wood chips for kraft and sulfite...
TRANSCRIPT
Wood Chips for Kraft and Sulfite
Pulping
Evaluation of Novel Forest-Industrial Drum-
Chipping Technology
Jessica Gard Timmerfors
Doctoral Thesis, Department of Chemistry
Umeå University, 2020
Responsible publisher under Swedish law: the Dean of the Faculty of Science and
Technology
This work is protected by the Swedish Copyright Legislation (Act 1960:729)
ISBN: 978-91-7855-234-4
Electronic version available at http://umu.diva-portal.org/
Tryck/Printed by: KBC Service Center, Umeå
Umeå, Sweden, 2020
Allt stort som skedde i världen skedde först i någon människas fantasi.
Everything great that ever happened in this world happened first in somebody’s imagination.
- Astrid Lindgren, 1958, reception of the H C Andersen Award
i
Table of Contents
List of Abbreviations iii
List of Publications iv
Enkel sammanfattning på svenska v
1. Background 1 1.1 Feedstocks in the forest industry 1 1.2 Wood structure and chemistry 3
1.2.1 Wood structure 4 1.2.2 Wood constituents and chemistry 5
1.3 Forest-industrial processes 7 1.3.1 Saw mills 8 1.3.2 Heat and power plants 8 1.3.3 Mechanical pulping 8 1.3.4 Chemical pulping 9 1.3.5 Sugar-platform processes 15
1.4 Wood preparation 16 1.4.1 Debarking 16 1.4.2 Wood chipping 17 1.4.3 Screening of wood chips 24
1.5 Impregnation 24 1.5.1 Methods for studying impregnation 27 1.5.2 Laboratory impregnator 28
2. Present Investigation 28 2.1 Aim of investigation 28 2.2 Wood chipping and characterization of wood chips 29
2.2.1 Chippers 29 2.2.2 Impregnation and cooking 32 2.2.3 Analysis methods for wood chips 33 2.2.3.1 Size distribution 34
2.3 Results and discussion 38 2.3.1 Full-scale demonstration drum chipper (Paper I) 38 2.3.2 Pilot drum chipper (Paper II) 40 2.3.3 Chipping different wood qualities (Papers III and IV) 42 2.3.4 Impregnation of wood chips (Paper IV) 44
3. Conclusions and Future Work 49
4. Acknowledgements 50
5. References 51
ii
Abstract
Wood chipping and the supply of high-quality wood chips are of
critical importance for most forest-industrial processes. The quality of
wood chips affects product yield, product quality, and processability.
Wood chips from a novel type of forest-industrial drum chipper, with
a large drum and specially designed wood-chip channels, were
evaluated with regard to wood chips for the Kraft and sulfite
processes. Wood chips from a full-scale demonstration version of the
drum chipper and from a conventional disc chipper at a Kraft mill
were compared. The average bulk density and the fractions of
oversized and overthick wood chips were similar, but the
demonstration drum chipper produced 51% more large accept chips,
11% more total accept chips, and 74% less pin chips and fines. A
pilot-scale drum chipper based on the new technology was used to
produce short wood chips designed for acidic processes. When the
drum velocity was 30-34 m/s and the average wood-chip length 21-22
mm, the fraction of pin chips and fines was 4.2% and the fraction of
total accept was 89-90%. When the average wood-chip length was
decreased to 17 mm, the fraction of pin chips and fines increased to
8.5% and the fraction of total accept decreased to 80-82%. The pilot
drum chipper was used to investigate the influence of using different
tree species (aspen, birch, pine, and spruce), processing of wood with
different moisture content, and frozen wood. For hardwood (aspen and
birch), the fraction of total accept reached ~90% when the average
wood chip length was 17 mm. The pilot drum chipper was also used
to generate wood chips of heartwood of pine for a comparison of 15
sulfite-process reaction conditions that differed with regard to
impregnation and cooking procedures. The analyses included
absorption of liquid in a specially designed impregnation reactor, pulp
yield, reject, viscosity, kappa number, brightness, fiber properties, and
chemical composition as determined using compositional analysis
based on two-step hydrolysis with sulfuric acid and pyrolysis-gas
chromatography/mass spectrometry. The results reveal in detail how
the individual wood constituents were affected by the different
treatments, and demonstrate the benefits of using a pressurized
impregnation step prior to sulfite cooking.
iii
List of Abbreviations
α Clearance angle/Pulling angle
β Sharpness angle/Knife angle
λ Complementary angle
ε Spout angle/Cutting angle
ε’’ Infeed angle
CHP Combined Heat and Power
CTMP Chemo-Thermo-Mechanical Pulp
DW Dry Weight
EA Effective Alkali
HMW High Moisture Wood
HW Heartwood
LMW Low Moisture Wood
m3fub Cubic meter solid volume excluding bark
m3sk Cubic meter standing volume
MCS Multi Channel Sweden AB
MMW Medium Moisture Wood
MS Mass Spectrometry
p Probability (level of significance in Student's t-test)
SCAN-CM Scandinavian Pulp, Paper and Board Testing Committee
test methods for chemical (C) and mechanical (M) pulps
and wood chips
SEM Scanning Electron Microscopy
SL Setting Length
SW Sapwood
TMP Thermo-Mechanical Pulp
iv
List of Publications
This thesis is based on the following four papers. In the text, the
papers are referred to by their Roman numerals.
Paper I Gard Timmerfors J, Sjölund T & Jönsson LJ. New
drum-chipping technology for more uniform size distribution of wood
chips. Holzforschung 2020, 74, 116-122.
Paper II Gard Timmerfors J & Jönsson LJ. Evaluation of novel
drum chipper technology: pilot-scale production of short wood chips.
TAPPI J. 2019, 18, 585-592.
Paper III Gard Timmerfors J, Salahi H, Larsson SH, Sjölund T
& Jönsson LJ. The impact of using different wood qualities and wood
species on chips produced using a novel type of pilot drum chipper.
Manuscript submitted to Nord. Pulp Pap. Res. J.
Paper IV Gard Timmerfors J, Gandla ML, Sjölund T & Jönsson
LJ. Evaluation of chipping and impregnation of Scots pine heartwood
with sulfite cooking liquor. Manuscript.
v
Enkel sammanfattning på svenska
När vi producerar massa till papper och kartong vill vi använda så
mycket av vedråvaran som möjligt, dvs. ha ett högt utbyte när råvaran
konverteras till produkt. Det är viktigt för att minimera transporter,
råvaruåtgång och kemikalieåtgång, vilket bidrar till en ekonomiskt
bärkraftig process. Ett av processens första steg är att sönderdela
stockarna till flis. Hur jämn flisen är i storlek, dess fukthalt och dess
packdensitet påverkar hur bra massa det blir.
Idag huggs nästan all flis som blir massa med skivhuggsteknik.
Skivhuggen uppvisar en hastighetsgradient, vilket leder till att en
större andel av råvaran blir till pinnflis och spån. Pinnflis och spån har
för små dimensioner för att fungera bra som råvara vid impregnering
med kokkemikalier och vid massakok.
En ny typ av trumhugg har utvecklats. Trumman har inte den
hastighetsgradient som skivhuggen har och borde teoretiskt sett kunna
producera en mindre andel pinnflis och spån. För att utvärdera den nya
tekniken så jämfördes en fullstor demonstrationshugg baserad på den
nya trumhuggstekniken med en konventionell skivhugg på ett
sulfatmassabruk. För att utvärdera effekten av trummans hastighet och
stockens inmatningsvinkel användes istället en pilothugg baserad på
den nya tekniken. Med hjälp av pilothuggen undersöktes också hur
fliskvalitén påverkades av stockarnas fukthalt, vilket trädslag det var
och om veden var fryst eller tinad.
Den nya trumhuggstekniken skapade flis som visuellt liknade den
från skivhuggen. För att få bra massa behöver en andel på ungefär
85% av flisen vara acceptflis, dvs. dess dimensioner ligger inom de
storleksintervall som industrin har valt att definiera som acceptabel.
Den nya trumhuggen gav en andel på 85% acceptflis direkt efter
huggning i demonstrationsskala, ett värde som brukar uppnås först
efter sållning av flis. Framför allt minskade andelarna av de fraktioner
som kan ses som förlust, pinnflis och spån. Dessa småfraktioner stör
massaprocessen och även om de kan användas som bränsle innebär
det en värdeförlust. Bra huggresultat gick också att få med andra
inställningar och varierande råvara. När flis som var betydligt kortare
än normalt producerades gick som väntat andelen acceptflis ner, dock
fortfarande inom gränser som är industriellt acceptabla. Detta pekar på
att den nya trumhuggstekniken kan vara speciellt fördelaktig för sura
processer där kortare flis är att föredra.
vi
Den nya flisningstekniken användes också för att producera flis från
kärnved av tall. En stor andel av vedråvaran i Sverige består av tall,
men det är utmanande att använda en stor andel tallkärnved i
sulfitprocessen. Flisen användes till försök där 15 olika
reaktionsförhållanden jämfördes. Skillnaden mellan de 15 olika
reaktionsförhållandena bestod i hur impregnering och sulfitkok
utfördes. Resultatet utvärderades genom analys av upptaget av
impregneringsvätska i en specialkonstruerad impregneringsreaktor,
bestämning av utbytet av massa, rejekt, viskositet, kappatal, ljushet
och fiberegenskaper, samt detaljerad analys av råvarans och de 15
produkternas kemiska sammansättning. Försöket gav detaljerad
information om hur veden påverkas under olika reaktionsbetingelser
och visar tydligt de positiva effekterna av att inkludera ett trycksatt
impregneringssteg innan sulfitkoket.
1
1. Background
In a global context, around 3300 million m3sk of wood is used
every year. The largest use of wood is as fuelwood in developing
countries (Gellerstedt 2009a). Around 45% of all felled trees are used
by the industry, mostly in developed countries (Gellerstedt 2009a).
The total felling in Sweden during the season 2016 amounted to 70.7
million m3fub. In addition, 7.8 million m3fub wood was imported and
0.7 m3fub exported (Skogsindustrierna 2017). In 2016, the Swedish
forest industry used 36.3 million m3fub wood for the saw mills and
35.3 million m3fub as pulp wood. From the saw mills, 9.9 million
m3fub wood was transferred to pulp production.
Sweden is the world's 3rd largest exporter of forest-industrial
products (pulp, paper, and sawn timber). In 2018, the export value of
forest-industrial products amounted to 124 billion SEK. In 2016, pulp
production in Sweden amounted to 11.3 million tons
(Skogsindustrierna 2017). In comparison, around 184 million tons of
pulp were produced globally during the same year (FAO 2019).
The focus of this thesis is on the first steps in the production of
pulp. In most forest-industrial processes, the wood logs are chipped
after debarking. The yield and quality of wood chips are important for
the industry. The cost for the feedstock is an important part of the
operating costs for the mills, and the quality of the wood chips that are
fed into the processes is critical for the function of digesters and for
the quality of the pulp.
1.1 Feedstocks in the forest industry
The total forest area in the world is probably around 4 billion ha.
Around 5% of the total forest area consists of plantations. The
countries with the largest plantations are China, USA, and India
(Skogsindustrierna 2020; Henriksson et al. 2009). More than half of
the total land area of Sweden (69%) is productive forest land. This
area corresponds to 23.6 million ha (Skogsdata 2019).
Woody species can be divided into softwoods and hardwoods.
Softwood comes from gymnosperm trees, i.e. conifers, and hardwood
comes from angiosperm trees, i.e. broad-leaved trees (Wiedenhoeft
2013). The wood of many softwoods is softer than that of many
hardwoods, but this is not always true. Whereas there are obvious
visual differences between conifers and broad-leaved trees, the
2
fundamental differences between softwood and hardwood are
apparent also at a cellular level.
The boreal forest, or taiga, stretches eastward from the
Scandinavian peninsula to Russia and northern China, and further on
to Canada and northern USA. It is dominated by relatively few species
of conifers (Henriksson et al. 2009). The temperate forests south of
the taiga consist of a more even mix of conifers and broad-leaved
trees, and the forest is more rich in different tree species. Tropical rain
forests contain a multitude of tree species, and they are dominated by
broad-leaved trees. Nevertheless, some conifers are abundant also in
southern areas, such as sub-tropical regions. In plantation forestry,
both broad-leaved trees, such as eucalypts, and conifers, such as pines,
are common.
The Forest Act from 1903 has protected Swedish forests through
mandatory replanting of forests after felling. There is more forest in
Sweden now compared to when collection of data started in the 1920s.
The standing volume has increased with around 80%. Around 25% of
the forest in Sweden is not used as productive forest land, and 9% is
officially protected (Skogsverige 2020). Protected areas include 30
national parks, 5000 nature reserves, and 8300 wildlife conservation
areas (Skogsindustrierna 2020). There are two certificates for
sustainably forestry: FSC (Forest Stewardship Council), with focus on
sustainable forests and forestry, and PEFC (Programme for the
Endorsement of Forest Certification), with focus on sustainability and
trackability from forest to product (FSC 1993; PEFC 2017).
According to data from 2013 (Sveaskog 2020), roughly 70% of the
Swedish forest land was certified through FSC and/or PEFC.
The standing volume in Swedish forests (2014-2018) consisted
mainly of Norway spruce (40.4%), Scots pine (39.3%), and birch
(12.5%) (Skogsdata 2019). Other common trees species include aspen
(1.7%), alder (1.7%), lodgepole pine (1.3%), and oak (1.3%).
Wood logs from typical long-rotation Swedish forestry are sold as
timber, pulp wood, or fuel wood. The price paid for timber is higher
than the price paid for pulp wood, which in turn is higher than the
price for fuel wood. Therefore, it is advantageous for forest owners to
sell their wood as timber, or at least pulp wood, rather than as fuel
wood, if it is possible. It is the quality of the wood logs that
determines this. The separation of wood logs into different qualities is
3
usually done in the forest after harvesting. Factors that affect wood log
quality include tree species, dimensions, straightness, time of storage
after felling, occurrence of cracks, residual protrusions left after
removal of branches, occurrence of forest rot at end surfaces,
occurrence of root bones, and occurrence of impurities, such as gravel,
in wood and bark (Biometria 2019; 2020).
Fuel wood is typically used in big heat and power plants, but to
some extent by households. Around 300,000 Swedish households
used wood burners or fireplaces for heating their homes
(Energimyndigheten 2003). Apart from fuel wood, a minor fraction of
the felling residues, such as branches and tops, are also utilized for
energy purposes.
1.2 Wood structure and chemistry
The tree is a woody material, which can be seen as having three
main parts: the branches and twigs, the stem, and the root system
(Henriksson et al. 2009). It is the stem that is used in pulp mills and
biorefineries. Some industries use feedstocks from non-wood
materials. For example, paper from straw of wheat and rice is made in
China, and cotton-based paper is used for making paper money.
Tree stems consist of a pith, wood (xylem), cambial zone, and bark.
However, the innermost part, the pith, is very small, and the bark is
removed during the debarking process (Sjöström 1993).
The cambial zone (cambium) is a thin layer of cells outside the
wood, but inside the inner bark. It consists of living cells and is the
growing zone generating new tissue (Sjöström 1993).
Around 15% of the dry weight of the tree consists of bark. The bark
can be divided into outer and inner bark (phloem) (Brännvall 2009b;
Sjöström 1993). The outer bark is dead tissue and serves as protection
for the tree, whereas the inner bark transports water and nutrients. The
composition of bark differs between species. The fibers in the bark
consist mostly of cellulose, hemicellulose, and lignin, as fibers in the
wood. The bark also contains cork cells, which die early and which
resist water and gas, and parenchyma cells, which store nutrients.
Generally, the bark contains high fractions of extractives and minerals
compared to the wood.
4
1.2.1 Wood structure
The wood can be divided into heartwood and sapwood. The
heartwood forms the inner part, outside the pith. It often identified as
darker than the outer part of the wood, which is the sapwood. It is
called sapwood as it distributes the sap through the tree.
In boreal and temperate forest land, the growth activity of the trees
varies over the season. This results in growth rings in the wood. The
springwood (earlywood) is formed during periods of rapid growth,
whereas summerwood (latewood) is formed during later seasons.
Earlywood and latewood differ visually, but also with regard to
mechanical properties and physical structure (Browning 1967;
Rydholm 1965). For example, there are typically large differences in
density. Earlywood has a density of 250 kg/m3 and latewood has a
density of around 750 kg/m3 (Hartler 1990).
There are significant differences in mechanical properties between
species, but also between trees of the same species. Not even the wood
from the same tree is of uniform quality. There is a mechanical
variation between the bottom and top of the stem, and also between
the center and the periphery of the log (Twaddle 1997). This variation
in wood quality and the angle between ring orientation and knife
create differences in the relationship between wood chip thickness and
length.
Most of the cells in the tree are elongated cells, oriented in the
direction of the stem (Fengel & Wegner 1989; Henriksson et al.
2009). The cells transport and store liquids, nutrients, and resins. The
name and structure of the cells differ between softwoods and
hardwoods (Henriksson et al. 2009). Softwood has a relative simple
structure compared to hardwood. Softwood contains 90-95% tracheid
cells, which provide mechanical strength and transportation of liquid,
and 5-10% parenchyma cells, which are involved in transport and
storage of nutrients (Fengel & Wegner 1989; Henriksson et al. 2009).
Hardwood consists mostly of libriform fibers, which provide
mechanical strength, vessels, which are involved in transport of liquid,
and parenchyma cells, which are involved in transport and storage of
nutrition. Hardwood typically has shorter wood fibers than softwood
(Rydholm 1965).
5
1.2.2 Wood constituents and chemistry
The main constituent of fresh wood is water. The water can be free
water in the cell wall and the lumen, and bound water in the cell wall.
The moisture content of the wood consists of the total amount of free
and bound water. The moisture content also varies between heartwood
and sapwood (Rowell 2013).
Dry softwood typically contains 40-45% cellulose, 25-30%
hemicelluloses, and 25-30% lignin. For hardwood, the corresponding
values are 40-45% cellulose, 30-35% hemicelluloses, and 20-25%
lignin (Henriksson & Lennholm 2009).
Cellulose is a long linear polymer consisting of β-linked D-glucose
units (Henriksson & Lennholm 2009). Cellulose typically has a high
degree of polymerization, and may consist of up to 15,000 glucose
units. This makes cellulose to one of the longest polymers in nature.
The secondary structure of cellulose is created by hydrogen bonds,
which stabilize the chain, make it stiff, and form the basis of cellulose
sheets (Henriksson & Lennholm 2009). The cellulose sheets are
stacked next to each other and are held together with van der Waals
bonds. The cellulose sheets form a long and thin fibril. The fibrils
consist of many cellulose chains and can be up to 40 μm in length.
Hemicellulose chains are shorter than cellulose chains. They consist
of around 200 units. The chains are branched and contain not only
glucose units, but also other sugar units and sugar acid units (Teleman
2009). Common sugar units in hemicelluloses include D-glucose, D-
mannose, D-galactose, D-xylose, and L-arabinose (Fig. 1). The most
common hemicelluloses are galactoglucomannan, glucomannan,
arabinoglucuronoxylan, arabinogalactan, and glucuronoxylan
(Teleman 2009). Hemicelluloses are usually found between the
cellulose fibers and form the bulk of the cell wall.
Pectins are sometimes classified as hemicelluloses, but usually not
(Teleman 2009). The ability to gel comes from the pectins and they
constitute only a few percent of the dry-matter of wood. Another
common plant polysaccharide is starch, which consists of glucose
units arranged as amylose (~20%) or amylopectin (~80%).
Lignin is a branched aromatic polymer consisting of phenylpropane
units (Sjöström 1993; Ralph et al. 2004). Lignin is synthesized from
monomeric precursors, referred to as monolignols. The most
important monolignols are p-coumaryl alcohol, coniferyl alcohol, and
6
sinapyl alcohol (Ralph et al. 2004; Henriksson 2009). p-Coumaryl
alcohol, which lacks methoxy substituents, gives rise to p-
hydroxyphenyl (H) units in lignin. Coniferyl alcohol, which has one
methoxy group, gives rise to guaiacyl (G) units. Sinapyl alcohol,
which has two methoxy groups, give rise to syringyl (S) units. The
composition of H, G, and S units differs depending on the biological
origin of the lignin. Softwood lignin consists predominantly of G
units, whereas hardwood contains SG-lignin. Lignin from grasses and
herbs may contain a considerable fraction of H units.
Lignin has a more branched structure than cellulose and
hemicelluloses (Ralph et al. 2004). The phenylpropane subunits of
lignin are commonly connected by ether bonds (-O-4, -O-4, and 5-
O-4, -O-) or carbon-carbon bonds (-5, 5-5, -1, and -) (Ralph et
al. 2004). The number of units in lignin is difficult to estimate, as
preparation of lignin would typically disrupt its structure. It is a
possibility that all lignin in a tree is one big molecule (Henriksson
2009).
The purpose of lignin is to give stiffness to the cell wall and glue
the different wood cells together (Henriksson 2009). It also makes the
wood hydrophobic and protects it against microbial degradation.
Extractives are small molecules in wood that can be extracted using
various solvents (Björklund Jansson & Nilvebrant 2009). The content
of wood extractives varies between softwood and hardwood, between
different tree species, and between different trees of the same species.
Water-soluble extractives do not cause as much problems in pulp and
paper production as lipophilic extractives (wood resin). Therefore,
most work on wood extractives has had focus on resins (Björklund
Jansson & Nilvebrant 2009).
Wood resins consist of fats, fatty acids, steryl esters, steroids,
terpenoids, and waxes (Björklund Jansson & Nilvebrant 2009).
Extractives also contain phenolic constituents, such as stilbenes,
lignans, hydrolyzable tannins, flavonoids, and condensed tannins
(Sjöström 1993). Pinosylvin and pinosylvin monomethyl ether (Fig. 1)
7
are stilbenes and well-known constituents of heartwood of pine (Sixta
2006).
Fig. 1. Common monosaccharides derived from wood (A-E) and pinosylvins (F-G):
(A) glucose, (B) mannose, (C) galactose, (D) xylose, (E) arabinose, (F) pinosylvin,
and (G) pinosylvin monomethyl ether.
1.3 Forest-industrial processes
In forest-industrial processes wood is utilized to produce sawn
goods, fiber boards, pulp and paper, tall oil, specialty cellulose,
lignosulfonates, and other bio-based commodities. In many cases,
residual fractions, such as bark and partially degraded lignin in black
liquor, are used for energy production. Pulp and paper processes are
traditionally divided into mechanical and chemical pulping processes.
Industrial plants that convert lignocellulosic feedstocks, such as wood,
to multiple products are referred to as biorefineries. One type of
biorefining process that is currently in the focus of much research and
development is the sugar-platform process. In a sugar-platform
process, lignocellulosic polysaccharides, such as cellulose and
hemicelluloses, are converted to sugars, typically by using cellulose-
degrading enzymes. The sugars can then be converted to desirable
A B C
D
F G
E
8
products using microbial fermentation processes or through chemical
catalysis. Wood and residual forest-industrial streams can also serve
as basis for various thermochemical processes including combustion,
gasification, pyrolysis, hydrothermal liquefaction (HTL), and
hydrothermal carbonization (HTC).
1.3.1 Saw mills
Sweden has about 140 saw mills (Skogsindustrierna 2020). At the
saw mills, the wood logs are debarked and the wood is then sawed and
dried.
A by-product from saw mills are wood chips that are produced from
the outer part of the wood logs, the sapwood. This makes wood chips
from saw mills, i.e. wood that mostly consists of the sapwood, slightly
different from wood chips made from whole logs, which consist of a
mixture of both sapwood and heartwood.
Saw dust is another by-product from saw mills. Saw dust is not
suitable for pulp and paper production, but it is sometimes made into
pellets that are used as a solid fuel. There are 58 pellet factories in
Sweden producing pellets with a total energy content of 9.3 TWh per
year (Bioenergitidningen 2020).
1.3.2 Heat and power plants
There are around 200 combined heat and power (CHP) plants in
Sweden (Skogen 2019). In Sweden, low-quality wood logs and other
residual woody biomass are common feedstocks for CHP plants.
Wood logs that are dry, damaged by rot, or have too small diameter
cannot be used by saw mills or pulp mills and are instead sold to a
lower price as fuel wood. If a pulp and paper mill does not have any
bark boiler, it can sell bark and fine fractions to CHP plants.
One of the most important parameters for energy production from
woody materials is the moisture content. Most plants work best with
wood with a moisture content of 20-30% (Skogen 2019). The typical
dimension of particles fed into CHP plants is usually up to 15-50 mm
(Skogen 2019).
1.3.3 Mechanical pulping
The principal of mechanical pulping is to grind wood or wood chips
to generate cellulose fibers. Whole wood logs are used to make
groundwood pulp, whereas in other mechanical pulping processes
9
wood chips are used to make refiner pulp (Brännvall 2009a).
Groundwood pulp was more common in the past, but it is still
produced by some mills. As for other mechanical pulping processes,
an advantage with the groundwood process is the high pulp yield,
which is due to that all major organic constituents, cellulose,
hemicelluloses, and lignin, remain in the pulp. A disadvantage with
mechanical pulping is the high energy demand (Gorski et al. 2010).
Refiner pulp is made by grinding wood chips between refiner discs.
The mechanical pulp consists of fibers released from the woody
material, but also of fines, smaller particles of broken fibers and other
material from the fiber walls (Brännvall 2009a).
To make thermo-mechanical pulp, TMP, the wood chips are
steamed before they are inserted into the refiner. To make chemo-
thermo-mechanical pulp, CTMP, a relatively mild chemical and
thermal treatment is carried out before the refining step (Brännvall
2009a).
1.3.4 Chemical pulping
Compared to mechanical pulping, chemical pulping provides a pulp
with more flexible fibers (Brännvall 2009a). By degrading the lignin
and a part of the hemicelluloses, the cellulose fibers can be released
and form chemical pulp. When the lignin is partially degraded in
chemical pulping, charged groups are introduced. This facilitates
solubilization of lignin fragments, which then can be washed away
(Brännvall 2009a) (Fig. 2).
Fig. 2. Delignification during chemical pulping. The upper path shows an alkaline
process (Kraft pulping), whereas the lower path shows an acidic process (sulfite
pulping).
10
Today there are two main chemical pulping processes (Sjöström
1993). The most common is the Kraft process, which was developed
from soda pulping. The other one is the sulfite process, which was
once very common but which has now decreased in importance. The
first sulfite mill was installed in Sweden 1874. The Kraft process to a
large extent replaced the soda process and then the sulfite process, in
particular after the development of multistep bleaching in the 1930s.
Organic solvents can also be used for delignification during
chemical pulping. In the organosolv process, ethanol, methanol, or
peracetic acid are used in the cooking step (Brännvall 2009a). Even if
this idea stems from the 1930s, it has not gone much further than
laboratory and pilot scale, which is due to problems associated with
solvent recovery (Sjöström 1993).
Other chemical processes that can be used for delignification and
for breaking free the cellulose fibers include treatments with oxygen
and steam explosion. Oxygen is a good delignification agent that is
used for bleaching, but not for full-scale pulping processes (Sjöström
1993). To use hot steam at high pressure and then suddenly reduce the
pressure to atmospheric conditions is referred to as steam explosion.
However, due to fiber damage steam explosion is not used for
production of pulp (Sjöström 1993).
To make chemical pulp, wood chips are packed into a reactor.
There are several types of reactors, but only two main cooking
procedures. These are based on batch-wise and continuous digesters.
The benefits of a batch reactor is a more reliable production, a more
flexible production allowing changes in pulp quality, and a more
efficient turpentine recovery. The most common batch reactor is a
stationary vertical cylinder with a conical or spherical bottom. The
reactor is filled with chips from the top and hot cooking liquid from
the bottom, and is heated with a heat exchanger. Mills usually have
more than one reactor and a normal size is 150-400 m3 (Brännvall
2009c). To stop the reaction when the desired delignification is
reached, the hot cooking liquid is removed by pumping a cooler
displacement liquid from the bottom (preventing the cooking liquid to
vaporize in the woody material). The pulp is discharged and pumped
from the pressurized digester to a flask tank (blow tank) with
atmospheric pressure (Fig. 3). After cooking, the fibers need to be
11
separated from each other. This is done mechanically in a defibrator (a
hot-stock refiner/blow line refiner).
Fig. 3. Schematic figure showing forest-industrial processes in which wood is
converted to pulp. The figure shows a wood log truck (A), a drum debarking system
(B), a disc chipper (C), screening of wood chips (D), a wood chip pile (E), batch (F)
and continuous (G) cooking systems, and washing and bleaching (H).
A
B
C
D
E
F
H G
12
The most common continuous digesters are tall and slim vertical
flow digesters with production rates of 1,000-30,000 air-dry metric
ton pulp/day (Brännvall 2009c). The process is continuous from the
chip bin to the pulp blow. The process is arranged as a single digester
or as two digesters depending on whether a separate vessel is needed
for impregnation or not. The continuous digester schematically
depicted in Fig. 3 has a separate impregnation reactor. The wood chips
are fed from the chip bin to a horizontal steaming vessel for pre-
steaming (removal of gases from inside the wood chips) and are then
fed into the top of the digester. To get a high pressure in the digester, a
high-pressure feeder is used. The wood chips are then transported by a
transportation liquid into the top of the digester, where a screw feeds
the chips into the digester. The wood chips fall down into the digester
onto the top of the chip column and then move continuously down
through the digester. The cooking zone of the digester has a
temperature of 160-170 C (softwood) or 150-160 C (hardwood).
The chips then enter a zone with washing liquid. After the passage
through the digester, the fibers need to be defibrated. This occurs in
the line defibrator, when the pulp is still under high pressure. The pulp
is thereafter screened in a deshiving refiner and then washed.
Sometimes the mill has a second refiner after the wash. There are also
continuous digesters that have blow units or blow tanks (Bryce
1980a). After cooking, the pulp can go through screening, washing,
oxygen delignification, and bleaching steps depending on the desired
pulp quality.
1.3.4.1 Kraft process
In a global context, the most common process is the Kraft process.
The cooking liquor in Kraft pulping, i.e. the white liquor, consists of
sodium hydroxide and sodium sulfide. The active cooking chemicals
are hydrogen sulfide ions (HS-) and hydroxide ions (OH-) (Brännvall
2009a). Hydrogen sulfide serves as the main delignification agent.
Reasons why the Kraft process is popular include that many wood
species can be used, that the cooking time is short, and that the pulp
has excellent strength (Bryce 1980a).
The Kraft process can be divided into two steps; an initial increase
in temperature (impregnation) followed by a period with high
temperature. It is desirable that the cooking liquid penetrates the wood
chips before the temperature reaches 140C. The final cooking
temperature depends on the wood species, and the yield depends on
13
the temperature. The strength of the pulp seems not to be affected as
long as the temperature is below 200C (Bryce 1980a).
The most important parameters of the wood chips used for Kraft
cooking is the thickness, length, and moisture content (Gullichsen
1992; Ressel 2006). The wood chips used for Kraft pulping are
relatively long to minimize fiber shortening, which would result in
weaker pulp. An even moisture content and an even thickness are
important to get an even impregnation process.
1.3.4.2 Sulfite process
Until the 1950s most pulp was produced using the sulfite process,
but the production of sulfite pulp has decreased as the Kraft process
became more and more common (Sjöström 1993). The reason why the
Kraft process became more common is two main disadvantages
associated with sulfite pulping, namely that it is limited to fewer wood
species and that the pulp is weaker compared to the pulp from the
Kraft process (Bryce 1980b). In recent years, the focus area of the
sulfite process has shifted from producing pulp and paper to
producing dissolving pulp and lignosulfonates. This is due to the
demands from textile and concrete manufacturing (Rødsrud et al.
2012; Sixta et al. 2013).
In acidic sulfite cooking, the active chemicals are sulfurous acid
(H2SO3) and bisulfite ions (HSO3-) (Brännvall 2009a). The counter
ion can be calcium, sodium, magnesium, or ammonium (Bryce 1980b;
Gellerstedt 2009b).
The sulfite system is based on two equilibria:
The equilibria are dependent on the temperature. At the temperature
used for pulping (130-170C), the pH will be higher than measured in
room temperature (Sjöström 1993; Gellerstedt 2009b).
To obtain a high level of sulfonation of lignin, bisulfite ions (Fig. 2)
need to be present in the liquor. If there are insufficient amounts of
bisulfite, the woody material will turn dark, i.e. a black cook will
occur, and there will be no efficient dissolution of the lignin
(Gellerstedt 2009b). By increased sulfonation (Fig. 2), the lignin will
successively become more and more hydrophilic and water soluble.
SO2H2O + H2O(l) ⇌ HSO3-(aq) + H3O+(aq) pKa=1.9 (1)
HSO3-(aq) + H2O(l) ⇌ SO32-(aq)+ H3O+(aq) pKa=7.0 (2)
14
Most sulfite processes are based on batch digestion systems, but
there are some mills that utilize continuous systems. The batch
digesters are typically 70-350 m3 (Bryce 1980b).
Acidic sulfite pulping is performed with a cooking liquid in the pH
interval 1.2-1.5 and there is an excess of free SO2 (Bryce 1980b). The
initial temperature is often around 70-80C to assure that the cooking
chemicals have completely penetrated the wood chip before the
temperature reaches 120C. If the temperature is too high under acidic
conditions, condensation of lignin will occur resulting in a black cook
(Bryce 1980b).
Bisulfite pulping is a process where the liquid has equal fractions of
free and bound SO2. The pH is in the range 3-5, and the duration is
typically 5-7 h. The temperature increases faster than in more acidic
sulfite cooking processes (Bryce 1980b).
Alkaline sulfite pulping, in which a combination of sodium sulfite
and sodium hydroxide is used, results in low yields and a brightness
that is similar to that of pulp from the Kraft process (Bryce 1980b).
However, the rate of pulping is rapid, and the pulps have high
strength.
Multistage sulfite pulping is an approach designed to get the
benefits that are associated with different pH intervals (Bryce 1980b).
The first step could be neutral or alkaline, and the second step could
be acidic. Alternatively, the first step could be acidic, and the second
step neutral or more acidic (Bryce 1980b; Sixta 2006).
Dissolving pulp is a pulp with special properties that make it
possible to use it for regenerated fibers that are used for textiles. The
pulp needs to have a low content of hemicellulose, and a high content
of alpha-cellulose. The final degree of polymerization of the cellulose
and the viscosity need to be carefully controlled. The temperature of
the process is usually higher than for other sulfite processes (150C or
higher) and cooking continues until low kappa numbers are achieved
(Bryce 1980b).
The desired size of wood chips for sulfite cooking depends on the
products. If the product is dissolving pulp that will be used to produce
viscose, the strength of the pulp is not important. Fiber shortening
caused by using short wood chips is then not a limitation. An average
length of 19 mm has been determined as suitable for having a good
15
packing degree (Howard 1951). If the sulfite process is used for
making paper, longer wood chips are needed for minimization of fiber
shortening. The thickness of the wood chips is, however, not as
critical as for the Kraft processes.
1.3.5 Sugar-platform processes
The fundamental difference between chemical pulping processes
and sugar-platform processes based on lignocellulose is that in
chemical pulping the cellulose is preserved in polymeric form,
whereas in sugar-platform processes the cellulose is converted to
sugar. Conversion of cellulose to glucose, i.e. saccharification of
cellulose, is typically catalyzed by cellulose-degrading enzymes. The
sugar is then utilized as substrate in a microbial fermentation process
or in a chemical conversion process. Typical products would be bio-
alcohols, such as ethanol or butanol, or bio-acids, such as lactic acid
or succinic acid (E4tech et al. 2015).
Another difference is that chemical pulping primarily targets lignin,
which is modified and at least partially degraded. In sugar-platform
processes, too extensive lignin degradation is avoided and polymeric
lignin, hydrolysis lignin, remains as a co-product (Ragauskas et al.
2014). Too high concentrations of lignin-degradation products, for
example phenolic substances, can inhibit cellulose-degrading enzymes
and microorganisms (Jönsson and Martín 2016).
Prior to saccharification of cellulose, a pretreatment is needed to
make the cellulose accessible to cellulose-degrading enzymes
(Arantes and Saddler 2011). There are many different types of
pretreatment (Sun et al. 2016; Gandla et al. 2018), but the
predominant technology is hydrothermal pretreatment, which is
typically carried out at temperatures between 160C and 240C (Sun
et al. 2016), and which primarily targets hemicelluloses (Gandla et al.
2018). To make it more efficient, hydrothermal pretreatment can be
combined with steam explosion and addition of acids, for example
sulfuric acid. Regardless of whether acid is added, the process will be
acidic, as degradation of hemicelluloses will result in formation of
carboxylic acids, for example acetic acid, formic acid, and levulinic
acid (Jönsson and Martín 2016).
Feedstocks for sugar-platform processes based on lignocellulose
can be agricultural residues, energy crops, and wood. In either case, a
diminution step is needed before the pretreatment. For wood, that
16
would typically imply wood chipping. As wood, and particularly
softwood, is relatively recalcitrant compared to many other
lignocellulosic feedstocks (Gandla et al. 2018), hydrothermal
pretreatment would typically commence with an impregnation of
wood chips with an acid.
1.4 Wood preparation
Regardless of whether it is a conventional pulp and paper process, a
biorefinery process, or a CHP plant using wood, the primary step in
the conversion of wood logs is mechanical disintegration of wood into
chips of dimensions that are optimized to suite a particular type of
process. Due to the large quantity of raw material that is processed
(Section 1), it is important that the first steps are handled as
effectively as possible and with minimal losses. The handling of the
wood logs will affect the processability, the end product, and the total
production yield. Wood logs are often stored under variable weather
conditions, both dry and wet, for several months. Long time storage
can negatively affect the yield and the demand of bleaching
chemicals, depending on the process (Ressel 2006).
Wood handling is made in wood yards, and between the steps
presented below there are also transportation and storage steps. The
different steps are also showed in Fig. 3. Transport and storage differ
between mill sites and are not in the focus of this thesis. Transport and
storage may, however, affect wood chipping, as the average width of
wood chips is often affected by cracking along the fiber direction.
1.4.1 Debarking
Debarking is not necessarily a step for CHP plants or for sugar-
platform processes. It is, however, an important step in pulp
production. In saw mills, the logs are also debarked. However, in saw
mills the bark is removed with a debarker that removes the bark from
one log at a time. Saw mills only remove bark if they sell wood chips
from the outer part of the wood to pulp mills.
For pulp mills, it is very important that bark does not enter the
process. Bark has a large content of extractives, which can negatively
affect the processes, and small pieces of bark can also result in dark
spots on the finished products. The wear of the knives of wood
chippers is highly influenced by stones and sand, and small stones and
sand are often embedded in the bark (Brännvall 2009b). However, for
17
the economy and for resource-efficient use of the feedstock it is
important that wood loss during debarking is minimized.
The force that is needed to remove the bark depends on the tree
species and on whether the bark is frozen. Debarking is typically
carried out in a rotating drum (Fig. 3). The wood logs are fed into the
drum and the bark is removed as the wood logs are rubbed against
each other and, for some debarkers, against the inner surface of the
drum (Ressel 2006; Brännvall 2009b).
1.4.2 Wood chipping
For most pulping processes, it is necessary to chip the debarked
wood logs to wood chips of a dimension that is suitable for that
particular type of process. The only exception is the groundwood
process, in which the logs are ground against a rotating stone
(Brännvall 2009b). For all other processes, wood chipping is
necessary even if it will damage the fibers causing lower strength and
decreased pulp yield. Wood chips may also come to pulp mills from
the saw mills.
Quality controls are needed to separate high-value wood chips from
fuel-grade wood chips and wood pieces. If wood chips are too small,
too dry, contain bark, or are too damaged, they are separated from the
other wood chips and are used for combustion to generate energy.
To get a uniform chemical reaction, all fibers in the wood need to
get their share of chemicals and heat. Deficiencies are shown as a
higher degree of shives in the pulp (Uhmeier 1995; Hartler 1996). In
the production of semi-chemical and chemicals pulps it is important to
have a short impregnation time. Different processes vary with respect
to sensitivity to uneven distribution of chemicals in the beginning of
the impregnation, to uneven wood chip dimensions, and to the
cooking temperature.
The dimensions of the wood chips are important for pulp
production and for heat production. In pulping, good wood chip
quality is commonly defined as chips that will give a uniform pulp,
and which have qualities such as narrow distribution in size, bulk,
mechanical properties, and moisture content (Hartler & Stade 1977;
Uhmeier 1995; Hartler 1996). For heat and power plants that utilize
biomass, wood chips are not the only feedstock and the material that
goes into the process is often referred to as particles.
18
With regard to pulp production, the impregnation process is
affected by the properties of the impregnation liquid and the
dimensions of the wood chips in relation to the direction of the fibers
(Hartler & Stade 1977). For a wood chip (Paper III, Fig. 1), the length
is the dimension that has the same direction as the fibers, the thickness
corresponds to the smallest dimension, and the width corresponds to
the third dimension.
Common dimensions for wood chips are: length 20-30 mm,
thickness 3-8 mm, and width 15-30 mm (Brännvall 2009b). However,
the dimensions of the wood chips depend on the chipping settings,
wood species, temperature (frozen wood), and moisture content.
It is important that wood chips have sufficient length. Direct fiber
shortening was noticeable for wood chips that were shorter than 20
mm, and below 15 mm the shortening was apparent (Hedenberg et al.
1999). There has been a trend that pulp mills utilize longer wood
chips, and that leads to less compact wood chip columns in the
digester (Hartler 1997). A less compact wood chip column gives a
lower radial and axial filtration resistance for the liquid flowing
through the column. When the wood chips are longer, the thickness
increases as well, which gives larger fractions of overthick chips that
need to be rechipped (Fig. 3) in order to avoid increasing the amounts
of shives.
Wood properties as porosity and density also vary. Formation of
individual chips depends on micro variation in wood properties
occurring at the cell structural level (Twaddle 1997).
1.4.2.1 Wood chippers
The chipping process strongly affects the quality of the wood chips.
The settings of the knife angles and the distance to the wear plate (i.e.
the T dimension) result in chips of a certain length (Paper II, Fig. 1).
The settings can be changed during maintenance to reduce the
influence of wood variation, for example the difference between
summer wood and winter wood, but that happens only rarely due to
the resulting loss in production time. Relevant knife angles are shown
in Fig. 4 and are also presented in Papers I and II (Paper I, Fig. 2, and
Paper II, Fig. 3). The knife penetrates a distance into the log,
depending on the T dimension, and the wood will experience both
cutting and shearing forces. The further the knife penetrates, the
higher is the build-up of shear force in the material. When the shear
19
force is big enough, the wood chips will be shaved off in the fiber
direction, giving the wood chip its thickness (Brännvall 2009b). The
thickness will vary, due to that the break occurs randomly. However,
the average thickness of the wood chips increases almost linearly with
increasing average length, in a relation determined by the
complementary angle (λ) (Fig. 4). Changes in this angle are due to
changes in spout angle (ε) or in the sharpness angle (β) (Fig. 4). The
spout angle influences the size distribution of the wood chips.
Increased spout angle gives increased fractions of small-sized chips,
increased wood chip thickness (with constant length), and increased
bulk density. A smaller spout angle is beneficial, but it will decrease
the bulk density and reduce the limits for the maximum diameter of
the logs. Increasing the spout angle from the commonly used value
30° to 40° or to 50° would increase cracking and decrease the
percentage of accept chips, as it would increase the fraction of both
overthick chips and pin chips (Hartler 1962; Hellström et al. 2011).
Fig. 4. Definitions of knife angles of wood chippers: ε, spout/cutting angle; α,
clearance/pulling angle; β, sharpness/knife angle; λ, complementary angle.
In order to obtain the desired spout angle, the direction, the speed,
and the angle of the incoming logs are of importance. A correct
positioning of the logs decreases the formation of small-sized chips.
This is achieved by having sufficiently many knives on the disc or
drum to make at least one knife engaged in the log all the time. A
20
narrow thickness distribution is achieved by having a low friction
between the tool and the wood (Hartler 1996).
The use of disc-chipping technology for making wood chips for
mechanical pulping has been studied previously. For example, by
changing the spout angle and thereby using more energy for the
chipping, more cracks were created and by decreasing the wood chip
size energy could be saved in the refining step (Hellström et al. 2011).
It is not only the dimension that is an important parameter.
Damaged ends and cracking also influence impregnation, due to that
the damage zones are easier accessible for the chemicals (Brännvall
2009b).
The cutting speed, also defined as laps of the knife disc or drum,
also influences the dimensions of the wood chips. Too low speed will
decrease the capacity of the chipper and decrease the wood chip
production rate. A study of a newly installed chipper showed that
decreasing the cutting speed (the speed of the knife disc) with 25%
reduced the fraction of small-sized material (pin chips and fines) to
half of that of the original value (Bergman 1987).
At lower temperatures the wood is more brittle, and the influence of
having a high cutting speed will be more pronounced. When the
temperature decreases, the fractions of pin chips and fines will
increase, and the fraction of wood chips that are too large will
decrease. At temperatures lower than 0 °C, the moisture content is the
most important factor that influences the mechanical behavior due to
the presence of frozen water in the log. For higher temperatures, the
basic density is the primary parameter rather than the moisture content
(Hartler & Stade 1977; Hartler 1996; Hernándes et al. 2014).
1.4.2.1.1 Disc chippers
There are two types of disc chippers; smaller mobile chippers that
are used at sites in the forest, and bigger chippers at heating plants and
pulp mills. The mobile chippers are traditionally used to produce
wood chips for the energy market. The focus of this work is big
chippers that are found at mill sites and that chip more than one wood
log at the time. The main part of the feedstock from forestry is
converted to wood chips using energy-consuming conventional disc-
chipping technology (Brännvall 2009b).
21
A disc chipper consists of a rotating disc with 10-16 radial knife
holders and knives. The log is fed endwise onto the disc through a
slot. Different disc chippers have different designs. This could be an
angled disc with horizontal infeeding, chippers that are fed
horizontally, as in Fig. 5, or the disc could be radial and the feeding in
an angle and the wood logs would fall into the slot through gravity
feed (Brännvall 2009b).
Fig. 5. Schematic views of (A) a disc chipper, and (B) the velocity gradient across
the disc (courtesy of Multi Channel Sweden AB).
There are certain drawbacks with disc-chipping technology. This
has to do with the placement of the wood raw material, the lack of
flexibility to handle wood of variable quality (such as summer and
winter wood, different wood species, variations in wood quality
caused by growth in different habitats, and variable log dimensions),
and large demands for installed power to drive the wood chippers. The
knife-angle settings of today's disc chippers cannot be changed during
operation, and stopping the chipper to change the settings to make
them optimal for the feedstock and for the process takes valuable time.
The design of the disc chipper results in a cutting gradient over the
disc. The cutting speed towards the center is lower, which generates
too large wood chips. Towards the periphery a higher speed is
obtained, generating pin chips and saw dust. This results in lower
product yield from the feedstock, which increases the cost and the
environmental impact.
1.4.2.1.2 Drum chippers
In the past, drum chippers have mainly been used to produce chips
for bioenergy power plants. These chippers are smaller and mobile
and can be used in the forest, close to the raw material. The chippers
A B
22
are usually used as tractor implements or placed on trucks. The drum
is usually stronger than small disc chippers, and is not only used for
logs both also wood residues. The biggest problem with the drum
chippers is the uniformity of the dimensions of the chips. This is due
to that a large T dimension is needed to prevent wood chips from
entering the middle of the drum or the pocket next to the knives (Fig.
6B). When wood logs with large diameters are chipped, changes in
spout angle, ε, also affect the uniformity (Paper I, Fig. 1).
Fig. 6. Schematic views of (A) the novel drum chipper (courtesy of Multi Channel
Sweden AB), and (B) an example of a drum of a conventional drum chipper.
Drum chippers have the same velocity over the knives and the same
distance between the knives, similar T dimension as traditional disc
chippers, and the same angle independently of where on the knife the
logs are chipped. On disc chippers, the parameters change along the
knives. That is not a problem when only one wood log is chipped, but
when several wood logs are chipped at the same time there will be
differences between wood logs that are close to the periphery of the
disc and the wood logs that are close to the center of the disc. This
problem will increase when the diameter of the disc increases.
The company MCS (Örnsköldsvik, Sweden) has developed a new
type of drum chipper that potentially offers several advantages
compared to conventional disc chippers. The new drum-chipper
design also differs from that of conventional drum chippers. The MCS
drum chipper has specially designed wood chip channels around the
knife holder (Fig. 6A) to facilitate the use of drum dimensions over 2
m. The new drum chipper does not transport the wood chips from the
23
middle of the drum or have big pockets next to the knives resulting in
a large distance between knives and wear plate. The knife-to-
anvil distance and the knife-to-wear-plate distance (the T dimension)
are similar to those of traditional disc chippers, and the wood chips
will fall down into a collecting vessel due to centrifugal forces and
gravitation. The channels around the knife holders lead the wood
chips out from the drum and not back to the knives (Fig. 6A).
The size of the drum is important for making the wood chips more
uniform in size and make them similar to wood chips produced using
conventional disc chippers. The use of smaller drum chippers can
result in formation of boat-formed wood chips and in large variability
with regard to length and thickness, which is due to the distance
between the wear plate and the knife. The size of the drum will also
affect the spout angle (ε). If a small drum is used, there will be a
noticeable difference between the angles (ε) with regard to the upper
part and the lower part of the wood log (Paper I, Fig. 1). The new
drum chipper has more knives than conventional chippers, and the
invariable distance between the knives gives a more uniform chip size.
It is the knives that feed the drum with wood logs, as the knives will
pull the wood logs closer to the drum.
The new wood chipper is flexible, as it allows the infeed angle
settings to be adjusted during running, which permits optimized
chipping without process interruption. The construction has potential
to provide a more even wood chip quality, minimization of the reject
fraction, increased yield, and energy savings. As the new chipper has
the same cutting speed across the knives, it has potential to generate
smaller fractions of chips that are too large, and less pin chips and
fines. This would result in a better use of the precious wood feedstock.
1.4.2.1.3 Other chippers
In saw mills, wood chips are produced from the outer parts of the
timber, while the interior is utilized for production of sawn goods. A
reducer, a disc with knives, is used to produce the wood chips.
Hand-cut laboratory chips are used in some studies. Laboratory
chips cannot be compared to chips produced with industrial-style
wood chippers. The laboratory wood chips have no damaged ends or
cracks. Impregnation is affected by the cracking of the wood chips,
and laboratory-cut wood chips for Kraft processes need to be much
thinner than wood chips produced in a normal way (Brännvall 2009b).
24
1.4.3 Screening of wood chips
All wood chips that are produced by the wood chippers are not used
in the digesters. The fractions with too big wood chips and too small
wood chips and particles can create problems in the production of
pulp. Therefore, the wood chips need to be screened before use. There
are different kinds of screening equipment, but they typically consist
of plates moving above holes. The plates can vibrate and rotate
depending on the model
Oversized wood chips (diameter > 45 mm) are usually rechipped
using a chip cleaver, or cracks are introduced in them using a chip
conditioner, before they go back into the production. The fractions
with too small material are sorted out, but in some mills the pin chips
are fed into the process in controlled amounts. In some mills, the pin
chips are sorted away together with the fines. Thereafter, they are sold
for their energy value or used as fuel in the bark boiler.
1.5 Impregnation
To get a uniform chemical reaction with the wood, all fibers in the
wood need to get their share of chemicals and heat. This step, the
impregnation, is vital for achieving uniform pulp, low proportions of
reject and shives, and a product of high quality. Different processes
exhibit different sensitivity with regard the need for a thorough
impregnation. The sulfite process is typically more sensitive than the
Kraft process (Rydholm 1965; Hartler 1990).
Impregnation is not only about penetration of wood chips by
pulping chemicals, as it also depends on outward diffusion of
entrapped air and gases, and dissolved organic matter. Impregnation is
also affected by inward diffusion during cooking (Rydholm 1965).
Penetration of wood chips by liquid is determined by the character
of the capillary cavities of the wood, the presence of trapped air, and
the pressure gradient of the penetrating liquid. The diffusion is
determined by the diffusion surface that is specific for different wood
species, and characteristic of the ions and the concentration gradient
of the diffusing agent (Hartler 1990).
Different tree species and parts of the same tree exhibit differences
with regard to the capillary structure. Thus, there are differences
between the capillaries of softwood and hardwood, sapwood and
heartwood, and earlywood and latewood. Reaction wood is also
25
different compared to normal wood. Normal pulpwood consists of 50-
70% of void spaces filled with water and gases (Rydholm 1965). The
difference between heartwood and sapwood will affect not only
impregnation, but also the packing degree. Due to differences with
regard to moisture content and wood properties, the wood chips will
be affected differently by the cooking process.
Theoretically, the use of very thin wood chips, i.e. 0.2-1 mm, would
be advantageous for alkaline processes, but this has not been possible
to implement in large-scale operations. In most mills, the normal
wood chip dimension is a length and width of 15-25 mm and a
thickness of 2-5 mm (Rydholm 1965).
The flow of liquid in completely soaked wood can be described
with the Poiseuille equation:
𝑉
𝑡= 𝑘 ∗
𝑛 ∗ 𝑟4 ∗ ∆𝑝
𝑙 ∗ 𝜂
.....where the volume, V, flows through n capillaries of the radius (r)
and length (l) at the time t if a pressure differential Δp is maintained
and the viscosity of the liquid is η. An increase in the temperature of
the liquid will increase the flow rate in proportion to the decrease in
viscosity. Increasing pressure differential will also result in increased
flow rate (Rydholm 1965).
Alkaline liquids, such as cooking liquid of Kraft pulping, cause a
swelling of the wood structure. This results in almost equal diffusion
rate in all dimensions. Neutral and acidic liquids diffuse more rapidly
in the longitudinal than in the transversal direction of the chips. The
diffusion rate for neutral and acidic liquids in the longitudinal
direction would be about half compared to that of water, and in the
transverse direction 3-6% compared to that of water (Rydholm 1965).
This is due to the total cross-sectional area of the capillaries that
control diffusion. The cross-sectional area of the longitudinal direction
is half of the total area. For the radial and transversal directions, the
rate of diffusion increases up to 40% of that of water if the pH is in
the range 12.8-13.5, whereas the diffusion in the longitudinal direction
would still be 50% of that of water (Rydholm 1965).
Thickness is the most important wood chip dimension in a Kraft
process. This is mostly due to the difference in dimensions, as the
26
length and width of the wood chips are 5-10 times greater than the
thickness. When the distance is shorter and the diffusion rate is almost
the same, impregnation in the width and length directions can be seen
as insignificant compared to impregnation in the thickness direction
(Rydholm 1965).
The initial impregnation can be described as penetration of air-filled
wood by liquid. Trapped air results in uneven impregnation rate.
Proper gas diffusion is important with regard to both entrapped gases
and gases that are produced in the chemical reactions. One method for
air removal is to soak the wood chips in water. However, soaking
large volumes of wood chips in water requires a lot of time and a lot
of clean water, which makes implementation of soaking in industrial
scale challenging. Using large amounts of water also has a negative
influence on recovery of chemicals and it is not good for the heat
balance of the pulp mill. Another way to remove air is through
vacuumation, which is based on a pressure gradient. The drawback is
that the low pressure needed to remove the trapped air is impractical
to use in a normal digester. A third method is replacement of air with
gases that are soluble in the penetrating liquid. This method is
commonly used and a lot of research has been made on pre-steaming
of wood chips with water vapor (Rydholm 1965).
Impregnation of saw dust differs from impregnation of wood chips
and therefore requires the use of special techniques. When saw dust is
in contact with a liquid, a compact saw-dust matrix that limits the
impregnation will form. The matrix limits the flow of liquid and
sometimes completely prevents impregnation. The saw dust will
consume more effective alkali than wood chips, which also results in a
lower yield. The impregnation time required to get the same EA
concentration in saw dust is twice as long as that of wood chips
(Korpinen et al. 2008).
When wood logs are chipped, the material becomes compressed,
which creates a plastic deformation. This deformation is manifested as
cracks, structural damage, or micro changes in the fiber wall. For a
single wood chip, one of the sides is subjected to compression and
damage, but the other side is more or less intact. This mechanical
effect seems to make the wood more sensitive to subsequent chemical
damages. The damages make it easier for the cooking chemicals to
impregnate the chips and the reaction rate is increased.
27
Cracking with a chip sizer (chip cleaver) is efficient to improve
impregnation. Impregnation of cracked and sized overthick wood
chips was found to be faster compared to accept chips (Määttänen &
Tikka 2008a).
1.5.1 Methods for studying impregnation
Different methods are used to study impregnation. The
impregnation is mostly influenced by the pH of the impregnation
liquid, the dimensions of the wood chips, and the degree of cracking.
A sinkage test is used to study impregnation by measuring the time
it takes for a wood block to sink into a liquid. However, the method
has poor reproducibility and accuracy (Malkov et al. 2003).
Another possibility is the so called "uptake-method". Here, the
treated wood chip sample is compared to an untreated or dried sample
by comparing the weight. However, this method also has poor
reproducibility and accuracy (Malkov et al. 2003).
Another approach is to use image analysis by analyzing and
digitizing photographs of sliced frozen chips after impregnation. Only
a rough estimation of the penetration degree can be achieved with this
method, because of interference between diffusion and penetration
(Malkov et al. 2003).
Specially designed penetration clamps have been used to study the
permeability of different wood specimens. A wood block is clamped
in a special cell, and water or air is being forced through the wood
chip sample (Malkov et al. 2003).
The wood chips can also be hung in a quartz spiral balance. The
method has been used to measure the flow of a liquid into a single
wood chip on continuous basis, the influence of steaming time,
hydrostatic pressure, moisture content, cooking liquid penetration, and
penetration rates (Woods 1956; Aurell et al. 1958; Malkov et al.
2003).
The way liquid penetrates wood can also be studied by using a
microtome (Zanuttini et al. 2000). Other techniques include scanning
electron microcopy (SEM), staining and precipitation techniques,
radioactive tracer technique, and nuclear magnetic resonance (NMR).
A SEM equipped for energy dispersive X-ray analysis was used to
determine the sodium content in a study of impregnation of wood by
sodium sulfite (Bengtsson & Simonson 1988). Confocal laser
28
scanning microscopy and a fluorescent dye was used to analyze the
flow path of water through radiata pine (Matsumura et al. 1999).
Autoclave cooking is commonly used to investigate the impact of
differences in impregnation. This might regard duration, temperature,
and the composition of the impregnation or cooking liquid. The results
obtained typically include the pulp yield, the content of shives, the
fraction of reject, and the kappa number (Wedin et al. 2010; Wedin et
al. 2012).
To quantify the degree of cracking in wood chips, a technique
referred to as the "water absorption test" can be used. The wood chip
sample is immersed into water, and the weight is recorded as a
function of time. The specific water absorption (the weight subtracted
by the DW divided by the DW) is plotted with the time using a
logarithmic scale (Hellström et al. 2011).
1.5.2 Laboratory impregnator
A laboratory impregnator with a special weight sensor has been
developed by researchers at Helsinki University of Technology (now
Aalto University, Espoo, Finland) (Malkov et al. 2001a; 2001b). This
method allows the direct measurement of the mass of the wood chips
during the impregnation process. The method was further developed
by Määttänen and Tikka (2008a; 2008b). The amount of wood chips
was increased to about 600 g of wood (DW) (Määttänen & Tikka
2008a; 2008b).
2. Present Investigation
2.1 Aim of investigation
A large amount of pulp wood is used by the Swedish pulp and paper
industry every year (Section 1). Therefore, an increase in yield by
only 1% would make a big impact, for example by reducing the cost
of energy for transportation of the wood logs. An increased share of
fiber-based products from the feedstock would represent a more
resource-efficient use of trees from Swedish forestry.
The new drum-chipper technology of MCS should theoretically
increase the fraction of useable wood chips (accept chips). The goal of
this thesis work was to evaluate the new drum-chipping technology
with regard to wood chipping for Kraft and sulfite pulping. A
29
challenge associated with research in this area is the scale of the
experiments, as small-scale wood chipping would differ from forest-
industrial wood chipping. Furthermore, full-scale wood chippers used
in production are too valuable for the mills to allow experimentation
that might affect the operation of the mills. A solution to this problem
was the installation of a pilot wood chipper based on the new
technology, a pilot of a size that would make comparisons with full-
scale chippers meaningful.
2.2 Wood chipping and characterization of wood chips
2.2.1 Chippers
2.2.1.1 MCS drum chipper
In the studies presented in Papers I-IV, the wood was chipped using
chippers based on MCS drum-chipping technology. In the study
presented in Paper I, the wood chips were produced with a
demonstration-scale drum chipper temporarily used for production at a
Kraft pulp mill. The design of a full-scale drum chipper from MCS is
shown in Fig. 7. The wood chips were compared with wood chips
from a conventional full-scale disc chipper used by the mill. In the
studies presented in Papers II-IV, a pilot drum chipper based on MCS
technology was used.
Fig. 7. Full-scale drum chipper (courtesy of Multi Channel Sweden AB).
30
2.2.1.1.1 Infeed equipment
The novel drum-chipping technology has a patented infeeding
system that can change the infeed angle (ε’’) (Fig. 1 in Paper II)
during processing. This makes it possible to rotate the infeed
equipment by changing the infeed angle, and displace the wear plate
(constant T dimension). The specially designed infeeding system has a
chain transporter with lowerable spikes. The spikes direct the wood
logs and prevent the log from moving sideways into the chipper. The
spikes are lowered if the wood logs get stuck or if a wood log with
unusually large diameter enters the chipper.
2.2.1.1.2 Drum
An important feature of the new chipping technology is that the
drum diameter is big enough so that the changes in ε are so small that
it could be assumed to be constant. The diameter of the drum depends
on the desired upper limit for wood log diameter. To comply with
current forest-industrial standards for wood logs, the diameter of the
drum should be at least around 4 m.
The width of the drum is determined by the required production
rate. A wider drum can chip more wood logs at the same time, but will
also require more energy. Therefore, the width should be wide enough
to meet the production demand of the mill, but not larger than that.
2.2.1.1.3 Knife and knife holders
The knife holders are designed to allow more ergonomic knife
changes. The operator that changes the knives can stand and do the
work. There are specially designed channels for the wood chips.
Without these channels, the wood chips would get stuck inside the
drum and there would be a build-up of wood chips behind the knife. If
the channels were not correctly designed, the wood chips stuck behind
the knives could destroy the knives and impede chipping. The wood
chips will fall down to a storage space below the chipper. From this
storage space, the chips are transported by a screw to a conveyer belt.
2.2.1.2 Demonstration drum chipper
The demonstration-scale drum chipper was installed at a Kraft pulp
mill in southern Scandinavia. The drum had a diameter of 4 m, the
width was 1 m, and it was equipped with 24 knives. The chipping
speed varied between 28 and 36 m/s depending on the production rate.
31
Standard cutting angles were used (ε was slightly above 30) and
the infeeding angle could be changed (±5) during processing. The
infeed angle was not changed during the study presented in Paper I.
2.2.1.3 Pilot drum chipper
A pilot drum chipper based on the new technology (Fig. 8A) has
been deployed at Domsjö Development Area (Örnsköldsvik, Sweden).
The drum has a diameter of 3 m, and a width of 20 cm. It is equipped
with 16 knives and knife holders. The distance between the knives is
the same as for a full-scale wood chipper (Fig. 8B). The pilot chipper
has certain limitations compared to the demonstration chipper. The
infeed angle can be changed, but not during operation. The infeed
equipment is not as movable, so a mechanic needs to change the wear-
plate position to make sure that the chipper has the correct angles and
the correct T dimension. The pilot wood chipper is also limited by the
size of the wood logs. The wood logs cannot have a larger diameter
than the width of the drum. Wood logs need to be positioned manually
on the infeed equipment. The chip bin is under the chipper, and the
wood chips are transported from the bin by a scraper conveyer. The
wood chips are collected on a 3 m × 3 m metallic plate, or, if not the
whole sample will be collected, they are collected just below the
scraper conveyer when the chips start falling.
Fig. 8. Pilot-scale drum chipper designed by Multi Channel Sweden AB: (A) Drum
of the pilot chipper (3 m in diameter), and (B) knives, knife holders, and specially
designed wood-chip channels (courtesy of SB Kommunikation AB).
A B
32
2.2.2 Impregnation and cooking
Impregnation, cooking, and processing of pulp was performed in
the laboratories of More Research (Örnsköldsvik, Sweden).
Impregnation can be studied in an impregnation reactor that is a
further development of the laboratory impregnator described in
Section 1.5.2. The reactor has a basket that can accommodate 500 g
(dry weight) of wood chips and that hangs freely in a weight sensor
(Fig. 9). The reactor has been modified so that it can handle both
acidic and alkaline cooking liquids, and withstand pressures up to 16
bar. The reactor is coupled to a system with five different tanks, so
that the liquid can be exchanged between impregnation and cooking.
Is also possible to steam the wood chips.
The principle of the impregnation reactor is to have the basket
immersed in cooking liquid, which is always in excess. Therefore, the
EA in relation to raw material and the added liquid in relation to raw
material would be different compared to industrial systems. The
weight sensor will show an increase upon attachment of the basket
(Fig. 10). When water/cooking liquid (16 L) is pumped into the
reactor, the weight sensor will show a decrease. When the reactor is
pressurized, the weight sensor will show an increase, and after that the
impregnation process can be monitored.
Fig. 9. Schematic view of the impregnation reactor (courtesy of MoRe Research i
Örnsköldsvik AB).
N2 in
N2 outSteam out
Steam in
Circulation pipe
Weighing device
Steam
Water
Cooking liquor
Heat exchanger
Drain
33
Cooking experiments were performed in autoclave reactors or in
pilot digesters. Each of the six autoclave reactors can accommodate
50-100 g wood chips (dry weight). The wood chips and the cooking
liquid are added manually and the reactors are then immersed into a
preheated polyethylene glycol bath where they are rotated to obtain an
even distribution of the liquor . Each of the three pilot digesters can
accommodate two baskets. Each basket can accommodate 1000 g (dry
weight) of wood chips. Duplicates are used to obtain samples for both
determination of pulp yield (after drying at 105 °C) and for
temperature-sensitive analyses (after drying at 40°C). The baskets
with wood chips and the cooking liquids are inserted manually. The
pilot digester is then heated to the desired temperature. The fourth
reactor in the laboratory can accommodate six baskets, and each
basket can accommodate 1000 g DW of wood chips.
Fig. 10. Changes recorded by the weight sensor in the impregnation reactor during
impregnation of spruce wood chips with water. Color-labeled time periods show:
orange, fixation of the wood-chip basket onto the weight sensor by lowering the lid
and fastening it; green, filling the reactor with water; red, increase of the pressure to
2 bar; blue, increase of the temperature from 20 to 120 °C.
2.2.3 Analysis methods for wood chips
There are many standardized methods that can be used to analyze
the quality of wood chips. These include moisture content and size
distribution. Other factors that are believed to influence the quality,
but which are harder to analyze, include the degree of cracking in the
wood chips.
0
1000
2000
3000
4000g
1 2
h
34
2.2.3.1 Size distribution
The quality of wood chips is commonly classified through chip-size
measurements. The size has a strong influence on mass transfer and
smaller dimensions give weaker pulp.
The industry commonly utilizes a standardized method, SCAN-CM
40:01, for measuring the size distribution of wood chips. Using this
method, the wood chip sample is shaken through five different screens
(with a standardized size and placement of holes and slots). The
classification of wood chip fractions is based on the screen that it
cannot pass. The two fractions of too large chips, i.e. oversized chips
and overthick chips, cannot pass 45 mm holes and 8 mm slots,
respectively. The two fractions of accept chips, large accept chips and
small accept chips, cannot pass 13 mm holes and 7 mm holes,
respectively. The two fractions of small-sized material are pin chips,
which cannot pass through 3 mm holes, and fines (the residue) (Fig.
11).
Fig. 11. Wood-chip size fractions in the order left-to-right: oversized chips,
overthick chips, large accept chips, small accept chips, pin chips, and fines.
The fractions of oversized and overthick wood chips should only be
a small percentage (around 3%) and for making good quality Kraft
pulp the fraction of pin chips and fines should have a maximum in the
interval 8-15% (Hartler 1996). The fines are often collected and used
for combustion to generate energy, which, however, lowers the overall
yield. Removal of fines is important, as pulp made from material with
too much fines has low quality and strength. Too high proportion of
fines may plug the liquid flow or build up flow resistance in the
digester, which would cause serious operating problems. Oversized
and overthick chips create heterogeneity during pulping. If they are
left as they are, the need for pulp screening and reject refining will
increase, and the risk for having lower pulp quality will be higher.
35
Oversized and overthick wood chips are therefore separated and
processed again by sizing.
The impregnation during Kraft cooking is influenced by the
thickness of the wood chips. The method used to investigate the
thickness of the wood chips is similar to the wood chip size
distribution analysis method. In the standard method for investigating
thickness and thickness distribution, SCAN-CM 47:92, the wood chip
sample is shaken through 5 different screens, which all have slots. The
distances between the slots in the screen trays are 10 mm, 8 mm, 6
mm, 4 mm, and 2 mm. In the bottom, there is a pan for collecting
pieces that are thinner than 2 mm.
In sulfite cooking, the length of the chips has more influence than
the thickness. There is a standard method for determining the length
distribution, viz. SCAN-CM 48:92. This method can also be used to
determine the width distribution, but that parameter is not as important
in an industrial context as length and thickness. Length distribution is
a time-consuming analysis method compared to the other methods
used for analysis of wood chip dimensions. The part of the wood chip
sample that is classified as accept is mixed and a 1-L sample is
collected, and the wood chips are then manually sorted with the help
of a device with 15 slots for different lengths. The wood chips are
divided into the following fractions (in mm): <10, 10-13, 13-16, 16-
19, 19-22, 22-25, 25-28, 28-31, 31-34, 34-37, 37-40, 40-43, 43-46,
46-49, and >49.
2.2.3.2 Moisture content, yield and drying
The dry-matter/moisture content of wood chips was determined
using the SCAN-CM 39:94 method. For determination of yield, the
woody material/pulp was dried at 105°C. Pulp or impregnated wood
chips that were stored for later analyses were dried at 40 °C. Except
with regard to determination of extractives, the samples were then
freeze-dried.
2.2.3.3 Packing degree
The bulk density and the basic density of the wood chips are
important for pulping. The bulk density and the basic density describe
the packing degree in the digester. Variations in bulk density can lead
to overuse of chemicals or to lower production rate.
36
The bulk density is influenced by the density of the wood, the
dimensions of the wood chips, and the organization of the wood chips
in space. The wood species has a large impact on bulk density.
Higher regularity will increase the bulk density. Uniform wood
chips have lower bulk density and thereby lower packing degree in the
digester. To achieve a high degree of packing, small-sized wood chips
have to fill the gaps between the larger ones. The bulk density is very
much influenced by the relationship between the shortest and the
longest dimensions. Another parameter that can create problem with
respect to the packing of the wood chips is the degree of curviness.
Wood chips are good if they are flat and have uniform shape. Having
too many curved chips can lead to bridging, and in very serious cases
this can completely plug the digester. Small-scale commercial drum
chippers that are used to produce wood chips for heating plants
typically provide more boat-shaped chips. The degree of curving is
thought to be due to the relation of the drum size and the dimensions
of the logs. If the drum is large enough, the angle of the drum is too
small to create a curvy, boat-shaped wood chip.
There is no standard to determine the curviness of the wood chips,
but for determination of the degree of packing the standard method for
bulk density (SCAN-CM 46:92) can be used, or the method for basic
density (SCAN-CM 43:95). MoRe Research investigated the
relationship between the basic density and the bulk density for various
tree species, and the result suggests that there is a linear relationship
(Fig. 12). With the help of this relationship, the bulk density of a small
portion of wood chips can be determined. The basic density is the
oven-dry mass of a wood sample divided by its green volume. The
green volume is the solid volume of the wood sample when it is in
equilibrium with water. The bulk density is the oven-dried mass of the
wood divided by the bulk volume of the sample.
The basic density is measured using a completely soaked sample of
accept and pin chips. An apparatus is used to measure the amount of
water that the wood sample is replacing. The sample is dried,
weighed, and the mass is divided with the green volume. The bulk
density is measured by dividing the mass with the bulk volume. The
equipment basically consists of a cylindrical tube on a weighing scale
where wood chips are falling down in a controlled way. Thereafter the
volume is calculated by measuring the height of the wood chip sample
37
in the cylindrical tube (average of measurements made at three
different positions).
Fig. 12. The relationship between bulk density and basic density (courtesy of MoRe
Research i Örnsköldsvik AB).
2.2.3.4 Image analysis
Conventional methods for determining the dimensions of wood
chips are time-consuming, and only a small fraction of the wood chips
produced by the wood chipper will be analyzed. The companies
PulpEye (Örnsköldsvik, Sweden) and Iggesund Tools (now part of
Andritz) have developed an optical analyzer of wood chips that can
make real-time evaluation of fractions of the wood chip stream in a
pulp mill (Bergman 1998). The instrument, which is called ScanChip
analyzer, has a setup for manual analysis of samples as well as an
automatic mode that is based on a wood chip sampling device that
regularly will take representative samples from the flow of wood
chips. The sample is then shaken to separate the wood chips, which
are scanned with the help of a laser and a camera for evaluation of the
size distribution of the wood chips in all three dimensions. The
ScanChip instrument is calibrated using the SCAN methods
mentioned previously so that the data should be comparable with the
standard methods. However, a study of saw mill wood chips with the
ScanChip analyzer suggested that the values for average thickness
y = 0.4x - 25,8
110
115
120
125
130
135
140
145
150
155
360 380 400 420 440 460
kg/m³
kg/m³
Bulk
density
38
were lower with that method than the average values obtained with the
SCAN standard method (Bjurulf 2005).
2.3 Results and discussion
2.3.1 Full-scale demonstration drum chipper (Paper I)
A full-scale demonstration chipper based on MCS technology was
installed at a Kraft pulp mill to evaluate the new drum-chipping
technology. Before installation of the drum chipper, wood chips
produced with a conventional disc chipper at the mill site were
characterized for comparison. Wood chips were collected on the
conveyer belt after the wood chipper according to standard methods.
Wood chip samples were taken on the conveyer belt going into the
digester as well. To investigate possible differences arising during the
cooking step, pulp samples were collected directly after the blow tank
and washed to remove black liquor. The sample was then screened to
determine reject.
Reference samples from the disc chipper were collected during one
week in June and during one week in July. The drum chipper was
tested during eight days in June. In the study presented in Paper I,
wood chips from the drum chipper are compared to wood chips
collected during the same period of the year (June). Wood chips from
a saw mill at the mill site were mixed with the other wood chips
before they entered the digester. As the proportions of saw-mill wood
chips were not well known, the evaluation of pulp quality was not
possible to conduct in a satisfactorily way. Therefore, evaluation of
pulp is not included in Paper I.
Initially, a visual inspection of the wood chips was carried out.
Wood chips from earlier drum chippers have not been used in
industry, mostly because small drums produce curvy chips, which
could create plugs in the digester (Rydholm 1965). In the visual
inspection of wood chips from the demonstration-scale drum chipper,
no curviness could be detected. The wood chips from the drum
chipper had an appearance that was similar to wood chips from the
disc chipper, except that they had a greater width.
Other factors that were investigated included the wear of the knives,
ergonomic aspects regarding the changing of knives, and production
rates for wood chips and pulp. To limit the variation in the wood chip
sampling process, the wood chips were always collected by the same
39
person, and 2-3 samples were taken daily during a working week (5
days). Three pulp samples were collected during daytime with approx.
two hours between each sampling time.
The results presented in Paper I and in Table 1 support the
hypothesis that the drum chipper would produce more uniform wood
chips than the disc chipper. The drum chipper produced a greater
fraction of large accept and smaller fractions of pin chips and fines (p
≤ 0.01). Specifically, the demonstration drum chipper produced 51%
more large accept chips, 11% more total accept chips, and 74% less
pin chips and fines. Size measurements verified that wood chips
produced by the drum chipper had greater average width (Paper I,
Table 1).
Table 1. Results from test of the demonstration-scale drum chipper (sampling after
chipper).
aConventional disc chipper at a mill site in southern Scandinavia.
bMSC demonstration drum chipper.
cAverage values for bulk density and size fractions were determined according to SCAN-CM
40:01 and SCAN-CM 46:92.
dSum of large and small accept chips
eAverage values for thickness, length, and width determined for the fraction that was defined
as wood chips (two cut edges) using the ScanChip analyzer.
Disc chipper
Junea
Disc chipper
Julya
Drum
chipperb
After chipper / SCANc
Bulk (kg/m3) 138±6 142±10 140±4
Oversized (%) 2.4±1.5 2.2±2.1 2.6±1.1
Overthick (%) 8.9±1.6 8.7±2.3 8.7±1.5
Large accept (%) 48.7±5.4 49.1±4.8 73.7±2.6
Small accept (%) 28.0±4.4 27.8±3.8 11.7±1.8
Pin chips (%) 10.7±2.7 10.6±2.0 2.7±0.6
Fines (%) 1.4±0.5 1.6±0.81 0.5±0.1
Total acceptd (%) 76.7±2.2 76.9±3.2 85.4±1.5
After chipper / ScanChipe
Thickness (mm) 4.6±0.3 4.6±0.2 4.3±0.1
Length (mm) 22.4±1.2 22.7±1.0 23.3±0.4
Width (mm) 16.7±1.1 17.1±0.8 24.3±2.4
Oversized (%) 0.2±0.4 0.1±0.3 1.4±0.7
Overthick (%) 8.6±2.3 9.0±2.2 9.4±1.9
Large accept (%) 48.4±5.0 50.5±2.4 74.4±2.3
Small accept (%) 27.5±2.8 26.9±2.0 11.6±1.0
Pin chips (%) 13.0±3.4 11.7±1.7 2.9±0.4
Fines (%) 2.4±1.0 1.9±1.2 0.3±0.1
Total acceptd (%) 75.9±3.3 77.3±2.0 85.9±2.2
40
The difference between the wood chips going into the digesters was
not as great as for samples taken after the chipper. Nevertheless, the
wood chips produced with the drum chipper were still more uniform
and had a larger fraction of large accept (72.3%) compared to the
wood chips from the disc chipper (50%) (Table 2).
Table 2. Results from test of demonstration-scale drum chipper (samples taken
before and after the digester).
aConventional disc chipper at a mill site in southern Scandinavia.
bMSC demonstration drum chipper.
cAverage values for bulk density and size fractions were determined according to SCAN-
CM 40:01 and SCAN-CM 46:92.
dSum of large and small accept chips.
eAverage values for thickness, length, and width determined for the fraction that was
defined as wood chips (two cut edges) using the ScanChip analyzer.
fReject defined as the fraction of material that could not be screened using a 0.15 mm screen
plate in relation to the pulp that was collected after the blow tank (calculated as DW of total
amount of screened pulp).
2.3.2 Pilot drum chipper (Paper II)
A pilot drum chipper based on the technology of MCS was
deployed at Domsjö Development Area in Örnsköldsvik. A first
Disc chipper
Junea
Disc chipper
Julya
Drum
chipperb
Before digester / SCANc
Bulk (kg/m) 142±10 139±6 143±4
Oversized (%) 1.0±1.0 1.1±0.8 0.8±0.8
Overthick (%) 9.5±2.2 10.0±1.8 7.1±1.5
Large accept (%) 51.3±3.8 49.3±4.1 71.9±2.2
Small accept (%) 27.7±4.62 28.5±3.1 16.7±1.7
Pin chips (%) 9.6±0.8 10.0±2.0 3.1±0.6
Fines (%) 1.0±0.2 1.1±0.3 0.4±0.2
Total acceptd (%) 79.0±2.6 77.9±1.9 88.6±2.1
Before digester / ScanChipe
Thickness (mm) 4.6±0.2 4.6±0.2 4.2±0.1
Length (mm) 22.5±1.0 22.9±1.0 23.1±0.2
Width (mm) 17.1±0.4 17.1±0.8 22.5±0.7
Oversized (%) - - 0.4±0.3
Overthick (%) 8.5±1.9 9.7±1.9 7.1±1.0
Large accept (%) 50.6±1.4 50.7±1.6 72.3±1.6
Small accept (%) 27.4±1.6 26.4±1.2 16.2±0.5
Pin chips (%) 11.7±1.1 11.4±1.5 3.7±0.1
Fines (%) 1.8±0.6 1.8±0.7 0.3±0.1
Total acceptd (%) 78.0±1.2 77.0±1,2 88.5±1.0
Pulp
Rejectf (%) 0.69±0.24 0.35±0.10 0.55±0.10
41
attempt to produce wood chips with the pilot drum chipper was made,
and the wood chips were compared to wood chips produced with a
conventional industrial disc chipper. Initially, the wood chips from the
pilot drum chipper were far too short (Table 3). The total accept
fraction was also too low (even if that could be explained by the very
short average length, Table 3). The experiment made it clear that the
settings of the pilot drum chipper needed to be changed for future
studies.
Table 3. First evaluation of pilot chipper
Pilot drum chipper Disc chipper
SCAN standarda
Bulk density (kg/m3) 131 125
Basic density (kg/m3) 395 407
Length (mm) 18.6 24.4
Thickness (mm) 5.2 5.0
Oversized (%) 1.5 3.7
Overthick (%) 12.1 12.8
Large accept (%) 61.4 72.5
Small accept (%) 17.6 7.3
Pin chips (%) 6.5 2.9
Fines (%) 0.9 0.8
Total accept (%) 79.0 85.5
Kraft cookingb,c,d
Kappa number 20.5 20.9
Viscosity (mL/g) 1060 1180
Yield (%) 47.0 47.1
Reject (%) 0.24 0.50
Sulfite cookingb,c,d
Kappa number 7.8 9.1
Viscosity (mL/g) 524 603
ISO brightness (%) 61.4 47.9
Yield (%) 40.0 42.5
Reject (%) 1.4 2.6 aAverage values for bulk density, basic density, length, thickness, and size fractions were
obtained by using SCAN-CM 40:01 and SCAN-CM 46:92.
bKappa number, viscosity, and ISO brightness were determined according to ISO 302:2004,
ISO 5351:2010, and ISO 2470-1:2016.
cYield determined as the fraction of pulp after cooking in relation to the DW of the added
wood chips.
dReject determined as the fraction of material that could not be screened through a 0.15 mm
screen plate based on the mass of the raw material (the DW of added wood chips).
After the first test of the pilot drum chipper, a study was conducted
in order to compare different velocities and infeed angles (Paper II). A
major challenge in doing studies with wood is the major difference
42
between wood logs. Due to the time-consuming work to debark,
transport, and chip the logs, and due to the need to transport, store,
and analyze the wood chips, the number of wood logs in a study has to
be kept low. In this study, Norway spruce was selected and 3-5 trees
per setting was used. Wood chips from the whole tree were collected
for the analyses.
One aim of the study presented in Paper II was to detect trends
resulting from varying the infeed angle and the velocity. However,
very little variation could be detected. The results verified the results
presented in Paper I, and indicated that the pilot drum chipper gave
similar results as the demonstration chipper. The fraction of total
accept varied between 78% and 87% (Paper II). The size fractions
were rather similar, with large accept in the range 67-75%. The fines
fractions (0.3-1.4%, Paper II) increased somewhat compared to the
study with the demonstration drum chipper. This can possibly be due
to that processing of small end pieces of wood logs was more difficult
with the pilot drum chipper.
The average length of the wood chips was decreased aiming
towards lengths often used for sulfite dissolving processes (setting
length SL23) and to the theoretically most suitable length for the
sulfite process as suggested by Howard (1951) (SL19) (Paper II). The
setting length SL23 resulted in wood chips with an average length of
21 mm. The accept fraction was 89%, with approximately 76% large
accept. The fraction of fines was below 1% (Paper II, Table II). The
average length of the wood chips produced with SL19 was only
around 17.5 mm. The accept fraction decreased to 75-82%, and the
fraction of large accept to only 59%. The fractions of pin chips and
fines increased to 7.9-9.7% and 0.9-1.8%, respectively.
The results indicate that the new drum-chipping technology would
be beneficial for production of wood chips for acidic and neutral
dissolving processes. Impregnation during acidic and neutral
conditions is more dependent on the length in the fiber direction and it
would be advantageous to use short wood chips. The results obtained
with the SL23 setting show that production of short wood chips
without high fractions of pin chips and fines is a realistic perspective.
2.3.3 Chipping different wood qualities (Papers III and IV)
In the study presented in Paper III, the pilot drum chipper was used
to investigate how the wood chip quality was affected by using
43
different wood species and different wood log qualities. As the wood
quality would not only affect wood chipping but also ensuing
processes, pilot cooking and chemical analysis were also conducted.
The first series of experiments was carried out with a set of Norway
spruce wood logs with different moisture content. The driest wood
came from two trees that looked dry as judged from visual inspection.
The average dry-matter content was 79% and they were referred to as
LMW. MMW wood logs came from two trees that looked normal and
grew in the interior of the forest. However, later analysis indicated
that the wood was dry (61% dry-matter content) compared to normal
industrial standards. Wood logs from two fast-growing trees growing
in the outer part of a forest yielded HMW, with a dry-matter content
of only 42%.
There was an obvious difference between the two drier wood
qualities, on one hand, and the fast-growing HMW, on the other hand.
Compared to HMW, the bulk density of wood chips from LMW and
MMW was low and the fractions of pin chips and fines were higher.
There were no major differences with regard to pulp yield, viscosity,
and kappa number.
Chipping of frozen and unfrozen wood was studied by cutting wood
logs into halves and letting one of the halves freeze. Although three
different velocities were tested, there were only minor differences
with regard to size distribution. There was a trend that the fraction of
large accept was larger for the frozen material, and that there was
more small accept for the unfrozen material. On an average, the frozen
samples were thinner (Paper III, Fig. 3). Previous studies of the
mechanical behavior of wood could explain the difference in thickness
(Hernándes et al. 2014), but indicate almost a doubling of small-sized
fractions when frozen wood was chipped (pin chips and fines) (Hartler
& Stade 1977; Hartler 1996). This effect could potentially be due to
extra wear of knives during winter due to increased amounts of stones
and sand entering the chippers and changing the sharpness angle.
When logs are debarked by hand, no stones would go into the chipper
and only a few logs are chipped.
Wood from four different tree species was chipped using two
different setting lengths, SL25 and SL19 (Paper III). The species
investigated (spruce, pine, birch, and aspen) are common in Nordic
forestry. There were noticeable differences between the hardwoods
44
and the softwoods. Hardwood species gave higher fractions of
oversized and overthick wood chips. This indicates that the settings
were more suitable for the softwoods, and that knife angles would
need to be adjusted to be more suitable for hardwoods.
The T dimension was changed to produce shorter wood chips. For
setting SL19, a fraction of 90% accept chips was achieved for
hardwood. Softwood showed the opposite trend and the fraction of
accept decreased compared to the production of longer wood chips.
A comparison of chipping of heartwood and sapwood of Scots pine
is presented in Paper IV. Two trees were cut into logs, and the
heartwood was separated from the sapwood. Chipping resulted in
significant differences, in the sense that heartwood chips were
significantly thicker and generally larger in size resulting in more
oversized wood chips and large accept. However, the wood pieces that
were fed into the chipper had different dimensions and it is possible
that the result was affected by that.
2.3.4 Impregnation of wood chips (Paper IV)
A first study of impregnation using autoclave cooking was made
with wood chips of different length and thickness. This study covered
both Kraft and sulfite cooking (Table 4). Three autoclave cooks were
made per setting. No linear relation was found for yield, kappa
number, viscosity, or ISO brightness.
Table 4. Autoclave cooking of 100 g (dry weight) portions of chipped softwood.a
aIndustrial mix of wood chips of Norway spruce and Scots pine.
bKraft cooking with four different thicknesses (2-4 mm, 4-5 mm, 5-6 mm, and 6-8 mm) and
length 25 mm. Sulfite cooking with 5 different lengths for 4-5 mm thick wood chips (19-22
mm, 22-25 mm, 25-28 mm, 28-31 mm, and 31-34 mm) and one with 6-8 mm thickness and a
length of 31-34 mm
c”Liquid" is the combination of cooking liquid and the moisture content of wood chips.
dFor Kraft cooking, effective alkali was based on DW of added wood chips. For sulfite
cooking, the fraction of SO2 was based on added cooking liquid.
eCooking in polyethylene glycol bath. One cook for each combination of thickness and length
was used for analysis of yield and the second was screened for content of reject and other
analysis.
Cooking
processb
Liquid-to-wood
ratioc (L:kg)
Cooking
liquord
Heatinge Cookinge
160 °C
Kraft 3.8:1 EA 20% 122-132 °C 40’ and
then 132-160 °C 40’
2 h 40’
Sulfite 4:1 Bound SO2 3.45%
Total SO2 6.9%
80-160 °C
1 h 45’
3 h 30’
45
There was a difference between Kraft and sulfite cooking with
regard to reject formation. The reject from sulfite cooking seemed to
consist of uncooked wood chip cores, whereas the reject from Kraft
cooking appeared as poorly separated fibers (Fig. 13). The length of
the reject core from the sulfite cooking was correlated with the length
of the wood chips (Fig. 14). This suggests that the impregnation
process was insufficient. The experiments were discontinued and
efforts were directed towards studying impregnation using the
impregnation reactor described in Section 2.2.2.
Fig. 13. Reject from autoclave reactors after (A) sulfite cooking, and (B) Kraft
cooking.
Impregnation and cooking of heartwood of Scots pine was studied
using the impregnation reactor and autoclave reactors (Paper IV). The
use of heartwood of pine in acidic sulfite processes is challenging.
One reason is the high content of extractives, particularly pinosylvins,
which contribute to problems associated with black cooks. The
heartwood also contains more lignin than the sapwood (Fig. 15A). As
pine is one of the most common tree species in Nordic countries, it
would be desirable to have processes that are suitable also for a
feedstock consisting mainly of pine.
Impregnation and acidic sulfite cooking of wood chips from
heartwood of Scots pine was studied using 15 different reaction
conditions (Table 2 in Paper IV). No impregnation was compared to
impregnation during five min or four hours. Most impregnation
reactions were pressurized (9 bar), but one set of reactions was
conducted close to atmospheric pressure. Normal impregnation liquid
was used for most impregnation reactions, but one set of reactions was
conducted using more acidic cooking liquid rather than impregnation
liquid. To evaluate the impregnation reactions, cooking was
performed for one, two, and four hours. As two autoclave reactors
A B
46
were needed to generate sufficient sample for analysis of each
condition, the investigation included 30 autoclave reactions.
Fig. 14. (A) Kraft cooking reject fractions for wood chips with a length of 25-28
mm and different thickness. (B) Sulfite cooking reject fractions for wood chips with
a thickness of 4-5 mm and five different lengths, and for wood chips with a
thickness of 6-8 mm and a length of 31-34 mm (asterisk). (C) Average length with
standard deviation of 10 pieces of reject from the reaction mixtures described in (B).
Fig. 15A shows compositional analysis of the starting material and
the pulp or woody material obtained after cooking. The contents of
hemicelluloses rapidly decreased for all samples (Fig. 15). Klason
lignin was gradually converted to ASL and was then solubilized and
0
1
2
3
4
5
0
5
10
15
20
25
%
A
19-22 22-25 25-28 28-31 31-34 31-34*
mm
mm
2-4 4-5 5-6 6-9
mm
%
%
B
C
19-22 22-25 25-28 28-31 31-34 31-34*
0.2
0.1
0
47
washed out from the pulp. Delignification was most efficient after
pressurized impregnation using the normal impregnation liquid. It is
interesting that the difference between five min and four hours
impregnation was very small.
Fig. 15. (A) Compositional analysis of wood chips, woody materials, and pulp
(percent DW). From bottom to top: glucan (light brown), hemicellulosic
carbohydrates (yellow), Klason lignin (dark brown), acid-soluble lignin (brown),
and ash (black). (B) Fraction of carbohydrates in wood chips, woody materials, and
pulp (percent of DW of total carbohydrates). From bottom to top: glucan (blue),
mannan (red), xylan (green), galactan (purple), and arabinan (orange).
48
Delignification is shown in Fig. 15A but it is also indicated by the
kappa number (Fig. 16A). It is obvious that pressurized impregnation
using normal impregnation liquid was superior with regard to
delignification. The relationship between pulp yield and viscosity was
almost linear (Fig. 16B). The relationship between pulp yield and
reject (Fig. 16C) demonstrates the very large reject formation for the
non-impregnated material. Also, impregnation at low pressure resulted
in relatively large fractions of reject. This indicates that without
pressurized impregnation the cooking liquid did not penetrate the
whole wood chips. High pressure was always 9 bar, and it would be of
interest in the future to investigate the effects of the pressure in more
detail.
Fig. 16. Pulp yield in relation to (A) kappa number, (B) viscosity, and (C) fraction
of reject: I0C2h (●), I5mC2hLP (●), I5C2h (●), I4hC2h (●), I4C2hAcid (●), I0C4h
(▲), I5mC4hLP (▲), I5C4h (▲), I4hC4h (▲), I4C4hAcid (▲).
49
3. Conclusions and Future Work
The cost of the feedstock is an important part of the operating
expenditures of the forest industry, an industry with relatively low
profit margins. In this context, the possibility to utilize the feedstock
in a more resource-efficient way and make more of the main product
from the same amount of feedstock likely appears as an attractive
opportunity. Furthermore, feedstocks from forestry can serve as
starting material for biorefineries that manufacture liquid biofuels and
bio-based chemicals and materials. There is a risk that limitations in
the feedstock supply hamper the development and make it difficult to
reach ambitious goals, such as a fossil-independent vehicle fleet by
2030 and no net emissions of greenhouse gases by 2045
(Naturvårdsverket 2020). The development of more resource-efficient
processes can facilitate the transition to a bio-based economy.
Despite its significance, forest-industrial wood chipping is currently
a neglected area in research and development. The novel drum chipper
showed promising results with regard to minimization of the fractions
of pin chips and fines. This has potential to increase the yield of pulp
from a given amount of feedstock. The new chipping technology
might be especially valuable for acidic processes, in which the use of
short wood chips would be advantageous. The effects of wood-chip
quality on impregnation and cooking processes need to be studied
further in the future. Reduction of the fraction of fines might have
positive effects on both processes and product quality. It would be of
considerable interest to test the short wood chips produced with the
new technology in an industrial environment and to analyze the
consequences all the way to the final product. It would also be of great
interest to study the new technology during operation in a mill for a
prolonged period of time. The current investigation has covered wood
chips for Kraft and sulfite processes, but it would also be of interest to
study the new technology in TMP and CTMP manufacture, and in
sugar-platform biorefinery processes.
The investigation included initial attempts to study different wood
qualities. This field of research may increase in importance in the
future. For example, the moisture content of the wood is known to
affect the chipping result. Increased problems with bark-beetle
damaged trees motivate further studies of dry wood qualities. The
effects of the amount of knots on the fraction of oversized wood chips
and on the product quality would also warrant future attention.
50
4. Acknowledgements
There are many to whom I own gratitude for their help,
contributions, and support for this thesis. First of all I have to give
credit to the person without whose contribution and guidance my
thesis would not be possible: my advisor, and co-author of all my
papers, professor Leif J. Jönsson. I would also like to thank my co-
supervisor David Blomberg Saitton, the rest of my colleagues at RISE
Processum, and especially Ing-Mari de Wall for administrative and
practical support. This thesis should not have been possible without
the support of Torbjörn Sjölund, who provided advice and practical
help with impregnation, cooking, and analysis. My project should
have been less fun without the positive energy and advice from Sture
Noréus, Stefan Svensson, and Anna Svedberg at MoRe Research. I
would also like to thank other personnel at MoRe Research for
helping me with the practical work, and for providing space both for
me and for all my wood chips. I would also like to thank the people at
Multi Channel Sweden AB, especially my wood-chipping partner Sten
Hägglund, and also Nicklas Boström and PG Jonasson. I would also
like to thank Sune Wännström at RISE Research Institutes of Sweden
for being a member of my reference group.
Thank you also to all colleagues within our research group, who
always welcomed me and helped me when I needed to perform
experiments outside Örnsköldsvik: especially Madhavi Latha Gandla
for good collaboration with the last paper in the thesis, and also
Stefana Ganea Kozin and Jenny Lundqvist for guiding me in the
laboratories at KBC. I would also like to thank my collaboration
partners at SLU in Umeå: Hamid Salahi and Sylvia Larsson. This
research was supported by Umeå University Industrial Doctoral
School, the Kempe Foundations, the Swedish Energy Agency,
Bio4Energy, and the RISE Processum R&D Council. The research has
also been facilitated by Domsjö Fabriker, who is hosting the pilot
wood chipper at their wood yard, and by Metsä Board Husum, by
providing access to their ScanChip analyzer. Last but not least I would
like to thank my office room partner and dear friend Dimitrios Ilanidis
for making these years a lot more fun and for all the help.
51
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