Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
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Journal of Analytical and Applied Pyrolysis
journal homepage: www.elsevier.com/locate/jaap
Evaluating fractional pyrolysis for bio-oil speciation into holocellulose and
lignin derived compounds
Héctor Hernando a, b, Gema Gómez-Pozuelo b, Juan A. Botas b, David P. Serrano a, b, *
a
b
Thermochemical Processes Unit, IMDEA Energy Institute, Móstoles, Madrid, Spain
Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, Móstoles, Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pyrolysis
Lignocellulose
Bio-oil
Fractionation
GC–MS
Fractional pyrolysis of lignocellulosic biomass, by staged thermal treatment, has been assessed as an in-situ
speciation method of the bio-oil components that could be highly beneficial for extracting valuable com­
pounds or for their subsequent catalytic upgrading. Wheat straw and pine woodchips were used as representative
biomasses. Based on the results of TG analyses in an inert atmosphere, 350 and 700 ◦ C were selected, respec­
tively, as operational temperatures for the fractional pyrolysis. Compared to single-step pyrolysis, fractional
thermal treatment of both biomasses led to some reduction of the bio-oil yield but with improved properties due
to their lower oxygen content. Sharp differences were observed in the bio-oil composition obtained at the two
steps of fractional pyrolysis. GC-MS analyses revealed that most of the compounds detected in the bio-oil ob­
tained at 350 ◦ C were products formed by the decomposition of polysaccharides, such as carboxylic acids, furans,
sugars, and light oxygenates. In contrast, the organic liquid phase obtained during the subsequent treatment at
700 ◦ C was rich in aromatic oxygenated compounds, coming from the lignin conversion. The content of oligo­
meric and heavy species, not detected by GC-MS, was much higher in the bio-oils obtained in the hightemperature step of fractional pyrolysis, denoting that they are largely formed from lignin. Significant
changes were also observed in the relative contribution of the deoxygenation pathways during the two steps of
fractional pyrolysis. Thus, dehydration was the predominant deoxygenation route during the degradation of the
holocellulose biopolymers at the low-temperature step, whereas the decomposition of the lignin-rich solid at the
high-temperature treatment proceeded with a significant contribution of decarbonylation and decarboxylation.
These results evidence the great potential of lignocellulose fractional pyrolysis to generate bio-oil streams with
high speciation of the components, facilitating sharply their further processing and upgrading.
1. Introduction
Lignocellulosic biomass is one of the most abundant renewable re­
sources, having great potential for the production of both biofuels and
bio-based chemicals. Among the different processes for lignocellulose
valorisation, pyrolysis is considered of high interest as it allows pro­
ducing a liquid phase (bio-oil) under relatively mild conditions. Biomass
pyrolysis proceeds usually at intermediate temperatures and atmo­
spheric pressure, giving rise to the formation of three fractions: noncondensable gases, liquid (bio-oil), and carbonaceous solids (char).
The bio-oil fraction is generated through fragmentation of the
lignocellulosic biopolymers (cellulose, hemicellulose and lignin), which
leads to a large variety of oxygenated compounds. Bio-oil can be
potentially used for obtaining transportation fuels, heat and electricity,
or as a source of valuable chemicals [1–3]. However, its high water
content, complex composition and poor properties (high viscosity,
acidic pH, low heat value and limited both thermal and chemical sta­
bility) hinder its use and further processing [4,5]. For this reason, a
variety of bio-oil valorisation routes have been earlier evaluated, such as
catalytic pyrolysis [6], aldol condensation [7], ketonisation, [8] esteri­
fication [9] or hydrodeoxygenation (HDO) [10–12] which are aimed
mainly to decrease its oxygen content, favouring also the water sepa­
ration. Unfortunately, these processes suffer from significant limitations
mainly due to the extensive formation of carbonaceous residues that
provokes a fast deactivation of the catalysts and a significant reduction
in the bio-oil yield [13]. These solid deposits are in a great part formed
by the occurrence of secondary reactions between the different families
of compounds present in the bio-oil fraction. Moreover, the effectiveness
* Corresponding author at: Thermochemical Processes Unit, IMDEA Energy Institute, Móstoles, Madrid, Spain.
E-mail address: david.serrano@imdea.org (D.P. Serrano).
https://doi.org/10.1016/j.jaap.2021.105019
Received 22 October 2020; Received in revised form 10 January 2021; Accepted 13 January 2021
Available online 21 January 2021
0165-2370/© 2021 Elsevier B.V. All rights reserved.
H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
of the catalysts is negatively affected by the presence of a high diversity
of oxygenated species in bio-oil. Different strategies to tackle this
problem, including the incorporation of additional active phases to the
catalysts with different functionalities [14] or the design of multi-stage
catalytic process [15], have proven to increase the effectiveness of the
process.
An increasing number of works in recent years have been conducted
for investigating the feasibility of bio-oil fractionation to recover high
added value compounds or to facilitate the subsequent processing of the
different oxygenated families. One of the most studied alternatives has
been the separation of pyrolytic bio-oil using extraction with solvents
[16–19]. Wang et al. [18] reported the separation of phenolic species
from the water-insoluble phase of bio-oil by extraction with acid and
alkaline solutions combined with organic solvents. Farag et al. [19]
employed switchable hydrophilicity solvents to extract phenols from
microwave-pyrolytic lignin. However, the use of significant quantities of
solvents hampers the large-scale implementation of extraction-based
methods. Molecular distillation has been also applied for separating
bio-oil components [20], which involves operating under extremely low
vacuum pressure, thus increasing significantly the cost of the process.
Other authors have studied the staged condensation of the vapours ob­
tained from biomass pyrolysis [21,22] by using an arrangement of
condensers in series operating at progressively lower temperatures.
However, fractional condensation is not completely useful for being
coupled with chemical upgrading routes since in many cases, such as
catalytic pyrolysis, they proceed in the vapour phase.
Accordingly, the development of new and efficient alternatives for
bio-oil speciation is needed. In this way, a potential route for facilitating
the separation of the products present in the bio-oil is the fractional
pyrolysis of lignocellulosic biomass. This strategy is based on the
different decomposition temperatures of the three biopolymers present
in lignocellulose. Thus, it has been reported that holocellulose can be
thermally decomposed at relatively low temperatures, whereas the
lignin fragmentation occurs at higher temperatures [21,23]. This strat­
egy has the advantage of affording the in-situ (during the own pyrolysis
process) fractionation of the bio-oil components, avoiding or minimizing
the need of additional separation steps with external solvents or re­
agents. Moreover, the different bio-oil streams so generated could be
easily coupled with vapour phase catalytic upgrading treatments to
overcome the losses of yield by charring that typically takes place during
bio-oil re-evaporation.
Despite its high interest and potential, fractional pyrolysis of biomass
has been relatively little investigated in the previous literature. Li et al.
applied a staged pyrolysis process using algae as feedstock that effec­
tively separated the bio-oil compounds in the range of 200− 500 ◦ C [24].
Westerhof et al. studied the influence of performing two-step lignocel­
lulose pyrolysis in a fluidised-bed reactor but without achieving enough
difference in the composition of the bio-oil streams to consider the
process fully successful [25]. Recently, Persson et al. investigated frac­
tional pyrolysis in two thermal steps, obtaining a bio-oil rich in phenolic
compounds and low acidity in the second one, with bare loss of mass
yields respecting the single-step process [26]. Zheng et al. studied the
effect of torrefaction at 240− 320 ◦ C prior to pyrolysis, focusing on the
characterisation of the bio-oil by multiple techniques [27]. Likewise,
Waters et al. [28] and Cai et al. [29] provided detailed information
about the bio-oil composition produced by fractional pyrolysis,
concluding that fractional pyrolysis is efficient at concentrating valuable
compounds in different streams. A similar conclusion was achieved by
Ouarzki et al., who compared direct and fractional pyrolysis [30].
However, a complete view on the fractional pyrolysis process is absent in
previous literature since important information is not taken into account
in most cases, such as the yield and composition of all the pyrolysis
products obtained at the different steps of fractional pyrolysis, which is
essential to establish a right comparison with conventional pyrolysis.
Additionally, most of the fractional pyrolysis publications do not pro­
vide information about the bio-oil content of heavy and oligomeric
compounds, which are not detectable by the GC-MS technique and may
represent an important percentage of the liquid fraction.
In this context, and aimed to shed more light on this interesting and
promising process, the present work reports a comprehensive study
about the fractional pyrolysis of representative biomasses from agri­
cultural and forestry residues (wheat straw and pine) in two sequential
steps at different temperatures. Thereby, fractional pyrolysis has been
assessed based on both TG and laboratory-scale reactor tests, allowing
the detailed characterisation and analyses of all the pyrolysis products
and the determination of the contribution of the different bio-oil deox­
ygenation routes. The results here reported show the benefits of frac­
tional pyrolysis in comparison with single-step pyrolysis, since it leads to
the in-situ speciation of the bio-oil components to facilitate their further
processing and upgrading.
2. Experimental Procedures
2.1. Biomass feedstock
The lignocellulosic biomasses employed in this study were wheat
straw (WS) and pine woodchips (Pine), obtained from Spanish agricul­
tural and forestry residues, respectively, which were ground into a
particle size of 0.5− 1 mm. To avoid the interferences of the high mineral
matter content present in the raw wheat straw, it was subjected to a deashing treatment by acid washing, as earlier reported [31,32]. Thereby,
a representative amount of biomass was dispersed into a 1 wt.% HNO3
solution (20 mLsol g− 1biomass) at 50 ◦ C for 2 h. The sample was filtered
afterwards in a Kitasato flask, washed with Milli-Q water until pH = 6,
and dried at 105 ◦ C for 2 days.
2.2. Biomass pyrolysis tests
The reaction set-up, illustrated in Fig. 1 consists of a stainless-steel
fixed bed reactor (16 mm i.d. and 400 mm length), heated by two in­
dependent electrical furnaces. The reaction temperatures are measured
by two internal type K thermocouples, placed at the middle height of
each furnace. The biomass (6 g) is placed in the feeding tank and kept at
room temperature. This feedstock was the corresponding lignocellulose
in the case of the first step of fractional pyrolysis, and the so generated
carbonaceous residue (partially decomposed biomass) in the second
Fig. 1. Schematic diagram of the experimental setup used for the pyrolysis
experiments.
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H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
one. Two internal stainless-steel tubes, having a metallic plate and a
quartz wool sheet on its top, are placed on the bottom part of the reactor
to separate the resulting char particles, which are retained inside the
reactor, from the pyrolysis vapours. The pyrolysis tests have been car­
ried out at atmospheric pressure and temperatures of 350 and 700 ◦ C for
the first and second pyrolysis steps, respectively. The residence time for
the pyrolysis vapours is estimated to be in the range of 3− 5 s, depending
on the temperature conditions.
Before each test, during the heating up of the reactor, the feeding
tank and all the reaction system are purged with a 100 cm3 min− 1 N2
flow, delivered by mass flow controllers to ensure an inert atmosphere.
Once the reactor temperature reaches the desired setpoint, the feeding
valve is open and the raw biomass falls into the reactor. The volatiles so
generated are swept by the N2 flow, being condensed through four 125
cm3 flasks connected in series and refrigerated by an ice-water bath (~
0 ◦ C). Permanent gases and light hydrocarbons (C1-C4) are finally
collected in a totaliser at the end of the line for 10 min, which measures
the total volume of the produced gases by water displacement at the
same time that allows their further analysis.
All fractions, including char, bio-oil, and permanent gases are
recovered independently for their quantification and analysis. The or­
ganics present in the liquid fraction, i.e. bio-oil on a water-free basis, are
denoted as bio-oil*.
aromatics (O-AR) and aromatic hydrocarbons (AR). Calibrations of
representative compounds detected in the GC-MS analyses were carried
out to estimate the corresponding response factors. Thus, a total of 14
compounds were calibrated (acetic acid, diethoxypropane, hydrox­
ypropanone, furfural, levoglucosan, phenol, guaiacol, cresol, creosol,
syringol, toluene, xylene, trimethylbenzene and napthtalene). The
concentration of the rest of the compounds was estimated by using an
average response factor for each family, determined from the calibrated
ones.
The molecular composition of the gas fraction was analysed in a
dual-channel Agilent® CP-4900 Micro Gas Chromatograph (μ-GC),
equipped with molecular sieve (Molsieve 5 Å) and HayeSep A columns
and a thermal conductivity detector (TCD), using Ar and He as the
carrier gas, respectively. The equipment was periodically calibrated
with standard gas mixtures of different gas concentrations containing
N2, O2, H2, CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8, C4H8, and C4H10.
Thus, the mass yield of the gas fraction and its elemental composition (C,
H, and O) could be calculated.
The high heating value (HHV) of the gases was determined from
those of the individual compounds. The HHV of the biomass, char, and
bio-oil were determined from the elemental composition using an
empirical correlation earlier reported [36].
By adding the measured weights of the different products, both total
mass and elemental (C, H, N and O) balances were closed with respect to
the raw biomass with an experimental error lower than 5 wt.% in all
tests. Additionally, the energy yield associated with a specific pyrolysis
product was calculated as the proportion of chemical energy (HHV)
retained in this product respecting that of the raw biomass. Details on
the equations used for the mass balance calculations can be found
elsewhere [37]. The results of both mass and energy yields were referred
to the amount of initial biomass. Finally, the selectivity corresponding to
the different deoxygenation pathways (dehydration, decarbonylation,
and decarboxylation) was calculated from the oxygen contained in the
H2O, CO and CO2 so produced, respectively.
2.3. Characterisation of the products obtained in the biomass pyrolysis
tests
The proximate analyses of the biomass, including moisture (UNE-EN
14774-1:2010), ash (UNE-EN 14775:2010), and volatile matter (UNEEN 15148:2010) contents, were performed following their respective
European standards. Moisture was calculated by weight difference after
introducing a representative amount of the as-received biomass sample
in an oven at 105 ◦ C overnight. Ash content was determined by weight
difference as well, after placing 20 g of the corresponding dried biomass
in a furnace at 815 ◦ C with a heating rate of 10 ◦ C min− 1 and for 2 h of
dwell time, under an airflow of 100 cm3 min− 1 to ensure the complete
combustion of the sample. Volatiles were determined in a NETZSCH 449
Thermobalance, by introducing a sample of dried biomass with a heat­
ing rate of 10 ◦ C min− 1 up to 900 ◦ C, and keeping constant this tem­
perature for 7 min, under a flow of 100 cm3 min− 1 of Ar to ensure an
inert atmosphere. The amount of Fixed Carbon was then calculated by
difference, taking into account the volatiles and the ash content. The
biopolymer composition of the biomass samples (cellulose, hemicellu­
lose, and lignin) was determined according to methods described else­
where [33].
The ultimate analyses of the biomass and pyrolysis products,
including char and bio-oil, were performed by Organic Elemental
Analysis (OEA) in a Thermo Scientific FLASH 2000 CHNS/O analyser,
measuring directly the C, H, N and S elements and determining O by
difference. The water content in the bio-oil was calculated following
American standards (ASTM E203-08) by volumetric Karl-Fischer titra­
tion using a Mettler-Toledo V20S compact titrator. From these analyses,
it was possible to estimate the H/C, O/C and “effective H/C” ratios. The
latter was defined, according to earlier works as H/Ceff =
(H–2O–3N–2S)/C [34,35].
The molecular composition of the bio-oil* fraction was analysed by
Gas Chromatography coupled to Mass Spectroscopy (GC-MS), using a
Bruker® SCION 436-GC (Electron Energy: 70 eV; Emission: 300 V; He
flow rate: 1 cm3 min− 1; Column: WCOT fused silica 30 m x0.25 mm ID
x0.25 μm). The identification of the compounds was performed
comparing the corresponding spectra with the NIST EI-MS spectral li­
brary (v2.0), using a minimum match score of 700. Due to the high
number and variety of molecules present in the bio-oil fraction, they
were classified into families according to their main functional groups:
carboxylic acids (AC), light oxygenates (LO: aldehydes, alcohols, ke­
tones, and ethers), furans (FUR), anhydrosugars (SUG), oxygenated
3. Results and discussion
3.1. Biomass properties
De-ashed wheat straw (WS) and pine woodchips (Pine) were the
feedstocks used in the pyrolysis tests, selected as representative of
agricultural and forestry lignocellulosic biomasses, respectively. Table 1
discloses the proximate and ultimate analyses of them, as well as their
composition in terms of biopolymer content. Agricultural residues are
known for having a higher ash content than the hardwood biomass and,
in particular, of alkali and alkaline earth metals (AAEM). The latter have
been reported to catalyse non-desired reactions during pyrolysis, such as
secondary cracking and carbonisation [13,38], that cause a reduction of
the bio-oil yield [31,32,39,40]. By means of the acid-washing treatment
applied to the WS biomass in this work, the amount of detectable AAEM
is reduced in more than 90 wt.%, which also provokes an increase of 6.2
wt.% in its content of volatile matter. The last finding suggests that a
partial degradation of the biopolymers may occur during the acid pre­
treatment generating smaller units (volatile matter), in agreement with
previous observations in similar processes [41]. Comparing both bio­
masses, WS and Pine, the major difference is observed in respect to the
biopolymer composition, with Pine having less cellulose and more lignin
than WS. This fact is an important factor during pyrolysis, as a direct
relationship exists between the lignin content and the formation of char
and heavy compounds [42,43].
3.2. Fractional pyrolysis tests using thermogravimetric analyses
In order to select the most convenient operating temperatures for the
lab-scale experiments, the thermal decomposition in an inert atmo­
sphere of the biomasses was first studied through TG analyses (Fig. 2 and
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H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
Table 1
Proximate, ultimate and biopolymer analyses of the raw (Raw-WS), acid-washed wheat straw (WS) and pine woodchips (Pine) biomass samples.
Sample
Proximate analysis, db (wt.%)
Ash
Raw-WS
3.3
WS
1.7
Pine
0.3
db: dry basis
Sample
Al
Raw-WS
0.94
WS
0.55
Pine
1.03
Volatiles
F. Carbon
74.6
80.8
79.3
22.1
17.5
20.4
Ba
0.1
0.02
0.03
Ca
28.62
0.54
3.66
Ultimate analysis, db (wt.%)
Fe
0.68
0.35
1.53
S
HHV
Biopolymers distribution (wt.%)
C
H
N
O
(MJ kgdb-1)
Cellulose
Hemicell.
Lignin
46.7
47.4
48.4
5.9
6.1
6.2
0.6
0.5
0.2
43.5
44.3
44.9
18.7
19.1
19.6
42.9
43.8
32.7
34.4
33.7
32.2
22.8
22.4
29.3
K
46.72
7.6
0.62
Ash elements (wt.% •10-2)
Mg
Mn
Na
P
5.92
0.18
3.78
2.14
0.15
0.02
0.02
0.59
0.79
0.9
1.29
0.3
0.15
0.02
0.35
Sr
0.05
0.02
0.17
Ti
0.05
0.02
0.02
Zn
0.12
0.02
0.02
0
0
0
Sn
Fig. 3). TGA results performed using one heating step are depicted in
Fig. 2a and Fig. 3a, showing that the decomposition of lignocellulose
takes place in a broad range of temperatures [44]. Thus, in the case of
WS, hemicellulose is degraded in the 200− 330 ◦ C range (peak at 295
◦
C), followed by cellulose in the 330− 350 ◦ C interval (peak at 348 ◦ C),
whereas lignin decomposition occurs in a quite broad range, between
200 and 700 ◦ C (peak at 395 ◦ C) [45]. The position of these peaks is
shifted at higher temperatures in the case of Pine with values of 327 ◦ C,
373 ◦ C, and 400 ◦ C for hemicellulose, cellulose, and lignin, respectively.
This fact is consistent with a larger molecular weight and/or a higher
degree of connectivity among them for Pine compared to WS [41]. On
the other hand, the relative area contribution of the deconvoluted curves
agrees reasonably well with the biopolymer content recorded in Table 1,
despite other compounds that could be present in the raw biomasses,
such as extractives, are not taken into account. After reaching a tem­
perature of 700 ◦ C, the weight variation is very small, denoting that the
pyrolysis process has been almost completed. The remaining weights at
the end of the test at 750 ◦ C correspond with the carbonaceous residue
(char), having values of 17 and 20.7 wt.% for WS and Pine, respectively.
The higher char formation for Pine would be in agreement with is larger
lignin content.
Since the curves of each biopolymer in Fig. 2a and Fig. 3a present
some overlapping, it can be anticipated that the complete separation of
the decomposition of the three biopolymers is not feasible. However, the
differences existing between the temperatures of the corresponding peak
maxima suggest the possibility of generating at least two bio-oil streams
of very different nature in the case of performing stepwise pyrolysis.
Fig. 2b and Fig. 3b gather the results obtained in TG analyses per­
formed using two heating steps, with a 30 min isotherm period at 350 ◦ C
between them. The results obtained show that a great part of the vola­
tiles (73.9 wt.% regarding the raw biomass) for WS evolve during the
first heating step, i.e. at temperatures below 350 ◦ C, which would
correspond mainly with the holocellulose decomposition. In contrast,
for Pine, the formation of volatiles is not completed in the ramp but it is
prolonged for about 20 min during the isothermal period. Moreover,
considering together the ramp and the isothermal period, the overall
formation of volatiles for Pine is 67.7 wt.%, referred to the raw biomass,
being lower than that obtained in the case of WS. These results agree
well with those obtained in the TG analyses using a single heating step,
suggesting the larger molecular weight and/or connectivity of the
Fig. 2. Thermogravimetric analyses of WS using one (a) and two heating steps
(b). Reaction conditions: 100 mL min− 1 Ar; heating rate of 10 ◦ C min− 1, from
room temperature to 750 ◦ C. TGA with two heating steps (b) follows the same
program with a 30 min isothermal period at 350 ◦ C.
Fig. 3. Thermogravimetric analyses of Pine using one (a) and two heating steps
(b). Reaction conditions: 100 mL min− 1 Ar; heating rate of 10 ◦ C min− 1, from
room temperature to 750 ◦ C. TGA with two heating steps (b) follows the same
program with a 30 min isothermal period at 350 ◦ C.
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H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
biopolymers in the forestry biomass. On the other hand, an additional
formation of volatiles occurs in the second heating step, showing peak
maxima in the DTG curves at about 430− 433 ◦ C with a shoulder at
526− 528 ◦ C and yields of 10.3 and 11.7 wt.% for WS and Pine,
respectively. This weight loss is originated by the further decomposition
of the carbonaceous residue formed after the treatment at 350 ◦ C, which
could be related mainly to the transformation of lignin-derived species.
The amounts and temperatures corresponding to the evolution of the
volatiles during the second heating step are very similar for both bio­
masses. This fact suggests that the solid residues formed after the
treatment at 350 ◦ C present quite similar properties, independent of the
lignocellulosic feedstocks.
the raw biomass.
Lab-scale results followed similar trends to those of the preliminary
TG analyses, indicating that a great part of the biomass decomposition
takes place already in the first thermal step. Thus, more than 60 wt.% of
volatiles (bio-oil*, water, and non-condensable gases) are formed during
the treatment at 350 ◦ C, while an additional 21− 24 wt.% of volatiles
evolve in the second step at 700 ◦ C. This behaviour is observed for both
biomasses, showing quite similar values. The final yield of char in the
fractional pyrolysis tests is somewhat larger for Pine compared to WS, as
it occurred in the TG analyses, which can be assigned to the higher lignin
content of the forestry biomass.
Interesting conclusions are derived when comparing the fractional
pyrolysis and the single-step pyrolysis tests. For each biomass, they
present a practically identical total yield of both volatiles and char, with
differences lower than 2 wt.%, but with significant variations in the
product distribution corresponding to bio-oil*, water, and noncondensable gases. The overall bio-oil* yield decreases for the frac­
tional pyrolysis tests, whereas the production of both water and noncondensable gases is enhanced, in comparison with the single-step py­
rolysis experiments at 550 ◦ C. These variations are more pronounced in
the case of WS, with a 13 wt.% decrease in the bio-oil* yield versus just a
reduction of 6 wt.% for Pine. On the other hand, it can be observed that
the production of water occurs mainly during the first step in the frac­
tional pyrolysis tests, whereas the opposite is denoted for the noncondensable gases, showing enhanced production during the second
thermal step. These results are in agreement with earlier results that
indicated how dehydration reactions prevailed at low temperatures,
while the formation of gases was more favoured when increasing the
pyrolysis temperature [37].
Important changes in the composition of non-condensable gases are
observed in Fig. 4b. The gases formed in the first step of the fractional
pyrolysis tests (at 350 ◦ C) are mainly formed by CO2, with minor con­
tributions of CO, whereas the presence of light hydrocarbons is
3.3. Fractional pyrolysis tests using a laboratory-scale reaction system
3.3.1. Products distribution and bio-oil oxygen concentration
Based on the results of the TG tests, temperatures of 350 and 700 ◦ C
were selected for carrying out the fractional pyrolysis experiments using
a laboratory-scale reaction system that operates with biomass amounts
large enough (about 6 g) to get a complete quantification and charac­
terisation of all the pyrolysis fractions. Thereby, as reported in the
experimental section, the carbonaceous solid obtained after the first
thermal treatment at 350 ◦ C was subjected to a second pyrolysis step at
700 ◦ C. In these tests, it was possible to collect and characterise inde­
pendently all the pyrolysis products: char, non-condensable gases, and
bio-oil. On the other hand, for both biomasses, a single-step pyrolysis
test was also performed to be used as a reference. A temperature of 550
◦
C was selected for this experiment with both biomasses since according
to a previous work it provided the maximum bio-oil yield [37]. The
results obtained in all the pyrolysis tests are illustrated as follows: yield
of the different fractions (Fig. 4a), the yield of the components in the gas
phase (Fig. 4b), and the oxygen content of bio-oil* (bio-oil in water
free-basis, Fig. 4c). In all cases, the yields are referred to the weight of
Fig. 4. Products yield distribution (wt.%) (a), gaseous components yield (wt.%) (b), and bio-oil oxygen concentration (wt.%, dry basis) (c) in the pyrolysis of WS and
Pine (single-step pyrolysis: 550 ◦ C; fractional pyrolysis: 350 → 700 ◦ C). GO: gaseous olefins (C2–C4); GP: gaseous paraffins (C2–C4).
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H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
Table 2
Properties of the char fractions obtained in the single-pyrolysis reactions at 550 ◦ C in comparison with those produced in the fractional tests at 350 and 700 ◦ C.
Sample
Char (WS / 550 ◦ C)
Char (WS / 1st step – 350 ◦ C)
Char (WS / 2nd step – 700 ◦ C)
Char (Pine / 550 ◦ C)
Char (Pine / 1st step – 350 ◦ C)
Char (Pine / 2nd step – 700 ◦ C)
Proximate analysis, db (wt.%)
Ultimate analysis, db (wt.%)
Volatiles
Ash
F. Carbon
C
H
N
O
HHV
(MJ kg−db1)
19.5
38.3
5.8
17.5
45.7
5.7
6.8
4.7
5.6
1.3
0.8
1.4
73.7
57.0
88.6
81.2
53.5
92.9
77.6
64.1
83.1
78.7
70.0
83.7
2.9
4.3
2.0
3.0
4.8
2.1
1.1
0.7
0.4
0.1
0.1
0.1
11.5
26.2
9.0
17.0
24.3
12.7
29.2
24.6
30.3
29.2
27.6
30.3
db: dry basis.
practically negligible. Since hemicellulose is a shorter chain polymer,
easier to defragment in small molecules, it is expected to be the major
contributor to the formation of CO2 and CO, especially at low temper­
ature, followed by cellulose [46–48]. In contrast, the gases coming from
the second step at 700 ◦ C show enhanced production of CO, as well as of
light hydrocarbons, especially methane, similar to past observations
[48]. Thus, the CO/CO2 ratio passes from 0.1 to 1.9 w/w in the case of
WS when comparing the first and the second pyrolysis step. For Pine, the
variation of this ratio is less pronounced but still significant, with values
of 0.2 and 1.3 w/w for the first and second pyrolysis steps, respectively.
The formation of methane and other light hydrocarbons can be attrib­
uted mainly to the cracking of O-alkyl groups in lignin fragments [44,
49], reaching yields of about 2 wt.% regarding the raw biomass. Like­
wise, a noticeable production of H2 takes place in the second step of the
fractional pyrolysis tests, probably due to the occurrence of dehydro­
genation reactions during the char formation. Finally, if the gases ob­
tained during the fractional pyrolysis tests in overall are compared with
those of the single-step pyrolysis, enhanced production of CO, H2, CH4,
and the rest of light hydrocarbons is observed in the former. This finding
is probably due to the high temperature (700 ◦ C) employed in the second
step of the fractional pyrolysis, which could promote severe cracking
and dehydrogenation reactions.
Coming back to the bio-oil* fraction, the tests using two thermal
steps lead to a lower yield compared with the single-step pyrolysis,
which in principle would be a negative effect. However, the quality of
the bio-oil* so produced must also be taken into account. In this way,
considering its possible application as fuel, it would be desirable that the
bio-oil* fraction presents an oxygen content as low as possible. Typi­
cally, the bio-oil* obtained in conventional pyrolysis, with no catalyst,
shows an oxygen content very similar to that of the initial biomass [14].
This is the case of the single-step pyrolysis tests here reported, producing
bio-oils* with 40− 42 wt.% oxygen content versus values of 44− 45 wt.%
in the raw lignocelluloses. In the case of fractional pyrolysis, the bio-­
oils* generated in the first step exhibit a relatively high oxygen content
(34 and 40 wt.% for WS and Pine, respectively). However, a sharp
decrease in the oxygen content is denoted for the bio-oils* produced in
the second thermal step, with values of 27 and 13 wt.% for WS and Pine,
respectively. These results could be assigned to the lower oxygen con­
tent of lignin compared to holocellulose as the decomposition of the
former occurs in a higher extension during the second pyrolysis step in
line with the TG analyses. Interestingly, the overall oxygen content of
the bio-oil* produced by fractional pyrolysis (considering together both
steps) is significantly lower than that of the organic liquid phases ob­
tained in the conventional pyrolysis tests at 550 ◦ C (Fig. 4c). Therefore,
it can be concluded that fractional pyrolysis has the advantage of leading
to partially upgraded bio-oils* in terms of oxygen content.
even material for electrodes [56], is a key factor to favour the process
economy. As the feasibility of these applications will depend on the char
properties, the results of the characterisation study of the carbonaceous
residues obtained in the present work are collected in Table 2.
The oxygen content of the char obtained after the first step in frac­
tional pyrolysis is about 24− 26 wt.%, clearly lower than in the raw
biomasses (43.5 and 44.9 wt.% for WS and Pine, respectively) but higher
than those of the carbonaceous residues generated in single-step pyrol­
ysis at 550 ◦ C (11.5–17 wt.%). Moreover, the elemental composition of
the solids produced in the first thermal treatment of the fractional py­
rolysis tests is relatively close to that of lignin [48], which is another
indication that the low-temperature step affects in higher extension to
the holocellulose components. On the other hand, the chars finally
produced in the second step of fractional pyrolysis show the lowest
oxygen content, and therefore, the largest HHV values. These results
denote how the solid fraction undergoes progressive deoxygenation
along the two-steps of the fractional pyrolysis process. Likewise, the
properties of the char produced using single-step pyrolysis are
comprised between those obtained in the first and second steps of
fractional pyrolysis. In this way, clear trends are observed in the
composition of the carbonaceous residues as a function of the pyrolysis
temperature corresponding to the final thermal step. Thus, the content
of volatile matter, i.e. that able to decompose in a non-reactive atmo­
sphere, decreases linearly with the pyrolysis temperature, from 79− 81
wt.% in the raw biomasses to less than 6 wt.% for the chars obtained at
700 ◦ C. Consequently, the opposite variation is observed in the case of
fixed carbon. Regarding the elemental analysis, as the reaction tem­
perature rises the solid samples become richer in carbon and poorer in
hydrogen and oxygen, due to a progressive condensation of the com­
ponents present in the carbonaceous residues [57,58]. This fact is re­
flected in the reduction of the O/C (from 0.8 to 0.3) and H/C (from 0.3 to
0.1) atomic ratios, denoting an increase in both the graphitic carbon and
the molecular weight of polyaromatic species, in agreement with pre­
vious studies [59].
The water content of the bio-oils are compiled in Table S1, including
also their elemental composition and high heating values (HHV). In the
case of the bio-oils produced in the second pyrolysis step, a separation in
two phases (water and organic-rich fractions) takes place induced by
marked differences in the polarity of the compounds so obtained. Both
phases were physically separated for their further analyses, although the
results here reported are a weighted average of them. As seen in the
previous section, dehydration reactions are more predominant at lower
temperatures, which leads to high water contents for the bio-oils ob­
tained in the first step of fractional pyrolysis, reaching values in the
range 37− 45 wt.%. On the other hand, the HHV of the bio-oil* fraction
is determined mainly by its oxygen content, a clear enhancement being
observed for the bio-oils* generated at 700 ◦ C. In overall, considering
together the liquid fractions obtained sequentially at 350 and 700 ◦ C, it
can be appreciated that the bio-oils* produced by fractional pyrolysis
exhibit larger HHV than that obtained in a single thermal treatment at
550 ◦ C.
The variation of the elemental composition of the bio-oil* fraction
can be visualized in the Van Krevelen graph presented in Fig. 5. In the
3.3.2. Proximate and ultimate analysis of the pyrolysis products
Char is an important pyrolysis by-product in the pyrolysis process
due to its considerable mass yield and, given their high heating values,
even larger energy yield [31]. Therefore, its energetic exploitation [50]
or valorisation as a commodity for refined products, such as solid
amendment [51,52], support for catalysts [53–55], adsorbent [52,53] or
6
H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
case of the single-step pyrolysis tests, a reduction in the O/C ratio is
observed for both biomasses, but they show opposite variations in terms
of the H/Ceff ratio, increasing for Pine but lowering for WS. In contrast, a
positive evolution in both parameters occurs in the fractional pyrolysis
tests. Thus, passing from the initial biomasses to the first pyrolysis step
involves a significant reduction of the O/C ratio and a noticeable in­
crease in the H/Ceff ratio. Subsequently, the second pyrolysis step
accentuated the decrease in the O/C ratios while the H/Ceff reached
values in the 0.51− 0.75 range. Accordingly, the fractional pyrolysis biooils* exhibit elemental compositions that increasingly approach the
composition typical of liquid fossil fuels. Moreover, if both low- and
high-temperature bio-oils* are considered together, it can be concluded
that they present a more favourable composition than those obtained in
the single-step pyrolysis tests. This finding denotes that fractional py­
rolysis is more efficient than single-step pyrolysis for producing partially
upgraded bio-oils*.
abundant compounds in each family, GC-MS analyses have been used to
get quantitative information (wt.% concentration and yield) of the
identified species, allowing also to estimate (by difference) the overall
share of components which are present in bio-oil* but are not detected
by GC-MS analyses [14,60,61]. The latter are considered to consist
mainly of oligomers and heavy species, coming from the partial frag­
mentation of the lignocellulose biopolymers or formed through
cross-reactions between the bio-oil components. The results derived
from the GC-MS analyses of the bio-oils* obtained in the present work in
the different pyrolysis tests are illustrated in Fig. 7, showing the values
of the mass yield considering together all the components (Fig. 7a) and
those corresponding to the different families of compounds (Fig. 7b).
Moreover, detailed information about the bio-oil* composition is pro­
vided in Table S2.
According to Fig. 7a, for the single-step pyrolysis tests at 550 ◦ C just a
minor part of the species present in bio-oil* are identified and detected
by GC–MS, representing 22.5 and 43.7 wt.% of the whole bio-oil* for WS
and Pine, respectively. These data denote that the thermal bio-oils are
formed in a great part by oligomeric and heavy compounds. The large
difference observed between both biomasses, regarding the yield of
GC–MS detected species, could be assigned to the presence of a higher
amount of mineral matter in the herbaceous feedstock, even after the deashing treatment, since inorganic components could catalyse conden­
sation reactions between the primary pyrolysis products. On the other
hand, significant variations in the relative proportion of GC–MS detected
components are denoted for each one of the steps in fractional pyrolysis.
Thus, the proportion of compounds identified by GC–MS is quite larger
in the bio-oil* produced in the first thermal step at 350 ◦ C, which are in
the range of 45− 47 wt.%, than in the second one at 700 ◦ C, 23− 30 wt.%.
This finding suggests that oligomers and heavy species are formed in a
greater extension from the lignin fraction and at high pyrolysis
temperatures.
Regarding the distribution of detectable compounds by families,
Fig. 7b reveals marked differences between the experiments. The most
abundant family in the reference experiment at 550 ◦ C with WS is SUG
(mainly levoglucosan) with a yield of about 6 wt.%, coming from cel­
lulose decomposition, while the rest of the families (AC, LO, FUR, O-AR)
are present in the range of 0.6–2.2 wt.%. In contrast, light oxygenates
are the predominant family (with a yield of about 12 wt.%) in the Pine
pyrolysis at 550 ◦ C. Propanol, hydroxypropanone, butenal, hydroxycyclopentenone and methoxybutene are some of the compounds pre­
sent in high concentration in this bio-oil* (Table S2). The enhanced
production of LO with Pine concerning WS can be related to its higher
hemicellulose/cellulose ratio (see Table 1). Likewise, the higher yield of
O-AR observed in the bio-oil* from Pine, in comparison to that of WS, is
well aligned with the larger content of lignin in the forestry biomass.
On the other hand, if the bio-oils* produced at the two steps of
fractional pyrolysis are considered together, important variations can be
observed regarding the reference WS pyrolysis test at 550 ◦ C. Thus, in
the case of stepwise pyrolysis, the yield of SUG decreases, whereas those
of the other families is enhanced. In particular, the formation of AC
(mainly acetic acid) and LO is strongly promoted in the first step of
fractional pyrolysis. These results suggest that holocellulose decompo­
sition may follow different pathways as a function of the reaction tem­
perature, leading to AC/LO or SUG, respectively. The fact that these
differences are quite less significant in the case of Pine is an indication of
the possible catalytic participation in those transformations of the
mineral matter still present in the WS after the acid-washing treatment.
Finally, although detected in small concentrations, it must be high­
lighted the presence of aromatic hydrocarbons in the bio-oils* produced
in the second step of fractional pyrolysis probably due to the high
temperature (700 ◦ C) reached during this final treatment. The AR yield
is higher in the case of Pine, denoting their formation from the lignin
fraction. As recorded in Table S2, the main components of this family are
polyaromatic hydrocarbons, such as naphthalene, methyl-naphthalene,
dimethyl-naphthalene, methyl-anthracene, and phenyl-naphthalene.
3.4. Deoxygenation selectivity
The selectivity corresponding to the three deoxygenation routes
(dehydration, decarboxylation, and decarbonylation) has been calcu­
lated from the overall yield of H2O, CO2, and CO, respectively, being
represented in Fig. 6. For both biomasses in the single-step pyrolysis
tests, dehydration is the major deoxygenation pathway with a selectivity
of around 70 %, followed by decarboxylation and decarbonylation. The
share of dehydration is even higher during the first step of fractional
pyrolysis, reaching selectivity values close to 90 %, with little contri­
bution of decarboxylation, whereas the extension of decarbonylation is
practically negligible. Accordingly, the decomposition of hemicellulose
and cellulose during the low-temperature step proceeds for both ligno­
cellulosic feedstocks mostly through dehydration reactions. However,
this is not the case in the second step of the fractional pyrolysis that
displays significant contributions of both decarboxylation and decar­
bonylation routes during the decomposition of the corresponding ligninrich carbonaceous residues.
3.5. Bio-oil* molecular composition
The composition of the bio-oil* at the molecular level has been
assessed by GC-MS, the detected compounds being grouped into car­
boxylic acids (AC), light oxygenates (LO; including mainly aldehydes,
alcohols, ketones, and ethers), furans (FUR), anhydrosugars (SUG),
oxygenated aromatic compounds (O-AR) and aromatic hydrocarbons
(AR). Based on the calculation of the response factors of the most
Fig. 5. Van Krevelen graph for the fast pyrolysis bio-oils* from WS and Pine
(single-step pyrolysis: 550 ◦ C; fractional pyrolysis: 350 → 700 ◦ C).
7
H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
Fig. 6. Deoxygenation selectivity in the fractional pyrolysis of WS and Pine (single-step pyrolysis: 550 ◦ C; fractional pyrolysis: 350 → 700 ◦ C).
The ultimate analysis of the bio-oils*, shown in Table S1, are in
agreement with their molecular composition. Thus, bio-oils* produced
at 350 ◦ C present values of oxygen concentration and O/C molar ratios
similar to those of furans and anhydrosugars, such as furfural (O ~ 33
wt.%; O/C ~ 0.4) and levoglucosan (O ~ 49 wt.%; O/C ~ 0.8). In
contrast, the elemental composition of bio-oils* obtained at 700 ◦ C is
lower, approaching that of oxygenated aromatic compounds, like cresol
(O ~ 14 wt.%; O/C ~ 0.1) or catechol (O ~ 29 wt.%; O/C ~ 0.3), which
is in line with the GC–MS observations.
Focusing on O-AR family, Fig. 8 depicts the mass yields of the
following sub-groups within this family: phenol, alkylphenols, guaia­
cols, catechols, anisols, syringols, and phenylketones. The larger dif­
ference comparing both biomass sources is observed in the guaiacol
group, showing an enhanced production in the case of Pine in compar­
ison to WS. This fact could be assigned to variations in the lignin
structure, i.e. to the presence of a higher concentration of methoxy
groups in the forestry feedstock [62]. Most of the O-AR components are
generated in the second thermal step at 700 ◦ C, which agrees well with
the lignin being decomposed mainly at higher temperatures compared
to holocellulose. However, guaiacols, and anisols, both containing
methoxy groups, are also detected in significant amounts in the first step
of fractional pyrolysis. On the other hand, phenol, alkylphenols, and
catechols formation, all possessing hydroxyl groups, stands out mainly
in the second step at 700 ◦ C. In this way, catechol and methyl-catechol
are the compounds detected with the highest concentration in the
bio-oils* produced at 700 ◦ C for both biomasses (Table S2).
Fig. 9 illustrates the large differences existing in terms of the
composition of the GC–MS detected matter between the bio-oils* pro­
duced in the two steps of fractional pyrolysis. Thus, bio-oils* obtained in
the low-temperature step present a small concentration of O-AR, being
the main components AC and LO, followed by SUG and FUR. Accord­
ingly, species derived from holocellulose represent about 95 wt.% of the
GC–MS detected matter in the bio-oils* produced at 350 ◦ C. In this way,
as above highlighted, the low-temperature bio-oil* in the case of Pine is
very rich in the LO family, with a share of 58 wt.% of the whole GC–MS
detected matter. In contrast, O-ARs are by large the main components in
the bio-oils* obtained at high temperature with a share up to about 75
wt.% in the case of Pine.
Finally, Fig. 10 displays how the different families of compounds are
distributed between the two bio-oils* generated in fractional pyrolysis.
Thereby, it has been represented the share of each family in the GC–MS
detected matter corresponding to both low- and high-temperature biooils*. The ratio between both concentrations can be considered a
partition coefficient, its values being provided in the inset of Fig. 10. The
points corresponding to AC, LO, FUR, and SUG are clearly located above
the diagonal in this figure, as they are present mostly in the lowtemperature bio-oil, having partition coefficients values from 1.7 (AC
in WS bio-oil*) up to 13.3 (FUR in WS bio-oil*). In contrast, the com­
ponents of O-AR and AR families appear preferentially in the bio-oils*
obtained in the second thermal step at 700 ◦ C, with very low partition
coefficients < 0.1.
In summary, these results confirm the high degree of speciation
Fig. 7. Total/GC–MS quantified bio-oil* components (a), and bio-oil* compo­
nents mass yields in terms of main organic compounds families (b) obtained in
the fast-pyrolysis of de-ashed wheat straw (WS) and Pine (single-step pyrolysis:
550 ◦ C; fractional pyrolysis: 350 → 700 ◦ C).
8
H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
Fig. 8. Quantified mass yields of bio-oil* oxygenated aromatic (O-AR) components obtained in the fast-pyrolysis of WS (a) and Pine (b) biomasses (single-step
pyrolysis: 550 ◦ C; 2 fractional pyrolysis: 350 → 700 ◦ C).
Fig. 9. Composition of the bio-oil* GC–MS detected matter obtained in the two steps of fractional pyrolysis of WS (a, b) and Pine (c, d).
achieved by fractional pyrolysis. Accordingly, it can be envisaged that
further processing of these two bio-oil* streams, by separation of valu­
able compounds or catalytic upgrading, would be significantly facili­
tated in comparison with the organic liquids obtained in single-step
pyrolysis. In particular, catalytic upgrading of the bio-oils* obtained in
the low-temperature step may be highly advantageous as they present
low amounts of oxygenated aromatics and polyaromatics, as well as of
oligomers and other heavy species. All these components are known to
cause a strong deactivation of the catalysts typically employed for biooil upgrading, like zeolite-based materials. Therefore, it should be ex­
pected that catalyst deactivation becomes quite less pronounced during
the upgrading of the low-temperature bio-oil* of fractional pyrolysis
with respect to conventional single-step schemes.
components, thus reducing the chemical complexity of the organic
liquid streams so generated as to facilitate their further processing and
upgrading.
Wheat Straw and Pine have been selected as representative feed­
stocks of agricultural and forestry residues, respectively. Based on TG
analyses under an inert atmosphere, temperature values of 350 and 700
◦
C were established as suitable to carry out the two thermal steps of
fractional pyrolysis. In this way, the carbonaceous solid obtained at 350
◦
C was subsequently subjected to a second pyrolysis treatment at 700 ◦ C.
Whereas most of the hemicellulose and cellulose were degraded
during the thermal treatment at 350 ◦ C, the decomposition of the lignin
fraction occurred in a higher extension during the high-temperature
step. Accordingly, clear differences were observed in the composition
of the different pyrolysis fractions obtained at the two thermal steps, as
well as regarding the contribution of the different deoxygenation routes.
Thus, dehydration reactions took place extensively during the lowtemperature step, whereas decarbonylation and decarboxylation
become significant in the pyrolysis treatment at 700 ◦ C. Moreover,
4. Conclusions
The results obtained in this work show that fractional pyrolysis is an
interesting process to achieve the in-situ speciation of the bio-oil
9
H. Hernando et al.
Journal of Analytical and Applied Pyrolysis 154 (2021) 105019
Declaration of Competing Interest
The authors declare no conflicts of interest.
Acknowledgements
The authors gratefully acknowledge the financial support from the
Spanish Ministry of Economy, Industry and Competitiveness, and the
Spanish State Research Agency through the project BIOCASCHEM
(CTQ2017-87001-R) and from the Regional Government of Madrid
through the project BIOTRES-CM (P2018/EMT-4344). CIEMAT (Spain),
and in particular Dr. Ignacio Ballesteros, are acknowledged for assessing
the biopolymers composition of the biomass.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jaap.2021.105019.
Fig. 10. Distribution of the bio-oil* component families between the two steps
of fractional pyrolysis, in terms of concentration in GC–MS quantified matter
(Ci), for both WS and Pine biomasses (350 → 700 ◦ C). The inset provides the
partition coefficients for each family.
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gaseous hydrocarbons was clearly enhanced due to the cracking of alkyl
groups in lignin-derived components.
In comparison with single-step pyrolysis at 550 ◦ C, sequential
treatment of the lignocellulosic biomasses in two thermal steps led to
enhanced production of water and gases, while the overall char pro­
duction was quite similar. Although fractional pyrolysis gave rise to
some decrease of the bio-oil yield, this fraction contained significantly
lower oxygen and, therefore, improved properties in comparison with
bio-oils obtained by single-step pyrolysis.
Drastic compositional differences were observed in the bio-oils
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during the low-temperature step of Pine pyrolysis.
Based on these results, it can be envisaged that catalytic upgrading of
the low-temperature bio-oils obtained in fractional pyrolysis would
proceed with significantly reduced catalyst deactivation as they present
lower amounts of oxygenated aromatics, polyaromatics, oligomers, and
other heavy species.
Author statement
Hereby you can find the statement of contributions of each author to
the manuscript entitled “Evaluating fractional pyrolysis for bio-oil
speciation into holocellulose and lignin derived compounds”:
CRediT authorship contribution statement
Héctor Hernando: Data curation, Formal analysis, Investigation,
Methodology, Resources, Validation, Visualization, Writing - original
draft, Writing - review & editing. Gema Gómez-Pozuelo: Investigation,
Resources, Visualization, Writing - original draft, Writing - review &
editing. Juan A. Botas: Conceptualization, Funding acquisition, Project
administration, Methodology, Supervision, Writing - original draft,
Writing - review & editing. David P. Serrano: Conceptualization,
Formal analysis, Funding acquisition, Project administration, Method­
ology, Supervision, Writing - original draft, Writing - review & editing.
10
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