Towards Liquid Fuels from Lignin

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Master Thesis

Towards Liquid Fuels from Lignin

Author:

Nicola Giummarella

Academic Supervisor:

Gunnar Henriksson, Professor in Wood Chemistry Supervisor:

Christofer Lindgren, CleanFlow Black AB

Examiner:

Mikael Lindström, Dean School of Chemical Sciences and Engineering

Pulp Technology, Fiber and Polymer Technology Department, School of Chemical Science and Engineering,

KTH Royal Institute of Technology, Stockholm, 2014

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TABLE OF CONTENTS

ABSTRACT 4

1. INTRODUCTION 5

1.1 THE NEED OF BIOREFINERY 5

1.2 LIGNIN: structure and properties of an underutilized resource 5

1.3 CHEMICAL PULPING: different ways to isolate lignin 8

1.3.1 Kraft Pulping 8

1.3.2 Sulfite pulping 9

1.3.3 Organosolv pulping 10

1.4 PRECIPITATION OF TECHNICAL LIGNIN 10

1.5 CONVERSION TO LIQUIDS FUELS 12

1.6 DISSOLUTION OF LIGNIN 13

1.6.1 Termodynamical Background 13

1.6.2 Hildebrand Solubility Parameters 14

1.6.3 Hansen Solubility Parameters 15

1.6.4 Lignin Solvents 16

1.6.4.1 METHANOL 17

1.6.4.2 MBO 17

1.6.4.3 FORMIC ACID: A H DONOR SOLVENT 18

1.6.4.4 FURFURAL 19

1.7 REHOLOGY: laws and tests 19

1.7.1 Hooke´s Law 20

1.7.2 Newton´s law 20

1.7.3 Dynamic Mechanical Testing 21

1.8 VISCOSITY 22

1.8.1 Non Newtonian Flow Behaviour 23

1.8.1.1 SHEAR RATE DEPENDENT FLOW BEHAVIOUR 23

1.8.1.2 SHEAR TIME DEPENDENT FLOW BEHAVIOUR 24

1.8.2 The Mark–Houwink–Sakurada Relationship 24

1.9 ASH IN LIGNIN 25

2 MATERIALS AND METHODS 27

2.1 δ HILDEBRAND VALUES CALCULATION 27

2.2 HANSEN PARAMETERS DETERMINATIONS 29

2.3 SOLUBILITY OF LIGNIN 30

2.3.1 Lignin samples 30

2.3.2 Experimental part 30

2.4 VISCOSITY AND RHEOLOGICAL EVALUATION 31

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2.5 BLACK LIQUOR TITRATION 32

2.6 WASHING LIGNIN 32

2.6.1 Preliminary test 32

2.6.2 Lowering ash 32

2.6.3 Scaling Up 33

2.7 ASH MOISTURE CONTENT ANALYSIS 33

2.8 FRACTIONATION OF LIGNIN 34

2.8.1 Methanol 34

2.8.2 MBO 34

2.9 SEC SET UP 35

2.9.1 Intrinsic viscosity by Mark–Houwink–Sakurada equation 35

2.10 HYDROGENATION OF LIGNIN 35

3 RESULTS AND DISCUSSION 37

3.1 LIGNIN SOLVENTS 37

3.2 SOLUBILITY EXPERIMENT 38

3.3 VISCOSITY OF LIGNIN SOLUTIONS 41

3.4 BUFFER CAPACITY OF BLACK LIQUOR 44

3.5 LOWERING ASH CONTENT 45

3.6 METHANOL FRACTIONATION OF LIGNIN 48

3.6.1 Lignoboost Lignin 48

3.6.1.1 RHEOLOGICAL ANALYSIS 50

3.6.1.2 SEC ANALISYS 51

3.6.2 Aspa Lignin 54

3.7 MBO EXTRACTION OF LIGNIN 55

3.8 VISCOSITY EXPLAINED BY SEC RESULTS 57

3.9 HYDROGENATION EXPERIMENT RESULTS 58

4 CONCLUSIONS 60

5 ACKNOWLEDGEMENTS 61

6 REFERENCES 62

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Abstract

The solubility of Lignoboost lignin was compared with softwood lignin precipitated from filtered black liquor and explained by Hilebrand as well as Hansen solubility parameters theory.

The ability to dissolve efficiently lignin rises as the hydrogen bonding capacities together with the polarity of the solvents increases; similarly, their solubility parameter, according to Hildebrand, lay within the range between twelve and fourteen.

Lower molecular weight lignin obtained by ultrafiltration is definitely more soluble than lignin obtained by Lignoboost process, especially at higher concentration.

In addition, viscosity measurements show that solutions obtained from low molecular weight lignin are always less viscous than Lignoboost solutions. The gap in viscosity, between two lignins, becomes even higher at high concentration.

The relationship between molecular weight of lignin and viscosity has been demonstrated by SEC analysis and application of Mark–Houwink–Sakurada equation.

By ash content evaluation it has been possible to find out the most efficient conditions to lower salts formation when lignin is burnt. Several washes carried on with cold and acidic water have decreased the amount of ash to a value lower than 0,5% of dry weight.

The effect of methanol fractionation on the molecular weight and its distribution of Lignoboost lignin has been investigated showing phase separation. The heavy and high lignin content fraction shows a pseudoplastic behaviour; however, its viscosity at low shear rate is too high to be interesting in a fuel production context and because the high volatility of methanol.

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1. Introduction

1.1 THE NEED OF BIOREFINERY

A biorefinery is a facility able to produce materials, fuels, power, heat and value added chemicals separating and modifying biological raw materials in order to obtain a spectrum of valuable half products. Efficiency, logistic and know how of petrochemical refinery must be transferred into this “greener” conversion process so that a complete use of the biomass feedstock and integration of pre existing equipment can be carried onⁱ. The concept of to use renewable raw materials improves our technical culture and knowledge in terms of sustainability and it can be seen as the real answer to environmental and social issues which are concerning our society.

Firstly, the depleting stocks of fossil fuels is increasing the demand of new oil deposits leading, for instance, to worldwide war as it has happened in Iraq, one decade ago, or in Kuwait in ‘90s. Furthermore, burning fossil fuels is increasing the emission of CO2 and greenhouse gases, which causes global warming as well as serious climate changes.

On the other hand, producing renewable liquid fuel for transportation engine, is a great challenge in biofuels area since requires sustainably managed source materials with no adverse socio economic side effects. From a technical point of view, the liquids products have to be fully compatible with the existing fuel technology and to supply infrastructure, which demands unpolar, low viscous liquids with relatively low oxygen content.

1.2 LIGNIN: structure and properties of an underutilized resource

Lignin is an amorphous 3D web polymer built on by end wise polymerization (Figure 1) in which the monolignols form covalent bonds (C-‐O-‐C or C-‐C) combining each other, first, by radical coupling and afterwards by either rearrangement or nucleophilic attack on the electrophilic α carbon.

Figure 1: Suggested lignin structure.ⁱⁱ

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Figure 2 shows the structure of the most common monolignols and their composition in different Phylum of plants.

As can be seen, these propyl phenol derivates cumaryl alcohol, coniferyl alcohol and sinapyl alcohol, respectively, vary upon the number of methoxy groups attached to the aromatic ring in ortho position. The lignin content in wood decreases with the following order: softwood>hardwood>grasses. In conifers lignin is mostly formed by coniferyl alcohol (guaiacyl structure), whilst proportions from almost equal amounts of coniferyl alcohol and sinapyl alcohol (syringyl structure) to three times higher levels of the latter, constitutes hardwood lignin. Grasses contain a mixture of all three main monolignols in different proportion.

Figure 2: Lignin content and chemical structures of the three primary lignin monomers of woodⁱⁱⁱ.

Lignin basically consists of a variety of C-‐C “condensed” and C-‐O “ether” linkages irregularly distributed. Typically, more than two thirds of the linkages in lignin are ether bonds. The most frequent coupling link in the lignin network is the β-‐O-‐

4 bond which accounts for at least half of all inter monolignol bonds and it is the most easily broken by chemical pulping. 4-‐O-‐5 connects two aromatic rings whereas the C-‐C bonds such as β−β, β−5 and 5-‐5 are more chemically stable (Figure 3).

Figure 3: Most common intermonolignols linkages in lignin: β-‐O-‐4, 5-‐5, α-‐O-‐4, β-‐5, β−β, 4-‐O-‐5, β-‐1. ^iv

Softwood 27-‐33 <5 >95 None

Hardwood 18-‐25 0-‐8 25-‐50 46-‐75

Grasses 17-‐24 5-‐33 33-‐80 20-‐54

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The different composition of monolignols in hardwood and softwood explains why delignification time of conifers is longer.

In softwood, for instance, the availability of position 5 on the ring (only one methoxy group is present in coniferyl alcohol at position 3) creates the possibility to form more condensed bonds in comparison with hardwood where, instead, β-‐O-‐4 links are more common and, as has been told, easily to cleave.

However, differences in the chemical structure of hemicellulose between hardwood and softwood can be as well a reason for different time of delignification required. Looking at Lignin Carbohydrate Complexes (LCC), the totally dominating xylan bonded to lignin in hardwood has lower content of condensed bonds differently from glucomannan in softwood where, instead, C-‐C linkages prevail.^v

On the contrary, in order to defibrillate hardwood fibers, the delignification process must be carried on to higher degree of delignification than softwood.

This is because lignin in the middle lamella of hardwood consists mostly of coniferyl alcohol. Therefore, differently from the homogeneous lignin composition of softwood, the “glue” of hardwood fibers (e.g. middle lamella), consists of more condensed network. Because of that, it is required a delignification to a lower residual content in order to obtain defibrillation.

Roughly 80% of lignin can be found in secondary cell wall even if the concentration is higher in the middle lamella where lignin acts as glue between the fibers. Due to presence of aromatic rings, lignin is hydrophobic and seems to play an extremely important role in the water transportation along the plant.

Furthermore, lignin stiffens the cell wall providing the typical mechanical properties of a woody plant; as a demonstration of this, non woody plants such as Herbs are soft and flexible since the lignin content is low. Lignin is thought to provide resistance to microbial attack. The higher degree of branching, due to impossibility of close packing and formation of crystals, makes lignin optically inactive and very amorphous. This last property explains the ability of lignin to act as interface between the cellulose and hemicellulose filling up all the cavities.

The 3D network of lignin, exactly the α carbon, is covalently linked either through esther or ether bond or by phenyl glycoside bond to different polysaccharides. The curing effect of LCC avoids swelling and increase the stiffness of wood.^vi

Lignin, being one of the three main components of woody plants, it’s the third most common biopolymer on Earth after cellulose and chitin. Moreover, lignin is the largest renewable source of aromatics on Earth with quite high specific energy content due to more reduced carbons (redox number ≈ -‐0.4).^vii than carbohydrates (redox number: 0-‐1) Furthermore, pulp and paper industries generate enormous amounts of lignin as by-‐product, which is mostly utilized as a low grade boiler fuel to provide heat or power in the pulp process.

On the one hand, looking at the chemical structure of lignin it can be seen that, if it could be broken into smaller molecular units, it might be a good source of valuable fuels. On the other hand, the hydrophobic complex 3D structure covalently crosslinked mostly to the hydrophilic hemicellulose makes impossible to extract lignin as it is in nature. Therefore, pretreatments as chemical pulping or steam explosion are condicio sine qua non to fractionate lignin even if chemical modifications make it dramatically different from the native. Furthermore, poor product selectivity of biorefinery process, formation of a more condensed

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structure during thermochemical process and ease of use as a solid fuel thanks to new technology (i.e. Lignoboost) are some of the biggest obstacle in the development of lignin as raw material of biorefining technologies^viii.

Finally, most of the biorefinery processes are more focused on utilizing easily convertible and more profitable fractions such as cellulose and hemicelluse (i.e generation of ethanol by fermentation) rather than unremunerative lignin which, at present state of art, is still underutilized.

1.3 CHEMICAL PULPING: different ways to isolate lignin

Lignin can be isolated from wood by broadly two main methods:

-‐ Solubilisation of hemicellulose and cellulose with precipitation of lignin as insoluble residue (i.e. Klason lignin or byproduct lignin produced from ethanol process in a biorefinery, e.g. EPL).

-‐ Dissolution, removal and recovery of lignin leaving most of cellulose and hemicellulose as insoluble residue. This happens during chemical pulping where depolymerisation, increasing lignin solubility and breaking of LCC^ix are the main steps adopted.

The amount of lignin obtainable from Kraft pulping is the highest ever, since it accounts for about 90% of the world production capacity.^x

In addition to that, technical lignin recovered is one of the most interesting as a fuel source being less oxidized than lignin obtained from bleaching but, on the other hand, it is not water soluble as lignosulfonates derived from sulfite pulping.

Therefore, the main methods of chemical pulping as isolation method of lignin is going to be further discussed, underlining, from a chemical point of view the different processes.

1.3.1 Kraft Pulping

The key reaction in alkaline condition (initial pH>13) and high temperature (>160°C) of Kraft pulping is the cleavage of non phenolic β-‐O-‐4 bonds with formation of phenolic residues. This step is important also in soda pulping which is mostly used for non woody plants (low lignin content) and therefore not interesting in this dissertation. During this relatively slow reaction, due to alkaline condition, ionization of α hydroxyl occurs (α alcohol is more reactive than alcohol in γ position since it is a secondary alcohol). Nucleophilic attack in β electrophilic carbon with formation of epoxide leads to depolymerisation due to the cleavage of a new phenolic group from β position (Figure 4).

Figure 4: Alkali depolymerisation of non phenolic β-‐O-‐4 bonds^xi.

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The phenolic lignin cleaved up is the starting point of the next peeling reaction where mono lignol residues are cut one by one. This step is faster and is strongly dependent on concentration of sulfide so that the selectivity and efficiency of delignification is strongly affected by concentration of HS^-‐. After deprotonation of phenolic end groups and due to the presence of LCC in α position, the zwitterion quinone methide, as intermediate resonance form, is created. Sulfide ion performs nucleophilic attack on α position, deprotonates and episulfide is formed between α and β carbon. Last, elementary sulfur is released forming an unsaturation on the aliphatic chain of the mono lignol structure (Figure 5).

Figure 5: Peeling reaction of monolignol units from phenolic end groups^xii.

However, during final delignification when the concentration of sulfide ions is low, unfavourable side reactions leading to “dead ends” become more and more common, decreasing the selectivity of process. Both enol ether formation with formaldehyde as leaving group and condensation by radical coupling stop the delignification process making it impossible to remove all of the lignin from a fiber. Last but not least, it has been shown that LCC is the biggest problem for complete delignification since some of them are strong enough to survive Kraft cook^xiii.

The reaction described in figure 5 has the same mechanism during anthraquinone and polysulfide pulping where one of the main differences regards the reactive species: respectively dehydroanthraquinone and polysulfide, both strong nucleophilic attacking α position in phenolic lignin. On one hand, both pulping methods are more efficient as well as selective than Kraft one since are able to oxidize the reducing end of polysaccharides slowing down the peeling reaction of carbohydrates. Furthermore, “dead end” stable in Kraft pulping as enol ethers can be to some extent cleaved in presence of polysulfide.

Nevertheless, the former has been discovered to be carcinogenic whereas the latter is less doable in practice than it is theoretically.

1.3.2 Sulfite pulping

The importance of sulfite pulping has dramatically decreased in the last years compared to Kraft one and only about 10% of the pulp is currently produced by this method^xiv.

During acidic sulfite pulping wood is heated in presence of HSO3-‐ anion with calcium, magnesium or sodium as counter ion. Lignin is removed by introduction of sulfonic acid (sulfonation) to the α carbon, which subsequently leads to solubilisation. Differently from Kraft lignin, which is only soluble at pH higher than 10, lignosulfonates are soluble over almost the whole pH range.

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However, the introduction of charged groups together with an amount of sulfur accounting for 4-‐8 wt.%,^xv twice as much as the amount in Kraft lignin, makes lignosulfonate soluble in cooking liquor when reach markedly higher average molecular weight than Kraft lignin. Therefore, the high content of sulfonate groups incorporated in the sulfite lignin, needs more intense flue gas purification when burning sulphite lignin.

Lastly, it must be noticed that average molecular weight of lignosulfonates from softwood (≈60kDa) is significantly higher than lignosulfonates from hardwood (≈12kDa)^xvi. The amphiphilic structure made by hydrophobic aromatic group and hydrophilic propane unit makes this biorefinery product widely used as industrial binder, dispersing agent in oil drilling and additive to concrete and asphalt. Finally, it must be mentioned that softwood lignosulfonates are the raw materials for synthesis of vanillin.

1.3.3 Organosolvpulping

The key concept is to increase the solubility of the hydrophobic lignin by using organic solvents such as ethanol, methanol, acetic or formic acid with ≈50% of water. Pulping can be carried out in alkaline as well as acidic catalyst condition.

In the first case, the chemical reactions are identical as those explained in Kraft pulping whilst at low pH, LCC linked by α ethers are broken by solvolysis combined with hydrolysis. Lignin is cleaved up at β position where C-‐O bounds are present. Using methanol as a solvent, methylated lignin is created with therefore more hydrophobic behaviour than Kraft lignin.

Finally, it is important to point out that the organosolv lignin is one of the most suitable types of technical lignin for further conversion and valorisation thanks to its sulfur free composition.

In spite of these advantages, organosolv pulping has not been yet widely adopted in a production scale mill due to expensive and difficult recovery of organic solvents, which affects the sustainability of the process^xvii.

1.4 PRECIPITATION OF TECHNICAL LIGNIN

Black liquor is obtained as by-‐product of chemical pulping. It mostly consists of inorganic elements (sodiumand sulphur) as well as a mixture of organic compounds such as dissolved lignin, extractives (volatile and hydrophobic compounds) and hydroxyl acids with acetate ions originating from unwanted peeling reactions and alkaline hydrolysis of carbohydrates.

However, the proportion of the organic and inorganic components in black liquor varies from mill to mill due to the natural variations of the organic constituents in wood species and the cooking conditions of each particular mill.

One important property of black liquor is that it contains several chemical species that act as buffers, such as OH^-‐, CO32-‐ (pKa2 ≈ 10.2), phenolic groups (pKa 9.4–10.8), HS^-‐ (pKa1 ≈ 7) and carboxylic groups (pKa ≈ 4.4) on lignin together with other organic acids^xviii.

Nowadays, the majority of the mills burn the organic content of black liquor in recovery boilers for the production of electricity and steam and very few Kraft mills (2%)^xix make commercial lignin for sale. However, lignin must be seen as an

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added value of chemical pulping and should be therefore recovered for at least two reasons:

• It has very high value as energy source (about 26 MJ/kg)^xx.

This is the main reason making lignin the best alternative to fossil oil based fuels, which, together with beck oils, are conventionally used in kraft mill lime kilns.

• When increasing production of existing chemical pulping mills leads to increased chemical recovery demand in the evaporation plant and recovery boiler. Nowadays, the recovery boiler is limited by the heat load and many mills run their boiler close to its maximum limit. Hence, removing lignin from black liquor, which stands for approximately 35% of the dry solids content and has the highest energy value, prior to combustion could be an effective solution. In this way the overall heating value of black liquor together with the demand on the recovery boiler decreases. This means that, at unchanged availability, the flow of black liquor can be increased and therefore also the pulp production.^xxi

Kraft lignin can be recovered by acidification of the black liquor in high yields.

Being soluble in basic condition, lignin precipitates at low pH due to protonation of phenols first and carboxylic groups afterwards with consequent losing of charges and thus solubility. Lignin starts to precipitate at pH of approximately 11.5, with the yield increasing to approximately 60% of the original lignin content at pH 10. (Figure 6)

Figure 6: Yield of softwood lignin precipitated with CO2 and H2SO4 at 80ºC at various pH. BL1 and

BL2 are different batches of black liquor from the same softwood kraft mill^xxii

The most common way to create acidic environment in black liquor is either adding sulfuric acid or reacting with CO2, which, in turn, creates carbonic acid.

The last method is more sustainable since the remaining liquid, after lignin precipitation, can be used in the chemical recovery system in order to regenerate white liquor without affecting the sodium to sulphur relationship. Conversely, it has been reported that lignin precipitation by the addition of a weak acid as CO2

gives slightly lower yield than precipitation with H2SO4.

Precipitated lignin, wet and dirty, must be afterwards washed and filtered.

However, filtration is not so easy to perform since lignin can dissolve again due to lowering of ion strength and excessive pH gradient between washing liquors.

Both effects form gels that clog the filter matrix and/or the filter media.

Furthermore, the case of more or less complete plugging, consisting in an extremely low flow of washing liquor through the cake, results in very high levels

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of impurities in the lignin.

Lignoboost process has solved this issue by resuspending the initial filter cake in acidified water. Thus, the right values of pH and ionic strength are created so that the precipitate is stable enough to be re filtered and lastly washed^xxiii. The clogging problem can be also avoided by ultrafiltration process since ceramic filters are not affected by temperature and pH variation. Depending on pore sizes, lignin with same molecular weight and more homogeneous composition can be separated in a cheaper way than Lignoboost.

1.5 CONVERSION TO LIQUIDS FUELS

Besides the widespread generation of energy by burning, lignin can be converted by thermochemical processes into liquid fuels for transportation engines.

However, due to the complex structure of lignin and the poor selectivity of conversion, these processes must be tailored by presence or absence of solvents, chemical additives, reaction condition and catalysts. Yields, composition of degradation products and severity factor are important parameters that vary on the process type and the condition applied.

Figure 7: H/C and O/C ratio for several solid fuels (Van Krevelen diagram)^xxiv. The ratio of technical

lignin are showed by the black arrow

Van Krevelen diagram plots the Hydrogen/Carbon molar ratio over Oxygen/Carbon molar ratio. The target of thermochemical process is to reach the composition of ideal hydrocarbon which, having a content of H twice as much as C and no presence of O, lays, in the diagram, at the top of left corner. As can be seen in figure 7, values of O/C in lignin, varying between 0,3 and 0,4 for coniferyl and sinapyl structure respectively, shows a definitely higher content of O than the desired one. Moreover, the presence of aromatic rings in lignin structure lowers the molar ratio of H/C in comparison with aliphatic hydrocarbons to a value of approximately of 1,2. Consequently, during conversion of lignin to liquid fuels, deoxygenation as well as hydrogenation must be carried on. The former may be done thermally since CO2 and H2O are released with heat; the latter needs special chemical condition as reducing agents or hydrogen donors such as formic acid^xxv. Furthermore, depolymerisation of the modified lignin resulting from chemical pulping is a condicio sine qua non for the conversion into liquid fuels.

Bond cleavages at the α aromatic ring bond^xxvi and β-‐O-‐4 are the most probable pathways that occur prior to demethylation of aryl methoxyl groups^xxvii, either

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directly, or after α−β dehydration. Lastly, C-‐C bonds in the “dead ends” such as enol ethers and stillbenes created by alkaline pulping must be cleaved off in order to increase the yield of conversion.

1.6 DISSOLUTION OF LIGNIN

1.6.1 Termodynamical Background

A solution is generally defined as a homogeneous blend consisting of one phase containing more than one component evenly distributed. This might be a polymer and a solvent or two different polymers.

Thermodynamically speaking, to fulfil the requirement of miscibility and to have a spontaneous solution process, the change in free energy ΔGmix upon mixing must be negative as shown in equation 1.

∇𝐺_!"# = ∇𝐻_!"#− 𝑇∇𝑆_!"# < 0

Equation 1: Free enegy Gibbs of mixing

∇𝐺_!"# is the change in Gibbs free energy on mixing, ∇𝐻_!"# is the enthalpy of mixing whereas ∇𝑆_!"# is the entropy of mixing. Lastly, T is the temperature in Kelvin. T∇𝑆_!"#is always positive since, by mixing, there is always an increase in entropy, therefore the sign of ∇𝐺_!"#depends not only on how large ∇𝑆_!"# is, but also on the magnitude of the enthalpy of mixing ∇𝐻_!"#. Obviously, the higher is the mixing temperature the more negative is ∇𝐺_!"#.

However, equation 1 is a necessary but not sufficient condition for solubility since the miscibility of a polymer in solution also depends on the molar fraction of the two components (𝑣!"#$%&', 𝑣!"#$%&'), the number of monomers 𝑥 in the polymer being dissolved and 𝑋_!" which is Flory-‐Huggins parameter interaction between polymer and the specific solvent. (Equation 2)

This last parameter, estimated from solubility parameters is not always concentration independent but, as a rule, an amorphous and linear polymer is soluble if 𝑋_!"is lower than 0,5. Lastly, R stands for the gas constant.

∇𝐺_!"#

𝑁 = 𝑅𝑇𝑋_!"𝑣_!"#$𝑣_!"#+ 𝑅𝑇(𝑣_!"#$𝑙𝑛𝑣_!"#$+ 𝑣_!"#

𝑥 𝑙𝑛𝑣_!"#) < 0

Equation 2: Flory-‐Huggins model for a polymer in solution

In equation 2, which is the Gibbs free energy of mixing for mole, the temperature dependence of a polymer in solution cannot be easily predicted as it happens for small molecules in solution. This is because polymers show both upper and lower critical temperature solution and often an increase in temperature leads to lower solubility. Looking at equation 2 this can be explained by the presence of the factor T (absolute temperature) also in the enthalpic term.

Moreover, a crucial role for the solubility of a polymer is played by its degree of polymerization expressed by the term 𝑥. Differently from low molecular weight substances, the entropy decreases with ^!_! making its contribute very small.

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Similarly, the longer is the polymer’s chain, the lower is the entropic term, which means that the enthalpic term must be either close to zero or negative to obtain

∇!_!"#

! < 0.

1.6.2 Hildebrand Solubility Parameters

The solubility of a mixture of a polymer as lignin in solution can be explained by Hildebrand solubility parameter theory. One of the simplest notion in chemistry

“like dissolves like” or, to be more scientific, the prediction of solubility can be explained qualitatively by the similarity in chemical groups between solvent and polymer whilst quantitatively by Gibbs free energy (equation 2).

As reported by Schuerch^xxviii, the ability of a solvent to dissolve lignin is upon to the cohesive energy density (CED) and molar volume, according to equations 3-‐5.

𝛿 = 𝐶𝐸𝐷 = (𝐸 𝑉)^!^!

Equation 3:δ Hildebrand value calculation

where:

𝐸 = ∆𝑒_!

!

Equation 4: Energy vaporization value

𝑉 = ∆𝑣_!

!

Equation 5: Molar volume value

E and V are obtained by the sum of atomic and functional group contributions for the energy of vaporization at zero pressure and molar volume, respectively, of the molecule.

To put it in other words, δ is a measure of the intermolecular attraction forces in a material provided by the cohesive energy. Denoting FAA the attraction forces between the molecules of material A (for instance lignin), FBB the forces between the molecules of another material (solvent); FAB represents, indeed, the attraction between A and B. A homogenous solution will result if FAB >FAA and FAB

> FBB. According to Schuerch^xxix it is possible to calculate the δ value also for low molecular weight solvents by equation 6:

𝛿 = [ ∇𝐻 − 𝑅𝑇 𝜌

𝑀_! ]^!^!

Equation 6: δ Hildebrand value calculation for solvents

What is more, δ value of an organic solvent water system can be calculated by empirical method described by the following equation:

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𝛿_! = 𝑥_!𝛿_! =

!

𝑥_!𝛿_!+ 𝑥_!𝛿_!+ ⋯

Equation 7: δ value for solvents mixture

Were 𝛿_! is the δ value of water solvent mixture, 𝑥_! is the volume fraction of i^th solvent and 𝛿_! is the δ value of the i^th solvent.

ΔH is the vaporization heat in cal/mol, T is the boiling point in K, ρ the density in g/cm³ and Mw the molecular weight in g/mol.

Finally, a good solvent for a certain solvent must have a Hilderbrand’s solubility parameter as close as possible to that of solute (Equation 8).

𝛿!"#$%&− 𝛿!"#$%&' ≤ 1

Equation 8: Solubility condition by Hildebrand

If different solvents have the same value, the higher is the capacity to form hydrogen bonding, the better is the ability to dissolve lignin. Despite this, the biggest limitation of Hildebrand’s theory regards the pertinence for only apolar and slightly polar systems.^xxx

1.6.3 Hansen Solubility Parameters

The breakthrough of Hansen solubility theory consists in the fact that the total cohesive energy (CED) of a solvent or polymer is split in three major intermolecular interactions:

• 𝛿! (nonpolar) dispersion forces

• 𝛿! (polar) permanent dipole–permanent dipole forces

• 𝛿! (polar) hydrogen bonding

The nonpolar cohesive energy 𝛿_! derives from induced dipole or van der Waals forces, which are also called atomic or dispersion interactions. All molecules contain these attractive forces.

Conversely, 𝛿_! is the polar cohesion energy, which results from inherently molecular interactions and is essentially found in polar (non centrosymmetric) molecules. The molecular dipole moment is the most important parameter accounting for this value. The last third and major cohesive energy source, δ_!, takes into account the hydrogen bonding ability which, according to a modern definition, deals with “attractive interactions between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H and an atom or a group of atoms in the same or a different molecule in which there is evidence of bond formation”^xxxi.

Therefore, as explained before, the energy of vaporization can be split into three individual energies that makes it up (Equation 9):

𝐸 = 𝐸_!+ 𝐸_!+ 𝐸_!

Equation 9: Contributions to energy of vaporization according to Hansen

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Dividing the heat of vaporization by the molar volume is obtained the square value of Hildebrand solubility parameter, which is expressed as the sum of Hansen’s components (Equation 10):

𝐸

𝑉 = 𝐸_! 𝑉 +𝐸_!

𝑉 +𝐸_!

𝑉 = 𝛿^! = 𝛿_!^!+ 𝛿_!^! + 𝛿_!^!

Equation 10: Relationship between Hansen and Hildebrand solubility parameters

According to Hansen theory, any molecular substance can be represented by a point in a tridimensional space, whose orthogonal axes are δ_!, δ_!, δ_!.

Within Hansen space, a polymer is represented not only by their axes, as with solvents, but also by an interaction radius (R0). Thus, a solubility sphere is created whose centre coordinates are δ_!, δ_!, δ_!. All good solvents for a polymer should stay within this sphere whereas the poor ones should be outside.

A useful parameter for comparing two substances is the solubility parameter distance (RA): it is based on respective Hansen components of polymer P and solvent S:

𝑅_!^! = (2𝛿_!,! − 2𝛿_!,!)^!+ (2𝛿_!,!− 𝛿_!,!)^!+ (2𝛿_!,!− 𝛿_!,!)^!

Equation 11: Solubility parameter distance by Hansen

Obviously, so that a solvent could stay inside the Hansen solubility sphere of the polymer, the condicio sine qua non is that the solubility parameter distance RA

must be smaller than R0 or, in other words, their ratio, the Relative Energy Difference (RED), should be less than a unit.

𝑅𝐸𝐷 =𝑅_!

𝑅_! < 1

Equation 12: Condition of solubility by Relative Energy Difference definition

1.6.4 Lignin Solvents

Regarding lignin, it is markedly known the ability to dissolve lignin of basic solvents as ammonia or sodium hydroxide, no wonder if this latter is used in Kraft and soda pulping. However, the presence of non organic elements as sodium, would increase, even more in the case of Kraft lignin, its presence in the composition of the mixture solvent lignin. As a result of this, the ash content resulting after combustion of the lignin fuel will be quite high due to formation of sodium salts. What is more, differently from organosolv process, Kraft lignin has a lower purity due to the presence of organic sulphur: covalent and strong bonds between lignin and sulphur, created in pulping condition.

Likewise, dissolving lignin in ammonia, even if successful in terms of solubility, would be meaningless in a fuel context due to the production of toxic mono nitrogen oxides NO and NO2 well known as NOX

Since lignin is slightly hydrophobic, water is obviously a poor solvent at least at room temperature and when lignin is protonated. However, it has been investigate that low molecular weight lignin can form a homogeneous solution

(17)

with quite acceptable viscosity at 60°C. Temperature of roughly 140°C is needed to obtain a quite similar liquid and homogenous solution from water and unfiltered lignin. Therefore, quite harsh conditions as well as pressurised environment are needed for high molecular weight lignin. Furthermore, it is commonly recognized that if water is used as solvent in fuel production, although costless and always available, will definitely lower the heating value of the mixture.

1.6.4.1 M^ETHANOL

Firstly, methanol must be considered from a biorefinery point of view since is a byproduct of the evaporation plant in a pulp mill during the rise of dry content of black liquor. Secondly, it is relatively cheap.

Moreover, it has been reported that methanol is not only a good solvent for efficient purification and fractionation of lignin but also that repeated MeOH fractionations of softwood Kraft lignin successfully remove the low molecular weight fractions. Remaining high molecular weight lignin shows a Tg and char formation much higher than original lignin. Conversely, the MeOH soluble fractions show a much lower molecular weight and Tg than the reference^xxxii. Last but not least, it is well known that methanol is used as an additional chemical in sulphite, Kraft and soda pulping and, together with ethanol, is the most common solvent used in organosolv pulping.

On the other hand, the toxicity and the low vapour pressure of methanol (Tb =63°C) are undoubtedly seen as cons in a fuel production and use. For instance, pressurized and thick wall pipes are needed in order to handle the high pressure created inside at high temperature.

1.6.4.2 MBO

Another interesting solvent found in literature is MBO, which stands for 2-‐

methyl 3-‐buthene 2-‐ol whose molecular structure is shown in figure 8.

Figure 8: MBO:2-‐methyl 3-‐butene 2-‐ol

As reported by Guay^xxxiii, MBO is a special lignin solvent due to its advantages in comparison to other organic solvent as ethanol or methanol since it is not only biologically produced but also has chemical properties more suited to fuel production and use.

To be more specific, speaking about the production of fuels from black liquor where protonated lignin is dissolved in aqueous solution, if ethanol is used to dissolve lignin, being miscible with water and not very volatile, requires energy intensive distillation to be recovered.

On the other hand, MBO is more volatile and has a density less than water (0,824 g/ml), making its separation less energy intensive or just by gravity. More specifically, the water, being denser than the solvent, naturally separates under the influence of gravity from the MBO lignin mixture greatly reducing the energy