2014 rug01-alkali metal ac

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Faculty of Bioscience Engineering

Academic year 20132014

The role of alkali and earth alkaline metals as

intrinsic catalysts in the fast pyrolysis of biomass

constituents

Jens Huyghe

Promotors: Prof. dr. ir. Frederik Ronsse

Prof. dr. ir. Wolter Prins

Tutor: Ir. Jop Vercruysse

Masters dissertation submitted in partial fulfillment of the requirements

for the degree of Master in Bioscience Engineering: Chemistry and

Bioprocess Technology


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De auteur en de promotoren geven de toelating deze scriptie voor consultatie beschikbaar testellen en delen ervan te kopiren voor persoonlijk gebruik.

Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met

betrekking tot de verplichting uitdrukkelijk de bron te vermelden bij het aanhalen van

resultaten uit deze scriptie.

The author and the promotors give the permission to use this thesis for consultation and to

copy parts of it for personal use.

Every other use is subject to the copyright laws, more specifically the source must be

extensively specified when using the results from this thesis.

Gent, 6 juni 2014,

De promotoren, De auteur,

Prof. dr. ir. Frederik Ronsse Prof. dr. ir. Wolter Prins Jens Huyghe


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SAMENVATTINGBio-olie als product van snelle pyrolyse biedt een groot potentieel als bron voor de productie

van chemicalin of zelfs als vervanger voor fossiele brandstoffen. Het voorbije decennium is

veel onderzoek gevoerd naar de mechanismen en de kinetiek van snelle pyrolyse van

biomassa en de vele mogelijkheden die de uiteindelijke producten kunnen leveren. In dezemasterproef werd het effect van natuurlijk voorkomende anorganische elementen onderzocht

op de snelle pyrolyse van de twee hoofdbestanddelen van biomassa, namelijk lignine en

cellulose. Zouten van kalium en calcium werden gebruikt als vertegenwoordigers van

respectievelijk de alkali-en aardalkalimetalen, terwijl een HZSM-achtige zeoliet aangewend

werd om het katalytisch effect van de intrinsieke anorganische componenten te kunnen

vergelijken met dit van een extern toegevoegde katalysator. Voor dit onderzoek werd de

techniek van micropyrolyse, gekoppeld met gaschromatografie en massaspectrometrie,

toegepast. Dit is een zeer nuttige manier omdat via deze techniek een onderscheid kan

gemaakt worden tussen primaire en secundaire afbraakreacties. Om de primaire reacties te

onderzoeken werden de katalysatoren gemengd met afwisselend lignine en cellulose voor ze

samen gepyrolyseerd werden (in-bed katalyse). Anderzijds werden de secundaire reacties

bestudeerd door de katalysator in een reactor na de pyrolysereactor te plaatsen, waar het de

dampen katalyseert (ex-bed katalyse). Eerst werden cellulose en lignine gepyrolyseerd bij

verschillende temperaturen om een onderscheid te maken in de degradatiemechanismen en

de optimale pyrolysetemperatuur vast te stellen. Een temperatuur van 500C werd gekozen

om de verdere experimenten uit te voeren aangezien bij deze temperatuur de opbrengst aan

bio-olie maximaal is. Vervolgens werden katalytische experimenten uitgevoerd met de twee

hoofdbestanddelen van biomassa gempregneerd met acetaat, citraat en chloride zouten van

kalium en calcium. Algemeen wordt aangenomen dat alkali-en aardalkalimetalen de primaire

afbraak van cellulose benvloeden. Zij worden beschouwd als inhibitoren van het breken van

de glycosidebindingen tussen de glucose units, wat leidt tot fragmentatie en dus een hogere

opbrengst van laagmoleculaire componenten. Dit effect werd bevestigd door onze resultaten.

Ook de daling in geproduceerd levoglucosaan bevestigt deze theorie. Een ander interessant

resultaat is het vermogen van calcium om de opbrengst van waardevolle chemicalin zoals

furfural en 5-hydroxymethylfurfural te verhogen. Het katalytisch effect op lignine was minder

intens. Er werd een kleine toename van de niet-gemethoxyleerde fenolische componenten

en aromaten waargenomen, wat wijst op een katalytisch effect op de demethoxylering.

Vervolgens werd ook ontdekt dat dezelfde katalytische invloeden zoals beschreven voor in-

bed katalyse werden teruggevonden in de resultaten van de ex-bed experimenten. Dit geeft

aan dat alkali- en aardalkalimetalen levoglucosaan en andere anhydrosuikers kunnen

destabilizeren en fragmenteren in componenten met een laag moleculair gewicht. Het effectvan de HZSM-5 katalysator was volledig verschillend dan de effecten geobserveerd bij de

experimenten met alkali- en aardalkalimetalen. HZSM-5 heeft een grotere invloed op de

secundaire reacties en zal vooral cyclisatiereacties leidend tot aromaten promoten.


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ABSTRACTFast pyrolysis bio-oils have huge potential as a source for the production of chemicals or

even as a replacement fossil fuels. Last decade lots of scientists studied the mechanisms

and kinetics of biomass fast pyrolysis and the possibilities the final products offer. In this

masterthesis the effect of naturally occurring inorganics on the fast pyrolysis of the two mainbiomass constituents, lignin and cellulose, is investigated. Salts of potassium and calcium

were used as representatives for respectively the alkali and earth alkaline metals, while a

HZSM-like zeolite was used to compare the catalytic effect of intrinsic inorganics with those

of extrinsic added catalysts. Therefore micropyrolysis coupled to gas chromatography and

mass spectrometry is applied as a useful approach because a distinction can be made

between primary and secondary thermal degradation reactions. Therefore the catalyst is

respectively mixed with the biomass compound (in-bed catalysis) and placed in a reactor

downstream the pyrolysis reactor where it catalyzes pyrolysis vapors (ex-bed catalysis).

First cellulose and lignin were pyrolyzed at different temperatures to differentiate thermal

degradation mechanisms and to define a proper pyrolysis temperature for the catalytic

experiments. A temperature of approximately 500C was chosen to execute the experiments

since it is the optimal temperature for maximizing the bio-oil yield. Next catalytic pyrolysis

experiments were conducted with cellulose and lignin impregnated with the acetate, citrate

and chloride salts of potassium and calcium. It is generally accepted that alkali and alkaline

earth metals have an influence on the primary decomposition of cellulose. They are

considered as inhibitors for glycosidic bond breakage, leading to a higher yield of low

molecular weight oxygenates. This effect was confirmed by our results, together with a

significant decrease in levoglucosan production. An interesting result is the potential of

calcium to increase the yield of furfural and 5-hydroxymethylfurfural, both valuable platform

chemicals. The catalytic effect on lignin was less intense, but a small increase in non-

methoxylated phenolic species and aromatics was observed, which indicates a catalytic

effect on the demethoxylation reaction. Furthermore it was interesting to discover that the

same catalytic influences as described for in-bed catalysis were also found back in ex-bed

catalysis. This indicates that alkali and earth alkaline metals are able to destabilize and

fragment levoglucosan and other anhydrosugars into low molecular weight compounds. The

effect of the HZSM-5 catalyst was totally different than this observed by the salts. The effect

of in-bed and ex-bed catalysis with HZSM-5 was similar with respect to the pyrolysis vapor

constituents formed, which proves that HZSM-5 merely acts on the secondary decomposition

of the already formed primary vapour constituents.


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TABLE OF CONTENTS

Preface .................................................................................................................................. V

Samenvatting ...................................................................................................................... VII

Abstract ................................................................................................................................ IX

List of Figures ..................................................................................................................... XIII

List of Tables ....................................................................................................................... XV

Introduction ........................................................................................................................... 1

Literature research ................................................................................................................ 3

1. First and second generation biofuels .......................................................................... 3

2. What is biomass ......................................................................................................... 4

2.1 Cellulose .................................................................................................................. 5

2.2 Lignin ....................................................................................................................... 6

2.3 Hemicellulose ........................................................................................................... 6

2.4 Extractives ................................................................................................................ 7

2.5 Ash ........................................................................................................................... 7

3. Conversion of biomass ............................................................................................... 8

3.1 Thermochemical conversion ..................................................................................... 8

4. Bio-oil ........................................................................................................................12

4.1 Applications .............................................................................................................14

5. Decomposition mechanisms of biomass constituents in fast pyrolysis .......................15

5.1 Cellulose .................................................................................................................15

5.2 Lignin ......................................................................................................................17

5.3 Hemicellulose ..........................................................................................................18

6. Catalytic upgrading ....................................................................................................19

6.1 Influence of alkali and alkaline earth metals ............................................................20

Objective ..............................................................................................................................23Materials and methods .........................................................................................................25

1. Sample preparation ...................................................................................................25

2. Micropyrolysis experiments (pyGC-MS) .....................................................................25

3. Catalytic pyrolysis experiments (pyGC-MS) ...............................................................26

4. GC-MS data processing .............................................................................................28

Results and discussion .........................................................................................................29

1. Temperature effect ....................................................................................................29

1.1 Cellulose .................................................................................................................29


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1.2 Lignin ......................................................................................................................32

2. Alkali and earth alkaline metal catalysis .....................................................................37

2.1 In-bed catalysis .......................................................................................................37

2.2 Ex-bed catalysis ......................................................................................................44

3. Zeolite catalysis .........................................................................................................49

3.1 Cellulose .................................................................................................................49

3.2 Lignin ......................................................................................................................52

Conclusions ..........................................................................................................................55

Recommendations for further research ................................................................................57

References ...........................................................................................................................59

Appendix ..............................................................................................................................65

1. List of pyrolysis products ............................................................................................65


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LIST OF FIGURESFigure 1:Technology pathways to transform biomass into biofuel. 4

Figure 2:Cellulose structure 5

Figure 3:Non-, mono- and dimethoxylated monoligols in lignin 6

Figure 4:Main constituents hemicellulose. 7Figure 5:Influence of temperature on product distribution.. 11

Figure 6:Biomass mass loss in function of temperature . 15

Figure 7:Proposed models for the degradation of cellulose 16

Figure 8:Mechanism for levoglucosan formation proposed by ponder et al. 17

Figure 9:Reactions associated with the catalytic upgrading of bio-oils.. 19

Figure 10:Cellulose degradation mechanism in presence of inorganics 21

Figure 11:Micropyrolysis setup at the department of bioscience engineering. 25

Figure 12:Reaction tube packed with catalyst. 27

Figure 13:Overview of the frontier micropyrolyzer. 27

Figure 14:TIC chromatogram of pure cellulose (Avicell) micropyrolysis of approx. 0.5 mg at

(upper) 300C, (middle) 400C and (bottom) 500C.. 29

Figure 15:Relative amount of oxygenated compounds, furans/pyrans and anhydrosugars in

function of pyrolysis temperature in pygcms 31

Figure 16:relative amount produced of the 6 most important cellulose pyrolysis degradation

products. Levoglucosan (LG) and 1,6-Anhydro--d-galactofuranose (agf) are examined on

the left axis, Hydroxymethylfurfural (hmf), levoglucosenone (lgo), furfural (FF) and

hydroxyacetaldehyde (ha) on the right axis. 32

Figure 17:TIC chromatogram of lignin micropyrolysis of approx. 0.5 mg at (upper) 300C,

(middle) 400C and (bottom) 500C.. 33

Figure 18:Relative amount of low molecular weight compounds, phenolas and aromatics in

function of temperature in pygcms. 34

Figure 19:Total peak area per mg sample calculated from the gas chromatographic analysis

of lignin and cellulose pyrolysis products in pygcms.. 35

Figure 20:Amount of non, mono and dimethoxylated phenols produced during lignin

pyrolysis in function of temperature in pygcms 36

Figure 21:Amount produced of twelve most important products of lignin pyrolysis in function

of temperature in pygcms 37

Figure 22:Classification of pyrolysis products of cellulose samples impregnated with various

concentrations of potassium citrate in pygcms at 500c. 38

Figure 23:Classification of pyrolysis products of cellulose samples impregnated with various

concentrations of Calcium citrate in pygcms at 500C 39Figure 24:Effect of different salts on five main cellulose pyrolysis products. 42

Figure 25:Relative amount of low molecular weight compounds, phenols and aromatics

during 500C lignin pyrolyis in function of added cacl2 concentration.. 43

Figure 26: Amount of non, mono and dimethoxylated phenols produced during 500C lignin

pyrolysis in function of the added metal concentration 43

Figure 27:Effect of different salts on main lignin 500C pyrolysis products 44

Figure 28:Classification of pyrolysis products of cellulose at 500C after vapour phase

catalysis with kcl and cacl2 in pygcms . 46

Figure 29:TIC total peak area per mg sample of the pyrolysis products of cellulose in in- en

ex-bed catalyzed mode 47


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Figure 30:tic total peak area per mg sample of the pyrolysis products of both cellulose and

lignin in non-catalyzed and in in- and ex-bed catalyzed mode 48

Figure 31:effect of different levels of hzsm-5 on the main cellulose 500C pyrolysis products

in pygcms. levoglucosan (lg) is examined on the left axis; toluene, furfural (ff), 2,3-anhydro-d-

galactosan and hydroxyacetaldehyde (haa) on the right axis..... 50

Figure 32:Absolute amount of cellulose pyrolysis products detected by the gc/ms in terms oftotal peak area 50

Figure 33:Absolute amount of lignin pyrolysis products detected by the gc/ms in terms of

total peak area 52


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LIST OF TABLES

Table 1: Elemental composition for different kinds of biomass 8

Table 2: Differences between conventional, fast and flash pyrolysis. 9

Table 3: Physical properties of bio-oil and heavy fuel oil. 12

Table 4: Characteristics of bio-oil.. 14

Table 5: Gas chromatographic analysis of pyrolysis products from cellulose at 500C. The

products determined at 300C are indicated with a star, those with one or more stars are

found at 400C.. 30

Table 6: Gas chromatographic analysis of pyrolysis products from lignin at 500C. the

products indicated with at least one star are also found in the distribution at 400C, the

products with only one star are those determined at 300C.. 33

Table 7: Relative amount (TIC relative abundance) of the main cellulose 500c pyrolysis

products after vapor phase upgrading .. 46

Table 8: Relative amount of the main lignin pyrolysis products after vapor phaseupgrading 48

Table 9: Gas chromatographic analysis of 500C pyrolysis products from lignin catalyzed

with hszm-5 in ex-bed mode 51


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INTRODUCTIONThe use of biomass as a source of renewable energy has become very important because of

the large opportunities it offers to solve environmental problems. Together with the living

standards the world population is increasing, which forces us to find alternatives to satisfy the

growing energy demand without contributing to the greenhouse effect originating from theuse of fossil fuels. The high and fluctuating prices of oil and the desire of countries to be

independent in their energy supply are economic reasons to justify the use of renewable

resources. The declining reserves of fossil fuels and the abundant availability of biomass

worldwide, coupled with the concerns about nuclear energy, are additional driving forces to

investigate the opportunities of biomass-derived energy. The most interesting advantage of

the use of biomass is the fact that there is no net contribution to the atmospheric CO2, one of

the most important gases causing global warming. Since the growing concerns about the

well-known greenhouse effect, scientists are attracted to study the possibilities of bio-energy

and in this way reducing the emissions of CO2.1 The European Union has decided that by

2020 a minimum of 20% of the consumed energy must be derived from renewable resourcesand furthermore they have set an obligatory target of 10% for biofuels2. These targets are

ambitious if you know that in 2010 only 12.4% of the energy was renewable. Belgium didnt

even reach 5%. Biomass is with 67% by far the most important source of renewable energy3.

Its particularly interesting because at this moment its the only renewable resource of liquid

transportation fuel. Fast pyrolysis is receiving attention as one of the most promising

processes to obtain bio-oil. During this process, the biomass constituents are thermally

cracked into numerous smaller components, which can be classified according to their

physical appearance at room temperature: liquid bio-oil, solid char and non-condensable

gases. Although a lot of research has been done during the last decades to improve the

quality of the bio-oil, still some problems exist in terms of viscosity, acidity, aging etc. Thatswhy knowledge about its chemical composition and consequently about possible ways to

improve bio-oil quality is of utmost importance. Biomass contains some natural catalysts

which can affect the pyrolysis process. Previous work has demonstrated the catalytic effect

of inorganic compounds and alkali metals and alkaline earth metals in particular. They have

a significant influence on the yield and composition of pyrolysis liquids as they catalyze

specific pyrolysis reactions. Hence the main purpose of this master thesis is the investigation

of the catalytic effect of alkali and alkaline earth metals on the primary and secondary

pyrolysis reactions of the major biomass constituents cellulose and lignin.


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LITERATURE RESEARCH

1. FIRST AND SECOND GENERATION BIOFUELS

The use of fossil fuels and the subsequent CO2 production can be decreased by applying

biofuels from renewable resources since the CO2 released by burning the biofuel is the sameamount that has been stored by the plant using photosynthesis. A distinction can be made

between biofuels of the first and the second generation. First generation biofuels are made

from sugar, stark or vegetable oils and are already commercially produced: over 100 billion

liters of the three main types (biodiesel, ethanol and biogas) are annually produced from

crops as sugarcane, corn, soy and palm4. But there are growing concerns about these

technologies. Because of the growing number of people in the world dealing with food

scarcity, it seems unethical to use food crops as energy crops or to even use productive land

for growing energy crops. Skeptics also mention the impact on the biodiversity and the cost

inefficiency of e.g. biodiesel.

A better solution can be offered by biofuels produced with lignocellulosic biomass, the so

called biofuels of the second generation. These lignocellulosic materials (e.g. switch grass,

miscanthus, poplar and agricultural wastes) have generated a lot of attention as they are

cheap and abundant and the plants dont compete with food crops. However they still might

compete in land use if production increases. Although its a very promising path to

investigate, at present it isnt cost effective due to some technical obstructions . More

investigation and development is required to identify the ideal characteristics of the feedstock

and especially to improve the pre-treatment methods for opening up the matrix of cellulose,

hemicellulose and lignin56.

As can be seen in Figure 1, a lot of different pathways originating from various types ofbiomass to a wide range of biofuels exist. One of the most promising and nowadays most

studied processes is pyrolysis. Pyrolysis is a thermochemical decomposition process of

organic material at elevated temperatures without the addition of oxygen7. A substantial

variety of biomass can be used to produce bio-oil through pyrolysis. The most important

feedstock in North-America and Europe are forest residues as bark, sawdust and shavings.

In the rest of the world agricultural residues and sugar cane bagasse are feedstocks of

choice. Nevertheless still plenty of lignocellulosic biomass sources have the potential to be

pyrolyzed, including bagasse, rice hulls, rice straw, peanut hulls, switchgrass, wheat straw

and wood8.


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FIGURE 5: TECHNOLOGY PATHWAYS TO TRANSFORM BIOMASS INTO BIOFUEL9

Lately the capability of algae as a feedstock for bio-fuel production has also been

investigated. This expansion of research area is logical since 70% of the earth surface is

covered by water. The results are hopeful since algae species that produce valuable

carbohydrates instead of lipids as their reserves were discovered9. If the crop harvest and

treatment is upgraded, we can refer to the algae fuel production as the third generation of

biofuels.

2. WHAT IS BIOMASS

The renewable energy directive (2009/28/EC) of the European parliament gives us thefollowing definition of biomass: "Biomass means the biodegradable fraction of products,

wastes and residues from biological origin from agriculture (including vegetable and animal

substances), forestry and related industries including fisheries and aquaculture, as well as

the biodegradable fraction of industrial and municipal waste"10. Biomass contains a lot of

energy, stored as chemical bonds which are formed through photosynthesis. In this process,

water and carbon dioxide from the air are converted into energy-rich carbohydrates using

solar energy.

Lignocellulosic biomass is essentially composed of cellulose, hemicelluloses, lignin,

extractives and minerals. The exact composition depends on various characteristics


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including the plant species, type of plant tissue, growth stage and growing conditions.

Usually, in woody biomass, a distinction is made between hardwood and softwood because

the properties of both groups are different, but even within these groups a large diversity is

observed11. These terms are quite misleading because hardwoods are not necessarily harder

or stronger, the terms just refer to the type of tree. The wood from conifers (gymnosperms),

trees with needle-like leaves, is usually called softwood while hardwoods comprise thebroad-leaved trees (angiosperms). These last mentioned are at most deciduous, but in more

tropical climates evergreen broad-leaved trees appear12.

2.1CELLULOSED-glucose polysaccharide is the most important structural component in lignocellulosic

biomass and contributes to approximately 40-45% of the plants dry weight. As a cell wall

component, the main function is to protect, stabilize and give structure to the plant. The

glucopyranose units are linked by -1,4-glycosydic bonds. Two adjacent glucose units are

rotated 180 to eachother and are defined as the disaccharide cellobiose 13. The degree of

polymerization DP is used to express the chain length, which can have a value from 800 to

10 000 according to the plant species. Every glucose subunit contains three hydroxyl groups

with a high reactivity when positioned in the equatorial plane. This is the basis for intra -and

extracellular hydrogen bonds, which promote the formation of a partially crystalline structure

and which give cellulose a multitude of different fiber structures and morphologies14. The

parallel aligned cellulose polymers form microfibrils, these in turn assemble as fibrils and

finally cellulose fibres are built from the fibrils. The non-uniformity of the cellulose fiber, with

amorphous regions on the one hand and regions of very high crystallinity on the other hand,

is the reason for the interesting opposing properties of cellulose such as stiffness and rigidity,

but also flexibility. The strong hydrogen bonds strengthen the structure and makes the

cellulose insoluble in most solvents 1.

FIGURE 6: CELLULOSE STRUCTURE15


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2.2LIGNINLignin is the second most abundant organic polymer in plants. Only vascular plants are

lignified but the amount of lignin present can differ a lot in plants. Woody species contain 20

to 40 wt.% lignin, while the content in aquatic and herbaceous species is less. The mainfunction of lignin is to give strength to the cell wall of vascular tissues, specialized for liquid

transport and mechanical strength. They also connect the cellulose fibers and hemicellulose.

The structural integrity of the cell wall is increased to such an extent that the plants get the

opportunity to grow upright15. Lignin is very heterogenic, has an extremely complex three

dimensional structure and consists of random connected substructures of monolignol

monomers which differ in their degree of methoxylation, namely p-coumaryl, coniferyl and

sinapyl alcohol. These monolignols are synthesized from phenylalanine in a multistep

proces16. In general, softwoods consist of guaiacyl lignin (with mainly coniferyl alcohol units)

and hardwoods are composed of guaiacyl-syringyl lignin (with both coniferyl and synapyl

units). In both cases, p-hydroxyphenyl (from p-coumaryl alcohol) appears in smallquantities17.In guaiacyl lignin of softwood, about 20-25% of the guaiacyl units are linked with

a stable 5-5 carbon linkage, while in hardwood this percentage doesnt exceed 10%. This is

because of the additional methoxygroup at carbon 5 of syringyl monomers in hardwoord. But

the most widespread links are -O-4 ether bonds. At last it can also bind with other

components like hemicelluloses, glycoproteins or tannins18.As a result of the random linkage

the polymers are retained to form crystalline structures. They have a high atomic mass, but

its very difficult to determine the degree of polymerization due to the irregularity of the

structure.

FIGURE 7: NON-, MONO- AND DIMETHOXYLATED MONOLIGOLS IN LIGNIN

2.3HEMICELLULOSEIn contrast with the homopolymer cellulose, which is strong and partially crystalline,

hemicellulose is a branched heteropolysaccharide with little strength and amorphous

structure. Also the degree of polymerization is up to 100 times lower. Moreover its qualified

as an alkali soluble material, where cellulose is insoluble2. Due to the short chain length, in

addition with the high rate of hydrogen bonds, they are capable of absorbing huge quantities


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of water. Hemicellulose exists of a highly substituted chain of sugar monomers, as there are

pentoses (-d-xylose, -l-arabinose), hexoses (-d-mannose, -d-glucose, -d-galactose)

and uronic acids (-d-glucuronic, -d-4-O-methylgalacturonic and -d-galacturonic acids).

Occasionaly other sugars such as -l-rhamnose and -l-fucose can be found in small

quantities19. Xylans and glucomannans are the most abundant hemicelluloses and theyre

typically separated into 6 main classes: glucoronoxylans, galactoglucomannans,arabinoglucuronoxylans, xyloglucans and arabinoxylans and complex heteroxylans19.

FIGURE 8: MAIN CONSTITUENTS HEMICELLULOSE

2.4EXTRACTIVESExtractives are the organic compounds besides the polymeric parts of the cell wall and can

be extracted by hot water or organic solvents. Primary metabolites like sugars, fats, amino

acids and carboxylic acids are always present, but also more complex secondary metabolites

such as phytosterols, terpenes and phenolic compounds can be found. The functionality is

diverse, for example fats are an energy source for the cells, while terpenoids and phenolic

compounds are protecting against microbiological invasion. Other components are important

for odor and color. Especially bark contains a lot of these extractives20.

2.5ASHThe alkali and alkaline earth metals are the most significant metals present in the ash, with

calcium being the most abundant, followed by potassium and magnesium. They are

incorporated in salts as carbonates, sulfates, silicates, oxalates and phosphates. In most

plants, these metals only make up approximately 1% of the dry wood weight, but they can

have a significant effect on the product distribution of biomass pyrolysis. Thats why its

important to take into consideration the exact amount of each metal present. The elemental

compositions of some biomass materials are shown in Table 1. Only trace amounts of heavy

metals are found20. Its important to realize that the ash content in fast growing energy crops

or in plants growing on contaminated soils can be significantly higher.


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TABLE 2: ELEMENTAL COMPOSITION FOR DIFFERENT KINDS OF BIOMASS22

3. CONVERSION OF BIOMASS

Energy can be produced from biomass by chemical, biological or thermal treatment and the

choice of an appropriate method depends on the biomass feedstock, the desired end-

product, economic conditions etc. Power and heat generation, transportation fuels and

chemicals are three main products which can be derived from biomass. The thermochemicalconversion processes, which are becoming very important nowadays, apply heat (and

eventually catalysts and pressure) to convert a broad range of biomass into these products.

The four most important methods (combustion, gasification, liquefaction and pyrolysis) will be

described further, pyrolysis in particular. Fermentation for the production of ethanol and

anaerobic digestion for the production of biogas (mainly containing methane and carbon

dioxide) are the best known biological conversion methods. A well-known chemical treatment

on the other hand is the trans-esterification of several specific kinds of biomass (e.g.

rapeseed oil) to produce bio-diesel21.

3.1THERMOCHEMICAL CONVERSION

3.1.1COMBUSTION

The most straightforward thermal conversion process is to completely oxidize the biomass by

combustion, providing heat which can be applied in heating systems or in steam production

for electricity generation. There are a couple of restrictions for the biomass when used as a

feedstock for combustion. First of all the water content has to be lower than 60 %. In addition

constituents apart from C, H and O should be avoided since they are associated with ash,

deposits and corrosion. Combustion of herbaceous biomass is not favorable because it

contains usually higher amounts of N, S, K and Cl, promoting higher emissions of NO xand

polluting particulates. Wood is better suited to the requirements for combustion. The process

itself involves drying, devolatilization, gasification, char combustion and gas-phase


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oxidation22.High air pollution and low energy yield have forced to shift to other degradation

processes.

3.1.2GASIFICATION

Gasification is just as combustion an exothermal process that partially oxidizes the solid fuel

(biomass) at temperatures in a range from 800 to even 1700C into a gaseous fuel namedproducer gas. The main constituents of this gas are CO and H 2, together with smaller

amounts of CO2, CH4 and N2. For an engine driven by a fuel gas, gasification is the

suggested conversion method. The obtained gas can also be used as a fuel gas for

combustion to provide heat, but the most efficient concept is the biomass integrated

gasification/combined cycle heat and power generation, where the produced gas is

converted in a turbine to obtain electricity. Further treatment of the gas by Fischer-Tropsch

process is a more recent application where methanol or other liquid hydrocarbons are

synthesized 6.

3.1.3PYROLYSISPyrolysis is derived from the Greek words pyro (fire) and lysis (decomposition). Its a

thermal decomposition process in the absence of oxygen that converts biomass into solid,

liquid and gaseous products. The process has been used for ages to produce charcoal and

in ancient Egypt it has been used to acquire tar for caulking boats.23 Earlier the preferred

product was solid char and the term pyrolysis referred to carbonization.8But the capability of

pyrolysis to produce liquids from biomass has attracted more and more attention in recent

years because these bio-oils could be profitable thanks to their advantages in storage,

transport and wide range of applications24. At the end of the 20th century scientists

discovered that through fast pyrolysis the yield of liquid products can be increased, simply by

adjusting some process parameters in the conventional slow pyrolysis process 25. Fast

pyrolysis, as a novel technology, was born, where a liquid yield as high as 72% can be

reached when the feedstock is heated rapidly and also the produced vapors are condensed

as fast as possible. The terms slow and fast are quite arbitrary as there are no precise

definitions of involved heating rates or times8. The process conditions of slow pyrolysis are

less extreme than those applied in the fast pyrolysis process, where liquid yield is

maximized. Flash pyrolysis is a type of fast pyrolysis where even higher temperatures are

used with smaller residence times. As a result the main products formed are gaseous,

therefore this method approaches gasification. A short overview of the differences is given in

Table 2 26.

TABLE 2: DIFFERENCES BETWEEN CONVENTIONAL, FAST AND FLASH PYROLYSIS28


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3.1.3.1FEED PREPARATION

Two important treatment operations of the feed have to be done to accomplish an effective

heat transfer and a smooth performance of the pyrolysis reactor. Both the biomass moisture

content and particle size should be reduced. In a fluidized bed pyrolysis reactor for example,

particles in a range of 2-6 mm are required to achieve a proper heat transfer and

consequently, to achieve sufficiently high reaction rates. Prior to grinding, the biomassshould be dried to a moisture content below 15 wt.%, as water has an adverse effect on the

bio-oil properties such as pH, viscosity, stability and corrosiveness27.It has been proven that

a drying temperature exceeding 200C decreases the bio-oil yield. The drying time on the

other hand has no impact on the liquid product quality28.

3.1.3.2PYROLYSIS PROCESS

Primary pyrolysis reactions start when the used feedstock is heated and volatiles are

released from the particles while char is formed. A chain reaction is started by the hot

volatiles which heat up the remained unpyrolyzed biomass and the volatilization is

maintained. In competition with the volatilizing pyrolysis reactions, some of the volatiles

condense in contact with colder parts of the biomass and produce tar. Depending on

temperature, pressure and residence time, other reactions such as thermal decomposition,

dehydrations, radical recombinations and water gas shift reactions are possible8.

Short residence time and a controlled temperature of 500C are necessary to maximize the

oil yield and reduce the further degradation of the vapors. This formation of low molecular

weight compounds and gases is called secondary cracking and takes place at temperatures

exceeding 500C. Fast condensation is required to eliminate these reactions. Condensation

can be realized by rapid cooling while aerosols can coagulate and agglomerate. Using a coldtrap is the easiest solution, but this method can cause liquid fractioning and even blockage of

piping in the condenser system because of the preferential deposition of highly viscous,

lignin derived components. Quenching of the vapors in product oil or in an immiscible

hydrocarbon is the generally accepted method. Electrostatic precipitation is widely applied at

smaller scales (up to pilot plant)29. Nevertheless eliminating all secondary decomposition

reactions is impossible because char particles contaminating the vapors need to be excluded

prior to condensation in a cyclone or another hot vapor filtration device, prolonging the

residence time.

At temperatures below 400C, decomposition reactions on the char take place. This

phenomenon can appear on the surface of the particles when the vapor residence time is too

long, but particularly takes place inside the larger particles. During pyrolysis, the particles are

vaporized more and more inwards, leaving behind char layers at the outside. The produced

vapors diffuse slower out of the particle and are subjected to condensation and

decomposition as the vapors are diffusing through the outer char layers30. Char also

contributes to a higher concentration of polycyclic aromatic hydrocarbons (PAHs), which are

atmospheric pollutants27. So for an optimal bio-oil yield, an efficient char removal is required.

The conventional method of char removal is by cyclones, although the smallest particles

cant be held back and when they collect in the pyrolysis liquid they have a negative

influence on the oil stability resulting in the problem of aging. Alternative approaches are

being developed, such as in-bed vapor filtration and rotary particle separation, but they tend


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to create filter clogging problems due to the complex interactions between char and pyrolysis

liquid and the subsequent formation of a gel-like phase29. Another issue is the liquid yield

reduction by 10 to 20% because of the catalytic cracking by the char accumulation on the

filter surface24.

3.1.3.3PYROLYSIS PRODUCTSThe yield of the products can be influenced by changing the temperature, since different

reactions occur at different temperatures. Figure 5 shows an overview of the yield of each

product fraction as a function of temperature. The moisture content is an additional important

factor in the product distribution as it accounts for water condensation in the liquid fraction,

hereby extracting water-soluble compounds from the gaseous phase and in this way

decreasing gas fractions31.

At higher temperatures the large molecules in char and tar are cracked to smaller, gaseous

molecules. Consequently, the yield of the char can be maximized at a low temperature and

low heating rate. The majority of the solid char left after biomass pyrolysis results from lignin

and hemicellulose degradation. If pyrolysis is complete, char exists nearly completely of

carbon, since the hydrogen and oxygen fractions have been removed from the biomass

feedstock. Other heteroatoms as N, S and P, which affect the properties of bio-char, can be

found as incorporations in aromatic rings, their degree of incorporation depending on the

pyrolysis conditions and feedstock type32. Also the biomass mineral fraction is retained in the

char.

FIGURE 5: INFLUENCE OF TEMPERATURE ON PRODUCT DISTRIBUTION29

Liquid products are gaseous when collected at pyrolysis temperature but are condensed at

room temperature to a dark brown viscous liquid. Liquid products from biomass pyrolysis are

discussed in more detail in the next paragraph.


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The main constituents of the produced gas, after removal of the condensable pyrolysis

vapors, are H2, CO, CO2, H2O, N2 and light hydrocarbons as CH4, C2H4, C2H6 etc33. H2

production originates from the further cracking of hydrocarbons at higher temperatures, while

CO and CO2 originate from the decarbonylation and decarboxylation of oxygenated

compounds. Extreme conditions such as temperatures exceeding 1200C ensure complete

cracking of the volatiles, only leaving H2and CO. During pyrolysis these temperatures arenever reached and therefore the gas contains more components, as mentioned before. The

pyrolysis-gas is called producer gas and can be cleaned to syngas, an equimolar mixture of

H2 and CO. Pure syngas can be used for power generation, for the production of

transportation fuels including methanol and FT-diesel (through Fischer-Tropsch synthesis)

and for synthesis of chemicals such as ammonia (for fertilizers) and hydrogen (used in

refineries). The main application of producer gas on the other hand is found in power and

heat generation by combustion. It isnt recommended for more advanced applications

because only 50% of the energetic value of the producer gas is derived from the syngas

components CO and H2. Only for the production of synthetic natural gas (SNG) the use of

production gas is desirable because in this case the presence of CH4 is beneficial forachieving a high yield 34.

4. BIO-OIL

Bio-oil is the liquid fraction obtained after condensation of the pyrolysis vapor phase. This

dark brown organic liquid is often referred to by many other names, like bio-crude-oil (BCO),

bio-fuel, liquid wood, liquid smoke, wood distillates, pyrolysis oil, pyrolytic oil etc. It is a

complex mixture that contains highly oxygenated products but the elemental composition

resembles that of the biomass it has been derived from29

7

. In Table 3 the major differencesbetween pyrolysis bio-oil and conventional heavy fuel oil are given.

TABLE 3: PHYSICAL PROPERTIES OF BIO-OIL AND HEAVY FUEL OIL7

More than 300 components can be identified in bio-oil and most of them possess oxygen,

leading to an oxygen content of approximately 35-40 wt.%. The high oxygen level is the key

reason for the differences in properties between bio-oil and conventional hydrocarbon fuels,

but the percentage can be adjusted by changing the pyrolysis conditions. Severe conditions

(high temperature, long residence time and high heating rate) favor the formation of


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secondary reactions and hence decrease the bio-oil yield, but the remaining organic liquid

contains less oxygen7. Reducing the oxygen content is an important purpose, knowing that it

causes the bio-oil to have a heating value which approaches only more or less 50% of the

heating value of conventional fuel oils. The many polar groups present also give rise to poor

chemical stability of bio-oil and high boiling points and viscosities30.

Water in bio-oil originates either from the moisture present in the initial feedstock, or it is

produced by dehydration reactions during the pyrolysis process. So the water content in the

bio-oil varies from 15 to 30 wt.%, depending on the biomass used and pyrolysis severity.

Thanks to the numerous polar compounds derived from carbohydrate decomposition, water

is in this concentration miscible with the oligomers resulting from lignin conversion. On the

one hand water is desirable because it reduces the viscosity (advantageous for combustion)

and it leads to lower NOx production and a more uniform temperature profile in diesel

engines. On the other hand with increasing water content the heating value and the

combustion rate reduces significantly3524.

Because of the wide range of feedstock and process conditions, the viscosity can vary from

25 cP up to even 1000 cP at 40C. But with higher temperatures the viscosity reduces

rapidly, unlike conventional fuels of which the viscosity decreases a lot slower under the

same conditions. Not only by water but also by mixing with other polar solvents like methanol

and acetone the viscosity can be reduced drastically7. Finally the increase of viscosity with

time is referred to as aging. This phenomenon is caused by formation of larger molecules by

condensation reactions among the bio-oil components36.

Another disadvantage of bio-oil is the high level of corrosiveness due to organic acids,

particularly formic acid and acetic acid, causing a very low pH of 2-3. As a result, the bio-oil

can affect construction materials as aluminum and carbon steel. At high temperatures andwith high water contents the oil is even more corrosive37.

At last, the combustion behavior of bio-oils is very unique. Due to the high amount of

nonvolatile species, significant energy is needed for ignition. The combustion of an oil droplet

begins with evaporation of water after which vaporization of light compounds and liquid-

phase pyrolysis of the heavier fraction occurs. This phase is characterized by some unique

processes such as swelling, shrinking and micro-explosions. After this phase, the initial blue

flame of quiescent burning is followed by a bright yellow flame with increased size. The last

step is the solid residual char burnout, with formation of cenosphere-like particles. Petroleum

oil only shows a quiescent, sooty burning after ignition38 39. An overview of the bio-oil

characteristics is given in Table 4.


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TABLE 4: CHARACTERISTICS OF BIO-OIL29

4.1APPLICATIONSCompared to gases and solids, liquid products are far easier to transport which is of major

importance in for example combustion equipment. Poor volatility, coking, high viscosity,

corrosiveness and the variability of its composition are factors which have limited the number

of bio-oil applications. However, lots of efforts have been made on the development of

pyrolysis oil and still many challenges are left, but progression has been made and plenty oforganizations are succeeding in giving bio-oil a place on the market.

Furnaces and boilers are usually less efficient than engines and turbines for heat and power

generation, but the huge variety of fuels that can be used in the former gives them a major

advantage. So bio-oil too has received interest to be used for example as a boiler fuel in

district heating, although a better quality and price is required to make this economically

feasible. The use as combustion fuel in diesel engines on the other hand is also being

investigated but until today a lot of difficulties exist with typical bio-oil properties such as its

difficult ignition and corrosiveness7. These characteristics are also the major drawbacks in

the use of pyrolysis liquids in turbines, however, with a proper accommodation of the

turbines, electricity generation is possible.

Bio-oil doesnt seem to be interesting for transportation fuels due to high viscosity and

corrosiveness, low stability and so on, but some of these properties can be improved by

numerous upgrading processes such as mixing with conventional fuels, deoxygenation by

hydrotreating or catalytic vapor cracking etc. The most promising application in this area is

the use as a source of hydrogen for a variety of petrochemical processes such as

hydrocracking and hydrogenation.

Production of a whole range of chemicals derived from bio-oil is also possible, but separation

techniques are very complex and most of these chemicals are now already being produced


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from coal, natural gas or crude oil at a much lower cost. However some typical and unique

pyrolysis chemicals such as glycolaldehyde and levoglucosan can be more easily isolated

and are derived from bio-oil with an acceptable yield.

In summary, bio-oil shows lots of opportunities but the challenges to be overcome are the

high cost, the lack of standards, the unavailability of materials, incompatibility withconventional fuels and the lack of familiarity to the user7.

5. DECOMPOSITION MECHANISMS OF BIOMASS CONSTITUENTS IN FAST

PYROLYSIS

Pyrolysis processes are very complex because there are a lot of variables to be considered.

Biomass has a very complicated structure and a large number of products are created

through thousands of different physical and chemical transformations during the process.

Since the first kinetic model of cellulose pyrolysis developed in 1960 by Broidos group40

,many extensive studies have exposed new insights in the different pathways and reactions.

Only later the pyrolysis of the other two important biomass components, lignin and

hemicellulose, were described. Figure 6 shows an overview of the mass loss rate for the

three biopolymers41.

FIGURE 6: BIOMASS MASS LOSS IN FUNCTION OF TEMPERATURE44

5.1CELLULOSECellulose pyrolysis already starts at temperatures as low as 150C. At this low temperature a

charred residue is formed and a very low yield of liquids is achieved. Depolymerization takes

place, besides the free radical formation and water elimination. Plenty of carbonyl, carboxyl

and hydroperoxide groups are formed as well, together with a small amount of gaseous

species as carbon monoxide and carbon dioxide. Once above 300C, lots of new reactions

are introduced which contribute to the formation of a liquid product42.The maximum weight

loss rate was reached between 315 and 400C. This single step-multiple reaction type

behavior was later described by an improved kinetic model by Shafizadeh43 who proposed


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the formation of an intermediate species called activated cellulose, a reactive component

resulting from depolymerization, from which two major pathways start: further

depolymerization through the 1-4 glycosydic bond scission and fragmentation of the

glucose rings, also known as ring-scission. The most typical compounds formed during

depolymerization of cellulose are levoglucosan and other anhydrosugars, furans, anhydro-

oligosaccharides, pyrans and cyclopentanones. The glucose units mainly exist as pyranoses,however lots of furan derivatives such as 5-hydroxymethyl furfural (HMF) are formed due to

the fact that these species are more stable and also kinetically favored. On the other hand

the more endothermic ring-scission reaction provides mainly linear alcohols and carbonyls,

such as hydroxyacetaldehyde and acetol, esters and others. The first proposed models do

not take secondary reactions into account, so later on a modified Shafizadeh-Broido model

was developed, according to which the primary products were produced by three competing

reactions, after which secondary char and gases were produced from the vapors43.

FIGURE 7: PROPOSED MODELS FOR THE DEGRADATION OF CELLULOSE

Levoglucosan is by far the principal cellulose pyrolysis product. A mechanism for

levoglucosan formation was proposed by Ponder et al.44as shown in Figure 8. Heterolytic

cleavage of glycosidic bonds leads to a glycosyl cation, which subsequently transforms into

an intermediate containing a terminal 1,6-anhydopyranose. A further cleavage of the

glycosidic bond results in a levoglucosan unit accompanied by a depolymerized unit26. A

study on the fundamental degradation mechanism of cellulose has shown that levoglucosan

is stable at 500C, which allows to exclude complex levoglucosan degradation reactions

when explaining the product distribution of cellulose pyrolysis. This result also confirms the


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modified model of Shafizadeh, stating that competitive reactions rather than sequential

degradation account for the formed species.

FIGURE 8: MECHANISM FOR LEVOGLUCOSAN FORMATION PROPOSED BY PONDER ET AL.47

5.2LIGNINLignin is the thermally most stable compound of biomass due to its complicated, highly

branched three-dimensional structure. Therefore its degradation is also the least understood,

its strongly influenced by plenty of parameters as temperature, biomass source and heatingrate. The high quantities of lignin residues available worldwide however have caused major

interest in its thermal behavior and in its potential to serve as a source for valuable

chemicals.

The temperature range over which lignin decomposes is wide because of the variety of

chemical bonds linking the aromatics, all with their own thermal stability and thus other

degradation temperatures. Low molecular weight products are produced by the cleavage of

the several functional groups, while at higher temperatures complete rearrangements of the

backbone occur, producing char and a series of volatile species. Also radical formation and

initially self-condensation occur, but the latter is inhibited by phenolic species leading to a

mass distribution equilibrium. Coniferous lignin is the most stable and accordingly causes the


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highest char yield. Lignin pyrolysis starts around the relatively low temperature of 200C, but

the main degradation processes occur at higher temperatures, with the formation of aromatic

hydrocarbons, (hydroxy)phenolics and guaiacyl-and syringyl-type components. Between 275

and 350C, the lignin monomers are cleaved, while demethylation of the dimethoxygroups

occurs at 350-450C, hereby increasing the conversion rate of phenols into pyrocatechols.

No demethylation or demethoxylation occurs at lower temperatures, so the substitutes fromthe resulting phenols arent changed and the substitution pattern of the native lignin can be

analyzed. The faster the heating rate, the greater the amount of hydrocarbons and alkyl-

phenol derivatives, while lower heating rates gives components with high oxygen ratios.

Unsaturated products (e.g. styrene, vanillin and eugenol) are formed during dehydration of

lignin, together with the production of substantial amounts of water. A temperature of 230C

induces the formation of CO, followed by other condensable gases such as CH4. At 500C

the main non-condensable gas produced is H2, released by rearrangements and

condensations of the aromatic rings. The further decomposition at temperatures exceeding

600C are called secondary reactions and they comprise the decomposition of the formed

intermediates to char and gases. The decomposition reactions of the strongest bonds suchas the oxygen functional groups at even higher temperatures are referred to as tertiary

reactions, producing more H2and CO.

Lignin and cellulose have a mutual effect when theyre pyrolyzed together. Lignin favours the

formation of low molecular weight products from cellulose, leading to a decreased char yield,

while cellulose also reduces the char formation from lignin by inhibiting secondary char

formation.

5.3HEMICELLULOSEThe varying properties and high complexity of hemicellulose are the primary reasons why the

mechanisms and kinetics of its thermal degradation are largely unexplored. A huge variety of

linkages and branches are present, depending on the species, the tissue location and even

the tissue age45. Due to their interaction witch cellulose and lignin, hemicellulose is very

difficult to obtain and as a result many studies used xylan or 4-O-methyl-D-glucurono-D-xylan

as a model compound to study pyrolysis behavior. Nevertheless hemicellulose consists of a

set of different polysaccharides which may have different properties. Unlike cellulose,

hemicellulose lacks crystallinity, the amorphous structure causes the hemicellulose to be less

thermally stable42.So thermal decomposition starts at a slightly higher temperature, namely

180C, but the greatest mass loss is reached at 280-340C, which is lower than cellulose

decomposition. At lower temperatures, dehydration and decomposition of the side chains,

which are very easy to remove are the most important processes, followed by breakage of

the glycosidic, C-C, C-O and C-H bonds of the individual sugars, depolymerization,

decarboxylation and decarbonylation. The non-condensable volatiles mainly consists of CO2,

CH4,CO, H2, C2H2, C2H4, and C2H6. CO2 is predominant in the early stages of pyrolysis (low

temperatures), but since its concentration decreases with temperature, H2and CH4become

more significant above 750C. The liquid products can be classified in following groups:

furans, cyclopentanes, alcohols, saturated fatty acids, cyclohexanes and aromatics. Between

300 and 600C, particularly oxygen compounds with low molecular weight (alcohols,

anhydrosugars), cyclopentanes, simple aromatics and fatty acids are formed, while the

pyrolysis at temperatures exceeding 800C yields specially phenolic and polynuclear


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aromatic compounds. The components produced in the early stages can also experience

condensation, hereby generating complex aromatic compounds such as benzofuran and

indene. Generally, the char yield of hemicellulose pyrolysis is larger than the one from

cellulose pyrolysis19.

6. CATALYTIC UPGRADING

The study of catalyzed biomass pyrolysis is becoming very popular, because of its huge

potential for modifying the pyrolysis products in order to provide a more efficient pathway for

chemical production or to improve the bio-oil quality. The main purpose for the latter is

producing a liquid transportation fuel from biomass that is able to replace fossil fuels, which

is essentially achieved by the removal of oxygen. By adding catalysts to the pyrolysis

process, reactions such as decarbonylation, decarboxylation, hydrocracking,

hydrodeoxygenation, and hydrogenationare enhanced. An overview of these different types

of reactions can be seen in Figure 946.The bio-oil properties are improved since oxygenatedcompounds are removed via H2O and CO2, making the oil less corrosive. Furthermore these

cracking reactions lead to a lighter, less viscous product and also the heating value of the

bio-oil is increased thanks to the formation of more valuable components resembling

petrochemical products. Numerous types of catalyst applications, each responsible for

another product distribution, have been studied in order to improve pyrolysis productivity.

They can be classified according to their application: one possibility is to mix the catalyst with

the biomass before being fed to the reactor, but if a more intense contact with vapor, solids

and char is needed they can also be fed directly to the reactor. The catalysis occurs in the

reactor and thats why it is called in-bed catalysis. Another option is to locate them in a

secondary reactor downstream of the actual pyrolysis reactor, the so-called ex-bedcatalysis27. In-bed catalysts mainly influence the primary reactions while the ex-bed catalysts

affect the secondary reactions (vapor phase upgrading).

FIGURE 9: REACTIONS ASSOCIATED WITH THE CATALYTIC UPGRADING OF BIO-OILS49


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Its very important to take into account that biomass itself contains components to which

catalytic effects can be ascribed. Common inorganics such as Na, K, Ca, Mg and silica have

been proven to alter the behavior of cellulose and lignin during thermal degradation. So when

the pyrolysis of biomass with additional catalysts is studied, a good understanding of the

effect of naturally occurring inorganics is essential47.

The two most important routes for bio-oil upgrading separated from the pyrolysis reactor are

hydrodeoxygenation and zeolite cracking. Hydrodeoxygenation is a catalytic hydrotreatment

of the bio-oil under high pressure in the presence of hydrogen or hydrogen and carbon

monoxide. Occasionally other hydrogen donor solvents are used. Sulfide/oxide and transition

metal catalysts are applied to promote the carbon-oxygen bond cleavage with hydrogen.

Nowadays cracking catalysts such as zeolites are receiving more interest since they operate

at atmospheric pressure and in the absence of hydrogen47. Zeolites are microporous

minerals with a complex three-dimensional structure and varying elemental composition. The

pore size and acidity of the zeolite are the main parameters which affect the reactivity. The

most widely applied zeolite, HZSM-5, is an aluminosilicate composed of several pentasil

units (eight five-membered rings). Its acidity depends on the Si/Al ratio; a low ratio indicates

high acidity, which provides a better adsorption of the oxy-compounds. A drawback of using

these catalysts is the formation of high amounts of coke, which deposit on the catalyst

surface and block the pores in this way, making the catalyst less reactive. To minimize

depositions and promote the desired cracking reactions, zeolites with a correct pore size and

well defined number of acidic reaction sites should be used46.

6.1INFLUENCE OF ALKALI AND ALKALINE EARTH METALS

The last two decades studies already have proven that high amounts of inorganiccompounds present in the biomass alter the product distribution and chemical speciation of

the bio-oil. More and more interest is focused on the effect of alkali and alkaline earth metals

(AAEM), since there is observed that especially these species have a strong catalytic activity.

Raveendran et al.48studied the effect of deashing on pyrolysis product distribution of twelve

types of biomass. In all cases the liquid yield increased in expense of the gas yield.

Varheghyi et al.49 used thermogravimetric analysis (TGA) to investigate the influence of

mineral matter on the mass loss rate in function of the temperature. They found that the

biomass decomposition already started at a lower temperature in presence of greater

amounts of inorganics. The study demonstrated also the accelerated secondary

decomposition resulting in a broad range of smaller products. Various studies made use ofTGA when studying the effect of metals on biomass pyrolysis, but this technique is not

capable to produce the same high heating rates as applied in fast pyrolysis, which is a

serious drawback. Obtained results from these studies could be wrong since lower heating

rates favor the production of char26. The investigation of the effect of indigenous or added

inorganics on biomass pyrolysis focused on the classification of the product yields in terms of

char, liquids and gases, while only a few studies investigated the chemical speciation as a

result of pyrolysis reactions. Piskorz et al.50pyrolyzed demineralized biomass in a fluidized

bed reactor and discovered an increase in the yield of anhydrosugars. He suggested two

possible mechanisms for cellulose pyrolysis depending on the presence of mineral matter.

Depolymerization leading to levoglucosan and other anhydrosugars is the main mechanism


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in demineralized biomass, while the presence of inorganics favors the fragmentation to low

molecular weight products. He reported that the C-O bond in glucose monomers is less

stable than the C-C bonds and consequently the two-, three- and four-carbon fragments

could be produced directly from cellulose in competition with levoglucosan formation. Eom et

al.51 and Nowakowski et al.52confirmed that these ring-opening reactions are catalyzed by

potassium in particular. Figure 10 shows the suggested mechanism in presence ofinorganics, while Figure 8 represents the cellulose degradation pathway in absence of

inorganics.

FIGURE 10: CELLULOSE DEGRADATION MECHANISM IN PRESENCE OF INORGANICS28

Patwardhan et al.53 pyrolyzed different glucose-based carbohydrates to investigate the

thermal degradation mechanisms and exposed numerous competitive degradation pathways

which are dependent on temperature and concentration of inorganic species. Other authors5455 indicate sodium and potassium as the strongest catalysts (stronger then calcium). The

anion counterpart also seems to play a significant role in the catalytic effect.54 Shimada et

al.56 confirmed that the production of low molecular weight compounds was increased by

AAEMs. In general the investigated salts induced the production of low molecular weight

species because the Ca2+ and K+ kations induced homolytic fission of glucose rings. The

complexity of these reactions has been proved by isotopic labeling studies. In the case of

glycerine up to four competing fragmentation reactions were proposed depending on the

presence of acid or basic catalysts. The degree of polymerization of the applied cellulose

also has a major influence on the type of reaction57.

The pathway for the effect of AAEMs on cellulose is better understood than for lignin.

Although Galgano et al.58reported an increase in total phenols, which are typical degradation

products of lignin. The presence of NaOH increased guaiacol and cresol production, while

KOH increased the formation of phenol and isoeugenol. Kleen et al.59 reported that sodium

has a beneficial effect on demethoxylation, demethylation and dehydration reactions, hereby

significantly changing the pyrolysis product distribution. Interestingly, the results of lignin

decomposition are modified by AAEMs in a comparable way, but more investigation is

required to clarify the complete mechanism60.


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OBJECTIVEThe objective of this masterthesis is to investigate the effect of alkali and earth alkaline

metals on the fast pyrolysis of biomass constituents. Previous work has shown the catalytic

influence of char on the thermal degradation of biomass, but less is known about the

complex mechanisms behind this process. Furthermore the pyrolysis products are generallyclassified in tar, char and gas but this kind of classification does not give information about

the different chemical species formed. A better understanding of the exact catalytic role of

the naturally occurring minerals on the chemical speciation can be achieved by using

micropyrolysis-GC/MS as analysis technique. Through pyrolyzing biomass constituents, with

focus on cellulose and lignin, mixed with various salts in different concentrations a better

insight in their effect on the final product distribution is received. Micropyrolysis together with

gas chromatography and mass spectrometry is for this purpose a very useful approach

because a distinction can be made between primary and secondary reactions. The complex

nature of pyrolysis reactions has led to a lack of distinguishing primary and secondary

reactions. However the residence time of vapors in the micropyrolyzer does not exceed 20

ms, which makes it possible to exclude the main secondary reactions. This allows us to study

the catalytic influence of metals on the primary reactions by mixing them with the biomass

samples before dropping them in the micropyrolyzer oven. On the other hand, by placing a

secondary reactor containing metals fixed in a bed downstream of the pyrolysis reactor, also

the effect on secondary reactions can be analysed.


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MATERIALS AND METHODS

1. SAMPLE PREPARATION

Avicel microcrystalline cellulose, purchased from EMC Biopolymer was used to run the

pyrolysis experiments. This commercially available cellulose is derived from fibrous plantsand is already partially depolymerized. Lignin was purchased from SigmaAldrich as alkali

lignin, separated from cellulose by the Kraft process. Five catalysts (CaCl2, calcium citrate,

calcium acetate, potassium citrate and potassium acetate) were also purchased from Sigma-

Aldrich as a powder, while for KCl a 3 M solution from VWR international company was

applied. The last catalyst used was a non-commercial, spray-dried heterogeneous ZSM-5

based catalyst, modified for enhanced mesoporosity and was supplied by Delft Solid

Solutions (Delft, The Netherlands). Cellulose and lignin samples doped with these catalysts

were prepared for in-bed catalytic pyrolysis. Impregnation was used in order to achieve a

good mixing of cellulose and lignin with the catalysts. Samples with different concentrations

(ranging from 0.1 to 22 wt.%) were prepared by impregnation. The precisely weighed amountof salt was dissolved in 25 ml deionized water and 2.5 g of cellulose was dispersed in this

solution. The well-mixed slurry obtained by good stirring for 2 hours was put in an oven at

110C to dry overnight. Approximately 0.5 mg of these samples were pyrolyzed and each

experiment was run in duplicate or triplicate.

2. MICROPYROLYSIS EXPERIMENTS (PYGC-MS)

The micro-pyrolysis set-up used to perform the fast pyrolysis experiments is shown in Figure

11. It consists of a micro-pyrolysis unit (FrontierLab Multi-shot pyrolyser EGA/PY-3030D)

connected to a gas chromatograph and a mass spectrometer (Thermo Fisher Scientific

Trace GC Ultra and Thermo ISQ MS). Xcalibur data processing software was used for the

final product identification and integration.

FIGURE 11: MICROPYROLYSIS SETUP AT THE DEPARTMENT OF BIOSCIENCE ENGINEERING


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A more detailed illustration of the micro-pyrolysis unit is given in Figure 13. It consists of a

sampler and a quartz pyrolysis tube that can be preheated by a furnace to the desired

temperature (500C). Furthermore a heated interface (350C) and deactivated needle (which

is directly inserted into the GC injector) can be distinguished.

Sample cups of deactivated stainless steel were loaded with approximately 500 g of the

finely ground lignin or cellulose sample. Roughly 2 mg of inert quartz wool was added on top

to prevent loss of sample material during transportation of the cup and particularly during the

free fall through the reactor tube. A Mettler Toledo microbalance with a sensitivity of 0.001

mg was used to weigh the cups accurately before they were dropped into the quartz

pyrolysis tube. The cup falls freely into the preheated furnace by gravity in a very short time

period of approximately 12-20 ms. In that way the sample is heated at a heating rate

exceeding 2000C/s, hereby ensuring fast pyrolysis. Its difficult to determine the yield of

liquids and solids by using micropyrolysis. Initially it was tried to estimate the amount of char

produced by weighing the cup before and after pyrolysis, but some loss of quartz wool during

the process is inevitable which leads to an underestimation of the produced amount of char.

But the gas chromatographic analysis results allowed us to determinate the different volatile

pyrolysis products. All of these experiments were conducted at least two times to confirm the

reproducibility of the tests.

Helium gas was used as the carrier gas to sweep all the produced vapours directly into the

GC. A split/splitless injection port with a split ratio of 1:100 was used to achieve a constant

helium gas flow of 1 ml/min in the capillary column, knowing that the original gas flow was

100 ml/min. The injection temperature was 300C. A Restek capillary column (Rtx-1707, 60m

L x 0.25 mm I.D. x 0.25 m df) with a stationary phase consisting of a crossbond 14%cyanopropylphenyl and 86% dimethyl polysiloxane was used to separate the injected

pyrolysis vapours. The GC oven temperature started with a 3 min. hold at 40C, followed by

heating to 280C at a heating rate of 5C/min. Finally the temperature was held constant for

1 min. After separation on the column, the species are broken down in the mass

spectrometer for further analysis.

3. CATALYTIC PYROLYSIS EXPERIMENTS (PYGC-MS)

Catalytic experiments were performed in both in-bed and ex-bed mode in a Tandem

Pyrolyzer. The in-bed mode experiments only require one reactor (Frontier single -reactor)

since the catalyst is mixed with the biomass constituents prior to dropping the cup in the

pyrolyzer, while for ex-bed mode a secondary tube furnace needs to be mounted

downstream of the pyrolysis reactor (Frontier tandem -reactor). In this way the vapors

produced by regular pyrolysis are contacted with the catalyst. The secondary reactor

contains a quick-change quartz catalyst reaction tube with an inner diameter of 5 mm of

which the temperature can be independently controlled, separate from the primary reactor.

The reaction tube is packed with the catalyst as shown in Figure 12. The usable vertical

length of the catalyst bed is 40 mm. This leads to a catalyst amount of approximately 5 mg

which is placed in the tube between small amounts of quartz wool and two coil springs to


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prevent the catalyst from shifting in case of sudden pressure changes. The catalysts used

were the same in both in-and ex-bed mode (see sample preparation).

FIGURE 12: REACTION TUBE PACKED WITH CATALYST61

FIGURE 13: OVERVIEW OF THE FRONTIER MICROPYROLYZER61


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4. GC-MSDATA PROCESSING

The peak areas were derived from the total ion current (TIC) chromatograms, which are

found in the quality browser of the Xcalibur software. The peaks represent the relative

abundance (the peak area of one component divided by the sum of all the individual peak

areas) of the detected components. The software allows us to perform an integration of thepeaks, applying restrictions as baseline window (set to 200) and area to noise factor (set to

100) to reduce the integration of the huge number of small peaks that arise in the spectra

due to noise. All components with a relative abundance greater than 0.09% were quantified

and identified. The identification of the individual components was performed by using the

National Institute of Standards and Technology (NIST) MS library. Subsequently the detected

and quantified compounds were grouped according to their chemical functionality (a

distinction was made in the following groups: carbon dioxide, aliphatic hydrocarbons,

aromatic hydrocarbons, alcohols, ketones, aldehydes, ethers, carboxylic acids, furans and

pyrans, phenolics and methoxylated benzenes, sugars, N-compounds and others). In

addition the phenolic species were grouped according to their degree of methoxylation (non-,

mono and dimethoxylated species). The most abundant components of each sample where

studied and compared to determine differences between the catalysts chemical activities.

These differences were highlighted with statistical analysis using SPSS software. Since its

hard to determine normality or homoscedasticity of a small group of data (only three values

coming from the repetitions of every sample), non-parametric tests were used to verify

significant differences. The Wilcoxon rank-sum was applied for comparing two populations,

while the Kruskal-Wallis test was used when more than two populations needed to be

compared.


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RESULTS AND DISCUSSIONAt first lignin and cellulose were pyrolyzed non-catalytically at different pyrolysis

temperatures to differentiate thermal degradation mechanisms and to define a proper

pyrolysis temperature for the catalytic experiments. This prior investigation was necessary to

study the effect of the catalysts on the first mentioned degradation reactions of both biomassconstituents. In the second part an overview is given of the catalytic effect of five different

potassium and calcium salts and the non-commercial HZSM-5 zeolite. A distinction was

made between their effect on primary and secondary reactions by comparing the pyrolysis

products from both in-bed and ex-bed catalysis.

1. TEMPERATURE EFFECT

1.1CELLULOSE

Pure cellulose was pyrolyzed at 300C, 400C and 500C. Figure 14 shows their respectivechromatograms. The decomposition products from the pyrolysis of pure cellulose at 500C

are given in Table 5. These 47 identified products make up to approximately 96% of the total

peak area in the TIC chromatogram. At 300C and 400C respectively only 13 and 22

products could be distinguished respectively. The products determined at 300C are

indicated with a star in Table 5, those with one or more stars are found at 400C.

FIGURE 14: TIC CHROMATOGRAM OF PURE CELLULOSE (AVICELL) MICROPYROLYSIS OF APPROX. 0.5 MG AT

(UPPER) 300C, (MIDDLE) 400C AND (BOTTOM) 500C


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TABLE 5: GAS CHROMATOGRAPHIC ANALYSIS OF PYROLYSIS PRODUCTS FROM CELLULOSE AT 500C. THE

PRODUCTS DETERMINED AT 300C ARE INDICATED WITH A STAR, THOSE WITH ONE OR MORE STARS ARE FOUND

AT 400C

Iupac name Apex RT

TIC Peak

Area % Iupac name Apex RT

TIC Peak

Area %

Carbon dioxide* 4,65 1,29 Cyclohexanol, 2,3-dimethyl- 26 0,16

Ethane 4,77 0,46 Maltol 26,36 0,05

Acetaldehyde 5,13 0,11 2-Hydroxy-gamma-butyrolactone 26,44 0,06

Furan 5,66 0,07 Cyclopentanecarboxaldehyde 27,2 0,06

2-Propenal 5,98 0,04 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-Pyran-4-one 27,7 0,08

Acetic anhydride* 6,26 0,58 Levoglucosenone** 27,93 0,18

2,3-Butanedione 7,86 0,08 3,5-Dihydroxy-2-methyl 4H-Pyran-4-one* 28,28 0,18

Hydroxyacetaldehyde* 9,06 0,76 2-Pentanol, 5-(2-propynyloxy)-** 30,43 0,45

Acetic acid 10,28 0,05 2H-Pyran-3(4H)-one, dihydro-6-methyl- 30,56 0,08

1-Hydroxy-2-propanone 11,41 0,17 2,3-Anhydro-d-galactosan 30,69 0,12

2(5H)-Furanone* 15,76 0,25 2,3-Anhydro-d-mannosan* 30,88 0,21

3-furfuraldehyde 15,88 0,05 1,4:3,6-Dianhydro--d-glucopyranose** 31,19 0,42

Methylpyruvaat* 16,32 0,20 2,4;3,5-Dimethylene-l-iditol* 31,78 0,21

Furfural* 16,86 0,42 5-hydroxymethylfurfural** 32,81 0,44

5-Methyl-2(3H)-furanone 18,28 0,05 4-Cyclopentene-1,3-diol 32,95 0,07

dihydro-4-hydroxy-2(3H)-furanone* 20,12 0,27 2-Butene-1,4-diol** 33,95 0,12

1,2-Cyclopentanedione* 20,42 0,21 2-Hydroxy-2-methyl-Butanedioic acid 34,84 0,14

5-Methylfurfural 21,36 0,10 Methyl--d-ribofuranoside** 35,08 0,56

1,2-Cyclopentanedione, 3-methyl-

23,53 0,22 Levoglucosan** 42,16 79,71

2,5-Dimethyl-4-hydroxy-3(2H)-

furanone* 25,02 0,35 1,6-Anhydro--d-galactofuranose** 44,9 8,66

Figure 15 shows the relative yield of the three main groups of products (low molecular weight

oxygenates, furans/pyrans and anhydrosugars) formed in function of the temperature. It is

clear that depolymerization reactions, leading to anhydrosugars and levoglucosan in

particular (mechanism is shown in Figure 8), are the most important, but at elevated

temperatures, i.e. from 400C on, the formation of oxygenated compounds and furan/pyranderivatives becomes more significant since they are produced by endothermic glucose ring-

scission reactions. Regarding the results in Figure 15, attention has to be paid to the fact that

the absolute amount of anhydrosugars still increased from 5.97*1010 to 4.03*1012 TIC (in

peak area per mg cellulose sampled), which indicates that the polymeric structure of

cellulose is more degraded into monomers at higher temperatures.


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FIGURE 15: RELATIVE AMOUNT OF OXYGENATED COMPOUNDS, FURANS/PYRANS AND ANHYDROSUGARS IN

FUNCTION OF PYROLYSIS TEMPERATURE IN PYGCMS

Levoglucosan (1,6-anhydro--D-glucopyranose, LG) has been demonstrated to be the major

product formed during cellulose pyrolysis2661. As can be seen in Table 5, the yield of 79.70%

at 500C confirms this theory. 1,6-Anhydro--d-galactofuranose is the second most abundantspecies formed (8.67%). The formation of this levoglucosan-isomer requires more energy,

leading to smaller yields which increase at higher temperatures62. Other products are formed

in much lower amounts, those most valuable for chemical production are shown in Figure 16.

One of them is levoglucosenone, but its amount reduces from 1.24% to 0.18% with

increasing temperature because it rapidly polymerizes. Levoglucosenone is an interesting

product since it contains easily modifiable functional groups, which can be very useful in the

production of several chemicals63. The amounts of hydroxyacetaldehyde and furfural also

increase significantly, while the amount of 5-hydroxymethylfurfural, a compound derived from

the dehydration of certain sugars, increases from 0.24% at 300C to 0.51% at 400C, but

remains constant at higher temperatures. This can be explained by the growing share offurfural, which is a degradation product of hydroxymethylfurfural63.

0

10

20

30

40

50

60

70

80

90

100

Oxygenatedcompounds

Furans / Pyran Anhydrosugars

TICrelativeabundance(peakarea%)

300C

400C

500C


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FIGURE 16: RELATIVE AMOUNT PRODUCED OF THE 6 MOST IMPORTANT CELLULOSE PYROLYSIS DEGRADATION

PRODUCTS. LEVOGLUCOSAN (LG) AND 1,6-ANHYDRO--D-GALACTOFURANOSE (AGF) ARE EXAMINED ON THE

LEFT AXIS, HYDROXYMETHYLFURFURAL (HMF), LEVOGLUCOSENONE (LGO), FURFURAL (FF) AND

HYDROXYACETALDEHYDE (HA) ON THE RIGHT AXIS.

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