Biermann Ch. Handbook of Pulping and Papermaking

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24 2. WOOD AND FIBER FUNDAMENTALS

the pulp mill is monitoring each supplier for chip

quality.

Slasher deck

The slasher is a deck where saws cut logs

into shorter lengths, such as 2.5 m (8 ft), for

easier handling. Because slashers require much

maintenance, many mills are going to tree length

handling of wood, although this is much more

difficult with hardwoods because of their tendency

to have several main trunks.

Barker

A barker (or debarker) is a device used to

remove bark from wood before chipping. Remov-

al of bark is necessary as it has negligible useful

fiber, darkens pulp, requires extra chemical usage,

and introduces contaminants such as calcium,

silica and aluminum into the chemical recovery

system. Bark adhesion is about 3-5 kg/cm^ during

the growing season and 2-3 times higher during

the dormant season (winter).

One type of barker, the drum barker, is a

large, rotating, steel drum 2.5-5.0 m (8-16 ft) in

diameter by 8-25 m (30-90 ft) long (12 ft in

diameter by 70 ft long is common), mounted with

the exit lower than the entrance to promote the

flow of logs (Fig. 2-11 and Plate 6). The drum

rotates at about 5 revolutions per minute. A dam

at the exit controls the log retention time, which is

on the order of 20-30 minutes. Debarking occurs

by mechanical abrasion of the logs against each

other. Typically the wood contains 0.5-1.0% bark

after this method, although this depends on the

type of

wood,

average log diameters, season of the

year, and other factors. Logs that are not ade-

quately debarked may be sent through the drum

again. Some disadvantages of

the

drum barker are

the relatively high power requirements, main-

tenance costs, and bruising of the logs.

The cambial shear barker works with a ring

of knives that peel off the bark as the logs are fed

individually as shown in Plate 7. Feeding the logs

individually increases the operating costs due to

the required supervision. This method does not

work well on frozen logs. The cutterhead or

Rosserhead barker works like a planar, where a

turning wheel that is toothed or spiked is held

against the log and removes the bark by abrasion

(see the insert of Plate 7). It has low capacity

since logs are fed individually, requires large

amounts of supervision, and has high wood losses.

It is used in low-production facilities such as small

saw mills. It is suitable for logs that are difficult

to debark such as frozen logs.

Hydraulic

barkers

operate by impinging high

pressure water jets onto the wood. With ring

barkers, logs 0.5-1.5 m (1-5 ft) in diameter travel

at 1 m/s (3 ft/s) while 1500 gallons per minute of

water at 1500 psi squirt out of four nozzles in a

rotating ring (16 stationary nozzles in older de-

signs) with a velocity of 400 ft/s. As much as

' Feed from hot pond

Motive force

Fig. 2-11. Schematic diagram of a barking drum. Redrawn from J. Ainsworth, Papermaking,

®1957 Thilmany Paper Co., with permission

WOOD CHIPS AT THE PULP IVflLL 25

Fig. 2-12. Bellingham hydraulic barker in

Crown Zellerbach Corp., with permission.

Fig. 2-13. A diagram of a flaU debarker.

Courtesy of Manitowoc Engineering Company.

2500 hp is required to drive the water pumps.

The water must be very clean for ring barkers as

grit and dirt quickly disintegrate the seal.

Single jet units such as the modified

Bellingham barker (Fig. 2-12) use a traveling

nozzle that moves up and down while the log is

moved and rotated using airplane-type controls

(joy sticks). Water is supplied by a six-stage

impeller pump to produce 1200 gal/min at a

nozzle pressure of 1400 psi. Numerous other

designs for hydraulic barkers are used as well.

The waste water from hydraulic barkers is high in

BOD (a type of pollutant) and color and must be

treated before release, a severe drawback. They

were once common on the West Coast of the U.S.

as they work well for debarking large logs.

Flail debarkers (Fig. 2-13) use a rotating

cylinder with numerous chains hanging from it to

delimb and debark small diameter material. It is

useful for in-the-woods operations for processing

precommercial thinnings that would otherwise be

unusable. Without debarking, whole tree wood

chips are less valuable. Plate 8 shows an in-the-

woods operation using a flail delimber/debarker.

Chipper

Chippers are devices used for mechanically

breaking down wood into chips (Fig. 2-14). Chip-

ping takes approximately 7-14 kWh/t (0.4-0.8 HP-

days/ton). Hardwoods generally are harder to

chip and generate fewer fine chips but more large

chips than softwoods. The traditional chipper for

log lengths of 1.5-3 m (4-10 ft) is the gravity feed

(or drop-feed) disk chipper, where the logs enter

through a spout mounted on the top (or sometimes

other) comer of the feed side. Mills that handle

tree length wood must use horizontal feed disk

chippers that have the feed at the top or the bot-

tom of the feed side. Horizontal chippers use

more knives than drop-feed chippers to avoid the

production of more fine and pin chips. Disk

chippers use gravity discharge, which increases

the initial capital cost due to the higher elevation

required for the chipper, or blowing discharge,

which increases the pins and fines content due to

the extra chip damage caused by the paddles

forcing the chips from the chipper.

The best chipping of softwood logs leads to

85%

accept chips, 4% overthick, 2% overlength

26 2. WOOD AND FTOER FUNDAMENTALS

Fig. 2-14. Drop feed (gravity) wood chipper.

Redrawn from J. Ainsworth, Papermaking,

®1957 Thilmany Paper Co., with permission

chips,

7% pin chips, and 2% fines (see below for

these definitions). A worn blower (for blower

discharge units) or excessive anvil gap leads to

more pins and fines.

Drum and double cone (V-drum) chippers

have very limited use for smaller sized wood.

Because of their design and because they are

processing small residues, the generation of pin

chips (15-25%) and fines (5% or higher) is much

higher than chipping softwood logs. Other chip-

pers are designed for specific purposes such as

veneer chippers that chip veneer residues from

plywood plants and core chippers that chip the

core of peeled logs used to make veneer.

Chip size sorting for production

Ideally all chips regardless of their source are

sorted (screened) at the mill into several fractions

according to their size to permit uniform pulping.

In the past, most mills classified wood chips by

size using oscillating, round-hole screens such as

those shown in Fig. 2-15. During the 1970s it

became apparent that for the kraft cooking pro-

cess,

chip thickness is of primary concern. Since

1980,

almost all kraft mills have installed equip-

ment that classifies the chips by thickness (Fig. 2-

16 and Plate 9) to remove the overthick chips.

Most mills use additional separations to remove

fines.

A few mills even separate pin chips going

to the process and meter them back into the

process. Most sulfite mills and sawmills continue

to use chip classification by round-hole screens.

Laboratory chip screening

Chip classification is also done in the re-

search and development laboratory (as opposed to

production quality control) for experimental

protocols and to determine the quality of chips

from the various chip vendors, although the

methods used are not designed for large numbers

of samples on a routine

basis.

Laboratory classifi-

cation was traditionally based on chip size using

round-holed screens and is known as the Williams

classification with pans containing 9/8, 7/8, 5/8,

3/8,

and 3/16 in. holes. This method is now

obsolete for most purposes. To more closely

duplicate screening at kraft mills, a thickness

screen is now included for laboratory screening.

The exact definitions of the following chip frac-

tions depend on the nature of the classification

scheme. The laboratory screen is shown in Fig.

2-17.

The following definitions are based on typical

laboratory screening. Overs are the oversized or

overthick fraction of chips and are retained on a

45 mm (1.8 in.) diameter hole screen and are

thicker than 10 mm for conifers or 8 mm for

hardwoods. [For sawdust, overs are retained on

a 12 mm (1/2 in.) diameter hole screen.] Accepts

are the chip fraction of the ideal size distribution

for pulping. These chips pass through an 8 or 10

mm slotted screen and are retained on a screen

with holes 7 mm (0.276 in.) or 3/8 in. diameter.

Pin chips are the chips that pass through a 7 mm

screen but are retained on a 3 mm (0.118 in.) or

3/16 in. hole screen. Fines (unders) are the

undersized fraction of chips or sawdust and are

collected in the bottom pan. The definition of

fines will vary with mill specifications, but fines

generally consist of material passing through a 3

mm screen.

Wood chip quality control at the mill

Chip quality control uses devices that are

designed to handle numerous samples quickly with

a minimum of operator time. It is important to

practice wood chip quality control for several

reasons. The most basic reason is that the amount

of dry wood must be determined for a truck,

railcar, or barge load so that the supplier can be

paid for the equivalent oven-dry wood. An equal-

WOOD CHIPS AT THE PULP IVDLL 27

Fig. 2-15. Vibratory, round-holed screens for chip size classification. The top shows oversized chip

removal; the bottom shows accept and pin chip fractions each at a different miU.

ly important reason to determine chip quality from

each load is so that the mill can work with each

chip supplier to insure that high quality chips are

supplied in terms of desired species, suitable chip

size distribution, and small amounts of dirt, bark,

and decayed material.

28 2. WOOD AND FIBER FUNDAMENTALS

Fig. 2-16. Chip thickness screen for overthick chip removal. The insert shows a close-up of disks.

Like any method to insure quality, a repre-

sentative sample from each truckload of incoming

chips must be obtained. Some mills use continu-

ous samplers to insure the sample represents the

entire truckload, but most mills use a simple

bucket sampler that is filled from the first 5% of

the truckload. Fig. 2-18 shows a continuous

sampler and a bucket sampler that grabs a sample

from one section of the truck. Plate 10 shows a

chip truck dumping in an arrangement that also

uses a bucket-type sampler. (Stories circulate

about unscrupulous suppliers that water-down or

use inferior chips in the part of the truck that they

know will not be sampled.) Many mills may also

have a person collect a sample every hour of the

chips going to the digester to see how the screen-

ing system is working. (Stories also circulate

about the person who collects all eight samples for

a given shift at one time and submits one sample

every hour.)

Ideally, laboratory determinations are made

for moisture content, chip size distribution, and

bark, rot, and dirt contents. Determinations of

wood species, extractives content, chip bulk densi-

ty, and chip damage are made less often. Chip

size distributions are determined in the laboratory

using oscillating screen systems, systems with

adjustable thicknesses between bars (Fig. 2-19), or

other suitable systems. Collection of representa-

tive wood chip samples from rail cars or barges

can be quite challenging (Fig. 2-20.)

Miscellaneous analyses

TAPPI Standard T 257 describes the sam-

pling and preparation of wood for analysis whether

logs,

chips or sawdust. The basic density and

moisture content of pulpwood is determined

according to TAPPI Standard T 258. In this test,

volume is measured by water displacement and

moisture content by the difference in mass before

and after drying at 105 °C ± 3^ The overall

weight-volume of stacked roundwood is deter-

mined according to TAPPI Standard T 268.

TAPPI Standard T 265 is used to measure the

natural (wood derived) dirt in wood chips. Dirt

originates from the outer and inner barks, knots,

WOOD CHIPS AT THE PULP IVflLL 29

QVER-SIZED (45 mm HOLES)

OVER-THICK (10 ^^ SLOTS)

ACCEPTS (7 rrim HOLES)

PINS (3 mm HOLES)

FINES

Fig. 2-17. Laboratory chip size distribution analysis. The top shows the pan configuration; the

bottom shows the actual equipment.

stains,

rot (decay), etc. Color photographs help determined by ignition in a muffle furnace at 575

with identification for the novice. The ash content ± 25°C as described by TAPPI Standard T 211.

(i.e.,

the mineral content including metals and TAPPI Standard T 263, with numerous dia-

their anions and silicates) of wood and pulp is grams and photomicrographs, covers identifi-

30 2. WOOD ANB FIBER FUNDAMENTALS

Fig. 2-18. Continuous sampling during chip truck emptying on the left; on the right a bucket that

is filled when it swings out, obtaining a sample that represents only one section of the load.

cation of wood and fibers from conifers. One

should consult the references listed in this method

for additional information and high quality photo-

micrographs unless experienced in microscopy

and, more particularly, wood anatomy.

The amount of hardwood and softwood in

wood chips is easily determined with the Maule

test (Section 21.18), which gives a purple color

for hardwoods while leaving softwoods uncolored.

and determine the bone dry weight using moisture

contents of the wood. Table 2-2 gives the conver-

sion factors for different units of solid wood and

wood chips that are described below.

Board foot

A board foot is a volumetric measurement of

solid wood. It is equal to 12 in. x 12 in. X 1 in.

or 1/12 cubic foot of solid wood.

2.4 SOLID WOOD MEASUREMENT

Wood is measured and sold on a variety of

bases.

The volimie of solid wood in stacked

roundwood can be determined by scaling (a labor-

intensive method of sampling volume and log sizes

and converting to solid wood volume with tables)

or by water displacement methods. In practice, it

is easier to weigh the wood (using truck scales)

Cord

A cord is stacked roundwood occupying a

total volume of 4 ft by 8 ft by 4 ft. Typically, a

cord of stacked wood contains 80-90 cubic feet of

solid wood, although this can vary widely, and

will yield about 500 bd. ft. of lumber or 1.2 BDU

of chips. (A face cord is used to sell firewood in

some locations; it is 4 ft by 8 ft by the width of

the wood pieces.)

SOLID WOOD MEASUREMENT 31

Fig. 2-19. Chip size classification (left) and moisture content determination (right) in a mill's wood

quality control laboratory.

Cunit

A cunit is 100 cubic feet of solid wood in

stacked roundwood. It is used to determine the

wood content of pulp logs.

Fig. 2-20. RaU cars ready to be unloaded.

They will be tipped back on the special platform

to discharge their contents. Over six cars per

hour can be unloaded by this method.

2.5 WOOD CHIP MEASUREMENT

Bulk density

The bulk density is the oven-dry weight of

chips (or sawdust, or other wood residue) con-

tained in a given volume of space. The bulk

density of the chips depends on the specific gravity

of the wood source, the chip geometry, and the

chip size distribution. For example, Douglas-fir

chips from roundwood are typically 192 kg/w? (12

Ib/ft^) (dry wood weights), while Douglas-fir chips

made from veneer are 184 kg/w? (11.5 Ib/ft^).

White fir and pine chips are about 168 kg/m^

(10.5 Ib/ft^), and those of redwood are about 160

kg/m^ (10 Ib/ft^). A rule of thumb is that 1 m^ of

wood yields about 2.6 m^ of chips (1 ft^ yields

about 2.6 ft^).

Bone dry unit, BDU

A bone dry unit is the equivalent of 2400 lb

of oven-dry chips, sawdust, or other wood parti-

cles.

A BDU of packed Douglas-fir chips occu-

pies approximately 200 cu. ft.

Unit

A unit is 200 cu. ft. of wood chips, sawdust,

or other wood particles. A 40 foot open-top chip

truck carries about 18 units. One unit of

Douglas-fir or western hemlock chips is about

32 2. WOOD AND FffiER FUNDAMENTALS

Table 2-2. Approximate conversion factors of wood of 0.45 specific gravity in various forms (for

ft^

of solid wood per cord and 2.6 ft^ of chips per

ft^

of solid wood). Entries vjith five significant

digits are exact for any wood. Conversion factors for wood varying from these parameters should

be calculated for individual situations. Board feet values do not include sawing loses.

Convert froml

1000 board ft

cord

cunit

^^^

unit

ft' chips

Convert

to 1

by multiplication

m' stacked

wood

3.55

1 3.6247

4.26

3.64

3.28

1 0.0164

m' of solid

wood

2.3598

2.41

2.8318

2.42

2.18

0.0109

MT of bone dry

wood

1.06

1.08

1.27

1.0886

0.98

0.0049

m' of wood

chips

6.1

6.3

7.4

6.3

5.6636 1

0.028318 1

0.85 cords of logs or 67 ft^ of solid wood. One

unit of Douglas-fir or western hemlock sawdust is

about 80 ft^ of solid wood.

2.6 WOOD CHEMISTRY

The composition of hardwoods and softwoods

by the class of compounds is given in Table 2-3.

The ultimate (elemental) analysis of wood is given

in Table 2-4.

Lignin is more highly concentrated in the

middle lamella and primary cell wall regions of

the wood fiber than any other part of the cell wall.

Most of the lignin, however, is actually in the

secondary cell wall since the secondary cell wall

accounts for most of the mass of the fiber. The

concentration of the major components with

varying cell wall position is shown in Fig. 2-21.

Cellulose

On the molecular level, cellulose is a linear

polymer of anhydro-D-glucose connected by

iS-(1^4)-linkages as shown in Fig. 2-22. The

degree of polymerization (DP), which is the num-

ber of units (glucose in this case) that make up the

polymer, is above 10,000 in unaltered (so-called

"native") wood, but less than 1000 in highly

bleached kraft pulps.

Table 2-3. Typical compositions

American woods (percent).

of North

Cellulose

Hemicelluloses

(Galacto)glucomannans

Xylans

Lignin

Extractives

Ash

Hardwoods

40-50

2-5

15-30

18-25

1-5

0.4-0.8

Softwoods

45-50

20-25

5-10

25-35

3-8

0.2-0.5

Table 2-4. The ultimate analysis of North

American woods in percent.

Carbon

Oxygen

Hydrogen

Nitrogen

49.0-50.5

43.5-44.5

5.8- 6.1

0.2- 0.5

Physically, cellulose is a white solid material

that may exist in crystalline or amorphous states.

Cellulose in wood is about 50-70% crystalline and

forms the "back-bone" structure of a wood fiber.

WOOD CHEMISTRY 33

Relative

oven-dry

mass,

Middle Lamena

Primary Wall

Secondary Cell Wall

Fig. 2-21. Typical composition of a conifer fiber across its cell wall. From Krahmer, R.L. and

Van Vliet, A.C., Eds., Wood Technology and Utilization, O.S.U. Bookstores, Corvallis, Oregon

1983.

Celloblose unit "

Fig. 2-22. The primary structure of cellulose.

Cotton is about 95% crystalline cellulose. The

crystalline form of cellulose is particularly resis-

tant to chemical attack and degradation. Hydrogen

bonding between cellulose molecules results in the

high strength of cellulose fibers.

Microfibrils are aggregations of cellulose

molecules into thread-like structures approximate-

ly 3.5 nm in diameter, containing both crystalline

and amorphous regions. Microfibrils occur in the

secondary cell wall. Microfibrils are oriented in

different directions in each of the three layers

within the secondary cell wall; the fibril angle is

measured from the longitudinal axis of the cell.

Hemicellulose(s)

Hemicellulose(s) are actually a class of

mate-

rials.

The plural form should be used to describe

them generically, but the singular form should be

used to describe a particular type such as the

hardwood xylan hemicellulose. Physically,

hemicelluloses are white solid materials that are

rarely crystalline or fibrous in nature; they form

some of the "flesh" that helps fill out the fiber.

Hemicelluloses increase the strength of paper

(especially tensile, burst, and fold) and the pulp

yield, but are not desired in dissolving pulps.

(Dissolving pulps are relatively pure forms of

cellulose used to make cellulose-based plastics.)

Starch is often added to pulp to increase the

strength of paper and probably has a very similar

mechanism of effect as the hemicelluloses.

Hemicelluloses chemically are a class of

polymers of sugars including the six-carbon sugars

mannose, galactose, glucose, and 4-0-methyl-

D-glucuronic acid and the five-carbon sugars

xylose and arabinose. The structures of these