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Mechanical pretreatments of lignocellulosic biomass:
towards facile and environmentally sound technologies
for biofuels production
Abdellatif Barakat, Claire Mayer, Abderrahim Solhy, Rick A. D. Arancon,
Hugo de Vries, Rafael Luque
To cite this version:
Abdellatif Barakat, Claire Mayer, Abderrahim Solhy, Rick A. D. Arancon, Hugo de Vries, et al.. Me-
chanical pretreatments of lignocellulosic biomass: towards facile and environmentally sound technolo-
gies for biofuels production. RSC Advances, Royal Society of Chemistry, 2014, 4 (89), pp.48109-48127.
�10.1039/c4ra07568d�. �hal-01135766�
Mechanical pretreatments of lignocellulosic
biomass: towards facile and environmentally sound
technologies for biofuels production
Abdellatif Barakat,
*
a
Claire Mayer-Laigle,
a
Abderrahim Solhy,
b
Rick A. D. Arancon,
c
Hugo de Vries
a
and Rafael Luque
c
The transformation of lignocellulosic biomass into biofuels represents an interesting and sustainable
alternative to fossil fuel for the near future. However, one still faces some major challenges for the
technology to be fully realized including feedstock costs, novel pretreatment processes, production,
transportation, and environmental impact of the full chain. The development of new technologies
focused to increase the efficiency of cellulose conversion to biofuels determines successful
implementation. Mechanical fractionation is an essential step in order to increase final carbohydrate
output, appropriate particle sizes and densification, enzymatic accessibility, and bioconversion affectivity
without the production of toxic side streams. In this review article, we surveyed a substantial amount of
previous work in mechanical fractionation or pretreatments of a variety of lignocellulosic biomasses;
these include numerous milling schemes and extrusions, and their impacts on the physical and
physicochemical properties of the lignocellulosic matrix (crystallinity, surface area, particle size, etc). We
have also compared results with other pure chemical and physicochemical pretreatments in order to
show the new aspects and advantages/disadvantages of such an approach. Last, but not least, the effect
Cite this: RSC Adv.,2014,4, 48109
Received 24th July 2014
Accepted 18th September 2014
DOI: 10.1039/c4ra07568d
www.rsc.org/advances
a
INRA, UMR 1208 Ing
´
enierie des Agropolym
`
eres et Technologies Emergentes (IATE) 2, place
Pierre Viala – 34060, Montpellier Cedex 1, France. E-mail: barakat@supagro.inra.fr; Fax:
+33 (0)4 99 61 30 76; Tel: +33 (0)4 99 61 25 81
b
Universit
´
e Polytechnique Mohammed VI, Lot 660-Hay Moulay Rachid, 43150, Ben
Guerir, Morocco
c
Departamento de Qu
´
ımica Org
´
anica, Universidad de C
´
ordoba, Campus de Rabanales,
Edicio Marie Curie (C-3) CtraNnal IV-A, Km 396, C
´
ordoba, Spain E-14014
Abdellatif Barakat is a Researcher
in Green Chemistry and Engi-
neering of Lignocellulosic Biomass
at INRA, JRU of Agropolymer
Engineering and Emerging Tech-
nologies (IATE, Montpellier,
France) which focuses on Dry
Fractionation of Agro-resources.
He has great experiences in the
eld of Biochemical and Physico-
chemical Characterization of
Biomass, Dry Fractionation,
Chemical and Physicochemical
Pretreatments and Bioconversion of Lignocellulosic Biomass to Bio-
fuels, Chemicals and Biomaterials. He received in 2007 his PhD in
Chemistry and Physicochemical of Lignocellulosic Biopolymers from
Reims University. Before joining the JRU IATE, he effected several Post
Doc in France, at Laboratory of Biotechnologies of the Environment
(LBE,INRA),inNarbonne(2009–2011), and at Institut Charles Ger-
hardt (MACS, CNRS) in Montpellier, (2007–2009).
Claire Mayer-Laigle is a Research
engineer and Director of the agro-
resources processing platform of
the JRU Agropolymer Engineering
and Emerging Technologies
(IATE), INRA Montpellier, France.
She received her PhD in process
and chemical engineering from
Toulouse University. She has
great experiences in development
and optimization of Dry Frac-
tionation of Agro-resources
Process and Physicochemical
characterization of powders. Before joining the JRU IATE, she worked
at RAPSODEE research centre, Albi, France and at Norais Technolo-
gies (SME), L'hermitage-Lorge, France.
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of mechanical treatment and physical properties on enzymatic hydrolysis and bioconversion has been
discussed, with potentially interesting dry lignocellulosic biorefinery schemes proposed.
1. Introduction
Mechanical size reduction is a crucial step for the trans-
formation of feedstock into energy and polymer biomaterials in
the eld of bio-based products (bioenergy and biomaterials)
from renewable biomass resources.
1–4
Size reduction has many
advantages: (i) it increases the volume caloric value of biomass
and simplies the densication processes,
5
(ii) it simplies the
supply chain of raw materials,
6
and their storage conditions,
(iii) it increases the total accessible surface area and, thus,
improves the bio-accessibility of constituents
7
and the conver-
sion of saccharides during hydrolysis and (iv) it reduces the
Abderrahim Solhy received his
PhD in materials chemistry and
heterogeneous catalysis in 2004
from the University of Hassan II
Mohammedia and in collabora-
tion with Universidad of Zar-
agoza, Spain. Right aer the
thesis, he passed one year in
Surface Treatment industry.
Then he effected several post-
doctorals, at Universidad Pub-
lica de Navarra in Spain, at
Laboratoire de Chimie de Coor-
dination in Toulouse, France and at Institut Charles Gerhardt in
Montpellier. Between July 2008 and February 2009, he worked at
MINATEC in Grenoble. Since March 2009, he is project-leader at
MAScIR Foundation. He is Laureate of the third edition of the
Grand Prize for Invention and Research in Science and Technology
in 2010. His current research interests include synthesis of mate-
rials and nanomaterials for catalysis and environment and waste
and biomass valorization, elds in which with co-workers he has
published over 40 papers and 3 patents.
HugodeVriesisDirectorofthe
Joint Research Centre IATE of
INRA, CIRAD, SupAgro and
University of Montpellier II on
Biopolymer Research and
Emerging Technologies. JRC is
IATE is focusi ng on dry fraction-
ation, process–structure–function
relationships, food-packaging
material science, white biotech-
nology and knowledge engi-
neering. He is Board member
EFFoST and of the Chair
UNESCO ‘alimentation durable’.HeisformerheadoftheFood
Technology Centre, WageningenUR,andhehascoordinatedthe
European Integrated Project NovelQ. He has a PhD in Physics.
Rick Arneil Arancon nished his
BSc in Chemistry at Xavier
University – Ateneo de Cagayan
in 2011 (Cum Laude). Aer
graduation, he moved to Ateneo
de Manila University to accept a
Junior Faculty teaching post. In
2013, he moved to the City
University of Hong Kong in the
group of Dr Carol Lin as a
Graduate Teaching Assistant/
Research Fellow for 6 months
and to the group of Dr Rafael
Luque at the Universidad de Cordoba, Spain. He is currently
pursuing his MSc studies under the LANEF Pre-Doctoral Fellow-
ship Program at the Universite Joseph Fourier in Grenoble, France.
Rafael Luque has extensively
contributed to the areas of
biomass and waste valorisation
practises to materials, fuels and
chemicals as well as nanoscale
chemistry over the past 10 years,
with >220 research articles, 3
patent applications and 7 edited
books as well as numerous
contributions to book chapters
and keynote and plenary
lectures in scientic events
worldwide. Among recent
awards, Rafael has received the RSC Environment, Sustainability
and Energy Early Career Award (2013), 2013 Distinguished Engi-
neering Fellow from CBME at HKUST in Hong Kong and currently
honoured as Chinese Academy of Sciences Visiting Professor at the
Changchun Institute of Applied Chemistry. Rafael combines his
academic duties with his activities as young entrepreneur aer co-
founding Green Applied Solutions S.L.U in Cordoba, Spain and
more recently involved in Posidonia Oceanica S.L.
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mass and heat transfer limitations during the hydrolysis reac-
tions
8
and consequently reduces energy inputs.
1
We can
distinguish different types of size reduction that are generally
differentiated, like cutting or crushing (meter to centimeter
range in size), coarse milling (cm to mm, cm to 500 mm),
intermediate micronization (cm to 100 mm), ne grinding (<100
mm), ultra-ne grinding (<30 mm) and nanogrinding (<1 mm).
1,9
However, nanogrinding could only be achieved through wet
grinding which is not addressed in this contribution due to the
associated energy consumption, particularly to dry biomass
aer the grinding step, far too substantial to consider it a
worthwhile pretreatment step (Fig. 1).
The re duction of raw material size is achieved using a
combination of different mechanical stresses such as impact,
compression, fri ction, and shear (Fig. 2) – all may coexist in one
commercial equipment.
1,10,11
For example, in a jet mill, the parti-
cles are projected against each other in an air stream; major
mechanical stresses generated are impact and friction between
particles (Fig. 2). Different mill tools are used to fragment and
dissociate lignocellulosic biomass: knife mill, hamm er mill, pin
mill and centrifugal mill, which consist of a rotor driving different
tools. The rotor speed is genera lly adjustable. A sieve or a screen
allows control of the particle size of the nal product. These mills
generate more impact and shear. In ball mills includin g vi bratory
Fig. 1 The different mechanical operations for size reduction of constituents related to plant structure.
Fig. 2 Schematic representation of some commercial milling equipment with the different mechanical stresses generated.
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ball mill and tumbling ball mills (or planetary ball mills), the raw
materials suffer impact and compression stresses when collisions
betweenballsandwallsoccur.Finallyinanextruder,themain
mechanical stress is shear occurring between the screw and the
walls of the extruder. The choice of equipment depends on many
parameters: physical and chemical properties of the biomass, the
moisture content, nal particle size, the particle size distributions
and application targ ets. Colloid mills and extruders are suitable
only for comminuting wet materials with moisture contents over
15–25%, whereas hammer and knife mills are suitable to pretreat
dry biomass with moisture contents up to (10–15%).
1,10
Extruders,
in comparison with disc and ball mills, have advantages in term s
of continuous processing, easy adjustment on-line, and usage in
large-scale applicatio ns with high throughput. The uid ized bed
as superne grinder has been widely used in vario us industrial
elds for its excellent ability to improve the surface area and
enhance the bioavailability of the materials thro ugh micro n-
izations, without sacricing the natural physical–chemic al
proprieties of the materials.
12–14
The energy requirement in relation to nal particle size is one
of the most important economical parameters in the choice of
milling equipment. It mainly depends on (i) machine specica-
tions such as motor speed, (ii) storage capacity of the milling
chamber, (iii) material throughput characteristics, (iv) initial
biomass structure and physical–chemical proprieties (moisture
content, chemical composition, tissue composition, post-
pretreatment etc.); and (vi) particle sizes.
1,4,6,10,15
However, the
equipment could also be selected for steering the reactivity of
biomass. As an example, several studies have shown that BM
could be described as a mechanical–chemical treatment because
the prolonged milling efficiently breaks chemical bonds between
lignin and hemicelluloses,
16
decrease particle size,
17,18
decrease
the CrI (from 69.9 for raw wheat straw to 23.7 aer a BM step),
19
increase enzymatic hydrolysis
16,17
and increase the SSA (from 0.64
for raw wheat straw to 2.3 m
2
g
1
aer a BM step).
19
In this review, we address unique features of extrusion and
mechanical size reduction as mechanical pretreatment in
lignocellulosic bioreneries. First of all, we outline
bioconversion pathways of lignocellulosic materials and we
discuss the effect of mechanical treatment compared to the
purely chemical and physicochemical treatments with respect
to surface area (in relation to enzymatic accessibility) and CrI. In
the second part, we discuss the effect of mechanical treatment
on enzymatic hydrolysis and the factors that can inuence the
performances of enzymatic hydrolysis and bioconversion.
2. Bioconversion of lignocellulosic
biomass: from heterogeneous particles
to biofuels
The bioconversion of lignocellulosic biomass has been exten-
sively studied in the past 30 years. In spite of such research
endeavors, enzymatic degradation of lignocellulose is still
poorly understood because of competing effects including
physical properties of the substrate, enzyme synergy and mass
transfer. The structural heterogeneity and complexity of cell
wall constituents such as crystallinity of cellulose microbrils,
specic surface area of particles and matrix polymers are
responsible of the recalcitrance of cellulosic materials (Fig. 3).
Biomass pretreatment is consequently an essential step in
order to increase its nal carbohydrate output, accessibility,
bioavailability and hydrolysis rate (Fig. 3). The objective of
pretreatments depends on the process type and biomass
structure. For instance, pretreatments aimed to produce bio-
fuels target changes in lignocellulosic matrix properties to make
the holocelluloses more accessible to enzymatic attack.
20–25
Pretreatment methods can be divided into different cate-
gories: mechanical, chemical, physicochemical and biological or
various combinations of these. Mechanical pretreatments allow
the separation of the main botanical parts of the crop into
different fractions (tissues, cell, polymers, etc.), to be used as
feedstock for various applications. Such pretreatment greatly
reduces biomass particle sizes and possibly affects its molecular
structure to facilitate enzymatic accessibility. Palmowski and
Muller
26
have studied the effect of mechanical operation on
Fig. 3 Different steps of biomass conversion and parameters influencing lignocellulosic particle reactivity.
48112
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