Biomass and Bioenergy 113 (2018) 31–44

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Biomass and Bioenergy

journal homepage: www.elsevier.com/locate/biombioe

Review

Fine grinding of wood – Overview from wood breakage to applications T

∗
Pasi Karinkanta, Ari Ämmälä, Mirja Illikainen , Jouko Niinimäki
Fiber and Particle Engineering, P. O. Box 4300, FIN-90014 University of Oulu, Finland

A R T I C LE I N FO A B S T R A C T

Keywords: Due to its abundance, wood is the pre-eminent lignocellulosic raw material for a sustainable bioeconomy based
Pulverization society. Wood is widely used as fuel, construction material, and raw material in cellulose and lignocellulose
Milling based products. Besides the established uses of wood powder, like co-firing with coal and biofuel production,
Size reduction there are also novel uses and process applications, e.g., advanced wood-plastic composites and biochemical
Wood powder
production are emerging for which the pulverization or fine grinding of wood is an essential pre-treatment step.
Energy consumption
Due to the tenacious nature of the wood matrix, size reduction is an energy intensive process and thermal or
Aspect ratio
chemical pre-treatment may be needed to improve economy.
This paper provides a broad overview of the fine grinding of wood. First, wood breakage mechanisms and the
mechanisms of size reduction are presented, followed by fine grinding techniques and wood pre-treatment
methods. A comparison of the specific energy consumption of wood fine grinding in both a gaseous and liquid
environment is illustrated. Additionally, examples are given of the role played by pre-treatment methods in
decreasing energy consumption. The particle aspect ratio is discussed briefly. Finally, the use and requirements
of wood powders in various applications are discussed.

1. Introduction grinding’ has been proposed for product sizes less than 100 μm [9] but
in other studies, the term has been used for product sizes up to 1 mm
Social and political incentives for a carbon neutral and en- [10–12]. In this review, fine grinding is considered as the size reduction
vironmentally sound society lie behind the impetus to develop more where the product mass median particle size is below 500 μm, which
sustainable products from renewable biomaterials. Because of its also means that virtually all particles are less than 1 mm. 500 μm is a
abundance, wood is the first choice as a renewable non-food source of practical limit in dry grinding that is difficult or even impossible to
lignocellulosic biomaterial. The FAO 2010 survey reports that 31% of achieve with moist wood because size reduction will be limited by the
the total land area of the Earth is covered by forests [1], accounting for agglomeration of particles, especially in mills where a particle bed is
about 50% of terrestrial gross primary production i.e. a carbon flux compressed, e.g., in ball mills and roller mills [13]. Additionally,
produced by terrestrial plants through photosynthesis [2] and 80% of 500 μm is the limit proposed for dividing biomass fuels into powders
total plant biomass [3]. and granular materials [14].
Wood is widely used as fuel, construction material, and raw mate- In fine grinding, it is also common practice to classify processes as
rial in cellulose and lignocellulose based products such as paper and wet or dry. Wet grinding typically means the grinding of a material
board. For many applications, wood has to be pre-treated by grinding it containing about 50% of uncombined water by volume [4], although
into a particulate form to produce ‘wood powder’ or ‘wood flour’. other liquids apart from water can also be used. In practice, wet
Mechanical grinding of lignocellulosic substances such as wood leads grinding involves the grinding of material that behaves like a liquid,
typically to a fine particle size, various particle shapes, high specific i.e., a viscous fluid under compressive and shear stresses. In wet mills
surface area, and sometimes low cellulose crystallinity, depending on (e.g. ball mills, roller mills, disc mills) grinding is performed in a par-
the energy and grinding mechanism applied as well as the grinding ticle bed in which repeating squeezing and consolidation of the bed is
conditions and raw material properties. Grinding has a considerable responsible for particle breakages [15]. Dry grinding is related to
effect on the storage and conveying properties of wood powder as well grinding where material behave more like a solid, i.e., a rigid body
as its processability and suitability for use in a given application. under compressive and shear stresses, although the formation of par-
The definition of ‘fine grinding’ varies between different industrial ticle bed is also possible.
areas [4–8]. For lignocellulosic biomasses, such as wood, the term ‘fine Today, the main use of wood powder is in energy production by co-

∗
Corresponding author.
E-mail address: mirja.illikainen@oulu.fi (M. Illikainen).

https://doi.org/10.1016/j.biombioe.2018.03.007
Received 21 September 2016; Received in revised form 21 December 2017; Accepted 17 March 2018
Available online 23 March 2018
0961-9534/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Fig. 1. Cellular structure of wood and an in-

dividual fiber. Crack paths are presented: (a)
crack advance by cell separation (intercellular/
interwall fracture), (b) crack advance by cell
fracture (intracellular/transwall fracture).

firing with coal; however, there are an increasing number of applica- tangential, and longitudinal compression [22]. The stress-strain curves
tions where wood can be used in powdered form, such as thermo- shown have some general features that have been found for various
plastics [16] and paper products [17]. Research is ongoing to find novel other wood species under compression [23]. Linear-elastic behavior
ways to utilize wood powders, including the enzymatic, chemical, and prevails at the beginning of deformation (A), but when the stress
thermochemical transformation of forest biomass for energy and che- reaches the peak value, plastic deformation and failures take place (B).
micals. The plateau of the stress-strain curves relates to a progressive crushing
As yet, there exists no comprehensive review of the fine grinding of of the wood, encompassing plastic deformation, during which a sub-
wood. This paper aims to give a broad insight into the topic. First, wood stantial growth in density is seen due to collapsing of the cells. This
breakage mechanisms and the mechanisms of size reduction are pre- allows the denser wood to resist compression, as can be seen in the form
sented, followed by fine grinding techniques and wood pre-treatment of strain toughening (C) in the stress-strain curve.
methods. A comparison of the specific energy consumption (SEC) of
wood fine grinding in both dry and wet is illustrated. Additionally, 2.2. Wood breakage
examples are given regarding the role of pre-treatment methods in
decreasing energy consumption. A brief discussion is given of particle At the macroscopic scale, fracture mechanics of wood is typically
aspect ratio. Finally, the use and requirements of wood powders in divided into three modes of loading that lead to different forms of
various applications are discussed. failure behavior [24] shown in Fig. 3. Pure tension failure is en-
ergetically more favored than pure shear failure in wood. A reason for
2. Mechanical properties and breakage behavior of wood this may be energy losses caused by friction between the fractured
surfaces brought about by shear loads [25]. On the other hand, mixed
Wood is a composite material both on macroscopic and microscopic mode loading (modes I and II) can lead to failure that is energetically
levels (Fig. 1). The fibers forms cellular structure glued together by more favorable than pure tension failure [26], since in this type the
lignin-rich middle lamella. Additionally, individual fibers are compo- initial crack propagation always takes place along the longitudinal axis
sites as such since cell wall is composed variously aligned layers of [27].
cellulose microfibrils - cross-linked together by hemicellulose - in a Fracture of wood studied at the macroscopic scale can be explained
lignin matrix. Rheologically wood is a viscoelastic material. The viscous by phenomena at the cellular level. However, knots, growth rings and
behavior causes internal friction, which converts most mechanical en- flaws may cause deviations and interpretation is not always straight-
ergy imposed on it into heat [18,19] that holds true also with brittle forward.
material [20]. The fracture force needed is dependent on the loading direction.
When stress is perpendicular to the longitudinal axis, the wood
breakage may occur as intercellular failure due to the separation of cells
2.1. Stress-strain behavior of wood from each other by peeling. This means the crack propagating mainly
via the compound middle lamella, (intercellular fracturing, see Fig. 1),
Due to the anisotropic structure of wood, the mechanical properties [28,29]. When tension is perpendicular to the longitudinal axis,
of wood vary depending on the loading direction (see Fig. 1, left) [21]. breakage of the wood requires stress more than 1 MPa, but much less
Fig. 2 illustrates typical stress-strain curves for spruce wood in radial, than 10 MPa [30–33]. In the case of tension parallel to the longitudinal
axis, for the breakage of the wood or of a single tracheid is required
stress over 30 MPa [30,34]. Transwall (intracellular) failure takes place
[35–38], i.e., the fracture path intersects the cell wall [39] (see Fig. 1).
Even though there are considerable differences in the breakage
behavior of wood in different directions, in practical grinding it is dif-
ficult to utilize this material property of wood, since wood particles are
typically more or less randomly oriented in the grinding zone. In par-
ticle beds, however, elongated particles have a tendency to be oriented
perpendicular to the stressing element.

2.3. Effect of moisture, temperature and time under load

As a viscoelastic material, wood response to mechanical treatment is

affected by temperature, moisture, time under load, and the number of
stressing events. Internal friction causes energy loss, which is sig-
nificantly higher for water-saturated than for air-dry wood, even at
temperatures below 30 °C [18]. The glass transition point of such wood
components as amorphous cellulose, hemicelluloses, and lignin has
significant dependence on the moisture mass fraction of the wood [31]
Fig. 2. Illustration of typical stress-strain curves for spruce wood under tangential, radial, while crystalline cellulose has not [40].
and longitudinal compression [22].
Above the glass transition temperature, the physical state of the

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Fig. 3. Illustration of different loading modes in fracture mechanics: a) mode I, tensile stress leads to tension failure, b) mode II leading to in-plane or forward shear failure, and c) mode
III, leading to out-of-plane or transverse shear failure [22].

polymer alters from a brittle glass-like material to a viscous rubber-like producing the particles having a fairly elongated shape. Regardless of
material [40,41]. This alteration occurs in hemicelluloses, amorphous the loading direction, ‘Intensity III’ can cause intense intercellular and
cellulose, and lignin at temperature around 200 °C in a dry state, but intracellular damages, which leads to more spherical particle shapes
when the moisture mass fraction of the wood increases, hydrophilic than with ‘Intensity II’. The theoretical strength of crystalline cellulose
hemicelluloses and amorphous cellulose can be softened at room tem- of 25 GPa [44] is many magnitudes higher than the stresses required for
perature [41,42]. Increasing moisture mass fractions lowers also the transwall failures in tracheids [34,45–47]. ‘Intensity IV’ characterizes
glass transition temperature of lignin so that in water-saturated con- an impact that is sufficient to decrease the degree of the cellulose
ditions lignin softens at 90 °C [43]. The stressing frequency affects the crystallinity. With smaller particle sizes, the stressed area decreases,
softening temperature of the wood. The higher is the stressing fre- possibly resulting in local stresses (Intensity IV) that are high enough to
quency, the higher is the softening temperature of lignin [43]. destroy crystallinity but too low to break particles. Thus, the occurrence
Depending on the moisture mass fraction of the wood, temperature, of ‘intensities I − IV’ in milling depends on the particle size range.
and stressing frequency, fracturing is more likely to occur in the lignin-
rich compound middle lamella or in the cellulose-rich cell wall. In fine 3. Fine grinding mills
grinding, the aim is typically to break the fiber cell walls to obtain fine
particulates as opposed to fibrous particles. Fine grinding mills can be categorized into five groups [48]: impact
mills, ball media mills, air jet mills, roller mills, and those of some other
2.4. Breakage mechanisms in fine grinding type, including disc mills. Impact mills and air jet mills are based on
single impacts, as presented in Fig. 4a, while ball media mills utilize
The breakage mechanisms of grinding can be characterized by double impacts between metal surfaces (Fig. 4b). Roller mills are based
considering the changes in particle size, shape, and the degree of cel- on compression (Fig. 4c) and disc mills on shearing impacts (Fig. 4d).
lulose crystallinity, all of which can be attributed to the mechanical
properties of the wood. Table 1 lists relations between breakage be- 3.1. Rotor impact milling
havior of wood and the developments of particle size, shape, and cel-
lulose crystallinity in wood exposed to different impact loading in- In rotor impact mills particle size is reduced by single impacts for
tensities [22]. which energy is produced by rotating disintegration elements. Typical
‘Intensity I’ characterizes wood loading with stresses that are not rotor impact mills are usually equipped with a screen or an integrated
strong enough to cause fracturing in any direction in a single stressing classifier (often referred to as classifier mills). In classifier mills ground
event. However, fatigue such as microcracks, kinks etc. can take place. particles are carried by air flow into the classification zone where ro-
‘Intensity II’ characterize wood loading situation in which interwall tating classifier wheel with slots allows fine particles to flow through
failures can occur but transwall failures do not take place. Cracks the slots while coarse particles are returned back to the grinding zone
propagate via the energetically most favored path, i.e. along the grain, for further grinding. Classifier mills produce the finest powder of all the

Table 1
Possible breakage behavior due to differences in ultimate strength in wood when exposed to a single impact of different intensity, and its influence on the size, shape, and relative degree
of cellulose crystallinity [22].

Impact intensity Breakage behavior Changes in size, shape, and cellulose crystallinity

I
(No failures) - No breakage - No changes
- Fatigue is possible
II
(Intercellular and intrawall failures) - Fractures depend on the orientation of the wood - Particle size decreases but tracheids can remain unchanged in length in the
- Intercellular and intrawall failures in tracheids longitudinal direction
- No transwall failures in tracheids - Shape changes but tracheids can remain elongated
- Fatigue - No changes in relative degree of cellulose crystallinity
III
(Transwall failures) - Fractures independent of the orientation of the - Particle size decreases
wood - Shape changes
- Intracellular and intercellular failures in tracheids - No changes in relative degree of cellulose crystallinity
- Fatigue
IV
(Amorphization of cellulose) - Fractures independent of the orientation of the - Particle size decreases
wood - Shape changes
- Intracellular and intercellular failures in tracheids - Relative degree of cellulose crystallinity decreases
- Amorphization of cellulose
- Fatigue

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Fig. 4. Illustrations of different stress types: a) single im-

pact, b) double impact, c) slow compression, and d)
shearing impact [22].

types of rotor impact mill, whereas classic hammer mills with a screen prolonged grinding media milling and may increase the apparent par-
or sieve are used for coarser powder production [49]. The residence ticle size [12,63,76].
time of the particles within the milling zone controlled by classifier is Grinding media milling can also change the macromolecular struc-
related to the number of stress events and thus the achievable particle ture of wood components. This can be observed as an increase in the
size of the product [50]. water-soluble oligomer content, the existence of mechano-radicals, and
Rotor impact mills are usually the first choice for the grinding of the lower degree of polymerization of lignin and cellulose
viscoelastic materials. They can be used to wood grinding if the target [64,66,77–81]. Mechano-radicals are generated as a consequence of the
median particle size is well over 100 μm [10,51–56]. Although in- cleavage of covalent bonds in lignin and cellulose [66,78].
tegrated screens or classifiers are used to adjust the fineness of the
product [40], the attainable particle size of moist wood is limited due to 3.3. Opposed jet milling
blinding of the screen leading to material build-up, excessive heat
build-up and equipment damages [55]. Thus, rotor impact mills are A jet mill is a fluid energy mill that uses high speed jets of com-
suggested only for grinding of lignocelluloses having a low moisture pressed air to apply impact energy of particles for size reduction [82].
mass fraction, less than 10–15% [57]. Besides air, also other inert gases like nitrogen and steam can be used as
Ground wood particles by rotor impact mills are reported to have the fluid. Jet mills can be further categorized as spiral, opposed, oval
more cylindrical than spherical in shape [56,58]. The size and shape of chamber and target jet mills [60,82]. Spiral and oval chamber jet mills
particles are dependent on the type of rotor impact mill, but clearer have been classified as attrition type jet mills [48].
differences in the morphology of wood particles are apparent when Jet mills are typically used in the final stage of the fine grinding
cutting mills are compared against rotor impact mills. In the latter case, process [48,82]. Opposed jet milling is applied for the production of the
wood particles are more elongated [54], but still less elongated than finest powders, where high-energy single impacts between particles
those ground with a disc refiner [55]. play a decisive role in their breakage [82]. Grinding pressure de-
termines the kinetic energy of the grinding air, which has a significant
influence on the particle breakage mechanism [83–85]. Opposed jet
3.2. Grinding media milling
milling to a median particle size of around 20 μm has a negligible effect
on the relative degree of cellulose crystallinity of wood and favors the
In grinding media mills, grinding media (usually balls or beads) are
production of elongated wood particles in comparison to vibration
driven by the movement of the mill casing or by an agitator [48]. Al-
milled wood [86].
though usually spherical balls or beads are used, there are normally no
restrictions to using non-spherical grinding media, which is a reason
why they are referred to as media mills or grinding media mills 3.4. Roller milling
[6,59,60] rather than ball mills. Size reduction is caused by impacts
when particles are caught either between grinding balls and the mill There are basically two roller mill types; ones that employ rollers
casing or between colliding grinding balls while colliding impacts be- rotating on a table or in a vessel (roller type) and ones in which the feed
tween the grinding media (balls) and particles have insignificant role. material is pulverized between cylindrical rolls (roll type) [48]. Fine
Grinding media mills disrupt cell walls more efficiently than non- grinding based on slow compression is possible with roller mills having
media mills, causing the fibrous structure of the wood to be destroyed smooth roll contours.
[61–64]. The fibrous structure has been reported to disappear faster in A special type of vertical roller mill was applied by Gravelsins [13]
planetary ball milling than in vibration or tumbling ball milling of air- in the milling of wood. Material is ground between the ridges of the
dried aspen sawdust [64]. rollers and the wall primarily by compressive stresses exerted by the
Besides effective fibrous structure destruction, grinding media mills rollers. It was shown that this kind of mill can be applied in the pro-
have been found to reduce the cellulose crystallinity of the particles. duction of wood powder with a median particle size of less than
Prolonged grinding of lignocellulose material having a low moisture 1000 μm, especially when grinding in an aqueous environment. It is
mass fraction brings about total amorphization of cellulose [63,65–73]. also one of the very few studies that has considered the agglomeration
A low moisture mass fraction of the wood in grinding media milling is of moist wood during milling in detail.
vital when there is the need to decrease the relative degree of crystal-
linity of cellulose [67,74]. 3.5. Disc milling
Vibration milling can be used to produce very fine wood powder.
Kobayashi et al. [12] used it to produce fine powder (median size of Disc mills, also known as disc refiners, are predominant technology
24 μm) consisting of rounded broken fibers with a smooth surface and in the pulp and paper industry for defiberizing moist wood chips as such
having a significantly reduced degree of cellulose crystallinity. The or after chemical modification to wood pulp, known as thermo-
reduced cellulose crystallinity by vibration milling is mainly due to the mechanical (TMP) or chemithermomechanical pulp (CTMP) [43]. Disc
mechanical action, whereas temperature and chemical effects are of mills have also been tested for wood pulverization [55].
only minor importance [75]. In vibration milling, the cell corners and In mechanical pulping, lignin is softened by high temperature or
compound middle lamella of wood seem to be the most resistant ele- chemicals, and wood chips are refined due to the shear and attrition
ments [63] while the outer secondary cell wall layer S1 is loosened at caused by patterned stator and rotor blades. Although a considerable
an early stage of grinding. The thick S2 layer is split into lamellae of amount of fines are generated (mass fraction of 20–30% below 74 μm),
varying diameters from 10 nm to 500 nm. The globular particles in fine the target is to produce fibrous material and therefore equipment and
powdered wood tend to aggregate (form agglomerates) during process conditions are not necessarily optimal for pulverization. The

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

high energy consumption typical of mechanical pulping (about becomes brittle at low temperatures [111]. There are some publications
1.2 MWh t−1 in the first stage TMP refiner [87]) has also been observed describing milling of wood with grinding media mills in cryogenic
in pulverization [55]. conditions i.e. in liquid nitrogen [63,66,79]. The size reduction kinetics
of dry wood has been reported to be similar both in gaseous and liquid
4. Pretreatments and special milling conditions nitrogen medium [63]. However, the milling temperature of −196 °C
weakens the ultimate strength of wood [112,113].
Thermal, chemical, and enzymatic pre-treatments of wood can be Moisture mass fraction has been found to be essential. The higher is
used in order to reduce the energy consumption of grinding the moisture mass fraction, the greater are changes in the ultimate
[43,56,88–93]. Thermal pre-treatment temperatures from 50 °C to strength of wood at low temperatures [112]. At the moisture mass
150 °C lead to drying in which wood loses moisture and shrinks [94] fraction of fresh wood, 40–50%, brittle behavior is generally observed
causing cracks and defects in the wood [10]. It also increases cellulose at temperatures below −40 °C [40]. Particle median size and the degree
crystallinity [95,96]. At higher temperatures, from 150 °C to 200 °C, of cellulose crystallinity of wood powder were found to be less de-
drying results in structural damages due to cell wall collapse, while at pendent on the moisture mass fraction in cryogenic grinding conditions
temperatures from 200 °C to 300 °C depolymerization and devolatili- than in ambient grinding conditions [114]. Interestingly, cellulose
zation take place [94]. crystallinity could be destroyed by the prolonged cryogenic grinding of
wood having a moisture mass fraction upto the fiber saturation point
4.1. Torrefaction (i.e., at the state in which fiber wall is saturated with moisture
throughout) and even above it, while in ambient conditions that was
Torrefaction is a thermal pre-treatment in an inert atmosphere at possible only for dry wood.
temperatures in between 200 °C and 300 °C causing chemical and
physical alterations in lignocellulosic material [94,97–99]. Wood loses 4.4. Other forms of pretreatments
its tenacious nature, which is mainly associated with the breakdown of
the hemicellulose matrix and depolymerization of the cellulose. Lignin It has been hypothesized that microwave radiation would induce
is relatively inert to torrefaction [100]. The anhydrous weight loss superheated conditions in the biomass matrix that could break down
caused by torrefaction due to slow thermal decomposition of wood the lignocellulosic structures [107]. Microwave treatment of wood logs
(slow pyrolysis) is related to increased brittleness of the lignocellulose before chipping has reduced refining energy 15% using a disc refiner
[94,97] and thus the reduction of the energy required for size reduction [115]. The reduction was assumed to cause rather by lignin depoly-
[10,11]. merization during microwave treatment than internal fracturing of
Torrefaction affects the wood powder morphology in grinding, re- wood, however. The assumption is supported by a study [116] which
sulting in better handling characteristics and flowability through pro- concluded that under unpressurized conditions microwave radiation
cessing and transportation systems [10,11,101–105]. There have been tends to cause primarily macroscopic breaks and do not sufficiently
observations that as the severity of torrefaction increases, the sphericity disrupt the cellular wood structure.
of particles increases [102,103]. On the other hand, it has also been The irradiation of wood chips (aspen, spruce) with high-energy
reported that the severity of torrefaction affects the particle size but not electrons has been studied in thermomechanical pulping. Energy sav-
the morphology, i.e., the circularity or aspect ratio under given ings of 30% with a disc refiner have been reported [117]. The effect was
grinding conditions [101]. This is partly supported by the results of assumed to relate to the depolymerization of the material.
Phanphanich & Mani [93], who found the sphericity of ground particles Various chemical agents such as acids, alkalis, and oxidizing agents
was changed only at higher temperatures. have been studied as pre-treatments [107,118]. One promising che-
mical pre-treatment with sodium bisulfite at low pH has been reported
4.2. Steam explosion to lower energy consumption considerably in size reduction using a disc
refiner [92]. The pre-treatment is generally used in the production of
Steam explosion is a pre-treatment process for a biomass, which chemi(thermo)mechanical pulp in order to soften lignin and thus re-
disintegrates the biomass matrix structure, opens up the fibers and ducing refining energy consumption and promote fiber separation [43].
breaks up the fiber structure. The pre-treatment increases grindability
and makes the biomass polymers more accessible to fermentation, hy- 5. Energy consumption vs. particle size
drolysis, or densification.
In general, moist woody biomass is treated with hot steam The size reduction of wood is an energy-intensive process. Due to
(180–240 °C) under pressure (1–3.5 MPa) for a few minutes, followed the viscoelastic nature of wood, the breakage of particles needs a lot of
by an explosive decompression of the biomass that breaks the rigid energy. Industrially, the grinding of wood and other lignocellulosic
structure of the biomass fibers [106,107]. Depending on the severity of materials has mainly been performed with a hammer mill, knife mill
the treatment, i.e., residence time and temperature, steam explosion and disc (attrition) mill. Some useful data is found in published studies
can result in transformations from small cracks in the wood structure to about grinding using hammer mills, knife mills, disc mills, vibratory
total fragmentation of the wood fibers [108]. Thus grinding may be mills, and roller mills.
needed to finalize the size reduction of wood. Lam [109] reported de-
creased particle size and aspect ratio of steam-exploded softwood par- 5.1. Dry grinding
ticles with increasing severity of steam explosion pre-treatment.
There is no data available on the effect of steam explosion on the Although data about specific energy consumption can be found in
grindability of wood although the effect has been studied for non-wood the literature, information about the particle size of the product is
material by Adapa et al. [110]. According to their data, steam-exploded missing in many studies. Hammer and knife milling results have often
straw biomass needed about 2/3 less grinding energy for grinding to a been presented and even modeled against the screen nominal aperture
given particle size than untreated straw biomass. size, which may give misleading information because the screen size
does not explicitly define the size of the product. The specific energy
4.3. Cryogrinding consumptions have been presented either as net or gross values, i.e., no-
load power has either been deducted or not. No-load power can be high
Cryogenic grinding is suggested as effective method for fine especially in small-scale devices and it varies between devices so it
grinding of viscoelastic materials because the materials usually would be preferable to present net specific energy consumption, which

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

would make it possible to compare results between studies. fraction of more than 95%, i.e., 5% wood mass fraction) is more energy
An extensive wood grinding study with various species and moisture efficient than high consistency refining in a gaseous medium, i.e., at a
mass fractions has been carried out by Temmerman et al. [119]. They moisture fraction of around 40–60% [125].
presented the size reduction of wood in a rotor impact mill (hammer Wet pulverization of wood in a roller mill has been studied by
mill) to follow the well-known Rittinger model, which states that the Gravelsins [13]. Selected data from his work is presented in Fig. 6
energy required for size reduction is proportional to the new surface (denotations are explained in Table 2). Wet grinding was shown to be
area generated. According to their data, at low moisture mass fraction more efficient than dry grinding. For example, in the grinding of pine
(< 10%), the least grinding energy for a given size reduction was sawdust in a water suspension with an SEC of 75 kWh t−1, a particle
needed for spruce, followed by pine, beech, and oak. For moist wood size of 350 μm was achieved; while in the grinding of air-dry and moist
(20–25%), however, the order was almost reversed; the lowest energy sawdust in a gaseous environment with the same SEC, sizes of 570 and
was needed for beech followed by oak, pine, and spruce. The increase of 990 μm were obtained, respectively. In the case of aspen chips, to
moisture from < 10% to around 20–25% increased energy consump- achieve a median size of 400 μm, for example, a SEC of 100 kWh t−1
tion around two-fold for hardwood species, four-fold for pine, and was required, while in dry grinding over three times more (320 kWh
seven-fold for spruce. t−1) was needed. Thus, having an incompressible fluid between parti-
Repellin et al. [10] also found slightly higher grinding energy cles that are being compressed by grinding elements seems to be ad-
consumption for dry beech than for dry spruce with a rotor impact mill vantageous.
(ultra centrifugal mill). The results of Gravelsins [13] showed that
spruce chips need a lower grinding energy than aspen chips in a roller
5.3. Effect of torrefaction on grinding energy
mill. Slight decaying lowered the energy required for grinding aspen
chips considerably.
Pre-treatment of wood by torrefaction decreases the energy re-
In Fig. 5, the specific energy consumption of dry grinding from
quirement of grinding by 80–90% [122]. It has been proposed that the
various sources [12,13,53,55,119–123] is illustrated as a function of
decrease in energy required is proportional to the anhydrous weight
the product mass median size. The denotations of the figure are ex-
loss caused by a slow thermal decomposition of wood. Fig. 7 shows the
plained in Table 2. It can be observed that there is a linear correlation
reduction in specific energy consumption as a function of the weight
between SEC and particle size in a log-log plot in an individual grinding
loss of wood. Torrefaction seems to be more advantageous for hard-
series with dry wood (moisture mass fraction of less than 10%) but a lot
wood than softwood. A weight loss of 25% decreases specific energy
of variation exists between series. This can be explained by the different
consumption by about 70% with softwoods (pine and spruce) and al-
grinding methods and material properties including the moisture and
most 90% with hardwood (beech). Data from Repellin et al. [10] in-
operating parameters used in the experiments. It is also possible that
dicates mild torrefaction to reduce grinding energy requirement even
grinding conditions were not optimal in each study. This can especially
without anhydrous weight loss while data from other sources
be seen in disc refining experiments with moist chips, where a large
[11,104,126,127] suggests SEC reduction to be related to anhydrous
variation exists between test points, indicating the importance of the
weight loss also after mild torrefaction.
optimization of grinding conditions.
Apart from SEC reduction, torrefaction results in a decrease in
particle size when grinding without torrefaction is carried out under
similar conditions. Fig. 8 presents the relative SEC consumption as a
5.2. Wet grinding
function of relative particle size for pine, spruce and beech (data from
Refs. [10,127]).
In general, wet grinding in a liquid medium like water is more en-
ergy efficient than dry grinding in a gaseous medium, typically air
[124]. Wet grinding of wood in an aqueous medium is scarcely reported 6. Aspect ratio vs. particle size
in the literature if mechanical pulp fiber production by low consistency
disc refining is excluded. It is generally known that the low consistency In addition to particle size, the aspect ratio affects powder handling
refining of pulp fibers in a continuous water phase (moisture mass and flowability properties as well as end product properties. For

Fig. 5. Specific energy consumption vs. mass median particle size of grinding of dry wood chips (moisture content less than 10%, except in the disc mill). Grinding conditions, references,
and notes are listed in Table 2.

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Table 2
Information related to Figs. 5 and 6.

Denote Mill type Raw material Moisture mass Feed (size) Reference Notes SEC net/gross
fraction, %

KM Knife mill Aspen 6 Chips [53] 1.59–6.35 mm grid net

DM Disc mill Poplar 60 Chips [55] gross
VM Vibration mill ? ? ? [12] SEC estimated by Kobayashi et al. gross
AM Attrition mill Aspen 6 Chips [53] net
HM1 Hammer mill Beech 8 Pre-ground chips (4–6 mm) [119] 2- 8 mm grid net
HM2 Hammer mill Oak 8 Pre-ground chips (4–6 mm) [119] 2- 8 mm grid net
HM3 Hammer mill Pine 8 Pre-ground chips (4–6 mm) [119] 2- 8 mm grid net
HM4 Hammer mill Spruce 8 Pre-ground chips (4–6 mm) [119] 2- 8 mm grid net
HM5 Hammer mill Aspen 6 Chips [53] 1.59–6.35 mm grid net
HM6 Hammer mill Spruce, Beech ? Pre-ground chips, 2–4 mm [120] probably dried, net
0.5 mm grid
HM7 Hammer mill Willow 1 ? [122] gross?
HM8 Hammer mill Poplar 5–7 Chips [123] 2 and 5 mm grid gross
RM1 Roller mill Aspen decayed 5 Small chips, 1.7 mm [13] net
RM2 Roller mill Aspen 9 Chips [13] Slightly decayed chips net
RM3 Roller mill Spruce 2–3 Chips [13] net
RM4 Roller mill Pine 94–84 Sawdust [13] Wet grinding (Fig. 6) net
RM5 Roller mill Pine 5 Sawdust [13] Dry grinding (Fig. 6) net
RM6 Roller mill Pine 34 Sawdust [13] Dry grinding (Fig. 6) net
RM7 Roller mill Aspen 93–83 Chips [13] Wet grinding (Fig. 6) net
HM + ACM Hammer mill + Air classifier mill Douglas fir <3 Chips [121] Pilot-scale net

Fig. 6. Wet grinding vs. dry grinding in a roller mill. Data from Ref. [13]. See also Table 2.

instance, in the pneumatic transport and fluidization of wood powder, a applications listed in 1960 [131]. As can be observed, most of the
low aspect ratio is an advantageous or even necessary criterion, proposals are still relevant while some have become outdated.
whereas in wood-plastic composites a high aspect ratio is preferable. Today there exists a need for the production of wood powders
Only in few publications [67,86,128–130] has the aspect ratio of finely having tailored properties to be used in specific high value-added
ground wood particles been studied systematically and quantitatively. products. Wood has a unique chemical composition and physical
The aspect ratio is somewhat dependent on grinding methods, properties that can be exploited in various products. The low density
moisture mass fraction, and wood species but the general prevailing and relatively high aspect ratio of wood powders can be utilized in
trend is a decreasing aspect ratio with decreasing particle size, as shown lightweight composites. They can be used as a rheology modifier in
in Fig. 9. The highest aspect ratios seem to be achievable with a jet mill. paints, pastes, putties, cements, and to prevent segregation, e.g., in
Other mills seem to give a more or less similar relationship between the asphalt. It has also been proposed that they could be used in 3D printing
aspect ratio and particle size, but data is scarce. According to the study together with binders [132].
with oscillatory ball milling [114], the moisture mass fraction of the In the following section, examples of established and evolving ap-
wood had no effect on the aspect ratio as long as the moisture mass plications are described and the requirements for the morphology of
fraction was less than 20%. A gradual increase was seen in the aspect fine ground wood powder are discussed.
ratio at a moisture mass fraction between 20 and 50%.
7.1. Combustion and co-firing
7. Wood powder in various applications
Presently, co-firing with coal in boilers is the most important ap-
Attempts have been made to find a use for wood powder for dec- plication of biomass, 64% of which originates from wood and wood
ades. Decades ago, wood powder was mainly a waste stream from wastes [133]. Biomass has to be pulverized before being fed to the
sawmills and the furniture industry. Table 3 shows some potential furnace, which is energy intensive and achievable only at very low

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Fig. 7. Change of specific energy consumption due to anhydrous weight loss by the torrefaction of wood [10,11,104,126,127].

capacity in traditional coal grinders. Particle size, shape, and density pulverized coal-fired boilers), the particle size requirements are
are important parameters in fuel handling and combustion. Wood pel- 0.5–20 mm for fluidized bed boilers while pulverized coal-fired boilers
lets are at present the state-of-the-art for co-firing because the particles need less than 6 mm (typically 0.1–1 mm) [14,136]. A low aspect ratio
are sufficiently small. Torrefied biomass, however, due to its high en- is preferable for powder flowability [54,137].
ergy density and hydrophobic nature, can be a good replacement for
wood pellets in co-firing and gasification plants [94]. Torrefaction (or
steam explosion), combined with pelletization, produces a biofuel 7.2. Densification
having properties similar to those of coal, thus enabling it to be ground
into a dustlike powder [134]. Thus, it is possible to substitute or sup- The low bulk density of wood chips (150-200 kgm-3) makes the
plement coal with biomass in coal-based power plants with existing material difficult to store, transport, and use. Low bulk density also
equipment [99]. Co-firing biomass with coal is a near-term, low-cost presents challenges for technologies such as coal co-firing, because the
option to produce electricity and/or heat. By substituting a part of coal bulk density difference between coal and wood causes difficulties in
with biomass in high-efficiency coal boilers, up to 15% of the total feeding the fuel into the boiler and reduces burning efficiency
energy can be renewable [135]. [138,139]. Densification is one option for overcoming these limitations.
Depending on the feeding system (transport from storage and in- Conventional processes for biomass densification can be classified into
jection into the boiler) and combustion technology (fluidized bed or three types: extrusion, roll briquetting, and pelletizing [140]. Pelleti-
zation increases bulk density to around 700 kgm-3 [141].

Fig. 8. Change of specific energy consumption and particle size by the torrefaction of wood in impact mills. Data from Refs. [10,127].

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P. Karinkanta et al. Biomass and Bioenergy 113 (2018) 31–44

Fig. 9. Aspect ratio of wood powders ground in different mills as a function of median particle size. Data from Refs. [67,86,128–130].

Table 3 In enzymatic hydrolysis, a finer particle size improves digestibility,

Variety of possible uses of wood flour as described in Ref. [131]. especially when the cell wall of the wood is disrupted. The use of wood
with a lower particle size increases the extent and rate of enzymatic
Activated carbon Filler for batteries Putty and adhesives
hydrolysis for a particle size range below 2 mm as determined by
Artificial wood Filler for paper and Sandpaper discs sieving [56,144]. Fukazawa [62] showed that the destruction of the cell
cardboard wall of wood is very important for enhanced reactiveness in enzymatic
Cleaning and polishing Filter paper Sweeping powder
conversions, which has been later confirmed [61]. Zhu et al. [145] have
agents
Cleaning products Glue extender Wallpaper coating
distinguished two size reduction classes for lignocellulosics due to their
Coating for baking trays Kneadable wood Wood plastic different influence in enzymatic hydrolysis. In Class I the size reduction
composites does not significantly compromise the physical integrity of the cell wall
Desiccant Linoleum Woodstone (Xyolith) whereas in Class II the biomass size is reduced to a level beyond fibers
Explosives and fuses Medical dressing
with significant physical destruction of the cell wall. Elongated wood
material
Feed extender Powder fuel particles (high aspect ratio) provide a higher specific surface area than
rounder shaped particles and therefore increase the enzymatic con-
version yield and rate [91].
In general, the density and durability of pellets are inversely pro- In chemical and enzymatic conversions the degree of cellulose
portional to particle size, since smaller particles have a greater surface crystallinity in wood, and also in other lignocellulosic substances, is
area during densification [139]. There is a critical particle size below considered to be important particle property [74,146–148]. Lowering
which pelletization is not commercially feasible in terms of technical the degree of cellulose crystallinity improves the rate and yield of en-
capability and the economics. For good quality pellets, a median par- zymatic conversion of celluloses [149] but in the case of lignocellulosics
ticle size of 0.5 mm and 95% below 1 mm have been proposed [139]. there are conflicting results, where some studies consider its contribu-
On the other hand, only 10–20% of the reported fines mass fraction tion to be very significant [150–153] while other studies consider it less
there should be below 0.5 mm, unless a binding agent is used [134]. significant or even negligible [61,70,154].
Pelletized wood usually has to be ground before use. Pellets tend to The presence of lignin and hemicelluloses in lignocellulosics re-
disintegrate into their constituent parts, i.e., to the particle size that the stricts the accessibility of celluloses in hydrolysis [74,155–157]. It has
wood powder had before pelletization [119]. The energy needed to been suggested that the removal of lignin from lignocelluloses is an
achieve this is very low: the net specific energy consumption in a important step for obtaining a better yield and rate in enzymatic hy-
hammer mill is 2–2.5 kWh t−1 [119]. drolysis [61,151–153,157], but the removal of hemicelluloses may be
even more important than the removal of lignin [145].
7.3. Conversion to chemicals and biofuels
7.3.2. Liquefaction
There are essentially three approaches to utilizing lignocellulosic Biomass carbon-based compounds can be liquefied by fast pyrolysis
biomass in the production of chemicals and biofuel: saccharification, or hydrothermal liquefaction (HTL) [142]. In fast pyrolysis, dry bio-
liquefaction, and gasification [142]. mass is heated to a temperature of 400–500 °C for a few seconds,
yielding 70–80% pyrolysis oil that can be refined for transport fuel. In
7.3.1. Saccharification liquefaction, a finely ground biomass feed of typically less than 2–3 mm
The cellulose and hemicellulose of biomass can be converted into is required in order to achieve high heating and heat transfer rates
simple sugars via acidic or enzymatic hydrolysis, and can then be fer- [158].
mented to ethanol or butanol. Sugars can be also converted into hy- In hydrothermal liquefaction, wet biomass is heated to close to the
drocarbon chemicals microbiologically or catalytically. The yield and supercritical point of water (280–350 °C, 10–24 MPa), resulting in the
rate of enzymatic hydrolysis of wood are dependent on the crystallinity rapid breakdown and liquefaction of the biomass. The yield of biocrude
and degree of polymerization of cellulose, the amounts of lignin and is around 35%. Size reduction in the HTL process is not so critical.
hemicelluloses, interactions between all major wood components, pore Essentially, the feed can be wood chips but the pumpability of biomass
size and volume, and the cell wall surface [143,144]. feedstock limits the larger particle size to the order of 2–6 mm [159].

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7.3.3. Gasification ratio in wood plastic composites, rather than the particle size, has the
In the gasification of biomass synthetic gas, ‘syngas’, i.e., a mixture greatest effect on strength and stiffness. A high aspect ratio is assumed
primarily of hydrogen and carbon monoxide, is produced by reacting to enhance stress transfer from the matrix to the wood particles. Ty-
biomass powder at temperatures above 700 °C without combustion. pically, the aspect ratio in wood flour varies between 1 and 5 [167].
Syngas is an intermediate in the production of synthetic chemicals and
fuels. 7.6. Organic filler or pigment
When considering applications involving thermochemical conver-
sion, particle shape and size influence the biomass particle dynamics, Wood powder is currently used in boardmaking as an organic filler
including drying, heating rate, and reaction rate [160]. Even in the to increase bulk and dewatering and drying properties [180,181]. By
simplified particle model for combustion of wood, particle size and comparison with commonly used inorganic fillers, organic fillers pro-
shape must be considered [161]. Near-spherical particles exhibit lower vide lower grammage (i.e. lighter products) and improved the recycl-
volatile and higher tar yields relative to aspherical particles with the ability of papers and boards. Nowadays, recycling sludge with high ash
same mass under similar conditions [160]. Decreasing the particle size content is a problem due to the large amount of inorganic waste to be
increases the yield of volatiles for particles of all shapes [160]. In landfilled. The use of an organic filler overcomes this problem because
combustion studies it has been reported that wood powders with 25% of its combustibility and biodegradability.
of particles less than 100 μm provide excellent flame stability and The requirements of a wood filler are related to size and retention
general performance of a swirl burner [162]. In a plane flame furnace property in the web during papermaking or boardmaking. The size of
the lower mean particle size shortens the ignition and burnout times of the wood flour has to be small enough so as not to have a detrimental
wood powders with a moisture mass fraction of 10% or 28% when effect on papermaking properties. If the target is higher quality paper,
burned under similar conditions [163]. then small size, high brightness, and light scattering ability are pre-
Gasification or fast pyrolysis in entrained flow reactors or co-firing ferable properties [182,183]. In specialty papers like filter papers, the
with coal in pulverized fuel boilers requires finely ground solid wood effect of wood flour on adsorption and permeability is important
biomass. In these applications wood powders also require good fluidi- [184,185]. Preferred particle size of organic filler varies approximately
zation properties to ensure a stable flame, low emissions, and the un- in between 10 and 100 μm.
disrupted smooth pneumatic transport of biomass into the reactor
where particle morphology has an important role [137]. Fluidization of 7.7. Adsorbents and ion exchangers
pulverized biomass is challenging because of the high cohesion forces
between particles and fibrous particle shapes that cause channeling of Wood powders (saw dust most often) have also been studied as
the gas flow. A particle size of 0.5–20 mm is usual in fluidized beds, biosorbents to remove pollutants from aqueous systems [186]. The
whereas less than 6 mm (typically 0.1–1 mm) is common for entrained excellent ability of wood powders to bind metal cations can be attrib-
flow gasifiers [136]. uted to the hydroxyl, carboxylic, and phenolic groups present in cel-
lulose and lignin structures [187]. Ground, sieved and chemically
7.4. Lignophenol production modified saw dust having median size below 200 μm has been studied,
for example, in the removal of heavy metals and nitrates [188,189]
Wood lignin is a complex polymer, which can be used as a precursor while saw dust with and without chemical modification in the removal
in many value-added products [164]. Nowadays, lignin is most often of organic pollutants [190] and heavy metals [191]. Sorption depends
separated from the black liquor from kraft pulp mills, which means that on particle size and accessible specific surface area as well as on ad-
lignin is not in its native form but is sulfonated and depolymerized. A sorption conditions such as pH, metal concentration, ligand con-
process has been developed for the separation of carbohydrates and centration, and competing ions. Adsorption capacity can be increased
lignin without unselective destruction of the lignin structure [165]. In by physical and chemical modifications including carbonization.
the phase separation process, finely ground wood powder having a high
specific surface is preferred to fractionate the raw material into water- 8. Concluding remarks
soluble carbohydrates and organic lignophenols.
Wood is often the first option for the sustainable production of en-
7.5. Composites ergy, chemicals, and biomaterials from renewable resources. The size
reduction of wood by fine grinding is a prerequisite in many applica-
Wood flour is the most commonly used material in the manu- tions, e.g., in pulverized firing, thermal or chemical conversions, and
facturing of wood-polymer composites. Wood flour possesses some wood-polymer composite manufacturing. However, there is no estab-
advantageous properties for use in composites: namely, low density, lished definition for the fine grinding of wood or other biomass. In this
flexibility, non-abrasiveness, and acceptable specific strength properties paper a particle median size of 500 μm was adopted as the limit that
[166]. Compared to wood fibers it is cheaper and has better flowability, divides material into pulverized or granular form.
making it easier to feed. Wood flour is available in various particle size In fine grinding, energy consumption and wood powder properties
ranges. The typical particle size of wood flour is between 180 and are closely related to the particle size of the product. The energy con-
840 μm [167]. sumption of fine grinding increases exponentially with decreasing
In wood-based composites, the effect of particle size is ambiguous. particle size. The aspect ratio and crystallinity decrease as the particle
Several researchers have reported that increasing the particle size or size decreases while the specific surface area increases. An unique
fiber length enhances the mechanical performance of wood-based correlation does not exist between energy consumption and wood
composites [168–171] but other researchers have reported enhanced powder morphology. Grinding mechanisms, the intensity of grinding,
mechanical properties when wood powders with smaller particle size the grinding environment (liquid or gaseous) and raw material prop-
are utilized [172–174]. These findings may indicate there can be found erties that depend on wood species and the pre-treatment of raw ma-
an optimal size. Indeed, the optimal size range of 200–300 μm has been terial such as by drying and torrefaction to affect the relationship.
reported [175]. Due to the apparent unsuitability of particle size for From an application point of view, the focus in terms of wood
characterizing mechanical properties of composites, several studies powders has recently been on co-firing and biofuel production. The
have concentrated on the shape of the particles and found that the production of fine-sized wood powder is advantageous, for instance, in
aspect ratio is an important particle property affecting these properties thermal conversion and hydrolysis processes. Size reduction is an en-
[176–178]. Stark and Rowlands [179] have concluded that the aspect ergy intensive operation and the electric energy used in fine grinding

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