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ISBN 978-952-62-0103-0 (Paperback)ISBN 978-952-62-0104-7 (PDF)ISSN 0355-3191 (Print)ISSN 1796-220X (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAA




OULU 2013

A 607

Matti Kuokkanen

DEVELOPMENT OF AN ECO- AND MATERIAL-EFFICIENT PELLET PRODUCTION CHAIN—A CHEMICAL STUDY

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF SCIENCE,DEPARTMENT OF CHEMISTRY

A 607

ACTA

Matti K

uokkanen

A C T A U N I V E R S I T A T I S O U L U E N S I SA S c i e n t i a e R e r u m N a t u r a l i u m 6 0 7

MATTI KUOKKANEN

DEVELOPMENT OF AN ECO-AND MATERIAL-EFFICIENTPELLET PRODUCTION CHAIN—A CHEMICAL STUDY

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in Wetteri-sali (Auditorium IT115), Linnanmaa,on 26 April 2013, at 12 noon

UNIVERSITY OF OULU, OULU 2013

Copyright © 2013Acta Univ. Oul. A 607, 2013

Supervised byDocent Toivo KuokkanenDoctor Tuomas Stoor

Reviewed byDocent Ari VäisänenDoctor Antero Moilanen

ISBN 978-952-62-0103-0 (Paperback)ISBN 978-952-62-0104-7 (PDF)

ISSN 0355-3191 (Printed)ISSN 1796-220X (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2013

OpponentProfessor Lauri Sikanen

Kuokkanen, Matti, Development of an eco- and material-efficient pellet productionchain—a chemical study. University of Oulu Graduate School; University of Oulu, Faculty of Science, Department ofChemistry, P.O. Box 3000, FI-90014 University of Oulu, FinlandActa Univ. Oul. A 607, 2013

Abstract

According to the EU’s strategy and the corresponding Finnish national strategy on wastematerials, all kinds of waste must be utilised primarily as material (reuse, recycling) andsecondarily as energy, and at the lowest level of waste hierarchy is their disposal usingenvironmentally friendly methods. Today material efficiency is an essential topic in promotingsustainable use of natural resources, industrial by-products and waste material. The present goalproposed by the EU sets the target for the total proportion of renewable energy as high as 38% by2020 in Finland. Up to 20 million tonnes of waste wood biomass per year are left unused inFinland, mainly in the forests during forestry operations, because supply and demand do not meet.As a consequence of high heat energy prices, the looming threat of climate change, the greenhouseeffect and global as well as national demands to considerably increase the proportion of renewableenergy, Finland currently has a tremendous interest in increasing decentralised pellet productionalongside of large-scale factories.

The aim of this thesis is to promote the development of eco-, material- and cost-efficientNordic wood-based pellet production and utilisation of pellet bio-ash by means of chemicalresearch. Using Finnish wood (sawdust and shavings) as a model raw material, the totalfunctionality of a pilot-scale pellet facility combined with an extensive chemical toolbox wastested in this study to promote development of an eco-, material- and cost-efficient wood-basedpellet production chain. The chemical toolbox includes measurements of moisture content,density, heat value, mechanical durability and particle size distribution, TG analysis andelementary analysis, as well as new applications for pellet biodegradation using BOD OxiTopequipment and optical microscopic staining methods.

To improve the quality of pellets, considering the profitability of production and occupationalsafety factors (wood dust exposure, fire and explosion risk), it is profitable to use different bindingagents, especially industrial by-products and locally utilisable residuals. Thus, lignosulphonate,residual potato flour and potato peel residue were used and tested as model adhesive bindingagents. The results showed that binding agents increased the quality of pellets and changed theirinorganic characteristics, but did not have a significant effect on their calorimetric heat values.Lignosulphonate even increased the rate of production. To characterise different starch-containingbinding agents, a new specific optical microscopic staining method was developed and tested, andthe initial results are presented in this thesis.

Wood pellet ash has potential as a liming agent, in soil remediation, as a soil fertilizer, and ingranulated form, in new applications such as road construction and waste water purification.Valuable information about raw materials, binding agents and the pelletizing process is necessarywhen developing good-quality pellets—a prime biofuel—from non-utilised low-value and/ormoist biomass that has undergone a cost-efficient drying process. This way pellet production willhave more essential importance in energy policy, especially in the European forest belt.

Keywords: binding agent, bio-ash, biodegradation, bioenergy, biomass, eco-efficiency,forestry, pellet production, wood pellet

Kuokkanen, Matti, Eko- ja materiaalitehokkaan pellettituotantoketjun kehittä-miseen liittyvä kemiallinen tutkimus. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Luonnontieteellinen tiedekunta, Kemian laitos,PL 3000, 90014 Oulun yliopistoActa Univ. Oul. A 607, 2013

Tiivistelmä

Vallitsevan EU:n sekä Suomen kansallisen lainsäädännön mukaan kaikenlainen jäte täytyy hyö-dyntää ensisijaisesti materiaalina (uudelleenkäyttö, kierrätys), toissijaisesti energiana ja jätehie-rarkiassa alimpana tasona on sen hävittäminen ympäristöystävällisin keinoin. Materiaalitehok-kuus on nykyään välttämätön aihe edistettäessä luonnonvarojen, teollisuuden sivutuotteiden jajätemateriaalien kestävää käyttöä. EU-strategian mukainen tavoite uusiutuvan energian osuudel-le kaikesta energiantuotannosta Suomessa on 38 % vuoteen 2020 mennessä. Jopa 20 miljoonaatonnia jätepuubiomassaa vuodessa jää käyttämättä Suomessa lähinnä metsänharvennustöidenyhteydessä, koska kysyntä ja tarjonta eivät kohtaa. Seurauksena korkeista lämpöenergiahinnois-ta, uhkaavasta ilmastonmuutoksesta, kasvihuoneilmiöstä sekä globaalisista ja kansallisista vaati-muksista lisätä uusiutuvan energian osuutta, Suomessa on viime aikoina noussut voimakas kiin-nostus lisätä hajautettua pellettituotantoa suurten pellettilaitosten rinnalle.

Väitöskirjan tarkoituksena on edistää ja kehittää pohjoismaista eko- ja kustannustehokastapuupellettituotantoa ja pellettibiotuhkan hyötykäyttöä kemiallisen tutkimuksen avulla. Käyttäensuomalaista puuta (sahanpurua ja kutterinlastua) malliraaka-aineina, tässä tutkimuksessa testat-tiin pilot-mittakaavan pellettilaitoksen toimivuutta yhdistettynä laajaan kemialliseen ”työpaket-tiin”, edistämään tulevaisuuden eko-, materiaali- ja kustannustehokkaan pellettituotantoketjunkehittämistä. Kemiallinen työpaketti sisältää kosteuden, tiheyden, lämpöarvon, mekaanisen kes-tävyyden ja partikkelikokojakauman määritykset, TG- ja alkuaineanalyysin, kuten myös uudetsovellukset pellettien ja niiden sideaineiden biohajoavuuden määrittämiseksi BOD OxiTop -lait-teistoilla sekä optisen mikroskooppivärjäysmenetelmän.

Pellettien laadun kohottamiseksi, ottaen huomioon myös tuotannon kannattavuuden ja työter-veydelliset ongelmat (puupölylle altistuminen, tulipalo- ja räjähdysvaara), on perusteltua käyt-tää sideaineita, erityisesti teollisuuden sivutuotteita ja paikallisesti hyödynnettävissä olevia jäte-materiaaleja. Täten lignosulfonaattia, jäteperunajauhoa ja perunankuorijätettä käytettiin ja testat-tiin liimaavina mallisideaineina. Tulokset osoittivat, että sideaineet nostivat pellettien laatua jamuuttivat niiden epäorgaanisia ominaisuuksia, mutta niillä ei ollut merkittävää vaikutusta määri-tettyihin lämpöarvoihin. Lignosulfonaatti lisäsi selvästi pelletoinnin tuotantonopeutta. Työssäkehitettiin pelleteille uusi spesifinen optinen mikroskooppivärjäysmenetelmä erilaisten tärkke-lystä sisältävien sideaineiden karakterisointiin ja ensimmäiset tulokset on esitetty tässä väitöskir-jassa.

Puupellettituhka on potentiaalinen kalkitus- ja maanparannusaineena, lannoitteena sekärakeistettuna uusissa sovelluksissa, kuten tierakentamisessa ja jäteveden puhdistuksessa. Arvo-kas informaatio raaka-aineista, sideaineista sekä pelletöintiprosessista on välttämätöntä kehitet-täessä tulevaisuudessa hyvälaatuisia pellettejä, ”priimaa” biopolttoainetta, hyödyntämättömästähuonolaatuisesta ja/tai kosteasta biomassasta, joka on ennen pelletointia käynyt läpi kustannuste-hokkaan kuivausprosessin. Täten voidaan olennaisesti lisätä pellettituotannon merkitystä ener-giapolitiikassa, erityisesti Euroopan metsävyöhykkeellä.

Asiasanat: bioenergia, biohajoavuus, biomassa, biotuhka, ekotehokkuus, metsätiede,pellettituotanto, puupelletti, sideaine

7

Acknowledgements

This work was mainly carried out at the Department of Chemistry, Laboratory of

Physical Chemistry, and at the Fibre and Particle Engineering Laboratory at the

University of Oulu. This research was financially supported by the University of

Oulu, Vapo Inc., Formados Inc., Naturpolis Oy, Kuusamon Energia- ja

Vesiosuuskunta, The Fortum Foundation, the Tauno Tönning Foundation and

Oulun Läänin Talousseuran Maataloussäätiö. All of these founding bodies are

greatly acknowledged. In addition, I would like to thank the following people

who have helped and encouraged me during this work:

To begin with, I would like to express my deepest gratitude to my

supervisors, Docent Toivo Kuokkanen (Ph.D.), Tuomas Stoor (Dr. Tech.) and

Jouko Niinimäki (Dr. Tech, Professor), for providing me with the opportunity to

work with this interesting subject. Toivo, Tuomas and Jouko have always

instructed me patiently and given me very important advice in relation to my

research. Special thanks go to Hanna Prokkola (M.Sc.) for her valuable

contributions with the experimental work. Also the co-authors of my articles, Veli

Pohjonen (Dr. Tech, Professor), Teemu Vilppo (M.Sc., Tech.), Jaakko Larkomaa

(M.Sc., Tech.), Leena Siltaloppi (M.Sc.), Hannu Nurmesniemi (Ph.D., Docent)

and Risto Pöykiö (Ph.D., Docent), are warmly acknowledged. In addition, I

would like to thank Suomen Ympäristöpalvelu Oy and Ilkka Välimäki (M.Sc.) for

the analyses of the bio-ash samples and Formados Inc. and especially CEO Kyösti

Nevala, who were involved in the full industrial scale manufacture of pellets at

their pellet plant in Kuusamo.

I also wish to thank Mr. Risto Ikonen from the University of Eastern Finland,

Ph.D. Juha Koskela, M.Sc. Juhani Kaakinen, M.Sc. Riitta Raudaskoski, M.Sc.

Henna Jokinen and M.Sc. Tommi Kokkonen from the University of Oulu and

M.Sc. Ritva Imppola, M.Sc. Mikko Aalto and M.Sc. Tech. Heikki Takalo-Kippola

from Oulu University of Applied Sciences and Ms. Reetta Kolppanen from the

Finnish Forest Research Institute Metla for their scientific co-operation and

Kotirannan Vihannesjaloste Oy and CEO Timo Jaakola, Evijärven Peruna Oy,

Liperin Juurespakkaamo Oy and Pinifer Oy for their binding agent co-operation.

Lastly, I want to thank my fellow students and especially my friends. My

warmest gratitude goes to my parents Vuokko and Toivo and to my brothers Mika

and Ville and their families. They all have encouraged and supported me during

my studies and reminded me about the significance of education and hard work.

Oulu, 2013 Matti Kuokkanen

9

Abbreviations and definitions

EU European Union

TG Thermogravimetry

BOD Biological oxygen demand

ICCE International Congress of Chemistry and Environment

ICSW International Conference on Solid Waste Technology and

Management

LCA Life cycle assessment

CEN/TS European Committee for Standardisation / Technical Specifications

PM Particulate matter

CHP Combined Heat and Power

REACH Registration, Evaluation and Authorisation of Chemicals

EC European Council

PPW Potato peel waste

SFS-EN Finnish Standards Association

d.w. Dry weight

ICP-OES Inductively coupled plasma optical emission spectrometry

VOC Volatile organic compound

BET (Brunauer, Emmett and Teller) Theory

SEM Scanning electronic microscopy

ISO International Organization for Standardization

BMA Biomaterial moisture analyser

DIN Deutsches Institut für Normung

ASTM American Society for Testing and Materials

EEC European Economic Community

OECD Organisation for Economic Co-operation and Development

BOD28 Biological oxygen demand in 28 days

ThOD Theoretical oxygen demand

TA Thermoanalytic

DSC Differential scanning calorimetric

EPA Environmental Protection Agency

L/S Liquid to solid ratio

VNa Valtioneuvoston asetus (in Finnish) (Government Decree)

PIMA Asetus pilaantuneista maista (in Finnish) (Assessment of Soil

Contamination and Remediation Needs)

PCC Precipitated calcium carbonate

10

PBPM Potential bioavailability percent

R&D Research and development

FINAS Finnish Accreditation Service

EAKR Euroopan Aluekehitysrahasto (in Finnish) (European Regional

Development Fund)

MC Moisture content

BOD7 Biological oxygen demand in 7 days

TOC Total organic carbon

COD Chemical oxygen demand

GC-MS Gas chromatography-mass spectrometry

PAH Polycyclic aromatic hydrocarbon

wt% Weight percent

vol-% Volume weighted percentage

LOI Loss on ignition

NV Neutralising value

BCR Community Bureau of Reference

SPE Suomen Pellettienergiayhdistys ry (in Finnish) (Finnish Pelletenergy

Association)

RME Rape methyl ester

11

List of original papers

This thesis is based on the following articles, which are referred to in the thesis by

their Roman numerals:

I Kuokkanen M, Kuokkanen T, Stoor T, Niinimäki J & Pohjonen V (2009) Chemical Methods in the Development of Eco-efficient Wood-based Pellet Production and Technology, Waste Management & Research 27: 561–571.

II Kuokkanen M, Vilppo T, Kuokkanen T, Stoor T & Niinimäki (2011) Additives in Wood Pellet Production – A Pilot-Scale Study of Binding Agent Usage, BioResources 6 (4): 4331–4355.

III Kuokkanen M, Prokkola H, Larkomaa J, Stoor T, Siltaloppi L & Kuokkanen T (2010) Specific Staining and Optical Microscopy – a New Method for Characterisation of Starch-containing Wood Pellets, Special Issue of Research Journal of Chemistry and Environment, Proceedings of ICCE-2009: 311–317.

IV Kuokkanen M, Kuokkanen T, Nurmesniemi H & Pöykiö R (2009) Wood pellet ash – a potential forest fertilizer and soil conditioning agent (a case study), The Journal of Solid Waste Technology and Management, Proceedings in ICSW 2009: 659–667.

V Kuokkanen M & Kuokkanen T (2009) Puu- ja turvepellettien sekä hakkeen lämpökeskus- ja pienpoltossa syntyvien tuhkien hyötykäyttöön liittyvä tutkimusraportti. University of Oulu, Report Series in Chemistry, Report No. 74.

The present author was the primary author and corresponding author of all articles

I–V of this dissertation. The design of analyses, the bulk of the experimental work

as well as the analysis of the results related to Articles I and III were carried out

by the present author. The design of the research and the analysis of the chemical

results as well as partly the experimental work of Article II were the work of the

present author. The design of the research and the analysis of the results but only

partly the experimental work of Articles IV and V were carried out by the present

author.

In addition, the results of the article “Kuokkanen, M., Vilppo, T., Kuokkanen,

T., Stoor, T. & Koskela, J. (2011) Pilot-mittakaavainen sekä kemiallinen tutkimus

eräiden lisäaineiden käytöstä puupellettituotannossa, EkoPelletti, Raportit ja

julkaisut, 21 pp. http://www.oamk.fi/hankkeet/ekopelletti/docs/pilot-

mittakaavainen.pdf” are discussed in this thesis. The article was jointly written by

the present author and co-author M.Sc. Teemu Vilppo.

13

Contents

Abstract

Tiivistelmä

Acknowledgements 7 Abbreviations and definitions 9 List of original papers 11 Contents 13 1 Introduction 15 2 Aims of the study 23 3 Materials and methods 25

3.1 Materials ................................................................................................. 25 3.1.1 Lignosulphonates .......................................................................... 25 3.1.2 Potato starch ................................................................................. 26 3.1.3 Potato peel residue ........................................................................ 27 3.1.4 Wood pellet ash ............................................................................ 28

3.2 Research methods ................................................................................... 28 3.2.1 Moisture content measurement ..................................................... 29 3.2.2 Density measurements .................................................................. 30 3.2.3 Calorific heat measurement .......................................................... 32 3.2.4 BOD measurements ...................................................................... 33 3.2.5 Mechanical durability measurement ............................................. 36 3.2.6 Microscopic structure analysis ..................................................... 36 3.2.7 TG analysis ................................................................................... 37 3.2.8 Elemental analysis ........................................................................ 38 3.2.9 Particle size distribution ............................................................... 39 3.2.10 Sequential leaching procedure ...................................................... 39

4 Pelletizing machinery 43 4.1 Delivery of raw material and receiving stock ......................................... 43 4.2 Drying unit and pre-treatment of raw material ........................................ 43 4.3 Pelletizing machine ................................................................................. 44 4.4 Pelletizing field tests ............................................................................... 47

5 Experimental work 51 6 Results and discussion 53

6.1 Moisture content ..................................................................................... 53 6.2 Calorimetric heat (I,II) ............................................................................ 54 6.3 BOD measurements (I,II) ........................................................................ 57

14

6.4 Binding agent usage (I, II)....................................................................... 60 6.5 Visual analysis of pellet sample images (I, II, III) .................................. 64 6.6 TG analysis ............................................................................................. 68 6.7 Particle size distribution (II) .................................................................... 71 6.8 Wood-pellet-based bio-ash (II,IV,V) ....................................................... 73

6.8.1 Utilisation potential of bio-ash ..................................................... 82 7 Conclusions 85 8 Future work 89 References 91 Original papers 99

15

1 Introduction

It is estimated that 15–20 million tonnes of waste wood biomass per year are left

unused in Finland, mainly in the forests during forestry operations, because

supply and demand do not meet. According to the present Finnish strategy on

waste materials—in accordance with the corresponding EU strategy—all kinds of

waste must be utilised primarily as material (reuse, recycling) and secondarily as

energy, and if neither of these utilisation methods is possible, they can be

disposed of using ecologically beneficial methods. Today material efficiency is an

essential topic in promoting sustainable use of natural resources, industrial by-

products and waste material, in agreement with the principles of sustainable

development and LCA (life cycle assessment). Recycling and exploitation of

waste are perceived as also belonging to the concept of material efficiency.

Formation of waste can be considered an indicator of ineffectiveness in industry

[1,2,3]. Eco-efficiency means optimal combined financial and ecological use of

natural resources and can be described as a management strategy of doing more

with less. It is achieved by increasing product or service value, optimizing the use

of resources and reducing environmental impact. In accordance with these goals

and as a consequence of high heat energy prices, the looming threat of climate

change, the greenhouse effect and global as well as national demands to

considerably increase the proportion of renewable energy, there are currently

goals of expanding the raw material base in Finland and also substantially

increasing pellet production. As a part of this European objective to increase the

eco- and cost-efficient utilisation of bio-energy from the European forest belt, the

aim of our research group is—by means of multidisciplinary research, especially

through chemical methods—to promote the development of Nordic wood-based

pellet production in both the qualitative and quantitative sense. It is noteworthy

that wood-based pellets are classified as an emission-neutral fuel, i.e. they are free

from emission trading in the EU.

Wood pellets, taking into consideration the large amounts of utilisable waste

wood material worldwide, have been developed and patented surprisingly late.

The first patent for wood pellet manufacturing was registered in 1976 in the USA

in the aftermath of the global oil crisis. Wood pellets are short cylindrical pieces

(usually with a diameter of 6 to 10 mm and a length of 10 to 30 mm) which are

produced mechanically by compressing a uniform material that has first passed

through a hammer mill or mills to provide a homogeneous particle size. This mass

is fed to a press where it is pressed through a die with holes of the size required.

16

Typically the diameter of pellets is 8 mm in the Nordic Countries, 6 mm in

Central Europe [4] and sometimes 15 mm and up to 25 mm according to CEN/TS

14961 [5]. The high pressure and friction of the press causes the temperature of

the wood to increase significantly, slightly plasticizing the lignin in a temperature

range of 100 to 130 °C [6] and forming a natural 'glue' that holds the pellet

together as it cools. At the same time pelletizing increases the density of the

biomass, which is especially important logistically when low-density biomass is

pelletized. The initial raw materials used in pellet production are sawdust and

cutter shavings, which are by-products of the mechanical forest industry. The raw

material used in pellet production is typically stem wood. It has been concluded

that utilisation of high-quality wood fuel, such as wood pellets produced from

natural, uncontaminated stem wood, would generate the least particulate matter

(PM) emissions compared with other wood fuel types [7]. Taking into

consideration the large amounts of utilisable residual wood materials worldwide

and the high price of crude oil, global interest in developing pellet technologies

and increasing wood-based energy production has grown in recent years. The

present goal suggested by the EU sets the total proportion of renewable energy as

high as 25% by 2020. As a consequence of the EU strategy, this proportion must

grow to even 38% in Finland by 2020.

The biggest biomass potential in the EU region lies in forested areas (see Fig.

1). It is worthwhile noticing that these are the most rural areas and there are no

other methods available there for utilising this biomass potential, because supply

and demand do not meet in these areas where biomass is growing perhaps more

than ever before (biomass potential in more detail in paper I). It is notable that if

the pellet market increases in the future, it will have much more financial

importance to rural areas than its production volume indicates, because pellet

prices are much higher than wood chip prices. Correspondingly, it has a stronger

employment factor for these areas than wood chip production. On the other hand,

in Finland these are the same areas that have potential in the strongly growing

mining business, especially in the Lapland, North Karelia and Kainuu areas. This

is important because wood pellets could be a partial source of energy in these

mines as the prime biofuel. Altogether this is a matter of cost-efficiency. Forest

economists have calculated that with present costs the maximum distance for

profitable transport of forest chips, sawdust and shavings in Finland is 100 km,

for round wood it is approximately 1000 km, but for wood pellets transported by

sea the figure is, however, even 5000 km because sea transportation is the

cheapest way to transport. Shorter-distance transports of biomass in land areas are

17

carried out by trucks and trains, which are more expensive, but faster on the other

hand.

Fig. 1. Forest map of the European region [8].

These calculations, in conjunction with the general demands to increase

utilisation of renewable energy as opposed to fossil fuels, explain why there is

strong pressure to increase production of wood-based pellets in particular (for

more details, see paper I). This biomass material has potential for utilisation by

pelletizing it, in both large-scale factories and decentralised small-scale

manufacture. A map of European pellet plants is presented in Fig. 2. This figure

combined with the information of Fig. 1 predicts that the areas with the biggest

growth potential in pellet production are the Northern countries and Russia.

18

Fig. 2. A map of European pellet plants [9].

The number of industrial-scale pellet factories in Finland today is about 25, with a

total annual production of about 300,000 tonnes in recent years, owing to the

narrowness of the raw material base. However, total production capacity is much

larger, even nearly 1 million tonnes per year [10]. Pellet production started in

19

Finland in 1998; the first plant was established in Vöyri, with annual production

of 10,000 tonnes. In 2001 total annual production in Finland was about 80,000

tonnes and in 2005 it was even about 200,000 tonnes, which shows the rapid

development in pellet production [10,11]. The aim in the future is to utilise more

low-value and/or moist biomass to expand the base of wood pellet raw material,

especially logging residue from thinning. For example, if only 10–15% of

unusable waste wood biomass from forestry operations could be utilised, it would

raise the annual wood pellet production capacity significantly, being about 2–3

million tonnes. However, there is reason to point out that the use of poor-quality

raw material requires pre-treatment methods such as drying, which raises the cost

as energy consumption increases. Nevertheless, since the purchase prices of these

poor-quality biomasses are very low, sometimes almost free, it doesn’t increase

the total cost of pellet production significantly.

To improve the quality of pellets, commercial and industrial by-product

materials are commonly used as binding agents in pellet production [4,6,12].

Also, considering the profitability of production and occupational safety problems

(wood dust exposure, fire and explosion risk, coherence, etc.), it is practical to use

binding agents. The use of binding agents increases the cost of production, but on

the other hand the increase in cost can be added to the price of a high-quality

product. Pellets are subject to mechanical loads and environmental factors during

production, transportation and storage, i.e. pellets are pumped into storage silos,

transported by truck or ship and delivered to the customer. During all these steps

there is the risk of mechanical failure of the pellet, resulting in fine particle and

dust formation, which have shown to have harmful effects on human health

[13,14]. It has also been noticed that pellets may decompose during storage,

forming gases such as carbon monoxide and hexanal, which are hazardous to

human health. Typically this happens with pellets made from softwoods.

Decomposition of pellet biomass may produce large quantities of carbon

monoxide and hexanal during pellet storage [15,16,17].

A partial aim of the present research is to find and test some eco- and cost-

efficient materials for binding purposes and thus improve the competitiveness of

pellet products as an alternative in energy production. Therefore, various locally

tailored binding agents, such as certain industrial by-products and residuals,

especially starch-containing waste materials, are currently under study. In this

thesis three potential binding agents, lignosulphonate, residual potato flour and

potato peel residue, were selected primarily because of their local availability and

secondly for their properties. Potato peel residue represents waste from important

20

production sector in Finland and its material-efficient utilisation applications are

under wide study (bioethanol, biogasification for CHP production, animal food,

pellet production, etc.). Utilisation of residual potato flour as binding agent

represents modern material-efficient use of waste or industrial by-product.

Although potato flour is commonly used commercial binding agent in pellet

plants, we focused our study on the use of residual material in details.

Lignosulphonate proved as functional binding agent in our preliminary

experiments and represents the use of industrial by-product or commercial

product from wood pulp production. However, there were no chemical studies

concerning the use of lignosulphonate as pellet binding agent.

Due to the low density of the biomass, pelletizing is required for efficient

transportation and utilisation in some cases, for example in long-distance

transports. To ensure a continuous supply of fuel and to maintain consumer

confidence in biofuel products, the pellet industry requires a broader raw material

base and less dependency on sawmill stem wood by-products [10]. Small-scale

pellet facilities are required in order to decrease low-density transportation from

outlying areas to centralised consumer markets and also for small-scale heating

applications. To support small-scale industry, independent research is required to

provide information on how to produce pellets that meet strict quality criteria. For

lower-quality raw material, this requires extensive knowledge about locally

usable and available binding agents that provide the means to produce high-

quality pellets.

As described earlier, according to the present EU and Finnish strategies on

waste materials, all kinds of waste must be utilised primarily as material (reuse,

recycling) and secondarily as energy, and if neither of these utilisation methods is

possible, they can be disposed of using ecologically beneficial methods. The

present goals of the Finnish waste strategy are given in the national waste plan

approved by the Finnish Council of State on the 10th of April 2008 [18] and in the

new waste law, approved on the 17th of June 2011; the changes took effect on the

1st of May 2012 [19].

The intent is to reach these goals by 2016. The key goals of this national

waste plan are to: 1) reduce waste, 2) increase waste material recycling and

biological reuse, 3) increase combustion of non-recyclable waste and 4) ensure

harmless treatment and final placement of waste. In addition, regional waste

plants supplement the above-mentioned national waste plan. According to the EU

and national waste strategies, the primary goal for ash is its utilisation as a

material. A major change in the status of bio-ash has happened during the last

21

years, as earlier the main treatment method for all kinds of ash was to place them

in landfill and dumping places—all kinds of ash were considered to be unusable

waste, even problematic hazardous waste. Wood and wood-based pellet ashes are

bio-ashes which are generated as by-products of biomass combustion or

gasification processes in heat and power generation. With the change in the

EU/Finnish waste strategy, there is currently a tremendous interest in Finland to

substantially increase the utilisation of bio-ash, develop new applications and

develop new bio-ash-based products, especially granulated bio-ash-based

products. Utilising pellet ash instead of disposing of it in landfills—with

increasing landfill costs and a new waste tax on all industrial landfills—will

increase the profitability of the whole chain of bioenergy production and thereby

that of pellet production.

According to the REACH regulation (1907/2006/EC), all chemical

substances of which at least one tonne per year is produced or imported must be

registered [20]. So, if ash materials are placed in landfills and dumps, they need

not be registered but they belong under waste taxation. Registration of European

wood-based ashes is just now underway, including altogether 71 ash producers in

the beginning of December 2010. This broad consortium indicates the importance

and topicality of the potential utilisation of bio-ashes and also wood pellet ashes.

For all the reasons presented above, pellet ash has been studied and treated widely

in this doctoral thesis.

23

2 Aims of the study

This study consists of five articles (I–V) in which chemical analyses are used for

the development of eco-, material- and cost-efficient pellet production and

technology. Pellet production and therefore scientific—especially chemical—

research on pellets are a very new topic area and thus there is also a strong

demand for information concerning pellets and pellet research. Roughly speaking,

as far as we know, there are no studies concerning the chemical perspective of the

pelletizing issue.

The first aim of this dissertation was to introduce general information on

pelletizing and chemical research on pelletizing development for scientific

purposes and the bioenergy sector. This topic is dealt with especially in Articles I

and II. In addition, this topic has been discussed in oral presentations at many

scientific forums. The following presentations are presented below:

1. M. Kuokkanen: Chemical methods in the development of eco-efficient pellet

production and utilisation of wood ash, HighBio-seminar, Luleå, Sweden

12.5.2009. URL: https://ciweb.chydenius.fi/project_files/HighBio-

Projektiseminaari%20120509/Matti%20Kuokkanen%20HighBio-

Lulea%201205-09%20Chemical%20methods.pdf

2. M. Kuokkanen: Ekotehokkaan puupohjaisen pellettituotannon kehittäminen,

EnePro-conference, Oulu, Finland 3.6.2009. URL: http://nortech.oulu.fi/

EnePro/Kuokkanen_EnePro.pdf

3. M. Kuokkanen: Chemical methods in the development of eco-efficient wood

pellet production – Specific staining and optical microscopy, ICCE 2009

conference, Ubonratchathani, Thailand 21.1.2010.

The second aim was to test and develop new chemical methods for use in pellet

research, in addition to methods used in the recent pellet standards. These new

methods, BOD measurement and an optical staining procedure, are included in

the chemical research toolbox which is presented and discussed in Articles I, II

and III. As far as we know, these methods have not been presented in pellet-

related literature earlier.

Today material efficiency is an essential topic in promoting sustainable use of

natural resources. According to the present Finnish strategy on waste materials,

and in accordance with the corresponding European strategy, all kinds of waste

must be utilised primarily as material (reuse, recycling) and secondarily as energy,

and if neither of these utilisation methods is possible, they can be disposed of

24

using ecologically beneficial methods. The current waste hierarchy is presented in

Fig. 3. Thus, the goals of utilising biomass as pellets, waste adhesive materials as

binding agents in pellet production and bio-pellet ash for different applications,

all represent the top global goals in utilisation of waste materials and industrial

by-products.

Fig. 3. Waste hierarchy [21].

Material efficiency in terms of industrial ecology principles is dealt with in all the

articles I-V, because pellets and some of their binding agents are recycled waste

materials. Respectively, utilisation of especially bio-ashes is a hot topic now in

the Northern countries, where ash materials are widely available. However, heavy

metal concentrations may limit the usability of the ashes. Keeping in mind the

utilisation potential, the chemical properties of wood pellet ashes are discussed in

Articles IV and V.

25

3 Materials and methods

3.1 Materials

The effect of binding agents on the operation of a pilot-scale pellet facility was

studied using Finnish conifer bark-free stem wood as model raw material and

lignosulphonate, residual potato flour and potato peel residue as binding agents.

The aim of this thesis was also to study the effect of binding agents on the various

properties of wood-based pellets.

3.1.1 Lignosulphonates

Lignosulphonates, or sulphonated lignins, are water-soluble anionic

polyelectrolyte polymers. They are by-products of wood pulp production that

employs sulphite pulping [22]. Most delignification in sulphite pulping involves

acidic cleavage of ether bonds which connect many of the constituents of lignin.

Electrophilic carbocations produced during ether cleavage react with bisulphite

ions (HSO3-) to give sulphonates [23].

The primary site for ether cleavage is the α-carbon (carbon atom attached to

the aromatic ring) of the propyl (linear three-carbon) side chain. The following

diagrams do not detail the structure, since lignin and its derivatives are complex

mixtures—the purpose is to provide a general idea of the structure of

lignosulphonates (Fig. 4). The groups labelled “Q” may be a wide variety of

groups found in the structure of lignin. Sulphonation occurs in the side chains, not

in the aromatic rings, like in p-toluene sulphonic acid [22].

26

Fig. 4. Preparation and structure of lignosulphonate [22].

The sites of potential cross-linking reactions in lignosulphonates are at

unsubstituted ring positions of ortho- to free phenolic hydroxyls, as in

phenolformaldehyde condensations, or direct reactions of free phenolic hydroxyls

with ether-forming agents, as in epoxide condensations. In either case, phenolic

hydroxyls are necessary, and the primary bond formed is very stable, a necessary

criterion for adhesive durability. Thus, the phenolic hydroxyl content of

lignosulphonates may be an important criterion of its potential reactivity [22,23].

3.1.2 Potato starch

Starch is a carbohydrate consisting of a large number of glucose units joined

together by glycosidic bonds. This polysaccharide is produced by all green plants

as energy storage. It is contained in such staple foods as potatoes, wheat, corn,

rice and cassava. Pure starch is a white, tasteless and odourless powder that is

insoluble in cold water and alcohol. Starch consists of two types of molecules:

linear, helical amylose (Fig. 5) and branched amylopectin (Fig. 6). Depending on

the plant, starch generally contains 20 to 25% amylose and 75 to 80%

amylopectin [24]. For potatoes, the values are 25% amylose and 75%

amylopectin. When starch is dissolved in warm water, it can be used as a

thickening, stiffening or gluing agent.

27

Fig. 5. Structure of amylose [25].

Fig. 6. Structure of amylopectin [25].

3.1.3 Potato peel residue

Potato peel waste (PPW) is a starch-rich residue [26] from the potato peeling

process. Disposal of this process residue is problematic due to its extremely high

organic content, which readily decomposes but has high nutritional value as

animal feed [27]. The residue is also rich in nutrients, especially potassium. Using

it in pellet production without a sterilisation process poses a significant disease

risk to potato plantations and an environmental risk if it is not properly processed

[28].

28

3.1.4 Wood pellet ash

Is it generally known and numerous studies have shown [29,30,31,32] that many

factors affect the composition and amount of bio-ash. The physical and chemical

qualities of ash vary significantly depending on factors such as the ratio of the

fuels burned, tree species, geographical location, growing site, climate, tree

components (e.g. bark, wood, leaves) and the season of cutting. Other factors

which also affect the characteristics of the ash are the size and age of the tree, the

manner of collection and storage and the burning technique, such as the

combustion temperature and type of boiler [33]. As examples, two samples of

wood-pellet-based bio-ash from Kuusamo and Jyväskylä are considered in this

summary part of the thesis. The corresponding results for the other analysed bio-

ashes as well as other ash results are presented in detail in papers II, IV and V.

Ash sample 1 was taken from a grate-fired boiler in a small-scale district

heating plant in Kuusamo, Eastern Finland (papers IV and V). The plant has a

100-kW boiler, and 100% of the energy produced by the boiler originates from

the incineration of commercial wood pellets. The ash—technically speaking

bottom ash—was sampled as dry from the outlet of the boiler. The boiler

incinerates ca. 40 tonnes (d.w.) of commercial wood pellets per year. The

commercial wood pellets incinerated in the boiler were made from cutter shavings

and sawdust (ca. 80–90% pine and ca. 10–20% spruce) originating from the

mechanical wood-processing industry in the Kuusamo region.

Ash sample 2 was taken from a grate-fired boiler in a small-scale district

heating plant at the Jyväskylä airport, Central Finland (paper V). The plant has a

700-kW boiler, and 100% of the energy produced by the boiler originates from

the incineration of commercial wood pellets. The ash is the mixture of bottom ash

and fly ash and it was sampled from the ash silo of the boiler, which was emptied

1.5–2 months earlier, so the compilation sample represents an approximately 1.5-

month burning period. The boiler incinerates ca. 330 tonnes (d.w.) of commercial

wood pellets per year.

3.2 Research methods

The most important chemical methods used in this pellet research are shown in

Table 1 and are described in following chapters.

29

Table 1. Chemical toolbox for pellet production and development research used in this

work.

Measurement Equipment Purpose of determination

Moisture content Heat oven and scales

Automatic Senfit method

Water content

Density Solid: Laboratory compacted bulk

density, SFS-EN-13040

Liquid: Densitometer

Compactness of materials

Heat value measurements Bomb calorimeter Energy content

BOD tests BOD OxiTop® equipment

for liquid and solid phases

Biodegradation/loss of material

Dust content, strength

properties

Vibrator, sieve analysis Mechanical stability of the pellet

Structure analysis Staining reagents and microscope Information about pellet structure

and cross-linking mechanisms

TG analysis Thermogravimetric analyser Volatilisation of water and VOCs

Elementary analysis ICP-OES Heavy metals and/or nutrients of

pellet and ash

Particle size distribution Laser diffraction particle size

analyser, image analysis

Information about particle size

distribution in wood and ash

In addition, BET for specific surface area determination and SEM (Scanning

electronic microscopy) have recently been used in studies of wood-based ash, but

these methods were not used in this thesis.

3.2.1 Moisture content measurement

Moisture content was measured according to the ISO 589 [34] and CEN/TS

14774-1 standards [35]. According to ISO 589, the samples were dried overnight

to a constant mass (16–24 h) in a drying oven at 105 ± 2 °C. After drying, the

samples were placed in cooling desiccators and weighed. The moisture content

could then be calculated based on the decrease in mass during the drying stage.

According to CEN/TS 14774-1, the samples were dried to a constant mass (up to

24 h) in a drying oven and immediately weighed when hot.

Measurement of moisture content according to the drying/weighing method

presented above is very simple and reliable and requires no expensive equipment,

and therefore it is generally used for biomass studies and also in trading of raw

materials of bioenergy. However, this method includes many disadvantages such

as: i) it is time-consuming (the temperature is only a little higher than temperature

30

of the water), ii) the amounts of samples are relatively small (the sample layer

cannot be very thick because of the “free” water vaporisation), iii) VOC

compounds can be vaporised during the drying process and iv) the measurement

is not reliable if the moisture is contained within the cells. For these reasons many

practical and automatic methods for determining the moisture content of biomass

quickly but faithfully have been developed and tested during the last decade.

During this thesis work automatic rapid analysis methods were generally used to

measure the moisture content of raw materials and pellets in full-scale pellet

factories. These methods are based on CEN/TS 14774-1 and they generally

measure the increase in electrical conductivity produced by water in the sample.

It is noteworthy to observe that the importance of using these automatic

methods increases all the time with increasing international trading of renewable

energy. The measurements of moisture content in this study were all carried out

according to the ISO 589 and CEN/TS 14774-1 standards, but later on some small

automatic devices have been fitted and tested in our laboratory. A new Finnish

patented invention—a biomaterial moisture analyser (BMA)—seems to be very

promising for large-scale biomass trading [36] and is therefore briefly described

below.

Measurement with a BMA is done by using a patented microwave resonator

structure [36]. Detecting the fraction of water in a vast variety of different

materials is based on the unique electrical properties of water in a high-frequency

electromagnetic field. The key parameter—the permittivity of water—differs

significantly from most other materials, changing the response of the resonator.

Microwave resonator sensors are commonly used in measurements of low water

content (< 30%). BMA measurement utilises a new method where a resonating

wave mode is selected to be useful also with very high water contents. The

BMA’s fully digital signal processing uses multi-parameter calculation. Special

algorithms are used to compensate for density changes [36].

3.2.2 Density measurements

The densities of liquids (binding agents in a liquid phase) were measured at room

temperature with a common procedure using an AP Paar DMA 40 digital

densitometer, based on an oscillating U-tube principle where the oscillating

frequency is dependent on the density of the liquid sample, not on its

composition.

31

Pellet bulk (solid phase) densities were measured using equipment with a

measuring cylinder of 1 L and a weight of 650 g (8 g cm-2) (Fig. 7), according to

standard SFS-EN-13040 [37].

Fig. 7. Bulk density test cylinder. 1) Plunger, 2) Supported sieve, 3) Funnel 4) Collar 5)

Test cylinder, d = diameter: 100 mm ± 1 mm, h = height: 127 mm ± 1 mm, d1 = diameter:

95 mm ± 1 mm [37].

32

3.2.3 Calorific heat measurement

Calorimetric heat measurements and calculation of net/gross calorific heat values

were performed according to standards DIN 51900 [38], ISO 1928 [39] and

ASTM D240 [40]. In this study calorific heat was determined for pellet samples

and binding materials with an IKA C5000 calorimeter at the Finnish Forest

Research Institute (Metla) Kannus Research Unit and with an IKA C200

calorimeter at the Department of Chemistry, University of Oulu (see Table 2,

comparison test). The weighed portion of the analysis sample of the solid biofuel

was burned in a high-pressure oxygen bomb in a bomb calorimeter under

specified conditions. The effective heat capacity of the calorimeter was

determined in calibration experiments by burning certified benzoic acid under

conditions similar to those specified in the certificate. The heat released from the

burned sample was observed with a digital thermometer. The heat values were

determined with at least duplicate measurements. The calorimeter constant C was

determined using equation (1):

C = (ΔH x m + Q) / ΔT1 (1)

where ΔH is the heat of combustion of the calibration compound (ΔH = 26.44

kJ/g for benzoic acid), m is the mass of the calibration compound [g], Q is the

heat of combustion of the fuse igniter (50 J) and ΔT1 is the temperature change in

calibration. After determination of the calorimeter constant C, the gross calorific

values of the air dry (i.e. “air wet”) samples, Qgr,ad [kJ/g], were calculated using

equation (2):

Qgr,ad = (ΔT2 x C - Q) / m2 (2)

where ΔT2 is the temperature change that occurs during sample burning [ºC] and

m2 is the mass of the sample [g]. Hence, the gross calorific heat value of the dry

basis Qgr,d [MJ/kg] can be calculated using equation (3),

Qgr,d = Qgr,ad x 100 / (100 - Mad ) (3)

where Mad is the moisture content of the air dry sample [wt%].

The net calorific value at constant volume of biofuel and the net calorific

value at constant pressure are both obtained by calculating the gross calorific

value at constant volume as determined from the analysis sample. Calculation of

the net calorific value at constant volume requires information about the moisture

and hydrogen contents of the analysis sample. The net calorific value of the dry

basis Qnet,d [MJ/kg] can be calculated from the Qgr,d value using equation (4),

33

Qnet,d = Qgr,d - 0.02441 x M (4)

where 0.02441 [MJ/kg] is the latent heat of vaporisation of water at +25 °C and M

is the hydrogen content of the moisture-free biofuel [wt%].

The net calorific heat value as received is calculated from Qnet,d using equation

(5), where Mar is the moisture content of the as received sample

Qnet,ar = Qnet,d x (100 – Mar) / 100 – 0,02443 x Mar (5)

3.2.4 BOD measurements

Biodegradation of matter means its degradation by bacteria and micro-organisms

in soil and water environments. Various fungi may also act as decomposing

organisms in wood material and in soil. Biochemical oxygen demand (BOD) is a

measure of the quantity of oxygen required for biodegradation of organic matter

(carbonaceous demand) in water and the amount of oxygen used to oxidise

inorganic material, including ferrous iron salts and sulphides [41]. The BOD

value is an important parameter for monitoring organic pollutants in a water or

soil environment due to the simple fact that easily degradable matter will cause no

long-term risk to the environment.

EU Directive 67/548/EEC (Annex V) lists the testing requirements for new

chemicals and all chemicals declared to be on the market in September 1981

[42,43]. The accepted methods for determining biodegradation are listed in Annex

V, Part C, and the respirometric testing method is one of the accepted testing

methods for determining biodegradation. Thus, BOD OxiTop methods used for

biodegradation in a solid phase and in solution are accepted testing methods.

Furthermore, when new pellet products that include new chemicals as binding

agents are manufactured in the future, EU legislation requires that the

biodegradation of these chemicals is determined using accepted testing methods.

Biodegradation tests in solution

The biodegradation tests were carried out in this study in OECD 301F standard

conditions (optimal solution conditions) using the manometric respirometric BOD

(Biological Oxygen Demand) OxiTop method [41,44,45,46,47] and OxiTop®

Control 6 instrumentation (see Fig. 8).

34

Fig. 8. BOD OxiTop apparatus used for respirometric manometric measurements in

solution (OECD 301F conditions, ground water and surface water).

This method is based on very accurate automatic pressure measurements in closed

bottles under constant temperature (here 20.0 ± 0.2 oC). When organic matter

degrades, it requires a certain amount of oxygen, Equation (6):

Corg + O2(g) → CO2(g) (6) CO2(g) + 2NaOH(s) → Na2CO3(s) + H2O(l) (7)

When oxygen is consumed from the gas phase, the pressure falls and carbon

dioxide gas is produced, but in this method the carbon dioxide is absorbed by

solid sodium hydroxide pellets and therefore does not affect the measured

pressure, Eq. (7). The measurement time can be selected by the user (the

maximum for one measurement period is 99 days, but repeat periods can be

used), and here in the OECD 301F tests BOD28 was determined. The

measurement is fully automated, and for measurements in solutions, the

instrument calculates the BOD value in the desired unit [mg/L] using Equation

(8):

BOD[mg/L] = M(O2)/RTm x [(Vtot - V1)/V1 + αTm /T0] x Δp(O2) (8)

M(O2) is the molecular weight of oxygen (32.00 g/mol), R is a gas constant

(83.144 l hPa mol−1 K−1), Tm is the measurement temperature [K], T0 is 273.15 K,

Vtot is the bottle volume [ml], Vl is the liquid phase volume [ml], α is a Bunsen

absorption coefficient (0.03103) and Δp(O2) is the difference in partial oxygen

pressure [hPa] as given by WTW.

35

Biodegradation tests in the solid phase

Pellet samples (about 50–100 g) with a normal moisture content were first

weighed and measured in MG 1.0 bottles (WTW Weilheim, Germany) using

OxiTop® Control B6M instrumentation (see Fig. 9) [41,44,46,47]. Then, a 50-mL

beaker filled with a 1-M sodium hydroxide solution was placed on a holder. The

bottles were held in an incubation cabinet at a temperature of 20.0 ± 0.2 °C for a

period of 99 days.

Fig. 9. OxiTop® Control B6M instrumentation for BOD measurements in the solid

phase.

The BOD OxiTop apparatus for solid phase measurements is also fully

automated, but it only calculates the difference in partial oxygen pressure Δp(O2)

[hPa]. After that, consumed oxygen Δm(O2) [mg] can be calculated using

Equation (9) when free gas volume Vf is first calculated.

Δm(O2) [mg] = Δp(O2) x Vf x M(O2) / (R x Tm) (9)

The degree of biodegradation of the substance in solution and the solid phase can

be calculated using Eq. (10),

Degree of biodegradation [%] = BOD [g/g] / ThOD [g/g] x 100% (10)

where ThOD [g/g] is theoretical oxygen demand, calculated from the mass and

carbon content of the sample. The BOD value in Equation (9) is calculated using

Eq. (11):

BOD [mg/mg] = Δm(O2) [mg] / msample) [mg] (11)

36

3.2.5 Mechanical durability measurement

The mechanical durability of the pellets was tested according to the CEN/TS

15210-1 standard [48]. Sieved double samples were tumbled in standard

dimension boxes at 50 ± 2 rpm for 500 rotations and sieved. An acceptable result

is more than 97.5% of the mass above the sieve. The average of the test results

was used.

3.2.6 Microscopic structure analysis

It has been observed that the most cost-efficient binding agents in pellet

production are starch-containing materials. Therefore, various locally tailored

binding agents, such as certain industrial by-products and solid starch-containing

waste materials, are currently being considered and studied. To improve the

quality of pellets we have developed and tested a new specific optical

microscopic staining method using starch-containing binding agents.

Characterisation of starch-containing wood pellets by optical microscopic staining

has been developed as a tool for the purpose of controlling and optimising pellet

production and is described in this thesis.

The first step in the staining procedure is to mould wood pellet samples in

epoxy resin to support the pellet structure. After this step is complete, the samples

are sectioned by grinding. Every sample moulding contains two orientations of

pellets: a cross-sectional cut and an axial cut. Then the surfaces of the pellet

sample mouldings are honed, carefully polished and left to cool down for about

one hour [Paper III]. The unstained pellet samples are then digitally imaged with

a Leica MZ FL III stereomicroscope. The staining procedure is carried out with a

reagent selective to starch compounds, such as potassium iodide (KI) [49].

Colour formation caused by a chemical interaction between starch and iodine

is an important property for starch characterisation. Iodine forms a complex with

α-1.4 linked glucans by penetrating the hydrophobic cavity of the linear glucan

helices. This complex is generated if the chains are sufficiently long. In

spectroscopic determination the maximum wavelength (λmax) of the formed

complex is situated in the visible region, and the value of λmax is dependent on the

length of the chain [49].

Potassium iodide is prepared by dissolving 4–5 g of potassium or sodium

iodide in a bit of water and adding approximately 130 mg of iodine. After

dissolution of the iodine, the solution is diluted into 100 mL of distilled water

37

[50]. Potassium iodide is then carefully placed on the surface of the pellet sample

and left to absorb for a period of five minutes. After that, the staining reagents

must be carefully flushed away from the pellet samples, first with water and then

with ethanol. The samples are then dried before the second microscopic

examination. The last step of the optical microscopic staining method included

examination of both the unstained and stained pellet samples with a Leica optical

microscope and digital imaging for visual analysis purposes. The microscope

images enabled us to analyse the binding structure of the pellet and starch

penetration.

In the microscope images obtained with the method we have developed,

starch compounds appear as small dark spots and areas. Microscope images

enable us to analyse the binding structure of the pellet and starch penetration.

However, in visual analysis it must always be noted that even after careful

flushing, pellet pores may contain some reagent residuals and wood material itself

may contain dark spots, and hence these effects must be eliminated with careful

flushing in the analysis stage. There is reason to point out that these conclusions

concerning the visual analysis of the pellet samples containing starch have been

examined from the real microscopic slides, and not from the images presented in

the results. In the future, computational image analysis will be integrated into the

structure analysis procedure, which transforms the data received from staining

distribution for numerical information. Currently image analysis is based on

observing the shape of the staining distribution, for example whether binding

agent is mixed for example as lumps, uniformly distributed or distributed only in

pellet surface. As more sample points are added in the analysis, one or two quality

parameters (e.g. mechanical durability) can be attached—the mechanical

durability results of high quality pellets compared with image analysis data—and

then investigate whether regularity in staining distribution can be found. The

information obtained with this method can be used for examining and planning

the feeding of binding materials in pellet plants (place, amount, functionality) and

for optimizing the pelletizing process.

3.2.7 TG analysis

Thermogravimetric analysis (TG) is a thermoanalytic (TA) method wherein the

mass of the sample vs. temperature is weighed when the sample is heated at a

constant heating rate. With the help of TG measurements it is possible to obtain

vital quantitative information about the weight loss of the sample. TG

38

measurements also supply us with direct stoichiometric data for the purpose of

analysing certain reactions [51,52]. Hence, TG is a useful tool for developing the

drying processes of different biomasses, such as sawdust, wood chips, peat and

potato peel residue used in this thesis work. TG measurements give a simple TG

curve as a result. From TG curves it is possible to estimate the mass change of

different compounds during drying. The components under study (such as VOC,

CxHx, NOx) are determined before measurement. A curve is obtained for each

determined component so that more detailed examinations can be done later. The

information obtained can then be used e.g., to optimise the drying temperature for

different types of wet biomaterials. The TG curve of a certain potato residue

material is given as an example in Fig. 10.

Fig. 10. Thermogravimetric (TG, green line) and differential scanning calorimetric

(DSC, blue line) analysis data for potato residue material dried to a constant weight

at 105 °C.

3.2.8 Elemental analysis

The samples were extracted with MARS5 microwave wet combustion equipment

using the EPA-3051 standard method [53]. Measurements were done with a

Project:Identity:Date/Time:Laboratory:Operator:Sample:

-sample15.10.2007 13:52:29ProcessmetallurgyRMA-, 49.800 mg

Material:Correction File:Temp.Cal./Sens. Files:Range:Sample Car./TC:Mode/Type of Meas.:

base-12102007.bsvlpt-dsccp-210607.tsv / sens-dsccp-210607.esv30/20.0(K/min)/1000DSC(/TG) HIGH RG 2 / SDSC-TG / Sample + Correction

Segments:Crucible:Atmosphere:TG Corr./M.Range:DSC Corr./M.Range:

1/1DSC/TG pan PtAir/50 / ---/--- / ---/---020/30000 mg020/5000 µV

Instrument: NETZSCH STA 409 PC/PG File: F:\Muut\Raudaskoski\sample-15102007.dsvMarko 2007-10-15 15:47 Main

100 200 300 400 500 600 700 800 900Temperature /°C

-5

0

5

10

15

20

25

30

35

DSC /(mW/mg)

10

20

30

40

50

60

70

80

90

100

TG /%

[1][1]

↑ ex

39

commercial plasma emission spectrometer (ICP-OES) and the results were

calculated against the sample’s dry weight (at 105 °C).

3.2.9 Particle size distribution

Particle size was determined with a Beckman Coulter LS 13 320 laser diffraction

particle size analyser. The range of measurement was 0.4 to 2000 µm. The

particle size analyser utilises laser diffraction as a measurement method. The laser

diffraction technique is based on the principle that a laser beam scatters light at an

angle that is directly related to particle size. The scattering angle increases

logarithmically when particle size decreases. Small particles scatter light at wider

angles with low intensity and large particles scatter light at narrow angles with

high intensity [54].

3.2.10 Sequential leaching procedure

As presented in the introduction and later in the results section, in EU countries

both utilisation and disposal of bio-ashes and other waste materials require

chemical analyses of the materials using different analytic methods. These

standard methods include determination of either total concentration or that of

dissolution in water (either a two-stage leaching test with L/S ratios 2 and 8,

cumulatively 10:1, or an up-flow percolation test, CEN/TS 14405:2004 [55]) for

different species, thus providing information on dissolution in only one selected

condition. The most important environmental legislation concerning bio-ash and

other waste materials in Finland today are: i) Waste Act 646/2011 [18], ii)

Ministry of Agriculture and Forestry Decree on Fertilizer Products 24/2011 [56],

iii) Government Decree VNa 591/2006 [57], or Utilisation of Certain Waste

Materials in Earth Construction Government Decree VNa 214/2007 [58], iv)

Assessment of Soil Contamination and Remediation Needs (“the Finnish PIMA

decree”) VNa 214/2007 [58] and v) Government Decree on Amendment of

Government Decision on Landfill Sites VNa 202/2006 [59]. Finnish

environmental legislation is in accordance with the corresponding EU legislation

[60,61], as well as in agreement with the principles of sustainable development,

LCA (life cycle assessment) and material efficiency. Finland also has

environmental legislation on some issues related to wastes that have not yet been

covered by EU legislation. Lastly, it is worth mentioning that in these

40

requirements, waste-related legislation in Finland has undergone an extensive

reformation during the last few years.

The single chemical determination methods used in EU and Finnish

environmental legislation and mentioned above give only poor information for

risk assessment of different species and tell nothing about the effect of the

environment on the solubility, mobility and bioavailability of harmful heavy

metals. This information is necessary if we wish to know the real environmental

risk of metals in different conditions possible in nature now or in the future, and if

we wish to substantially increase utilisation of waste materials according to EU

and national strategies and in tandem reform environmental legislation.

Therefore, during the last couple of decades many different sequential leaching

procedures have been developed—most of which are based on the method

initially developed for sediments by Tessier et al. [62]—and used to determine

trace elements and sulphur in different materials. During the last decade, a

research group at the University of Oulu has studied and published many papers

on the use of a sequential 3–5-stage procedure for estimating the bioavailability,

mobility and thus risk assessment of many different, especially bio-based, Finnish

waste materials and industrial by-products, such as fly or bottom ash from

industrial and municipal district heating plants [Paper V,63,64], bottom or fly ash,

paper mill sludge, green liquor dregs, PCC waste and lime waste from pulp and

paper mills [65,66,67,68,69,70,71] and waste rock material originating from a

Finnish zinc mine and used as railway ballast in Northern Finland [72,73].

The five-stage sequential leaching procedure (see Fig. 11) generally used to

determine the distribution of 11 metals (Cd, Cu, Pb, Cr, Mo, Zn, As, Co, V, Ni,

Ba), and sulphur (S) in the studied materials into the following fractions: 1) a

water-soluble fraction (ultrapure distilled water acidified with HNO3, pH = 4.0),

2) an exchangeable fraction (CH3COOH), 3) an easily reduced fraction (NH2OH-

HCl), 4) an oxidizable fraction (H2O2 + CH3COONH4) and a residual fraction

(extraction with HF + HNO3 + HCl). The conditions in the last step—three strong

acids and strong, long-lasting shaking—are never possible in nature, and therefore

the residual fraction is called the inert phase.

Since the conditions in stages (1)–(4) represent realistic possible risk

conditions at least in the future, the potential bioavailability percent PBPM [%]

can be calculated from the sum of the concentrations of M in stages (1)–(4), or

(2)–(4) if the water extraction stage (1) is missing (as in paper V, and the total

concentration ctot(M), by using equation (12), where ci(M) is the concentration of

metal M in stage i [Paper V, 69,70]. The value of ctot(M) can be calculated either

41

from the sum of the concentrations in all stages or directly by some standard

determination method. The values of residual percentage and concentration can be

calculated with equations (13) and (14).

( )

( )

4

1 100%ii

Mtot

c MPBP

c M== ×

(12)

[ ]% 100% Mresidual PBP= − (13)

( )( ) 100residual M totc M PBP c= − × (14)

Fig. 11. Five-stage sequential leaching procedure [Paper V].

43

4 Pelletizing machinery

A pellet factory usually includes certain unit processes relative to pelletizing

(pelletizing machine, cooler, conveyors, recycling system, mixers). However, a

drying unit has only recently been in use in a few factories. Sawdust may be

brought to a pellet factory already dried, which increases the cost of the raw

material, or it may be pelletized without discrete drying, which lowers the quality

of the pellet product. The main unit processes of a pellet factory are described in

the following chapters.

4.1 Delivery of raw material and receiving stock

The location of a pellet factory is ideal if there is a sawmill, for example, located

nearby. In that case the source of raw material is close and the operation is

logistically cost-effective. The by-products of a sawmill (sawdust and cutter

shavings) can be utilised as the raw material of pellet manufacture, and thus

closely located factories create a symbiosis that is ecologically beneficial to both

sides. Furthermore, if the pellet plant has a drying unit, the drying process can be

performed by this unit and the pellet factory can receive moist raw material.

As the first step in a pellet factory the undried raw material is transported by

a wheel loader into receiving silos in the factory where the raw material is

preheated by recycling waste heat from the later stages of the process. This is

necessary especially in winter, because the raw material may be covered with

snow and heating it before it is transferred to a drying unit saves energy in the

drying phase and minimises the blockages in the later process stages. The

preheated biomass is fed forward from the receiving silos by a pneumatic blower.

4.2 Drying unit and pre-treatment of raw material

A pneumatic blower moves sawdust from the receiving silos to a conveyor which

transports the preheated to a drying unit. Wet sawdust is fed into the feeding tube

of a drum drier by a screw conveyor. In the drying unit moisture is removed from

the raw material and the generated waste heat is used to heat the receiving silos.

The typical parameters of a drum drier (diameter, length, permeability, drying

time, drying temperature) depend on the moisture and quality of the raw material

being dried. Drying is performed as a continuous process. The drying temperature

44

of a drum drier can be between 100 °C and 380 °C. Drying is carried out until the

required moisture content in the pelletizing process (less than 15%) is achieved.

After the drying process and before the pelletizing machine, the first

separation of dust is done by a cyclone. The separated dust is salvaged by the

cyclone and recycled back to the receiving silos of the process. Multiphase

separation of dust is necessary for a good-quality pellet product, and drifting of

dust among the pellet product must be avoided. Dust causes significant problems

in the combustion process, such as explosions and a decrease in combustion

efficiency.

Before being fed to the pelletizing machine, the dried raw material is

collected into an intermediate mixing chamber. The raw material is crushed by a

crusher in to make the material homogenous enough to be fed into the pelletizing

machine. This is done to avoid blockages in the pelletizing machine. Blockages

cause both interruptions in production and abrasion of the machinery. The

intermediate mixing chamber is meant especially to prevent interruptions in

production. To keep the raw material from getting lumpy, the intermediate mixing

chamber contains a discrete mixer.

4.3 Pelletizing machine

The dried raw material is fed by conveyor to a pelletizing machine. The

pelletizing machine is the most essential stage of the pelletizing process in terms

of the quality of the pellet product. The pelletizing machine feeds the raw material

into a pellet press, possibly adds binding agents and presses the pellets.

Pelletizing is a mechanical pressure operation. The high pressure and friction of

the press causes the temperature of the wood to increase significantly, thus

slightly plasticising the lignin. This phenomenon is very important, especially in

forest biomasses with large lignin contents. Hardwood contains ca. 20% lignin

and softwood even more, ca. 30% [74]. As a rule of thumb, a pelletizer requires

between 50 and 100 kW of electrical demand for every ton per hour of production

capacity. In addition, electricity is usually needed to operate any chopping,

grinding, drying, cooling, and bagging equipment that is in use. Electrical demand

is dependent on many factors such as raw material, matrix channel and binding

agent properties.

Two different types of pelletizing machines are commonly used in the pellet

industry: a plane matrix machine (Fig. 12) and a core matrix machine (Fig. 13).

45

Fig. 12. Plane matrix pelletizing machine [75].

In a plane matrix machine pressing occurs through a round planar matrix where a

rolling press rolls against its surface (Fig. 12). The pressing mechanism is based

on either a rotating matrix and stationary rolls, or a stationary matrix and a

rotating roller head, where the rolls rotate axially around a main shaft. In a plane

matrix machine feeding is done by force of gravity from above. The most

important advantage compared with a core matrix machine (see Fig. 13) is that in

this case the matrix is easy to clean.

In the second and most common type of pelletizing machine pressing occurs

through a core matrix (Fig. 13). The pellet pressing mechanism is based on a

stationary core matrix and revolving press rolls inside the inner circle or a rotating

core matrix with stationary axially rotating press rolls. In a core matrix pelletizing

machine the amount of raw material fed and the binding mechanism are the two

most decisive factors in achieving high efficiency as well as low, uniform wear of

the matrix.

46

Fig. 13. Core matrix pelletizing machine [75].

47

4.4 Pelletizing field tests

It is generally known that the external conditions in a pellet plant (temperature,

atmospheric moisture, etc.) can vary strongly between different seasons and even

during the same day, which can strongly affect the success of wood pelletizing.

However, it is necessary to also obtain results from pelletizing in large-scale field

conditions in addition to pilot-scale pelletizing experiments at constant

conditions. Therefore, the samples used in this thesis work were manufactured by

different pellet facilities, from pilot-scale to industrial-scale processes.

Fig. 14. Process description of the Formados Inc pellet plant in Kuusamo [76].

In our field tests we used three different pellet plants to manufacture the pellet

samples. In the Formados Inc. pellet plant in Kuusamo, binding materials were

added to a continuous flow of raw material just before the pressing phase in the

pelletizing machine (Fig. 14). The core matrix compressed pellet samples were

then collected into sample trays for analysis. To calculate the amount of binding

material added, the pellet plant’s raw material input was estimated as being

48

constant during the addition periods, and the output per hour was estimated from

the pellet plant’s production rate. The production capacity in the field tests of the

Formados pellet plant was about 1500 kg/h.

In the Mekrijärvi Research Station experiments (Fig. 15), additives are added

to a continuous flow of raw material on a collector screw before the mixer buffer

silo. Pellet samples were collected in sample trays for analysis after the cooling

phase. To calculate the amount of binding material added, the pellet plant’s raw

material input was estimated as being constant during the addition periods, and

the output per hour was estimated from the pellet plant’s production rate. The rate

of addition of binding material was calculated based on calibration to correspond

to the required binding material contents (1% and 2%). The production capacity

in the tests of the pilot pellet plant in the Mekrijärvi Research Station was about

300 kg/h.

Fig. 15. Pellet facility used in this study. 1) Drier carriage and container, separate

locations, 2) Rough sieve (30-mm square holes) + metal trap, 3) Rough sieve bypass

flow, 4) Hammer mill (Miller 20), 5) Spiral conveyor from hammer mill to raw material

silos, 6a-c) Raw material silos 1–3 (3.5 m3 each), 7a) Collector screw, 7b) Collector

screw operation at raw material silo calibration, 8a&b) Additive addition 9) Buffer

mixer silo, 10) Pellet press feed screw conveyor 11) Pellet press, 12) Belt conveyor to

cooler 13) Cooler 14) Vibration sieve conveyor (5-mm round holes). Automation, dust

extraction and blowers are not shown [Paper II].

In the EkoPelletti R&D project’s pilot pellet plant at Oulu University of Applied

Sciences, School of Renewable Natural Resources, binding materials were added

to a continuous flow of raw material just before the pressing phase in the

pelletizing machine (Fig. 16). Core matrix compressed pellet samples were then

collected into sample trays for analysis. To calculate the amount of binding

49

material added, the pellet plant’s raw material input was estimated as being

constant during the addition periods, and the output per hour was estimated from

the pellet plant’s production rate. The production capacity in the tests of the pilot

pellet plant at Oulu University of Applied Sciences was about 150 kg/h.

Fig. 16. Pilot pellet plant at Oulu University of Applied Sciences, School of Renewable

Natural Resources.

51

5 Experimental work

The research of this thesis work was carried out mainly at the University of Oulu

in close co-operation with Finnish pellet factories, forestry experts and Finnish

industry. In the preliminary stage, full industrial-scale manufacturing of our

sample pellets was carried out at the Formados Inc. pellet plant (see Fig. 14) in

Kuusamo, in North-East Finland (Paper I). In the preliminary tests it was

observed that external conditions (temperature, atmospheric moisture, etc.) can

vary strongly between different experiments, and therefore later samples were

manufactured in pilot-scale pellet facilities at constant conditions. The samples of

pellets manufactured in these preliminary tests were used also in characterisation

of wood and wood-based bio-ash (Paper IV and V).

Pilot-scale manufacturing of our sample pellets for paper II was carried out at

the Mekrijärvi Research Station pellet facility in Ilomantsi (see Fig. 15). Some

supplementary tests and pellet manufacturing were carried out at the pilot pellet

plant in Oulu University of Applied Sciences, School of Renewable Natural

Resources (see Fig. 16). The pellet samples for staining method analysis for paper

III were manufactured from softwood from Central Finland at a typical pellet

plant by Vapo Inc., which is the largest pellet producing company in Finland.

The moisture content and BOD measurements (Paper I and II) were carried

out at the Department of Chemistry. The calorimetric heat measurements (Paper I

and II) were carried out at the Department of Chemistry as well as in

Finnish Forest Research Institute Metla in Kannus. Staining method analysis

(Paper III) was carried out in co-operation with Vapo Inc. at the Department of

Chemistry and at the Fibre and Particle Engineering Laboratory at the University

of Oulu. Particle size distribution measurements (Paper II) were done at the Fibre

and Particle Engineering Laboratory. TG analysis measurements were carried out

at the Laboratory of Process Metallurgy at the University of Oulu. Sequential

leaching procedure and solubility tests of bio-ashes (Papers IV and V) were

carried out at Suomen Ympäristöpalvelu Oy, a FINAS-accredited test laboratory.

53

6 Results and discussion

This chapter presents the chemical results of our studies on the development of

eco- and material-efficient pellet production. The results are collected from

analyses in pre-experimental pelletizing research, the publications (I-V) of this

thesis and in addition some new results from the EAKR EkoPelletti R&D project.

Some pre-experimental tests have been carried out just recently. There is reason to

point out that in addition to these practical pelletizing results, we have developed

and tested new chemical methods for use in pellet research, in addition to the

methods used in current pellet standards. An optical staining procedure and an

automatic respirometric BOD OxiTop method are the most important of these

new chemical methods used for pelletizing studies.

6.1 Moisture content

Drying of biomass has increased during the last decades and with a growing pellet

market it will increase further. The fresh wet raw material of wood pellet

production contains about 50–55% water. For pellet production, manufacturers

normally dry the raw material to a water content of 8–12% before the pelletizing

process begins [77]. Drying of wood-based biofuels is important, since wet wood

results in low combustion temperatures, low energy efficiency and high emissions

of hydrocarbons and particles compared with, for example, pellets. If biofuels are

dried and compressed into pellets or briquettes, the fuel will have controlled

moisture content (MC) and a higher energy density and it is easier to transport. It

will also take up less room and is also less biodegradable and less susceptible to

mould and insect attacks during storage.

Samuelsson et al. found that the moisture content of sawdust was the

dominant factor of bulk density and the pelletizer motor current; they both show

low values at high moisture contents due to the lubricating property of water,

which lowers friction in the pelletizing process. The increase in binding strength

with longer storage time is explained by a reduction in extractive content which

contains molecules that can block binding sites on the material surface. Optimum

pellet quality was obtained when storage time exceeded 120 days and with

sawdust moisture content within the range of 11–13% [78].

In this thesis work the raw materials of the pellets manufactured were already

dry. We modelled the drying process of different biomasses (sawdust, wood chips,

peat) used in pellet manufacture. The results showed up as asymptotic curves for

54

each raw material (see Fig. 17). For peat the curve was almost linear. Wet sawdust

(moisture content 51.5%) needed to be dried 60 minutes to achieve the required

pelletizing dryness. Fresh wood chips (moisture content 54.4%) needed to be

dried up to 150 minutes to achieve the required pelletizing dryness. For the peat

the required pelletizing moisture content was achieved in 300 minutes. Drying

temperature in the test was 105 °C.

Fig. 17. Drying curves of pellet raw materials at drying temperature T = 105 °C.

6.2 Calorimetric heat (I,II)

All the experiments showed that lignosulphonate, potato flour/waste potato flour

or potato peel residue did not have any significant effect on the calorimetric heat

value of wood-based pellets. This is logical, due to the low content of the binding

material in the pellets and the differences between the heat values of the analysed

wood and binding materials. Calorimetric heat results in this study are presented

as gross calorific heat (Eq. (2)), net calorific heat (Eq. (3)) and net calorific value

in received moisture (Eq. (4)). The heat values of wood pellets containing bark-

free stem wood and different binding material are presented in Fig.18. Gross

calorific heat values amongst the different pellet samples are not proportional to

changes in moisture content between the samples. Therefore, the results presented

55

as gross caloric heat values are usually the most representative amongst all the

samples.

The results show that additives generally comprise less than 2% of total mass,

and therefore they have only little effect on the combustion of pellets. Even if the

maximum amount of a high-energy-content additive, like vegetable oil, were

used, the total effect would be a less than 3% increase in heating value [79].

Fig. 18. Heat values [kJ/g] of wood-based pellets containing stem wood and binding

material.

However, owing to the accuracy of the combustion heat values observed—for

example lignosulphonate-containing pellets with an increasing quantity of

lignosulphonate—the results can be explained by the slightly lower gross calorific

value of lignosulphonate compared with the corresponding value of the studied

stem wood: Qgr,d (stem wood) = 20.31 kJ/g > Qgr,d (lignosulphonate 1%) =

20.21 kJ/g > Qgr,d (lignosulphonate 2%) = 20.12 kJ/g > Qgr,d (lignosulphonate)

= 17.06 kJ/g.

All the experiments show that measurement of the calorific heat value with

an automatic calorimeter is very accurate. However, there is an interesting

question: Is there uniformity in the calorimetric heat values determined by

different calorimeters and their users? Therefore, we carried out an inter-

56

calibration study to examine this problem. This is especially important in order to

be able to compare different heat value results determined in different

laboratories. The results of the inter-calibration study are presented in Table 2

[unpublished results].

Table 2. Qgr,ad results of an inter-calibration study with different calorimeters [MJ/kg].

Sample IKA C200

purchased 2011

IKA C2000 Basic

purchased 2008

IKA C5000

purchased 2005

Parr 6200

purchased 2010

Oat straw pellet 19.02 18.99 18.88 18.79

Sawdust pellet 20.67 20.87 20.83 20.81

Chips/straw/oat pellet 19.80 19.86 19.83 19.68

Biodiesel 39.82 39.68 39.9 39.84

Chain oil 39.50 39.32 39.5 39.46

Butanol 35.97 35.63 35.9 35.77

The results of the inter-calibration study [unpublished data] showed that:

1. The heat values Qgr,ad of the studied liquid biofuel samples are significantly

higher than the heat values of the solid samples.

2. The liquid samples are homogenous, and so the heat values of different

laboratories are very uniform, dispersion between the different liquid samples

being only ± 0.1, ± 0.1 and ± 0.2 [MJ/kg].

3. Considering the practical point of view (biomasses are not totally

homogenous), the solid samples were not strictly homogenised, but still

dispersion between the results was remarkably small (all three ± 0.1

[MJ/kg]), being of the same magnitude as that of the homogenous liquid

biofuels.

4. Because of their inhomogeneity, it is not reasonable to express the results of

the natural products such as solid biofuels more than with only one decimal

of accuracy. In addition, the sampling error of solid material [80,81] also

affect to determined heat value and must be taken into account in result

accuracy.

5. The brand, model (price), age nor performer of the determination have no

significant impact on the heat value results, as long as the researcher does all

the determinations carefully.

6. The results showed that all the heat values of the four different laboratories,

determined by bomb calorimeter for the liquid and solid biofuels, are

57

remarkably uniform, which knowledge can be utilised in collaborative studies

in the future.

7. These Qgr,ad results can be used as an approximation of biofuel heat values of

liquid (in Table 2: 38 ± 2 MJ/kg) and solid biofuel samples (in Table 2: 20 ± 1

MJ/kg).

6.3 BOD measurements (I,II)

Biodegradation of pellets and their raw materials is a very important factor in full-

scale industrial manufacture of pellets, as high biodegradability may cause

considerable economic losses during storage and transportation. Potato peel

residue is moderately biodegradable when measured in OECD 301F conditions.

Its degree of biodegradation is 27% in 5 days (Fig. 19). Even though potato peel

residue itself has high biodegradability, it seems to even decrease the

biodegradation of pellets when it is used as a binding material. This effect can be

explained by the prevention of oxygen gas adsorption in pellet pores. The effect

of lignosulphonate is slight but opposite, it increases the biodegradation of pellets.

Lignosulphonate is not biodegradable when measured in OECD 301F conditions.

Its degree of biodegradation is 1.75% in 7 days of BOD7 measurement. Potato

flour is slightly biodegradable when measured in OECD 301F conditions. Its

degree of biodegradation is 4% in 7 days of BOD7 measurement. The graph of

potato peel residue turns downward after 5 days. This may be explained by

moulding or some other reactions in the potato peel residue, which causes gas

formation (also observed as malodorous gases). This causes an increase in

pressure during the period from 5 to 16 days, after which biodegradation or other

reactions that form carbon dioxide are dominant.

58

Fig. 19. Biodegradation of binding agents in solution in OECD 301F test conditions

within 28 days.

Fig. 20. Biodegradation of pellets with no binding agent and lignosulphonate pellets in

the solid phase within 30 days.

When biodegradation was measured in solid-phase pellets without a binding agent

and in pellets containing lignosulphonate as a binding agent, they did not degrade

at all (Fig. 20). Fresh wood chips with a moisture content of 50% were used as a

59

reference material. Pellets containing potato peel residue and potato flour

measured in the solid phase did not degrade at all, as can be seen in Fig. 21. The

biodegrability results of this study show that BOD reactions in the analysed

wood-based pellets do not have any practical effect on their mechanical

durability, nor do they cause mass loss even during a relatively long period of

storage. However, it is generally known that different fungi and other micro-

organisms decompose moist wood in the solid phase.

Fig. 21. Biodegradation of potato peel residue pellets and potato flour pellets in the

solid phase within 30 days.

The biodegrability of condensate water from drying of mixed wood chips was

also determined for some pre-experimental cases (Fig. 22) [unpublished results].

This knowledge is very important for future consideration of condensate water

from drying of mixed wood chips on an industrial scale. Also pH, TOC, COD and

electrical conductivity were determined. GC-MS results showed that the

condensate water in question did not contain any PAH compounds. The degree of

biodegradation (BOD28) of organic matter in condensate water was 41% (average

of 3 determinations). The BOD28 value shows that condensate water from drying

of mixed wood chips is moderately biodegradable and purification of condensate

water from wood chips is spontaneous. All these results—which indicated a lack

of heavy metals in these waste waters—suggest that at least the analysed

condensate water from drying of Finnish wood chips is pure and its treatment

causes neither problems nor costs [unpublished results].

60

Fig. 22. Biodegradation of condensate water from drying of mixed wood chips (pine

20% and aspen 80%) determined by the BOD OxiTop method at 20 oC.

6.4 Binding agent usage (I, II)

Several explanations have been proposed for the mechanisms by which particles

in binderless wood composites, including wood pellets [82], are bonded, such as

thermal softening of lignin [83], pectin bonding [84], bonding through

hemicellulose degradation products [85] or mechanical particle interlocking [86]

in which the size, shape and surface roughness of the wood particles influence

bonding strength [87]. In this thesis work we concentrated on studying the effects

of using a binding agent to improve pellet compactness and quality.

The influence of raw material extractives and binding agents on pellet

characteristics such as density, compression strength and moisture sorption is not

well understood. However, it has been observed that active sites on the particles

may be blocked by lipophilic substances that have migrated to the particle surface

during drying and hot pressing, thus obstructing wettability and reducing bond

strength [87,88]. It may be assumed that similar hydrophobic extractive migration

to the surface also occurs during sawdust drying and pelletizing, thus affecting

particle bonding in the pellet [89]. This view is supported by the fact that pellets

pressed from fresh sawdust compared with those pressed from stored sawdust

-5

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30

BOD/ThOD*100%

time [d]

61

show differences in durability. Pellets pressed from stored sawdust or from a

mixture of fresh and stored sawdust generally have higher durability and bulk

density than pellets pressed from entirely fresh sawdust [90,91]. During the

storage of sawdust, VOC compounds may evaporate and lipophilic extractives are

broken down through microbiological and auto-oxidative processes [91,92,93],

resulting in increased particle surface wettability and improved particle bonding

[87,88]. Stronger bonding between particles will most likely minimise springback

after compression, thus resulting in higher density [89]. Knowledge about the

influence of extractives and binding agents on the pellet’s physical characteristics

could make it possible to fine-tune the raw material handling process in the

pelletizing industry, resulting in less variation in pellet quality.

Additives or binding agents may be added to improve the pellet’s mechanical

properties, i.e. to increase strength and density, improve pelletizing throughput or

improve moisture resistance [94]. Another reason to include additives is to

improve combustion properties, e.g., the ash melting point, slagging and

corrosion [95]. Binding agents that improve inter-particle bonding may be in

liquid or solid form. It is reported that roughly 50 organic and inorganic

compounds have been used in densified biomass products [96]. In feedstock

pellets, a wide range of additives are used, including different starch-containing

compounds, molasses, proteins, modified cellulose, lignosulphonate [Paper II],

kraft lignins and inorganic clay minerals [6,97,98]. Other additives used in wood

pellets are paraffin, stearin and cellulose fibres [94]. It has to be noted that there

are national differences on what additives and binding agents are allowed in fuel

pellets. However, from the environmental, economic and logistical points of view,

the most important binding materials are industrial by-products and locally

utilisable residuals.

The aim in the future is to expand the base of wood pellet raw material by

utilising more low-value and/or moist biomass, especially logging residue from

thinning. To improve the quality of pellets, commercial and industrial by-product

materials are commonly used as binding agents in pellet production [4,6,12].

Also, considering the profitability of production and some occupational safety

problems (wood dust exposure, fire and explosion risk, coherence, etc.), it is

practical to use binding agents. By using some new binding materials, such as

potato peel residue, it seems to be possible to prevent pellet biodegradation [Paper

I], which can cause substantial economic losses in full-scale pellet production.

The aim of the present research was to find and test some eco- and cost-efficient

materials for binding purposes and thus improve the competitiveness of pellet

62

products as an alternative in energy production. Therefore, various locally tailored

binding agents such as certain industrial by-products and residuals, especially

starch-containing waste materials, are currently under study. In this thesis

lignosulphonate, residual potato flour and potato peel residue were used and

tested as adhesive binding agents. Using lignosulphonate, potato flour and potato

peel residue as model agents in pelletizing, in this thesis we tested the total

functionality of a pilot-scale pellet facility combined with an extensive chemical

toolbox (see Table 1) to promote future development of eco- and cost-efficient

wood-based pellet production in both the quantitative and qualitative sense.

The power consumption of the press was measured, and its efficiency can be

estimated by comparing its power consumption to the feed rate of material to the

pellet press. Sample pellets were produced with the settings described in Table 3.

Potato peel residue was problematic in terms of moisture. The 5% potato peel

residue mixture was excessively dry, which increased power consumption. To

produce good-quality pellets from a 20% potato peel residue mixture, a slower

feed rate was required to let moisture evaporate from the pellets during the

pressing process. Lignosulphonate has the best properties as an additive; while

improving pellet quality, power consumption does not significantly increase

compared with native wood. Adding potato flour does increase power

consumption, but it also improves pellet quality while having no noticeable effect

on inorganic content.

Table 3. Efficiency: power consumption, feed rate to the pressing chamber and pellet

outputs.

Power consumption (kW) Feed Power/feed Pellet output (kg/h)

Native wood (no additive) 21.8 7 3.1 144

Potato flour 1% 21.7 6 3.6 a

Potato flour 2% 23.5 5 4.7 a

Potato peel residue 5% 21.4 5 4.3 116

Potato peel residue 10% 19.0 5 3.8 a

Potato peel residue 20% 17.6 3 5.9 a

Lignosulphonate 1% 22.2 7 3.2 180

Lignosulphonate 2% 23.5 7 3.4 180

a Not determined

63

In terms of mechanical durability, native wood pellets without additives and 5%

potato peel residues (0.5 % starch) have substandard quality [48] (97.5% above

sieve). It is possible to achieve standard-quality pellets from Scotch pine without

additives, but it requires optimisation of the matrix channel length. In this thesis

work the matrix channel length was kept constant to preserve the comparability of

the results. A major factor in the case of 5% potato peel residue pellets is the

excessive dryness of the pelletized mixture, which also caused reduced output.

Fig. 23. Results of the pellets’ mechanical durability [Paper II].

All three tested binding materials had a positive effect on pellet compactness.

Lignosulphonate is known to improve pellet quality as well as compactness

[99,100]. The results of the pellets’ mechanical durability are presented in Fig. 23.

The results show that the technically optimal proportion of the analysed binding

compounds in the pellets is ca. 1 to 2%. These results are in accordance with

other experiments in both pilot and full-scale pellet plants [Paper I]. The lowest

64

effective limit for each additive needs further research in regard to elemental

composition and to obtain optimal economy. As mentioned, all the additives had a

positive impact on the pellets’ mechanical durability. In addition, lignosulphonate

seems to have a lubricating effect, which reduced the power consumption of the

pellet press during the experiments. Lignosulphonate raised the production rate of

pellets, bringing energy and cost savings, but the amount must be ≤ 0.5% in order

to keep the sulphur concentration of the pellets below the normative limit of

CEN/TS 14961 [5].

As seen in the results of paper II, potato peel residue, as an agricultural

biomass, typically has a very high potassium content [101]. According to a new

Decree on Fertilizer Products [55], the sum ratio of potassium (K) and

phosphorus (P) combined should be at least 2%, and this high potassium content

in potato peel residue can be utilised in ash products in the future. Potato peel

residue is an attractive additive from the economic and environmental points of

view, despite problems in logistics. The current cost situation related to potato

peel residue in Finland is sensitive to freight costs, whether the buyer or seller

pays for it. Potato peel residue itself has zero value and it mostly goes to cattle

feed or bioethanol production. If potato peel residue could be used as a pellet

additive, then the potato operator could reduce the need for waste management

and would have the option of selling this by-product. This requires good logistics

or local solutions, as the functional component in potato peel residue is starch,

which is quickly lost due to biodegradation and other chemical reactions. Also the

risk of earth contamination has to be taken into account, as potato peel residue is

not a purified product [28]. Potato flour is a much more refined product than

potato peel residue, but it is a standard additive in current pellet production [6].

6.5 Visual analysis of pellet sample images (I, II, III)

In the microscope images based on the method we have developed, starch

compounds appear as small dark spots and areas. Microscope images enable us to

analyse the binding structure of the pellet and starch penetration. However, in

visual analysis it must always be observed that even after careful flushing, pellet

pores may contain some reagent residuals and wood material itself may contain

dark spots, and hence these effects must be eliminated in the analysis stage. Some

images of starch-containing pellets obtained with the optical microscopic staining

method are presented in the following chapters. In the future, computational

image analysis will be integrated into the analysis procedure. In order to examine

65

microscopy photographs in more detail, specific starch penetration analysis

technologies have been developed that provide reliable information on

penetration [102]. Once more, there is reason to point out that these conclusions

concerning visual analysis of the pellet samples containing starch are based on

examinations of real microscopic slides, and not of the images presented in the

results in the following chapters.

The reference sample (see Fig. 24) contains no binding agent. As there are no

dark spots or areas to be observed in the images, this observation combined with

later results for starch-containing pellet samples show that our characterisation

method is proven to function selectively for starch compounds.

Fig. 24. Reference samples unstained (left) and stained with KI (right).

Solid starch waste pellets with binding agent contents of 1 % and 3 %,

respectively, are presented in Figs. 25 and 26. Starch compounds appear in the

images as small dark spots and areas. It can be observed that starch compound

penetration is relatively uniformly distributed in both samples. It can also be

observed that the pellet samples with a binding agent content of 3% have

distinctively more dark spots compared with a binding agent content of 1%. Thus,

our characterisation method appears to function quantitatively correct in terms of

binding agent content examination.

66

Fig. 25. Solid starch waste pellets with 1% binding material content; unstained (left)

and stained with KI (right).

Fig. 26. Solid starch waste pellets with 3% binding material content; unstained (left)

and stained with KI (right).

Liquid starch waste pellets are presented in Fig. 27. Starch compounds appear in

the images also as small dark spots. Starch compound penetration in the liquid

waste pellet samples is observed to have a more varied distribution compared

with the solid starch waste pellet samples (see Figs. 25 and 26).

67

Fig. 27. Liquid starch waste pellets; unstained (left) and stained with KI (right).

Lignosulphonate pellets and solid starch-containing potato waste pellets were

both manufactured in two different process conditions. The aim was to

manufacture pellets of both high and poor quality. After that, both samples were

visually analysed and compared. In order to obtain information on their quality

parameters and mechanical stability, the pellet samples were vibrated, sieved and

weighed. The proportions were calculated from the weighed mass in each sieve.

The conclusion can be drawn from the results of the sieve tests that the more the

cumulative proportion of the particle size exceeds 4 mm, the better the quality and

compactness of the pellet will be. In comparison to pellets without binding agents,

the effect of the different binding agents on the mechanical stability of the pellets

was concluded based on the sieving results. The moisture contents of the pellet

samples were also measured, because this is known to have an effect on pellet

quality. In our preliminary research, we observed that the poor quality of pellets is

clearly visible from optical microscopic images for example as transparency and

irregularity of shape. Some of these results are presented in Table 4. The effect on

pellet quality (transparency and irregularity of shape) was particularly observed

(see Figs. 28 & 29). These effects can be observed much more clearly from real

size microscopic slides.

Table 4. Moisture content and sieving results of two different process conditions; high

quality (sample 1) and poor quality (sample 2) pellets.

Binding agent Measurement Sample 1 Sample 2

Lignosulphonate Moisture content 6.5% 9.5%

Sieving (> 4 mm) 99.7% 96.9%

Solid potato waste Moisture content 8.0% 10.0%

Sieving (> 4 mm) 99.6% 95.2%

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Fig. 28. Lignosulphonate pellets; high quality (left) and poor quality (right).

Fig. 29. Solid potato waste pellets; high quality (left) and poor quality (right).

6.6 TG analysis

Thermogravimetric (TG) measurements results provide vital quantitative

information about weight loss in the sample, wherein the mass of the sample vs.

temperature is weighed as the sample is heated at a constant heating rate. From

the TG curves it is possible to estimate the change in mass of different gaseous

compounds during drying. The information obtained can then be used e.g., to

optimise the drying temperature of different types of wet biomaterials.

To model the drying process and optimise the drying temperature, TG

measurements were carried out for two different wood pellet raw materials: wet

sawdust and wood chips. Before thermogravimetric measurements the wood

pellet raw materials were artificially moisturised to about a 40% moisture content,

prepared for thermoscale sample (particle size 6 mm) and placed in the corundum

69

(Al2O3) beaker. Measurements were carried out under air (20% O2 / 80% N2)

atmosphere and sample heated at rate of 10 °C/min, from 25 °C to a final

temperature of 400 °C.

Fig. 30. Thermogravimetric mass curve (wt-% vs. temperature) of wet sawdust raw

material.

The thermogravimetric mass curve of sawdust raw material is presented in Fig.

30. From the graph it can be observed that elevating the drying temperature first

extracts only water (moisture) from the sawdust material, which is indicated as an

initial weight decrease in the sawdust material mass curve. This decrease in the

mass of the wet sawdust material takes place until about a 100 °C drying

temperature in the conditions of this TG study. The next sharp weight decrease in

the mass curve indicates the drying temperature which must not be exceeded

during the drying process, because in that case the sawdust material will catch

fire. For the wet sawdust material this temperature is achieved at about 230 °C.

Based on the thermogravimetric mass curve results, the maximum drying

temperature of the wet sawdust material in this study is about 200 °C. On the

other hand, it is observed from the graph (Fig, 30) that the optimum drying

temperature for wet sawdust material is about 100 °C, where most of the moisture

is evaporated but the sawdust raw material will not yet catch fire and volatile

70

compounds with a high heat value are not lost. Our results were similar with

earlier TG study of biomasses, for example Darvell et al. [103] have indicated

that the first mass loss at about 120 °C and below is from moisture evaporation,

followed by a larger mass loss due to the volatile matter being released. The

remaining residue is char. Darvell et al. also found that higher total mass losses

were obtained for the biomass fuels with higher volatile contents [103].

Fig. 31. Thermogravimetric mass curve (wt-% vs. temperature) of wet wood chip raw

material.

The TG mass curve of wood chip raw material is presented in Fig. 31. As earlier,

from the graph it can be observed that elevating the drying temperature first

extracts only water (moisture) from the wood chip material, which is indicated as

an almost linear weight decrease in the wood chip material mass curve. This

decrease in the mass of the wet wood chip material takes place until about a 120

°C drying temperature in the conditions of this study. These results are in

accordance with corresponding observations of Darvell et al. [103]. The next

sharp weight decrease in the mass curve indicates the drying temperature which

must not be exceeded during the drying process, because in that case the wood

chip material will catch fire. For the wet wood chip material this temperature is

71

achieved at about 230 °C. Based on the thermogravimetric mass curve results, the

maximum drying temperature of the wet wood chip material in this study is about

220 °C. On the other hand, it is observed from the graph (Fig, 31) that the

optimum drying temperature for wet wood chip material is about 120 °C, where

most of the moisture is evaporated but the wood chip raw material will not catch

fire and valuable volatiles are maintained. In addition, drying costs are low.

6.7 Particle size distribution (II)

It is generally known that the particle size of biomass and biomass-based

products, and therefore the particle size distribution of different materials, is

nowadays under wide study. So an important matter for the success of pelletizing

and understanding some physical and chemical properties of pellets is the particle

size of the materials used. For example, the optimal particle size of the raw

material affects the strength and processing properties of pellets. In this study the

milled raw material with a particle size of over 2000 µm was sieved off with a

Hosokawa Alpine Air Jet Sieve e200LS. The proportion of sieved material was

about 3.3% of the whole sample mass. Twenty mL of sample were taken for the

analysis.

Bergström et al. have indicated that particle size distribution has some effect

on current consumption and compression strength, but no evident effect on single

pellet and bulk density, moisture content, moisture absorption during storage and

abrasion resistance [104]. Differences in average total conversion time

determined for pellet batches tested under the same combustion conditions was

less than 5 % and not significant. The results are of practical importance,

suggesting that grinding sawdust particle sizes below 8 mm is probably needless

when producing softwood pellets. Thus, it seems that less energy could be used if

only oversized particles are ground before pelletizing [104].

In general, it has been observed that broad variation in particle size is best

with respect to pellet quality [105]. An overly high proportion of fine particles

(particle size with a diameter smaller than 0.5 mm) in the raw material has a

negative impact on both friction and pellet quality. The optimum particle size

depends on the densification process, for pellet production particles are usually

below 5 mm in diameter, depending on the material. As a rule of thumb, the

amount of fine materials should not exceed 10 to 20% unless a binding agent is

added [105].

72

The particle size distribution of milled wood (bark-free Scotch pine sawdust

and shavings) pellet raw material is presented in Fig. 32. Particle diameter in

micrometres is on the x-axis and volume weighted percentage (vol-%) is on the y-

axis. According to the particle size distribution plot of pellet raw material, it is

clear that the pellet wood material is very fine and quite homogeneous. For

comparison, the particle size distribution of corresponding Finnish wood ash from

the burning process is presented in Fig. 33. The results show that the particle size

distribution of wood ash is quite different from that of the raw material.

Fig. 32. Particle size distribution of milled pellet raw material (bark-free Scotch pine)

[Paper II].

73

Fig. 33. Particle size distribution of typical bio-ash from the burning process [106].

6.8 Wood-pellet-based bio-ash (II,IV,V)

The chemical and physical properties of two model wood-pellet-based bio-ashes

are described in this chapter. The corresponding results concerning the other

studied bio-ashes are quite similar and are presented in detail in papers II, IV and

V.

According to Table 5, the pH of the wood pellet ash sample 1 and 2 solutions

(wood pellet ash from Kuusamo and Jyväskylä) was 13.3 and 13.4, respectively,

and they were very strongly alkaline, in accordance with corresponding results for

other wood-based ashes [Papers IV,V,31,32,66,107]. This means they have a

strong liming effect. An alkaline pH indicates that at least part of the dissolved

metals in the ash occur as basic metal salts, oxides, hydroxides and/or carbonates

[65,66,67]. Thus, the proportion of soluble basic metal salts, oxides, hydroxides

74

and carbonates in the wood ash obviously outweighs the proportion of soluble

acidic components, and the wood pellet ash subsequently generates an alkaline

pH. Furthermore, according to the electrical conductivity value (58 mS cm-1)—an

index of the total dissolved electrolyte concentrations—the leaching solution of

the wood pellet ash has a relatively high ionic strength. This also indicates that at

least part of the dissolved metals occur as dissolved basic metal salts, e.g. oxides

and hydroxides. The dry matter content of wood pellet ash sample 1 was very

high (99.7%). From an environmental and health point of view, this can be

problematic because it may increase the amount of dust present during handling.

The dry matter content of ash sample 2 was also high, but a little lower (97.5%).

The slightly elevated LOI (5.3%; d.w.) and TOC (5.7%, d.w.) values of ash

sample 1 as well as the LOI (2.7%; d.w.) and TOC (4.0%, d.w.) of sample 2

indicate that samples 1 and 2 contain some unburned organic material due to

incomplete combustion of the fuel in the boiler. The easily soluble Ca

concentration of 100 g kg-1 (d.w.) in sample 1 was ca. 62.5 times higher than the

Ca concentration of 1.6 g kg-1 (d.w.) measured in typical coarse-textured mineral

soil in Finland. In sample 2 the corresponding ratio was 27. The concentrations of

easily soluble Mg (16.5 g kg -1; d.w. and 8.61 g kg-1; d.w.) and P (0.08 g kg-1; d.w.

and 0.02 g kg-1; d.w.) were correspondingly ca. 82.5 and 8 times higher in sample

1 and ca. 43 and 2 times higher in sample 2 than the typical value of Mg (0.2 g

kg-1; d.w.) and P (0.01 g kg-1; d.w.) in Finnish soil. The elevated Ca, Mg and P

concentrations in model wood pellet ash samples 1 and 2 indicate that wood-

pellet-based ash is also a potential agent for soil remediation and for improving

soil fertility. It would therefore be ecological beneficial if also wood-pellet-based

bio-ash could be returned to the forest ecosystem.

The relatively high, easily soluble concentrations of K in wood pellet ash

samples 1 and 2 (70 g kg-1 d.w. and 82 g kg-1 d.w., respectively), which contribute

to improving soil fertility, are also worth noting. In addition, it is worth noting

that the amounts of potassium alone in ash samples 1 and 2 obviously exceed the

limit value given in the new Ministry of Agriculture and Forestry Decree on

Fertilizer Products [56]. According to this decree the sum ratio of potassium (K)

and phosphorus (P) combined should be at least 2%, determined by microwave

assisted digestion using HNO3 as a digestion solution [53]. According to Tulonen

et al. [108], this kind of ash is especially recommendable for nitrogen-rich

peatlands, which suffer from a shortage of other nutrients.

The liming effect (neutralising value NV) is one of the most important

indicators in evaluating the agricultural value of ash, because ash acts as a liming

75

agent in acidic soil. The capacity of a liming agent to neutralise soil acidity

depends on its proportion of soluble and hydrolysable bases such as oxides,

hydroxides, carbonates and silicates. Cations such as calcium, magnesium and

potassium are interactive counter-ions. Liming capacity—the calculated

equivalence a neutralising material with a corresponding value of commercial

lime 38 [% Ca]—was calculated to be 1.1 t/t and 1.05 t/t, respectively, for wood

pellet ashes 1 and 2. These results mean that only 1.05–1.1 t of wood pellet ash is

needed to replace 1 t of commercial lime in liming applications, or the

neutralising value of the model pellet ashes is similar to that of commercial lime.

Omitting peat pellet ash, the corresponding values of liming capacity of the bio-

ashes studied in this work [Paper IV] changed from 1.05 to 3.6, indicating that all

the studied bio-ashes are potential neutralising agents for acidic soils.

Table 5. Physical-chemical properties and easily soluble nutrient concentrations in

wood pellet ash sample 1 from Kuusamo and wood pellet ash sample 2 from

Jyväskylä.

Parameter / nutrient Unit Wood pellet ash 1 Wood pellet ash 2

pH (1:5) --- 13.3 13.4

Electrical conductivity (EC) mS cm-1 58 10

Dry matter content (105 °C) % 99.7 97.5

Loss on ignition (LOI, 550 °C) % (d.w.) 5.3 2.7

Total organic carbon (TOC) g kg-1 (d.w.) 57 40

Neutralising value (NV) % (Ca; d.w.) 34 36

Reactivity value (R) % (Ca; d.w.) 26 25

Liming capacity a % (Ca; d.w.) 1,1 1,05

Ca g kg-1 (d.w.) 100 43

Mg g kg-1 (d.w.) 16.5 8.61

Na g kg-1 (d.w.) 5.0 21.9

K g kg-1 (d.w.) 70 82

S g kg-1 (d.w.) 6.7 4.4

P g kg-1 (d.w.) 0.08 0.02

Mn mg kg-1 (d.w.) 1370 597

Cu mg kg-1 (d.w.) 23 25

Zn mg kg-1 (d.w.) 350 177 a Equivalence with the neutralising value (38%) of commercial lime [t/t].

The total heavy metal concentrations in wood pellet ashes are expressed on a dry

weight (d.w.) basis. According to Table 6, the total heavy metal concentrations in

wood pellet ash 1 (Kuusamo) and wood pellet ash 2 (Jyväskylä) were lower than

76

the current Finnish limit values for maximal allowable heavy metal

concentrations in forest fertilizers [56]. On the other hand, the heavy metal

concentrations in both wood pellet ash 1 (Kuusamo) and wood pellet ash 2

(Jyväskylä) were higher than the current Finnish limit values for maximal

allowable heavy metal concentrations in agricultural use [56], but only for

cadmium. It should be noted that the old limit values are presented and discussed

in papers IV and V. In Finland the limit values in agricultural use have stayed the

same, but the new limit values for heavy metal concentrations in fertilizers used

in forestry came into force in September 2011 [56]. In this context it is worth

noting that we did not determine e.g. the toxicity of the wood pellet ash. This is

due to the fact that, according to Finnish legislation, information about the

toxicity is not necessary when ash is utilised. However, in many cases the

competent authority may decide that toxicity has to be determined with toxicity

tests before the ash can be utilised as fertilizer. A comprehensive review of the

Finnish limit values and legislation concerning the use of materials as forest

fertilizers in Finland is given in the new Ministry of Agriculture and Forestry

Decree on Fertilizer Products 24/11 [56].

Table 6. Total heavy metal concentrations (mg kg-1; d.w.) in wood pellet ash sample 1

(Kuusamo) and wood pellet ash sample 2 (Jyväskylä) and the current Finnish limit

values (mg kg-1; d.w.) for wood-, peat- and biomass-derived ashes used as forest

fertilizer and in agricultural use, determined by microwave-assisted HNO3 acid

digestion [55].

Metal Wood pellet ash 1 Wood pellet ash 2 Limit value

forest fertilizer

Limit value

agricultural use

Cd 16 9 25 2.5 a

Cu 160 180 700 600

Pb 12 15 150 100

Cr 180 84 300 300

Zn 640 360 4500 1500

As < 3.0 < 3.0 40 25

Ni 110 30 150 100

Hg < 0.03 < 0.03 1.0 1.0

a For ash-based fertilizer.

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Solubility results for different heavy metals in 3-stage BCR extraction in wood

pellet ash sample 1 are presented as histograms in Fig. 34, in which the presented

total concentrations are determined by microwave-assisted acid digestion (HCl +

HNO3) [53]. Solubility results of different heavy metals in 3-stage BCR

extraction in wood pellet ash sample 2 are presented as histograms in Fig. 35. The

percentages of metals presented in the figures and the BCRtotal values can be

changed into metal concentrations. Furthermore, potential bioavailabilty

percentages PBPM can be calculated with Eq. (12) and are given for the different

metals in Figs. 34 and 35.

Fig. 34. Distribution of heavy metals in wood pellet ash sample 1 (Kuusamo) in 3-stage

BCR extraction between exchangeable (BCR S1), easily reduced (BCR S2) and

oxidizable (BCR S3) fractions.

79

Fig. 35. Distribution of heavy metals in wood pellet ash sample 2 (Jyväskylä) in 3-

stage BCR extraction between exchangeable (BCR S1), easily reduced (BCR S2) and

oxidizable (BCR S3) fractions.

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6.8.1 Utilisation potential of bio-ash

As presented earlier, as part of the EU and Finnish national waste strategies, the

primary goal also for bio-ash is its utilisation as material. Therefore, there is

currently tremendous interest in the EU countries in substantially increasing the

utilisation of bio-ash, developing new applications and developing new bio-ash-

based products, especially granulated bio-ash-based products. Many factors, such

as combustion technology, affect the properties of bio-ashes. As in the case of

wood pellets, binding agents can slightly affect the composition and properties of

ash. For example, when lignosulphonate is used as a binding agent, it slightly

increases the sulphur content of pellets [Paper II]. However, it is generally known

that wood biomass is very pure bioenergy and thus the results obtained in this

thesis work support the assumption that wood and wood-based pellet ash have

great utilisation potential and they are suitable for use in new eco-, cost- and

material-efficient applications even in the near future as part of extensive bio-ash

utilisation.

Granulated ash-based products in particular seem to have great potential. Ash

can also be pelletized, but it is not economically profitable, contrary to granulated

ash products. Lately there are many studies in progress or under consideration

concerning this area [109]. In addition to agricultural and forestry fertilizers, there

are also several other utilisation applications for wood-based bio-ashes, such as

liming and soil conditioning, road construction and landscaping, combustion, as

adsorbents in industrial and municipal waste water treatment, in concrete industry

and in new applications [106]. One of the new applications for granulated ash is

polymeric geo-ash, which can be used for manufacturing different geo-polymers.

Utilisation of steel slag—a chemically pure, voluminous and thus economical

industrial by-product—mixed with bio-ash has also been under study [32]. There

have also been promising results in using granulated ash to remove phosphorus in

waste water treatment. Because wood-based pellet ashes are strongly alkaline, it

can be assumed that they can be used for heavy metal removal in waste water

treatment applications. It is noteworthy to point out that the new ash-based

environmentally friendly products can also have utilisation potential at the global

level. In this thesis work only pine-based ash was studied, but it would be

interesting to also examine ashes from other wood species.

The results of sequential leaching of wood-pellet-based bio-ash in this thesis

show, in agreement with earlier studies on other ashes, such as bottom or fly ash,

paper mill sludge, green liquor dregs, PCC waste, lime waste from pulp and paper

83

mills and waste rock material originating from a zinc mine

[65,66,67,68,69,70,71,72], that conditions have a large effect on the solubility of

all heavy metals, and therefore on their mobility, bioavailability and

environmental risk. In addition, the total concentration of every element is much

larger than its solubility in each potential bioavailability fraction, because the

highest concentrations of all metals occurred in the residual fractions; see

equations (13) and (14). Thus, the total concentrations of heavy metals, which

generally have been used in the legislative procedure in e.g. utilisation of bio-ash

for agricultural and forest fertilizers [56] and in assessment of soil contamination

and remediation needs [58], is a poor measure of real environmental risk.

Sequential leaching provides such information for utilisation purposes, which is

necessary if we wish to know the real environmental risk of metals in different

conditions possible in natural conditions now or in the future, i.e. not only in

terms of the conditions related to permit applications.

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7 Conclusions

Using Finnish wood, mainly Scotch pine (bark-free sawdust and shavings) as a

model raw material, the total functionality of a pilot-scale pellet facility combined

with an extensive chemical toolbox was tested and examined in practice in this

thesis work. The chemical toolbox includes measurements of moisture content,

density, heat value, mechanical durability and particle size distribution, TG

analysis and elementary analysis as well as new methods for pellet biodegradation

using the BOD OxiTop method and an optical microscopic staining method. To

improve the quality of pellets, considering the profitability of production and

occupational safety factors, it is practicable to use binding agents, and to

characterise them a new specific optical microscopic staining method using

starch-containing binding agents was developed and tested in this thesis work.

The aim in the future is to expand the base of wood pellet raw material by

utilising more low-value and/or moist biomass to, especially logging residue from

thinning. For example, if only 10–15% of unusable waste wood biomass from

forestry operations could be utilised, it would raise annual wood pellet production

capacity significantly, being about 2–3 million tonnes. The purchase prices of

poor-quality biomasses are very low, sometimes almost free, so it doesn’t increase

the total cost of pellet production significantly, which makes them economically

beneficial and interesting.

In pellet production the manufacturers normally dry the raw material to a

water content of 8–12% before the pelletizing process begins. Optimum pellet

quality is obtained with a sawdust moisture content in the range of 11–13%. In

experiments, wet sawdust (moisture content about 50%) needed to be dried 60

minutes to achieve the required pelletizing dryness and fresh wood chips

(moisture content about 50%) needed to be dried 150 minutes. For peat the

required pelletizing moisture content was achieved in 300 minutes when the

drying temperature was 105 °C.

Binding agents lignosulphonate, residual potato flour or potato peel residue

did not have any significant effect on the calorimetric heat value of the wood-

based pellets, due to the low content of the binding material in the pellets and the

differences between the heat values of the studied wood and binding materials.

Additives or binding agents, < 2% of total mass, have only little effect on

combustion. Even if the maximum amount of a high-energy-content additive were

used, the total effect would be a less than 3% increase in heating value.

86

High biodegradability of raw materials, pellets and binding agents may cause

considerable economic losses in wood pellet production during storage and

transportation and may also cause health problems in pellet warehouses.

However, the concentrations of hazardous volatile compounds (carbon monoxide,

hexanal) are low when pellet biodegradation does not take place. Potato peel

residue itself has high biodegradability, but it seems to even slightly decrease the

biodegradation of pellets when it is used as a binding material. The effect can be

explained by the prevention of oxygen gas adsorption into pellet pores. Pellets

containing lignosulphonate, potato peel residue and residual potato flour

measured in the solid phase did not degrade at all. The BOD reactions in the

studied wood-based pellets do not have any practical effect on their mechanical

durability, nor do they cause mass loss even during a relatively long period of

storage. However, various fungi and other micro-organisms decompose moist

wood in the solid phase. The content of condensate water from drying of Finnish

wood chips is biodegradable and purification of condensate waters of wood chips

is spontaneous, and thus condensate water from drying is pure and its treatment

causes neither problems nor costs.

Binding agents or additives may be added to improve the pellet’s mechanical

properties, for example to increase strength and density, improve pelletizing

throughput or improve moisture resistance. From the environmental, economic

and logistical points of view binding materials should be industrial by-products

and locally utilisable residuals. For example, logging residue from thinning has

great potential for utilisation when expanding the raw material basis of

pelletizing, but usable binding agents are needed for low-value and moist

biomasses. Lignosulphonate, residual potato flour and potato peel residue were

used and tested as model adhesive binding agents. All three tested binding

materials had a positive effect on pellet compactness. Lignosulphonate seems to

have a lubrication effect, which reduced power consumption of the pellet press

during experiments. Lignosulphonate also raised the production rate of pellets,

but the amount must be ≤ 0.5% in order to keep the sulphur concentration of the

pellets below the normative limit. By using potato peel residue as a binding

material it is possible to prevent harmful pellet biodegradation. Adding potato

flour does increase power consumption, but it also improves pellet quality while

having no noticeable effect on inorganic content.

In the microscope images based on the optical microscopic staining method,

starch compounds appear as small dark spots and areas. Microscope images

enable us to analyse the binding structure of the pellet and starch penetration. This

87

characterisation method was proven to function selectively and quantitatively

correctly for starch compounds in terms of binding agent content examination.

The poor quality of pellets is also visible from optical microscopic images, as the

effect of pellet quality (transparency and irregularity of shape) can be visually

observed from the images. In the future, computational image analysis will be

integrated into the analysis procedure. In conclusion, the optical microscopic

staining method was shown to be a useful method for characterisation of starch-

containing binding agents in wood pellets. The information obtained with the

staining method and microscopic structure analysis can be used for planning the

feeding of binding materials in pellet plants (place, amount) and also for

optimizing the pelletizing process.

From thermogravimetric (TG) curves it is possible to estimate the mass

change of different compounds during drying. The information received can then

be used to optimize the drying temperature for different types of wet biomaterials.

For modeling the drying process and optimizing the drying temperature TG

measurements were carried out for two different wood pellet raw materials: wet

sawdust and wood chips. The maximum drying temperature of the wet sawdust in

this study is about 200 °C and the optimum drying temperature for sawdust is

about 100 °C, when most of the moisture is evaporated, but volatile compounds

with high heat value are not lost and drying costs are low. The maximum drying

temperature of the wet wood chips raw material is about 220 °C and the optimum

drying temperature for wood chips is about 120 °C. TG analysis was shown to be

useful method for optimizing and modeling the drying process of different

biomass raw materials for pelletizing.

Particle size distribution has some effect on current consumption and

compression strength but no effect on single pellet and bulk density, moisture

content, moisture absorption during storage and abrasion resistance. Grinding of

sawdust particle sizes below 8 mm is not needed when producing softwood

pellets. The optimum particle size depends on the biomass densification process,

for wood pellet production particles are usually below 5 mm in diameter,

depending on the wood material used. The presence of fine materials (particles

smaller than 0.5 mm in diameter) has a negative impact on pellet quality. The

amount of fine materials should not exceed 10 to 20% unless a binding agent is

added.

The results show that only 1.05–1.1 t of wood pellet ash is needed to replace

1 t of commercial lime in liming applications. Wood-pellet-based ash is also a

potential agent for soil remediation and for improving soil fertility. It is worth

88

noting that the amounts of potassium alone in wood ash samples obviously

exceed the limit value given in the new Ministry of Agriculture and Forestry

Decree on Fertilizer Products, since according to this decree the sum ratio of

potassium (K) and phosphorus (P) combined should be at least 2 %. Because

wood biomass is very pure bioenergy, wood and wood-based pellet ash have great

utilisation potential and they are suitable for use in new eco-, cost- and material-

efficient applications in the future. Granulated bio-ash-based products in

particular have great potential. There are several potential utilisation applications

for these wood-based bio-ashes, such as agricultural and forestry fertilizers,

liming and soil conditioning, road construction and landscaping, combustion,

concrete industry and other new applications, but above all they have economic

potential as adsorbents in industrial and municipal waste water treatment.

89

8 Future work

The aim of multidisciplinary pellet research in the future is to widely utilise non-

utilised low-value and/or moist biomass, especially logging residue from

thinning, and thus substantially expand the base of wood pellet raw materials.

Because these raw materials are mainly situated in rural areas, this decentralised

pellet production will also have an employing effect in these areas. Using short

rotation tree species, such as willow, aspen, alder and eucalyptus as well as other

tree species, e.g. spruce and birch, as a wood pellet raw material in pelletizing

needs to be examined and tested in more detail. Also other biomasses and

industrial by-products in Finland have potential for utilisation as pellets; for

example, agricultural materials such as various straws (barley, oat, wheat, grass),

peat and reed canary grass. A negative aspect with these materials is, however, the

problems in combustion that arise because of the large amount of ash, which is

usually even 5–10%.

In the present research the aim was to find eco- and cost-efficient binding

materials and thus improve the competitiveness of pellet products. Therefore,

various locally tailored binding agents such as certain industrial by-products and

residuals, especially starch-containing waste materials and lignin, are currently

under study and their usage potential needs to be researched further in more

detail. Industrial use of potato peel residue as a pellet binding agent and in many

other applications is not possible in practice without functional drainage, because

of its rapid biodegradation and anaerobic decomposition. Thus, development of

cost-efficient methods, e.g. enzymatic or mechanical drainage of this waste

fraction, is under study for many purposes. One of the new binding agent

applications that will require further research is enzymatically and mechanically

processed fibre, so called zero fibre, which is a by-product of the chemical pulp

industry.

In the future, computational image analysis will be integrated into optical

microscopic staining procedure analysis. Further research based on microscopic

images concerning pellet quality is needed. The same kind of functioning

selective characterisation method has to be developed also for other binding

agents, such as lignosulphonate.

Wood-based pellet ash as a pure product has great utilisation potential and is

suitable for use in new eco-, cost- and material-efficient applications in the future

as part of extensive ash utilisation. Granulated ash products in particular will be

of great interest. Only pine-based wood pellet ash was studied in this thesis work,

90

but in the future it would be interesting to examine pellet ashes also from other

tree species, such as hardwood (birch) and short rotation tree species (willow) as

well as ashes from agricultural raw materials.

From the environmental, energy-related and social (employing effect) points

of view the goal is to develop decentralised pellet production alongside of large-

scale pellet factories, so that pellet production will have more essential

importance in energy policy than it does today. Therefore, much more

multidisciplinary research is needed to achieve this goal, for which purpose the

developed chemical toolbox is a very practicable tool.

91

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99

Original papers

I Kuokkanen M, Kuokkanen T, Stoor T, Niinimäki J & Pohjonen V (2009) Chemical Methods in the Development of Eco-efficient Wood-based Pellet Production and Technology, Waste Management & Research 27: 561–571.

II Kuokkanen M, Vilppo T, Kuokkanen T, Stoor T & Niinimäki (2011) Additives in Wood Pellet Production – A Pilot-Scale Study of Binding Agent Usage, BioResources 6 (4): 4331–4355.

III Kuokkanen M, Prokkola H, Larkomaa J, Stoor T, Siltaloppi L & Kuokkanen T (2010) Specific Staining and Optical Microscopy – a New Method for Characterisation of Starch-containing Wood Pellets, Special Issue of Research Journal of Chemistry and Environment, Proceedings of ICCE-2009: 311–317.

IV Kuokkanen M, Kuokkanen T, Nurmesniemi H & Pöykiö R (2009) Wood pellet ash – a potential forest fertilizer and soil conditioning agent (a case study), The Journal of Solid Waste Technology and Management, Proceedings in ICSW 2009: 659–667.

V Kuokkanen M & Kuokkanen T (2009) Puu- ja turvepellettien sekä hakkeen lämpökeskus- ja pienpoltossa syntyvien tuhkien hyötykäyttöön liittyvä tutkimusraportti, University of Oulu, Report Series in Chemistry, Report No. 74.

Reprinted with permission from SAGE Publications (I), BioResources (II),

Research Journal of Chemistry and Environment (III), Widener University School

of Engineering (IV) and Oulu University Press (V).

Original publications are not included in the electronic version of the dissertation.


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HUMANIORA

TECHNICA

MEDICA

SCIENTIAE RERUM SOCIALIUM

SCRIPTA ACADEMICA

OECONOMICA

EDITOR IN CHIEF

PUBLICATIONS EDITOR

Senior Assistant Jorma Arhippainen

University Lecturer Santeri Palviainen

Docent Hannu Heusala


University Lecturer Hannu Heikkinen

Director Sinikka Eskelinen

Professor Jari Juga


Publications Editor Kirsti Nurkkala

ISBN 978-952-62-0103-0 (Paperback)ISBN 978-952-62-0104-7 (PDF)ISSN 0355-3191 (Print)ISSN 1796-220X (Online)





OULU 2013

A 607

Matti Kuokkanen

DEVELOPMENT OF AN ECO- AND MATERIAL-EFFICIENT PELLET PRODUCTION CHAIN—A CHEMICAL STUDY

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF SCIENCE,DEPARTMENT OF CHEMISTRY

A 607

ACTA

Matti K

uokkanen

OULU 2013 ACTAjultika.oulu.fi/files/isbn9789526201047.pdf · TAMPERE 2013 Opponent Professor Lauri...

Documents

Transcript of OULU 2013 ACTAjultika.oulu.fi/files/isbn9789526201047.pdf · TAMPERE 2013 Opponent Professor Lauri...