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Correspondence to: Paul. J. De Wild, Energy research Centre of the Netherlands (ECN), Biomass & Energy Efficiency,

Westerduinweg 3, 1755 LE Petten, The Netherlands. E-mail: [email protected]

645 © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

Perspective

Lignin pyrolysis for profi table lignocellulosic biorefi neriesPaul J. De Wild and Wouter J. J. Huijgen, Energy Research Centre of the Netherlands (ECN), Petten, the NetherlandsRichard J.A. Gosselink, Wageningen UR, the Netherlands

Received November 1, 2013; revised and accepted January 14, 2014 View online March 7, 2014 at Wiley Online Library (wileyonlinelibrary.com); DOI: 10.1002/bbb.1474; Biofuels, Bioprod. Bioref. 8:645–657 (2014)

Abstract: Bio-based industries (pulp and paper and biorefi neries) produce > 50 Mt/yr of lignin that results from fractionation of lignocellulosic biomass. Lignin is world’s second biopolymer and a major potential source for production of performance materials and aromatic chemicals. Lignin valoriza-tion is a key-issue for enhanced profi tability of sustainable bio-based industries. Despite a myriad of potential applications for lignin and decades of research, its heterogeneity and recalcitrance still pre-clude commercial value-added applications. Most lignin is utilized for heat and power. Unconventional solutions are needed to better exploit lignin’s potential. Organosolv lignins are especially suitable as feedstock for high-value chemicals. At ECN, a lignin biorefi nery approach (LIBRA) has been developed, involving a dedicated lignin pyrolysis protocol that is robust, continuous, and capable of processing different lignins. Typical product yields are 20% gas, 35% char, and 45% oil. The oil contains approximately 45% oligomeric phenolic substances, 23% monomeric phenols, and 33% water. The future perspective is scale-up of the process to produce larger lignin pyrolysis oil samples for separation, purifi cation, and industrial application tests. Presently, small lignin pyrolysis oil sam-ples are investigated as feedstock for extracting high-value chemicals, as a substitute for phenol in several applications, and as a feedstock for hydrotreating. The biochar is tested as growth enhancer and as substitute for carbon-black in rubber. Regarding the large lignin side streams from (future) bio-based industries, the LIBRA pyrolysis technology has ample potential to increase the profi tability of lignocellulosic biorefi neries provided that for both the liquid product and the solid char value-added applications are developed. © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

Keywords: lignin; lignocellulosic biorefi neries; valorization; pyrolysis; phenolics

Introduction

What is lignin, where does it come from, and what are its current uses?

Lignocellulosic biomass is a composite based on intertwined biopolymers that on a dry basis consists of 35–45% cellulose, 25–30% hemicellulose, and

25–30% lignin.1 Lignin is an amorphous and highly cross-linked polymer consisting of phenolic substructures that originate from three aromatic alcohol precursors (Fig. 1). Th ese substructures are called p-hydroxyphenyl (H, from coumaryl alcohol), guaiacyl (G, from coniferyl alcohol), and syringyl (S, from sinapyl alcohol) moieties. Th ese sub-structures are interconnected by a variety of bonds with approximately 50% of the β-O-4 ether type.1

646 © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 8:645–657 (2014); DOI: 10.1002/bbb

PJ De Wild, WJJ Huijgen, RJA Gosselink Perspective: Lignin pyrolysis for profitable lignocellulosic biorefineries

Lignosulfonates are water-soluble anionic polyelectrolytes that contain a great variety of functional groups such as phenolic hydroxyl, carboxyl, and sulfur-containing groups. In contrast to these sulfur containing lignins, soda lignin from annual crops such as straw and grasses is sulfur-free and contains more minerals (e.g. silica), nitrogen (proteins), hydroxyl units, and carboxyl groups than wood lignins.5 Hydrolysis lignins originate from acid or enzymatic hydrolysis of biomass.5 Compared to ligno-sulfonates and Kraft lignin, enzymatic hydrolysis lignin shows higher reactivity and may be more suited for poly-meric materials.5 Finally, organosolv lignin is produced from lignocellulosic biomass by pulping with a mixture of water and organic solvent(s). Its homogeneity and purity are generally higher than that of other technical lignins. Organosolv pulping produces a high-quality lignin that is free of sulfur and ash and low in residual carbohydrates. It contains a lot of reactive side chains that are available for further chemical reactions such as cross-linking.5

In Europe, a limited number of biorefi neries that use lignocellulosic biomass for the production of second-gen-eration bioethanol are currently in operation (Table 1).

Th e CIMV plant diff ers from most other lignocellulosic biorefi neries, because it is more focused on the production of value-added chemicals instead of bioethanol such as a high-quality lignin as a separate marketable product.6,7

Lignin can be used for various applications but pres-ently residual lignin and lignin containing side-streams from the pulp and paper sector and the abovementioned biorefi neries are mostly deployed for the generation of heat and power. Th is rather low-value application is a serious drawback for the profi tability of lignocellulosic bio-based industries. Th e value of lignin when used in chemical conversions or as performance products

Soft wood lignins mainly contain guaiacyl units while hardwood lignins contain both guaiacyl and syringyl units. In the lignin from herbaceous plants, all three mon-olignols H, G, and S occur in signifi cant amounts.1 Lignin is associated with carbohydrates via covalent bonds, especially with hemicellulose that presumably provides the interconnection between cellulose and lignin.1 Lignin is the world’s second most abundant biopolymer and the major potential renewable source of aromatic chemicals.

Th e pulp and paper sector and (future) lignocellulosic biorefi neries2,3 produce large lignin-containing side streams resulting from the fractionation of lignocellulosic biomass by technologies such as Kraft -, sulfi te- and soda-pulping in the paper sector and organosolv, acid-hydroly-sis, and steam explosion treatments in biorefi neries.4

Th e technical lignins from these processes originate from diff erent sources and separation processes and diff er from each other to a signifi cant extent. When compared with other technical lignins, Kraft lignin contains an increased amount of phenolic hydroxyl groups and sulfur.

Table 1. Examples of European lignocellulosic biorefineries.

Company Feedstock Fractionation process / scale Main product Lignin use

ABNT* (Spain)www.abengoabioenergy.com

Wheat straw Steam explosion/ demo***

Bioethanol Fuel, feed additive

Chempolis (Finland)www.chempolis.com

Agricultural residues

Organosolv/ demo***

Bioethanol Fuel

Inbicon (Denmark)www.inbicon.com


Hydrothermal / demo***

Bioethanol Fuel

Beta Renewables (Italy)www.betarenewables.com/proesa


Steam explosion / commercial*** Bioethanol Fuel

CIMV** (France)www.cimv.fr

Wheat straw Organosolv/ pilot***

Cellulose, C5 sugars, lignin

Performance materials

*ABNT: Abengoa Bioenergy New Technologies, **CIMV: Compagnie Industrielle de la Matière Végétale, ***commercial: >1000 kg/hr feedstock intake, demo: ~1000 kg/hr intake, pilot: ~100 kg/hr.

Figure 1. Lignin monolignols.1

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Perspective: Lignin pyrolysis for profitable lignocellulosic biorefineries PJ De Wild, WJJ Huijgen, RJA Gosselink

© 2014 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 8:645–657 (2014); DOI: 10.1002/bbb

In addition, lignin is a potential renewable source for (aromatic) chemicals.12 Figure 2 presents possible uses for lignin and lignin derivatives as a qualitative function of market size and price illustrating the large variety of potential applications.

Lignin production costs range from 50 €/t for low grade lignin-rich side-streams such as black liquor and non-fer-mentables up to 750 €/t or beyond for high purity lignins.14 Figure 3 presents into some detail current and potential combinations between several lignin types and applica-tions.14 Near horizontal arrows indicate a limited added-value, while a signifi cant added-value is anticipated for the more upward directed arrows. However, suitable technolo-gies that enable value-added applications for lignin are not always available or industry-ready. Th erefore, costs that are associated with the development of these technologies should be counterbalanced with the expected maximum revenue for the specifi c lignin application. In addition, the production volumes of specifi c types of lignin for specifi c value-added applications should match the demand from the market.

Figures 2 and 3 clearly challenge the oft en-heard state-ment that ‘everything can be made from lignin except money’. Th e possible value increments for certain lignin applications show that ample opportunities exist to make

would be much higher. Unconventional solutions are clearly needed to exploit the lignin potential as a indus-trial source for valuable materials. Presently only small amounts of lignin are used for commercial purposes, despite the fact that the current production of lignin eas-ily exceeds 50 Mt/yr8. Almost all lignin that is currently commercially used in non-energy applications is ligno-sulfonate produced from spent sulfi te pulping liquors in cement applications.

The economic perspective for lignin; everything can be made from lignin except money?

Currently, lignin attracts a lot of attention from academia, research institutes, and industry because of its potential as a renewable source for aromatics and probably also because of the increasing market price of aromatic (petro)chemicals. Th e latter is, among others, caused by the increasing production of shale gas in the USA.9 Examples of lignin applications include, but are certainly not limited to, polymer additive (e.g. in polyolefi ns like PP and PE), bitumen, cement, carbon fi bres, agrochemical applications (e.g. soil improver), phenolic resins and adhesives, e.g. for coating applications.2,5,10,11

Figure 2. Potential market applications for lignin and lignin-derived products.13



Th en, detailed physico-chemical (including chemical reactivity) and biochemical (including biochemical reac-tivity) characterization is needed to assess the specifi c character of the lignin.16 Following a detailed characteri-zation, preferred applications that match lignin’s charac-teristics have to be found. Finally, the suitability of lignin for a particular use should be assessed by industrial appli-cation trials.

Because of its relatively higher homogeneity, purity, and reactivity, organosolv lignins are promising and probably ‘fi rst choice’ candidates for direct applications and further processing. Th e next section briefl y describes the organo-solv process and a current European project centered on a pilot-scale development of organosolv fractionation and valorization of the lignin.

Lignin production by organosolv fractionation; the BIOCORE project

Organosolv is a promising technology for the fractionation of lignocellulosic biomass. It is based on the treatment of the biomass with an (aqueous) organic solvent at elevated temperatures.17–19 Commonly used solvents are ethanol, methanol, acetone, and organic acids like acetic acid and formic acid or combinations thereof.19 Organosolv

money out of lignin14 although a real large-scale market breakthrough still has to take place.

What are the major barriers for industrial lignin applications and how can they be tackled?

As a renewable aromatic polymer, lignin is a promising feedstock for the replacement of synthetic (petrochemi-cal) polymers and for the production of fuels, performance materials, and aromatic chemicals that demand high mar-ket prices.12 However, the use of lignin has been generally limited.15 Lignin’s heterogeneity and recalcitrance still pre-clude commercial value-added applications, despite decades of research and several promising small-scale activities.

Due to the diff erences between the technical lignins, each lignin type should be considered individually for pos-sible value-added applications. While (technical) problems with recovery and purifi cation of lignin can be overcome in due time, lignins heterogeneous structure and reactivity is a fundamental challenge for successful industrial appli-cations.5 Cost-eff ective recovery and subsequent purifi ca-tion of lignin from the primary side-streams in pulp and paper factories (black liquor) and biorefi neries is manda-tory for further high-level valorization.

Figure 3. Lignin production and potential lignin-derived product market and value.14

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for such complex by-products have been identifi ed other than combustion (CHP). When compared to other techni-cal lignins, distinguishing features of organosolv lignins are a relatively low molecular weight (500–5000 Da) and a high chemical purity22 (>90%). Organosolv lignin can be used as a functional high-quality additive in inks, var-nishes, and paints23 and it can be used for basically the same applications as Kraft and soda lignins.5 In addition, the high-quality organosolv lignin might also be a prefera-ble candidate for phenolic resins and polyurethane foams. For example, up to 30 wt% of phenol has successfully been replaced by organosolv lignin in a phenol-formaldehyde resin24 and, more recently, successful attempts have been made to replace petrochemical polyols by organosolv lignin in the manufacture of rigid polyurethane foams. Th e foams contained up to 25–30 wt% of organosolv lignin (produced from hardwood).25 Other examples are the use of organosolv lignin in blends with polyethylene oxide,26 as radical scavenger (anti-oxidant)27 and as matrix material in bio-based composites reinforced with sisal

processes delignify lignocellulose, with the organic solvent functioning as lignin extractant while the hemicellulose is depolymerized through acid-catalyzed hydrolysis.20,21

In general, organosolv processing aims to fractionate the lignocellulosic biomass as much as possible into its indi-vidual major fractions in contrast to currently practiced fractionation technologies such as steam explosion and dilute acid hydrolysis. Th ese technologies merely liberate the cellulose fraction for further processing without spe-cifi c recovery of lignin.

Organosolv lignin has a high purity (sulfur-free, limited amounts of residual carbohydrates and m inerals) due to its isolation process. Consequently, the application spectrum for the purer organosolv lignin is broader compared to the more impure lignin-containing residues derived from conventional pre-treatments which are targeted primarily toward the production of cellulose for paper or second-generation bioethanol. Th ese residues are a complex mix-ture of unconverted carbohydrates, lignin, minerals, and process chemicals or microbial residues. No applications

Figure 4. CIMV’s organic acid-based organosolv biorefi nery for lignocellulosic biomass.6



from lignocellulosic biomass by organosolv fractiona-tion, in particular the CIMV organosolv fractionation technology. Figure 4 presents a schematic diagram of the CIMV biorefi nery concept.

BIOCORE aims to show how diff erent lignocellulosic bio-mass can be mobilized and supplied to biorefi neries. Other goals are implementation of organosolv technology for bio-mass fractionation, development of multiproduct manufac-turing pipelines, design of a working biorefi nery concept, and demonstration of sustainability and benefi ts for the whole of society. Figure 5 presents the BIOCORE project.

Lignin pyrolysis for value-added products

Lignin pyrolysis for biorefi neries

When it is desired to produce aromatic chemicals, lignin should be depolymerized by appropriate technology. Since all lignins consist maximal of only three basic building blocks (p-hydroxy-phenyl, guaiacyl, and syringyl), an eff ective depolymerization of diff erent lignins will result in a more homogeneous phenolic product when compared to the original lignins.

fi bers.28 Finally, as can be seen from Fig. 3, organosolv lignin also is a candidate for high-value applications such as carbon fi bers and aromatic (specialty) chemicals. For the latter application the organosolv lignin should in gen-eral be depolymerized via appropriate technology.

Within the European Seventh Framework project BIOCORE (Biocommodity refi nery, 2010–2014) sev-eral value-added applications for organosolv lignins are under investigation, such as phenol replacement in phenol-formaldehyde resins. In addition, valorization of the lignin fraction is studied via the production of functionalized lignins by synthetic chemical methods and via conversion into oligomeric and monomeric phenols by pyrolysis. Th e monomeric phenols produced consist of several high-value phenols that might fi nd applications in the pharmaceutical, food, and fragrance industrial sectors. Th e use of oligomeric phenols as well as functionalized lignins is studied for applications such as manufacture of bio-based polyesters, composite res-ins and polyurethane foams and coatings as well as for the synthesis of bio-based thermosetting resins for the production of wood panels. Th e BIOCORE project is co-ordinated by INRA, France, and focuses on the effi cient production and utilization of the fractions produced

Figure 5. Schematic of the BIOCORE project.

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its versatility and the advantage of a storable and trans-portable bio-oil product that permits economies of scale.37

Th e pyrolysis of lignin is diffi cult because of the challeng-ing processing of the recalcitrant nature and thermoplastic behavior of lignin, resulting in relatively low yields of valu-able products. To enhance the yield of monomeric phenols, several pyrolysis approaches have been pursued31 and from a recent study, it was concluded that typical state-of-the-art reactors for biomass fast pyrolysis cannot eff ectively pyrolyze pure lignins.32 Because of the problems that have been encountered in the fast pyrolysis of lignin, we have developed a novel (catalytic) pyrolysis technology to pro-vide added-value to the whole lignocellulosic biorefi nery by turning a lignin side-stream into saleable products (phenolic bio-oil and biochar) while maintaining the pos-sibility for generating heat and power. 22,35 Th e pyrolysis of organosolv lignin yields combustible gas, char, and a lignin pyrolysis oil from which individual chemicals can be obtained. Th e fi nal market value of the desired products should signifi cantly outweigh the costs that are associated with the pyrolysis of the lignin and subsequent separation and purifi cation steps. In addition, to achieve an environ-mentally sound process, side-streams from the pyrolysis of the lignin should be addressed properly such as waste-water treatment by anaerobic digestion for heat and power.

LIBRA (Lignin biorefi nery approach)

LIBRA is a lignin biorefi nery approach under development at ECN and involving a pyrolysis process that is robust

Lignin can be broken down into monomeric compounds by a variety of routes, such as (alkaline) oxidation or hydrol-ysis, hydrogenolysis, hydrotreatment such as hydrodeoxy-genation, gasifi cation, and pyrolysis methods.29 In general these methods use high pressure and/or expensive reactants and/or catalysts (e.g. in catalytic hydrogenation or conver-sion in supercritical fl uids). For example, until now only the Japanese Noguchi process33 (catalytic hydrogenolysis of desulfonated lignin aft er lignin liquefaction by a treatment with phenol) and the American Lignol process34 (liquid phase catalytic hydroconversion) have been practised as the two main industrial processes to produce chemicals from lignin.16 Both processes were abandoned a long time ago for various reasons such as low yields, high lignin production costs, and high capital and/or operational costs.

Pyrolysis (thermal degradation of biomass in the absence of molecular oxygen) at atmospheric pressure has been acknowledged as one of the economic ways to utilize biomass for biofuels and biochemicals.30 Pyrolysis is a conversion technology that can be applied in a biorefi nery as a primary unit to convert biomass into combustible gas, char and bio-oil as feedstocks for further processes.36 In biorefi neries such as in the abovementioned CIMV process that fi rst fractionate the biomass in hemicel-lulose, cellulose, and lignin, pyrolysis can play a central role as processing unit for one or more of these fractions. Finally, pyrolysis can be used as a more peripheral unit, for example to treat side-streams that originate from other processes within the biorefi nery. It is expected that pyroly-sis will play an important role in biorefi neries because of

Figure 6. Lignin Biorefi nery Approach (LIBRA) for a second-generation bioethanol process.39



produced by the CIMV organosolv process from wheat-straw using a mixture of acetic and formic acid), and coniferous lignin (Kraft lignin from soft wood). All lignins were more than 90 wt% pure (dry basis). Th e lignins were used as 2 mm particles that were prepared from a mix-ture of a natural mineral additive and the lignin. Lignin content in the extrudates amounted to approximately 90 wt%. Fed-batch experiments were conducted with 150 g of feedstock using an atmospheric pressure, 1 kg/h bubbling fl uidized bed test facility at 500°C (Fig. 8), featuring fully automated operation and data acquisition (pressure, tem-perature, permanent gases). Solid feed rate was approxi-mately 10 g/min. Liquid and gaseous products were ana-lyzed using off -line GC-MS for most GC-detectable organ-ics, gravimetry for GC-undetectables, NDIR spectrometry for on-line monitoring of CO, CO2, CH4 and CxHy and Karl-Fischer titration for the water content in the liquid organic samples.

From all three lignins, a two-phase oil was collected. Figure 9 presents the pyrolysis yields of the major prod-uct fractions for the three lignins, the composition of the organic fraction in the oil and a breakdown of the mono-meric phenol fraction. In general, the mass balances are around 95% closed.

Gas, oil and char yields are around 15%, 44%, and 36% respectively (in wt% of the dry lignin intake). Th e organic fraction in the oil is highest for Alcell hardwood lignin (29%) and lowest for the straw-derived Biolignin (22%). Although the diff erences in the yields of the organic oil

and can be operated continuously. LIBRA involves a smart combination of lignin pre-treatment, lignin feeding to the reactor, lignin pyrolysis, recovery of the pyrolysis prod-ucts, and optional upgrading. Basically, LIBRA can be applied to various types of lignin with or without the use of catalysts. As an example how LIBRA can be deployed in a typical second-generation bioethanol biorefi nery, Fig. 6 presents a general scheme of a typical steam-explosion-based biorefi nery in which pyrolysis is incorporated to valorize lignin. Integration of the lignin-pyrolysis section with the other biorefi nery sections enables an optimal use of material and heat.38 Th e black parts in Fig. 6 depict the current state-of-the-art processing of biomass for the production of bioethanol. Lignin is separated by means of a caustic wash, in which pH is increased to dissolve lignin. Th en solid-liquid separation takes place to separate dissolved lignin, which is aft erwards precipitated by acid addition. Th e bio-oil and the biochar are renewable (aro-matic) precursors for a variety of value-added applications (e.g. bio-bitumen, resins, fuel additives, pharmaceuticals, fragrances, carbon-black, growth enhancer).

In addition, Fig. 7 presents a general scheme of a possible future organosolv-based biorefi nery in which pyrolysis is deployed as a separate unit to process the organosolv lignin.22

Th e LIBRA process has been verifi ed at ECN for diff erent types of lignin such as deciduous lignin (Alcell®, prepared via the Alcell aqueous ethanol organosolv process from a mixture of hardwoods), herbaceous lignin (BioligninTM,

Figure 7. Pyrolytic valorization of organosolv lignin in a lignocellulosic biorefi nery.22

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phase are quite small, they might be attributed to diff er-ences in condensation degree between the three lignins, as a result of the diff erent lignin production processes.

From Fig. 9 the yield of monomeric phenols for the lignins is 6–7% (wt% based on the dry lignin intake). Th e diff erence in the organic oil yield (18–27%) is mainly caused by the oligomeric fraction that is highest for Alcell. Methanol and acetic acid are the main detected non-aromatic compounds. Biolignin yields the most acetic acid and Alcell the most methanol. Th e high acetic acid yield for Biolignin (2%) is probably due to residual acetic acid from the organic acid organosolv fractionation and thermal degradation of lignin acetyl groups that were formed during the organosolv fractionation (acetylation of hydroxyl groups). Lignin was partly demethoxylated dur-ing pyrolysis and the high methanol yield (2%) from Alcell is due to the fact that hardwood lignin contains more methoxy groups when compared to soft wood and straw lignin.

Diff erences in the amounts of identifi ed phenols refl ect the variety in the composition of the tested lignin types. For instance, soft wood-derived Kraft lignin consists almost exclusively of guaiacol units, while hardwood Alcell lignin contains a high level of syringol units, too. Th e relatively high yields of alkylphenols and cat-echols are probably due to secondary thermal cracking reactions whereby the primary methoxy-phenols are

demethoxylated. Th is implies that a secondary non-cata-lytic thermal treatment might be deployed to tune the fi nal composition of the lignin pyrolysis oil towards demeth-oxylated phenols.

To test the robustness of the pyrolysis part within LIBRA, continuous pyrolysis experiments were conducted with Biolignin™, the straw-derived organosolv lignin of CIMV. Continuous processing of lignin in a thermochemical con-version process is a challenge because lignin has a strong tendency to melt at elevated temperatures prior to depo-lymerizing in the pyrolysis reactor. As a result of the melt-ing behavior, clogging of the lignin feed-line might occur as well as incomplete conversion. Using a dedicated feeding system in combination with specifi c pyrolysis conditions and product recovery protocols, the Biolignin could suc-cessfully be pyrolyzed. Approximately 1 kg of pure lignin particles (1–2 mm) was continuously pyrolyzed at 500°C, using a solid feed rate of 10 g/min. Figure 10 presents the product yield results of a representative experiment.

When compared to the fed-batch experiments, the con-tinuous processing of the Biolignin produced somewhat more char and somewhat less phenols, probably due to the catalytic cracking eff ect of char build-up in the reac-tor bed. From an ultimate analysis of the dry oil a heating value of 31 MJ/kg was found. In addition, the oil appeared to contain 70% C, 20% O, 8% H, 2% N, and a negligible ash content.

Figure 8. Schematic overview of the lignin pyrolysis set-up.



Valorization possibilities for lignin pyrolysis oil and lignin biochar

Lignin pyrolysis liquid is a complex mixture of many phenolic compounds, some of which are reactive and/or unstable. In addition, the oil can contain substantial amounts of water and carboxylic acids. In general the

liquid – as such – is not directly suitable for transporta-tion fuel applications.30 Dewatering, removal of acids and treatment with hydrogen are techniques to upgrade the lignin pyrolysis liquid to a quality that might eventually be blended with fossil (transportation) fuels.16,30

In our earlier work we have shown that hydrodeoxy-genation (HDO) of deciduous-derived organosolv lignin pyrolysis oil (Alcell®) using a ruthenium on carbon cata-lyst is a viable way to upgrade the oil.35 Th e lignin pyrol-ysis oil could be converted by an HDO reaction with Ru/C as a catalyst into cycloalkanes, alkyl-substituted cyclohexanols, cyclohexanol, and linear alkanes. Th is implies that ruthenium on carbon is a too active catalyst for the hydrogenation of the lignin pyrolysis oil towards low molecular weight phenolics. However, the HDO oil can still be used as a fuel additive in the form of an oxy-genate in engines. Th ese preliminary experiments also indicate the possibility to use a straightforward pyrolysis as a pretreatment for subsequent hydrotreatment such as HDO. It is likely that the partially cracked/depolymer-ized lignin can be hydrotreated under less severe condi-tions, when compared to the hydrotreatment of the origi-nal lignin.

Figure 9. Pyrolysis results for organosolv Biolignin, Kraft lignin and organosolv Alcell lignin.

Figure 10. Continuous pyrolysis results for organosolv Biolignin. Numbers represent weight % based on the dry lignin intake.

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In general, the organic fraction of the lignin pyrolysis oil contains low boiling organic components such as acetic acid and methanol, several monomeric phenols and a rela-tively large portion of oligomeric substances, unidentifi ed lignin degradation fragments. It is obvious that the lignin pyrolysis oil contains quite some specifi c phenolic mono-mers that represent a very high market value if they could be separated from the oil matrix.

For example, guaiacol is a precursor for the production of vanillin, a high value chemical. Syringols, catechols and alkylphenols all have signifi cant market value as specialty chemicals in for example the food, fragrance, and phar-maceutical industries.40 Th e oligomeric fraction (oligophe-nols) also is an interesting feedstock for basically the same applications as the whole unconverted lignin. Diff erence is the reactivity which may be signifi cantly higher than unconverted lignin due a lower molecular weight (typi-cally 1000–3000 g/mol), a lower polydispersity and the presence of more active sites, for example for crosslinking reactions.

An actual challenge is the cost-eff ective separation of the oligomeric fraction from the monomeric fraction. Applicable technologies might be dedicated fractional distillation,41 membrane separation,42 steam distillation,43 reactive solvent extraction,44,45 and the like. An alternative approach is to separate certain phenolic fractions during the pyrolysis itself. An option would be to use a staged condensation approach in which condensable phenolic species are trapped as function of their dew-point. In the LIBRA process this is used to separate most of the reaction water from the organics.

Next to combustible gas and liquid, lignin pyrolysis always yields a signifi cant amount of char, typically 30–50% based on the dry lignin weight. Th is char can be used as a solid fuel to generate process heat. However, for an economically viable process, higher value applications are desirable, for example as soil-improver and/or plant-growth enhancer. Other options are the use of lignin-derived char as precursor for activated carbon, carbon-black or as fi ller material in bitumen.

Conclusion; lignin valorization future perspective

Currently, lignin is attracting a lot of attention because of its potential as a renewable source for aromatics and per-formance materials such as resins and polymers. It seems that there exists a myriad of value-added applications for lignin. However, lignin’s complex chemical structure, heterogeneity, unique reactivity, and recalcitrance still

preclude commercial use, despite several small-scale activ-ities and decades of research. Most lignin is utilized as a cheap fuel for generating process heat. Unconventional solutions are clearly needed to exploit lignins potential as a rich source for valuable materials.

Most of the applications for lignin as such have a limited value. Higher values can be obtained from individual and/or groups of individual aromatic chemicals. Especially pure sulfur-free lignins such as organosolv lignins are suitable as feedstock for the production of high-value chemicals. Th ese can be obtained by thermochemical depolymeriza-tion methods such as pyrolysis. At ECN, a lignin biorefi n-ery approach (LIBRA) has been developed that involves a dedicated pyrolysis protocol for conversion of pure lignins. Th e protocol is robust, can be operated continuously and is capable to process diff erent pure lignins. Typical product yields are 45% oil, 35% char, and 20% gas. Based on the dry lignin weight, the oil contains up to 30% phenolic sub-stances with up to 10% monomeric phenols.

Th e future perspective is the scale-up of the process to produce larger lignin pyrolysis oil samples for separation and purifi cation trials and for industrial application tests. Presently, small lignin pyrolysis oil samples are under investigation as a feedstock for extracting high-value chemicals, as a substitute for phenol in several applica-tions and as a feedstock for hydrotreating. Th e biochar is tested as growth enhancer and as substitute for carbon-black in rubber. Regarding the large lignin waste streams from (future) bio-based industries, the LIBRA pyrolysis technology has ample potential to increase the profi tability of lignocellulosic biorefi neries provided that both for the liquid product and the solid char product value-added applications are developed.

Acknowledgements

Th is work was carried out within the EU FP7 project BIOCORE that is supported by the European Commission through the Seventh Framework Programme for Research and Technical development under contract No. FP7-241566. Additional funding from the Dutch Ministry of Economic Aff airs as part of the Biomass research pro-gram of ECN is acknowledged. Compagnie Industrielle de la Matière Végétale (CIMV) from Pommacle, France, is thanked for providing the Biolignin™ that was used in some of the work described in this paper.

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Dr Paul J. de Wild

Paul de Wild works as a research sci-entist at the Energy Research Centre of the Netherlands (ECN), on ther-mochemical conversion of biomass within the biorefinery approach. Paul obtained a PhD in Biomass Pyrolysis for Chemicals from the University of Groningen, the Netherlands in 2011.

Dr ing. Richard J.A. Gosselink

Dr ing. Richard Gosselink works as a senior scientist at Wageningen UR Food & Biobased Research in the field of biomass characterization, fractiona-tion, and valorization of cellulose and lignin. Richard obtained his PhD on Lignin Valorization at Wageningen Uni-versity, the Netherlands in 2011.

Dr Ir. Wouter J.J. Huijgen

Wouter Huijgen works as a research scientist at the Energy Research Centre of the Netherlands (ECN). His research focuses on biorefinery proc-esses for lignocellulosic biomass and seaweeds. Wouter obtained a PhD in Environmental Sciences from Wage-ningen University in 2007.

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