pulp & paper reference book

Promotion of Benchmarking Tools for Energy Conservation in Energy Intensive Industries in China

Pulp and Paper

A reference book for the Industry

Pulp and Paper Industry – A Reference Book for the Industry

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Imprint

Contract: Promotion of Benchmarking Tools for Energy Conservation in Energy Intensive Industries in China

Contract No.: EuropeAid/123870/D/SER/CN

Contractor: The Administrative Centre for China’s Agenda 21 (ACCA21)

Room 609, No. 8 Yuyuantan South Road, Haidian District, Beijing, P.R. China, Postal Code: 100038

Partners: CENTRIC AUSTRIA INTERNATIONAL (CAI)

Beijing Energy Conservation & Environment Protection Center (BEEC)

Disclaimer

This publication has been produced within the frame of the EU-China Energy and

Environment Programme project “Promotion of Benchmarking Tools for Energy

Conservation in Energy Intensive Industries in China”. The EU-China Energy and

Environment Programme (EEP) was established to correspond to the policies of the

Chinese Government and the European Commission to strengthen the EU-China co-

operation in the area of energy. The project was formally started on the 1. September

2008. The total duration is 12 months. The contents of this publication are the sole

responsibility of the project team and can in no way be taken to reflect the views of

the European Union.

Beijing, 2009


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Table of Contents

Summary and Acknowledgments.............................................................................. 4

1 Introduction ............................................................................................................ 6

1.1 Pulp and Paper ............................................................................................. 6

1.2 History of papermaking................................................................................ 7

1.3 Worldwide production - reference to China ......................................... 11

1.4 Pulp and paper manufacturing - Overview........................................... 12

1.5 Categories of pulp and paper mills......................................................... 13

2 Non-wood pulping .............................................................................................. 15

2.1 Non-wood materials and processes........................................................ 15

2.2 International best practice ....................................................................... 17

2.3 Energy use and energy benchmarks ...................................................... 18

2.4 Energy saving measures ............................................................................ 18

3 Kraft pulping ......................................................................................................... 19

3.1 Process flow.................................................................................................. 19

3.2 Consumptions and emissions.................................................................... 22



4 Sulfite pulping ....................................................................................................... 27

4.1 Processes ...................................................................................................... 27

4.2 Consumptions and emissions.................................................................... 29



5 Mechanical pulping............................................................................................ 32

5.1 Processes ...................................................................................................... 32



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6 Recycled fibre mills.............................................................................................. 36

6.1 Processes ...................................................................................................... 36



7 Papermaking ........................................................................................................ 41

7.1 Processes - Overview.................................................................................. 41

7.2 Stock preparation ....................................................................................... 42

7.3 Paper Machine............................................................................................ 43



8 Integrated pulp and paper mills ....................................................................... 53

8.1 Processes ...................................................................................................... 53



9 Worldbest practice and energy benchmarks................................................ 54

10 Model case benchmarks............................................................................... 57

11 References and Links...................................................................................... 59


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Summary and Acknowledgments

This reference book for the pulp and paper industry is a compilation of best available

technologies methodologies and future development in this industry. A very useful

source for this work was the “Reference Document on Best Available Techniques in

the Pulp and Paper Industries (BREF)” published by the Integrated Pollution

Prevention and Control (IPPC) Board of the European Commission (2001). Particular

reference is also made to the model case study on “Energy and Cost Reduction in

the Pulp and Paper industry – A Benchmarking Perspective (2002)”. Done by the

Pulp and Paper Technical Association of Canada, which is one of the leading

institutes for this sector. These documents and a number of further links can be

downloaded from the internet for free (see references and links).

As part of the BMT-Tool set (BMT = Benchmarking – Monitoring – Targeting) this

reference book provides sector specific information regarding the pulp and paper

industry in general, frequently used technologies, energy consumption of key

processes and other relevant aspects connected with the energy and environment

performance of paper making. Energy benchmarks are discussed as typical ranges

of energy consumption (MJ or kg standard coal) per production unit (ton of pulp or

paper). References are made to world best performances. A larger part of this

reference book is dedicated to describing the ample options for energy improvements

which exist for this industry, even within relatively modern and advanced plants.

The report is divided into a number of Chapters as follows:

• The first chapter provides an overview of pulp and paper production worldwide

and the increasing dominant role of China in this context; to enhance the readers

background a historical discourse of papermaking was inserted.

• The chapters two to eight provide detailed discussions of the frequently used

process lines of pulp and paper making; starting with non-wood pulping as a

special feature to Chinese prospects (chapter 2), followed by several more

conventional technologies of pulping such as Kraft pulping (chapter 3), Sulfite

pulping (chapter 4), Mechanical pulping (chapter 5), Recycled fiber mills (chapter

6); and completed by the distinct process of Paper making in stand-alone paper

mills (chapter 7) and a summary regarding Integrated pulp and paper mills

(chapter 8). Each chapter is structured in a description of the relevant processes,


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and the related use of energy. Where applicable and available energy

benchmarks and energy saving measures are introduced.

• Chapter 9 provides an overview of selected energy intensity values drawn from

worldbest practices.

• Chapter 10 invites to further studying and reading along existing case studies and

a huge amount of references.

• Chapter 11 provides a number of selected links. The later include also

discussions of further important topics, which could not handled within the frame

of this reference book such as papermaking additives and chemicals, water

consumption and saving, and numerous further details to the various production

steps of pulp and paper.

Finally, it has to be noted that this reference book cannot answer all questions

related with energy efficiency/intensity in the pulp and paper industry. In fact, the

paper industry could be generally described as energy intensive. In Europe energy is

the third highest cost in the papermaking process, accounting for approximately 8%

of turnover. This is inconsistent with the fact that less is published on specific energy

requirement on process level than on water management for instance. Therefore, it is

rather difficult to find qualified information on energy consumption related to different

paper grades and product qualities, energy efficient technologies, and energy

practices and usage within the pulp and paper industry.

In many countries, information on energy balances of paper mills is poorly available

in public. Different reporting schemes, if any, for energy consumption are used.

Energy demand also depends on the product quality (especially in tissue mills) and

partly on local conditions. Therefore, it is difficult to present energy consumption

values associated with the use of BAT. The ranges of energy consumption of paper

mills shown in this reference book should only be taken as an indication about the

approximate need of process heat and power at energy efficient paper mills.

As it is in life, also for the pulp and paper industry and the specific operation, the truth

cannot be found in books but only in the practice by doing and self critic. Again it can

be found that besides external benchmarking the internal procedures of monitoring

and targeting are of prior importance. It is hoped that this reference book will be of

some guide towards this prospect.


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1 Introduction

1.1 Pulp and Paper

Papermaking today is a large, capital-intensive industry, characterized by high-speed

machines and complex systems of control for manufacturing to close tolerances

thousands of products vital to education, communications, marketing, packaging,

construction, etc.

The pulp and paper industry comprises manufacturing enterprises that convert

cellulose fibre into a wide variety of pulps, papers and paperboards, such as:

• Newsprint

• Packaging paper boards

• Uncoated printing and writing papers

• Liner and fluting

• Coated printing and writing papers

• Tissue

• Packaging papers

• Speciality papers

Each of these categories demands specific properties of the product and the most

appropriate manufacturing route to these products may differ substantially. For

instance, newsprint is a product required in high volume on a regular basis but is only

required to have moderate strength, opacity, printability and a relatively short life.

Thus a manufacturing route which involves a high yield of pulp at the expense of

maximum achievable strength, brightness and texture can contribute to the efficient

use of raw materials.

In contrast, the critical quality of packaging papers is their strength if they are to be fit

for their intended use. In this case it is necessary to accept a lower yield inherent to a

different manufacturing route in order to achieve this strength. Printing and writing

papers need a different balance of brightness, texture and strength, and some can be

required to last for great many years. Tissue papers are made to have good dry and


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wet strength for their weight and typically will be used once and not re-enter the fibre

cycle.

The raw material comes from wood (up to 95 % in Europe and North America) or

from straw and other plants residues, wastepaper and a very small quantity of linen

and cotton rags. The raw materials, e.g. wood, are reduced to fibre by mechanical

means or by cooking in chemicals. The fibres are then mixed with water, adhering to

one another as the water is removed by pressure and heat. This is the fundamental

principle of papermaking, discovered by the Chinese nearly 2000 years ago and

brought to Spain by the Moors, probably during the 12th century.

1.2 History of papermaking

Paper has a long history, beginning with the ancient Egyptians and continuing to the

present day. For thousands of years, hand-made methods dominated and then,

during the 19th century, paper production became industrialised. Originally intended

purely for writing and printing purposes, a wide variety of paper grades and uses are

now available to the consumer.

Of all the writing and drawing materials that people have employed down the ages

(since 3000 B.C.), paper is the most widely used around the world. Its name derives

from papyrus the material used by the ancient Egyptians, Greeks and Romans.

Papyrus, however, is only one of the predecessors of paper that together are known

by the generic term ‘tapa’ and are mostly made from the inner bark of paper

mulberry, fig and daphne. The fibres normally used for textiles, like flax and hemp,

also served as substitutes for bast. In later times, the fabric was replaced by fine

bamboo sticks, which freed the papermaker of the need to let the paper dry naturally

in the mould, since the poured or ladled sheet could be ‘couched’ off.

In AD 105, the Chinese court official, Ts'ai Lun, invented papermaking from textile

waste using rags. This can be considered as the birth of paper as we know it today.

Later, Chinese papermakers developed a number of specialities such as sized,

coated and dyed paper, and paper protected against ravages by insects, but they

had great problems satisfying the growing demand for paper for governmental

administration. They also used a new fibre-yielding plant - bamboo - which they de-

fibred by cooking in lye.


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Chinese papermaking techniques reached Korea at an early date and were

introduced to Japan in the year 610. In these two countries, paper is still made by

hand on a large scale in the old tradition, preferably from the fresh bast fibres of the

mulberry tree (kozo in Japanese). Following the cooking process, the long, uncut

fibres are merely prepared by beating, which gives the paper its characteristic look

and excellent quality. The latter is due, among other things, to multiple, rapid

immersions of the mould, which results in a multi-layer fibre mat.

Very soon, knowledge of papermaking spread to Central Asia and Tibet and then on

to India. When the Arabs, in the course of their eastern expansion, neared Samarkan

they too became acquainted with the production of paper and paper mills were

subsequently set up in Baghdad, Damascus and Cairo, and later in Morocco, Spain

and Sicily. Owing to the lack of fresh fibres, the raw material used by the Arabs was

made almost entirely from rags: however, their defective and poorly designed

processing equipment (such as breaker mills) produced a rather inferior ground pulp.

But, by using this method, with screens made of reeds, thin sheets were made and

then ‘coated’ with starch paste. This gave Arabian paper its good writing properties

and fine appearance.

The export of Arabian-made paper, along with the secrets of its production, to

Europe, especially to Italy, has been well documented. From the 13th century

onwards, papermakers at two early Italian centres, Fabriano and Amalfi, tried to

improve the Arabian technique. Their efforts focused not on the raw material but on

its preparation and the actual papermaking process was improved. The Italian

papermakers developed: the use of water power; the stamping mill (derived from the

stampers and milling machines used in textile handicrafts); the mould made of wire

mesh (as a result of progress in wire production), which triggered the introduction of

couching on felt; the paper press (screw press) with slides for feeding in the material;

drying the sheets on ropes; dip sizing.

In the course of the rapid expansion of trade in the late Middle Ages, more and more

merchants dealt in the commodity called ‘paper’ that was growing in importance for

public and intellectual life. The Nuremberg councillor Ulmann Stromer (Stromeir)

mulled over the advantages of making his own paper and, with the help of skilled

workers from Italy, transformed the ‘Gleismühle’ by the gates of his home town into a

paper mill. The dates noted in his diary, 24 June 1390 (start of work on the

waterwheel) and 7 and 11 August 1390 (oaths sworn by his Nuremberg foremen),

are the first assured records of papermaking on German soil.


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The advantages of this mill-based papermaking technique, which spread throughout

Europe in the 15th and 16th centuries far outweighed the disadvantage of

considerable outlays of time and capital for building and fitting out with new

machinery and equipment. However, the change in the production process, thanks to

the division of labour, boosted output and improved quality. And it could certainly

generate a profit, as some examples prove. On the other hand, there was a growing

risk of an imbalance between costs and earnings, a state of affairs noted in the

numerous reports of business failures among papermakers.

The tremendous upsurge in papermaking during the Reformation in the 16th century,

coupled with the introduction of printing with movable type, soon led to a serious

shortage of raw materials and to regulations governing the trade in rags. The

systematic search for substitute materials met with little immediate success. In the

early 18th century straw was certainly used as a raw material but failed to make

headway on quality grounds. Only the invention of groundwood pulp by Saxon Keller

(1843) and of chemical pulp (first patented in 1854 by Mellier Watt) solved this

problem.

During the 18th century there had been some concentration of craft activities in large

operations, the ‘manufactories’, which were dependent on skilled papermakers

organised into craft groups. The efforts made to step up production as much as

possible and to have many of the jobs done by machine (partly to get round the

constraining rules of papermakers' craft ‘usages’) culminated in the design and

construction of paper making machines. The initial model was the vat that was used

by J.N.L. Robert, who built the first flat-screen papermaking machine in 1798. This

was further developed in England, mostly by Donking and the Fourdrinier brothers.

Shortly afterwards other types appeared, like the Dickinson’s cylinder machine, and

machines which filled wire moulds transported on an endless chain and couched the

sheets on an continuous felt. Flat screen and cylinder machines, which were first

seen in the 19th century, were continually improved and extended to include a dryer

section. This soon led to a considerable widening of the paper web and to an

increase in production speeds. It also heralded industrialisation.

The history of the paper industry in the 19th and 20th centuries can be broken down

into five partly overlapping periods, each marked by definite trends. In the first stage

(from about 1800 to 1860), all work sequences previously performed by hand were

mechanised. This included the rag preparation, the use of fillers, pulp beating, the

paper machine with its various parts, and the machines required for finishing the


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paper (the headbox, wire section, press section, dryer section, units for reeling,

smoothing and packaging). During the second stage (about 1840 to 1880), efforts

were made to obtain rag substitutes on an industrial scale (groundwood pulp and

chemical pulp) and appropriate industrial plants (groundwood and chemical pulp

mills) were developed. The third stage (1860 to 1950) was marked by the

enlargement of the web width, an increase in working speeds, the introduction of

electric drive and further improvements to various machine parts. Machines designed

specifically for the production of particular paper and board grades (for example the

Yankee cylinder and multi-cylinder machines) were also developed. The web working

width grew from 85 cm (1830) to 770 cm (1930), while production speeds rose from 5

m/min. (1820) to over 500 m/min. (1930). The fourth stage (1950 to 1980), which was

still dependent on the old methods as far as the mechanics were concerned, brought

unprecedented changes in papermaking. Alongside further increases in web width

and working speeds, there was the use of new materials (thermo-mechanical pulp,

deinked recovered paper, new fillers, processed chemicals and dyes), new sheet

forming options (e.g. by twin-wire formers), neutral sizing, greater stress on ecology

(closed loops) and, most of all, automation. The operational impact of these changes

was: specialisation in certain paper types; development of new paper grades (LWC -

lightweight coated paper); corporate mergers; company groups with their own raw

material supply and trading organisations; closure of unprofitable operations. 1980

onwards, the fifth stage leads into the future. The evolution of new sheet-forming

principles (with fluid boundaries between paper and non-woven fabrics) and chemical

pulp processes have been the main process improvements. However, the situation

on the global market (increased demand, above all in the Third World, trends in

chemical pulp prices, problems of location), are again raising capital intensity and

encouraging the formation of big company groups with international operations. At

the same time there are definite opportunities for smaller, local firms satisfying

specific needs.

The new Millennium is dominated by the tremendous progress that has been made in

computer science, thus triggering a complete change in our commercial and private

communication and information behaviour. Does this mean that the paper era will

come to an end? The answer is most definitely "No". Clearly there will be a huge

amount of data being generated electronically, but the issue is how to preserve it.

The difficulties of data storage over a long period of time are well known (for

example, the durability of disks; frequent changes of hard and software, electronic

breakdowns etc.). Once again, paper offers the most convenient and durable storage


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option. The advance in technology will affect only the printing of items like short-lived

handbooks and encyclopaedias. Reading a book will remain a great pleasure into the

future and paper, as a ubiquitous material with its many uses, will continue to play an

influential role. Many artists will continue to express themselves by using this most

versatile material.

1.3 Worldwide production - reference to China

The global pulp and paper industry is dominated by North American (United States,

Canada), northern European (Finland, Sweden) and East Asian countries.

Australasia and Latin America also have significant pulp and paper industries. The

Asian region accounts for around 31% of the world’s consumption of paper. In Asia

Japan is the highest consumer of paper products per capita, followed by Singapore

and Malaysia. But both India and China are expected to be keys in the industry's

growth over the next few years. Accounting for over 50 % of the world’s overall

growth in paper and paperboard production since 1990, China is now the second

largest producer globally, surpassed only by the United States. In 2007 worldwide

paper production was 395 million tons. In this connection the USA was leading,

followed by China, Japan and Germany.

China’s unprecedented economic growth over the last 15 years has led to a sharp

increase in demand for paper and paperboard products. During this period, the

country’s aggregate consumption of paper and paperboard has grown by 9.6 nearly

10 % per year, rising from 14.6 million tonnes in 1990 to 48.0 million tonnes in 2003.

To meet this demand, domestic production of paper and paperboard has grown at a

similar pace, expanding from 13.7 million tonnes in 1990 to 43.0 million tonnes in

2003.

Baseline projections suggest that China’s aggregate demand will grow from 48.0

million tonnes in 2003 to 68.5 million tonnes per year in 2010. With domestic

production projected to reach 62.4 million tonnes per year, China is expected to

dominate global capacity expansion for most major grades. China’s annual demand

for fiber furnish across all grades is projected to rise from 40.2 million tonnes in 2003

to reach 59.6 million tonnes by 2010. Of this, approximately 58 % will come from

recovered paper, 25 % from wood-based pulp, and 17 % from nonwood pulp. This


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rapid growth has far-reaching implications for forest sustainability and rural

livelihoods both within China and throughout the Asia-Pacific region. It will place new

strains on China’s domestic wood supply and may exacerbate forest conversion and

illegal logging in key supplier countries, in addition to providing both threats and

potential income opportunities for smallholder tree growers.

Consequently there is a trend towards the production of homegrown, waste based

material rather than imported virgin fiber kraftliner.

China’s paper industry is already unique in that; it is one of the largest users of non-

wood fibers. In fact, the share of wood fiber used in China has declined since the

1990s to about 7% of the input of the paper production, with recovered paper

representing 36%, and the remaining 57% covered by imported waste paper and

nonwood fiber. In the late 1990s there were over 5,000 pulp and paper mills in China,

of which over 70% used non-wood fibers, mainly straw. Most non-wood fiber mills are

small scale. In 1998 there were only 43 non-wood mills with a capacity exceeding

30000 t/year, and the vast majority produced less than 10000, or even 5000 t/year.

Since 2000, the Chinese government has started to close down the small polluting

and inefficient mills. In recent years modern large-scale paper machines have been

installed in China (e.g. Hebei Norske Skog Long 300,000 t/year plant, Dagang’s

paper machine 3 with a capacity of 1.1 million t/year). Non-wood fibers are expected

to continue to play an important role in China’s future paper industry.

1.4 Pulp and paper manufacturing - Overview

Paper is essentially a sheet of cellulose fibres with a number of added constituents to

affect the quality of the sheet and its fitness for intended end use. The two terms of

paper and board generally refer to the weight of the product sheet (grammage) with

paper ranging up to about 150 g/m2 and a heavier sheet regarded as board

(paperboard).

The pulp for papermaking may be produced from virgin fibre by chemical or

mechanical means or may be produced by the re-pulping of recovered paper (RCF).

In the pulping process the raw cellulose-bearing material is broken down into its

individual fibres. Wood is the main raw material but straw, hemp, grass, cotton and

other cellulose-bearing material can be used. The precise composition of raw


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material will vary according to the type and species but the most important

constituents are cellulose, hemicelluloses and lignin.

Wood naturally contains around 50% water and the solid fraction is typically about

45% cellulose, 25 % hemicelluloses and 25% lignin and 5% other organic and

inorganic materials. In chemical pulping, chemicals are used to dissolve the lignin

and free the fibres. The lignin and many other organic substances are thus put into

solution from which the chemicals and the energy content of the lignin and other

organics may be recovered. The extent of this recovery is dependent upon the

chemical base used and the process configuration. In mechanical pulping processes

mechanical shear forces are used to pull the fibres apart and the majority of the lignin

remains with the fibres although there is still dissolution of some organics.

Pulps produced in different ways have different properties, which make them suited

to particular products. Most pulp is produced for the purpose of subsequent

manufacture of paper or paperboard. Some is destined for other uses such as thick

fibreboard or products manufactured from dissolved cellulose.

Paper produced by the use of recovered paper as fibre source will involve some

cleaning of contaminants prior to use and may involve de-inking depending upon the

quality of material recycled and the requirements of the end product the recycling

process. The fibres are reusable a number of times depending on the quality of the

recycled material and the purpose of the end product. The paper product may also

comprise up to 45% of its weight in fillers, coatings and other substances.

1.5 Categories of pulp and paper mills

As pulp and paper products are highly diverse and applied processes even for one

and the same product may vary greatly, many factors of production technology must

be taken into account to guarantee a high level of environmental protection. The best

techniques for the pulp and paper industry cannot be defined solely by describing

unit processes. Instead, the whole installations must be examined and dealt with as

entities

The process of pulp and papermaking consists of quite many stages. Besides the

fibrous material different chemicals and a great amount of water and energy in the


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form of steam, fuel oil or electric power is required in the process. The wide range of

processes involved in the manufacture of pulp and paper can be broken down into a

number of unit operations for the sake of discussion. A sequence of operations can

be described from raw materials to product but individual processes will not involve

all the operations and some are mutually exclusive alternatives.

The following chapters discuss the main processes applied for pulp and paper

making, such as:

• Non-wood pulping

• Kraft pulping

• Sulfite pulping

• Mechanical pulping

• Recycled fibre mills

• Papermaking

• Integrated pulp and paper mills


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2 Non-wood pulping

The pulp and paper industry converts fibrous raw materials into pulp, paper, and

paperboard. The processes involved in papermaking include raw materials

preparation, pulping (chemical, semi-chemical, mechanical, or waste paper),

bleaching, chemical recovery, pulp drying, and papermaking. The most significant

energy-consuming processes are pulping and drying. Globally, wood is the main fiber

source in the paper industry and most mills are quite large, producing over 300,000

t/year for typical paper mills.

China’s paper industry is already unique in that; it is one of the largest users of non-

wood fibers. In fact, the share of wood fiber used in China has declined since the

1990s to about 7% of the input of the paper production, with recovered paper

representing 36%, and the remaining 57% covered by imported waste paper and

nonwood fiber. In the late 1990s there were over 5,000 pulp and paper mills in China,

of which over 70% used non-wood fibers, mainly straw. Most non-wood fiber mills are

small scale. In 1998 there were only 43 non-wood mills with a capacity exceeding

30000 t/year, and the vast majority produced less than 10000, or even 5000 t/year.

Since 2000, the Chinese government has started to close down the small polluting

and inefficient mills. In recent years modern large-scale paper machines have been

installed in China (e.g. Hebei Norske Skog Long 300,000 t/year plant, Dagang’s

paper machine 3 with a capacity of 1.1 million t/year). Non-wood fibers are expected

to continue to play an important role in China’s future paper industry.

2.1 Non-wood materials and processes

It is well known that due to a plentiful supply and reasonable costs, the economics of

pulp and paper production in Europe and North America, and in most other industrial

countries, have favored wood as the fibrous raw material, in the past and up to the

present time. However, many of the developing countries, as well as a few of the

industrial countries, do not have adequate supplies of wood, but they do have large

quantities of non-wood plant fibers available. Fortunately, by choosing the proper

blend of non-wood plant fibers, almost all grades of paper and paperboard—ranging


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from tissue to linerboard, including newsprint—can be produced with as much as

100% fibrous content of these materials. Furthermore, every type of reconstituted

panelboard - ranging from insulating board to hardboard, including medium-density

fiberboard - can likewise be produced, and is being produced, from non-wood plant

fibers.

Generally, nonwood plant fiber pulps can be grouped into two broad categories:

• Common nonwoods or hardwood substitutes such as cereal straws,

sugarcane bagasse, bamboo (shorter fiber species), reeds and grasses,

esparto, kenaf (whole stalk or core fiber), corn stalks, sorghum stalks etc.

• Specialty nonwoods or softwood substitutes such as cotton staple and linters;

flax, hemp and kenaf bast fibers; sisal; abaca; bamboo (longer fiber species);

hesperaloe etc.

As with wood, there are differing chemical and physical properties within the two

groups depending on the nonwood fiber raw material, and the current uses of

nonwood pulps include virtually every grade of paper produced.

Typically, common nonwood pulps or hardwood substitutes are produced in

integrated pulp and paper mills, and softwood kraft or sulfite pulp is added to provide

the strength requirements to the paper. However, specialty nonwood pulp may be

used instead of softwood kraft or sulfite pulp thus producing a 100% nonwood paper.

And, in some cases, wastepaper pulp may be blended in the furnish. The nonwood

portion of the furnish typically varies from 20 to 90% and can be even up to 100%

depending on the paper grade and required quality. The possible combinations are

endless and can be adjusted to meet market requirements.

Furthermore, it is possible to add small quantities (up to 20 - 30%) of common

nonwood pulps to primarily woodpulp-based papers without impairing paper

properties or paper machine runnability. This provides wood-based mills which are

hardwood deficient but located within a region with available nonwood fiber

resources such as cereal straw or corn stalks with the option of adding-on a nonwood

pulping line to supplement their fiber requirements.

Typically, the specialty nonwoods have physical properties superior to softwoods and

can be used in lower amounts in the furnish when used as a softwood substitute.

Specialty papers such as currency, cigarette papers, tea bags, dialectric paper etc.

may be made from a furnish of 100% nonwood specialty pulps. Specialty pulps also

may be used in combination with woodpulp to produce lightweight and ultra-


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lightweight printing and writing papers.

Combinations of common and specialty nonwood pulps will permit the production of

virtually any grade of paper to meet any quality requirements demanded in the global

market. Adding possible combinations which include wood pulp, nonwood pulp and

recycled wastepaper pulp increases the possibilities for developing paper with

specific sheet properties designed to meet specific customers needs.

2.2 International best practice

International best practice energy use in pulp and papermaking technology is based

on wood-based fibers. Hence, the identified best practice technologies may not be

applicable to non-wood fiber based pulp mills. Most papermaking technology is

developed and manufactured in Europe (Metso and Voith) and Japan (Mitsubishi),

and specialized products from North America (e.g. felts). There is limited experience

with non-wood fiber outside of China and India, with the last mills in Europe closing

down (Dunavarosc in Hungary (1980s), Fredericia in Denmark (1991), and SAICA in

Spain (1999) due to tightening environmental regulations. Even though there is

increased interest in the use of non-wood fibers internationally, only a few best

practice technologies are available. Only one “non-wood” best practice pulping

technology outside China has been identified by a recent report (Ernst Worell at al,

1970). Other clean modern non-wood pulping technology has not yet been

demonstrated on commercial scales. Although the use of non-wood fibers may affect

the characteristics of the pulp (e.g. runnability, water retention), it is hard to evaluate

ex-ante the impact on the energy use of the paper machine. The energy use of the

paper machine is generally dependent on the pulp quality and paper grade produced,

and hence the best practice values apply to paper machines, indiscriminate of the

source of the virgin pulp (given a specific quality). Note that the variation of the pulp

characteristics and paper grades is so large, that it will affect the best practice energy

intensity values.


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2.3 Energy use and energy benchmarks

The current international best practice is based on the Chempolis process developed

in Finland. It provides a clean process that recovers the chemicals. A first

demonstration plant with a capacity of 65,000 air dry t (ADt)/year has been designed

for construction in China, but construction has been delayed. The pulp has similar

characteristics as hardwood pulp, resulting in similar behavior (e.g. runnability, water

retention) in the paper machine (see below).

The design assumes a steam consumption of 5 to 6 t/ADt pulp, or equivalent to

approximately 10.5 to 12.6 GJ/ADt (358 to 430 kgce/ADt) pulp. These values vary

with the process lay-out. The above values assume conventional water treatment,

and exclude pulp drying (for preparation of market pulp). Electricity consumption is

estimated to be 400 kWh/ADt. Note that the process uses no fuel directly, as there is

no need for a calcination kiln (as with kraft pulping). The lignin generates steam of

about 7 to 9t/ADt pulp, depending on the lignin yield, required steam pressure and

feed water temperature. Hence, the plant can have an excess steam production of 2

to 3 t/ADt, equivalent to approximately 4.2 to 6.3 GJ/ADt (143 to 215 kgce/ADt) that

can be exported for use in the paper machine.4

2.4 Energy saving measures

For further measures to reduce energy consumption that are generally applicable to

pulp and paper mills it is referred to the references and links in chapter 11.


19

3 Kraft pulping

The kraft process (also known as kraft pulping or sulfate process) describes a

technology for conversion of wood into wood pulp consisting of almost pure cellulose

fibers. The process entails treatment of wood chips with a mixture of sodium

hydroxide and sodium sulfide that break the bonds that link lignin to the cellulose.

The process name is derived from German kraft, meaning strength/power; both

capitalized and lowercase names (Kraft process and kraft process) appear in the

literature, but "kraft" is most commonly used in the pulp and paper industry.

The sulphate or kraft process accounting for ca. 80% of world pulp production is the

most applied production method of chemical pulping processes.

3.1 Process flow

Woodchips are fed into vessels called digesters that are capable of withstanding high

pressures. Some digesters operate in a batch manner and some in a continuous

process, such as the Kamyr digester. Digesters producing 1000 tonnes of pulp per

day and more are common with the largest producing more than 3500 tonnes of pulp

per day. Wood chips are impregnated with the cooking liquors. The cooking liquors

consist of warm black liquor and white liquor. The warm black liquor is the spent


20

cooking liquor coming from the blowing. White liquor is a mixture of sodium hydroxide

and sodium sulfide, produced in the recovery process. In a continuous digester the

materials are fed at a rate which allows the pulping reaction to be complete by the

time the materials exit the reactor. Typically delignification requires several hours at

130 to 180°C. Under these conditions lignin and some hemicellulose degrade to give

fragments that are soluble in the strongly basic liquid. The solid pulp (about 50% by

weight based on the dry wood chips) is collected and washed. At this point the pulp is

quite brown and is known as "brown stock". The combined liquids, known as black

liquor (so called because of its color), contain lignin fragments, carbohydrates from

the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other

inorganic salts.

One of the main chemical reactions that underpin the kraft process is the scission of

ether bonds by the nucleophilic sulfide (S2-) or bisulfide (HS-) ions.

The excess black liquor is concentrated in multiple effect evaporators to 60% or even

80% solids ("heavy black liquor") and burned in the recovery boiler to recover the

inorganic chemicals for reuse in the pulping process. Higher solids in the

concentrated black liquor increases the energy and chemical efficiency of the

recovery cycle, but also gives higher viscosity and precipitation of solids (plugging

and fouling of equipment).

The recovery boiler also generates high pressure steam which is led to

turbogenerators, reducing the steam pressure for the mill use and generating

electricity. A modern kraft pulp mill is more than self-sufficient in its electrical

generation and normally will provide a net flow of energy to the local electrical grid.

Additionally, bark and wood residues are often burned in a separate power boiler to

generate steam.

The finished cooked wood chips are blown by reducing the pressure to atmospheric

pressure. This releases a lot of steam and volatiles. The steam produced can then be

used to heat the pulp mill and any excess used in district heating schemes or to drive


21

steam turbine to generate electrical power. The volatiles are condensed and

collected, in the case of northern softwoods this consists mainly of raw turpentine.

The brown stock from the blowing goes to the washing stages where the used

cooking liquors are separated from the cellulose fibers. Normally a pulp mill has 3-5

washing stages in series. Washing stages are also placed after oxygen deliginfiation

and between the bleaching stages as well. Pulp washers uses counter current flow

between the stages such that the pulp moves in the opposite direction to the flow of

washing waters. Several processes are involved: thickening / dilution, displacement

and diffusion. The dilution factor is the measure of the amount of water used in

washing compared with the theoretical amount required to displace the liquor from

the thickened pulp. Lower dilution factor reduces energy consumption, while higher

dilution factor normally gives cleaner pulp. Thorough washing of the pulp reduces the

COD. Several types of washing equipment are in use: Pressure diffusers;

Atmospheric diffusers; Vacuum drum washers, Drum displacers; Wash presses.

In a modern mill, brownstock (cellulose fibers containing approximately 5% residual

lignin), produced by the pulping is first washed to remove some of the dissolved

organic material and then further delignified by a variety of bleaching stages.

In the case of a plant designed to produce pulp to make brown sack paper or

linerboard for boxes and packaging, the pulp does not always need to be bleached to

a high brightness. Bleaching decreases the mass of pulp produced by about 5%,

decreases the strength of the fibers and adds to the cost of manufacture.

A number of process chemicals are added to improve the production process:

Surfactants, Anthraquinone, emulsion breaker, defoamers, dispersing agents and

complexing agents, fixation agents

Pulp produced by the kraft process is stronger than that made by other pulping

processes. Kraft pulp is darker than other wood pulps, but it can be bleached to

make very white pulp. Fully bleached kraft pulp is used to make high quality paper

where strength, whiteness and resistance to yellowing are important. The kraft

process can use a wider range of fiber sources than most other pulping processes.

All types of wood, including very resinous types like southern pine and non-wood

species like bamboo and kenaf can be used in the kraft process.


22

3.2 Consumptions and emissions

In kraft pulping the wastewater effluents and the emissions to air including

malodorous gases are the centres of interest but in the next years it is also expected

that waste will become an environmental issue of concern. The most relevant

consumption of raw materials and emissions to water, air and soil (waste) as well as

energy aspects are covering the following aspects:

• Wood consumption

• Water consumption and wastewater emissions from different process steps

- Wood handling

- Condensates from evaporators

- Spillages

- Washing losses

- Bleaching

- Discharges of nutrients

- Discharges of metals

• Emissions to the atmosphere

- from the recovery boiler

- from the lime kiln

- from auxiliary boilers

- Malodorous gases

- Chlorine compounds from bleaching and bleaching chemical

preparation

• Solid waste generation

• Consumption of chemicals

• Use of energy

• Noise

Pulp mills are almost always located near large bodies of water due to their former

substantial demands. Delignification of chemical pulps released considerable


23

amounts of organic material into the environment, particularly into rivers or lakes. The

wastewater effluent can also be a major source of pollution, containing lignins from

the trees, high biological oxygen demand (BOD) and dissolved organic carbon

(DOC), along with alcohols, chlorates, heavy metals, and chelating agents. Reducing

the environmental impact of this effluent is accomplished by closing the loop and

recycling the effluent where possible, as well as employing less damaging agents in

the pulping and bleaching processes. The process effluents are treated in a

biological effluent treatment plant, which guarantees that the effluents are not toxic in

the recipient.


The major part of heat energy is consumed for heating different fluids and for

evaporating water. Heat energy is also used to accelerate or control chemical

reactions. Electrical energy is mostly consumed for the transportation of materials

(pumping) and for the operation of the paper machine (only in integrated pulp-mills).

The manufacturing of bleached kraft pulp consumes about 10-14 GJ/ADt of heat

energy (steam for the production of electrical power not included). The consumption

of electrical energy is 600-800 kWh/ADt, including the drying of pulp. The energy

consumption for pulp drying is about 25% of the heat energy and 15-20% of the

electrical energy. Over 50% of the electrical energy consumption are used for

pumping.

The energy consumption depends on the process configuration, process equipment

and process control efficiency.

A best practice Kraft mill (as reported by Ernst Worrell et al, 2007) produces excess

electricity that can be exported. The export is the result of balancing the energy used

in the pulping process and the energy recovered from the black liquor recovery

process (combusting the lignin). The energy consumption of the process itself varies

between 10 and 12.2 GJ/ADt (341-416 kgce/ADt) pulp, while electricity use is around

610 kWh/ADt (75 kgce/ADt). The lime kiln uses 1.2 GJ/ADt in fuels and 30

kWh/ADt54 for total energy consumption of 11.2 GJ/ADt (382 kgce/ADt) in fuels and

640 kWh/ADt in electricity. However, the recovery process is a net producer of 15.8

GJ/ADt of steam. It is assumed that the steam is used in a back-pressure steam

turbine to generate electricity (around 655 kWh/ADt), resulting in a net export of


24

power of 15 to 20 kWh/ADt. This leads to a total overall energy consumption value of

11.1 GJ/ADt (380 kgce). Research and development in black liquor gasification has

not yet resulted in a commercially operating process, and is hence not included in the

best practice energy consumption figures. However, when this technology is

available it could result in significant energy savings, due to large amounts of excess

power production.

A similar conclusion is drawn by the BAT surveys of the EU: Chemical pulping plants

are energy-intensive installations that consume high amounts of energy but at the

same time produce steam and electrical power on site by use of regenerative fuels.

Thus, modern non-integrated kraft pulp mills are energy self-sufficient mainly

because of efficient energy recovery by burning 50% of the incoming wood in the

recovery boiler (strong black liquor) and the use of bark as auxiliary boiler fuel.

Furthermore, secondary energy from different process steps can be recovered as

warm and hot water (40-80o C). Fossil fuels are mainly used as support fuel (e.g. oil

in the lime kiln).


In order to reduce the consumption of fresh steam and electric power, and to

increase the generation of steam and electric power internally, a number of

measures are available.

• Measures for a high heat recovery and a low heat consumption:

- High dry solids content of black liquor and bark

- High efficiency of steam boilers, e.g. low flue gas temperatures

- Effective secondary heating system e.g. hot water about 85 °C

- Well closed-up water system

- Relatively well closed-up bleaching plant

- High pulp concentration (MC-technique)

- Pre-drying of lime

- Use of secondary heat to heat buildings

- Good process control


25

• Measures for low consumption of electric power

- As high pulp consistency as possible in screening and cleaning

- Speed control of various large motors

- Efficient vacuum pumps

- Proper sizing of pipes, pumps and fans

• Measures for a high generation of electric power

- High boiler pressure

- Outlet steam pressure in the back-pressure turbine as low technically

feasible

- Condensing turbine for power production from excess steam

- High turbine efficiency

- Preheating of air and fuel charged to boilers

In many European countries, information on energy balances of whole pulp and

paper mills is poorly available in public. In Europe, different reporting schemes are

used. Energy balances also depend to a certain extent on local conditions. Therefore

the ranges of energy consumption of pulp mills shown in the table below should only

be taken as an indication about the approximate need of process heat and power at

energy efficient Kraft pulp mills.

Type of mill Process heat consumption (net) in GJ/ADt

Power consumption (net) in MWh/ADt

Remarks

Non-integrated bleached Kraft pulp

10-14 GJ/ADt About 2-2.5 GJ/ADt can be used for power generation giving a heat surplus of 0.5-1.0 GJ/ADt.

0.6 -0.8 MWh/ADt Modern pulp mills are power self-sufficient

An integrated mill reported 10 GJ/ADT heat consump-tion in the pulp mill (pulp at 2.2% consist.) 1

Integrated bleached Kraft pulp and uncoated fine paper

14 - 20 GJ/ADt 2) There is a heat surplus of 4.0-4.5 GJ per tonne of pumped bleached pulp which is used in the paper mill

1.2 -1.5 MWh/ADt There can be a surplus power from the pulp mill of 0.1 - 0.15 MWh per tonne of pumped bleached pulp which is used in the paper mill

The surplus of electricity, if any, depends on if back-pressure turbines are installed

Integrated kraftliner, Unbleached

14.0 -17.5 GJ/ADt There is a heat surplus of 1.5-2 GJ per tonne of pumped unbleached pulp which is used in the paper mill

1.0-1.3 MWh/ADt There is a surplus power from the pulp mill of 0.15 -0.2 MWh per tonne of pumped unbleached pulp which is used in the paper mill

Integrated sackpaper, Unbleached

14.0 -23 GJ/ADt 1.0 - 1.5 MWh/ADt


26

The units can be converted from MWh to GJ according to 1 MWh = 3.6 GJ and 1 GJ = 0.277 MWh Notes: 1) including activated sludge treatment 2) Paper drying is more energy consuming than pulp drying

The effect of this energy saving measures can often not be easily been shown in

form of values because improvements depend on the situation of the mill before the

measures were implemented, which shows the importance of internal benchmarking.




27

4 Sulfite pulping

The first pulp mill using the sulfite process was built in Sweden in 1874 and used

magnesium as the counter ion. Calcium became the standard counter ion until the

1950s. Sulfite pulping was the dominant process for making wood pulp until it was

surpassed by the kraft process in the 1940s. Sulfite pulps now account for less than

10% of the total chemical pulp production, and sulphate pulps are more used in

special purposes in papermaking rather than being an alternative market pulp grade

for kraft pulps. Mainly bleached sulphite pulp is made and the yield is a little higher

which can be attributed to the lower pH in the cooking.

The main reasons of the limited applicability of sulphite pulps are as follows:

• It is not possible to use pine as raw material in the acid cooking process

which limits the raw material base of sulphite pulping

• The strength properties of the pulps as measured by the papermaker are

generally not as good as those of kraft pulp, although for some speciality

pulps these properties may be equally good or even better

• Environmental problems have in many cases been more expensive to solve

and this has decreased the cost-competitivity compared to the kraft pulping

(e.g. pH-regulation of evaporation condensates, minimisation of sulphur

emissions and removal of organic compounds).

4.1 Processes

The sulfite process produces wood pulp which is almost pure cellulose fibers by

using various salts of sulfurous acid to extract the lignin from wood chips in large

pressure vessels called digesters. The salts used in the pulping process are either

sulfites (SO32−), or bisulfites (HSO3−), depending on the pH. The counter ion can be

sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+) or ammonium

(NH4+).

The sulfite process is acidic and one of the drawbacks is that the acidic conditions

hydrolyze some of the cellulose, which means that sulfite pulp fibers are not as


28

strong as kraft pulp fibers. The yield of pulp (based on wood used) is higher than for

kraft pulping and sulfite pulp is easier to bleach. Apart from printing and specialty

papers, a special grade of sulfite pulp, known as "dissolving pulp" is used to make

cellulose derivatives. Lignosulfonates are an important byproduct of sulfite bleaching.

These materials are used in making concrete, drilling mud, and drywall and so on.

The sulphite process is characterised by its high flexibility compared to the kraft

process, which is a very uniform method, which can be carried out only with highly

alkaline cooking liquor. In principle, the entire pH range can be used for sulphite

pulping by changing the dosage and composition of the chemicals. Thus, the use of

sulphite pulping permits the production of many different types and qualities of pulps

for a broad range of applications. The sulphite process can be distinguished

according to the pH adjusted into different types of pulping the main of which realised

in Europe are compiled in the table below.

Process pH Base Active reagent

Cooking temperature C

Pulp yield %

Applications

Acid (bi)sulphite

1-2 Ca2+ , Mg2+, Na+

SO2*H2O, H+, HSO3 -

125-143 40-50 Dissolving pulp, tissue, printing paper, special paper

Bisulphite 3-5 Mg2+, Na+ HSO3 -, H+ 150-170 50-65 printing paper, (Magnefite) tissue Neutral sulphite (NSSC)2

5-7 Na+, NH4 +

HSO3 -SO3 2-

, 160-180 75-90 Corrugated medium, semi-chemical pulp

Alkaline 9-13.5 Na+ SO3 2-, OH- 160-180 45-60 Kraft-type pulp sulphite

Table 5.1: Main sulphite pulping processes in Europe (according to Uhlmann, 1991)

The sulphite cooking process is based on the use of aqueous sulphur dioxide (SO2)

and a base - calcium, sodium, magnesium or ammonium. The specific base used will

impact upon the options available within the process in respect of chemical and

energy recovery system and water use. Today, the use of the relatively cheap

calcium base is outdated because the cooking chemicals cannot be recovered. In

Europe there is still one mill (FR) using ammonium as a base. The dominating

sulphite pulping process in Europe is the magnesium sulphite pulping with some mills

using sodium as base. Both magnesium and sodium bases allow chemical recovery.

The lignosulphonates generated in the cooking liquor can be used as a raw material

for producing different chemical products. Because of its importance in terms of

capacity and numbers of mills running in Europe in the following the focus is on


29

magnesium sulphite pulping.

Sulphite pulping consists of three main entities: the fibre line, recovery of chemicals

and energy (excluding calcium sulphite pulping where recovery is not possible but

where the spent cooking liquor can be evaporated and the components used for

other purposes) and external wastewater treatment. As in kraft pulping some

auxiliary systems like reject handling, manufacturing of bleaching chemicals and

auxiliary power generation are connected to the main departments.

In many respects the kraft and sulphite processes have similarities not least

regarding the possibilities of taking different internal and external measures to reduce

emissions to environment.

4.2 Consumptions and emissions

An overview of the raw material and energy input as well as the output of products,

by-products and the major releases (emissions, waste etc.) of the production of

sulphite pulp is presented in the figure below.

Using the mass stream overview, specific raw material, energy and water

consumption and specific emission per tonne of product can be calculated.

Because of the diverse and individual situation of each plant no general propositions

are possible to this topic. Internal benchmarking remains the only option.


30


Information is generally much weaker for sulphite mills than for kraft pulp mills.

Therefore, from the limited information only a few techniques could be described to

the same extent as for kraft pulping. The process can be operated to produce a wide

range of specialty products, which will also result in a wide range of energy use.


31

Due to the variety of pulps to be produced, steam use is estimated to be 16 to 18

GJ/ADt (546 to 614 kgce/ADt) and electricity use to be 700 kWh/ADt.


The available data set is relatively small. This could be partly compensated because

of the inherent similarities between sulphite and kraft pulping. A number of

techniques for pollution prevention and control for kraft pulping are also valid in most

respects for sulphite pulping.

Energy can be recovered from the “green liquor”, similar to the black liquor recovery

process, producing about 15 GJ/ADt (512 kgce/ADt) pulp. The best practice assumes

optimisation of power use, state-of-the-art controls, efficient evaporation and

concentration of the green liquor.




32

5 Mechanical pulping

In mechanical pulping the wood fibres are separated from each other by mechanical

energy applied to the wood matrix. The objective is to maintain the main part of the

lignin in order to achieve high yield with acceptable strength properties and

brightness.

5.1 Processes

There are several types of processes that can be used for mechanical pulping, i.e.

groundwood (GW), thermo-mechanical pulping (TMP) and chemi-thermo-mechanical

pulping (CTMP).

The earliest mills used sandstone grinding rollers to break up small wood logs called

"bolts", but the use of natural stone ended in the 1940s with the introduction of

manufactured stones with embedded silicon carbide or aluminum oxide. The pulp

made by this process is known as "stone groundwood" pulp (SGW). If the wood is

ground in a pressurized, sealed grinder the pulp is classified as "pressure

groundwood" (PGW) pulp. Most modern mills use chips rather than logs and ridged

metal discs called refiner plates instead of grindstones. If the chips are just ground up

with the plates, the pulp is called "refiner mechanical" pulp (RMP), if the chips are

steamed while being refined the pulp is called "thermomechanical" pulp (TMP).

Steam treatment significantly reduces the total energy needed to make the pulp and

decreases the damage (cutting) to fibers.

Some mills pretreat wood chips or other plant material like straw with sodium

carbonate, sodium hydroxide, sodium sulfite and other chemical prior to refining with

equipment similar to a mechanical mill. The conditions of the chemical treatment are

much less vigorous (lower temperature, shorter time, less extreme pH) than in a

chemical pulping process, since the goal is to make the fibers easier to refine, not to

remove lignin as in a fully chemical process. Pulps made using these hybrid

processes are known as chemi-thermomechanical pulps (CTMP). Sometimes a

CTMP mill is located on the same site as a kraft mill so that the effluent from the

CTMP mill can be treated in the kraft recovery process to regenerate the inorganic

pulping chemicals.


33


Energy use in mechanical pulping is determined by the wood type used and the

“freeness” of the pulp. The “freeness” is an expression for the fiber quality and water

retention. Hence, energy use may vary widely on the basis of the desired pulp quality

given a specific wood type used.

Mechanical pulp mills use large amounts of energy, mostly electricity to power

motors which turn the grinders. A rough estimate of the electrical energy needed is

10000 mega joules (MJ) per ton of pulp (2750 kWh per ton) in international best

practice average.

Energy efficient mechanical pulp and paper mills consume heat and power as

follows:

• Non-integrated CTMP: For pulp drying recovered process heat can be used

i.e. no primary steam is needed. The power consumption is 2 - 3 MWh/ADt.

• Integrated newsprint mills consume 0 - 3 GJ/t process heat and 2 - 3 MWh/t

of electricity. The steam demand depends on the fibre furnish and the degree

of steam recovery from the refiners.

• Integrated LWC paper mills consume 3 - 12 GJ/t process heat and 1.7 - 2.6

MWh/t of electricity. It has to be noted that the fibre furnish of LWC consists

usually only of about one third of PGW or TMP the rest being bleached kraft

pulp and fillers and coating colours. If the production of bleached kraft pulp is

carried out at the same site (integrated) the contribution of the energy

demand of kraft pulping have to be added according to fibre furnish mix

manufactured.

• Integrated SC paper mills consume 1 - 6 GJ/t process heat and 1.9 - 2.6

MWh/t of electricity.

As mentioned there are several types of processes that can be used for mechanical

pulping, i.e. groundwood (GW), thermo-mechanical pulping (TMP) and chemo-

thermo-mechanical pulping (CTMP). The best practice assumes TMP. TMP allows

the recovery of heat from the process in the form of hot water and steam, as only a

fraction of the energy is actually used to separate the fibers. TMP allows the recovery


34

of 60-65% of the heat generated in the process (45% as steam, 20% as hot water).

However, a TMP mill consumes more power than a groundwood mill. The best

practice integrated TMP newsprint mill consists of a pressurized TMP mill consuming

about 2190 kWh/ADt and generating 1.33 GJ/ADt (45 kgce/ADt) of heat. For a non-

integrated pulp mill electricity use is estimated at 2420 kWh/ADt with generation of

5.5 GJ/ADt of steam.


In mechanical pulping and chemo-mechanical pulping the wastewater effluents and

consumption of electricity for the drives of grinders or refiners are the centres of

interest. The main raw materials are renewable resources (wood and water) and

some chemicals for bleaching (for CTMP also for chemical pre-treatment of the

chips). As processing aids and to improve the product properties (paper auxiliaries)

various additives are applied during paper manufacturing.

A lot of options for energy saving in many stages within the manufacturing process

are available. Usually these measures are linked with investments to replace, rebuild

or upgrade process equipment. It should be noticed that energy saving measures are

mostly not applied only for energy saving. Production efficiency, improvement of

product quality and reduction of overall costs is the most important basis for

investments.

In order to reduce the consumption of fresh steam and electric power the following

measures are available:

• Implementation of a system for monitoring energy usage and performance.

Based on reliable energy performance information appropriate action can be

taken. Energy management includes setting, controlling, reviewing and

revising energy performance targets.

• Upgrading of equipment. When replacing equipment less energy consuming

equipment with possibilities for automated process control instead of

conventional manual systems. Automated controlled systems are more

efficient to control and can result in more accurate processing and energy

savings.


35

• Minimisations of reject losses by using efficient reject handling stages and

reject refining. If fiber bundles (shives) and undeveloped coarse fibres are

removed from the main process line, the energy, which was already spent for

first grinding/refining would be lost. The reduction of reject losses after

grinding and mainline refining and the further refining of shives and coarse

fibres to develop the properties to the quality level needed thus decrease

specific energy consumption.

• Use of effective heat recovery systems (applies only for TMP and CTMP).

Large volumes of steam are generated in TMP refining because around two-

thirds of the refining electrical energy is transformed in thermal energy in the

form of hot saturated steam. The impurities present in the TMP steam make

the heat recovery unit necessary because the direct use of TMP steam is not

possible. The TMP steam is separated from the fibres in cyclones and then

condensed in the reboiler against vaporising clean steam. The clean steam is

normally used in the paper machine department.

• Application of co-generation of heat and power where the power/steam-ratio

allows it.




36

6 Recycled fibre mills

Recovered fibre has become an indispensable raw material for the paper

manufacturing industry, accounting about one-third of the total raw materials because

of the favourable price of recovered fibres in comparison with the corresponding

grades of market pulp and because of the promotion of wastepaper recycling by

many European countries. In Europe there is an average utilisation rate of recovered

paper of 43 %. But is has to be taken into account that the maintenance of the fibre

cycle relies on the feed of a certain amount of primary fibres to ensure the strength

and other properties of the paper to be produced.

For effective use of recovered paper it is necessary to collect, sort and classify the

materials into suitable quality grades. Therefore, after collection recovered paper is

brought to the collection yards where it is sorted. Detrimental substances as e.g.

plastics, laminated papers etc. are removed before balling as well as possible. The

sorted recovered paper is usually compacted by baling machines. Industrial

recovered paper from large generators is usually delivered to and processed in

recovered paper yards integrated in the paper mill.

There are three categories of paper that can be used as feedstocks for making

recycled paper: mill broke pre-consumer waste, and post-consumer waste. Mill broke

is paper trimmings and other paper scrap from the manufacture of paper, and is

recycled internally in a paper mill. Pre-consumer waste is material that was discarded

before it was ready for consumer use. Post-consumer waste is material discarded

after consumer use such as old magazines, old telephone directories, and residential

mixed paper. Paper suitable for recycling is called "scrap paper".

Fiber recycling is an important option for reducing pulping energy use. China also

imports paper from other countries (notably the US and Europe) to provide its fiber

needs. Recycled fiber has become a global market in which China is an important

consumer.

6.1 Processes

The used fibers are pulped and (optionally) de-inked before being fed to stock

preparation for the paper machine. A concern about recycling wood pulp paper is that


37

the fibers are degraded with each and after being recycled 4-6 times the fibers

become too short and weak to be useful in making paper.

The recovered paper is put into a pulper together with hot water or fresh water, and

pulped with mechanical and hydraulic agitation resulting in their disintegration into

fibres. After repulping the recovered paper has a pulping consistency for subsequent

treatment. Some chemicals such as deinking agents and NaOH are often added as

pulping additives.

Deinking is the industrial process of removing printing ink from paperfibers of

recycled paper to make deinked pulp. The key in the deinking process is the ability to

detach ink from the fibers. This is achieved by a combination of mechanical action

and chemical means. In Europe the most common process is froth flotation deinking.

Ink removal is necessary in plants manufacturing paper grades where brightness is

important e.g. for newsprint, printing and writing paper, tissue or light top liner of

recovered paper based carton boards. The main objectives of deinking are

increasing of brightness and cleanliness and reduction of stickies. It should be noted

that the difference between de-inked and non de-inked grades is in the process and

not in the product itself. Depending on the quality of the recovered paper used,

market requirements or production needs, also packaging papers and boards could

be de-inked.

Contaminants and clusters are removed continuously during operation by a dirt trap

(e.g. screen plate) and are sent to a reject conveyor, in order to avoid the

contaminants breaking into small pieces or accumulating in the pulper. There is an

increasing use of secondary pulpers for further defibration, deflaking and cleaning

from heavy-weight (HW) and lightweight (LW) dirt. The installations trade under

different names but are based on similar functioning. Also screening drums are used.

Before entering a storage tower the pulp is often bleached by use of bleaching

chemicals. Generally hydrogen peroxide (P), hydrosulphite (Y) or formamidine

sulfinic acid (FAS) are used. Bleaching chemicals are added directly in the disperger

to maintain or increase the brightness. The reaction itself takes place in a bleaching

tower ensuring a sufficient dwell time. Any possible increase in brightness depends

on the raw material and on the pre-treatment of the stock. Hydrogen peroxide

bleaching is carried out in the presence of NaOH, sodium silicate and sometimes

chelating agents. For almost wood-free secondary fibre stock so-called

unconventional bleaching chemicals oxygen and ozone can be used.

Normally, the water for disintegration is totally re-circulated process water that comes


38

as white water from the paper machine. The figure below is a flow sheet of an

example for a stock preparation plant concept for processing recovered paper for

case making material (2-ply testliner). (HW = heavy-weight impurities; LF = Long

fibre fraction; SF = Short fibre fraction)

Various product characteristics require different cleanliness and brightness properties


39

from the RCF pulps and the process concepts vary accordingly. For example, de-

inking is not required in many board grades. On the contrary, a very efficient

multistage process is required for high speed paper machines, thin paper or for

grades where brightness is important. The degree of sophistication of the whole

process depends on the furnish used and the paper grade to be manufactured.

Therefore it is not reasonable to describe "one typical" recovered paper processing

system.


Energy consumption is reduced by recycling, although there is debate concerning the

actual energy savings realized. The EIA claims a 40% reduction in energy when

paper is recycled versus paper made with unrecycled pulp, while the Bureau of

International Recycling, BIR, claims a 64% reduction. Some calculations show that

recycling one ton of newspaper saves about 4000 kWh (14 GJ) of electricity,

although this may be too high. This is enough electricity to power a 3-bedroom

European house for an entire year, or enough energy to heat and air-condition the

average North American home for almost six months. Recycling paper to make pulp

may actually consume more fossil fuels than making new pulp via the kraft process,

however, since these mills generate all of their energy from burning waste wood

(bark, roots) and byproduct lignin. Pulp mills producing new mechanical pulp use

large amounts of energy; a very rough estimate of the electrical energy needed is 10

gigajoules per ton of pulp. Recycling mills purchase most of their energy from local

power companies, and since recycling mills tend to be in urban areas, it is likely that

the electricity is generated by burning fossil fuels.

As a fact, paper and board mills require substantial amounts of steam for heating of

water, pulp, air and chemicals to the demanded process temperature and above all

for drying the paper. Besides, large quantities of electricity are required for driving the

machinery, pumping, vacuum, ventilation and wastewater treatment. In paper mills

energy is usually the main factor in operating costs. Because the secondary fibres

have already passed through stock-preparation equipment when the original paper

was made, RCF pulping require comparatively less total energy for processing then


40

is needed for chemical and especially for mechanical pulping.

Based on the performance of Swedish mills, the best practice is estimated to be 0.3

GJ/ADt (10 kgce/ADt) use of steam and electricity use of 330 kWh/ADt.

In many European countries, information on energy balances of paper mills is poorly

available in public. Different reporting schemes, if any, for energy consumption are

used. Energy demand also depends on the product quality (especially in tissue mills)

and partly on local conditions. Therefore, it is difficult to present energy consumption

values associated with the use of BAT. The ranges of energy consumption of paper

mills shown in the table should only be taken as an indication about the approximate

need of process heat and power at energy efficient paper mills.

Type of mill Process heat consumption (net) in

GJ/t1)

Power consumption (net) in MWh/t 1)

RCF based testliner and wellenstoff,

6.0 - 6.5 0.7 - 0.8

without de-inking RCF based cartonboard or folding boxboard, without de-inking

8.0 - 9.0 0.9 - 1.0

RCF based newsprint, de-inked 4.0 - 6.5 1.0 - 1.5 RCF based tissue, de-inked 7.0 - 12.0 2) 1.2 - 1.4 Explanatory notes: The units can be converted from MWh to GJ according to 1 MWh = 3.6 GJ and 1 GJ = 0.277 MWh 1) All data from [J. Pöyry, 1998]

2) In tissue mills the energy consumption depends mainly on the drying system used. Through air drying and re-creping consume significant additional energy. Tissue mills use primary fuel instead of steam in drying (most hoods are direct gas fired). Table 7.2: Indication for heat and power consumption associated with the use of BAT

for different types of recovered paper production per tonne of product


In order to reduce the consumption of fresh steam and electric power the following

measures are available:

• Implementation of a system for monitoring energy usage and performance.

• Upgrading of equipment.

• Use of effective heat recovery systems.

• Application of co-generation of heat and power.




41

7 Papermaking

For the production of different paper grades either virgin fibre (chemical or

mechanical pulps) or recycled fibres or a mixture of fibrous materials are used as

main raw materials. The composition of raw materials used for paper manufacturing

(e.g. fibrous material, mineral fillers, coating) has a major effect on the total

production costs, the product quality and the environmental impact of the process.

The manufacturing of fibres used for papermaking has been described in the

previous chapters. In the following, paper and board manufacturing is described

independent from pulp manufacturing. This approach is considered to be reasonable

because the same unit processes around the paper and board machine are required

in every paper mill. The description of papermaking as part of integrated pulp mills

(see next chapter) would increase the complexity of the technical description. Finally,

many paper mills are non-integrated mills; they are stand-alone paper mills where

pulp is purchased as raw material from other factories (e.g. from stand-alone pulp

mills).

7.1 Processes - Overview

In the following the basic units of paper manufacturing are described. Although there

is a big variety of paper products and different process lay-outs in paper mills almost

all types of paper and board-making processes have the following basic units:

• Stock preparation

• Approach flow system

• A paper and board machine consisting of

- Head box introduces the suspension of fibres to the wire and creates a

uniform dispersion of fibres across the total width of the wire belt

- A wire section drains paper web to around 12 - 20% solids

- A press section removes more water out of the web by pressing down to

about 50% moisture content


42

- A drying section removes rest of moisture by heating the web with drying

cylinders

- A reeler reels the paper web into a roll

• Depending on the paper and board grade there are additional process units

(optional) like calenders, coaters, a coating colour kitchen, winders, rewinders

and a roll wrapping station.

7.2 Stock preparation

Stock preparation is conducted to convert raw stock into finished stock (furnish) for

the paper machine. The pulp is prepared for the paper machine including the

blending of different pulps, dilution and the addition of chemicals. The raw stocks

used are the various types of chemical pulp, mechanical pulp, and recovered paper

and their mixtures. The quality of the finished stock essentially determines the

properties of the paper produced. Raw stock is available in the form of bales, loose

material, or, in case of integrated mills, as suspensions. Stock preparation consists of

several process steps that are adapted to one another as fibre disintegration,

cleaning, fibre modification and storage and mixing. These systems differ

considerably depending on the raw stock used and on the quality of furnish required.

For instance, in the case of pulp being pumped directly from the pulp mill, the

slushing and deflaking stages are omitted.

Stock preparation is based on the removal of impurities, the conditioning of the

strength properties of the fibres (refining) and the addition of chemicals to aid the

process and affect the final quality of the paper sheet (resins, wet strength agents,

colours, fillers). In non-integrated mills the fibres are received dry. They are

suspended in a pulper to create a suspension that can be pumped. Then,

undissolved impurities are removed from the slurry by screening (screens) and

cleaning (centrifugal cleaners). The objective of screening is the removal of

interfering substances from the fibres. The fibre suspension is passed through a

screen with apertures in the form of slots or round holes, and the impurities to be

separated are rejected by the screen.

Cleaning is the separation of impurities from the fibre suspension in a centrifugal


43

field. Cleaning is carried out in centrifugal cleaners. A distinction is made between

heavy-particle and light-particle cleaners, depending on the purpose of separation.

Most cleaners are multistage systems (up to 5 stages).

To improve the bonding ability of the individual fibres of the finished paper refining

may be carried out (optional). The refining has the purpose of conditioning the fibres

to create the required properties of the finished product. Refining is carried out in

refiners equipped with e.g. a rotating disk that is pressed on a stator.

Complete stock preparation for a paper machine usually consists of several lines, in

each of which different raw stocks are prepared

7.3 Paper Machine

In the paper machine the paper is formed and most of the properties of the paper are

determined. The paper machine is actually a large de-watering device consisting of a

head box, a wire section, press section and dryer section. The most common

machine design still to recent times is the Foudrinier process in which the sheet is

formed onto a continuous wire or fabric onto which the suspension of fibres is

introduced from the headbox. Recently twin wire formers have been used for web

formation and they have become the state-of-the-art design. In twin wire formers, the

fibre suspension is led between two wires operating at the same speed, and is

drained through one or both sides. There are different types of twin wire formers (e.g.

gap formers. In gap formers the diluted stock is injected directly into the gap between

the two wires) and combinations of Fourdrinier and twin wires (hybrid formers).

Figure 7.2 shows the key features of such a paper machine.


44

Figure 7.1: Key features of a twin wire paper machine

The four main sections to the Fourdrinier:

Wet end

The first section is typically known as the wet end. Pulp may be delivered to the

Fourdrinier machine in a slurry form (a mixture of fiber and water) directly from the

pulping process. Alternatively, pulp may be supplied in dried sheets which are then

broken down in water to produce similar slurry, before being fed to the refiners in the

wet end where the fibers are subjected to high pressure pulses between bars on

rotating refiner discs. This action causes the fibrils of the fibers to partially detach and

bloom outward. After refining the pulp is mixed with some of the following: sizing,

fillers, colors, retention aid and waste paper called broke to a stock, and passed on.

Washing is done in pressurized screens and hydocyclones and also deaeration is

done.

The stock then enters the headbox, a unit that disperses the stock and loads it onto a

moving wire mesh conveyor with a jet from an opening called the slice. The

streaming in the jet makes some fibres align. This alignement can partly be taken

away by adjusting the speed difference between the jet and the wire. The wire

revolves around the Fourdrinier table, from breast roll under the headbox over the

couch to the forward drive roll, foils under the wire are creating low pressure pulses

that will vibrate and partly deflocculate the fibres while water is removed. Later on


45

Suction boxes below the wire gently remove water from the pulp with a slight vacuum

and near the end of the wire section the couch will remove water with higher vacuum.

Press section

The second section of the Fourdrinier machine (or any modern papermachine) is the

press section, which removes the most water via a system of nips formed by rolls

pressing against each other aided by press felts. This is the most efficient method of

dewatering the sheet as only mechanical pressing is required. Press felts historically

were made from cotton. However, today they are nearly 100% synthetic. They are

made up of a polyester woven fabric with thick batt applied in a specific design to

maximise water absorption.

Presses can be single or double felted. A single felted press has a press felt on one

side of the press, the sheet being exposed to a felt on one side and a smooth roll on

the other. Double felted is where both sides of the sheet are in contact with a press

felt. Single felted nips are useful when mated against a smooth top roll, which adds a

two-sidedness—making the top side appear smoother than the bottom. Double felted

nips increase roughness, as generally, press felts.

Conventional roll presses are configured with one of the press rolls is in a fixed

position, with a mating roll being loaded against this fixed roll. The felts run through

the nips of the press rolls and continue around a felt run, normally consisting of

several felt rolls. During the dwell time in the nip, the moisture from the sheet is

transferred to the press felt. When the press felt exits the nip and continues around, a

vacuum box known as an Uhle Box applies vacuum (normally -60 kPa) to the press

felt to remove the moisture so that when the felt returns to the nip on the next cycle, it

does not add moisture to the sheet.

Pickup roll presses are vacuum assisted rolls loaded against plain press rolls (usually

a roll in a centre position). While out of favour, these are generally found in machines

built in the 1970s–1980s. Pickup roll presses normally have a vacuum box that has

two vacuum zones (low vacuum and high vacuum). These rolls have a large number

of drilled holes in the cover to allow the vacuum to pass from the stationary vacuum

box through the rotating roll covering. The low vacuum zone picks up the sheet and

transfers, while the high vacuum zone attempts to remove moisture. Unfortunately,

centrifugal force usually flings out vacuumed water—making this less effective for

dewatering. Pickup presses also have standard felt runs with Uhle boxes. However,

pickup press design is quite different, as air movement is important for the pickup


46

and dewatering facets of its role.

Crown Controlled Rolls (also known as CC Rolls) are usually the mating roll in a

press arrangement. They have hydraulic cylinders in the press rolls that ensure that

the roll does not bow. The cylinders connect to a shoe or multiple shoes to keep the

crown on the roll flat, to counteract the natural "bend" in the roll shape due to

applying load to the edges.

Extended Nip Presses (or ENP) are a relatively modern alternative to conventional

roll presses. The top roll is usually a standard roll, while the bottom roll is actually a

large CC roll with an extended shoe curved to the shape of the top roll, surrounded

by a rotating rubber belt rather than a standard roll cover. The goal of the ENP is to

extend the dwell time of the sheet between the two rolls thereby maximising the

dewatering. Compared to a standard roll press that achieves up to 35% solids after

pressing, an ENP brings this up to 45% and higher—delivering significant steam

savings or speed increases.

Dryer section

The dryer section of the Fourdrinier machine, as its name suggests, dries the pulp by

way of a series of steam-heated rollers that stretch the web somewhat, removing the

moisture. Additional sizing agents, including resins, glue, or starch, can be added to

the web to alter its characteristics. Sizing improves the paper's water resistance,

decreases its ability to fuzz, reduces abrasiveness, and improves its printing

properties and surface bond strength. Some paper machines also make use of a

'coater' to apply a coating of fillers such as calcium carbonate or china clay.

Calender section

The calender stack is a series of rollers that the web is run between in order to

further smooth it out, which also gives it a more uniform thickness. The pressure

applied to the web by the rollers determines the finish of the paper, and there are

three types of finish that the paper can have. The first is machine finish, and can

range from a rough antique look to a smooth high quality finish. The second is called

a supercalendered finish and is a higher degree for fine-screened halftone printing.

The third type of finish is called a plater finish, and whereas the first two types of

finish are accomplished by the calender stack itself, a plater finish is obtained by

placing cut sheets of paper between zinc or copper plates that are stacked together,


47

then put under pressure and perhaps heating. A special finish such as a linen finish

would be achieved by placing a piece of linen between the plate and the sheet of

paper, or else an embossed steel roll might be used. The web is then wound onto a

roll after calendering, with a moisture content of about 6%, and stored for final cutting

and / or shipping. The environmental impact of (super)calendering is mainly the

energy consumption needed for running the machine and heating the rolls


Energy use in the paper machine is determined by the specific grade of paper to be

produced and the fiber quality (e.g. water retention) in the pulp. Moreover, not all

energy efficient technologies are suitable for all paper grades. The best practice

values assume that an effective control system is in place, long nip (or shoe) press is

being used (not suitable for tissue mills), use of efficient motors, condensate

recovery, a closed hood for heat recovery, as well as integration of the various steam

and hot water flows in the mill. Note that small scale mills may have a steam

consumption that is 10-25% higher and an electricity consumption that is 5-20%

higher than the average figures of bigger plants.

The total demand for energy (consumption) in the form of heat (steam) and electric

power for a non-integrated fine paper mill has been reported to consume

• Process heat: 8 GJ/t (� 2222 kWh/t)

• Electric power: 674 kWh/t. 14

This means that about 3 MWh electricity and steam/ton product is consumed. When

considering the primary energy demand for converting fossil fuels into power a total

amount of 4 MWh/t of paper is needed.

The figures represent a modernised mill, like a mill built in the 1970s and since then

modernised. The values include all the stages from disintegration of fibre raw

materials to the final paper product and include also necessary service departments.

Energy consumption for the coating process is included in the case of the production

of coated paper (the mill is considered to have a capacity of 125000 t/a of coated fine

paper from market pulp. The paper has a pigment content of 39% and a moisture


48

content of 4.5%).

Table 7.4a shows the energy consumption in the form of heat and electric power for

this nonintegrated fine paper mill in a little more detail.

Department Process heat Electric power [MJ/t] [kWh/t]

Stock preparation 0 202

Paper machine 8000 350

Coating kitchen 0 118

Total paper mill 8000 670

Effluent treatment 0 4

Total consumption per tonne of paper 8000 674 Turbine generator 0

Total external supply 8000 674

Table 7.4a: Energy consumption in a non-integrated coated paper mill with a production capacity of 125000t/a [SEPA-Report 4712-4, 1997]; The external supply

figure shows the amount of the total demand that is purchased from external sources as fuel oil, coal and gas and electricity

Electricity (Power)

The electricity consumption depends to a certain extent on the paper grade

produced. The lowest values correspond to packaging paper or corrugated base

paper that consumes about 500 kWh/t, whereas printing and writing paper account

for about 700 - 800 kWh/t. The highest power demand, up to 5600 kWh/ADt, is

needed for some special paper grades. The power is mainly consumed by more

intensive refining.

Electric power in paper industry is mainly consumed for the operation of various

motor drives and refining in stock preparation. The motors are used for running fans,

pumps, compressors, stirrers, paper machines, presses, vacuum systems, various

conveyors, etc. For refining the electrical energy is primarily used to drive the rotor in

the refiner. The energy usage varies by product with filter and blotting papers

requiring least energy and tracing papers requiring the highest input.

Process heat

Heating of water and liquors, wood or pulp, air and chemicals to the process

demanded temperatures. Pulp and white water systems can often be kept warm

enough without addition of steam. It is essential to minimise the use of fresh water


49

and increase the use of white water from an energy point of view. In non-integrated

paper mills except addition of warm fresh water, circulation of the white water through

heat recovery is often necessary to keep the temperature at a sufficient level in the

white water system.

Evaporating water: In papermaking paper drying is the most energy demanding stage

during which the major amount of heat is consumed to evaporate water in the paper

sheet. It is important to minimise the amount of water to be evaporated by

mechanical measures (pressing). Development of the press section (use of twin-wire

and extended nip press) has resulted in somewhat lower moisture levels of the paper

entering the drying section (this does not apply to tissue paper). In the case of

surface sizing or coating the dried paper has to be dried again after adding surface

glue or coatings to the paper web. Higher concentration and temperature of these

chemicals result in reduced heat consumption.

Covering the heat loss to the surroundings: The major part of heat losses with the

humid exhaust air from the drying section is compensated by inlet dry air that has to

be heated again. The heat requirements can be reduced by reducing the airflow

through the drying section. This gives also a higher humidity of the outlet air, which

increases the value of air as a source of secondary heat. Heat recovery through heat

exchanger between the outlet humid air and the inlet dry air reduces also the heat

consumption.

Conversion into electric power: An increasing number of mills have installed co-

generation of heat and power plants.


As seen, the papermaking processes can be divided into the main areas: stock

preparation, wet end, dry end including pressing and coating (optional). These can

be further sub-divided into main process units. Table 7.5 shows the role of energy in

each process and the potential for energy savings in these stages.

For a non-integrated paper mill using chemical pulp, refining (as part of the stock

preparation) will represent the largest use of electrical energy (drying being the

largest use of heat). The electrical energy used in refining is usually in the range


50

between 100 and 500 kWh/t for most papers but can be up to 3000 kWh/t for

speciality papers. Practically all of the energy input to this refining will be turned into

heat and there is no option here for energy recovery although this heat generated

contributes to the elevated temperature sought in the process.

The potential for energy savings will be high in many cases. For example, many

refiners are incorrectly sized or not well maintained and this results in a high no load

power, which reduces refiner efficiency. Incorrect refiner fillings will cause an

increased use of energy to achieve a given property. New refiners with enhanced

efficiency can also save energy because of the very low no load power associated

with this type of refiner.

Main processes

Main process units Type and role of energy in each process

Potential for energy saving

Stock preparation

Slushing Up to 60 kWh power /t to break up dry pulp Moderate

Cleaning/screening The amount of pumping energy and stock heating depend on the number of stages required and they type of fibre (recycled fibre needs more than virgin); About 5 kWh/t is used for virgin stock

Low for virgin fiber

Refining Very energy intensive. Electrical energy is mostly used to drive the rotor in the refiner. Depends strongly on the paper properties to be achieved; 100 - 3000 kWh/t

High

Wet end Forming and draining

It uses large amounts of electricity for machine drive and vacuum processes. Energy efficient design of the headbox and twin wire machine leads to power savings; About 70 kWh/t is used for vacuum systems (varies with grade and porosity)

Moderate

Dry end Pressing It is not energy intensive in itself but efficient dewatering can give very large energy savings in the dryers

Moderate

Drying Apart from refining it is the most energy intensive process in papermaking. Mostly heat energy

Very high

Size press and 2nd dryer section

Heat energy for after size press drying low

Calendering Electrical energy for machine drives and pressing

low

Coating Coating and dryer Electrical and heat energy for re-drying low Table 7.5: Role of energy in the main papermaking stages and potentials for

improvement

Electricity

For a very detailed discussion on the consumption of electricity in paper mills and the

technical background that build the basis for improvements and the application of

energy efficient technologies it is recommended to study the relevant chapters of the


51

BAT report on pulp and paper industry of the EU (see list of references and links at

the end of this report).

Heat recovery

The purpose of the heat recovery system is to lower the mill's consumption of primary

energy by utilising waste energy from the process in an economically profitable way.

Nearly all the heat energy consumed in a paper mill is used for paper drying, making

the dryer section easily the biggest energy consumer in a paper machine. Roughly

80% of the energy needed in the dryer section is brought as primary steam to the

dryer cylinders, the rest coming as drying and leakage air and with the paper web.

Nearly all energy leaving the dryer section is exhausted with the exhaust air. About

50% of this energy i.e. something like 620 kWh per ton of paper can be recovered by

an efficient heat recovery system (at winter conditions).

Typical applications are using either air-to-air heat exchangers or air-to-water heat

exchangers both of plate design (some applications use also scrubbers). The former

is mainly used for heating hood supply air and machine room ventilation air. The

most common application for the latter is the heating of circulation water and process

water respectively. These heat exchangers are part of heat recovery towers. In

Figure 7.5 an example is given for combined heat recovery where first the hood air

supply and secondly the circulation water is heated.

Figure 7.5: Example for a heat recovery tower

Usually only part of the heat recovered is led back to the dryer section with the hood

supply air. Most is used outside the dryer section to heat process water, wire pit


52

water and machine room ventilation air.

For a very detailed discussion of this topic it is recommended to study the relevant

chapters of the BAT report on pulp and paper industry of the EU (see list of

references and links at the end of this report).


53

8 Integrated pulp and paper mills

8.1 Processes

An integrated pulp and paper mill is an operation where both main processes, the

pulping and the papermaking, are integrated at one production site.

In consequence the processes and technologies which were discussed above are

combined in various ways according to the specific raw materials in use and

manufactured products.


Integrated mills can be more energy efficient than stand-alone mills, as no drying

energy is needed for the intermediate drying of the pulp. This will result in energy

savings at the pulp mill. Furthermore, process integration of the different processes

may result in a further optimization of the steam use on site. Finally, while stand-

alone pulp mills may have excess steam that cannot be used (due to black/green

liquor recovery or from heat recovery of the TMP), an integrated mill can use this

excess heat to serve the additional heat use of the paper machine.


Please refer to the distinct processes and technologies which have been discussed

before and which will constitute an integrated pulp and paper mill.


54

9 Worldbest practice and energy benchmarks

The following tables show the international best practice values of energy intensity in

the pulp and paper industry. The values have been compiled from mainly North

American and European experiences (taken from a study by Ernst Worrell at all,

2007). Energy values are given for final energy use only; for the adequate numbers

of gross energy (primary energy) the conversion factors for electricity generation

could be considered and self calculated.

The tables below respect the frequent practice and numbers are given separately for

• Integrated pulp and paper mills (comparing wood and recovered paper)

• Stand-alone pulp mills (comparing different technologies)

• Stand-alone paper mills (comparing different grades of papers)

Please note that these reported benchmarks are only indicating a direction of

potential improvements and that the individual conditions of a specific plant might be

very diverse from these numbers.

Table 9.1. World Best Practice Final Energy Intensity Values for Stand-Alone Pulp Mills (values are per air dried metric tons)

Raw Product Process Fuel Use for Steam Steam Exported Electricity Electricity Total

Use Produced Material

GJ/ADt kgce/ADt GJ/ADt kgce/ADt kWh/ADt kWh/ADt GJ/ADt kgce/ADt

Non-wood Market Pulp Pulping 10.5 358 -4.2 -143 400 7.7 264

Wood Market Pulp Kraft 11.2 382 640 -655 11.1 380

Sulfite 16 546 700 18.5 632

Thermo-mechanical -1.3 -45 2190 6.6 224

Paper Recovered

Pulp

0.3 10

330

1.5 51

Table 9.2. World Best Practice Final Energy Intensity Values for Stand-Alone Paper Mills (values are per air dried metric tons)

Fuel Use for Steam Electricity Use Total Raw Material Product Process

GJ/ADt kgce/ADt kWh/ADt GJ/ADt kgce/ADt

Pulp Uncoated Fine (wood free) Paper Machine 6.7 229 640 9.0 307

Coated Fine (wood free) Paper Machine 7.5 256 810 10.4 355

Newsprint Paper Machine 5.1 174 570 7.2 244

Board Paper Machine 6.7 229 800 9.6 327

Kraftliner Paper Machine 5.9 201 535 7.8 267

Tissue Paper Machine 6.9 235 1000 10.5 358

Table 9.3. World Best Practice Final Energy Intensity Values for Integrated Pulp and Paper Mills (values are per air dried metric tons)

Fuel Use for Steam Electricity Total Raw

Material

Product Process

GJ/ADt kgce/ADt kWh/ADt GJ/ADt kgce/ADt

Bleached Uncoated

Fine Kraft 14 478 1200 18.3 625

Kraftliner

(unbleached) and

Bag Paper

Kraft 14 478 1000 17.6 601

Bleached Coated

Fine Sulfite 17 580 1500 22.4 765

Bleached Uncoated

Fine Sulfite 18 614 1200 22.3 762

Newsprint TMP -1.3 -44 2200 6.6 226

Magazine Paper TMP -0.3 -10 2100 7.3 248

Wood

Board 50%

TMP 3.5 119 2300 11.8 402

Recovered

Paper

Board (no de-

inking)

8 273 900 11.2 384

Newsprint (de-

inked)

4 137 1000 7.6 259

Tissue (de-inked) 7 239 1200 11.3 386


57

10 Model case benchmarks

The Pulp and Paper Technical Association of Canada, which is one of the leading

institutes for this sector, performed in 2002 a model case study on “Energy and Cost

Reduction in the Pulp and Paper industry – A Benchmarking Perspective”. The

complete study can be downloaded from the internet for free (see references and

links).

The purpose of this study was to discus the motivation for developing an energy

efficiency program for the industry. In particular, it addresses two questions:

1. What is the potential for energy use reduction in pulp and paper mills?

2. How can this potential energy use reduction be achieved?

Benchmarking provides a means to determine the potential for energy reduction. A

benchmarking study is a comparison of the competitive situation among similar types

of mills producing the same product. The energy use for a particular mill can be

compared with that for similar mills or with that for a model mill representing the

current best practice.

To illustrate the potential to reduce energy consumption and greenhouse gas (GHG)

emissions, benchmarking studies were performed for the two largest production

segments of the Canadian pulp and paper industry:

• Kraft market pulp

The model kraft market pulp mill produces fully bleached market pulp from

wood chips transported from local sawmills. It utilizes the most energy-

efficient unit operations that have been proven technically feasible. The power

boiler uses hog fuel, and condensing-extracting steam turbines are used to

produce electricity. The total liquid effluent from the mill would be

approximately 35 m3/Airdried tonne (ADt).

• Newsprint

The model newsprint mill consists of a pulp mill and paper machine along with

an effluent treatment facility. The fibre furnish for a modern newsprint mill

would consist of thermomechanical pulp (TMP) and/or recycled fibre


58

depending on the fibre availability and market requirements.

In each case the energy consumption for a modern mill was determined using current

proven technology and compared with that for existing Canadian mills.

The authors draw the conclusions that there is considerable potential for energy use

reduction in pulp and paper mills.

• The model kraft mill can be operated with 9.9 GJ/ADt process steam, 1.2

GJ/ADt process fossil fuel and 578 kWh/ADt process electricity. The process

steam and electricity demands can be met by burning the spent pulping liquor

and the hog fuel associated with the chip supply. Thus, the only purchased

energy is the 1.2 GJ/ADt of fossil fuel needed for the lime kiln. The average

existing kraft mill, however, purchases considerably more energy: 5.99

GJ/ADt of fossil fuel and 272 kWh/ADt of electricity.

• The model newsprint mill had a fibre furnish of 80 percent TMP and 20

percent DIP. It can be operated with 0.2 GJ/ADt steam and 2430 kWh/ADt

electricity. The main steam demands are provided internally by heat recovery

from the refiner steam. The purchased energy is primarily electricity, 2430

kWh/ADt, with only 0.8 GJ/ADt fossil fuel needed for the continuous steam

demand of 0.2 GJ/ADt and to provide backup steam during upset conditions

in the TMP mill. The average existing newsprint mill purchases 2850 kWh/ADt

electricity and considerably more steam and fossil fuel, 4.46 GJ/ADt.


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11 References and Links

BEST AVAILABLE TECHNIQUES FOR THE PULP AND PAPER INDUSTRY, CEMBUREAU 2000 http://www.cembureau.be/Documents/Publications/CEMBUREAU_BAT_Reference_Document_2000-03.pdf

"Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies"

World Best Practice Energy Intensity Values for Selected Industrial Sectors - Ernst Worrell, Maarten Neelis - Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 2007 - http://ies.lbl.gov/iespubs/62806.pdf

Energy and Cost Reduction in the Pulp and Paper industry – A Benchmarking Perspective, Canada 2002 http://oee.nrcan.gc.ca/publications/infosource/pub/cipec/pulp-paper-industry/pdf/pulp-paper-industry.pdf

More information can be found by searching the cited links.

http://www.paperonweb.com/pmake.htm

http://www.paperonline.org/history

http://inventors.about.com/library/inventors/blpapermaking.htm#general

http://www.environmentaldefense.org/pdf.cfm?ContentID=1626&FileName=WP5.pdf.

http://www.hurterconsult.com/nonwood_uses.htm (non-wood pulping)

http://www.chempolis.com/index.html (non-wood pulping)