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10
1.2
Comminution as one unit operation in the Biofuel supply chain
When biomass is used as an energy source in most of the cases comminution is
necessary. It is possible to use biomass in the forms of the full logs, and it has been done in a
small scale home appliances. But because of the low efficiency and other problems like high
level of CO and volatile emissions it's definitely not recommended option.
It's justified to say that comminution is placed in biofuel supply chain and it's always
placed in-between biomass harvest and combustion stage. As previously stated some form
of comminution is necessary to achieve efficient combustion. It goes well with common
sense because combustion is a chemical reaction and comminution enables new surface for
that reaction to happen so achieving better efficiency of the reaction is totally logical
conclusion.
In general any other operations between biomass harvest and utilisation are aiming in
enabling biomass to be used by the technology of the device where biomass is utilised -
mostly boiler. The goal is to utilise it in the most efficient way. Combustion reaction seems
to be quite simple when one uses macroscopic approach and analyses input and output
only, without detailed look into things that happen inside reactor - namely combustion
chamber. To make reaction happen two reactants must be at hand - fuel and oxygen. Both
need to be delivered into reactor in a way that allows to control amount of both in order to
control the reaction.
To make it work proper feeding mechanism is necessary. That's the main place in the
supply chain where comminution is necessary. Size of output material has to be adjusted to
the feeding mechanism - utilising device technology. In case when next stage of supply chain
is not combustion but for example densification of biomass, in order to make transportation
more efficient by f. ex. pelletizing, same general rule applies. It's because pelletizer has
acceptable size range for biomass particles and only within that range can work properly.
On the other side of the comminution as an operation there is input size of the
material. That depends highly on technology of the comminution device itself and would be
a subject of more detailed discussion in further chapters of this study.
It's possible that size difference between material from the first operation (harvest) and
final operation is too big and more than one operation of comminution needs to be
introduced because there is no suitable comminution device that can handle that difference
singlehandly. There is also a possibility that second stage of comminution is introduced
separately in order to use residues from the main process (Fig. 1.1).
In general no operation is 100% efficient and there are always residues available.
Residues are present at basically every stage - even harvesting. Ratio of residues to output
material is very operation dependent. In some cases amount of residues is big enough to
make usage of those residues profitable.
It seems necessary to mention that the need of comminution might not be determined
by purely technical issues. Sometimes comminution is chosen only to introduce residues
into existing technology and is a cheaper substitute for right choice of the final utilisation
unit in order to reduce the investment cost.
11
Figure 1.1 - Example of placing comminution operations in supply chain (L.J. Naimi, 2006)
Table 1.1 - Different type of devices utilising biomass with respect to the input material
requirements (L.J. Naimi, 2006)
12
Table 1.1 shows some examples of input material size and properties for different
devices. It shows high variability in terms of the acceptable input size. Other thing it shows is
high variability in required moisture content. That means the drying as an operation in
supply chain could also be present. That also indicates that biomass is highly variable
material generally speaking.
One of the main question this study aims to answer is an existence of qualitative way to
determine possibility to optimize biofuel supply chain by lowering energy consumption
during the comminution stage.
13
1.3
Structure of biomass - wood as an example
Biomass has a composite structure. It consists of fibres which are made of cellulose and
matrix that binds fibres together. Matrix consists of hemicelluloses and in case of ligno-
cellulosic (woody) biomass also lignin. Biomass is highly anisotropic material which means
that it has different properties, strongly depending on coordinates - namely fibre (cell wall)
direction.
The most important thing about wood that should be understood is a basic fact that it
has evolved for millions of years to serve three main functions in a plant as an organism
(U.S. Forest Products Labolatory, 2010):
· conduction of water and nutrients from the roots to the leaves
· mechanical support of the plant body
· storage of bio-chemicals
"There is no property of wood, no matter physical, mechanical, chemical, biological or
technological - that is not fundamentally derived from the fact that wood is formed to meet
the needs of the living tree. By understanding the function of wood in the living tree, we can
better understand the strengths and limitation it presents." (U.S. Forest Products
Labolatory, 2010)
In most of the cases wood is used as a material in terms of trees, when stumps and
leaves are usually not utilised. In Bioenergy segment this statement is also true and in case
of herbaceous biomass stalk is the main part being used (straw) and although it looks little
bit different it's designed by nature to meet the similar needs. Properties concerning
comminution of woody biomass are to some extend true also for other types of biomass as
well as other fibrous materials which are mostly of biomass origin.
Trunk of the tree (stem) is composed of various materials present in the concentric
bands (U.S. Forest Products Labolatory, 2010):
· Outer bark (Fig. 1.2 - ob) provides mechanical protection of the softer inner bark
and also helps to limit evaporative water loss.
· Inner bark (Fig. 1.2 - ib) it's the tissue through which sugars (food) produced by
photosynthesis are translocated from the leaves to the roots or growing parts of the
tree. Minerals and nutrients are also transported from the roots to the green parts.
· Vascular cambium (Fig. 1.2 - vc) is the layer between bark and the wood that
produces both of these tissues each year.
· Sapwood the active tissue which is responsible not only conduction of sap and
water but also for storage and synthesis of photosynthate like starch and lipids.
· Heartwood is a darker-coloured wood in the middle of most trees. It's not
conductive and functions as a long term storage of biochemicals (extractives).
Extractives are formed by parenchyma cells at the heartwood-sapwood boundary
and then exuded through pits into adjacent cells (U.S. Forest Products Labolatory,
2010)
1
.
·
Pitch (Fig. 1.6 - p) is located at the very centre of the trunk and is the remnant of early
growth of the trunk before it was formed.Figure 1.2 - Macroscopic view of a transverse
section of a trunk (U.S. Forest Products Labolatory, 2010)
1
refers to Hillis 1996
14
Among the woody biomass we can distinguish two major types softwood and
hardwood.
Softwood are those species that come from gymnosperms (mostly coniferous). They
have more simple basic structure than hardwoods because they have only two cell types
and relatively little variation in structure between these cell types.
Hardwoods come from angiosperm. They are much more complicated in terms of their
structure because they have greater number of basic cell types and far greater degree of
variability within the cell types.
There are two basic cell orientation systems in wood structure - axial and radial. Axial
cells have their long axes running parallel to the long axis of the organ (stem). It's being used
as a long distance transport. Radial cells are oriented like radius in a circle, from pitch to the
bark.
Figure 1.3 - Growth of wood scheme (J.M. Dinwoodie, 1996)
In wood science there are three main perspectives distinguished that are being used in
description of wood:
15
· Transverse plane of section (the cross section) which shows face that is exposed
when a tree is cut down (Fig. 1.5 - H).
· Radial plane runs in pitch to bark direction and is parallel to the axial system. It
provides information about longitudinal changes in the stem from pith to bark (Fig.
1.5 - A).
· Tangential plane is parallel to any tangent line that would touch the cylinder and it
goes along the length of the cylinder (Fig. 1.5 - A).
Other concept which is often used in wood science descriptions is grain. It's a direction
of longitude axis of cell walls which is in most cases parallel to the longitude axis of a stem.
Figure 1.4 - Different sections of wood (J.M. Dinwoodie, 1996)
Cell wall give wood majority of its' properties (U.S. Forest Products Labolatory, 2010),
(J.M. Dinwoodie, 1996).
It consists of three main regions:
· middle lamella
· primary wall
· secondary wall (S1, S2 and S3 layers)
16
Figure 1.5 - Macroscopic and microscopic view of different planes in the wood (U.S. Forest
Products Labolatory, 2010)
17
Figure 1.6 - Cut away drawing of cell wall (U.S. Forest Products Labolatory, 2010)
In each region cell wall consists of three major components: cellulose, hemicelluloses
and lignin.
Cellulose contains repeating units of 1-4 linked D-glucose - is a glucose polymer.
Number of glucose units (degree of polymerisation) is variable and depends on the region of
the cell. In secondary cell wall it could be 8 000 - 10 000 (Dinwoodie, 2000)
2
, while in
primary cell wall degree of polymerisation varies between 2 000 and 4000 (Dinwoodie,
2000)
3
. Cellulose is a core and dominant in quantity part of microfibrill which have
threadlike shape. Cellulose mostly formed in crystalline structures is binded with
hemicelluloses, with lignin on the outer surface. Microfibrills are differently oriented in
different parts of cell wall and they may have different angle of orientation with respect to
the cell long axis.
Cell wall has a composite structure itself - microfibrills (that consist mainly of
cellulose) are placed in the matrix that consist of hemicelluloses and lignin (Fig 1.7).
Hemicellulose is heterogeneous class of polymers containing glucose, galactose,
mannose, xylose and other sugars (A. Bruce, 1998). Both degree of crystallisation and the
degree of polymerisation (approx. 200) of hemicellulose are generally low (Dinwoodie,
2000).
Lignin is a complex, three dimensional, aromatic molecule that consists of phenyl
groups. It is non crystalline, hydrophobic and its main constituent of composite matrix of
woody biomass. Lignin is a brittle material and its' presence in middle lamella provides
adhesion between the cells.
2
refers to Goring and Timell 1962
3
refers to Simson and Timell 1978
18
Figure 1.7 - Models of a microfibrill (Dinwoodie, 2000)
Table 1.2 -Microfibrillar orientation and percentage thickness of the cell wall layers in spruce
(Picea abies) (Dinwoodie, 2000)
Wall layer
Approximate thickness
(%)
Angle to longitudal axis
P 3
random
S1 10
50 70
S2 85
10 30
S3 2
60 90
Table 1.3 - Chemical composition of wood (Dinwoodie, 2000)
Component
Mass
Polymeric state
Molecular
derivatives
Function
Softwood Hardwood
Cellulose 42
2 %
45 2 %
crystalline, highly
oriented, large
linear molecule
glucose fibre
Hemicellulos
e
27 2 %
30 5 %
semicrystalline,
smaller molecule
galactose,
mannose,
xylose
matrix
Lignin 28
3 %
20 4 %
amorphous, large
3-D molecule
phenylpropane matrix
Extractives 3
2 %
5 2 %
principally
compounds
soluble in organic
solvents
terpenes,
polyphenols,
stilbenoids
extraneous
19
The primary wall is characterised by random orientation of cellulose microfibrills, where
any microfibrill angle from 0 to 90 with respect to long axis of the cell, ma be present. In
cells in wood the primary cell wall is thin and generally speaking indistinguishable from the
middle lamella. Middle lamella of two adjacent cells cannot be cannot be distinguished (U.S.
Forest Products Labolatory, 2010).
The remaining cell wall domain is called secondary cell wall. It's composed of three
layers:
S1 is characterised by high microfibrill angles and is quite thin. Cellulose microfibrills are
laid down in a helical fashion and the angle between the mean microfibrill direction and the
long axis of the cell is between 50 to 70 .
The next layer - S2 - is arguably the most important cell wall layer in determining the
properties of the cell and, thus, the wood properties at a macroscopic level (U.S. Forest
Products Labolatory, 2010)
4
. This is the thickest secondary cell wall layer. It's characterised
by a lower percentage of lignin and a low microfibrill angle - 5 to 30 .
S3 is a relatively thin layer with high microfibrill angle and the lowest percentage of the
lignin. It's because there has to be adhesion between the water molecules and the cell walls
to conduct water. Lignin is a hydrophobic macromolecule so its low concentration in S3
makes adhesion of water possible and thus facilitates transpiration (U.S. Forest Products
Labolatory, 2010).
It seems to be quite evident that properties of wood as a material would have ultimate
meaning in terms of energy expense in the comminution process. It is quite clear that
woods' mechanical properties are highly determined by its fibrous and porous structure.
Figure 1.8 - The transverse and tangentiallongitudinal faces of Sitka spruce. Microscope
magnification x60 (Moore, 2011)
4
refers to Panshin and deZeeuw 1980 and Kretschmann and others 1998
20
1.4
Elementary mechanics in the comminution process
Reduction of the material's particle size means that large particles or lumps are
fractured into smaller particles. Fractures have to be initiated i.e. external forces have to be
applied to the particle. The actual size reduction depends on the amount of stress applied to
the particle, the rate at which it's applied and the manner in which it's applied (Size
reduction solutions for hard to reduce materials, 2002).
It's well known from material sciences that there three fundamental types of stresses:
compression, tension and shear. It happens a lot that they occur in a kind of typical
configuration that could be distinguished from any other. Bending might be considered as
one of them - in microscopic scale it's just combination on compression stresses on one side
of the material sample and tension stress on the other. Since it's easy to distinguish and
appearance in real life cases is pretty common, bending stress is recognised in material
science.
There are few types of actions that may be used to apply stress necessary to inflict
fracture to the particle. Each of them is a combination of fundamental stresses. They could
be distinguished during conceptual studies, although it's not so easy in terms of real life
comminution machinery, since they tend to occur together during the process. This would
be discussed further in the study in the part that describes comminution machineries at
present.
One may distinguish (I. M. Petre, 2006):
· cutting
· shearing
· tearing
· impact stress
· compression and friction (f. ex. in a disc milling)
Comminution process in any of machinery available nowadays involves at least one.
Usually it's a combination of few. There is no possibility at present to quantify the exact
influence from each of the actions in real device comminution process, but seems possible
to estimate which could be dominant just by analyzing geometry of the tools in a
comminution device and the way they interact with comminuted material.
Figure 1.9 - Types of actions and corresponding particle shapes (I. M. Petre,
2006)
21
Cutting:
The difference between cutting and shearing is defined by both the amount of
deformation that occurs in the cross-section of the material and the way stress is being
applied. Stress applied in direction parallel to the surface unit vector (perpendicular to the
surface) by sharp knife edge is usually very big (very small surface where the force is
applied). Fracture of the material is the result of splitting effect of the knife. Deformation of
the material occurs locally and progressively, close to the tool tip and as e result cross-
section of cut material is relatively smooth (I. M. Petre, 2006)
5
. That is the ideal case of
cutting, where shear stress and tensile stress are applied locally, near the edge of a knife,
and material is not moving due to underlying support. In reality difference between cutting,
shearing and tearing is not so clear and much depends on the viewer. Main things that
should be taken into consideration when classifying the performed operation are: tool
sharpness/bluntness, position of the support with respect to cutting plane and the angle
between the incoming blade and the comminuted material surface.
Shearing:
Working tools with a wedge angle of 75 to 90 apply the shearing. During shearing
action performed in the comminution equipment fracture of material is a result of shear and
to some extend tensile stresses. Deformation zone extends before fracture , between
wedges of cutter head and stationary knife (L.J. Naimi, 2006)
5
. In shearing there is a
distance between vertical plane along the tool that is moving and the edge of the
supporting "anvil". In shearing deformation energy is applied across a lot bigger volume of
material so it could be justified to assume that it is more energy consuming operation.
Tearing:
Tearing action involves combining tensile stresses with bending and torsion (I. M.
Petre, 2006)
5
. Particles that are a result of tearing are very un-uniformed in shape. Tearing
should be dominant when tool hits the material in angle much smaller than normal to the
material's surface. Also tensile stresses seem to be much more significant comparing to
cutting and shearing. Since biomass tensile strength is dominant comparing to compression
and shear strength it seems logical to assume that this operation would be more energy
consuming in comparison with cutting and shearing.
Milling:
Particles that come as a result of compression and friction are quite uniform in shape.
Compression of the material and friction against the tool implies internal friction working in
the material as well.
5
refers to Schubert and Bernotat 2004; Woldt et al. 2004
22
Impact:
Impact occurs when a moving tool, such as hammer, hits the material with a certain
velocity. Then the material usually absorbs part of the tool kinetic energy which inflicts
fractures and makes particle to break and go through a fixed rigid target such as perforated
surface of the sieve (I. M. Petre, 2006)
6
.
Biomass is as stated in
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a highly
anisotropic, composite material with properties strongly dependent to coordinates. It seems
to be quite clearly stated by this paragraph that different way to apply stress may lead to
the different result both in terms of energy necessary to break the structure and to particle
shape achieved as a result of the operation. That indicates comminuted material properties
(
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) and used machinery (
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) would have a meaning in terms of energy used
in comminution. Next paragraph will approach machinery in more detailed manner.
Figure 1.10 - Cell walls collapsing under compression (F. Stefansson, 1995)
6
refers to Schubert and Bernotat 2004; Woldt et al. 2004
23
1.5
Comminution machinery
There are many different designs and types of machines, that can handle high variability
of different input and output sizes and there is no one clear classification method. It's good
to make an overview and describe shortly most common types with special emphasis of
magnitude of input and output sizes as well as types of stress that they apply. Few
tabularised summaries are in Appendix A.
Figure 1.11 - Possible pathway of size reduction processes of agricultural residues (M.
Hoque, 2007)
One of the most popular machines used for a first stage comminution are chippers. They
are rotary devices that have knives attached to the rotating part like drum or disk. Heavy
rotating part (drum/disc) plays the role of flywheel - every time knife cuts out the new chip
part of energy is lost. Input material is usually big in size - f. ex. whole logs. Rotating knives
perform cutting and shearing action. Chips are cut from unprocessed material which is
supported with the in-feed spout (anvil). Output material are chips - pieces that are more or
less uniform in shape, size 5 - 50 mm (S.van Loo, 2008). Chips thickness is usually
significantly smaller than two other dimensions which both are quite similar in magnitude.
Figure 1.12 - Different chipper designs - disc and drum chipper (S.van Loo, 2008)
Figure 1.13 - Different chipper designs - disc chipper seen from different angle [ (L.J.
Naimi, 2006) refers to Hakkila 1989]
24
Figure 1.14 - Different chipper designs - cylindrical drum chipper(a) and V-drum chipper
(b) [ (L.J. Naimi, 2006) refers to Hakkila 1989]
Figure 1.15 - Wood chips - CEN (see APPENDIX B) (E. Alakangas, 2007)
Other group of devices that offers little bit bigger and differently shaped output product
are chunkers. Chunk wood is defined as short, thick pieces of wood, where the majority of
particles have a relatively uniform length of 50 250 mm in the grain direction and a
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