Article

Thermochemical Conversion of Biomass into 2nd

Generation Biofuel
Tomáš Giertl 1 , Ivan Vitázek 1, * , Ján Gaduš 2 , Rastislav Kollárik 1 and Grzegorz Przydatek 3

1 Institute of Agricultural Engineering, Transport and Bioenergetics, Faculty of Engineering, Slovak University
of Agriculture in Nitra, Tr. A. Hlinku 2, 949 76 Nitra, Slovakia
2 Institute of Sustainable Regional and Local Development, Faculty of European Studies and Regional
Development, Slovak University of Agriculture in Nitra, Tr. A. Hlinku 2, 949 76 Nitra, Slovakia
3 Department of Management and Production Engineering, Faculty of Engineering Sciences, Academy of
Applied Sciences in Nowy Sacz, Zamenhofa 1a, 33-300 Nowy Sacz, ˛ Poland
* Correspondence: ivan.vitazek@uniag.sk

Abstract: Bioenergy is considered the largest contributor to the renewable and sustainable energy
sector worldwide, playing a significant role in various energy sectors such as heating, electricity
supply, and even in replacing fossil fuels in the transportation sector. As part of renewable, low-carbon
energy systems, bioenergy can also ensure atmospheric carbon sequestration, provide numerous
environmental and socio-economic benefits, and thus contribute to achieving global climate change
goals, as well as broader environmental, social, economic, and sustainable development objectives.
The use of bioenergy can significantly reduce our carbon footprint and thus contribute to improving
the environment. While bioenergy conversion of biomass produces some amount of carbon dioxide,
similar to traditional fossil fuels, its impact can be minimized by replacing forest biomass with fast-
growing trees and energy crops. Therefore, fast-growing trees and energy crops are the primary raw
materials for bioenergy. The results of the research in this publication confirm the high efficiency of
biomass depolymerization through thermochemical conversion. The principle of continuous biomass
conversion was used at a process temperature of 520 ◦ C. The experiments were carried out in the
Biomass Gasification Laboratory at the AgroBioTech Research Center of the Slovak University of
Agriculture in Nitra. The biomass used for the experiments was from energy-producing fast-growing
willows, specifically the varieties Sven, Inger, and Express. The aim was to determine the amount
Citation: Giertl, T.; Vitázek, I.; Gaduš, of biochar produced from each of these tree species and subsequently to investigate its potential
J.; Kollárik, R.; Przydatek, G.
use for energy purposes. During the experiments, 0.106 kg of biochar was produced from 1 kg
Thermochemical Conversion of
of Inger willow biomass, 0.252 kg from 1 kg of Express willow biomass, and 0.256 kg from 1 kg
Biomass into 2nd Generation Biofuel.
of Sven willow biomass. A subsequent goal was to determine the production of gas, which can
Processes 2024, 12, 2658. https://
also be used for energy purposes. The biofuel samples obtained were subsequently subjected to
doi.org/10.3390/pr12122658
thermogravimetric analysis to determine moisture content, volatile matter, and ash content. The ash
Academic Editor: Francesca content in dry matter ranged from 6% to 7.28%, while the volatile matter in dry matter was between
Raganati
92.72% and 94%. The moisture content in the samples ranged from 1.7% to 2.43%. These results
Received: 30 September 2024 may contribute to innovative insights into biomass depolymerization and help define optimized
Revised: 1 November 2024 parameters for thermochemical conversion, as well as the required biomass composition, with the
Accepted: 21 November 2024 goal of generating second-generation biofuels in the most cost-effective way.
Published: 25 November 2024
Keywords: bioenergy; biochar; depolymerization; pyrolysis; thermogravimetric analysis

Copyright: © 2024 by the authors.

Licensee MDPI, Basel, Switzerland.
1. Introduction
This article is an open access article
distributed under the terms and Biomass is biological material directly or indirectly produced by photosynthesis. This
conditions of the Creative Commons includes wood and wood residues, energy crops, plant residues, and organic waste/by-
Attribution (CC BY) license (https:// products from industries, agriculture, landscaping, and households. Advanced technolo-
creativecommons.org/licenses/by/ gies convert biomass energy into solid, liquid, or gaseous fuels that can be used for heating
4.0/). and/or electricity generation, or as alternative fuel in transportation [1]. Bioenergy is

Processes 2024, 12, 2658. https://doi.org/10.3390/pr12122658 https://www.mdpi.com/journal/processes

Processes 2024, 12, 2658 2 of 12

currently the main source of renewable energy, and studies by the International Energy
Agency [2] suggest that modern bioenergy is a key component of the future low-carbon
global energy system, necessary to meet climate change commitments.
Bioenergy will play an essential role in many scenarios for developing a low-carbon
economy, especially in replacing liquid fossil fuels in transportation, where sufficient
volumes of alternative fuels are not available.
Biomass is one of the key renewable energy sources (RESs) and plays a significant role
in achieving European climate goals by 2030. According to EU Directive 2023/2413 on the
promotion of energy from renewable sources, the renewable energy target for 2030 has been
increased from 32% to 42.5% (with a goal to increase it to 45%). EU member states follow
specific pathways to fulfil their obligations, as defined in national action plans based on
respective energy markets and available resources. In 2022, the share of renewable energy
in the EU accounted for 23% of gross final energy consumption in the EU, with bioenergy
contributing more than 60% of that [3].
Fast-growing trees and energy crops serve as the primary raw materials for bioenergy.
Bioenergy can be categorized into two main types: “traditional” and “modern”. Traditional
bioenergy mainly involves burning biomass, such as wood, animal waste, and traditional
charcoal. Modern bioenergy technologies include the production of liquid biofuels from
purpose-grown biomass in biorefineries, biogas from anaerobic digestion of agricultural
and food industry residues, and municipal waste. They also include wood chip and pellet
heating systems and technologies like pyrolysis.
Approximately three-quarters of the world’s renewable energy consumption is bioen-
ergy, with more than half provided by traditional biomass use technologies [4]. Biomass,
therefore, has significant potential and can be directly burned for heating or energy genera-
tion or converted into substitutes for oil or gas. Liquid biofuels are a suitable renewable
alternative to gasoline and diesel in the transportation sector.
The energy use of biomass in Slovakia holds great potential due to its geographical
conditions, with about 47% of the territory being agricultural land and approximately
41% forest land. This presents a challenge for Slovak farmers as, alongside food production,
there are opportunities in energy production and energy carriers, which could contribute
25–30% of total revenues in the future [5].
According to the World Bioenergy Association (WBA) [6], fossil fuels still dominate
global energy supplies. In 2021, the total primary energy supply worldwide was 618 EJ,
with fossil fuels accounting for 80% (coal 27%, oil 29%, and gas 24%). Nuclear energy
contributed 30.7 EJ, representing 5% of the total energy mix. Renewable energy technologies
such as solar, wind, hydro, biomass, and geothermal accounted for 15% of the primary
energy supply in 2021.
Coal remains a significant contributor to the global energy mix. In 2020, 35% of the
world’s electricity was generated from coal-based sources, totalling 9452 TWh. Of the
26,833 TWh of global electricity generation in 2020, 29% came from renewable sources,
driven by solar and wind energy, as well as contributions from hydropower and biomass.
Hydropower was the largest renewable electricity source, with a 58% share, followed
by wind energy at 21%. Bioenergy was the fourth-largest renewable electricity source,
producing 658 TWh [7].
To meet international energy and climate goals, renewable energy technologies need
to grow at a much faster pace, but efforts are also needed to secure a strategy for phasing
out fossil fuel production and use worldwide. The development of total primary energy
supplies globally since 2000 is shown in Figure 1.
However, to use woody biomass as an energy source, it is necessary to use it rationally
and in a sustainable way [8–10]. On the other hand, there are still some problems in current
biomass combustion furnaces, where kinetic analysis is a suitable tool to solve them [11–13].
 Processes2024,
Processes 2024,12,
12,2658
x FOR PEER REVIEW 33 of
of 12
13

Figure1.1. Global
Figure Global primary
primaryenergy
energysupplies
suppliesin
inthe
theworld
worldfrom
fromall
allsources
sources(edited
(editedWBA,
WBA,2023
2023[6]).
[6]).

However,
Various to use
studies woody
[14–20] biomass
observe thatasthe
an thermal
energy source, it is necessary
decomposition to use
of the main it ration-
wood con-
ally and in
stituents, a sustainable
such way [8–10].
as hemicelluloses, On theand
cellulose, other hand,
lignin, canthere are still into
be divided some twoproblems
steps. In in
current
the biomass
first step, combustion
in the temperature furnaces,
range of where
~200–400kinetic◦ C,analysis is a suitable
the combination tool hemicel-
of total to solve
lulose and cellulose decomposition with partial lignin decomposition occurs. The second
them [11–13].
step could
Various be studies
described as theobserve
[14–20] decomposition
that the of remaining
thermal lignin and the
decomposition combustion
of the main wood of
char residues [14,15].
constituents, such as The reason why cellulose,
hemicelluloses, lignin decomposes
and lignin, slowly
can be under
dividedthe whole
into two thermal
steps.
decomposition
In the first step, temperature range is that
in the temperature it contains
range aromatic
of ~200–400 °C, matrix which itselfofincreases
the combination the
total hemi-
degree
cellulose of the
andcondensation reaction [21–23].
cellulose decomposition withBrodin et al.
partial [24],decomposition
lignin in a study whereoccurs. decomposition
The sec-
of
ondlignin
stepfrom
could different originsaswas
be described theobserved, came to
decomposition ofthe conclusion
remaining thatand
lignin a loss
theofcombustion
~40% took
place in a temperature range of 200–600 ◦ C with maximum material loss at ~400 ◦ C.
of char residues [14,15]. The reason why lignin decomposes slowly under the whole ther-
mal One of the products
decomposition resulting range
temperature from biomass
is that itprocessing is biochar.matrix
contains aromatic It is produced by the
which itself in-
pyrolysis of sustainably sourced biomass under controlled conditions
creases the degree of the condensation reaction [21–23]. Brodin et al. [24], in a study where using clean technology and
isdecomposition
used for any purpose of lignin that doesdifferent
from not involve its rapid
origins was mineralization
observed, came toto
CO 2. The
the condition
conclusion is
that
that theof
a loss carbon
~40%content in biochar
took place must be higher
in a temperature thanof
range 50% of dry matter
200–600 °C with (DM) [25]. material
maximum
It has
loss at ~400 °C.the ability to filter water and retain nutrients and can also be used for filtering
contaminants in soil [26,27]. Adding biochar to soil offers potential
One of the products resulting from biomass processing is biochar. It is produced environmental benefits byby
preventing nutrient loss and thus protecting water quality
the pyrolysis of sustainably sourced biomass under controlled conditions using [28]. Biochar is not only capable
clean
of strongly adsorbing
technology and is used manyfor cationic
any purposechemicals, suchnot
that does as ammonium
involve its rapid ions and various metal
mineralization to
ions [29], but can also effectively remove anionic nutrients,
CO2. The condition is that the carbon content in biochar must be higher than such as phosphate, from50%aqueous
of dry
solutions
matter (DM) [30].[25].
Pyrolysis is a thermochemical conversion process that occurs in the absence
of oxygen.
It has theWhile bothtoexothermic
ability filter waterand andendothermic
retain nutrients reactions
and can occur,
also the latterfor
be used dominate,
filtering
requiring external heat. Pyrolysis produces solid (biochar), liquid (bio-oil), and gaseous
contaminants in soil [26,27]. Adding biochar to soil offers potential environmental benefits
(synthesis gas) products. Despite some final oxidation products like water and CO2 , reduced
by preventing nutrient loss and thus protecting water quality [28]. Biochar is not only ca-
forms such as H2 , CO, and hydrocarbon gases are present in the pyrolysis gas. Unlike
pable of strongly adsorbing many cationic chemicals, such as ammonium ions and various
combustion, pyrolysis avoids emissions of SO2 and NOx, but it produces H2 S, NH3 , HCN,
metal ions [29], but can also effectively remove anionic nutrients, such as phosphate, from
and HNCO—highly toxic substances. However, not all nitrogen and sulfur from the raw
aqueous solutions [30]. Pyrolysis is a thermochemical conversion process that occurs in
material transfer to the pyrolysis gas, as some remain in the biochar and oil [31]. The nitrogen,
the absence of oxygen. While both exothermic and endothermic reactions occur, the latter
sulfur, and chlorine content in various biomass types is presented in Table 1.
dominate, requiring external heat. Pyrolysis produces solid (biochar), liquid (bio-oil), and
gaseous (synthesis gas) products. Despite some final oxidation products like water and
Table 1. Content of nitrogen, sulfur, and chlorine in some types of biomasses [32].
CO2, reduced forms such as H2, CO, and hydrocarbon gases are present in the pyrolysis
gas. Unlike combustion, pyrolysis N *avoids emissions ofSSO * 2 and NOx, but it produces Cl * H2S,
Biomass
NH3, HCN, and HNCO—highly (mass.toxic
%) substances. (mass. However, %) not all nitrogen (mass.and%) sulfur
fromWheatthe raw material transfer to
straw the pyrolysis gas, as0.07
0.98 some remain in the biochar 0.450 and oil
[31]. TheCornnitrogen,
husks sulfur, and 0.42 chlorine content in various 0.04 biomass types is0.426 presented in
Table 1. Sunflower 0.5 0.1 0.856
Cotton stalks 0.92 0.11 0.159
Wood from fruit trees 0.62 0.06 0.049
* Related to the basis without moisture and ash.
Processes 2024, 12, 2658 4 of 12

Unlike combustion, pyrolysis retains ash in the coke, preventing the formation of fly
ash. The transfer of metals and their oxides into pyrolysis oil or gas is negligible, so they
remain in the pyrolysis coke [33]. While pyrolysis is considered an energy recovery process,
it consumes heat rather than producing it. Pyrolysis gas, a flammable mixture of H2 , CO,
CO2 , H2 O, CH4 , and other gases, can reach a calorific value of up to 15 MJ/Nm3 , which
generally increases with higher pyrolysis temperatures. This gas is mostly used to heat the
pyrolysis reactor itself [34].
Pyrolysis oil contains organic acids, sugars, alcohols, ketones, aldehydes, furans, and
hydrocarbons such as toluene and naphthalene. It can serve as an alternative liquid fuel
with a calorific value ranging from 15 to 20 MJ/kg, often similar to that of the lignocellulosic
feedstock used. Being liquid, it is easier to handle than solid fuels. Higher hydrogen
and carbon content in the feedstock enhances the bio-oil’s calorific value, acidity, and
stability [34]. However, materials high in lignin yield lower oil quantities compared to
cellulose-rich feedstocks. By separating the components of pyrolysis oil, pure chemicals
can be produced.
Pyrolysis coke consists of fixed carbon, inorganic impurities, and condensed pyrolysis
products within its porous structure. It has a calorific value of around 34 MJ/kg, comparable
to black coal [35]. Due to its large surface area, pyrolysis coke can also be used as an
adsorbent to remove dyes or heavy metals from water [36] or as a cost-effective catalyst for
cracking tars during gasification [37].
The main advantage of pyrolysis is its ability to produce fuels with higher energy
content than the raw material and to convert biomass into various chemicals. However,
pyrolysis remains less explored compared to other technologies, and converting pyrolysis
products into heat or electricity requires commercial combustion systems. Ongoing research
and development in biomass pyrolysis and other renewable resources are critical for
creating the ecological technologies of the future [38].

2. Materials and Methods

The organic carbon content in biochar ranges between 35% and 95% of the dry matter,
depending on the biomass feedstock and pyrolysis temperature. For example, the carbon
content of pyrolyzed straw is usually between 40% and 50%, while wood ranges between
70% and 90% [39].
The aim of the paper is the material (energy) recovery of fast-growing tree species. The
research was conducted in collaboration with SUA in Nitra and the AgroBioTech Research Center.
For the material conversion of biomass, a low-temperature decomposition method of
organic matter without the presence of oxygen was applied. This technology is primarily
used to produce fuels for combustion engines, but also for the material (energy) recovery
of biomass. A schematic of the equipment is shown in Figure 2.
The thermal treatment of organic matter in the reactor leads to the decomposition
of the input material into three main output components, gas, liquid, and biochar, with
the target product of the experiment being biochar and its subsequent use as biofuel. The
device is shown in Figure 3.
During the experiment, the process parameters were also verified: process temperature,
measured electronically (◦ C), and gas flow rate (m3 /h).
The selected input materials were chips from the fast-growing willow varieties Express,
Inger, and Sven. From each of the studied willow biomass samples, 250 to 300 kg were
collected. The Swedish willow variety Inger is a hybrid of the Russian clone collected from
the Novosibirsk region in 1992 and the Jorr variety. It grows better on dry soils, forms
a denser stand with a large number of lateral shoots, and is resistant to leaf rust, rarely
affected by insects, and less resistant to frost [40]. The Swedish willow variety Sven is a
hybrid of the Swedish varieties Jorunn and Björn. It has fewer upright shoots, lance-shaped
leaves, is almost entirely resistant to leaf rust, and is less resistant to pests attacking the shoot
tips and frost [40]. The Hungarian willow variety Express was bred at the Szilvánus nursery
Processes 2024, 12, x FOR PEER REVIEW 5 of 13

Processes 2024, 12, 2658 5 of 12

Processes 2024, 12, x FOR PEER REVIEW station in Kapuvár, Hungary. The variety requires warm climatic conditions,5 has
research of 13
a
long growing season, grows quickly, and provides a high yield [40].

Figure 2. Technological diagram of the pyrolysis unit.

The thermal treatment of organic matter in the reactor leads to the decomposition of
the input material into three main output components, gas, liquid, and biochar, with the
target 2.product
Figure of the diagram
Technological experiment thebeing
of the biochar
pyrolysis unit.and its subsequent use as biofuel. The de-
Figure Technological diagram of pyrolysis unit.
vice is2.shown in Figure 3.
The thermal treatment of organic matter in the reactor leads to the decomposition of
the input material into three main output components, gas, liquid, and biochar, with the
target product of the experiment being biochar and its subsequent use as biofuel. The de-
vice is shown in Figure 3.

Figure 3.
Figure 3. Assembly
Assemblyof
ofthe
thepyrolysis
pyrolysisunit.
unit.1—biomass
1—biomassconveyor, 2—reactor,
conveyor, 3—carbon
2—reactor, conveyor,
3—carbon 4—
conveyor,
condenser.
4—condenser.

The biomass
During was then stored
the experiment, in a hall,parameters
the process where it was chipped
were to the required
also verified: processfraction
tempera-of
10 to 15 mm to ensure smooth material movement in
ture, measured electronically (°C), and gas flow rate (m /h). the 3reactor using screw conveyors. A
Figure 3. Assembly of the pyrolysis unit. 1—biomass conveyor, 2—reactor, 3—carbon conveyor, 4—
critical parameter in biomass gasification is the dry matter content. Dry matter
The selected input materials were chips from the fast-growing willow varieties Ex-
condenser.
analysis
was conducted
press, Inger, and using
Sven.a Kern
FromDBSeachmoisture analyzer
of the studied (KERN
willow & SOHN
biomass GmbH,
samples, 250Balingen-
to 300 kg
Frommern,
wereDuring Germany).
collected. The The
Swedishorganic
willow dry matter
variety content
Inger is a was
hybrid determined
of the according
Russian clone
the experiment, the process parameters were also verified: process tempera- to STN
collected
EN
from14775
the [41].
Novosibirsk region in 1992 and the Jorr variety.
ture, measured electronically (°C), and gas flow rate (m /h). 3 It grows better on dry soils, forms
The selected input materials were chips from the fast-growing willow varieties Ex-
press, Inger, and Sven. From each of the studied willow biomass samples, 250 to 300 kg
were collected. The Swedish willow variety Inger is a hybrid of the Russian clone collected
from the Novosibirsk region in 1992 and the Jorr variety. It grows better on dry soils, forms
 conditions, has a long growing season, grows quickly, and provides a high yield [40].
The biomass was then stored in a hall, where it was chipped to the required fraction
of 10 to 15 mm to ensure smooth material movement in the reactor using screw conveyors.
A critical parameter in biomass gasification is the dry matter content. Dry matter analysis
Processes 2024, 12, 2658 was conducted using a Kern DBS moisture analyzer (KERN & SOHN GmbH, Balingen- 6 of 12
Frommern, Germany). The organic dry matter content was determined according to STN
EN 14775 [41].
Wood
Woodisisthe
themost
most well-known lignocellulosic material.
well-known lignocellulosic material.Wood
Woodpyrolysis
pyrolysisusually
usually be-
begins
gins
at 200–300 ◦ C [42].
at 200–300 °C [42].
The The experiments
experiments presented
presented werewere conducted
conducted at a temperature
at a temperature of 520of◦ C.

The
520 biomass
°C. feeding
The biomass rate into
feeding the
rate reactor
into was setwas
the reactor to 0.5
set kg/90 s.
to 0.5 kg/90 s.
Thethermogravimetric
The thermogravimetricmethod methodwas wasused
used toto determine
determine the
the moisture,
moisture, volatile
volatile matter,
matter,
andash
and ashcontent
contentininthe theexamined
examined solid
solid biofuels.
biofuels. TheThe laboratory
laboratory equipment
equipment consists
consists of aof
a Nabertherm L9/11/SW/P330 (Nabertherm GmbH, Lilienthal,
Nabertherm L9/11/SW/P330 (Nabertherm GmbH, Lilienthal, Germany) furnace and ac- Germany) furnace and
accessories,
cessories, whichwhich include
include a laboratory-scale
a laboratory-scale KernKernEGEG 420-3NM
420-3NM (KERN&&SOHN
(KERN SOHNGmbH GmbH,
Balingen-Frommern, Germany)
,Balingen-Frommern, Germany) and aa control
controlcomputer.
computer.The Thebuilt-in
built-inP330
P330controller
controller allows
al-
the programming
lows the programming of selected heating
of selected and retention
heating profiles.
and retention Parameter
profiles. settings
Parameter can also
settings canbe
adjusted
also manually;
be adjusted however,
manually; we used
however, wethe control
used computer.
the control The furnace
computer. modification
The furnace modifi-al-
lows the measurement of the weight change of the examined sample
cation allows the measurement of the weight change of the examined sample during the during the experiment
using a digital
experiment usingscale. The scale.
a digital deviceThe
is connected to a computer,
device is connected where thewhere
to a computer, set andthemeasured
set and
data are used to plot the temperature and weight curves on the computer
measured data are used to plot the temperature and weight curves on the computer screen screen for the
selected
for time intervals.
the selected time intervals.
A picture ofofthe
A picture thedevice
deviceisisshown
shownininFigure
Figure4.4.TheThemain
maincomponents
componentsare arethe
thefurnace,
furnace,
corundum rod, and digital
corundum rod, and digital scale. scale.

Figure
Figure4.4.The
Theapparatus
apparatusused
usedininthermogravimetric
thermogravimetricstudy.
study.

Theproportions
The proportionsof
ofthe
the individual
individual components
components are
aredetermined
determinedaccording
accordingtotothe
thefollow-
fol-
ing relationships.
lowing relationships.
contentw:w:
Moisturecontent
Moisture m − m2
w= 1 (1)
m1
Ash content in original sample A′ :
m3
A′ = (2)
m1

Ash content in dry matter pps :

m3
p ps = (3)
m2
Combustible content in original sample h′ :
m4
h′ = (4)
m1
Processes 2024, 12, 2658 7 of 12

Combustible content in dry matter phs :

m4
phs = (5)
m2

where: m1 —original weight of sample, g; m2 —weight of dry matter, g; m3 —weight of ash,

g; m4 —weight of combustible content, g.
The technical standard STN EN 15148 [43] is used for determining the volatile matter
content in solid biofuels. For determining the ash content in the same materials, the stan-
dards STN EN 14775 [41] and STN ISO 1171 [44] are applicable. The experiments conducted
were performed in accordance with these standards. At the end of the experiment, the
sample was incinerated at a temperature of 815 ◦ C for 60 min. The required temperatures,
heating rate, and retention time were set in the respective program for device control.
Table 2 presents the parameters during the thermogravimetric measurements.

Table 2. Parameters for gravimetric measurement procedure.

Time Interval
1 2 3 4 5 6
Impact period, minute 60 120 60 60 60 60
Temperature, ◦ C 20–105 105 105–500 500 500–815 815
Source: EN 14775:2010 [41], and ash content determination—STN ISO 1171:2003 [44].

At the beginning of the experiment, the sample under investigation is first heated to
105 ◦ C ± 2 ◦ C for 60 min and then dried for another 120 min. The weight loss during this
interval is used to calculate the moisture content. The weight loss in the fourth interval
(500 ◦ C) is considered the volatile combustible matter, while the remaining mass at the end
of the experiment consists of ash.

3. Results and Discussion

The result of the experiment on the depolymerization of wood chips is the production
of biochar. The wood chips, which were pre-processed into a fraction with a maximum
Processes 2024, 12, x FOR PEER REVIEW 8 of
size of 12 mm, were fed into the reactor at regular intervals. Both the wood chips and 13
the
produced biochar are shown in Figure 5.

Figure5.5. Willow
Figure Willowsample
samplebefore
beforeand
andafter
afterprocessing.
processing.

After
After depolymerization,
depolymerization,we we obtained
obtained the
the results
resultspresented
presentedin inTable
Table3.3. Given
Given that
that our
our
goal
goalis
is biochar
biochar production,
production,the
thehighest
highestamount
amountof ofbiochar
biocharwas
was produced
producedfrom
fromthethe Swedish
Swedish
willow
willowvariety
varietySven.
Sven.The
Theyield
yieldand properties
and of of
properties biochar areare
biochar significantly influenced
significantly by the
influenced by
pyrolysis method
the pyrolysis usedused
method and and
the temperature [45,46].
the temperature [45,46].

Table 3. Flow of gas and production of biochar.

Biomass Express Inger Sven

Average gas flow [m3/h] 4.54 8.73 3.07
Production of biochar from 1 kg biomass 0.252 0.106 0.256
Processes 2024, 12, 2658 8 of 12

Table 3. Flow of gas and production of biochar.

Biomass Express Inger Sven

Average gas flow [m3 /h] 4.54 8.73 3.07
Production of biochar from 1 kg biomass 0.252 0.106 0.256

The elemental analysis of the biochar, presented in Table 4, revealed that the carbon
content in the sample was nearly 80%. During each experiment, 20 kg of biomass was
processed. After each experiment was completed, the produced biochar was weighed on
a Kern platform scale with a resolution of d = 20 g. Subsequently, a sample of the dry
biochar was taken for elemental analysis. According to [47], biochar produced from plant
material contains 70% carbon, with the remainder consisting of nitrogen, oxygen, and
hydrogen. Similar results for the analysis of biochar from woody materials are presented
in a study by Bird (2017), where the carbon content in biochar from pine chips was 81%,
from eucalyptus wood 74%, and from a mixture of softwood 83.7% (Bird 2017). Biochar
from willow was also analyzed in a study by Mašek (2013), which found that increasing
the pyrolysis temperature leads to higher carbon content, with a carbon content of 70.7% at
a temperature of 350 ◦ C. Rasa et al. (2018) pyrolyzed willow at a temperature of 320 ◦ C,
and the elemental analysis of the biochar demonstrated a carbon content of 74%. Our
results show a carbon content in biochar of nearly 80%, which is likely due to the higher
temperature used for biomass processing, as noted in Mašek’s (2013) study [48–50].

Table 4. CHNS analysis of biochar from biomass.

Biomass N% C% H% S%
Sven 1.3 79.3 2.6 0.2
Express 1.0 78.0 2.7 0.2
Inger 1.0 78.3 2.7 0.3

A very important and controlled parameter during depolymerization is the process

temperature. Biochar is produced by the pyrolysis of biomass, a process in which organic
materials decompose at temperatures ranging from 350 ◦ C to 1000 ◦ C in a low-oxygen
environment [25]. Figure 6 illustrates the temperature profiles during the process of each
sample. At a set temperature of 520 ◦ C during the thermochemical conversion of the
Express willow, the process temperature fluctuated between 510 ◦ C and 524 ◦ C. During
the depolymerization of the Inger willow, the temperature ranged between 515 ◦ C and
529 ◦ C, and for the Sven willow, it ranged between 513 ◦ C and 528 ◦ C. Yang (2024) discusses
the impact of pyrolysis temperature on the physical and chemical properties of biochar,
highlighting how lower temperatures can influence porosity and biochar stability [51].
The average gas flow rate produced from various types of willow is illustrated in
Figure 7. The figures show the average gas flow rate during the steady-state process, where
0.5 kg of biomass is fed into the reactor at regular intervals (every 90 s). As indicated by the
flow profiles, the lowest average gas flow rate was observed during the thermochemical
conversion of the Sven willow variety, 3.07 m3 /h. The average gas flow rate during the
depolymerization of the Express willow variety was 4.54 m3 /h, and the average gas flow
rate during the breakdown of the Inger willow variety was 7.83 m3 /h.
Figure 8 depicts the gas production process from 0.5 kg of input biomass at a processing
temperature of 520 ◦ C. As can be seen from the comparison of the gas flow profiles, there is
a difference in gas production as well as in the gasification time for 0.5 kg of biomass. The
shortest time, as well as the least gas production, was recorded for the Sven willow variety.
The depolymerization time was 1.9 min, and the maximum gas flow rate was 1.84 m3 /h. In
the case of the Express willow variety, the decomposition time was 3.2 min, and the maximum
gas flow rate was 2.7 m3 /h. The Inger variety required the longest depolymerization time,
with a gasification time of 3.8 min and a maximum gas flow rate of 7.5 m3 /h.
 Express willow, the process temperature
Express Sven fluctuated
Inger between 510 °C and 524 °C. During
the depolymerization of the Inger willow, the temperature ranged between 515 °C and
538
529 °C, and for the Sven willow, it ranged between 513 °C and 528 °C. Yang (2024) dis-
cusses the impact of pyrolysis temperature on the physical and chemical properties of

TEMPERATURE [°C]
Processes 2024, 12, 2658 9 of 12
biochar, highlighting how lower temperatures can influence porosity and biochar stability
528 [51].

518 Express Sven Inger

538
508
TEMPERATURE [°C]

0 10 20 30 40 50 60
528 TIME [min]

Figure 6. Process temperature profile of each sample.

518
The average gas flow rate produced from various types of willow is illustrated in
508 Figure 7. The figures show the average gas flow rate during the steady-state process,
0 where 0.5
10 kg of biomass
20 is fed into30the reactor at40regular intervals
50 (every 9060s). As indicated
by the flow profiles, the lowest average
TIME [min] gas flow rate was observed during the thermo-
chemical conversion of the Sven willow variety, 3.07 m3/h. The average gas flow rate dur-
ing the6.depolymerization
Figure Process temperatureof the Express
profile willow variety was 4.54 m3/h, and the average
of each sample.
Figure 6. Process temperature profile of each sample.
gas flow rate during the breakdown of the Inger willow variety was 7.83 m3/h.
The average gas flow rate produced from various types of willow is illustrated in
Figure 7. The figures show theSven
Express average gasIngerflow rate during the steady-state process,
10 where 0.5 kg of biomass is fed into the reactor at regular intervals (every 90 s). As indicated
by the flow profiles, the lowest average gas flow rate was observed during the thermo-
8 chemical conversion of the Sven willow variety, 3.07 m3/h. The average gas flow rate dur-
GAS FLOW [m3/h]

ing the depolymerization of the Express willow variety was 4.54 m3/h, and the average
6 gas flow rate during the breakdown of the Inger willow variety was 7.83 m3/h.
4
Express Sven Inger
102

80
Processes 2024, 12, x FOR PEER REVIEW 10 of 13
GAS FLOW [m3/h]

0 10 20 30 40 50 60
6 TIME [min]

4 depolymerization time, with a gasification time of 3.8 min and a maximum gas flow rate
Figure 7.
Figure 7. Flow
Flow rate
rate of
of produced
produced gas
gas of
of each
each sample.
sample.
of 7.5 m 3/h.

2
Figure 8 depicts the gas production process from 0.5 kg of input biomass at a pro-
cessing temperatureExpress
of 520 °C. AsSven Inger
can be seen from the comparison of the gas flow profiles,
0
80 there is
10a difference20 in gas production
30 as well as
40 in the gasification
50 time for
60 0.5 kg of bio-
7 mass. The shortest time, as TIME well[min]
as the least gas production, was recorded for the Sven
6 willow variety. The depolymerization time was 1.9 min, and the maximum gas flow rate
GAS FLOW [m3/h]

was 1.84
Figure m3/h.rate
7. Flow In of
the case of gas
produced the of
Express willow variety, the decomposition time was 3.2
each sample.
5
min, and the maximum gas flow rate was 2.7 m3/h. The Inger variety required the longest
4
Figure 8 depicts the gas production process from 0.5 kg of input biomass at a pro-
3
cessing temperature of 520 °C. As can be seen from the comparison of the gas flow profiles,
2 there is a difference in gas production as well as in the gasification time for 0.5 kg of bio-
1 mass. The shortest time, as well as the least gas production, was recorded for the Sven
0 willow variety. The depolymerization time was 1.9 min, and the maximum gas flow rate
0 was0.5
1.84 m3/h.1 In the 1.5
case of the2 Express2.5
willow variety,
3 the
3.5 decomposition
4 time was 3.2
min, and the maximum gas TIME flow[min]
rate was 2.7 m3/h. The Inger variety required the longest

Figure 8.
Figure 8. Gas
Gas production profile from
production profile from 0.5
0.5 kg
kg of
of each
each sample.
sample.

Thermogravimetric analysis
Thermogravimetric analysisusing
usingthe
thedevice
deviceshown in in
shown Figure 4 and
Figure calculations
4 and ac-
calculations
cording to to
according Equations (1)(1)–(5)
Equations to (5) yielded
yielded the
the data
data presented
presented in
in Table
Table 5.
5. Each sample was
examined multiple times, and the average values using descriptive statistics are reported.
Table 5 presents the average values of the relative moisture content in the original
sample and the ash and volatile matter content of the studied dry samples.

Table 5. Moisture, ash, and volatile matter content in biofuels.

Biomass Express Inger Sven

Processes 2024, 12, 2658 10 of 12

Table 5. Moisture, ash, and volatile matter content in biofuels.

Biomass Express Inger Sven

Moisture content wet basis, % 2.001 2.428 1.674
Ash content in dry matter, pps, % 7.210 6.008 7.279
Volatile matter content in dry matter, phs, % 92.790 93.992 92.721

Table 5 presents the average values of the relative moisture content in the original
sample and the ash and volatile matter content of the studied dry samples.
The results indicate that the moisture content was very low. The samples were pro-
cessed using thermochemical conversion and stored in sealed containers. The average dry
matter content in the Inger willow sample was 91.83%, and the average organic dry matter
content in the dried sample was 99.56%. The average dry matter content in the Sven willow
sample was 91.27%, and the average organic dry matter content in the dried sample was
98.33%. The average dry matter content in the Express willow sample was 91.02%, and the
average organic dry matter content in the dried sample was 98.43%. The ash content is
higher compared to, for example, softwood pellets [8,52].

4. Conclusions
The use of bioenergy can significantly reduce our carbon footprint and thus contribute
to improving the quality of the environment. Depending on the desired product at the
output, the biomass depolymerization parameters are set. During thermochemical con-
version, it is not possible to eliminate the production of coal or gas. In the mentioned
experiments, we produced biochar and gas at a temperature of 520 ◦ C in the quantities
shown in the results. When evaluating the results, we found that about 25% of biochar was
produced from 1 kg of input biomass from the willow of the Express and Sven varieties,
and it was about 10% from the willow of the Inger variety. If the emphasis was placed on
the production of biochar, the willow varieties Sven and Express are more suitable for this
purpose than the Inger variety. If the goal were to produce gas, the willow of the Inger
variety is more suitable than the Express and Sven varieties because the average flow of gas
produced from the willow of the Inger variety was 8.73 m3 /h, which is more than that of
the Express variety (4.54 m3 /h) and the variety Sven (3.07 m3 /h). From the point of view of
the speed of depolymerization, it is most advantageous to use willow biomass of the Sven
variety, which had the shortest decomposition time, which affects the energy requirement
of the thermochemical conversion. The disadvantage is that the average gas flow was the
lowest among the studied varieties. Subsequently, the moisture, volatile matter, and ash
content were determined through thermogravimetric analysis. We consider the ash content
in the dry matter to be significant, with the lowest being in the Inger willow at 6%. The
Express and Sven varieties (which are more suitable for biochar production) showed an
ash content of 7.2%. The high ash content is primarily due to the pyrolysis process itself,
where the proportion is even higher than what is reported by Lokwahrar (2024) [53]. These
data affect the choice of combustion equipment and its operation.

Author Contributions: Conceptualization, I.V.; Methodology, T.G.; Validation, I.V. and G.P.; Investi-
gation, T.G. and I.V.; Resources, T.G., I.V. and J.G.; Data curation, J.G.; Writing—original draft, I.V.;
Writing—review & editing, R.K.; Visualization, J.G. and R.K.; Supervision, I.V.; Funding acquisition,
G.P. All authors have read and agreed to the published version of the manuscript.
Funding: This publication was supported by the Operational Program Integrated Infrastructure
within the following project: Demand-driven Research for the Sustainable and Innovative Food,
Drive4SIFood 313011V336, cofinanced by the European Regional Development Fund.
Data Availability Statement: Data are contained within the article.
Conflicts of Interest: The authors declare no conflict of interest.
Processes 2024, 12, 2658 11 of 12

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