energies

Perspective
Advanced Applications of Torrefied Biomass:
A Perspective View
Tharaka Rama Krishna C. Doddapaneni * and Timo Kikas *

Chair of Biosystems Engineering, Institute of Forestry and Engineering, Estonian University of Life Sciences,
Kreutzwaldi 56, 51014 Tartu, Estonia
* Correspondence: tharaka.doddapaneni@emu.ee (T.R.K.C.D.); timo.kikas@emu.ee (T.K.)

Abstract: Because of the social, economic, and environmental issues linked with fossil resources,
there is a global interest in finding alternative renewable and sustainable resources for energy and
materials production. Biomass could be one such renewable material that is available in large
quantities. However, biomass physicochemical properties are a challenge for its industrial application.
Recently, the torrefaction process was developed to improve the fuel characteristics of biomass.
However, in recent days, energy production has slowly been shifting towards solar and wind, and
restrictions on thermal power plants are increasing. Thus, there will be a need to find alternative
market opportunities for the torrefaction industry. In that regard, there is a quest to find alternative
applications of torrefaction products other than energy production. This paper presents a couple
of alternative applications of torrefied biomass. Torrefaction process can be used as a biomass
pretreatment option for biochemical conversion processes. The other alternative applications of
torrefied biomass are using it as a reducing agent in metallurgy, as a low-cost adsorbent, in carbon-
black production, and as a filler material in plastics. The use of torrefied biomass in fermentation
and steel production is validated through a few laboratory experiments, and the results are looking
attractive. The lower sugar yield is the main challenge in the case of the microbial application of
torrefied biomass. The lower mechanical strength is the challenge in the case of using it as a reducing
agent in a blast furnace. To date, very few studies are available in the literature for all the highlighted
applications of torrefied biomass. There is a need for extensive experimental validation to identify
the operational feasibility of these applications.

Citation: Doddapaneni, T.R.K.C.;

Keywords: biomass torrefaction; reducing agent; fermentation; adsorbent; carbon black; torrefied
Kikas, T. Advanced Applications of
biomass applications; thermochemical conversion; biochar; biocoke
Torrefied Biomass: A Perspective
View. Energies 2023, 16, 1635.
https://doi.org/10.3390/en16041635

Academic Editor: Fernando Rubiera

1. Introduction
González
Today, global warming is one of the major threats the world is facing. The conse-
Received: 2 January 2023 quences of global warming are the melting of glaciers, uneven weather patterns, droughts,
Revised: 1 February 2023 and rising sea levels. The widely accepted reason for global warming is the increased
Accepted: 2 February 2023 greenhouse gas emissions into the atmosphere. The commonly known greenhouse gases
Published: 7 February 2023 are CO2 , NO2 , CFC, and water vapor, and among these CO2 is playing a major role in the
global warming. The usage of fossil resources in fuel and material production is the main
reason for the ever-increasing CO2 level in the atmosphere. Today, the global economy
depends on fossil resources. However, measures are being taken over a long time in order
Copyright: © 2023 by the authors.
to reduce society’s reliance on fossil resources. At the regional level, the EU is aiming to
Licensee MDPI, Basel, Switzerland.
reduce the CO2 level by 85% compared with the pre-industrial level. To achieve these ambi-
This article is an open access article
distributed under the terms and
tious targets, fossil resources need to be replaced with renewable resources. In that regard,
conditions of the Creative Commons
biomass is being considered as a renewable resource. Even today, biomass is mankind’s
Attribution (CC BY) license (https:// primary resource for several applications. However, these applications may differ between
creativecommons.org/licenses/by/ different countries depending on their socioeconomic development. In developing coun-
4.0/). tries, biomass is being used for primary energy applications, and in developed nations,

Energies 2023, 16, 1635. https://doi.org/10.3390/en16041635 https://www.mdpi.com/journal/energies

Energies 2023, 16, 1635 2 of 8

the same is being used for advanced applications such as biochemicals and bioenergy
production [1].
Although biomass is renewable and available in large quantities, it also possesses
some drawbacks which include high moisture content, reluctant nature, hydrophilicity,
ash, and microbial degradation. In order to overcome these issues, biomass needs to be
pretreated. The selection of these pretreatment methods depends on the type of conversion
process and the desired end product. Torrefaction is one such pretreatment method that
improves the biomass properties to the level of competing with coal. In the last decade,
research activities on torrefaction have gained bigger momentum. At the moment, a couple
of commercial scale operations are also established around the globe. The primary intended
application of torrefaction is to produce torrefied biomass for energy applications. However,
around the globe, there is a shift in energy production, and in the future, solar and wind
can play a major role. At the same time, the restrictions on thermal power plants are also
increasing globally [2]. However, the closing down of the power plants will still take a long
time. In addition, the demand for torrefied biomass pellets exists in other industries such
as pharmaceuticals, cement, and food industries, where process heat energy is required.
Still, to improve business prospects in the future, the torrefaction industry needs to find
alternative applications of torrefaction products.
The majority of the previous studies on biomass torrefaction focused on the proper-
ties of the solid torrefied biomass and its subsequent applications in thermal conversion
processes such as combustion, pyrolysis, and gasification. Few studies focused on the
diversified applications of torrefaction products. Technically, the torrefaction process can
also be integrated with other biomass conversion processes, and torrefaction products
can be used in different applications other than energy production. In this regard, the
perspective applications of torrefied biomass are presented in this paper. The main aim of
this paper is to highlight the applications of torrefied biomass other than energy production.
The challenges and future research directions are also highlighted.

2. Torrefaction Process
Torrefaction is a low-temperature thermochemical conversion process, which is carried
out in the temperature range of 200–300 ◦ C. Generally, torrefaction is carried in the inert
environment using either N2 and/or CO2 . However, some researchers also studied oxida-
tive torrefaction in a reducing environment (i.e., low oxygen). The main intended purpose
of the torrefaction process is to improve the biomass fuel characteristics. For example,
torrefaction reduces the volatile matter content of the biomass and increases the heating
value. It increases the hydrophobicity of the biomass by removing the O-H groups in the
biomass. The increased energy density is one of the major advantages of the torrefaction
process, which helps to improve the transportation economics of the biomass [3].
Detailed information on product distribution and mass and energy balances during
biomass torrefaction can be found in [4,5]. During torrefaction, the volatile fraction of
the biomass is released through different reaction mechanisms. The solid (i.e., torrefied
biomass) and gases (torrefaction volatiles) are the products of the torrefaction process.
During torrefaction, biomass undergoes drying, depolymerization, devolatilization, and
carbonization [6]. The extent of these processes depends on the operating conditions such
as temperature (i.e., severity of the torrefaction process). Deoxygenation is one of the
advantages of the torrefaction process. During torrefaction, the oxygen content of biomass
is released as volatiles, mainly in the form of water, CO2 , CO, and other condensable
products (i.e., organic acids).
Torrefaction volatiles contain both condensable and uncondensable gases. The tor-
refaction condensate mainly contains water and organic acids (i.e., acetic acid, propionic
acid, formic acid), furfural, furans, and phenols. Although the torrefaction condensate
contains several compounds, water and acetic acid are the major compounds. The uncon-
densable gases mainly contain CO2 , and CO, and the other gases such as CH4 and H2 in
low concentrations [7].
Energies 2023, 16, 1635 3 of 8

The solid, i.e., torrefied biomass is the main intended product of the torrefaction.
The other products, i.e., torrefaction volatiles released during torrefaction, are combusted
in a boiler together with utility fuel to match the heat energy requirement. The yield of
the solid and volatiles varies in the range of 40–80 wt.% and 20–60 wt.%, respectively,
depending on the operating parameters. Using it as a solid fuel in energy production
is the main intended application of the torrefied biomass. Because of the migration of
oxygen in the form of volatiles, the carbon content in the biomass increases relatively. This
increases the C/O and C/H ratios and ultimately results in the increased energy content
of the biomass. The heating value of the torrefied biomass varies in the range of 15 to
25 MJ/kg depending on the operating conditions and type of biomass. The fuel ratio of
the torrefied biomass varies in the range of 0.2 to 0.7 [2]. In terms of fuel characteristics,
the torrefied biomass can be compared with low-rank coals such as bituminous and lignite.
As the intention of this paper is to present the futuristic applications of torrefied biomass,
a basic overview of torrefaction is presented here. The readers are guided to check the
recent torrefaction review papers [6,8] for a detailed understanding of the process. For
more information on the current status of torrefaction process, authors suggest to check the
publication [8] from International Biomass Torrefaction Council (IBTC).

3. Perspective Applications of Torrefied Biomass

As an alternative to energy production, torrefied biomass can be used as a source
of sugars in fermentation, as a low-cost adsorbent in soil amendment applications, as
a supporting material in microbial processes (i.e., anaerobic digestion), and as a reducing
agent in metallurgy applications.

3.1. Fermentation of Torrefied Biomass

Lignocellulosic biomass mainly contains cellulose, hemicellulose, and lignin. Gener-
ally, the sugars derived from cellulose are interesting for microbial conversion. However,
these polymers are in a highly ordered complex matrix and reluctant to microbial degrada-
tion. Thus, it is essential to alter the structure and fractionation of lignocellulosic biomass
components prior to their application in microbial processes. There are several processes
previously developed to pretreat biomass which include milling, extrusion, microwave
treatment, acid treatment, alkali treatment, organosolv, fiber explosion, and steam explo-
sion [9]. During the pretreatment, the lignin is degraded and disintegrates. Hemicellulose
is degraded into multiple compounds such as organic acids and furans. Pretreatments
also alter cellulose’s crystalline structure and improve the enzymatic degradation in subse-
quent hydrolysis. All these conventional treatments have their own operational challenges.
Alternatively, torrefaction could also be considered as a treatment option to alter the
lignocellulosic biomass structure for its subsequent application in microbial processes.
In the case of torrefaction, hemicellulose is the mainly degraded biomass polymer, thus
by optimizing the torrefaction temperature, the selective removal of hemicellulose can be
achieved with minimal degradation of cellulose and lignin. The conventional pretreatments
are single-pot processes; therefore, the removal and recovery of hemicellulose and lignin
degradation compounds are challenging especially for chemical pretreatments. In the case
of torrefaction, hemicellulose and lignin are degraded in the form of volatiles which can be
condensed and collected separately. Torrefaction can also be considered as a chemical-free
treatment, such as a steam explosion. The operating temperature of the torrefaction is in
the range of 200 to 300 ◦ C. However, to minimize cellulose degradation, the maximum
temperature can be between 250 and 275 ◦ C. This is not much higher compared with other
pretreatments, for example, acid treatment (100 to 200 ◦ C) and steam explosion (170 to
210 ◦ C) [9].
On the other hand, biomass needs to be milled and grounded prior to pretreatment
to increase the surface area, especially in the case of chemical treatment. However, be-
cause of its fibrous nature, the grinding of biomass is energy-intensive. As torrefaction
increases, the brittleness of the biomass, grinding energy could be reduced by multiple
Energies 2023, 16, 1635 4 of 8

times [8]. Considering the above advantages, the torrefaction process could be considered
as a pretreatment option prior to the microbial conversion of lignocellulosic biomass. Some
research activities are already focused on this direction. Recently, Li et al. [10] studied tor-
refaction as a pretreatment option for the biohydrogen production from the corn stover. The
authors observed that the reducing sugars’ yield and biohydrogen production increased
significantly with torrefaction treatment compared with the untreated corn stover. For
example, the cumulative hydrogen yield increased from 362 mL to 618 mL for the corn
stover torrefied at 200 ◦ C. However, the authors observed that the optimum temperature
was 200 ◦ C, and the further rise in the torrefaction temperature resulted in a significant
decrease in the hydrogen yield.
Recently, Tripathi et al. [11] studied the influence of alkali treatment before and after the
torrefaction on the glucose yield during hydrolysis. The authors reported that torrefaction
severity had a significant influence on sugar yield. Finally, the authors concluded that
alkali treatment of the torrefied biomass could increase the glucose yield from the torrefied
biomass. In another study, Normark et al. [12] also observed that treating the torrefied
biomass with ionic liquids had a higher yield of sugars. A few other studies [13–15] also
evaluated the feasibility of torrefaction as a pretreatment option for biochemical conversion.
All these studies commonly reported that the sugar yield is lower during the hydroly-
sis of torrefied biomass. Generally, the thermal degradation range of cellulose is between
260 and 320 ◦ C. Thus, the loss of cellulose during torrefaction, especially when the temper-
atures are below 250 ◦ C, is low. The previous studies also observed the same. For example,
Cahyanti et al. [3] reported a cellulose loss of 14% at a torrefaction temperature of 275 ◦ C for
forestry wood waste. The morphological changes in the cellulose structure and/or biomass
matrix could be the possible reason for the lower sugar yield for the enzymatic hydrolysis
of torrefied biomass. The water biomass interactions and further mass transfer implications
within the solid biomass particle play an important role in the hydrolysis yield [16]. In the
case of torrefaction, the hydrophobicity of the biomass starts increasing with increasing
temperature because of the reduced hydroxyl groups. This increased hydrophobicity of the
torrefied biomass could be having a negative effect on the enzymatic hydrolysis. Further
extending the discussion, the cellulose crystallinity also shows an effect on the enzymatic
hydrolysis efficiency. The biomass contains both amorphous and crystalline cellulose with
varying concentrations depending on the type of biomass. Because of the lower thermal
stability, the amorphous cellulose is mainly degraded during torrefaction, compared with
crystalline cellulose, and thereby the crystallinity of the torrefied biomass increases [17].
According to Fan et al. [18], the hydrolysis rate and yields were more than 100 times lower
in the case of crystalline cellulose hydrolysis compared with amorphous cellulose. Thus,
the increasing crystallinity could also be the possible reason for the lower sugar yield
during torrefied biomass hydrolysis.
According to the above discussion and based on previous studies, torrefied biomass
must be treated to modify the cellulose structural changes prior to its enzymatic hydrolysis.
The literature survey shows that treating crystalline cellulose with ionic liquids significantly
reduces the cellulose crystallinity. Previously, the authors of [12] reported that treating
torrefied spruce with ionic liquid increased the sugar yield by 647% compared with the
hydrolysis of non-treated torrefied biomass.

3.2. As an Adsorbent
The research interest in using torrefied biomass as a low-cost adsorbent is increasing
in recent days. Few attempts were made to understand the feasibility of using torrefied
biomass as a low-cost adsorbent. Recently, Lee et al. [19] studied the feasibility of spilled
diesel oil recovery using torrefied spent coffee grounds. Interestingly, the authors observed
a high adsorption capacity (i.e., 1.36 times higher) for the spent coffee grounds torrefied
at 300 ◦ C compared with activated carbon. In another study, Lu et al. [20] studied a com-
parative analysis on the adsorption of uranium and methylene blue using torrefied and
pyrolyzed antibiotics’ production fermentation residue. Interestingly, the torrefied residue
Energies 2023, 16, 1635 5 of 8

showed better performance, and the authors attributed the superior adsorption capac-
ity of torrefied residue to the availability of high oxygen and nitrogen functional groups
compared with the pyrolysis char. Another study also showed that torrefied biomass had
a higher adsorption performance (i.e., 2-times higher) compared with the pyrolysis char
produced from the same biomass [21]. Recently, another study used torrefied Cyprus cone
as an adsorbent for the adsorption of oil contaminants from oil spills. The authors observed
that torrefied biomass had superior adsorption properties with a removal efficiency of
92% [22].
The literature data showed that torrefied biomass has superior adsorption properties
compared with pyrolysis char and activated carbon. This is mainly because of the presence
of high-oxygen functional groups. Previous studies strongly advocated the application of
torrefied biomass as a low-cost adsorbent for the removal of pollutants. However, there is
a need for more studies to further understand the relation between torrefaction operating
conditions and morphological changes with an aim of higher adsorption efficiency. At the
same time, there is also a need to analyze the economic feasibility.

3.3. As a Reducing Agent in Metallurgy

Using torrefied biomass as a reducing agent in metallurgical applications (steel-
making) is another interesting application. In order to reduce the negative environmental
impacts, the steel industries are looking to replace fossil coke with biobased reducing
agents. Previously, the biochar from pyrolysis and gasification was mainly considered as
the biomass-based reducing agents. In recent days, the interest in torrefied biomass as
a bio-reducing agent is increasing. A few studies have already focused on this direction.
The EU-funded Torero project is one such practical example [23]. Compared with either
conventional coke or with pyrolysis and gasification-derived char, torrefied biomass could
possess some advantages. According to Konishi et al. [24], the rate of iron oxide reduction
could be much higher for bio-coal with higher volatile matter than that of coke or coal-
derived char because of the release of the reducer gases, i.e., H2 and CO. The gasification
rate of torrefied biomass is also higher because of the porous structure. In another study,
Ubando et al. [25] observed that torrefied biomass with high volatile content decreased the
Wustite formation temperature. These preliminary experimental data showed the better
performance of torrefied biomass than conventional coke and/or pyrolysis chars. However,
there is a need for extensive studies to better understand the overall feasibility of using
torrefied biomass as an alternative reducing agent.

4. Challenges and Future Opportunities

The preliminary data show that torrefied biomass can be used in applications other
than energy production and/or in subsequent thermal conversion processes. However, the
literature survey shows that very few studies are available on the alternative application
of torrefied biomass. The fermentation of torrefied biomass could be one interesting
application to be considered. As presented in Section 3, the torrefaction process presents
several operational advantages compared with other biomass pretreatments. In addition,
lower energy input and superior supply chain benefits give torrefaction an edge over other
pretreatment processes. At the same time, torrefaction process can also be integrated with
the fermentation process either in terms of energy integration or handling the fermentation
residue. As with every biomass pretreatment process, torrefaction also has negative effects
when considered as a pretreatment option for fermentation.
The sugar loss at higher torrefaction temperatures is one challenge. The increasing
crystallinity and hydrophobicity of biomass play a negative effect and result in a low
sugar yield during hydrolysis. The sugar loss (cellulose degradation) can be controlled
by optimizing the torrefaction-operating parameters. The sugar yield during hydrolysis
can be improved by treating the torrefied biomass through chemical treatment (acids or
ionic liquids). Although the operational costs of the torrefaction are comparable with
other biomass pretreatments, the initial investment costs can be higher for torrefaction
Energies 2023, 16, 1635 6 of 8

compared with some of the pretreatment processes. So far, very few research studies are
available on this topic. Thus, there is a need for extensive experimental studies to better
understand the fermentation of torrefied biomass. The future studies must focus on relating
to the morphological changes of biomass during torrefaction and overall fermentation
efficiency. The operating parameters also need to be optimized relating the ash content
and the fermentation efficiency. The ash present in biomass could significantly influence
sugar degradation during torrefaction. At the same time, the economic and environmental
feasibility of integrating torrefaction with fermentation needs to be explored.
The other applications such as using it as an adsorbent and reducing agent are also
interesting applications with high market potential. This is especially true as a reducing
agent, as the steel industry is aiming to use sustainable carbon in their processes. Regarding
the excess heat energy available in the steel industry, for example, the heat energy recovered
from slag could be used for the torrefaction process, which reduces the utility fuel input
and improves the feasibility of the torrefaction process. Interestingly, the torrefied biomass
showed better performance compared with pyrolysis-derived or coal-derived cokes. This
is mainly because of the positive effect of the higher volatile matter present in the torrefied
biomass (i.e., lower C/O ratio). However, there are several other physiochemical properties
of reducing agents which can influence the blast furnace operations, for example, compres-
sion strength, reactivity, and density. Generally, a reducer with high compression strength is
required for efficient blast furnace operations [26]. Previously, Adrados et al. [27] reported
that bio-reducers (pyrolysis chars) are not suitable to use as a top burden in blast furnaces
because of their lower mechanical strengths, and suggested injecting through tuyere. As
stated in the previous sections, there are only a countable number of studies on these
advanced applications of torrefied biomass. The future studies must focus on establishing
the relationship between the physiochemical properties of the torrefied biomass and the
efficient blast furnace operational requirements.
The other application includes the production of nanocellulose and black carbon from
torrefied biomass. The torrefied biomass can also be used as a filler material in plastics and
as a supporting material in anaerobic digestion. However, these applications are still in
their infancy stage.

5. Conclusions
In terms of operational feasibility today, torrefaction is already an established process
and is being operated on a commercial scale. In a view of changing energy scenarios
and increasing restrictions on thermal power plants, there is a need to find alternative
applications of torrefied biomass. In that regard, the perspective application of torrefied
biomass is presented in this paper. The alternative applications of torrefied biomass are
microbial conversion into biochemicals; as a bio-reducer in the steel industry; low-cost
adsorbent, black-carbon production; and as a filler material in plastics. The fermentation of
torrefied biomass and using it as a reducing agent are validated through a few studies, and
the preliminary data look promising. However, it is difficult to conclude on the feasibility
of using torrefied biomass in fermentation and metallurgical applications because of the
lack of experimental data. There are several challenges that still need to be addressed
through extensive experimental validation.

Author Contributions: Conceptualization: T.R.K.C.D. and T.K.; Methodology: T.R.K.C.D.; Investi-

gation: T.R.K.C.D.; Supervision: T.K.; Writing—Original Draft: T.R.K.C.D.; Writing—Review and
Editing: T.K. All authors have read and agreed to the published version of the manuscript.
Funding: This project has received funding from the Estonian Research Council under ResTA
program grant agreement No [RESTA5].
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare that there are no conflict of interest to declare.
Energies 2023, 16, 1635 7 of 8

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