Energy 202 (2020) 117717

Contents lists available at ScienceDirect

Energy
journal homepage: www.elsevier.com/locate/energy

Upgrading of green waste into carbon-rich solid biofuel by

hydrothermal carbonization: The effect of process parameters on
hydrochar derived from acacia
Małgorzata Wilk a, *, Aneta Magdziarz a, Izabela Kalemba-Rec a,
Monika Szyman
ska-Chargot b
a
AGH University of Science and Technology, Mickiewicza 30 Av., 30-059, Krakow, Poland
b
Institut of Agrophysics PAS, Doswiadczalna 4 Str., 20-290, Lublin, Poland

a r t i c l e i n f o a b s t r a c t

Article history: This study investigated the influence of operating parameters on the physical and chemical properties of
Received 18 February 2020 solid and liquid products after the hydrothermal carbonization (HTC) of green waste, e.g. - acacia:
Received in revised form residence time (2 and 4 h), biomass to water ratio (1:10, 1:8, and 1:5) and temperature (180, 200 and
15 April 2020
220
C). The hydrochars had a higher carbon content (from c.a. 50e67%) which resulted in a two times
Accepted 23 April 2020
Available online 28 April 2020
higher fixed carbon and calorific value. Additionally, the structural features were investigated by a
scanning electron microscope, van Soest fibre analysis (hemicellulose, cellulose and lignin), and Fourier
Infrared Spectroscopy to confirm any changes in the chemical structure. Moreover, the thermal behav-
Keywords:
Acacia
iour of the hydrochars under combustion conditions were studied. Collaterally, the liquid products were
HTC analysed by measuring pH and conductivity, which confirmed their acidic and polar character. Chemical
Hydrochar Oxygen Demand with very high values was conducted indicating that the liquid phase contained a high
TGA concentration of organic matter and nutrients. The collected and analysed data revealed that residence
Fibre analysis time and reaction temperature were the most prominent factors which caused enhancement of the
FTIR energetic properties of the hydrochars. Whereas, biomass to water ratio was found to be negligible.
© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).

1. Introduction physical properties and to homogenize and make the material

uniform. These include torrefaction, hydrothermal carbonization
The focus of scientific research on renewable fuels is due to the and slow pyrolysis processes, which are effective ways to densify
necessity for cutting fossil fuels and to increase the share of alter- the energy content of renewable fuels [2e5] (Table 1). Torrefaction
native fuels in the global share of energy for energy safety and is commonly used to pretreat biomass (with a moisture content less
climate change mitigation policies. The properties of biomass used than 20%) which is described as having a rather homogeneous
as a renewable fuel is dependent on its origin and source. The most consistency (mainly wood chips) in order to obtain solid biofuel
common kinds of biomass used in the energy sector are wood similar to coal. On the other hand, the main disadvantages of tor-
biomass, agriculture residue, food industry waste and sewage refaction is the requirement for biomass feedstock, the need for
sludge. Despite the many advantages of biomass as a renewable energy input through preheating, and the production of biofuel
fuel, it also has some weak points, e.g. high moisture content, with a high tendency to autoignite as well as its explosive prop-
biodegradability, hydrophilic character, low grindability, and low erties [6]. Pyrolysis is more likely to be used when the pre-dried
energy density [1]. Therefore, pretreatment processes of low biomass is characterized by a more diverse size. During pyrolysis,
quality biomass are necessary in order to upgrade its chemical and and depending on the process parameters, the solid, liquid and
gaseous products are produced with differing efficiency. The solid
product is called charcoal or biochar, which can be used for pro-
duction as a Terra Preta type of soil. The pyrolysis gas can be used to
* Corresponding author.
E-mail addresses: mwilk@agh.edu.pl (M. Wilk), amagdzia@agh.edu.pl produce process heat, but in some cases it is difficult to clean, which
(A. Magdziarz), kalemba@agh.edu.pl (I. Kalemba-Rec), m.szymanska@ipan.lublin. can be a disadvantage of this method [7]. The HydroThermal

ska-Chargot).
pl (M. Szyman

https://doi.org/10.1016/j.energy.2020.117717
0360-5442/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
2 M. Wilk et al. / Energy 202 (2020) 117717

Table 1
Comparison of thermal pretreatment methods [5,6,10,22,36].

Advantages Disadvantages

Torrefaction
 dedicated to lignocellulosic biomass mostly wood and crop residues with low water content  pre-drying of the feedstock is required to obtain less than 20%
 improved physical and chemical properties of torrefied product compared to raw biomass: moisture content
- increased specific energy density  dedicated to more uniform size of biomass
- increased friability  weak strength and low durability of pellet
- easy to grind  low grindability and bulk energy density compared to coal
- reduced moisture content  calorific value of torrefied biomass is lowly graded
- hydrophobic properties  high inorganic metallic content in ash
- improved resistance to biodegradability  risk of explosion
- increased bulk energy density  high tendency to autoignite
 possibility of pelletizing  thermal stability and combustion profile may be affected by high
 possibility of being co-fired with coal O/C ratio and volatile content
Hydrothermal carbonization
 dedicated to wet biomass (e.g. sewage sludge, food and agriculture waste, green waste, algae  utilization of post-processing water with inorganic and organic
biomass etc.) dissolved compounds
 dedicated to more diverse size feedstock  separation of the solid and liquid products is necessary
 lack of energy intensive drying processes  the choice of adequate filtration process
 improved physical and chemical properties of hydrochar compared to raw biomass:  drying of solid product
- homogeneous
- energy dense form
- reduced O/C ratio
- highly hydrophobic
- friable
- good grindability
- increased percentage of lignin
- biologically sterilized
- relatively superior handling
- low ash content
 increased percentage of aqueous soluble materials
 high conversion efficiency
 relatively low operating temperature and residence time range
 demineralization of the elemental inorganic composition by precipitating the minerals in the
liquid by-product
 mitigation of exploitation problems
 pellets of hydrochar are more easily formed than pellets of torrefied product
 more rapid high heat transfer compared to torrefaction
Pyrolysis
 dedicated to a more diverse size feedstock  pre-drying of the feedstock is required to obtain less than 20%
 three kind of products are obtained: solid (biochar), liquid (bio-oil) and syngas moisture content
 application of different process conditions (residence time, temperature, and heating rate)  low hydrogen content of syngas
depending of feedstock and further application  problem with impurities of syngas
 low operating temperatures and slow heating rate support solid product yield
 high operating temperature and high heating rate influence on the carbon percentage, calorific
value and specific surface area of biochar
 application as fertilizer for biological sequestration of carbon
 application as activated carbon
 bio-oil used as diesel engine and gas turbine fuels
 bio-oil can be converted into value-added chemicals
 lower capital investment compared to other processes

Carbonization (HTC) process is dedicated to wet biomass that does organic acids. The solid product, hydrochar, is an energy-dense
not need to be dried to obtain a dehydrated hydrochar which, after solid fuel, which is homogenized and carbon-rich, and has much
drying, becomes a carbon-like (hydrophobic) solid biofuel. The better grinding properties compared to feedstock. Moreover, due to
main disadvantage of this method is the resulting post-processing the thermal treatment, it is biologically sterilized. That is why
water with its significant content of mineral parts and dissolved hydrochar has the potential to be used as solid fuel or as feedstock
organic compounds, which often prevents its use as a liquid fer- for pellets. It has also been suggested that it can be used as a soil
tilizer [8]. The research problem encountered is with the selection supplement due to its porous structure [9]. During the HTC process,
of such conditions for conducting the HTC process so that the post- by-products are produced that can also be used successfully for
processing water does not contain unwanted compounds and to both heat and power generation, for example, from biogas obtained
obtain a high energy solid product. by anaerobic digestion or from residual carbon found in the liquid
Hydrothemal carbonization is a thermochemical process which phase [10,11]. The process is specially dedicated for producing
involves the application of heat and pressure to convert raw biomass with a high amount of moisture mainly: municipal waste
biomass in the presence of water into carbonaceous biofuel. The (e.g. sewage sludge studied by Ref. [12]), different kinds of food
HTC process results in three types of products: gases, aqueous factory waste (e.g. beetroot, citrus, orange, herbal tea, pulp mill
chemicals, and a solid hydrochar. The gas product is approximately waste, olive mill or wine industry [13e19], tobacco stalks [20],
1e3% of the raw material, consisting mainly of CO2. The aqueous agriculture waste (e.g. wheat, straw [21], lignocellulosic biomass
extractive compounds can easily be filtered from the reaction so- [12,22], and also algae biomass [23,24]. The greatest advantage of
lution, thereby avoiding complicated drying schemes and costly hydrothermal carbonization compared to other carbonization
isolation procedures. They are primarily inorganic salts, sugars, and thermal processes, is regarding the use of wet biomass, including:
 M. Wilk et al. / Energy 202 (2020) 117717 3

omitting a high and costly drying scheme, relatively low operating range of approximately 0.5 mm. Hydrothermal treatment was
temperatures and high conversion efficiency. The following re- carried out in a Zipperclave Stirred Reactor. The reactor volume was
actions occur during the hydrothermal carbonization: hydrolysis, 1000 ml, it was made of stainless steel and was equipped with a
condensation, decarboxylation, and dehydration [25]. During HTC MagneDrive Agitator (Parker Autoclave Engineers). Maximum
conditions, and in the presence of water, hydrolysis above 150
C temperature was limited to 220
C due to technical construction of
causes fragmentation of large biomass molecules by the disinte- apparatus. A full description of the laboratory set and the meth-
gration of cellulosic and hemicellulosic fractions into smaller odology used can be found in Refs. [12,37]. For each experiment,
fragments. The removal of carboxyl groups and release of CO2 from acacia was dispersed in deionised water at a specific biomass to
biomass is called decarboxylation. Dehydration is the process that water ratio, and later weighed and placed into the vessel. Then, the
is relevant for water removal from biomass leading to hydroxyl reactor was sealed and heated to a specific temperature for 60 min
group elimination. The efficiency of the HTC process depends on and maintained for the desired time. When the reaction time was
the reaction temperature, reaction time, biomass to water ratio and finished, the reactor was cooled down to room temperature with
pressure. The HTC process is usually carried out within a temper- cooling water. The solution in the reactor was stirred by a magnetic
ature range of 150e300
C. The various pressures are maintained stirrer (150 rpm) during the entire process to avoid local over-
above the saturation pressure to ensure the liquid state of water, heating. The pressure was stabilized as per the requested temper-
and 2e12 h of residence time is used [26,27]. The researchers have ature (up to max. 2 MPa). Preliminary HTC runs were carried out at
paid a great deal of attention to the HTC process, because the residence time, 2 and 4 h and with a 1:10 biomass to water ratio at
process is energy-efficient, simple and low cost. Moreover, the 200
C. Next, two different biomass to water ratios: 1:5, 1:8 were
required energy to heat the reacting water is very low in compar- applied. It was found that the optimal biomass to water ratio was
ison with traditional thermochemical processes [28]. During the 1:10 due to the obtaining of an easy to stir solution, whereas some
HTC studies the researchers were mainly focussed on: different operational difficulties occurred with 1:5 and 1:8 biomass to water
kinds of feedstock, the variables influencing the process (temper- ratios. For those reasons, the influence of temperature on solid and
ature, pressure, residence time and ratio of biomass/water) liquid HTC products was investigated in detail for only the 1:10
[29e32], the reaction of natural complex biomass, the chemical and biomass to water ratios. Based on ultimate and proximate analysis
structural properties of hydrochars as well as the degradation of the 4-h period was chosen as a reference time for further analysis.
products [33], and also the addition of chemicals to the reaction The temperatures applied were 180, 200, and 220
C.
media [34], including the enhancement of textural properties and The solid product was separated from the liquid phase by a
the structural order of hydrochar for supercapacitor applications vacuum filtration process after being evacuated from the reactor.
[35]. The solid dark brown hydrochar was oven dried at 105
C for one
Acacia trees (Robinia pseudoacacia) can be found in Poland as day then weighed and stored in sealed containers for further
well as around Europe mainly due to the ease of growth in unde- analysis. The dark yellow liquid phase product from the HTC ex-
sirable places. They are usually planted along streets in big cities, periments was stored in sealed containers in the refrigerator.
because they can grow very fast on poor soil and have a great ability 100 cm3 of liquid was distilled under low pressure. 90% of distillate
to tolerate pollution. On the other hand, the acacia is an invasive was clear and transparent, whereas the remaining 10% was dark
species which spreads rather too easily and creates problems for brown. It was dried at 105
C and analysed using an elemental
other species. Therefore, the municipal greenery management have analyser to determine the carbon balance.
to deal with its wet (approximately 50% of moisture) residue Hydrothermal carbonization conditions including residence
including leaves, branches, limbs, and roots. The implementation of time, temperature and biomass to water ratio are summarized in
hydrothermal carbonization could avoid the high cost of the drying Table 2.
process and convert city greenery residue into clean solid coal-like
fuel. Another advantage of this method is a much better grind- 2.2. Solid product characteristics
ability of the solid product compared to the raw material, a strong,
hard, wood which is problematic during milling. To the best of the 2.2.1. Proximate and ultimate analysis
authors’ knowledge, research reports focussing on acacia HTC have The proximate analysis including moisture, ash, and volatile
not been widely presented. matter contents were determined according to PN-EN ISO
The main aim of this work was to solve the existing challenges of 18134e1:2015e11, EN 15403:2011, EN 15402:2011, respectively.
the HTC process thus the impact of HTC main parameters, e.g. Additionally, the fixed carbon was determined by difference. The
residence time, temperature and biomass to water ratio on the ultimate analysis (carbon, hydrogen and nitrogen content) was
hydrochar which leads to the conversion of acacia, a green waste, to measured by Elemental Analyser Truespec CHNS Leco (CHNS628)
a renewable solid fuel. The fuel characteristics, fibre, chemical according to PKNeISO/TS 12902:2007. The sulphur content was
bonds and morphology of hydrochars as well as the thermal determined by use of the analyzer CHS580 ELTRA using the com-
combustion behaviour of solid hydrochars were investigated and bustion of organic material with non-dispersive infrared detection.
compared to raw biomass. The multiphase wide approach by The higher and lower heating values of raw samples were deter-
application of advanced instrumental techniques gives a detailed mined by the use of a KL-10 bomb calorimeter.
description of the solid product of HTC derived from green waste.
The results could provide a better understanding of hydrothermal 2.2.2. Specific surface area
carbonization in order to successfully optimise this pre-treatment The multipoint BET adsorption method (ASAP 2010, Micro-
process. meritics Inst) was used to determine the specific surface area (SSA)
of both raw and pre-treated samples. At the beginning, the samples
2. Materials and methods were degassed overnight at 100
C and then adsorption occurred at
a temperature of 77 K using nitrogen as an adsorber.
2.1. HTC experiments
2.2.3. Fibre analysis
The wood biomass feedstock used in this study was Robinia In order to determine hemicellulose, cellulose and lignin frac-
pseudoacacia, known colloquially as acacia, with a particle size tions, a modified Van Soest analysis method was applied [36,38].
4 M. Wilk et al. / Energy 202 (2020) 117717

Table 2
Experimental conditions of the hydrothermal carbonization process performed for acacia.

Feedstock Residence time, h Temperature,
C Biomass to water ratio

HTC_200 C_1:10_2h 2 200 1:10

HTC_200 C_1:10_4h 4 200 1:10
HTC_200 C_1:8_4h 4 200 1:8
HTC_200 C_1:5_4h 4 200 1:5
HTC_180 C_1:10_4h 4 180 1:10
HTC_220 C_1:10_4h 4 220 1:10

Firstly a neutral detergent solution for removing pectic poly- 3. Results

saccharides, phenolic compounds, proteins and sugars (later called
extractives) was administered. Then, an acidic detergent solution 3.1. Physical and chemical properties of solid samples
was used to remove hemicellulose. At the end, the cellulose was
solubilised in a 72% sulphuric acid solution to separate lignin 3.1.1. Proximate and ultimate analysis
fractions. As presented in Table 3, the proximate and ultimate analysis of
raw acacia and hydrochars were affected by different conditions of
the process. For instance, fixed carbon and carbon contents were on
2.3. FTIR spectroscopy
the rise with increases in residence time, temperature and biomass
to water ratios, whereas the volatile matter significantly decreased
Fourier Infrared Spectroscopy (FTIR) was studied using a Nicolet
indicating that only part of the light organic compounds converted
6700 FTIR spectrometer (Thermo Scientific, Waltham, MA, USA)
into the liquid and gaseous phases of HTC. It was observed that
equipped with a Smart iTR attenuated total reflectance (ATR)
hydrochar carried out at 2 h (200
C and 1:10) had 9% and 6.5%
sampling accessory. Raw acacia samples as well as hydrochars were
lower fixed carbon and carbon content values compared to
placed directly on an ATR crystal and measured. The spectra were
hydrochars obtained at 4 h (200
C and 1:10). The fuel ratios (FC/
collected over the range of 4000e650 cm1. For each material, 3
VM) showed the same tendency and increased by 14%, but volatile
samples under the same conditions were examined. For each
matter decreased by 4%. The effect of temperature on the FC/VM
sample, 200 scans at a spectral resolution of 4 cm1 were averaged.
ratio was observed. The FC/VM increased with temperature rising
The final average spectrum was then calculated for each given
from 0.26 to 0.60 for 180e220
C, respectively. Hydrogen contents
material. All further spectral manipulation and display were carried
in both samples were at a similar level, whereas the oxygen content
out using Origin Pro 8.5 (OriginLab Corporation, USA).
measurement for the 2 h treated sample decreased by 12% over a
4 h duration time. Moreover, its chemical properties, as depicted on
2.3.1. SEM-EDS the van Krevelen diagram (Fig. 1), and based on H/C and O/C atomic
Scanning electron microscope (SEM) images were taken for all ratios, confirmed its less carbonaceous properties. They were also
studied samples using a FESEM Nova NanoSEM 450. Particles of the found to be comparable to those determined for hydrochar samples
raw acacia and hydrochars were mounted on metal stubs by double performed at 4 h, 180
C, and 1:10 biomass to water ratio. It was
sided carbon adhesive discs. The samples were observed in a high already confirmed that a longer reaction time affects the conver-
vacuum mode by secondary electrons (SEM) using an accelerating sion of biomass and defines the product composition [26]. More-
voltage - 1 kV. Additionally, EDS analysis of hydrochar ashes over, it was also proven [39] that when residence time was longer
compared to raw acacia ash were investigated in order to provide than 4 h, O/C ratio obtained stability, suggesting a longer residence
elemental composition analysis inside an SEM. time had no significant effect on O/C. Additionally, Gao et al. [40]
confirmed that the influence of residence time (4, 6, 8 and 10 h) on
the yield, properties and thermal behaviour of hydrochar was not
2.3.2. TGA significant. That is why 4 h was used as a reference time for more
Thermal analysis (TGA) was performed. The TG, DTG and DSC advanced analysis to ascertain the impact of temperature and
depicted raw acacia and HTC_200 C_1:10_4h, HTC_200 C_1:8_4h, biomass to water ratio. At 200
C when the biomass to water ratio
HTC_200 C_1:5_4h, HTC_180 C_1:10_4h, and HTC_220 C_1:10_4h. increased, the carbon content upgraded, e.g. for 1:10 biomass to
The TGA instrument was calibrated with indium, zinc and water ratio compared to 1:5 c.a. 1%. However, the temperature
aluminium, with accuracy equal to 106 g. For the thermogravi- impact on hydrochars produced at 1:10 biomass to water ratio was
metric analysis, the samples were placed in an alumina crucible. A a more prominent factor than the biomass to water ratio, e.g. from
mass sample of approximately 15 mg was heated from an ambient 180
C to 220
C, which increased by approximately 9%. Taking into
temperature up to 700
C at a constant 10
C/min, in 40 ml/min account the increase of the biomass to water ratio at 200
C on fixed
flow of air. carbon the effect is negligible, whereas the increase of temperature
up to 220
C for 1:10 water ratio is significant causing a 40% higher
FC value. In the van Krevelen diagram (Fig. 1) the regression line of
2.3.3. Liquid product characteristics
the H/C and O/C ratios was also plotted for all samples tested at 4 h
The liquid product from the HTC experiment and distillate
showing a good linear correlation by coefficient of determination
samples was characterized by pH, and conductivity, and oxygen
R2 ¼ 0.989. It suggested a strong relationship between the thermal
demands were measured by Multifunction Laboratory Meter CX-
degradation of biomass shifting the H/C and O/C atomic ratios from
505 ELMETRON. Chemical Oxygen Demand (COD) analysis using
the upper right to the lower left with increasing temperature and
potassium dichromate Cr2O2 7 , to oxidize the organic matter in the
biomass to water ratio mainly caused by dehydration and decar-
solution into carbon dioxide and water under acidic conditions, was
boxylation reactions [41]. For this reason O/C decreased from 0.46
also performed, according to PN-ISO 6060:2006. An acid-base
to 0.29 when temperature increased, whereas for biomass to water
titration of the distillate was carried out to determine the con-
ratio the effect was found to be almost negligible. During the HTC
centration of acetic acid.
 M. Wilk et al. / Energy 202 (2020) 117717 5

Table 3
Proximate and ultimate analysis.

Proximate analysis, as received, % Ultimate analysis, dry basis, %

FC VM Ash M FC/VM C H N S Oa O/C H/C

Acacia 16.04 76.58 0.16 7.22 0.21 50.60 5.62 0.120 0.01 43.49 0.65 1.33
HTC_200 C_1:10_2h 23.50 74.94 0.10 1.46 0.31 56.87 5.76 0.167 0 37.10 0.49 1.22
HTC_200 C_1:10_4h 25.63 71.85 0.26 2.26 0.36 60.60 5.87 0.192 0 33.08 0.41 1.16
HTC_200 C_1:8_4h 26.47 71.13 0.47 1.93 0.37 61.37 5.87 0.185 0 32.11 0.39 1.15
HTC_200 C_1:5_4h 27.72 70.85 0.19 1.24 0.39 61.56 5.77 0.200 0 32.28 0.39 1.13
HTC_180 C_1:10_4h 21.09 77.82 0.09 1.00 0.27 57.70 6.04 0.185 0 35.99 0.46 1.23
HTC_220 C_1:10_4h 36.80 61.49 0.10 1.61 0.60 66.70 5.7 0.223 0 27.28 0.29 0.99
a
HTC - hydrochar; FC - fixed carbon; VM - volatile matter; M  moisture; C - carbon; H - hydrogen; N - nitrogen; S - sulphur; O - oxygen; - calculated by difference.

HHVhydrochar
EY ¼ MY, ,100;% (2)
HHVraw

where: HHVhydrochar e high heating value of hydrochar, HHVraw e

high heating value of raw sample.
Energy densification ratio (EDR):

HHVhydrochar
EDR ¼ (3)
HHVraw
For this study, HHV and LHV, and energy yield were almost at
the same level when residence time was increased, but the densi-
fication ratio decreased from 1.2 to 1.17. In the case of temperature
impact, the energy parameters and energy densification ratios
increased following the same tendency, which had already been
revealed for FC and carbon content for hydrothermally tested
samples. The energy densification ratio rose with an increase in
biomass to water ratio (Fig. 2a), but the influence of temperature
was stronger (Fig. 2b). The energy densification ratio rose by
approximately 6% with an increase in biomass to water ratio from
1.17 to 1.24 and with a rise in temperature of approximately 15%
from 1.17 to 1.32. Additionally, when the higher temperature was
applied, a lower solid product was obtained, which was confirmed
by the mass yield, and resulted in a higher production of liquid
Fig. 1. Van Krevelen’s diagram. product.

3.1.3. Structural and fibre analysis

process a dehydration reaction occurred causing a decrease in the
It was found that in all the studied samples H/C and O/C ratios of
H/C, from 1.23 to 0.99, when the temperature increased
hydrochar products were lower compared to the raw biomass
(180e220
C), and from 1.16 to 1.13, when the biomass to water
samples. It is certain that this was related to the evolution of H2O
ratio also rose (1:10 to 1:5). Overall, the chemical properties of the
and CO2 in dehydration and decarboxylation reactions causing
obtained hydrochars versus raw biomass, clearly depicted in the
damage and changes in the structure and functional groups of
diagram, indicated the upgrading character of the HTC process and
hydrochar [40]. For this reason the specific surface area of hydro-
the spectacular temperature impact leading the hydrochar pre-
chars pretreated at 4 h were investigated in order to assess their
treated at 220
C and 1:10 in the direction of coal.
adsorptive properties (Table 5).
It was found that the hydrochars had a relatively small specific
surface area thus indicating that their porosity was also low, but
compared to the raw acacia this increased 5 to 9 times depending
on the temperature of the reaction. Additionally, fibre analysis was
3.1.2. Mass and energy yield performed using the van Soest method as depicted in Fig. 3. As can
Two important parameters were determined according to the be seen in all cases the hemicellulose and cellulose were
following Eqs. (1) and (2), namely mass and energy yields, which completely degraded and became volatile products. The obtained
gave an indication of the energy densification ratio preserved in the hydrochars were nearly consistent with lignin and NDS fractions.
hydrothermal carbonized samples defined as Eq. (3) (Table 4). The lignin content in all hydrochars was found at c.a. Similar level. It
Mass yield (MY) was calculated according to: should be emphasised that experimental conditions (temperature,
pressure and residence time, biomass to water ratio) did not lead to
mhydrochar lignin decomposition. Moreover, the increase in the percentage of
MY ¼ ,100;% (1) this fraction may suggest that apart from lignins there are other
mraw
compounds which are not removed by extraction conditions (72%
where: mhydrochar e mass of hydrochar, mraw e mass of raw sample. sulphuric acid). Therefore, it can be concluded that in both factors
and energy yield (EY) neither temperature nor biomass to water ratio significantly
6 M. Wilk et al. / Energy 202 (2020) 117717

Table 4
Mass and energy parameters.

Mass Yield, % HHV, MJ/kg LHV, MJ/kg Energy Yield, % Energy Densification Ratio

Acacia e 18.08 16.77 e e

HTC_200 C_1:10_2h 71 21.63 20.34 86 1.20
HTC_200 C_1:10_4h 73 21.81 20.55 86 1.17
HTC_200 C_1:8_4h 70 21.51 20.26 84 1.19
HTC_200 C_1:5_4h 70 22.44 21.20 87 1.24
HTC_180 C_1:10_4h 75 20.92 19.91 87 1.16
HTC_220 C_1:10_4h 58 23.89 22.64 77 1.32

HHV - higher heating value; LHV - lower heating value.

Fig. 2. Energy densification ratio of hydrochars pretreated at 4 h depending on a) biomass to water ratio, and b) temperature.

Table 5
Specific surface area of acacia and hydrochars pretreated at 4 h.

Acacia HTC_200 C _1:10 HTC_200 C _1:8 HTC_200 C _1:5 HTC_180 C _1:10 HTC_220 C _1:10

SSA, m2/g 0.336 2.01 2.05 2.48 1.78 3.02

SSA - specific surface area.

affected the fibre compounds of hydrochars. In analysis of the

decomposition of hemicellulose and cellulose, it is known that
hemicellulose decomposes at 230e300
C and cellulose at
300e400
C [42,43]. For this study it can be stated that pressure
was the factor which determined the decomposition of these
components.
In Fig. 4, FTIR spectra of raw acacia and hydrochars are pre-
sented. The spectrum of the raw acacia sample presented typical
bands characteristic of wood samples (Fig. 4 a). A broad band with a
maximum of around 3334 cm1 is characteristic of the stretching of
the OeH group present in water, but also in hydrogen bonds [44].
Another band between 2950 and 2800 cm1 is attributed to sym-
metric and asymmetric stretching of the CeH group in aromatic
methoxyl groups [45]. The band at 1730 cm1 is characteristic of
the stretching of the C]O group of alkyl esters present in hemi-
cellulose [45,46]. Additionally, bands with a maximum of 1600 and
1506 cm1 are characteristic of C]O and C]C groups stretching
vibrations in lignins, respectively, but the band at 1600 cm1 can
also be assigned to stretching of the carboxylate esters [45e47].
Bands with a maximum of 1457 and 1323 cm1 were previously
found in the spectra of pure lignin samples [48]. While bands at
1423 and 1366 cm1 were assigned to CH2 bending in cellulose and
Fig. 3. Van Soest analysis for hydrochar pretreated at 4 h, where: NDS - neutral
detergent soluble, H - hemicellulose, C - cellulose, L - lignin.
hemicelluloses [46]. The region 1250e1000 cm1 is dominated by
ring vibrations and the stretching of (CeOH) side groups and the
(CeOeC) glycosidic bond vibrations in cellulose and hemicelluloses
 M. Wilk et al. / Energy 202 (2020) 117717 7

some aromatic compounds resulting from hydrothermal decom-

position of cellulose and hemicelluloses. Temperature had the
greatest influence on this process.
Fig. 5 presents the structural changes in biomass material and its
ash caused by the carbonization process carried out over a period of
4 h. The structure of hydrochars confirmed the impact of the HTC
process. For HTC_180 C_1:10, small openings appeared and were
irregularly distributed compared to the raw sample. A further in-
crease of temperature (HTC_200 C_1:10) caused a more porous
structure with a regular distribution of openings which were a
result of volatile matter release. The HTC_220 C_1:10 hydrochar is
characterized by the most destroyed structure resulting from
degraded fibre. Based on the morphology of the study of ashes it
can be observed that for raw biomass ash spherical particles were
not observed, whereas for hydrochar ashes particles could be
identified. Spherical particles could be associated with the presence
of alkali silicates. The chemical composition of ashes were deter-
Fig. 4. FTIR spectra of raw acacia (a) and hydrochars (bef) over the range
3750e2750 cm1 and 1800e650 cm1. Region between 2750 and 1800 cm1 pre- mined using EDS analysis. The dominant element of the ashes is
sented only small bands attributed to noise and therefore is not shown in the figure. calcium.

[44,49]. However, the band at 1233 cm1 probably collapsed at 3.1.4. TGA
1264 and 1224 cm1, characteristic of the G-ring or C]O vibration Figs. 6e10 present the results of thermogravimetric analysis in
in lignin and syringil ring or CeO stretch in lignin and xylan, the form of TG, DTG, and DSC curves. Fig. 6 depicts the thermal
respectively [47]. The band at 896 cm1 can be assigned to b- behaviour of raw acacia in air atmosphere (corresponding to
glycosidic linkage in cellulose and/or hemicelluloses [46,49]. combustion). The combustion process of raw acacia based on the
Spectra of hydrochars differ from the raw acacia spectrum. The obtained curves can be divided into four stages. The first stage re-
most striking difference is the shift of band at 1730 cm1 to flected moisture release and took place up to 120
C (mass loss was
1700 cm1 (Fig. 4 b-f). Kruse and Zevaco (2018) [48] attributed this observed on the TG curve and consequently a small peak on the
band to the stretching vibration of C]O and the formation, by DTG curve). The main combustion process took place within a
crosslinking, of new quinones, carboxylates and aromatic com- temperature range of 230 up to 490
C, where volatile matter was
pounds after thermal treatment. This suggests that some compo- released (the maximum reaction rate was at 325
C) and char was
nents of raw acacia started to degrade with simultaneous combusted at c.a. 450
C. It should be emphasised that on the DTG
polymerization [50]. An interesting observation is the relative in- curve the inflexion point was visible within a temperature range of
tensity depending on the initial HTC conditions of temperature 260e280
C, which could have been the result of light volatile
(higher intensity for higher temperatures, Fig. 4 b, c, d) and wood matter release. These reactions were confirmed by two significant
biomass to water relation (intensity for sample 1:10 was higher exothermic peaks due to energy release. Above 500
C mass and
than for 1:8 or 1:5 at the same temperature, Fig. 4 c, e, f). Another heat changes were not observed and only a very small amount of
difference is the shift of the band at 1593 to 1602 cm1 which was solid residue was found.
previously attributed to the stretching vibration of C]C samples Figs. 7e9 present the thermal behaviour of obtained hydrochars
[48]. The maximum band in the range, 1515-1506 cm1, can be under different conditions. In all studied hydrochars the effect of
assigned to the C]C stretching vibration of aromatic ring in lignin hydrothermal carbonization was observed on TG, DTG, and DSC
skeleton thus shifting from 1506 cm1 (raw acacia) to 1515 cm1 curves. Based on TG and DTG curves it was confirmed that hydro-
(HTC samples) which is evidence of the removal of the amorphous chars were characterized by low-moisture content (only a small
fraction of wood components and perhaps partial decomposition of amount of water was observed). Moreover, the effect of HTC tem-
lignins [47]. Also, the splitting of the band at 1233 cm1, charac- perature was confirmed by the lack of decomposition of light vol-
teristic to inter alia cellulose, to two bands at 1274 and 1206 cm1 atile matter. In comparing the shape of DSC curves between
was observed during the thermal conversion of carbohydrates to hydrochars and raw acacia it is visible that the first exothermic peak
hydroxymethylfurfural which is a product of cellulose thermal is much higher for hydrochars, whereas the second one is lower. In
hydrolysis [51]. Bands in the region of 1200e900 cm1 are sharper analysis of DSC for hydrochars no significant differences are
and more intense compared to the spectrum of raw acacia. How- observed.
ever, Arellano et al. [52] attributed sharp bands in this region with Fig. 10 depicts TG curves for all the studied samples to aid better
the CeO group vibration of carboxyl, ester and ether groups. The comparison. The effects of hydrothermal carbonization is well
characteristic for b-glycosidic linkage in cellulose and hemi- observed on the TG and DSC curves. The temperature impact on
celluloses band at 896 cm1 disappeared. This may be proof of the hydrochars was found. The hydrochars obtained at a higher
decomposition of those polysaccharides by the breaking glycosidic carbonization temperature were characterized by the lower rate of
of linkage which appears regardless of the temperature used and combustion (this was observed in the second stage), whereas the
biomass to water ratio. Moreover, in the region 3750e2750, sharper biomass to water ratio impact on the combustion profile was
bands are visible, especially at 2850 (stretching vibration of CeH in negligible.
alkyl fragments) and 2901 cm1 (stretching of CeH) which may Additionally, the combustion parameters based on TG and DTG
suggest degradation of hemicelluloses and cellulose leading to the data were determined and summarized in Table 6. The following
formation of aromatic compounds in HTC samples [48]. This sup- temperatures were analysed: the ignition temperature (Ti), the
ports the Van Soest analysis result: in HTC samples, the main maximum combustion rate temperature (Tm) and the burnout
fraction in the so called L% fraction which apart from lignin contains temperature (Tb) [53,54]. The combustion index (Di) was calculated
according to the following formula:
8 M. Wilk et al. / Energy 202 (2020) 117717

Fig. 5. SEM analysis of raw acacia, hydrochars and its ashes.

higher Ti and Tb when compared to raw samples. It confirms that

Rm the HTC process enhanced the combustion behaviour of hydrochars
Di ¼ (4)
ðtm $ti Þ moving them towards a higher temperature and prolonging the
combustion process (Fig. 9). The effect of temperature on hydro-
where: Rm e maximum weight loss rate, %∙
C1; tm e time of the thermal carbonization is also visible: the higher the temperature of
maximum weight loss rate, min; tm e time of the ignition tem- the process, the lower the combustion parameters Ti, Tm, Rm, Di and
perature, min. S. This behaviour resulted from the reduction of volatile matter and
The comprehensive combustibility index was used to describe the enlarged value of fixed carbon in hydrochars. The hydrochar
the reactivity of the sample given by: produced at the highest temperature (220
C) shows more
favourable combustion reactivity among the others confirmed by
Rm ,Rmean the lowest Ti and the highest comprehensive combustibility index
S¼   ;% (5)
T2i ,Tb S, but on the other hand, the best combustion performance among
hydrochars was found for the sample pretreated at 180
C with the
highest S and Di values resulting from the highest VM content. The
where: Rm e maximum weight loss rate, %∙min; Rm e average
values of the comprehensive combustibility index for raw acacia
weight loss rate, %∙min.
and the hydrothermally pretreated sample at 180
C are close to
It was observed that pretreated samples via HTC methods had
 M. Wilk et al. / Energy 202 (2020) 117717 9

Fig. 6. TG, DTG and DSC curves of raw acacia in air atmosphere 10
C/min heated up to 700
C.

Fig. 7. TG, DTG and DSC curves of HTC_180 C_1:10 in air atmosphere 10
C/min heated up to 700
C.

Fig. 8. TG, DTG and DSC curves of HTC_200 C_1:10 in air atmosphere 10
C/min heated up to 700
C.

each other due to a very similar volatile matter content in the would be longer and less violently combusted compared to the raw
samples. Apparently, the combustion of the two above mentioned material, and their flames would be more stable.
samples was similar, and those results were in good accordance In brief, it can be summarized that the hydrochar pretreated at a
with the results discussed by Zhu [55]. Nevertheless, according to higher temperature, 220
C with a 4 h residence time and a 1:10
He et al. and Xu et al. [56,57] when taking into account the increase biomass to water ratio, has the best combustible properties among
of Tb for the hydrochars it would be expected that hydrochars the other discussed hydrochars, confirmed by both its physical and
10 M. Wilk et al. / Energy 202 (2020) 117717

Fig. 9. TG, DTG and DSC curves of HTC_220 C_1:10 in air atmosphere 10
C/min heated up to 700
C.

Fig. 10. TG and DSC curves of acacia and hydrochars pretreated at 4h in air atmosphere 10
C/min heated up to 700
C.

Table 6
Combustion parameters for acacia and pretreated hydrochar at 4 h.

Feedstock Ti,
C Tm,
C Tb,
C Rm, %∙
C1 Di∙102 S, %∙min3∙
C3

Acacia 288 325.21 485 1.12 0.15 5.45

HTC_180 C_1:10 308 322.16 514 1.50 0.19 5.84
HTC_200 C_1:10 304 318.00 522 1.13 0.15 5.07
HTC_220 C_1:10 298 315.79 515 0.80 0.10 3.31

Ti - ignition temperature; Tm - maximum combustion rate temperature; Tb - burnout temperature; Rm - maximum weight loss rate; Di - combustion index; S - comprehensive
combustibility index.

chemical properties. organic compounds, including valuable carbon, were dissolved in

the liquid phase. Moreover, the liquid product was characterized by
a high toxicity which may cause problems when it comes to
3.1.5. Liquid product characteristics
disposal. On the other hand, the liquid phase has some potential for
A study of the liquid phase of HTC and its distillates included pH,
use as a substrate of biogas or biofuel as well as and biofertilizer
conductivity, density and Chemical Oxygen Demand (COD)
[58,59].
(Table 7). The pH for the hydrochars was acidic, c.a. 3.2, but for the
distillates was lower, c.a. 2.7, those results were similar to others
previously reported in the literature [13,54,58]. The conductivity of 4. Conclusion
the distillates was 2 times lower than those measured for the HTC
liquid. This indicated that there were some polar compounds in the The solid hydro-products of HTC were characterized as above
solution. The results suggested that most inorganic compounds did 20% carbon content for 200
C and below 40% for 220
C process
not distillate. The resulting values of COD were very high indicating temperature. Hemicellulose and cellulose in acacia had degraded
that the HTC liquid contained a high concentration of organic after the HTC process which was evidently confirmed by fibre
matter and nutrients. COD values of their distillates were 2 times analysis, structural studies (SEM) and thermal analysis. Moreover,
lower. The density was at a similar level in all samples. Generally, analysis FTIR spectra showed that hemicellulose and cellulose
the results confirm the overall suggestions [60,61] that many present in acacia during the HTC process underwent degradation
 M. Wilk et al. / Energy 202 (2020) 117717 11

Table 7
Physical and chemical properties of liquid and their distillates from the HTC of acacia.

Parameters HTC_200_C_1:10_2h HTC_200_C_1:10_4h HTC_200_C_1:8_4h HTC_200_C_1:5_4h HTC_180_C_1:10_4h HTC_220_C_1:10_4h

pH 3.29 3.27 3.21 3.23 3.16 3.14

pHd 2.83 2.83 2.77 2.74 2.86 2.89
conductivity, mS/cm 1.72 2.07 2.17 2.64 1.52 1.90
conductivityd, mS/cm 1.10 1.10 1.20 1.28 0.96 1.12
density, g/cm3 1.0050 1.0045 1.0046 1.0049 1.0050 1.0035
densityd, g/cm3 1.0008 1.0021 1.0022 1.0025 1.0027 1.0009
COD, gO2/dm3 26.30 19.90 27.03 29.34 24.20 23.50
CODd, gO2/dm3 13.40 9.90 14.40 14.60 13.30 5.97
acidityd, mol/dm3 0.109 0.112 0.133 0.178 0.091 0.107

d - distillate; COD - Chemical Oxygen Demand.

(disappearance of bands connected with glycosidic bonds in those appeared to influence the work reported in this paper.
polysaccharides) and were transformed to aromatic compounds,
while lignin underwent only a partial decomposition. Additionally, Acknowledgements
the TGA results showed that the combustion of hydrochars took
place at higher temperatures than the raw material providing an This work was supported by the Ministry of Science and Higher
effective combustion performance. Taking into account the pre- Education, Poland [grant AGH no 11.11.110.663]. The authors
sented results it can be concluded that temperature was un- gratefully acknowledge EKOPROD Ltd. the proprietor of the HTC
doubtedly the main parameter influencing the physical and apparatus used in the presented study. The authors also wish to
chemical properties of hydrochars, and the best properties were thank Ph.D. Jadwiga Krop and M.Sc. Jo
zef Marszałek for their
achieved for hydrochar pretreated at 220
C and 4 h and 1:10 exceptional experimental help.
biomass to water ratio, while biomass to water ratio was found to
be insignificant. References

Credit author statement [1] Piekarczyk W, Czarnowska L, Ptasin
ski K, Stanek W. Thermodynamic evalu-
ation of biomass-to-biofuels production systems. Energy 2013;621:95e104.
https://doi.org/10.1016/j.energy.2013.06.072.
Authors confirm their following contribution in the preparation [2] Szwaja S, Magdziarz A, Zajemska M, Poskart A. A torrefaction of Sida her-
of the manuscript: maphrodita to improve fuel properties. Advanced analysis of torrefied prod-
ucts. Renew Energy 2019;141:894e902. https://doi.org/10.1016/
1. Małgorzata Wilk - corresponding author, conceptual design j.renene.2019.04.055.
and idea of investigations concerning the detailed description of [3] Botelho T, Costa M, Wilk M, Magdziarz A. Evaluation of the combustion
hydrotermal carbonization products, development and design of characteristics of raw and torrefied grape pomace in a thermogravimetric
analyzer and in a drop tube furnace. Fuel 2018;212:95e100. https://doi.org/
methodology of hydrothermal carbonization, conducting a 10.1016/j.fuel.2017.09.118.
research and HTC investigation process, specifically performing the [4] Magdziarz A, Wilk M, Wadrzyk M. Pyrolysis of hydrochar derived from
experiments, and data collection, analysis of results, preparation of biomass e experimental investigation. Fuel 2020;267:117246. https://doi.org/
10.1016/j.fuel.2020.117246.
manuscript.
[5] Colantoni A, Evic N, Lord R, Retschitzegger S, Monarca D. Characterization of
Conceptualization, Formal analysis, Investigation, Methodology, biochars produced from pyrolysis of pelletized agricultural residues. Renew
Project administration, Writing - original draft, Writing - review & Sustain Energy Rev 2016;64:187e94. https://doi.org/10.1016/
editing, Supervision. j.rser.2016.06.003.
[6] Van der Stelt MJC, Gerhauser H, Kiel JHA, Ptasinski KJ. Biomass upgrading by
2. Aneta Magdziarz - conceptual design and idea of investiga- torrefaction for the production of biofuels: a review. Biomass Bioenergy
tion, development and design of methodology of thermal analysis, 2011;35:3748e62. https://doi.org/10.1016/j.biombioe.2011.06.023.
conducting a research and TGA investigation, specifically per- [7] Sipra AT, Gao N, Sarwar H. Municipal solid waste (MSW) pyrolysis for bio-fuel
production: a review of effects of MSW components and catalysts. Fuel Pro-
forming the experiments, analysis of SEM-EDS results, and data cess Technol 2018;17515:131e47. https://doi.org/10.1016/
collection, analysis of results, preparation of manuscript. j.fuproc.2018.02.012.
Conceptualization, Formal analysis, Investigation, Methodology, [8] Zhang Y, Jiang Q, Xie W, Wang Y, Kang J. Effects of temperature, time and
acidity of hydrothermal carbonization on the hydrochar properties and ni-
Project administration, Writing - original draft. trogen recovery from corn stover. Biomass Bioenergy 2019;122:175e82.
3. Izabela Kalemba-Rec - development and design of method- https://doi.org/10.1016/j.biombioe.2019.01.035.
ology of SEM-EDS investigation, specifically performing the ex- [9] Fang J, Gao B, Chen J, Zimmerman AR. Hydrochars derived from plant biomass
under various conditions: characterization and potential applications and
periments, and data collection, analysis of SEM-EDS results, impacts. Chem Eng J 2015;267:253e9. https://doi.org/10.1016/
preparation of manuscript. j.cej.2015.01.026.
Conceptualization, Formal analysis, Investigation, Methodology, [10] Kumar M, Oyedun AO, Kumar A. A review on the current status of various
hydrothermal technologies on biomass feedstock. Renew Sustain Energy Rev
Project administration, Writing - original draft, Visualization.
2018;81:1742e70. https://doi.org/10.1016/j.rser.2017.05.270.
4. Monika Szyman
ska-Chargot - development and design of [11] De la Rubia MA, Villamil JA, Rodriguez JJ, Mohedano AF. Effect of inoculum
methodology of van Soest and FTIR investigations, specifically source and initial concentration on the anaerobic digestion of the liquid
fraction from hydrothermal carbonisation of sewage sludge. Renew Energy
performing the experiments, and data collection, analysis of van
2018;127:697e704. https://doi.org/10.1016/j.renene.2018.05.002.
Soest and FTIR results, preparation of manuscript. [12] Wilk M, Magdziarz A, Jayaraman K, Szyman
ska-Chargot M, Go€ kalp I. Hydro-
Conceptualization, Formal analysis, Investigation, Methodology, thermal carbonization characteristics of sewage sludge and lignocellulosic
Project administration, Writing - original draft, Visualization. biomass. A comparative study. Biomass Bioenergy 2019;120:166e75. https://
doi.org/10.1016/j.biombioe.2018.11.016.
[13] Erdogan E, Atila B, Mumme J, Reza MT, Toptas A, Elibol M, Yanik J. Charac-
Declaration of competing interests terization of products from hydrothermal carbonization of orange pomace
including anaerobic digestibility of process liquor. Bioresour Technol
2015;196:35e42. https://doi.org/10.1016/j.biortech.2015.06.115.
The authors declare that they have no known competing [14] Basso D, Patuzzi F, Castello D, Baratieri M, Rada EC, Weiss-Hortala E, Fiori L.
financial interests or personal relationships that could have Agro-industrial waste to solid biofuel through hydrothermal carbonization.
12 M. Wilk et al. / Energy 202 (2020) 117717

Waste Manag 2016;47:114e21. https://doi.org/10.1016/ doi.org/10.1016/j.jechem.2017.04.011.

j.wasman.2015.05.013. [36] Kambo HS, Dutta A. Comparative evaluation of torrefaction and hydrothermal
[15] Zhuang X, Zhan H, Song Y, He C, Huang Y, Yin X, Wu C. Insights into the carbonization of lignocellulosic biomass for the production of solid biofuel.
evolution of chemical structures in lignocellulose and nonlignocellulose bio- Energy Convers Manag 2015;105:746e55. https://doi.org/10.1016/
wastes during hydrothermal carbonization (HTC). Fuel 2019;236:960e74. j.enconman.2015.08.031.
https://doi.org/10.1016/j.fuel.2018.09.019. [37] Wilk M. A novel method of sewage sludge pretreatment - HTC. E3S Web of
[16] Wikberg H, Ohra-aho T, Honkanen M, Kanerva H, Harlin A, Vippola M, Laine C. Conf 2016;10:00103. https://doi.org/10.1051/e3sconf/20161000103.
Hydrothermal carbonization of pulp mill streams. Bioresour Technol [38] Reza MT, Uddin MH, Lynam JG, Coronella CJ. Engineered pellets from dry
2016;212:236e44. https://doi.org/10.1016/j.biortech.2016.04.061. torrefied and HTC biochar blends. Biomass Bioenergy 2014;63:229e38.
[17] Petrovic J, Perisi N, Dragisi Maksimovic J, Maksimovic V, Kragovic M, https://doi.org/10.1016/j.biombioe.2014.01.038.
Stojanovic M, Lausevi M, Mihajlovic M. Hydrothermal conversion of grape [39] Gao P, Zhou Y, Meng F, Zhang Y, Liu Z, Zhang W, Xu G. Preparation and
pomace: detailed characterization of obtained hydrochar and liquid phase. characterization of hydrochar from waste eucalyptus bark by hydrothermal
J Anal Appl Pyrol 2016;118:267e77. https://doi.org/10.1016/ carbonization. Energy 2016;97:238e45. https://doi.org/10.1016/
j.jaap.2016.02.010. j.energy.2015.12.123.
[18] Pham TPT, Kaushik R, Parshetti GK, Mahmood R, Balasubramanian R. Food [40] Gao Y, Wang X, Wang J, Li X, Cheng J, Yang H, Chen H. Effect of residence time
waste-to-energy conversion technologies: current status and future di- on chemical and structural properties of hydrochar obtained by hydrothermal
rections. Waste Manag 2015;38:399e408. https://doi.org/10.1016/ carbonization of water hyacinth. Energy 2013;58:376e83. https://doi.org/
j.wasman.2014.12.004. 10.1016/j.energy.2013.06.023.
[19] Volpe M, Fiori L. From olive waste to solid biofuel through hydrothermal [41] Liu F, Yu R, Ji X, Guo M. Hydrothermal carbonization of holocellulose into
carbonisation: the role of temperature and solid load on secondary char for- hydrochar: structural, chemical characteristics, and combustion behavior.
mation and hydrochar energy properties. J Anal Appl Pyrol 2017;124:63e72. Bioresour Technol 2018;263:508e16. https://doi.org/10.1016/
https://doi.org/10.1016/j.jaap.2017.02.022. j.biortech.2018.05.019.
[20] Cai J, Li B, Chen C, Wang J, Zhao M, Zhang K. Hydrothermal carbonization of [42] Lo
pez-Gonz
alez D, Fernandez-Lopez M, Valverde JL, Sanchez-Silva L. Ther-
tobacco stalk for fuel application. Bioresour Technol 2016;220:305e11. mogravimetric-mass spectrometric analysis on combustion of lignocellulosic
https://doi.org/10.1016/j.biortech.2016.08.098. biomass. Bioresour Technol 2013;143:562e74. https://doi.org/10.1016/
[21] Reza MT, Rottler E, Herklotz L, Wirth B. Hydrothermal carbonization (HTC) of j.biortech.2013.06.052.
wheat straw: influence of feedwater pH prepared by acetic acid and potas- [43] Watkins D, Nuruddin M, Hosur M, Tcherbi-Narteh A, Jeelani S. Extraction and
sium hydroxide. Bioresour Technol 2015;182:336e44. https://doi.org/ characterization of lignin from different biomass resources. J Mater Res
10.1016/j.biortech.2015.02.024. Technol 2015;4:26e32. https://doi.org/10.1016/j.jmrt.2014.10.009.
[22] Ma €kela
€ M, Benavente V, Fullana A. Hydrothermal carbonization of lignocel- [44] Szyman
ska-Chargot M, Chylin
ska M, Pieczywek PM, Zdunek A. Tailored
lulosic biomass: effect of process conditions on hydrochar properties. Appl nanocellulose structure depending on the origin. Example of apple paren-
Energy 2015;155:576e84. https://doi.org/10.1016/j.apenergy.2015.06.022. chyma and carrot root celluloses. Carbohydr Polym 2019;210:186e95.
[23] Yuan C, Wang S, Cao B, Hu Y, Abomohra AE, Wang Q, Qian L, Liu L, Liu X, He Z, https://doi.org/10.1016/j.carbpol.2019.01.070.
Sun C, Feng Y, Zhang B. Optimization of hydrothermal co-liquefaction of [45] Viet DQ, Son Tho VD. Study on characteristics of acacia wood by FTIR and
seaweeds with lignocellulosic biomass: merging 2nd and 3rd generation thermogrametric analysis. Viet J Chem 2017;55(2):259e64. https://doi.org/
feedstocks for enhanced bio-oil production. Energy 2019;173:413e22. 10.15625/2525-2321.2017-00456.
https://doi.org/10.1016/j.energy.2019.02.091. [46] Szyman
ska-Chargot M, Zdunek A. Use of FT-IR spectra and PCA to the bulk
[24] Yuan C, Wang S, Qian L, Barati B, Gong X, Abomohra AE, Wang X, characterization of cell wall residues of fruits and vegetables along a fraction
Esakkimuthu S, Hu Y, Liu L. Effect of cosolvent and addition of catalyst (HZSM- process. Food Biophys 2013;8:29e42. https://doi.org/10.1007/s11483-012-
5) on hydrothermal liquefaction of macroalgae. Int J Energy Res 2019;43(14): 9279-7.
8841e51. https://doi.org/10.1002/er.4843. € Ozgenç,
[47] O € Durmaz S, Kuştaş S. Chemical analysis of tree barks using ATR-FTIR
[25] Nakason K, Panyapinyopol B, Kanokkantapong V, Viriya-empikul N, spectroscopy and conventional techniques. BioResources 2017;12:9143e51.
Kraithong W, Pavasant P. Characteristics of hydrochar and liquid fraction from https://doi.org/10.15376/biores.12.4.9130-9142.
hydrothermal carbonization of cassava rhizome. J Energy Inst 2018;91: [48] Kruse A, Zevaco TA. Properties of hydrochar as function of feedstock, reaction
184e93. https://doi.org/10.1016/j.joei.2017.05.002. conditions and post-treatment. Energies 2018;11(674):1e12. https://doi.org/
[26] Nizamuddin S, Baloch HA, Griffi GJ, Mubarak NM, Bhutto AW, Abro R, 10.3390/en11030674.
Mazari SA, Ali BS. An overview of effect of process parameters on hydro- [49] Chylin
ska M, Szyman
ska-Chargot M, Zdunek A. FT-IR and FT-Raman charac-
thermal carbonization of biomass. Renew Sustain Energy Rev 2017;73: terization of non-cellulosic polysaccharides fractions isolated from plant cell
1289e99. https://doi.org/10.1016/j.rser.2016.12.122. wall. Carbohydr Polym 2016;154:48e54. https://doi.org/10.1016/
[27] Tag AT, Duman G, Yanik J. Influences of feedstock type and process variables j.carbpol.2016.07.121.
on hydrochar properties. Bioresour Technol 2018;250:337e44. https:// [50] Guo S, Dong X, Liu K, Yu H, Zhu C. Chemical, energetic, and structural char-
doi.org/10.1016/j.biortech.2017.11.058. acteristics of hydrothermal carbonization solid products for lawn grass. Bio-
[28]

Sabio E, Alvarez-Murillo A, Rom
an S, Ledesma B. Conversion of tomato-peel Resources 2015;10(3):4613e25. https://doi.org/10.15376/biores.10.3.4613-
waste into solid fuel by hydrothermal carbonization: influence of the pro- 4625.
cessing variables. Waste Manag 2016;47:122e32. https://doi.org/10.1016/ [51] Galaverna R, Breitkreitz MC, Pastre JC. Conversion of D-fructose to 5-
j.wasman.2015.04.016. (hydroxymethyl)furfural: evaluating batch and continuous flow conditions by
[29] Volpe M, Goldfarb JL, Fiori L. Hydrothermal carbonization of Opuntia ficus- design of experiments and in-line FTIR monitoring. ACS Sustainable Chem Eng
indica cladodes: role of process parameters on hydrochar properties. Bio- 2018;6:4220e423. https://doi.org/10.1021/acssuschemeng.7b04643.
resour Techol 2018;247:310e8. https://doi.org/10.1016/ [52] Arellano O, Flores M, Guerra J, Hidalgo A, Rojas D, Strubinger A. Hydrothermal
j.biortech.2017.09.072. Carbonization (HTC) of corncob and characterization of the obtained hydro-
[30] Missaoui A, Bostyn S, Belandria V, Cagnon B, Sarh B, Go €kalp I. Hydrothermal char. Chem Eng Trans 2016;50:235e40. https://doi.org/10.3303/CET1650040.
carbonization of dried olive pomace: energy potential and process perfor- [53] Peng C, Zhai Y, Zhu Y, Xu B, Wang T, Li C, Zeng G. Production of char from
mances. J Anal Appl Pyrol 2017;128:281e90. https://doi.org/10.1016/ sewage sludge employing hydrothermal carbonization: char properties,
j.jaap.2017.09.022. combustion behavior and thermal characteristics. Fuel 2016;176:110e8.
[31] Sermyagina E, Saari J, Kaikko J, Vakkilainen E. Hydrothermal carbonization of https://doi.org/10.1016/j.fuel.2016.02.068.
coniferous biomass: effect of process parameters on mass and energy yields. [54] Mihajlovi
c M, Petrovi
c J, Maleti
c S, Kragulj Isakovski M, Stojanovi
c M,
J Anal Appl Pyrol 2015;113:551e6. https://doi.org/10.1016/ Lopi ci

c Z, Trifunovi

c S. Hydrothermal carbonization of Miscanthusgiganteus:
j.jaap.2015.03.012. structural and fuel properties of hydrochars and organic profile with the
[32] Wang T, Zhai Y, Zhu Y, Lia C, Zeng G. A review of the hydrothermal carbon- ecotoxicological assessment of the liquid phase. Energy Convers Manag
ization of biomass waste for hydrochar formation: process conditions, fun- 2018;159:254e63. https://doi.org/10.1016/j.enconman.2018.01.003.
damentals, and physicochemical properties. Renew Sustain Energy Rev [55] Zhu G, Yang L, Gao Y, Xu J, Chen YH, Zhu Y, Wang Y, Liao C, Lu C, Zhu C.
2018;90:223e47. https://doi.org/10.1016/j.rser.2018.03.071. Characterization and pelletization of cotton stalk hydrochar from HTC and
[33] Xiao L-P, Shi Z-J, Xu F, Sun R-C. Hydrothermal carbonization of lignocellulosic combustion kinetics of hydrochar pellets by TGA. Fuel 2019;244:479e91.
biomass. Bioresour Technol 2012;118:619e23. https://doi.org/10.1016/ https://doi.org/10.1016/j.fuel.2019.02.039.
j.biortech.2012.05.060. [56] He C, Giannis A, Wang J. Conversion of sewage sludge to clean solid fuel using
[34] Mumme J, Getz J, Prasad M, Lüder U, Kern J, Masek O, Buss W. Toxicity hydrothermal carbonization: hydrochar fuel characteristics and combustion
screening of biochar-mineral composites using germination tests. Chemo- behaviour. Appl Energy 2013;111:247e59. https://doi.org/10.1016/
sphere 2018;207:91e100. https://doi.org/10.1016/ j.apenergy.2013.04.084.
j.chemosphere.2018.05.042. [57] Xu X, Tu R, Sun Y, Wu Y, Jiang E, Gong Y, Li Y. The correlation of physico-
[35] Sinan N, Unur E. Hydrothermal conversion of lignocellulosic biomass into chemical propoerties and combustion performance of hydrochar with fixed
high-value energy storage materials. J Energy Chem 2017;26:783e9. https:// carbon index. Bioresour Technol 2019;294:122053. https://doi.org/10.1016/
 M. Wilk et al. / Energy 202 (2020) 117717 13

j.biortech.2019.122053. j.cej.2020.124201.
[58] Paul S, Dutta A, Defersha F. Biocarbon, biomethane and biofertilizer from corn [60] Atallah E, Kwapinski W, Ahmad MN, Leahy JJ, Al-Muhtaseb AH, Zeaiter J.
residue: a hybrid thermo-chemical and biochemical approach. Energy Hydrothermal carbonization of olive mill wastewater: liquid phase product
2018;165:370e84. https://doi.org/10.1016/j.energy.2018.09.182. analysis. J Environ Chem Eng 2019;7:102833. https://doi.org/10.1016/
[59] Usmana M, Ren S, Ji M, O-Thong S, Qian Y, Luo G, Zhang S. Characterization j.jece.2018.102833.
and biogas production potentials of aqueous phase produced from hydro- [61] Poerschmann J, Baskyr I, Weiner B, Koehler R, Wedwitschka H, Kopinke F-D.
thermal carbonization of biomass - major components and their binary Hydrothermal carbonization of olive mill wastewater. Bioresour Technol
mixtures. Chem Eng J 2020;388:124201. https://doi.org/10.1016/ 2013;133:581e8. https://doi.org/10.1016/j.biortech.2013.01.154.