3 Wood Handling (1)

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3. WOOD HANDLING

Table of content

WOOD HANDLING ....................................................................................................................................................1DEBARKING ............................................................................................................................................................. 1

Purpose of debarking .........................................................................................................................................2Wood losses during debarking ...........................................................................................................................4

CHIPPING .................................................................................................................................................................5CHIP SCREENING ......................................................................................................................................................9

SAWMILL CHIPS......................................................................................................................................................11CHIP STORAGE .......................................................................................................................................................12QUESTIONS ............................................................................................................................................................ 14

Kaj Henricson

Professor Pulping TechnologyLappeenranta University of Technology

August 2004

Educational course material and only for internal and personal use during

the course: An introduction to chemical pulping technology.


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Wood handling

The trees are cut in the forest into logs of pulpwood. The length of the logs varies between

countries and depending on the type of wood raw material. Log debarking is done in the forest

or as the first process step at the mill.

The debarked logs are cut into chips 20mm to 30mm long. The chips are screened to remove

fines and oversize particles. The accepted chips are transported to the chip storage at the pulp

mill from where the chips are taken to the digester for cooking. Picture 1 shows the wood

handling process for pulpwood. In mills, where also mechanical pulp is used, the wood handling

line is common for both pulping types.

Chips are also brought to the mills mainly from sawmills, where the chips are made from

sawmill residues. In Finland, about 20% of the raw material for fiber production comes from

sawmills. In the USA, the proportion is about 40%.

TREECROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENINGTREE

CROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENING

Picture 1. Wood handling process. The process at the pulp mill often starts with precut logs coming

to the mill.

SCREENING

CHIPPING

BARKING

LOGS

CHIP

STORAGEBARK

STORAGE

SAWMILL

CHIPS

SCREENING

CHIPPING

BARKING

LOGS

CHIP

STORAGEBARK

STORAGE

SAWMILL

CHIPS

Picture 2. Single-line wood handling

Picture 2 shows a wood handling department consisting of a conveyor feeding the pulpwood to

the debarking drum, debarking drum, chippers, bark presses, bark storage, chip piles and chipscreening.

Debarking

Wood is debarked before chipping and cooking. Bark does not contain proper fibers and cancause problems in the fiber line. Bark contains extractives, which can cause deposits on the

machinery and lead to dirt particles in the final pulp. Especially hardwood bark may result in

dirt particles in the pulp, and a high degree of debarking is needed for most hardwoods.

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When bark is cooked together with the wood, much of the bark material is dissolved duringcooking and can be found in the black liquor going to evaporation and combustion. The

dissolved organic material will cause an extra load on the recovery boiler, and the bark willcause extra need of cooking chemicals. A more efficient way to burn bark is in a separate bark

boiler after pressing of bark, where the content of water in bark is reduced.

There are differences in debarking techniques depending on the country and type of wood. Some

tropical hardwoods are debarked in the forest in cases where the bark is easily removed from thefresh wood. In Scandinavia, most of the pulpwood is debarked in debarking drums at the mills

prior to chipping.

Purpose of debarking

The main purpose of debarking is to remove bark to the extent necessary for the quality of thefinal product. The debarking degree is a measure of the bark removal efficiency. This gives the

percentage of debarked surface or bark content in chips, which expresses the share of bark in the

weight of the chips. In Scandinavia for example, a debarking degree of 95% in pulpwood

corresponds to a bark content of about 0.5%.

Picture 3. Debarking drum with easy roll bearing

Picture 4. Typical drum design

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Picture 5. Tumble debarking drum Picture 6. Parallel debarking drum

The pulp industry usually debarks all round wood that arrives at the pulp mill. Dry debarking is

used in areas where thawing or deicing of logs is unnecessary. Both wet and dry debarking is in

use in areas where thawing of logs is necessary. Dry debarking is today the preferred alternativefor new installations.

There are two types of debarking drums; tumbling (Picture 5) and parallel (Picture 6). In tumble

debarking drums, the log length is smaller than the diameter of the drum. In parallel debarking

drums, the log length is greater than the drum diameter.

The logs can tumble freely in the tumble debarking drum. The drum diameter is 4m-6m and thelength is 20m-40m. The peripheral speed of the drum is 1.5m/s-2m/s corresponding to a rotating

speed of 5.7rpm-7.6rpm with a 5m drum. In the tumble debarking drum, the logs move

randomly through the drum, which rotates around its axis in a straight or a slightly inclinedposition. The logs become debarked by rubbing against each other.

Parallel debarking is used for full length trees but is not as common as tumble debarking inScandinavia. The debarking forces are weaker than in tumble debarking and the drums are

longer and have a smaller diameter.

Table 1. Effect of bark-to-wood adhesion on debarking

Bark-to-wood adhesion

kg / cm3 lb. / sq.in

Debarking Examples of wood species

3-4 4055 easy to debark southern pines, maple

56 70110 normal to debark white pine, hemlock, spruce, beech

8-10 140 difficult to debark Elm, birch

1214 170200 very difficult to debark black poplar, ironwood

> 20 Approx. 300 almost unable to debark basswood, hickory

Different wood species have different debarking characteristics. The thickness and structure of

the bark have a significant effect on debarking. Table 1 shows the effect of bark-to-woodadhesion on debarking for some wood species. The cutting season greatly influences the

debarking degree. For example, spruce bark may attach to wood 100% tighter and poplar 250%

tighter in the dormant season than in the growing season.

Fapet 6A: p. A355

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Table 2. Required debarking times of some wood species and qualities

Wood quality Wood species Required debarking

time in minutes

trees easy to debark, for

pulpwood

oak, maple, southern pine, most

tropical hardwood species

10-15

normal to debark, softwood

for pulpwood

most northern pine species,

spruce

20-25

normal to debark, hardwood

for pulping

e.g. birch 30

fresh groundwood e.g. spruce 30-40

dry hardwood for pulping e.g. aspen, birch 40-60

dry groundwood e.g. aspen, spruce 60

In the northern regions, debarking becomes increasingly difficult as log temperatures fall and thelogs become frozen. The bark may adhere to frozen logs with a force 2.5-5 times greater than

that of thawed logs. Table 2 shows debarking times for some wood species.

Picture 7. Debarking capacity

The cleanliness requirement for bleached sulfate pulp is high (Picture 7). The debarking degreerequirements for softwoods are usually 85%-92%. This leaves less than 1% by weight of bark on

the log. Bark extractives are a problem especially in the manufacture of bleached hardwood

sulfate pulp. The debarking degree for pulpwood should be higher with bleached hardwood

pulps than with bleached softwood sulfate pulps.

Wood losses during debarking

Some wood is always lost during debarking. The amount varies considerably depending on the

quality of wood to be debarked, required degree of wood cleanliness after debarking, debarking

equipment and local conditions. Typically, wood losses as percentage of wood feed are smallwhen large-diameter and uniform pulpwood logs are debarked, as can be seen in Picture 8.

Fapet 6A: s. A355

Fapet 6A: p. A362


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Log diameter, mm

%

Log diameter, mm

%

Picture 8. Effect of diameter of log on wood losses

It is possible to reach a wood loss of about 1%-1.5% in wood weight with correct debarkingconditions. Fresh and short pulpwoods that debark easily have wood losses that can be about

1%-2%. Debarking dry hardwood or mixed wood, which consists mainly of thin logs and some

big logs, leads to large wood losses, which can be up to 4% -5% of the wood weight.

Chipping

The wood is cut into chips in order to makemass and heat transport possible during

cooking. The chips have to be small enough

so that chemicals and heat can penetrate anddiffuse into the wood material so that the

whole chip is cooked in a homogeneous way.

In case of too large wood pieces, the center ofthe chip will be at least partly uncooked.

On the other hand, the chips have to be large

enough so that liquids can circulate in thedigester during cooking around the chipswithout causing too much flow resistance and

pressure gradients.

The size and shape of the chips is determined

by the raw material used and the geometrywhen cutting the chips. A chipper cutting

geometry is shown in Picture 9. The formula

is + + + = 90, where is feed angle, is pull-in angle, is knife angle, is

complementary angle, c is knife clearance andu is knife distance from disc cut.

Picture 9. Chipper cutting geometry

The complementary angle influences the chip length-to-thickness ratio and can be used tocontrol the chip geometry at least to some extent. Another import parameter is the speed of

cutting that influences the formation of oversize chips and fines. Wood species, cutting

conditions, condition of cutting edge and size of the logs are other parameters influencing the

result when producing chips.

Fapet 6A: p. A371

PV

Wood

Loss


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Picture 10. Effect of complementary angle on

length-to-thickness relationship ofchips

Picture 11. Effect of chipper cutting speed onchip quality

The size of the chip influences how the cooking liquor can penetrate and diffuse into the woodmaterial. In Picture 12 is shown how the cooking liquid penetrates and diffuses into a thin wood

plate. The wood plate is 1mm thick and 40mm long. The fibers are oriented in the longitudinaldirection. The cooking liquid can penetrate along the fibers in the longitudinal direction. The

mass transfer is mostly controlled by diffusion. The penetration and diffusion of cooking

chemicals into the wood material is called impregnation.

40 mm

1 mmA chip

5 min.

0.00.0

1.01.0

40.0

0.5

0.7

0.940 mm

1 mmA chip

5 min.

0.00.0

1.01.0

40.0

0.5

0.7

0.9

Picture 12. Transfer of cooking chemicals into a thin wood plate during impregnation. The

concentration of chemicals inside the wood plate relative to the surrounding liquid after fiveminutes of impregnation.

Picture 13 shows the impregnation of a wood chip 25mm long and 5mm thick. The fibers areoriented in the longitudinal direction of the chip and the mass transfer is faster in the

longitudinal direction of the fibers. An impregnation time of 5 minutes is not enough to transfer

the chemicals into the center of the chip. A minimum impregnation time of about 20 minutes is

Fapet 6A: p. A372

Fapet 6A: p. A374

Henricson


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required in order to have the cooking chemicals penetrated and diffused into the center of thewood chip with a thickness of 5mm. The size of the chip has to be small enough to ensure good

impregnation of cooking chemicals. The most critical dimension is the thickness of the chip.

5 mm

25mmA chip

0.9

0.7

0.5~2 mm

1.05.0

0.0

0.0 25.0

20 min.

5 min.

0.9

0.70.5

5 mm

25mmA chip

0.9

0.7

0.5~2 mm

1.05.0

0.0

0.0 25.0

20 min.

5 min.

0.9

0.70.5

Picture 13. Chip impregnation; transfer of cooking chemicals into a chip. Concentration of chemicals

inside the chip relative to the surrounding liquid after five and 20 minutes of impregnationtime.

During chipping, physical damage to the fibers has to be minimized. The longer the chips are,

the fewer number of fibers are cut by the knife. Picture 14 demonstrates the relationship betweenaverage fiber length in the chip, the fiber length in wood and chip length. When choosing thelength of the chip, a balance has to be found between the number of fibers cut in the chipper and

the thickness and length of chips. A longer chip will also mean a thicker chip and a long and

thick chip will be more difficult to impregnate.

ll

Picture 14. Fiber length in chips as a function of chip length

Fibers are also damaged due to compression forces when the chipper knife hits the log. Thisdamage is demonstrated in Picture 15. The compression damage depends on the chipper design,

condition of the knife and knife angle.

Fapet 6A: p. A246

Softwood

Hardwood

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LENGTH

THICKNESS

COMPRESSION

DAMAGE

WIDTH

LENGTH

THICKNESS

COMPRESSION

DAMAGE

LENGTH

THICKNESS

COMPRESSION

DAMAGE

WIDTH

Picture 15. Compression damage of a chip

In a chipper, the log is fed at a controlled angle towards the rotor with the cutting knifes.Industrial chippers are usually equipped with multiple motors to balance the momentum with

which the rotor rotates while cutting the log into chips.

Picture 16. Chipper knifes cutting a log

Picture 17. Industrial chipper

PV: p.184

PV: p.195

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Chip screening

Picture 18 shows a wood chip. The chip is characterized by length; industrial chips are typically

20mm-30mm long, and thickness; industrial chips have a thickness typically below 8mm. Width

is as such not critical for cooking. Mass and heat transfer in the thickness and width directionswill be mostly controlled by diffusion and, as thickness is smaller than the width of the chip, the

thickness will be the dimension controlling the time needed by impregnation.

Picture 18. Wood chip

The chips are screened usually after chipping and before transfer to the chip storage in agyrating screen to remove oversize particles and fines. Oversize particles are particles not able

to pass through a hole of a certain size, usually a hole with a diameter of 45mm. Fines are

particles that can pass through a hole of a certain size, usually a hole with a diameter of 3mm.These oversize particles and fines are not considered good for cooking. Oversize and undersized

fractions can also cause mechanical problems in the digester. Another objective of chip

screening is to remove some of the small size contaminants, such as bark, sand and grit.

Chips are also screened at some mills after the chip storage and before the transfer of chips to

the digester. The chips are usually screened with respect to thickness in this position. A chipthicker than 8mm is often considered overthick and can be partly uncooked in the digester and

cause reject after cooking. Disc screens are used in the thickness screening of chips.

Picture 19. Chip size distribution before and after screening

Fapet 6A: p. A369

KP: p.41


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Picture 22. Disc screen

Oversize particles separated during chip screening are crushed or cut into smaller size andreturned to the chip screens to be re-screened. There are many technical alternatives how this

can be done. The target is to avoid crushing or cutting the oversize chips into too small particles.

Fines can either be taken to a special boiler, for instance the bark boiler, and used for energy

production or taken to a digester to be cooked into pulp. Some mills have sawdust digesters orother digesters suitable for cooking small material, where the fines can be cooked into pulp.

Chips are also screened in laboratories in order to classify the chips. Picture 23 shows a typical

laboratory chip classifier arrangement. The amount of chips being retained on each screen gives

the size distribution of the chip sample.

Picture 23. Chip classifier SCAN-CM

Sawmill chips

Sawmill residue wood is used in the form of sawdust or sawmill chips for fiber production. The

sawdust and chips coming from sawmills are usually screened at the pulp mill before cooking.The sawdust has to be free of large wood particles before being taken into a sawdust digester.

Fapet 6A: p. A388

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The sawmill chips have to be screened to remove fines and oversize wood particles beforecooking in a digester for chips.

Sawmills use logs with a large diameter to produce sawn goods, and much of the residual woodfrom sawmills is from the outer part of the logs and thus contains long mature fibers. Sawmill

chips are a source of raw material for pulps with good strength properties. Sawmill chips are

treated and stored separately at many mills and added to the chip stream going to the digester in

a controlled way. The portion of sawmill chips added to the chip flow will influence theproperties of the pulp produced especially in the case of softwoods.

Chip storage

Chips are stored at pulp mills in piles or silos. The chip storage is a buffer area between wood

handling and cooking. The amount of chips stored varies but can correspond to a few weeks of

pulp production. Different types of chips are usually stored in separate piles or silos.

Picture 24. Screw reclaimed chip pile

The chip storage is used to homogenize the flow of wood material to the digester. It is also usedto control the portion of various chip types being fed to the digester. This way, the quality of the

pulp produced can be kept stable and controlled with respect to the type of raw material used.

The wood material in the chip storage is influenced by bacterial and fungi activities. Due to this,

the temperature in the chip storage will increase with time and there will be some wood losses.

Wood losses can be reduced by cooling and covering the chip piles with snow.

Picture 25. Temperature profile in a chip pile PV: p. 278

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During chip storing, especially the extractive content of the wood material will decrease. Thisphenomenon is used by pulp mills to lower the extractive content of hardwoods in particular.

Mills may store the hardwood for a minimum time of some weeks to get the content ofextractives low enough to enable easier processing of hardwood at the mill. On the other hand,

the extractives lost from softwoods will lead to a reduced production of turpentine and tall oil.

Chips are transported in the chip handling area with belt conveyors (Picture 26) or pneumatic

systems. In pneumatic systems, the chips are transported inside pipes by air blowing. The chippiles (Picture 24) and silos are equipped with screws or conveyors that take the chips out of the

lower part of the storage in a controlled way.

Picture 26. Belt conveyor

Picture 27. Wood losses during chip storage inScandinavia

Picture 28. Losses of resinous materials

Modified by KH

Fapet 6A: p. A423

Modified by KH

Fapet 6A: p. A421

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Questions

1. Wood losses during wood handling. / Puuhvit puunksittelyss.2. Storage of chips at pulp mills. / Hakkeen varastointi sellutehtaalla.3. Problems related to the debarking of pulpwood with low diameter from thinnings. / Ohuen

harvennuspuun kuorintaan liittyvt ongelmat.

4. The effect of chip size on pulp quality. / Miten hakkeen koko vaikuttaa massan laatuun.5. Effect of chipper cutting geometry on the size and quality of chips. / Hakun tern geometrian

vaikutus hakkeen kokoon ja laatuun.

6. Drum debarking technology. / Rumpukuorinta tekniikka.7. Chip screening at pulp mills. / Hakkeen seulonta sellutehtaalla.

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