacts in the biomass production countries [52].
Industrialized countries will ensure by policies a sustainable biomass growth and
charcoal production, whereas developing countries may have challenges to execute the
policies at district or regional level [
79
]. However, markets and policies are driven by global
factors and create the opportunities and constraints for the new land users [
87
]. To ensure
the sustainable ferromanganese alloy production, global factors, as well as natural and
social sciences in foreign countries have to play an increasing role in the process chain from
cradle-to-gate. Fuel and power supply in developing countries will be a source of GHG
emissions for mining and railway transportation. However, the energy demand for both
process stages is small compared to the metallurgical coke and ferromanganese production.
Replacement ratios of fossil fuels by bioenergy can significantly reduce the emission factors
in each process stage [17].
4. Socioeconomic Effects
Charcoal is mainly produced in Africa and Asia by households with low income. In
some countries the charcoal market accounts for more than 2% of the GDP [
111
], and the
biomass demand is greater than the sustainable biomass growth [
82
]. To supply biomass
for the industrial scale production, ecological and ethical criteria must be followed to
avoid social tension. Local policy, structured governance, and well-funded instructions can
improve the long-term success of sustainable charcoal production in these countries [
79
].
The charcoal production costs in Brazil are
≈
200 EUR/t of charcoal, whereas production
costs in Finland or Austrilia are between 270–480EUR/t without considering the by-
products as value-added compounds [
17
]. Bio-oil from flash pyrolysis ranges at 200–
300 EUR/t [17], indicating an economical value of the liquid by-products.
Simple retorts can be constructed for
≈
300 EUR [
41
], whereas industrial retorts such
as Lambiotte retorts have an capex of 0.5–2 million EUR [
112
]. The high capex of industrial
Processes 2021,9, 185 12 of 19
retorts is not economically reasonable for local farmers in developing countries. From
socioeconomic perspective, centralized charcoal production units can be provided as a
development assistance to minimize emissions from charcoal production, create local
jobs, ensure high conversion rates and stable charcoal properties for a long period. The
production in industrial retorts would increase the available charcoal by a factor of
≈
2
without increasing the demand in the raw feedstock.
Biomass certification such as the Programme for the Endorsement of Forest Certifi-
cation (PEFC), Forest Stewardship Council (FSC), or European Biochar Certificate (EBC)
can support local farmers with the licensing of sustainable wood sourcing. The sustainable
biomass growth in combination with the industrial charcoal production will maximize
CO
2
emission saving potential and concomitant reduce local toxic emissions formed by
classical charcoal production. An adequate income will be provided to farmers for a sus-
tainable biomass growth, and the additional charcoal yield will at least partially cover the
carbon demand of ferromanganese alloy production without increasing social tension in
the charcoal producing countries.
5. Discussion
Previous studies have shown that charcoal has the potential to reduce the GHG
emissions for the integrated steel making route by 31–74%, respectively, up to a CO
2
neutral production by EAF [
14
]. The power supply in Norway makes it most likely that
ferromanganese alloys can be produced CO
2
neutral by compensating direct emissions
from SAFs by renewable reductants. Other ferromanganese alloy-producing countries can
increase the renewable energy production in their energy mix to reduce indirect emissions
from power supply [
9
,
10
]. When the properties of carbon reductants inhibit a complete
replacement of fossil fuel-based reductants, CCS and CCU can be used to compensate
remaining emissions. Previous studies have shown that CO
2
neutral reductants can be used
for silicon and silicomanganese production [
113
], whereas closed hearth SAF still rely on
metallurgical coke due to the low volatile matter content. Post-treatment of charcoal may
improve physicochemical properties to replace fossil fuel reductants in the future [
28
,
29
,
55
].
The possible impacts of a classical and sustainable charcoal production on the LCA of
ferromanganese alloy production are summarized in Table 2.
Mining and metallurgical coke production are the main GHG emitters in the process
chain upstream of the metallurgical industry. Environmental effects such as dust formation
and acidification are often not considered in the life cycle analysis, and will further im-
prove the application of renewable reductants in ferromanganese alloy production. Land
consumption for mining, biomass growth, or road construction for transportation have
received more attention in recent studies, but the environmental impact is considered negli-
gible compared to the coke production for the metallurgical processes. While renaturation
of coal mining can take centuries, sustainable biomass growth can provide biodiversity in
the regions. Short rotation coppice or classical forestry can provide biomass with low ash
content. Transport of biomass and coal has only a minor impact on the GHG emissions
which are often neglected in LCA studies.
Biomass growth and charcoal production are the key process variables to decrease
GHG emissions during ferromanganese alloy reduction. Sustainable biomass growth
requires large areas, and large areas of monocultures should be avoided for plant diversity.
A stem wood production in Southern Norway was estimated to 18 m
3
/(ha a) of solid
wood [
114
], approximately 8 t/(ha a). An average growth rates of 10–15 t/(ha a) (dry basis)
can be realized in temperate climate for short rotation coppice [
88
,
115
]. However, the
lower material density of short rotation coppice such as willow may limit the transport
distance and storage time of this biomass [
116
]. Thus, GHG emissions by transport may
be covered by the additional biomass growth in southern European region for selected
biomass species.
Processes 2021,9, 185 13 of 19
Table 2. Impact of classical and sustainable charcoal production on the LCA in the ferromanganese alloy production.
Process Stage Current Situation and Main Impacts Changes by Classical Charcoal Production Changes by Sustainable Charcoal Production
Ore mining
Dust emissions, water pollution (acidifica-
tion), land transformation, and destruction
of local ecological environment.
No changes expected by renewable reductants.
Coal mining
Dust, CH
4
and CO
2
emissions, water pollu-
tion (acidification), fossil fuel depletion, land
transformation, and destruction of local eco-
logical environment
Reduced coal demand to a fully replacement of coal by charcoal as a renewable
reductant results in a decreased fossil fuel depletion, reduced air and water
pollution and avoids land transformation.
Biomass production Partly deforestation and soil degradation
Additional land occupation, deforestation
and reduced biodiversity.
Reduced demand of additional biomass by an effi-
cient and sustainable charcoal production, as well
as consideration of socioeconomic factors.
Transport (local)
Mainly conveyor belt and railroad trans-
port for coal, respectively truck transport for
biomass and charcoal.
Increased diesel consumption by truck transport for biomass and charcoal
transport expected.
Coke production
Local emissions, resulting in air and water
pollution.
Charcoal can replace up to 20% in bio-coke
production, resulting in a reduced volatile
matter.
Bio-cokes and tailor made charcoal may fully re-
place metallurgical coke in long-term.
Charcoal production
Incomplete combustion and release of volatile
matter, resulting in air and water pollution,
photochemical oxidant formation and human
toxicity.
Increased biomass demand can result in an
increased non-sustainable production and
additional local emissions.
Improved conversion technologies result in an
increased conversion efficiency, by-product uti-
lization and improved charcoal quality. The
greater conversion efficiency can compensate the
increased land demand for sustainable biomass
production.
Transport (international)
Emissions by ship and railroad transport
(<5% of total emissions)
Emissions may increase due to the lower
bulk density of charcoal and the volume lim-
ited transport (emissions may increase by a
factor up to 2)
By-products may be utilized as fuel for transport,
making long-distance transport more sustainable
Smelting (SAF) CO2, CO and dust emissions
CO
2
emissions from charcoal are considered CO
2
neutral, additional gas clean-
ing required for high volatile matter content
Processes 2021,9, 185 14 of 19
About 50% of the carbon content in the raw biomass can be converted to charcoal in
industrial retorts [
117
], resulting in a carbon yield of
≈
2.5–5 t/(ha a). The low conversion
efficiency of earthmound kilns and pits in combination with the incomplete combustion of
the volatile matter makes these technologies counterproductive for the large replacements
of fossil fuel-based reductants in ferromanganese alloy production. Replacing 20% of the
metallurgical coke by bio-coke can reduce the GHG emissions by 10–15% for classical char-
coal production, whereas the sustainable charcoal production would further reduce local
emissions and environmental impact by acidification and water contamination. In addition,
the sustainable charcoal production would further reduce the resource consumption by the
twice to three times greater charcoal yield compared to classical charcoal production.
An ensured price range by certified biomass can convince local farmers to produce
biomass sustainable. Centralized industrial retorts can create high-quality local jobs and
improve the quality of charcoal concomitant to the increased charcoal yield. The current
charcoal production of
≈
55 Mt [
85
] would be increased to
>
75 Mt without the consumption
of additional biomass. The high capex of industrial retorts can be covered by government
subsidies or financial aid.
6. Conclusions
Renewable reductants can decrease the direct and indirect GHG emissions from ferro-
manganese alloy production. The sustainable charcoal production can reduce the indirect
GHG emissions due to the improved feedstock growth and additional area occupation,
whereas the direct emissions can be decreased by the integration of energy efficient pyroly-
sis retorts. GHG emissions generated due to the transportation of feedstocks and charcoal
are expected to be greater than emissions from transportation of metallurgical coke and
fossil fuels due to their low bulk density. The utilization of pyrolysis by-products with the
concurrent production of biofuels can further decrease the emissions related to transporta-
tion. In the next few decades, the integration of renewable charcoal reductants is expected
due to the increased governmental requirements and policies towards development of
environmental and sustainable processes in metallurgical sector within a circular economy.
Overall, the authors believe that ferromanganese alloys can be manufactured in a CO
2
neutral way using carbothermal process with the addition of charcoal that has properties
striving metallurgical coke and electricity provided by renewable sources.
Author Contributions: Conceptualization, G.R.S., A.T., and M.T.; methodology, G.R.S.; validation,
G.R.S., A.T., and M.T.; writing—original draft preparation, G.R.S.; writing—review and editing,
G.R.S., A.T., and M.T.; visualization, G.R.S.; supervision, M.T.; project administration, M.T.; funding
acquisition, M.T. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Research Council of Norway grant number 280968. The APC
was funded by Norwegian University of Science and Technology.
Institutional Review Board Statement: Not applicable
Informed Consent Statement: Not applicable
Acknowledgments: The authors gratefully acknowledge the financial support from Research Council
of Norway (Project Code: 280968) and our industrial partners Elkem ASA, TiZir Titanium & Iron AS,
Eramet Norway AS, Finnfjord AS and Wacker Chemicals Norway AS for financing KPN Reduced CO
2
emissions in metal production project.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
BM Biomass
capex capital expenditures
CC Charcoal
Processes 2021,9, 185 15 of 19
CCS carbon capture and storage
CCU carbon capture and utilization
EBC European Biochar Certificate
FeMn ferromanganese
FSC Forest Stewardship Council
GHG greenhouse gas
LCA life cycle assessment
ORC organic Rankine Cycle
PAHs polycyclic aromatic hydrocarbons
PEFC Programme for the Endorsement of Forest Certification
SAF submerged arc furnace
SiMn silicomanganese
tonne metric ton
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