an overview of possible uses of sawdust 1985

7/27/2019 An Overview of Possible Uses of Sawdust 1985

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ATMICROFICHEREFERENCELIBRARYA project of Volunteers in Asia

By: Ir G.J. Arends & Dr. S.S. Donkersloot-Shouq

I

Published by: TOOL FoundationEntrepotdok 68 a1018 AD AmsterdamTHE NETHERLANDS

In conjunction with:CICATDelft Univ. of TechnologyP.O. Box 50482600 GA DelftTHE NETHERLANDS

Available from: TOOL FoundationEntrepotdok 68 a1018 AD AmsterdamTHE NETHERLANDS

Reproduced with permission.

CICAICMPEindhoven Univ. of TechnologyGebouw 0 Kamer 1P.O. Box 5135600 MB EindhovenTHE NETHERLANDS

Reproduction of this microfiche document in any form is subject to the samerestrictions as those of the original document.


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T6OL tech&ddavdopmentdavdopbrg-t-CICAT cientmm~mational~aml~~CICA ct3mmee orht~~coapsratknactvitlr#r

ANOVERVIIEWffmssllm USESOF

SAWDUST-

:i*

;. .y.+ &.,

k GJ. ARENDS

t)r SS DONKERSDOT-SHOUQ


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AN OVERVIIEW05LFvzEssll~ USES05SAWDUSTComplled.by IL LT. Arends

Delft Univereity of TechnologyDepartment of civil engineeringStevinweg 1P.O. Box 50482600 GA DelftThe Netherlands

Dr. S.S. Donkersloot - Shouq

Laboratory of chemical technologyDepartment of chemical engineeringElndhoven University of TechnologyP.O. Box 5135600 MB EindhovenThe Netherlands

May 1985

TOOL techmicallwdpmM dewelopIng countriesCICAT centm for memamaCICA

co-operation and appra@M technologycommittee fof international co-opevation actMtkt8


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Published by

Commissioned by

Typist

Lay out &cover design by

Prepared forpublication by

TOOL oundation

Entrepotdok 68 a1018 AD AmsterdamThe Netherlands

CICAT

Delft University of TechnologyP.O. Box 50482600 GA DelftThe Netherlands

CICA CMP

Eindhoven University of TechnologyGebouw0 Kamer 1P.O. Box 5135600 MB EindhovenThe Netherlands

The Directory General forEnvironmental Hygienics of theMinistry of Housing, PhysicalPlanning and Environment

Sandra Twisk

Albert Jan van Weij

Bees Hendriks


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TOOL

TOOL is a Dutch foundation linking el*:ven(non-profit) groups which together involveabout 400 volunteers based in universities,technical colleges and consulting engineeringfirms.

CICAT

The Centre for International Co-operation andAppropriate Technology is a mu&i.+-disciplinarycentre of the Delft University of Technology.

CICA/CMP

The Office for Development Co-operation isestablished by the Eindhoven University ofTechnology as supporting office for theInternational Co-operation Group.

Numerous people in developing countries findthemselves in a very difficult economic andsocial predicament. Appropriate Technology-h in many cases, solve their problems. Byplacing at their disposal knowledge und techno-108Y, appropriate to the local circumstances,

the above mentioned organisations wish to helpimprove the position of the less fortunate insociety.


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SATIS classificationotrm TOOL 550/222.1/222.5/424.1/632/?25

titk AII overview of possible uses of saubuat.A survey of applicable technologies.

utlnr Arends G 3, Donkereloot-shouq S S+MIUI TOOL, Entrepotdok 68~/69A, 1016 AD Amsterdam, the

N&htWlandS

~~~LIJO,SBN 90-70857-02-2 pgr 197 dh 1985 price . . .htws Bnglih ilk. 57 rdr. 160

utility book/scientific/research/international/sketches/general

lbsmn Kenya/fuel uses: direct cambustiont briquettingt carboni-@eyw0rW ation8 gasification/ agricultural uses: litter for

poultry arid cattle; fertilieer; animal feed/ buildingmaterial uses: insulationi reinforoement; particle boardmaking/ chemical uses: wood pretreatment8 pulp- and paper-

tmkingl miscellorntous uses

CIP-GEGEVENSONINKLIJKEBIBLIOTHEEK,DENHAAG

Overview

An overview of possible uses of sawdust: a survey

of applicable technologies / camp. by G.J. Arends,S.S. Donkersloot-Shouq. - Amsterdam: TOOL ;Eindhoven: CICA ; Delft: CICAT. - III.Commissioned by: The Directory General forEnvironmental Hygienics of the Ministry of Housing,Physical Planning and Environment.ISBN 90-70857-02-2SISO 670.1 UDC620.282-035.38 UGI 770Trefw.: zaagsel ; recycling.

The use of data, methods and/or results, given inthis publication is at your own risk. The publishersdeclare themselves not responsible for any damagearising from the use of these.


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CONTENTS

Summaryand conclusions1. Fuel from sawdust2. Agricultural uses of sawdust3. Sawdust in building materials4. Sawdust in chemical industries5. Miscellaneous uses of sawdust

PAGE

112373

103143189


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FUEL FROMSAWDUST

INTRODUCTION

1.1 DIRECTCOHBUSTIONSF SAWDUST 281.1.1 Sawdust as home fuel 29la1.2 Industrial combustion 311.1.2.a The fixed-bed combustors 321.1.2.b The spreader-stoker colvrbustors 331.1.2.~ The suspension combustors 351.1.2.d The fluidized bed combustors 35

1.2 DRIQUBTSFROMSAWDUST381.2.1 Pressing without a binder 39

1.2.1.a Handpresses 401.2.1.b Applied compaction machines (India) 411.2.1.~ Automotive compaction machines 421.2*2 Pressing with a binder 451.2.3 Charcoal briquets 46

1.3 CARBONIZA!lXONF SAWl)UST1.3.1 Fluidized bed carbonization1.3.2 Mobile pyrOli8i8 8y8telU

1.4 CASIPICATIONOF SAWDUST 551.4.1 Fixed-bed gasifier 561.4.2 Co-current bed gasifier 581.4.3 Fluidized bed gasifier 601.4.4 Imbert Stationary and mobile gasifiere 601.4.5 Lambiotte gasifier 621.4.6 Gunnermangasifier 621.4.7 An urban waste-wood-waste blend gasifier 64

1.5 coNcLus1:oN 65

APPENDICES

page

25

474751

66

68


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AGRICULTURALSESOF SAWDUST

INTRODUCTION

2.1. SAWDUSTS LITTER2.1.1 Deep-litter poultry system2.1.2 Sawdurtt a8 dairy bedding2.1.3 Other uses as litter

2.2 SAWDUSTS FERTILIZERAND SOIL CONDITIONER2.2.1 Sawdust compost in Kenya2.2.2 Composting system "Wilde"2.2.3

Sawdust compost with nutrients2,2.4 Compost of aged sawdust2.2.5 Other uses of sawdust for compost

2.3 FEEDFROM AWDUST

2.4 coNcLus1oN

REFERFWESANDBIBLIOGRAPRy

page

75

77777879

818485

889193

94

98

99


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SAWDUSTN BUILDINGMATERIALS

INTRODUCTION 105

'3.1 SAWDUSTSRD N ITS N6TURAL ORM 1063.1.1 Insulation material 1063.1.2 Reinforcement material 1063.1.3 Climate control material for fresh concrete 106

3.2 SAWDUSTN STONTMATEXIALS 1073.2.1 Sawdust in brick8 1073.2.2 Sawdust in mortar 107

3.2.3 Woodconcrete 1083.2.4 Woodgranite 109

3.3 SAWDUSTN BOARD RODUCTS 110110113115117119

119121122123124125126128128130

3.3.1.1 Particle board3.3.1.2 Particle board8 from sawdust3.3.1.3 The extrusion process3.3.1.4 Low cost particle board in India3.3.2 Fiberboard

3.3.2.la The Asplund-prOce883.3.2.lb The Masonite-proce883.3.2.2 From pulp to board83.3.2.3 Dry and semi-dry processes3.3.2.4 Propertie of fiberboard3.3,2,5 Sawdust in fiberboard production3.3.3 Medium Density Fiberboard8 (MDF)3.3.3.1 The Miller Hofft Process3.3.3.2 MDF or interior u8e

3.3.3.3 MDF or exterior u8e


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3.4 USE OF FINER SAWDUSTND WOOD'LOUR3.4.1 Wood lour production3.4.2.1 Molded articles

3.4.2.2 Plastic wood3.4.2.3 Linoleum3.4.2.4 Wall paper

3.4 USE OF FINER SAWDUSTND WOOD'LOUR3.4.1 Wood lour production3.4.2.1 Molded articles

3.4.2.2 Plastic wood3.4.2.3 Linoleum3.4.2.4 Wall paper

3.5 CONCLUSION.5 CONCLUSION

-s ANDBIBLIOGRAPHYBFBBENCIZSNDBIBLIOGRAPHY

As RAW

Page

133133135

137137137

139

140


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,,_

SAl$DUSTN CHEMICALNDUSTRIES

4.1 CHEMCALDESCRIPTION F SAUDDST4.1.1 Introduction

4.1.2 Classification of wood4.1.3 Chemical composition of wood4.1.4 Possible uses of wood constituents

on the chemical industry4.1.5 Separation of sawdust into it8 component

4.2 TECHNOLCXXF WOOD YDROIXSIS4.2.1 Introduction4.2.2 Conventional methods of wood hydrolysis

4.2.3 New methods for agriculture residuesand wood waste4.2.4 Sawdust hydrolysis, a pilot-plant study4.2.5 Derived chemicals:4.2.5.1 Ethyl alcohol4.2.5.2 Yeast *

4.2.5.3 Furfural4.2.6 Conclusion

4.3 MISCEuANEouS168

4.3.1 Woodextractives 1684.3.2 Tannins 1684.3.3 Turpentine and Rosin 1694.3.4 E88entiti Oils 1694.3.5 Vanillin from eawdust 1694.3.6 Oxalic acid 171

page

145145

146147

150152

156156158

159161

162164164167


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page

4.4 TBCHNOLoGyOFPULP-mPLLp-4.4.1 Introduction4.4.2 General method8 of pulpprocessing4.4.2.1 Chem.ical.pulping4.4.2.1.1 The sulphite process4.4.2.1.2 The sulphate or kraft process4.4.2.2 Mechanical pulping4.4.2.3 Thermomechanical pulping (T&P,)4.4.3.4 Semichemical pulping (S.C.P.)4.4.3 The conversion of sawdust into pulp4.4.4 Constraints and opportunities for

mechanical pulp4.4.5 Conclueion

172172174174174175176177177178

180183

184


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11

SJMMARYNDCONCLUSIONS

Forests in developing Countries are an important naturalre80urce: they provide wood for fuel and for buildingmaterial. A by-product Of WOOdprOCe88iag 8 8awdu8t. In Kenyafor instance, in the non-densily populated areas, where themajority of the ,SaWmillS 18 situated, the sawdust isconsidered fairly USele88 and ia therefore dumped in thedirect eurroundings of the mill8 and burned. Huge pile8 ofamouldering sawdust are the result. It can be assumed that asimilar 8ituation exist8 in developing countries withcomparable forestry resources.

The department of Social Housing, Physical Planning and theEnvironment of the Government of the Netherlands invited theTOOL Foundation to make a short survey of the possible u8e6of sawdust in developing countries, with special reference toKenya. TOOL made this survey in cooperation with the Centrefor International Cooperation and Appropriate Technology(CICAT) of the Delft Univereity of Technology and theCommittee for International Co-operation Activities (CICA) ofthe Eindhoven University of Technology.

In the survey the following pOSSibilitie6 tou8e sawdust aredescribed:

l Fuel from aawduat ( Chapter 1)l Agricultural uses of sawdust (Chapter 2)I Sawdust in building material8 (Chapter 3)I Sawdust in chemical industries (Chapter 4)l Miscellaneous u8e8 of sawdust (Chapter 5)

The chapter6 1, 2 and 3 are written by ir. G,J. Arends of theDclft University of Technology, Chapter 4 by dr. S.S. Donker-sloot-Shouq of the Eindhoven University of Technology andChapter 5 by the TOOLstaff.


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12

The survey is probably not exhaustive: from the Dutch view-point it seems impossible to get a complete list of all theuses of sawdust that have been researched, developed anddiscovered in -the world. However, the survey gives a fair

idea of what things one can do with sawdust.The processes and techniques used In the treatment of sawdustshow a wide variety: from use in its natural form to subjec-ting it to sophisticated processes; from labourintensive tocapital-intensive techniques; from the use of sawdust aloneto its use in combination with all kinds of other materials;from low energy input to high energy Input; from a smallsupply of sawdust to a large supply; etc.This variation makes it difficult to classify all the pro-

cesses described in the survey. The more so because theavailable material does not describe all possible aspects toa comparable extent. Sometimes the mechanical aspects of aprocess prevail, sometimes the chemical, sometimes theeconomical. This imbalance of information (for which nobodycan be blamed, of course) is reflected in the survey.

The first and foremost conclusion that can be drawn from thesurvey seems fairly obvious: sawdust is useful.

Sawdust is a very useful byproduct of woodprocessing and itwould be a pity if it would remain what it is often consider-ed to be: waste. There are simple sawdust-treatment techni-ques that can be used straightaway in developing countries.To avoid possible misunderstandings: many of these techniquesare already in use there.On the other hand one has to realise that the choice ofappropriate techniques In general is highly dependent uponlocal conditions. Only a detailed insight in local circum-stances makes relevant decisions on what to do with thesawdust possible.This detailed knowledge was not -yet- available for Kenya.Therefore the special reference to Kenya unfortunately willnot be so special as it should be.


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13

The second general conclusion is: transportation costs will bethe most decisive factor in the choice of sawdust-processingtechniques. Sawdust has a low specific gravity, is bulky and

is therefore expensive to transport. Sawdust should thereforebe processed In the immediate surroundings of its production-source: most of the products that can. be made with or of saw-dust are easier to transport than sawdust itself.

The information in the survey has to be read from the pointof view of appropriate and Intermediate technology. TOOL seessuch a technology as small scale, based on local resources,labour-intensive, easy to manage and, within this context,

economically viable.Just as it was impossible to make a proper classification ofthe sawdust-treatment technologies on the basis of theavailable material, it proved equally Pmposslble to makestatements on which process or technique should be consideredappropriate or Intermediate and which should not.

Modern high technology asks for an extensive infrastructurein terms of transport, marketing- and management-facilities.This is not the infrastructure found in developing countries.Also the oversupply of labour and the undersupply of capitalare not exactly prerequisites of high technology. This doesnot automatically mean, however, that for example a high-techmobile pyrollsis system to produce charcoal should be excludedbeforehand. It only means that detailed studies of the localconditions have to be made before appropriate decisions can bemade on what to do with these smouldering stacks of sawdust.


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1. FUEL FBOMSAWDUST

Wood Is the first and oldest means for firing. Sincesawdust is finely subdivided wood-fibre, it can be usedfor fuel, like wood. The most obvious way to do so is toburn it in its natural form: direct combustion.Some direct combustion drum-stoves have been developed fordomestic use. Sawdust can be directly combusted for indus-trial purposes inl fixed-bed combustors

. l spreader-stoker combustorsl suspension combustors andl fluidized-bed combustors.

Sawdust Is bulky and consequently expensive to store andto transport. Besides, the heating value is relatively low.Briquetting is an obvious way to decrease the bulk and toincrease the heating value. Other methods to increase theheating value of sawdust are carbonization andgasification.The borderline betwe& direct combustion, carbonization

and gasification is difficult to draw. The EngineeringExperiment Station of the Georgia Institute of Technologyhas developed a system which produces charcoal, oil andgen-gas at the same time.Briquetting can be done with and without a binder.Briquete can be pressed without a binder by manual, animaland mechanical power. There is quite a variety of auto-motive compaction machines on the market. Sawdust-briquetscan also be pressed with organic, inorganic and fiber-

binders. Charcoal briquets can be made either by pressingprepared charcoal or by carbonization of -,~ooZ riquets.Carbonization is the transformation of wood into charcoal.There are two systems of sawdust-carbonization: fluidized-bed carbonization and the mobile pyrolisis system.


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15

Sawdust can also be converted, by gasification, in the so-called gen-gas . Gen-gas can be used for heating and as fuelfor an internal combustion engine.Gasification systems are:I the fixed-bed gasifierB the co-current bed gaslfierl the fluidized-bed systeml the Imbert stationary and mobile gasifiersD the Lambiotte gasifierl the GunnermangasiflerI an urban waste-woodwaste blend gasifier.

The state 02 the art of the different uses of sawdust and

their technologies varies considerably.Direct combustion and briquetting is already commonpracticein many places in the Third World. The techniques usedcomply to some extent with the criteria for appropriatetechnology. There is an ample room for improvement andfurther research, however, for instance the direct-combus-tion drumstoves can be improved considerably. Directcombustion for industrial purposes could be considered whenthere is a large and steady supply of sawdust. Its most

obvious application is of course supplying the energy forthe sawmill itself.It seems appropriate to stimulate the use of briquettingtechniques in order to facilitate handling of the fuel anddecrease transportation costs. When binders are being used,possible negative effects on the environment have to betaken into account.The capital and infrastructural investments of the carboni-zation and gasification techniques are, on the whole,

considerable.More research and fieldtesting is necessary beforedecisions on the appropriateness of these techniques fordeveloping countries can be taken.


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16

2. AGRICULTURALSESOF SAWDUST

Sawdust is an organic material, so the search for profit-

able applications in agriculture is obvious.Agricultural uses of sawdust are: litter and bedding,fertilizer, sofl-conditioner and feed.The use of sawdust as litter and bedding is literallywidespread. The deeplitter poultry system is an economicmethod of converting sawdust and shavings into a usablecompost in temperate zones. In Zambia such a system, appro-priate for a tropical climate, was developed. In Malawi amethod has been developed to obtain good fertilizer by

using sawdust as litter in a cattle corral.There is an increasing use of sawdust as mulch, whichretards erosion, hinders weed growing, reduces water evapo-ration, Insulates the soil and keeps plants and fruitsclean.Sawdust can be used as fertilizer, although chemical sub-stances have to be added and the composting time Is 6 to 8months. Quite some research on the conversion of sawdustinto fertilizer and soilconditioner is being done in coun-

tries In the temperate as well as in the tropical climatezones. Wood is a potential source of energy for ruminants.Of course, it has to be made digestible first.

Generally many of the agricultural uses of sawdust can beconsidered appropriate for developing countries. Thisapplies especially for the use of sawdust as litter andfertilizer, as the required technologies are fairly cheapand easy to apply.The production of cattle-feed from sawdust requires a muchgreater capital investment. Besides, research in zhis fieldhas to be geared more to the situation in tropical areas.


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3. SAUDUSTN BUILDINGMATERIALS

Wood s the most prevalent building material. Since sawdustis a byproduct of woodprocessing, a lot of research has

already been done and is still being done on the use ofsawdust as building material. In its natural form sawdustcan be used for insulation (thermic isolation as well asnoise prevention), for reinforcement and for climatecontrol on fresh concrete surfaces.Sawdust is also being applied in stony materials. Mixedwith clay it is used in the production of bricks todecrease the weight and to increase their isolatingcapacity. Sawdust can be used as filler in mortar, which

than becomes ighter and cheaper.It is also possible to use sawdust in wood concrete andwood granite. Until recently the quantity of sawdust thatcould be used for the production of boardplates was limitedto 2 20%. Techniques have been developed by which platescan be made nearly completely out of sawdust.In India experiments are carried out with lowcost particleboards of sawdust and fibrous agricultural waste.In order to use sawdust in fiberboards it has to be pulpedfirst. Various methods for pulping sawdust have been des-cribed in chapter 4. In the USA fiberboards are made almostexclusively on a sawdust basis, while in Sweden mediumhardboard is made of up to 100% of sawdust with a littleglue added. Sawdust is a good raw material for the produc-tion of medium-density fiberboards, which can be used bothin interior and exterior construction.

Many sawduet applications in the building industry areknown and have proved their viability.Some of these techniques are cheap and easy to handle,especially when the oawdust is used in its natural form orin stony materials.However, the unavoidable transportation costs reduce theattractiveness of these uses.


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Application of sawdust in board products is a complicatedand capital-intensive process. The choice for boardproduc-tion seems only appropriate when large amounts of sawdust

are regularly available and when the market for the board-products is close by.

4. SAWDUSTN CHEMICALNDUSTRIES.

It is possible to convert wood or sawdust into a number ofuseful products by chemical processes. Various methods toachieve this purpose have been described in this chapter.

These methods will now be briefly evaluated.Woodpretreatment and fractionation

In this process sawdust is separated into its primaryconstituents. These can be converted into desired products,using different conversion processes.

This highly promising method has been, and still is thesubject of extensive research. However, the process ofautohydrolysis, i.e. pretreatment with steam, has beendemonstrated to produce high-energy ruminant feed from awide range of crop-residues and hardwoods. At presentcontinuous autohydrolysis systems are producing cattlefeed at two locations. It is high technology (high-pressure steaming) but not too complicated. It could beadopted in some developing countries.

Woodhydrolysis

Sawdust or wood on acid hydrolysis yields a mixture ofsugars, which can be used directly as a feed (molasses)for ruminants, or can be converted into other products.Due to the recent interest in the utilization of wastewood, research on wood hydrolysis is being carried out in


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a number of countries. At the moment, however, the productsare not sufficiently valuable to pay the cost of collec-ting, handling and processing of raw material. At the sametime interesting developments are being reported, forinstance production of yeast as an animal feed.

Pulping of eawduet and mod

Sawdust also seems to be a potential raw material forproducing pulp. The important processes for pulping woodand sawdust are given in section 4.4 and constraints andopportunities for mechanical pulping are discussed there.In principle sawdust can be used to produce pulp of accep-table quality. Sawdust pulp can easily be blended with alonger-fibre pulp in various ratios, depending upon thedesired end-products, or used as a filler to producevarious paper and paper-board grades. Bleached sawdustpulp (10-20X) can be .blended to produce a paper of goodquality.The choice of pulping method depends on the local treespecies, the price and the availability of base chemicalsfor chemical pulping and of energy for mechanical pulping.

Sawdust is not an easy material for pulpmaking andsuccessful utilization has only developed at places whereexperience and expertise in' this technology go togetherwith good research facilities. The latter are especiallyimportant as the applied method has to be optimized withrespect to local conditions and local wood varieties. Inorder to reduce transport problems (as often encounteredin developing countries) it seems desirable to set up smallintegrated sawmill/pulpmill/papermill combinations.

Environmental considerations demand a mainly mechanicalpulping method.


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..

20

Woodextractives

Small amounts of valuable substances can be extracted from

wood or sawdust from special trees. Extr-,action methods aresimple and well known.These methods are given in section4.3. However, they can be applied to only a few woodspecies.These methods are already practised in some developingcountries. In Kenya, for example, tannins, sandal oils andcedar oils are extracted from special trees.

The overall conclusion is that at present no chemical

process is directly applicable in developing countries.However, from the point of view of the precarious foodsituation existing in large parts of Africa at the moment,the potential of woodwaste and sawdust as a nutrient forruminants must not be ignored.It is possible to convert sawdust into animal feed: Somemethods have been described in chapters 2 and 6. Thesemethods are simple and easy to handle. However, they couldbe improved further. A suitable and simple method of pre-

treatment should be evaluated for use in developingcountries.In some countries like Pakistan intensive research isalready going on* to convert agricultural waste into rumi-nant feed. Also feeding tests on animals are performed.In such developing countries, where expertise and facili-ties are available a collaborative R and D prograAi@ onthe utilization of sawdust as a raw material could becarried out.

* From author's visit to Pakistan.


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21

5. MISCELLANEOUSSESOF SAWDUST

Sawdust compacted under high pressure with glue supplies a

moulding material for the production of interior buildingelements and household appliances. It can also be used inthe production of linoleum and wallpaper.however,

These uses,require complicated production processes and con-

siderable capital investment.More appropriate, i.e. for situations in which no highcapital investments are possible, are the following uses.Sawdust is used in the leather industry to facilitate thestaking and tacking of skins and in the fur industry toget the pelts into a pliable condition and to clean them.Selected sawdust is frequently used for curing meat andfish.Sawdust is a good packing medium.It can also be used for fire extinction, filtering,stuffing, cleaning, fire-lighters and bottle-stoppers.In France it is sometimes even used as a substitute forbran in bread-making.


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1

FUEL FROM SAWDUST

ir g.j. arends


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25

1. FUELFROM AWDUST

INTRODUCTION i

Wood is the first and oldest means for firing. Since sawdustis just finely subdivided wood-f ibre, it can be used ,f or fuel'too. Table 1.1 gives a typical analysis of hogged fuels (6)*.There are some problems however. Sawdust forms a layer which

* Numbers mentioned between brackets refer to the referenceslisted in the bibliography.

Table 1.1Neatern Hemlock Douglas Fir Fine Sawduet

Moieture aa received X s7.9 35.9Moisture air dried X 7.3 6.5 6.3Prorimte Analyeia, dryfuel Vol8tile matter X 74.2 82.0 79.4

Fired carbon X 23.6 17.2 20.1Aeh X 2.2 0.8 0.5

Ultimate Analyele. dryfuel Hydrogen % 5.8 6.3 6.3

Carbon X 50.4 52.3 51.8Nitrogen % 0.1 0.1

0.1Oxygen X 41.4 40.5 41.3Sulfur X 0.1 0.0AehX

Heating value, dry (Ml/kg) 2:::0.8 E

21.1 21.2

is very impervious to air when it is burned in a normal stoveor hearth, and because of its fineness' it falls through thefire-grate.

Over the years many ways of economical ways to use sawdust asa fuel were developed.

Besides the possibility of the direct combustion of sawdust,several methods to convert sawdust in one or more eminentfuels were developed.A more physical method is to compress sawdust to briquets,with or without a binder. Chemical methods are carbonization,gasification and distillation. Table 1.2 shows the heatingvalue of several wood-based fuels.


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./

.I

----faaumRAoam

lwutmRoaur-8!ltmnNo~&

I\

27

Figure 1.1 The Tech-Air system (27)

Analysis of the wood-based oil produced in the Tech-Air faci-lity (figure 1.1) shows,that the heat values of the heavierwood oils range from about 60% to 70% of the heating valuesof fuel oils. The wood based oils contain no sulphur, so theydo not create any eulphur emission problem when burned (20).Oil is mostly a by-product of gasification or carbonization,However, liquid fuels can be stored and transported easily,so they are the most versatile forms of energy.At the Pittsburgh Energy Research Center (Penn., USA) a system

is developed to convert wood-waste into a bitumen or heavyoil by processing it with water, sodium carbonate and carbonmonoxide at temperatures of 250 to 400C and pressures of10.5 to 24.6 ma.


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_.

28

1.1 DIRECT COMBUSTION OF SAWDUST

One mthad to burn sawdust in it8 natural form is to adapt an

incinerator.

Generally sawdust has a low heating value, because of itsusually high moisture content (varying from 30% to 60% of itsweight)(l8). 1Kg of dry sawdust has a heating vague of almost20 MegaJoule. This is comparable with about 0.7 kg of coal,about half a litre of fuel oil, or 0.5 cubic metres of naturalgas (6,8). The heat value of hogged wood with a moisture con-tent of 50% of weight is half the heating value of wood waste

with a moisture content of 10% (6). Combustion of sawdust atthe sawmill is the simplest digestion method.Table 1.3 gives a comparison of the cost per heat value ofseveral fuels with respect to the cost of sawdust (ex mill),before and after the energy crisis.

Table 1.3 Approximate cost of energy from various fuel8 inrelation to the cost of sawdust

FIlCl 1971 1) 1981 2)

Smduot l x DillDo plum 100 kmcartageChipr ex lillBark ax millCoalFuel 011LFCNatural r)aeElectricity

1) U.S.A.I), 2) Australia (13)

1

1.70.7

2.34.1 12.3

13.23.3

2:::

When discussing the direct combustion of sawdust a distinc-tion has to be made between the use of sawdust as home fueland industrial combustion.


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The following figure showe a very simple stove:

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l *-.-.-

.

m-:. l *

l . .*a

l l. .* l *.

. l .*

**.:. .*.*

.*..i .-

Figure 1.2 A simple stove (17)

It can be made of a 5 gallon square can in the following way(25):

a. Drill a hole in the bottom;b. Put a broomstick in the hole;c. Fill up the can with sawdust. The sawdust has

to be moistened and rammeddown after each inch;d. Remove he broomstick and place the filled can

on some bricks; 1e. Sprinkle some diesel oil or kerosene on the top

of the hole;f. Kindle the lightly oiled area.


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The sawdust will burn for 6 to 7 hours, while the burning ratecan be controlled by moving the bricks at the bottom of thecan, to vary the airflow through the hole.

Figure 1.3 shows an improved version, which is safer and canbe used inside a house. Instead of one hole, the stove hasfour holes. The burning takes place wholy inside the stove,while the flue gases are removed through the exhaustpipe. Thestove is removable to enable filling outside the house.Filling and igniting is done similarly to the previous stove.2 Kg of sawdust will give 4 to 5 hours of good cooking fire(26) l

4d No 1

No 2No 3No 4No 5No 6No 7No 8No 9No 10No 11No 12No 13No 14

Five gallon cmCrlllPotSupport (rod 8 dh)Support (rod t dla)Support (2) (rod # dla)Reducer flttlng 10x18 to 8n16 CQReducer flttlng 10x18 to 7x15 cmDuct of 10x18~45 cmDuct of 10x18~70 cmDuct of 10x18x57 cmSuet lon exhwt hood of 10x30 dlaDuct of 10x16~107 cmCap

Figure 1.3 A simple home cooking stove (17,261


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On this basis several stoves were developed, such as thedouble drum stove (figure 1.4) with a removable inner drum,and made from heavy sheet steel. This one is more suitable

for heating a room (34).

Figure 1.4 The double drum sawdust stove with innerdrum A andrampole B (34)

1.1.2 INDUSTRIALCOMBUSTION

Formerly, in order to cut down on fuel-expenses, most sawmillsheated their dry-kilns by burning wood wastes.Many sawmills and other woodworking plants used their woodwaste to feed industrial and central heating boilers, Lateron, sawdust was used as fuel too. This trend was caused bythe increasing demand for solid wood waste by other isdus-tries, the possibility of automatic feeding of the incine-rators, and the high cost of disposing of this waste (13).There are four general types of industrial wood-fired furnaces(6,8,13):

a) The fixed-bed combustors;b) The spreader-stoker combustors;c) The suspension combustors;d) The fluidized-bed combustors.


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32

a) THE FIXED-BEDCOMBUSTORS

In the fixed or packed-bed system, the fuel is dropped

through a hole so that a pile is formed, which is suppor-ted by a grate through which air is passed (figure 1.5).Before the material drops through the grate as ash, thewood waste goes through the successive drying, carboniza-tion and gasification zones. When there is insufficientair for complete combustion, extra air is added above thepile (or into the second chamber) to enable the combustiblegases and vapours to burn.Variations can be found in the way the pile is fed andwhether or not there is a second compartment to burn thegases in.The oldest and most simple, though common system is theDutch Oven, which coneists of two chambers (figure 1.5).In the first one the wood waste (sawdust and other hoggedwood waste) is dried and gasified. In the second one, thecombustion is completed under a boiler (6,13,8). DutchOvens can meet most anti-pollution requirements.

Figure 1.5 A Dutch Oven with steam boiler (6)

More recent combustors are the sliding grate combustor,the underfeed system combustor and the endless screwcombustor (figure 1.6) (19).


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1. Sliding grate 2. Underfeed system 3. Endless screw

Figure 1.6 Some ixed-bed systems (19)

The sliding grate combustor has a mechanically movable irongrate, with a drying zone, a combustion zone and a postcombustion zone. It even can be fed with sandy and humidwood waste. The ash is removed automatically.The underfeed system combustor is fed by a 8crew conveyorso that it look8 like a mole hill. Primary combustion airis blown through the fuel, while secondary air is added atthe top of the hill for combustion. The combustion effi-ciency 18 optimal if the total added air is slightly abovethe requirements. The maximum admissible humidity is 40%.An example of this system is the Kara-M.I.N. combustor,constructed by Kara, Almelo, the Netherlar&. Kara alsomanufactures smaller, hand-filled sawdust stoves (19).The endless screw combustor is fed by one or more screws.It can be used for fuels with a high ash content, and amaxim*unhumidity of 40%. Primary combustion air is blownthrough the blades of the screw(s) and secondary air isadded just above the glowing solids.

b) THE SPREADER-STOKEROMBUSTORS

Spreader-stoker or stoker feeder furnaces are fed by apneumatic or mechanical spreader system. It is more orless an intermediate form between the fixed-bed and the


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34

suspension combustors. A portion of the sawdust (or otherfine wood waste) is .burned in au8pension, while the restis spread in a thin bed over the grates where the combus-tion is completed (6,8,13). They are very popular but thepneumatically fed combustors especially produce a largequantity of flue gases, which reduces the profit (19). Tomeet rigid air pollution standards, an expensive, high-efficiency collection equipment would be required (8).A typical arrangement of a complete direct combustionsystem with a spreader stoker is shown in figure 1.7A. Adetail is given in figure 1.7B.

A B

Figure 1.7 A. A typical arrangement for a complete combustionsystem with a spreader stoker (32)

B. A detail of the spreader stoker (19)

This system is built by the American Fyr-Feeder Engineers, atDe8 Plaines (Ill., USA) (36). A German version is the Medio-plan, made by Mittelmann und Stephan at Laasphe (FRG) (16).See figure 1.8.


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. L .._j :- .-, , _) I< , z.-1,

:

35

, Figure 1.8 The Medio combustor (16)I

c) THE SUSPENSIONOMBUSTORS

During the 1960's, cyclonie-type furnaces appeared thatcould burn sawdust for steam raising. In the most usualtype, pneumatically conveyed sawdust was blown tangentiallyinto the upper section of a cylindrical combustion chamber(the vortex chamber), thereby creating a strong swirl,which on encountering the high temperatures, resulted inrapid combustion. These furnaces were designed to burn onlyfine particle-like wood residues, mostly with a lowmoisture content (13). Some will burn sawdust of up to 45%humidity of weight.

d) THE FLUIDIZED-BEDCOMBUSTORS

Fluidized or moving bed systems usually employ a bed ofhot sand; onto which the sawdust is fed. Air is blownthrough distributors, located at the base of the bed, tocause a violent motion of the sand and wood particles. Thewood material undergoes dehydration, carbonization, gasi-fication and combustion within the bed. Energy is provided


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36

-

1.

2.

3.4.5.

6.-

Advaatqaa ol FIuid Bed Iedncnton for Waste WomI Combustion

Durlu green hv+gtd wood waste wilhout cxpcnsive pledtying Mpulverizing-Z?xh cuber or finer-up to 55% mobtun/4S%bona dry mod.

Combustbn ot wood b self+ustalning. No supplcmcn tal rueI isrequired artrr bricf,stadup.Unique automatic 00 or p&cd standby system.ftolkr eflkicncy comparabk to oil-or gns4ircd equipment.Factory prcfabrlcated modular conntructbn reduces onsiteerectbn time, whkh can mull in substantial savings in construc-tbn I&or cost&Stack emissions @emrally meet all pollutbn rquiremcntr.

Figure 1.9A The fluidized-bed system (19,6)

in the form of a hot gas, which can be utilized in an iate-grated boiler, or by an exchanger in the bed (6,13,19).The velocity of the blown air is a function of the size,shape and density of the bed medium. Small and light parti-cles need lower velocity than large and heavy ones.Velocities higher than the minimum value required forfluidization do not necessarily improve operation, but caneven reduce the efficiency of the fluidized bed by loca-

lized spouting, excessive bed material carryover, and ashorter time for proper combustion to take place. Theminimum velocity needed, can be determined.Since the air expand8 when it passes up through the heatbed, the velocity increases. When the bed Is hot, less airis needed then when it is still cool. The effective air-fuel ratio depends also on the moisture content. Wettersawdust needs less air per kg of fuel, because there isless combustible material. For an economical process the

moisture content must be under 55% of weight.Before a wood particle begins to burn, the absorbed waterhas to evaporate. During this time, the temperature doesn'tgo higher than lOO"C, which is relatively cool. Much energy18 lost in this way.


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KLWK IKSULATlWMSTARLK MFMcmRY

..+-OYRRFLW DBDDISCHMCL

y ORtIIcK NW mrIcu)I-DRY HsD NWSACYORY

37

AIR

&LRM?McroRY~CASTARLE tNSUlATtOn

Figure 1,9B The fluidized-bed system (1)

When the quantity of the air blown through the bed, neces-sary for proper moving, is less than the air needed forcomplete combustion, air must be added above the bed.Capital costs of modem woodwaste-fired systems are high,because of the strong design requirements on emissionscontrol and safety, and the automation. However, once inoperation, they require minimal attention.The energy produced by these combustors can be used forsteamboilers, hot water boilers, heat exchangers (in whichheat transfering oil is circulated in a closed system, attemperatures of 180=26OC), direct heating with hot combus-tion gases (for the kilns), and exchangers transfering heatfrom the hot combustion gases to the air (13).


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38

1.2 BRIQUETS FROM SAWDUST

Sawdust in its natural form is a very bulky material with arelatively low heating value and high transport- and

storagecosts. A method to get a more profitable product andto facilitate its retail Value is to compress the sawdustinto briquets. Figure 1.10 shows some forms of briquets,

Figure 1.10 Forms of briquets (34)

Sawdust briquets are easy to kindle, give abundant heat in ashort time and are very clean (18, 23). Therefore one iswilling to pay the price which is comparable to that of coal.It will be clear, that the more expensive the other fuels(including solid firewood) are, the more economical it will

be to make briquets of sawdust.The briquets must have a sufficient toughness to withstandexposure to weather and shock6 during transportation. Duringcombustion, the exposure to heat must not cause disintegra-tion (23).For a profitable production, continuous operation is desi-rable. Large and continuous supply of waste must be availablefor industrial production, and the briquetting machinery mustbe located at the source of the waste (23).The waste mu;st have a moisture content of less than 10% toget the right strength. If the humidity is higher, the wastehas to be dried in drums on steam heated plates, by steampipes over which the waste is cascaded (23).


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39

When here is not enough sawdust available, it should be mixedwith other combustible solid waste for a profitable produc-tion. It can be mixed with bark, alfalfa, peat, coal, etc. Anexample is a briquetting process, which combine8 sawdust andcoaldust with chemicals. The mixture is compressed into logs100 mm n diameter and about 600 mm ong (23).

As early as the beginning of the 19th century, people triedto make briquets from sawdust (33). First binders such as tar,resins, clay, etc. were used. However, none of these processeshave attained any particular importance, because of the costinvolved (4,18). In those days, briquets pressed without abinder mostly were usually not 6uccesfu1, because temperatureand pressure were too low. In the 19506 several economicalmethods were developed to make briquets without a binder (23).

1.2.1 PRESSINGWITHOUT BINDER

After the First World War, a high grade sawdust briquet wasdeveloped by the Alabama Polytechnic Institute in Auburn(uSA)(23). The sawdust was preheated up to 275"C, to destroythe elasticity of the wood. This preheating also causes anevaporation of the moisture and most of the bounded ox;'.genand hydrogen from the wood, thus decreasing the weight withabout one third and almost doubling the heating value perkg. The sawdust gets a brownish colour, because it ha6 beenpartly charred. The preheated sawdust is briquetted at apressure of about 46 MPa and a temperature of 100C, withouta binder. Moisture must be added. The obtained (semi char-coal) briquets are proof against rough handling and weather-ing s if protected from rainfall. According to an extensiveinve8tigation, the above mentioned temperatures and pressures

give the best result (23).It has been found later, that at temperatures above theminimum plastic temperature (163OC), wood 16 more or le66


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40

self-bonding. The combination of preseure, cohesion of theinterfaces, interlocking of vibrous parts of the particlesand possible adhesion of the heat-softened lignin, cause8 abinding action. The briquets have to be cooled under pressure

(4).1.2.1.a HANDPRESSES

During the Second World War, diverse hand presses weredeveloped in Germany. Figure 1.11 Show8 two presses m$de byRebello.

A B

Figure 1.11 Singular (A) and plural (B) press (34)

The singular press ha8 a capacity of 100 to 150 briquets perhour. The size of the briquets varies from 100 to 200 mm indiameter and 10 to 100 mm in thickness, depending on thepress. The weight varies from 0.1 to 2 kg.The plural press has a capacity of 600 to 650 briquets perhour, each press action delivers 6 pellets. The pellets havea diameter of 90 mmand a weight of 80 to 700 grams (34).Figure 1.12 show6 a pressing machine, which can exercise 4 to5 times as much pressure. To reach this pressure a relative"long" time is needed. However, the machine also has thepossibility to deliver low pressure briquets in a shortertime. The latter were made by means of the handwheel for quickpressing, while the high pressure briquets were made by movingthe lever for high pressure pressing up and down fourteentime8 (34).


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Handwheelfor quickpressingLever forhigh-pre68urepressingDedutchinglever

Figure 1.12 A high pressure or fastpressure handpresa (34)

1.2.l.b APPLIEDCOMPACTIONACHINESINDIA)

After the world-wide energy crisis, the School of Applied

Research, Vishrambag at Lang11 (India), developed threeversion6 of compaction machines: one operated manually, oneoperated by bullock and a mechanically operated one. Besidessawdust, also agricultural wastes can be compacted with these(2) l

The manual version is specially useful for individuals orfamilies in rural areas. The hand press can provide poorfamilies with domestic fuel. By selling the fabricatedbriquets they do not need themselves, they can earn theirlivings.

The manually operated press work6 on the principleof a reciprocating engine. It consists of a flywheel, to berotated by hand, mounted on a crank-shaft with bearings onboth ends. The crank-shaft drive6 a plunger to compress thewaste into briquets with a diameter of about 30 mm and 10 mmthick. The rate of production is about 25 briquets per minuteor over 6 kg per hour.


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42

The bullock version can be used by farmers during the time ofthe year when they do not need their bullocks for normalfarmwork. The press, operated by a single bullock, has two

aeta of dies and punches, which are activated as the animalrotates, by two sets of simple side and face cams, driven bya central shaft. The machine is fed automatically, and has arotation speed of about 4 rpm. The output is '2 briquets perrevolution, each briquet weighing 30 to 50 grams, dependingupon the diameter of the used die (SO mm or 60 mm) and theused agrowaete. The capacity is about 20-25 kg/hour.

The powered version can be used there where usable agrowastes

are available in large quantities. This machine is driven byan AC motor.

1.2.1.~ AUTOMOTIVEOMPACTIONACHINES

To process large quantities of sawdust etc. an automotivepressing machines can be used.

Figure 1.13 Sawdust pressing plant (34)


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43

Figure 1.13 on the foregoing page 8hOWS uch an automotivepressing machine, made in Germany before World War II. Fromthe feedhopper B, the sawdust is carried to the drier D. Theevaporated water leaves the drier through the stack E, whilethe dry sawdust drops in the pelleting machine G. The pelletsare carried off through a cooling gutter (34). The plant ismade by Ganz 6 CO. at Ratibor (FRG).

In the USA a type of machine named "Pres-to-log", made byWood Briquettes Inc., et Lewiston (Id., USA)(23) is popular.Sawdust and the finer chips are compressed in a compressionchamber by means of a feedscrew,N/liUl12.

with a pressure of about 21At the outlet from this chamber, the compacted mate-

rial is cut into a spiral ribbon and forced into a cylindricalhole of a mold, under a local pressure of 175 to 210 MPa, Thefriction at this pressure generates sufficient heat to producethe neceersary plasticity for self-bonding. The mold8 with adiameter of about 100 mm are spaced at regular intervals inand extending through the rim of a large wheel (about 300 mmin diameter). The ases of the molds are parallel to the axisof the wheel. The bottom of the mold cavity is closed by ahydraulically operated piston. The piston retracks duringfilling and supplies the necessary resistance. The watercooled moldwheel revolves to bring the next mold into linefor filling. The resistance piston ejectes the cooled briquetbefore the mold reaches the fill location, The 100 by 300 mmbriquets are suitable for handfiring. The production rate isabout 500 kg/hour. One man can handle two machines (23).In case of mechanical stoking there is a machine availablethat extrude8 the selfbonded material through a cluster of

eight 25 mm wide round holes, to form continuous rods, whichare cut by rotating kniVc8 into pellets with a length of25mm. The production rate is about 800 kg/hour (23).


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44

In a Swiss patent, the "Glomera" process, briquets are com-pressed under a preseure of 120 MPa. The eawduet is forced bypistons or ram. The adjustable slight taper provides suffi-cient resistance to the flow of the material to develop high

pressure under the piston8 and thus causing the necessaryheat. Since the cohesion between the successive charge8 isless than the cohesion within each charge, the briquets tendto separate into disks. The obtained pellets have a diameterof about 80 to 90 mm and are about 6 to 25 mm thick. Thedouble delivery briquet-press produces 100 to 120 briquets aminute or 1 ton per hour, and can be tend by one man. Thepellet8 are not very suitable for mechanical stoking. In newermodels, the charge8 are precompressed to give greater density

(18,23),An American extrusion machine with 8 or 16 tubes, developedby W.W. Lette from Northvill (N.Y., USA) and manufactured byLandy Hill Iron and Brass Work8 at Hudron Falls (N.Y., USA),produce8 longer peilets. The pressure face of the piston hasa dimpled center and scalloped radial grooves, for a greaterinterlocking between the successive loads. One man can super-vise four 8-tube machines, with a total production grade of1.2 to 1.8 tons/hour (23).

In a process used by the California Pellet Mill Co. of SanFransisco @a., USA), the hogged wood waste is fed into a diecup with tapering orifices in its bottom. A roller in thebottom of the die cup, revolving under heavy pressure, forcesthe material through the orifices to form rode of dense mate-rial which are cut into short lengths by a rotating knifeunder the die cup. This machine has a production of about2000 kg/hour (23).

Recently developed presses are the Turbo briquetting press(made by Weima in the FRG), the German "Sp&nex" briquettingpress and the "Volmac" briquetting press from "Tukker Houtbe-werkiagSmachine8" in Amersfoort, The Netherlands. The latterproduce8 very dense briquets with a density of 1400 kg/m3 anda heat value of 17 to 18 MJ/kg (16).


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45

1.2.2. PRESSINGWITHA BINDER

When briquets are made with a binder, the binder must not

cause smoke or gummy deposits, while dustforming should beavoided. The binder must have a heat value which is at leastas high as wood (23). Environment pollution by the additionshave to be avoided.There are three classes of binders: inorganic materials(cement, sodium silicate, etc.), organic (tar, pitch, resinsglues, etc.) and fiber. Cement increases ash, decrease:combustibility and disintegrates during burning. Organicbinders usually increase the heat value and do not enlargethe ash quantity. Some of them do not disintegrate duringcombustion either. The best binders are usually too expensivefor economical use. Self-contained extractable binders ofwood are tars, formed in destructive distillation, and theresins in a few kinds of wood (23,341.The so-called Miller-process make8 briquets with hydrated woodfibers. These fibers should then be added in relatively smallproportions to the material and subsequently compressed by awet method combined, for economy, with air drying (33,341.

P.J. Weytmans Houthandel BV in Udenhout, The Netherlands,developed a process to make briquets consisting of 40 to 60%by weight of sawdust, 1 to 10% of eiac cinders, and the restof Bright Stork Slackwax. The latter is a good and cleanlyburning binding agent, while the cinders are added to getincombustible particles in the briquet to prevent the formingof carbonaceous crust. This crust hinders complete combusti-bility. As these briquets became too expensive, the produc-tion was stopped. Moreover the cinder8 can pollute the envi-ronment. Plants will not grow when there is too much zincaround.


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46

1.2.3. cBcLBca6~ rm&ms *

Charcoal briquets can be made either by pressing of. preparedcharcoal, or by carbonization of wood briquet8 (23).During World War II, Daeore and Moore developed a method inwhich dry sawdust was heated in molds, 80 that partial carbo-nization under the weight of the mold piston only, took place.Under a pressure of 2.5 Mpa, the sawdust is carbonized com-pletely. To drive off smoke-producing volatiles, the briquetsmust be heated further. (23).In the so-called Seaman process, distilled sawdust is mixedwith wood tar produced in the distillation pocese, and after-

wards briquetted and reheated in a retort, where the lighterfractions of tar are recovered and the charcoal particles arebound firmZly together. A very dense briquet results (23).A comparable system, the so-called Licalit-process (Germanpatent DRP 650045), was used in Germany during the secondWorld War. Here the evaporating gases were used as fuel todrive the pelletiter . The obtained briquets do not contain8ulphUr and phosphor and have a heating value of 31 to 33.5Iwkg. These briquete are especially suited to melt high

quality iron and steel (34).


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47

1.3 CARBONIZATPON OFSAWDUST

Carbonization is the conversion of wood into charcoal. Whendry wood is heated to temperature8 over 27O"C, pyrolysis orthermal decomposition take8 place to form charcoal and vola-tile matter.Charcoal by pyrolisis of wood has been done for centuries.Carbonization OfSawdUSt 18 a more recent idea.charcoal 18 a SmOkeleSS, clean-burning fuel and has a calo-rific value three times higher than wood and similar to thatof high quality coal: over 30 B&J/kg. A high proportion of thisenergy is emitted as radiant heat. The yield of charcoal byweight Is about 20 to 30% of the dry weight of the woodused,and by volumes about 50% (9).Mixing charcoal with fuel is being studied. Table 1.1 (on page1) gives the heating values of two mixtures in the ratio lot90and 20280 by mass. This slurry fuel has been used succeafullyin Australia and Papua New Guinea. However, provisions wouldhave to be made for the selection of pumps, valves, etc. andfor more frequent cleaning of fire-tubes, because charcoal/oilhas a higher ash content than oil alone (13).Besides fuel, charcoal can also be used a8 an absorbent (whentreated with steam or chemicals, activated charcoal can beproduced); as a carbon source (as rubber filler or in carbonelectrode8 for aluminum production); a8 a reducing agent(inthe processing of quartzite into silicon metal)and for horti-cultural applications (as potting medium for orchids etc.)(12).

1.3.1. FLUIDIZRD-RRD ARRONIZATION

The fluidized-bed carbonization plant, recently developed byCSIRO n Australia (12,13), is a combustor to produce charcoalfrom sawdust and other hogged wood waste. The sawdust is fedinto a sand bed, which is initially heatedwith preheated airup to a temperature of SOO"C, and kept moving by air injec-tion.


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The sawdust dries very quickly and carbonizes into charcoaland volatile gases. The latter are burnt in the fluidized bedby blown air. The liberated heat is more than sufficient tomaintain the required temperature in the bed. The process isthen self sustaining without any external heat (12,13).The light charcoal particles that are formed are lifted up bythe flue gases, and by way of a cyclone separator dropped in acharcoal hopper. The lump charcoal can be recovered by screen-ing the bed material and putting the sand back on the bed.The energy, released by the burning of the volatiles, which isneeded to maintain the required bed temperature can be usedfor the heating of oil in a heat exchanger, or for direct

steam production. For that purpose a heat exchanger, consis-ting of a coil, is placed in the moving bed. The heat transfercoefficient for the bed heat exchange is very high: up to0.47kW/m2/OC.In the case of oil-heating: oil, circulating in the coilwithin a closed system, is heated to temperatures of 180 to26O*C at low pressure and used for steam production in aboiler (figure 1.14, next page).

Steam can also be produced directly. The hot steam can beused both for heat and power. The latter by running a steamengine or generation of electricity by a turbine alteration.Figure 1.15 on page 69 Show8 a fluidized bedsystem with heatrecovery and electricity generation.


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2:56

x9

10111213141516

1718

iiTcPC

P

Iloidlred bedsewlwt hopperVerlebel lpeed DC motorBed hut uchaqerArch breakerSrev leederAir cwpteeeorAir filterCycloneDiaea@~ewnt rectioaPlenu chember TC:Porous dirtrlbutor pletc011 PUPOil filterBoiler091 expeaeiou twk

011 bypuo18aition burnerAit ptelmterOrifice plateTherencouplePrerrure dropPreroure 6*u6r

t FLUEGAS

9 16J-

izzA'. Cr-cc

LI ";i*qk*.---

-10OIL

x

HEATe

.-

TRANSFER*'

.tiuim,.HARCOALw&bl 5$4....*;y.3;. STEAM. * :-:, 4

49

*WATkR

Figure 1.14 Experimental fluidized-bed charcoal plant withhot-oil heat recovery at CSIRODivision ofBuilding Research (13)


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Figure 1.15 A fluidiaed bed system for charcoal production,heat recovery and power generation from sawdust(12)

A commercial fluidized bed plant for a typical sized sawmillwith dry kilns would be designed to convert 17.000 tonnes/

year of wood waste to 2300 tonnes of charcoal. Tabel 1.4show8 the capital cost8 and the DCF (Discounted Cash Flow)for plants with various options (12).

Table 1.4 Capital costs and Discounted Cash Flow analysis offluidieed bed plant options

Plant Option

1 Charcoal production only

EC1 1) DCF retura after tax

2 Charcoal prod. with heat recovery 22 x

3 Charcoal prod. with heat recovery30 x

and tlcctricfty geaeratloa $ 495 000 34 x

Charcoal price $ bO/tonne, diapoeal cost ravings of $ 3.SO/tonne, heatsaving0 0.74 c/I&I for mill61 requirement8 of 7.5 x 10 klfi (optione 2 6 3)electdclty price 4.74 c/kWh, annual output 2.4 CUh (option 31, plant liie15 yr.

1) ECP Estimated Capital Investment


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1.3.2. mqna PYBOLISIS Yst'm

51.

The Engineering Experiment Station in Atlanta (USA) developed

a mobile pyrolysis system. The whole system is mounted on twotrailers (30,321. It can be fed with sawdust and other solidcombustible forestry and agricultural wastes. The end productis charcoal mixed with pyrolysis oil. The start-up would beaccomplished using propane, but after that, the process iscompletely self-sustaining. Figure 1.16 shows the process ina flow diagram .

Figure 1.16 Process flow diagram of a mobilepyrolysis unit (30)

The sawdust is put in a feedhopper or receiving unit bin by afront end loader. Coarse material needs a feedhopper with ahammer mill. The dryer is heated by a part of the hot gases,produced in the burner. After drying, the material is conveyedto the convertor. The required process air is injected by ablower. The obtained char is emptied into a mixer where it iscombined with the pyrolysis oil.


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The latter is separated from the off-gases in a condenser. Inthis condenser the temperature has to be above the off-gasdeoipoint to avoid condensation of moisture. The part of theoff-gasles, to be used in the drier, are led through a burnerto burn the combustible gases. The rest of the off-gases isused to generate electricity.

Figure 1.17 shows the plant in the operational mode. A per-spective picture of an earlier version is shown In figure1.18.

1 hoot md lomder2 Rla coomyor3 R~cclvi~ bla4 co8vayoc ml115 l!hwr millS Dtlor7 Feed eonvworR9

10

ifl31415161716io202122232425

::28

Conr~rtor -CycloneCondcamorC0ad*nm0t coollrq fmDraft IanCcmbumtloa l lt Iluof L--n bummrDrier LmmIumer exbumtDrlmf ~hmmt duct.cyc1omhocwa air blourCemratortJll@ecooliq water radlmtorCompmawrConveyorChar 011 mixerChar stomp bloControl momA&ltbtar

29 Front rnd loader30 cat walk51 EO@W blomr

Figure 1.17 The mobile plant in operational mode (30)


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53

1. Loader

2. Yaace rerelvtn& bin3. clmpprr(r. Dryer5. Wasteonvertrr6. Cyclon* l*parator7. Condenrm6. Char-all mtr*r9. Char atoragmlo. 011 OtOta6*

11. Draft Cm12. GM wt~lll~13. Gancrrtor14. GM burner15. Control shed16. Loader rtarrm

Figure 1.18 Perspective of a mobile pyrolysis wasteconversion system (32)

All the required components, except the char storage bin, canbe placed on two trailers. For example the receiving bin canbe placed above the generator and the engine, by a collapsableramp stowed underneath. The trailers are fitted outside withmetal platforms on all sides, which can be fold out to get aworking space for the operating crew, consisting of two men,and local filler personnel.Since a ceramic insulator for the convertor is very heavy andfragile, the necessary insulation is obtained by making "charshelves" on the innerside of the convertor walls. Theseshelves will catch enough downward moving material to form agood insulator because of its relatively low conductivity. Afibrous insulator is used as a back-up system.


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an overview of possible uses of sawdust 1985

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