Benefits and drawbacks of mechanical
• + The material yield (pulp/wood) of
chemical pulping is 40-50% when in
mechanical pulping it can be 85-95%.
• + Low cost
• + High ink absorbency, compressibility,
opacity, and bulk.
• + Heat recovered can be used in paper
• - Low strength, low permanence,
tendency to yellow with time (primarily
caused by high levels of lignin).
• - Paper made with mechanical pulps
also contain shives, or incompletely
ground fiber bundles.
• - Needs a lot of electricity
Mechanical pulping methods
• Mechanical pulp is the oldest pulping process. New mechanical
pulping methods were developed to reduce disadvantages without
compromising the advantages of mechanical pulping.
• Thermomechanical pulping (TMP) operates under higher temperature
and pressure that further soften the lignin to make fiber separation
easier. Thermomechanical pulp is stronger than mechanical pulp, and
still retains the high-yield of mechanical pulps.
• Chemi-thermomechanical pulps use mild chemicals to increase pulp
brightness and reduce shive content.
• Nowadays pulps are custom made for specific end uses by affecting
the degree of refining, chemical treatments of chips, and peroxide
Mechanical pulping methods
• Some terminology
• TMP Thermomechanical pulp
• CTMP Chemithermomechanical pulp
• GW Groundwood pulp
• Stone groundwood (SGW) pulp: small logs are ground against
artificial bonded stones, high yield, short fibers-> weak
• Refiner mechanical pulping (RMP): similar to SGW, but can use
chips longer fibers and stronger than SGW
• PGW Pressurized groundwood pulp
• NSSC Neutral sulfite semichemical pulp
• CMP Chemimechanical pulp
Chipping and chip storage
• After barking chips are made
• Important for pulp quality
• Chips should be homogenous
• Screened chips are then wetted
and heated rapidly in the
• Short storing time
• Chips dry fast (high
temperature in storage)
• accelerates discoloration and
decay of the wood
• Sand, pieces of metal, etc.
carried along with the wood
can accelerate the wear on
the refiner discs.
• The chips are washed with
Steam treatment of chips
• Chips are moistened and heated (+
chemical treated in CTMP) in
preheater, which causes the lignin
bond between the fibers to soften.
• In CTMP chips are given a 20-30
minute steaming, followed by
impregnation with sodium sulfite
• The quantity of sodium sulfite,
reaction time and temperature can
be used to adjust pulp properties. A
typical sulfite dosage is 2-3% of the
• The chips are defiberized and refined to
finished pulp in a refiner. This takes
usually place in two stages.
• The wood chips are fed between the
refiner discs of refiner. At least one of
the discs rotates at a speed of 1500 to
2300 rpm. The wood fibers are
separated as a result of the action of
• A majority of the electricity used by the
refiners is converted to steam by the
shear forces the pulp is exposed to.
• Steam is sent to heat recovery to
generate fresh steam and used. e.g. in
pulp drying or in a paper machine.
• Todays refiners are almost always
• Advantages include reduced
volumetric steam flow and
improved stability of the refiner
load easier production of
• Largest single refiners operate at a
rating of about 40 MW.
• Refining to finished pulp usually
takes two stages arranged in
series. A reject refiner is used for
refine fiber bundles that pass main
• Lot of steam is produced in the
making of mechanical pulp.
• Recovering the heat formed in the
process is essential for plant
• The most of the power used is
spent on vaporizing of water in the
refiner and about ton of steam is
generated per MWh of the energy
• The steam is first separated from
the fibers and then condensed in
the reboiler to raise clean steam.
• Removal of fiber curling, which
occurs at high-consistency
refining, by means of mixing
the pulp a certain period of
time at a lower consistency (2
to 4 %) and a temperature of
70 to 80 0C.
Screening and rejects treatment
• After refining, the pulp always
contains shives and coarse
fibers, which must be treated
according to the requirements
of each paper grade.
• Brightness (57 to 64 %) of mechanical pulp is
very near the brightness of the wood material
OK for newsprint
• Bleaching needed if higher demands on
brightness is required.
• Peroxide is the most common chemical used for
• Pulp is bleached usually in two stages, first in
medium consistency and latter in high
• Depending on its intended use, CTMP can be
bleached to a brightness of as much as 80%. The
maximum brightness for aspen CTMP is 85%.
• As most of the lignin still stays in the pulp (to
maintain high yield), the whiteness achieved in
bleaching is temporary as exposure to air and
light can turn paper to yellow.
Rejects vs production
0 2 4 6 8 10 12
Case TMP energy efficiency analysis
Data ja efficiency metric
• TMP-data from mill:
• Year 2017, 8760 h
• ~ 200 parameters
• 15 parameters were chosen for more in detail analysis
• The energy efficiency metric was theoretic heat recovery potential
• The temperature of the incoming district heating flow was not studied
(untypically in this mill the steam from refiners didn’t go to the paper mill,
but into heating of district heat)
Case TMP: Initial results
• From the data a clear reason for the variation of energy efficiency was
• One of the refiners produced substantially less steam than the other refiners
energy efficiency decreased
• Following studies:
• Why did the temperature of district heating flow coming into the TMP process
varied so much
• Why was the steam production of one of the refiners so much less than the
• What improvements and/or investments are needed in order for the faulty refiner
to achieve the same steam production as the other refiners?
• If there is no chance to affect the district heating temperature, where else could
the steam from the refiners be used?
• In cases where pulp mill is integrated with a
paper mill, pulp is pumped through a pipe
from the pulp mill
• For a non-integrated mill, pulp is delivered in
the form of pulp bales, which must be
disintegrated (slushed) in a bale pulper.
• Fibers are the most important component of
paper and board.
• Properties of wood fibers vary to great extent
in different pulp types, and the use of
mechanical, chemi-mechanical, chemical and
recycled pulps is very much subject to paper
and paperboard grades.
• Other raw materials are fillers, adhesives,
chemical additives and coating agents.
• The main grades of magazine paper are wood-containing, light-weight coated
paper (LWC) and supercalendered or calendered, but uncoated paper (SC).
• Special papers include papers processed as household and sanitary paper,
wallpaper base paper, label paper, sack paper, wrapping paper, packaging paper,
envelope paper, flexible packaging paper or industrial papers. Insulation paper,
such as cable paper, is used to insulate electric cables.
• Fine paper is made of pulp which generally contains no more than one tenth of
mechanical pulp. Good-quality printing and writing papers and copier papers are
examples of fine papers.
• Hygiene papers are soft grades called tissues – such as toilet and kitchen paper
and hand or facial towels – which have a good absorption ability, disintegrate
rapidly in water and are recyclable as biomass.
• Wood-free pulp or paper is a product whose raw material does not include
PRODUCTS: Printing and fine papers
• Printing paper grades are manufactured
in the largest possible production units
using the most economic possible raw
• The main raw material is mechanical
pulp; for newsprint also recycled fibre pulp
• A typical basis weight range of paper is
from 35 to 55 g/m2, the production speed
from 1200 to 1600 m/min, and the web
width exceeds eight meters.
• Fine paper machines also belong to the
same size class as the machines for wood
• A typical production speed range is from
1000 to 1300 m/min and the basis weight
range from 50 to 90 g/m2 .
PRODUCTS: Special papers
• In the production of fine and special
paper grades chemical pulps are mainly
used, which are more expensive than
• Production output of special papers are
normally small; a lot of flexibility is
required from the production line and
the machines are generally slower and
• The basis weight range of special
papers is extremely wide varying
between 40 and 250 g/m2 .
• The running speeds range from 400 to
1100 m/min and the web width from
three to eight meters.
• Packaging boards include: folding boxboard,
white lined chipboard, liquid packaging board,
solid bleached sulfate board, carrier board; and
container boards: kraftliner, testliner, fluting
• The number of special board grades is very
high, but production volumes (compared to the
previously mentioned grades) rather small.
• Production of board machines are usually very
high because of the high basis weight. For
packaging box board the basis weight range is
between 170 and 450 g/m2, and for liner board
between 100 to 330 g/m2
• Packaging boards are normally formed in three or
• Due to high surface quality requirements the
surface ply is produced from chemical pulps
and in most cases is double coated.
• For the centre ply, less expensive mechanical
pulp is used
PROCESSES: Stock preparation
Stock preparation is located
between the pulp mill & the
• Integrated pulp mills do not
require treatment & pulping
system for bales, but pipe
stock is refined in accordance
with the paper grade.
• For a non-integrated mill,
bales are disintegrated in a
PROCESSES: Web forming
• The web forming section on a paper
• the headbox approach piping
• the headbox and
• the forming section.
• The tasks of the headbox and the
approach piping are:
• Uniform distribution of stock suspension
• Stabilize pressure variations and pulsation
in the infeed flow
• Produce suitable turbulence level in the
stock suspension for fibre floc dispersal
• Produce a stock suspension jet in the
forming section with a desired consistency
(typically from about 0.5 to 1.0 %), speed
PROCESSES: Press section
• The function of the press section is
• remove maximum amount of water
from the web and to compress it, and
• to achieve sufficiently high wet
strength to ensure that the web is
transferred to the dryer section
without any breaks.
• The dry content of the web is 17 -
20% in the beginning, and 35 - 50 %
after the press, depending on the
paper grade and press design.
• 1 % increase in dry content at the
press will diminish the dryer-section
steam consumption by 3-4 %.
• Raising the web dry content to
50 % is economically and
technically possible by using
methods (centrifugal force &
under-pressure at the wire
section, pressing at the press
• Water is then removed by
evaporation, in order to reach
the desired final dry content.
Factors contributing to drying
section energy economy
• There are three different methods normally
applied to paper and board drying:
• contact or cylinder-drying
• radiation drying
• Common to all these methods is that the web
is provided with external energy that
evaporates water away from the web.
• Then evaporated water exits the dryer with
• Main difference in drying methods is the way
energy is supplied, which leads to different
type of equipment being used.
• The heat transfer of air dryers is
performed by blowing air through
nozzles against the web.
• Drying is performed by means of
convection in the air dryer, allowing
hot air jets to be discharged against
the paper web surface .
• The web is supported and stabilized
by blowing air so as not to let the
dryer parts touch the web. The air
dryer's air circulation system
circulates the impingement air.
• The exhaust air system ensures low
moisture & heat loads for the
• Thermal radiation serves as the
• When radiation encounters
another material layer, part of it
penetrates it, part is reflected
back and part is absorbed into
the layer itself.
• This absorbed part of radiation is
utilized for infrared drying. The
radiating surface can be heated
either electrically or with the
combustion flame of gas.
• Finishing after paper or board
machine depends on grade
requirements. For example, the
product can be:
• Surface sized. Surface sizing is
common with fine papers, coated raw
• Coated. For example, printing papers
(LWC, MWC) and some boards.
• Calendered. Uncoated SC grades are
finished by calendering.
• Coated with plastic and aluminum
extrusion. For example some liquid
• The paper machine needs around 2500 – 5500 MJ thermal
energy and around 1800 – 2200 MJ electrical energy for
producing one ton of paper, depending on the grade,
machine type and process energy efficiency.
• Nearly all the energy removed from the dryer section is
transferred along with the exhaust air
• Main purpose of heat recovery
system is to replace primary energy
with recovered heat from the process
in an economically profitable way.
• Total heat recovery in a modern
paper or board machine can be over
• This is equivalent to 86 t/h of steam,
therefore the energy flows are
• It is important to utilize this energy
efficiently. Heat recovery solutions
provide excellent possibilities for
optimizing the energy and air flows in
paper and board making.