generalelectrolysis gasification separation syngas separation hydrolysis pyrolysis, htu organic...

S E C T I O N I

GENERAL

C H A P T E R

Principles of BiorefiningFrancesco Cherubini*, Anders H. Str�mman

Department of Energy and Process Engineering, Norwegian University of Science and Technology

(NTNU), NO-7491 Trondheim, Norway

*Corresponding author: E-mail: francesco.cherubini@ntnu.no

1 INTRODUCTION

1.1 Background

Driven by the increase in industrialization and population, the global demand for energyand material products is steadily growing. Since the world primary sources for energy andchemicals are fossil fuels, this growth raises important issues at environmental, economic,and social levels. Petroleum is exploited at a much faster rate than its natural regenerationthrough the planet C cycle, and the larger part of petroleum and natural gas reserves islocatedwithin a small group of countries. This production and consumption pattern is unsus-tainable because of equity and environmental issues that have far-reaching implications. Inaddition, there is a common increasing perception that the end of the cheap fossil era isaround the corner, and prices for crude oil, transportation fuels, and petroleum-derivedchemicals are likely to steadily increase in the years to come (Bentley et al., 2007; Greene,2004). Climate experts widely agree that emissions of greenhouse gases (GHG), such as car-bon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), arising from fossil fuel combus-tion and land-use change as a result of human activities, are perturbing the Earth’s climate(Forster et al., 2007). Global warming and other issues can be mitigated by shifting from fossilsources to renewable energy resources, which are more evenly distributed than fossilresources and cause less environmental and social concerns.

Among the other energy sources, biomass resources are extremely promising since theyare widespread and cheaply available in most of the countries. Today, biomass constitutesabout 10% of the global primary energy demand, and it is mainly used in inefficient and tra-ditional applications in developing countries (GBEP, 2007; IEO, 2009). Modern uses of bio-mass are restricted to developed countries to produce space heating, power, transportationbiofuels (mainly bioethanol and biodiesel), and few chemical products. Given the varietyof applications for biomass sources, it is extremely important to select the most promising

options under environmental, economic and resource perspectives. Electricity and heat can beprovided by several renewable alternatives (wind, sun, water, biomass, and so on), while bio-mass is very likely to be the only viable alternative to fossil resources for productionof transportation fuels and chemicals. Today, more than 90% of the fossil carbon is used onlyfor its energy content (Marquardt et al., 2010). This pattern is not likely to be followed in thefuture for biomass because of the lower efficiency in converting biomass into energy and thelower energydensityofbiomass than fossils. Stemming fromtheseconsiderations, someauthorsconvincingly argued that electricity should be produced by an increasing share of rene-wable sources, and the use of biomass be restricted to the production of transportation biofuelsand carbon-based chemical products (Agrawal and Singh, 2010; Marquardt et al., 2010).

1.2 The Biorefinery Concept

The sustainable use of bio-based carbon suggests integratedmanufacturing in biorefineriesto selectively transform the variety ofmolecular structures available in biomass into a range ofproducts including transportation biofuels, chemicals, polymers, pharmaceuticals, pulp andpaper, food, or cattle feed (Cherubini, 2009,, 2010; Kamm et al., 2006a).

The biorefinery concept embraces a wide range of technologies able to separate biomassresources (wood, grasses, corn, etc.) into their building blocks (carbohydrates, proteins, fats,etc.) which can be converted to value-added products, biofuels, and chemicals. A biorefineryis a facility (or network of facilities) that integrates biomass conversion processes andequipment to produce transportation biofuels, power, and chemicals from biomass.Figure 1 gives an overview of the possible conversion pathways to produce the desiredenergy and material products from different biomass feedstocks, through jointly appliedtechnological processes (Cherubini et al., 2009). The biorefinery concept is analogous totoday’s petroleum refinery, which produces multiple fuels and products from petroleum.

Biomass is constituted of an enormous variety of plant species with varying morphologyand chemical composition. However, regardless of the phenotype, five main biomasscomponents can be identified worldwide: lipids, starch, cellulose, hemicelluloses, lignin,and proteins. The average biomass available in the world is reported in Figure 2. It clearlyappears that lignocellulosic biomass components such as cellulose, hemicelluloses, and ligninare by far the most abundant. Since they can be even gathered from waste streams (e.g., cropresidues, paper and wood industries), or directly harvested from forests or biomass standsthrough sustainable management, their price tend to be lower than other biomass sourceswhich need a dedicated agricultural plot. For this reason, this chapter has a special focuson the possibility to produce commodity chemicals from lignocellulosic sources, which havethe largest chances for a massive market penetration in the near future.

2 FROM FOSSIL TO BIOMASS RAW MATERIALS

The elemental and chemical structure of biorefinery raw materials differs from that onwhich the current fossil refinery and chemical industry is based. Chemical and elementalcomposition of petroleum is compared with some lignocellulosic biomass feedstocks in

4 1. PRINCIPLES OF BIOREFINING

Grain Straw

Biogas

PlatformMechanical/

Physical process

Chemicalprocess

Biochemicalprocesses

Thermochemicalprocess

Upgrading

Steamreforming

Pressing/desruption

Estherification

Link among biorefinery pathways

Pretreatment

Fiber separation

Fractionation and/or pressing

Chemicalreaction

Methanisation

Fiberseparation

C6 sugars

Water gasshift

Hydrogenation/Upgrading

Extraction

Fermentation

Waterelectrolysis

Gasification

Separation

Syngas

Separation

Hydrolysis

Pyrolysis, HTU

Organic residuesand others Grasses

Sugarcrops

Starchcrops

Lignocellulosiccrops

Lignocellulosicresidues Oil crops

Oil basedresidues

Biomethane

BiodieselElectricityand heatFertilizer Glycerin

Polymers andresins

Feedstock

Materialproducts

Legend

Energy products

Synthetic biofuels(FT, DME…)

Anaerobicdigestion

Organicjuice

Lignin

Upgrading

Pyrolyticliquid

Marinebiomass

Biomaterials

C5 sugars

Bio-H2

Chemicalreaction

Animalfeed

Chemical reaction

Bioethanol

Chemicals andbuilding blocks

Electricity& heat

FIGURE 1 Main conversion routes for production of biofuels, energy, and chemicals from different biomasssources.

FIGURE 2 World average composition of theabove ground standing biomass.

Table 1. Crude oil is a mixture of many different organic hydrocarbon compounds. The firststep in oil refinery consists in the removal of water and impurities, and then distillation of thecrude oil into its various fractions as gasoline, diesel fuel, naphtha, kerosene, lubricating oils,and asphalts is carried out. The relative volumes of the fractions formed depend on theprocessing conditions and the compositionof the crudeoil. Thenaphtha fraction is subsequentlyused as a feedstock for the production of just a few bulk chemicals from which all the majorcommodity chemicals are subsequently derived. An important characteristic of the naphtha

TABLE 1 Average Composition of Some Lignocellulosic Sources and Petroleum

Parameter

Hardwood

(Poplar)

Softwood

(Pine)

(Switchgrass)

Crop Residue

(Corn Stover) Petroleum

LHV MJ/kg 19.5 19.6 17.1 16 42.7

Cellulose % 42.9 44.5 32.0 37.7

Glucan (C6) % 42.9 44.5 32.0 37.7

Hemicellulose % 20.3 21.9 25.2 25.3

Xylan (C5) % 17.0 6.30 21.1 21.6

Arabinan (C5) % 1.20 1.60 2.84 2.42

Galactan (C6) % 0.70 2.56 0.95 0.87

Mannan (C6) % 1.42 11.4 0.30 0.38

Lignin % 26.6 27.7 18.1 18.6

Acids % 3.11 26.7 1.21 3

Extractives % 4.70 2.88 17.5 5.61

Hydrocarbons %

Praffins % – – – – 30

Naphthenes % – – – – 49

Aromatics % – – – – 15

Asphaltic % – – – – 6

Elementalcomposition

C % 49.4 50.3 47.3 47 83-87

H % 5.75 5.98 5.31 5.66 10-14

O % 43.3 42.1 41.6 41.4 0.1-0.5

N % 0.19 0.03 0.51 0.65 0.1-0.2

S % 0.02 0.01 0.1 0.06 0.5-6

Minerals % 2.43 0.32 5.95 10.1 0.1

feedstock is that, unlike biomass, it is very low in oxygen content. Themost important chemicalproducts currently derived from oil and natural gas refinery are shown in Figure 3.

This figure shows that today’s chemical industry processes fossil resources into a limitednumber of bulk chemicals from which a wide spectrum of secondary commodity chemicalsare produced. These commodity chemicals havemany applications in almost all the sectors ofour society as textiles, plastics, resins, food and feed additives, and others. The bulk chemicalsfrom which the majority of commodity chemicals can be produced are ethylene, propylene,batanes/butadiene, and the aromatic benzene, toluene, and xylene (BTX).

The composition of biomass is less homogeneous than petroleum. The share of biomasscomponents in the feedstock can change and the elemental composition is a mixture of C,H, and O (plus other minor components such as N, S, and other mineral compounds). If com-pared to petroleum, biomass generally has less hydrogen, more oxygen, and a lower fractionof carbon. The compositional variety in biomass feedstocks is both an advantage and a

Benzene36.5 Mton/ap-Xylene

35 Mton/a

Toluene10 Mton/a

Butane

Butadiene9 Mton/a

Benzenederivatives

Foampolyuretanes

Naphtha(petroleum)

Vinyl chloride31.1 Mton/a

Acrylonitrile4.5 Mton/a

Propylene glycol1.5 Mton/a

Ethylene glycol6.7 Mton/a

Propene52.8

Terephtalic acid30 Mton/a

Acetone3 Mton/a

Phenol7 Mton/a

Caprolactam2 Mton/a

Adipic acid2.5 Mton/a

Styrene12 Mton/a

Ethene107 Mton/a

Natural gas

Polyestersfibers and films Polystyrene Nylon 6

Nylon 6.6

Solvent

Resins

Polybutadiene,rubbers

Resins,polyester films

Solvent Polypropylene

Polyvinilchloride

Fibers, plastics

FIGURE 3 Schematic flow diagram of petrochemical production from fossils. The world market production isbeneath the chemical name. The most common industrial applications for the specific chemical are even reported.

drawback. An advantage is that biorefineries can make more classes of products than canpetroleum refineries and can rely on a wider range of raw materials. A drawback is that arelatively larger range of processing technologies is needed, and most of these technologiesare still at a precommercial stage (Dale and Kim, 2006). Another difference with petroleumresources concerns the seasonal changes which biomass suppliers have to face, sinceharvesting is usually not possible throughout the year. A switch from crude oil to biomassmay require a change in the capacity of chemical industries, with a requirement to generatethe materials and chemicals in a seasonal time frame. Alternatively, biomass may have to bestabilized prior to long-term storage in order to ensure continuous, year-round operation ofthe biorefinery (Clark et al., 2009).

More difficult is to adapt chemical processes to act on nonhomogeneous substrates, sincethe chemical industry has been built largely on the use of uniform and consistent rawmaterials (Hatti-Kaul, 2010). It is unlikely that this will change, so technologies will needto be developed to precondition biomass feedstocks to make their properties and reactivitypatterns more stable, consistent, and uniform. One concept that may be of value is to separatethe different biomass components early in biorefinery operations, so to make a distinctionbetween those which are subject to energy uses (whose quality can be degraded) and thosedestined to chemical applications (which need high degree of purity and should be subject tomilder process conditions to conserve the original structure).

3 BIOMASS PROCESSING IN BIOREFINERY

3.1 Basic Elemental Conversions in Biomass Processing

In order to be used for production of biofuels and chemicals, biomass needs to bedepolymerized and deoxygenated. Deoxygenation is required because the presence of Oin biofuels reduces the heat content of molecules and usually gives them high polarity, whichhinders blending with existing fossil fuels (Lange, 2007). Chemical applications may requiremuch less deoxygenation, since the presence of O often provides valuable physical and chem-ical properties to the product. Biomass feedstocks usually have an amount of carbon whichmust be retained throughout the value chain, few hydrogen, which must be added, and toomuch oxygen, which must be rejected along with other undesirable elements (such as nitro-gen and sulfur). Hydrogen is usually added aswater (H2O), even if this implicates an additionof extra oxygen, which must be rejected. The addition of hydrogen as H2 is more attractiveand efficient (using proper metal catalysts) but underprivileged by the fact that elementalhydrogen is not present in nature and energy must be invested to produce it. Oxygen isrejected either as CO2 or H2O. In both cases, there are elemental issues: in the first case everymole of oxygen removes half a mole of carbon (thus reducing carbon efficiency), while in thesecond case 1 mol of oxygen removes 2 moles of hydrogen (which, contrarily, needs to beadded). It would be most desirable to reject oxygen as O2, but this is not a typical outputof any biomass conversion process. The other undesired elements, sulfur and nitrogen, areusually rejected in their oxide forms (SO2 and NO2, respectively), thus contributing to rejec-tion of excess oxygen.

3.2 Biomass Conversion Through Thermochemical or (Bio-)Chemical Processes

Biomass can be converted to chemicals through thermochemical or (bio-)chemical pro-cesses. The most promising thermochemical process is direct gasification of biomass, wherethe whole feedstock is kept at high temperature (>700 �C) with low oxygen levels to producesyngas, a mixture of H2, CO, CO2, and CH4 (Gassner and Marechal, 2009; van Vliet et al.,2009). These C-1 building blocks are then reassembled into the desired functional molecules.Other common thermochemical processes are pyrolysis and combustion for heat and power.These thermochemical approaches do not consider the complex molecular structuressynthesized by nature, since they destroy the original biomass structure, which should berather used to the maximum possible extent (Marquardt et al., 2010). Contrarily, the targetof (bio-)chemical processes is to access the rich molecular structure already available inbiomass without significant degradation of the basic components. For this purpose, thepretreatment step of lignocellulosic biomass is particularly important, since the three mainbiomass components must be efficiently separated into independent flows, lignin, cellulose,and hemicellulose, to be further processed (Fernandes et al., 2009; Kaparaju and Felby, 2010;Sun and Cheng, 2002). After pretreatment, these highly functionalized polymers have to beselectively depolymerized. Next, the resulting molecular structures need to be isolated andcatalytically re-functionalized into target molecules. Such an advanced strategy offers thechance to establish conversion processes with higher carbon efficiency and lower entropiclosses compared to conventional thermochemical processes. Although conceptually attrac-tive, its implementation requires the tailoring of the industrial value chains to the natureof the bio-based raw materials. Preserving the natural molecular structures in the rawmaterials requires a shift from gas-phase reactions at high temperatures, prevalent in petro-leum-based chemical engineering, to liquid-phase reactions at lower temperatures. Likewise,low-temperature separation technologies should be favored over classical distillationif possible. Higher viscosities of the process media and the management of large amountsof water are inevitable side effects offering their own challenges (Marquardt et al., 2010).

4 LIGNOCELLULOSIC MOLECULAR COMPONENTSAND THEIR DERIVATIVES

4.1 Lignin

The structure of lignin (see Figure 4) is complex and changes with the type of biomasssource. Lignin is composed of phenylpropenyl (C9) randomly branched units. Thephenylpropenyl building blocks, like guaiacols and syringols, are connected throughcarbon-carbon and carbon-oxygen (ether) bonds. Trifunctionally linked units providenumerous branching sites and alternate ring units (Holladay et al., 2007). Lignin offers a signif-icant opportunity for enhancing the operation of a lignocellulosic biorefinery. Today, lignin isused as a source of heat and power for the processing plant (e.g., pulp and paper industry), butthis approach seems to be shortsighted: lignin’s native structure suggests that it could play acentral role as a new chemical feedstock, particularly in the formation of supramolecularmaterials and aromatic chemicals. All current commercial nonenergy uses of lignin, exceptcombustion and production of synthetic vanillin and dimethylsulfoxide (DMSO), take

94 LIGNOCELLULOSIC MOLECULAR COMPONENTS AND THEIR DERIVATIVES

advantage of lignin’s polymer and polyelectrolyte properties. These are primarilyapplications targeted at dispersants, emulsifiers, binders, and sequestrants. Generally, ligninis used in these applications with little or no modification other than sulfonation or thiohydroxymethylation. These uses mainly represent relatively low value and limited volumegrowth applications. An economic study shows that when lignin is used for purposes otherthan power, the overall revenue improvement of a biorefinery concept is between $12 and$35 billion (Holladay et al., 2007).However, aswill be shownhereinafter, significant technologydevelopments are required to capture the lignin value benefit.

Besides the immediate opportunities for heat and power production, the specific types ofproducts which can be produced from lignin can be grouped in two main categories:

1. Syngas-derived chemicals (near-term opportunity)2. Aromatics (medium/long-term opportunity)

Olignin

CH3H3CO

H3CO O

lignin

FIGURE 4 Chemical structure of softwood lignin.

4.1.1 Syngas-Derived Chemicals

Gasification produces syngas, amixture ofH2, CO, CH4, and other light gases. Technology toproducemethanol or dimethyl ether (DME) from syngas is well established (Li and Jiang, 1999;Peng, 2002; Sai Prasad et al., 2008). These products can be used directly or may be furtherconverted to green gasoline via the methanol to gasoline process or to olefins via the methanolto olefins process (Cui et al., 2006; Lee, 1995). Because of the high degree of technology devel-opment in methanol and DME catalysts and processes, this conversion pathway is extremelypromising. The main drawback for this technology is the purification of biomass-derivedsyngas, which is capital intensive, and demonstration that gasification can proceed smoothlywith biorefinery lignin. Another promising use of syngas is the production of Fischer-Tropsch(FT) fuels (Wang et al., 2009). FT processes are well established but their application to biomassis still at a precommercial stage, due to the expensive purification of syngas streams and theneed for catalyst and process improvements able to reduce unwanted side-products such asmethane and higher molecular weight products such as waxes. Syngas can also be convertedto mixed alcohols (like ethanol and other alcohol chemicals), but this technology has not beencommercializedyet.Major challenges concern the catalyst andprocess improvementsneeded toincrease the selectivity and consumption rate of the catalysts (Holladay et al., 2007). Finally,although syngas production via gasification is a well-developed technology for coal (andnatural gas), there is continuing controversy over gasification economics at the scale neededfor the lignocellulosic biorefinery.

4.1.2 Aromatics

Lignin is the most abundant renewable source which has aromatic units in its structure.As shown in Figure 3, the world demand for aromatics is consistent and increasing overthe years. The possibility to establish a direct and efficient conversion of lignin to high-volume, low-molecular weight aromatic molecules is therefore extremely attractive. How-ever, there are important technological barriers which must be overcome, given the resistantand robust lignin structure.

The basic chemical units of lignin shows very high potential for making BTX chemicals(Figure 5). Technologies able to efficiently depolymerize the polymer by breaking the C–C andC–O bonds are necessary. An aggressive, nonselective, depolymerization would bring to amixture of BTX, phenols, and aliphatic fractions (C1-C3). These chemicals should be suitablefor being directly used by the conventional petrochemical processes which convert the bulkaromatics into nylons, resins, polymers, and others. Development of the required aggressiveand nonselective chemistries is part of the long-term opportunity but is likely to be achievablesooner than highly selective depolymerizations (presented below; Holladay et al., 2007).

A related technological challenge for the production of chemicals from lignin is the elabo-ration of proper separation techniques for themixture intermediates fromwhich the aromaticchemicals are to be isolated (Huang et al., 2008).

4.2 Cellulose and Hemicellulose

Carbohydrates are obtained from lignocellulosic resources afterdepolymerizationof celluloseand hemicelluloses. Glucose (a sugar containing six carbons) is produced via hydrolysis ofcellulose, whereas xylose and mannose are the main products obtained by hydrolysis ofhemicellulose.

114 LIGNOCELLULOSIC MOLECULAR COMPONENTS AND THEIR DERIVATIVES

Carbohydrates have the possibility to be converted to a wide spectrum of products bymeans of biochemical (e.g., fermentation) or chemical transformations. Fermentation ofsugars to ethanol is already established in the market: nowadays more than 90% of the worldethanol production is derived from biomass feedstocks, while the remaining 10% is producedfrom oil or gas refinery (Patel, 2006). Further promising sugar derivatives through fermenta-tion are organic acids like succinic, fumaric, malic, glutamic, aspartic, and others (Werpy andPetersen, 2004). Because of their functional groups, organic acids are extremely useful asstarting materials for the chemical industry and may act as intermediate to production of finechemicals. For many organic acids, the actual market is small, but an economical productionprocess will create new markets by providing new opportunities for the chemical indus-try (Sauer et al., 2008). For example, succinic, fumaric, and malic acid could replace thepetroleum-derived commodity chemical maleic anhydride in its applications. The marketfor maleic anhydride is huge, whereas the current market for the organic acids mentionedis small owing to price limitations. Once a competitive microbial production process forone of these acids is established, the market for that acid is expected to consistently increase.The technological barriers which keep these conversion routes at a precommercial stage con-cern microbial biocatalysts, which need to be improved to simultaneously reduce formationof byproducts and increase yields and selectivity. Issues of scale-up and system integrationare also to be addressed (Werpy and Petersen, 2004).

In addition to microbial conversions, there are several catalytic transformations forcarbohydrates, like oxidations, dehydration, hydrogenations, alkylations, among others,which are industrially feasible. Oxidation leads to valuable intermediates like gluconic acid,which is used for synthesis of pharmaceuticals, food additives, cleaning agents, and others(van Bekkum, 1998). Dehydration of sugars is a promising option for producing importantplatform chemicals like levulinic acid (LA; from glucose) and furfural (from xylose) whichcan be converted into a large portfolio of chemicals havingmany applications in the chemicalindustry and transportation sector (i.e., fuel additives; Bozell et al., 2000; Hayes et al., 2006).The technical barriers for this pathway concern the necessity to increase yields through moreselective dehydration processes, perhaps supported by the development of new catalysts.Catalytic hydrogenation of sugars gives sugar alcohols, such as xylitol and sorbitol. Sorbitolis used as a sweetener as well as an intermediate for synthesis of vitamin C, food additives,

technology

Lignin

Benzene Toluene p-Xylene

FIGURE 5 The production of BTX from lignin requires the development of a new technology.

and C4-C6 polyols for synthesis of alkyds (Blanc et al., 2000). Alkyds are polyesters formedvia esterification between polyhydric alcohols and di- or poly-basic carboxylic acids or theiranhydrides (Maki-Arvela et al., 2007). These reaction pathways have a larger degree of devel-opment than fermentation routes, and some of them are already at a commercial stage. Forinstance, the production of sorbitol is practiced by several companies and has a productionvolume on the order of 0.1 million tons/years (Werpy and Petersen, 2004). These productionsare usually based on batch technology, and the only technical development needed would bethe use of a continuous process.

5 BIOREFINERY TO REPLACE EXISTINGFOSSIL BULK CHEMICALS

Over the lastdecade,prices of fossil fuel feedstockshave increased,whereaspricesof biomassresourceshaveslowlyandsteadilydecreased.This situationmakes thepossibility toproduce theexisting bulk chemicals from biomass rather than fossils an attractive option. In the followingparagraphs, the current state of the art in the production of the bulk chemicals previouslyhighlighted is investigated. The possible reaction pathways are summarized in Figure 6.

5.1 Ethylene

The production of this chemical from biomass sources can be achieved through dehydra-tion of ethanol. This dehydration is favored at high temperatures (300-600 �C) and can be car-ried out over a wide variety of heterogeneous catalysts (Arenamnarta and Trakarnpruk, 2006;Takahara et al., 2005). There are no technological barriers to be faced for the production ofethene from ethanol at a commercial scale; this production is initially most likely to happenin regions with cheap and easy access to bioethanol (Haveren et al., 2008).

5.2 Propylene

Direct production of propene from sugars can be carried out via fermentation (Fukuda et al.,1987). Product yields are very low: the productivity needs to be improved by orders of magni-tude to make this process economically viable (Haveren et al., 2008). An alternative productionpathway consists in the dehydration of 2-propanol, which is produced by reduction of acetone.The latter can be obtained via the acetone, butanol, ethanol (ABE) fermentation process, whichis largely studied in the scientific and industrial community (Ezeji et al., 2007). In addition,propene can be produced from dehydration of 1,2-propanediol (either called propene glycol).This glycol can be effectively produced from reduced sugars as sorbitol and xylitol or lacticacid, and such conversion routes have strong commercial potential (Haveren et al., 2008).

5.3 Butane and Butadiene

Starting from biomass, butadiene potentially can be produced from ethanol: ethanol isfirstly dehydrogenated to acetaldehyde, which is then followed by aldol condensation anddehydration over a catalyst to form butadiene, with an overall yield of 70% (Weissermeland Arpe, 2003). Butadiene can subsequently be converted to butane by reduction.

135 BIOREFINERY TO REPLACE EXISTING FOSSIL BULK CHEMICALS

5.4 Aromatics (BTX)

If the conversion of carbohydrates to oxygen-containing chemicals has been largelyinvestigated, the replacement of bulk aromatic petrochemical compounds has received sofar relatively little attention and limited success. Fermentation of glucose to a number of aro-matic structures has been described in the patent literature. However, these aromaticstructures themselves were neither bulk products nor the desired end product of the fermen-tation process (adipic acid; Haveren et al., 2008).

Utilization of specific terpenes could offer potential for the production of aromaticcompounds such as, for example, substituted phenols or terephthalic acid and fine and

FIGURE 6 Main conversion pathways for producing the existing bulk chemicals in fossil refinery from lignocel-lulosic biomass.

specialty chemicals to be applied in the chemical or pharmaceutical industry (Costantinoet al., 2009). However, current production volumes of terpenes are rather in the range ofhundreds of thousands of tons instead of the million tons needed to substitute a significantamount of aromatics production.

Thanks to its original structure, the most promising feedstock for production of aromaticsfrom biomass is lignin. The ideal conversion pathway would include the possibility to effi-ciently and selectively depolymerize lignin and separate from the resulting mixture thecomponents of interest (e.g., BTX). Prior to be able to isolate aromatics and phenols fromlignin, major technological improvements are needed. Another long term possibility to syn-thesize aromatics from biomass is the Diels-Alder cyclo-addiction of butadiene over acatalyst. Clearly, this route relies on an economic production pathway to butadiene priorbeing industrially taken into consideration.

5.5 N-containing Chemicals

The production of N-containing bulk chemicals from biomass is at a later stage of devel-opment than oxygenated chemicals. Geneticallymodified plants may produce elevated levelsof amino acids, like lysine, which can be converted to caprolactam (a precursor of nylon),while fermentation of glucose can lead to N-containing compounds like glutamic acid andaspartic acid (see Figure 7). Other nitrogen-based chemicals could be produced by using pro-tein waste streams from bioethanol and biodiesel production chains. Aspartic acid is anamino acid that can be produced by reaction of ammonia with fumaric acid, which can betheoretically produced from glucose fermentation. In order to be produced on a large scale,a direct fermentation route from glucose to aspartic acid is fundamental. Aspartic acid has

FIGURE 7 Schematic production of N-based chemicals from glucose.

155 BIOREFINERY TO REPLACE EXISTING FOSSIL BULK CHEMICALS

large potential to be converted into a wide spectrum of N-containing chemicals (asparticanhydride, pyrrolydone, and others).

Fermentation of sugars may even lead to the N-containing chemical glutamic acid.Glutamic acid is a five-carbon amino acid and has the potential to be a novel building blockfor five carbon polymers. The building block and its derivatives have the potential to buildsimilar polymers but with new functionality to derivatives of the petrochemicals derivedfrommaleic anhydride (Werpy and Petersen, 2004). These polymers could include polyestersand polyamides. Themajor technical hurdles for the development of glutamic acid as a build-ing block include the development of very low-cost fermentation routes. There are currentlyseveral fermentation routes for the production of sodium glutamate. One of the majorchallenges for the development of a low-cost fermentation is to develop an organism thatcan produce glutamic acid as the free acid.

Ingeneral, there is amidtermpotential forproductionof acrylic acid andotherN-containingbulk chemicals like acrylonitrile, acrylamide, and caprolactam. The production of N-basedchemicals frombiomass is expected to becomecompetitive in themarketwhen largequantitiesof proteins (as a byproduct of biofuel production chains) will be available at affordable prices.

6 BIOREFINERY TO PRODUCE ALTERNATIVE PRODUCTS

In the previous section, the possibility to replace existing bulk chemicals from fossil refin-ery with the same bulk chemicals from oil refinery has been investigated. Unlike few cases,possible market penetration of biochemicals in the near term is limited and major technologi-cal barriers exist, especially in the production of aromatics. Rather than a head-to-head sub-stitution of petrochemicals with biochemicals, biomass resources can be used to produceplatform chemicals which better reflect the initial biomass composition and are easier tobe achieved. At the same time, the products must ensure to meet the same functionalproperties expected by the consumers. The head-to-head substitution of petrochemicals withbiochemicals is consistently disadvantaged by the presence of large quantity of oxygen inthe biomass feedstock. Future product trees should accommodate the native oxygen contentof biomass to reduce the need for deoxygenation. These considerations imply the need for aradical shift from petroleum-based to biomass-based chemical engineering aiming at newvalue chains with a new range of oxygenated products, novel production routes, andintegrated biorefineries built from intensified unit operations which operate at moderateconditions (Marquardt et al., 2010).

6.1 New Bulk Chemicals from Lignin

Lignin has potential for a very selective depolymerization leading to a wide spectrum ofoxygen-containing aromatics which are difficult to make via existing petrochemical routes(see Figure 8). These products preserve the ligninmonomer structure and can be highly desir-able if produced in reasonable quantity with an economic process. The major barrier of thisconversion concerns the development of a technology that would allow highly selective bondscissions to maintain the monomeric lignin block structures (Holladay et al., 2007). In addi-tion, proper markets and industrial applications for those aromatics which are related to theoriginal lignin building blocks need to be established. Figure 9 shows the potential reaction

Lignin

Newtechnology

Coumaryl alcohol

Sinapyl alcohol

Coumaric acid

Hydroxycinnamic acid Ferulic acid

Coniferyl alcohol

FIGURE 8 Products that preserve lignin original basic structure.

FIGURE 9 Potential reaction products from lignin decomposition at different reaction conditions (Haverenet al., 2008).

products from the decomposition of lignin via high temperature thermal processes (Haverenet al., 2008). This “cracking” of lignin results in a complex mixture of polyhydroxylated andalkylated phenol compounds, where the abundance and type of products are influenced byreaction conditions. Clearly, improved separation techniques for aromatic lignin monomersmust be achieved.

6.2 New Bulk Chemicals from Carbohydrates

Figure 10 shows the selected new bulk chemicals and derivatives which can be producedfrom biomass. A total of 13 intermediates are identified as potential bulk chemicals fromwhich awide spectrum of products can be obtained. They are specified according to the num-ber of C atoms:

• C2: ethanol• C3: acetone, lactic acid, 3-Hydroxypropionic acid (HPA)• C4: succinic acid• C5: furfural, itaconic acid, xylytol, and LA.• C6: sorbitol, HMF, 2,5-Furan dicarboxylic acid (FDCA), and gluconic acid.

FIGURE 10 Scheme of the selected bulk chemicals obtained from carbohydrates and their main derivatives.

6.2.1 C2 Bulk Chemicals

Besides its uses as transportation biofuel, ethanol also has interesting applications as bulkchemical fromwhich C2 derivatives can be produced. In particular, ethanol can be convertedvia dehydration to ethene, one of the bulk petrochemicals, which has a world production of107 million tons/year. Once produced from bioethanol, ethene can be then used for the pro-duction of other important chemicals like 1,2-dichloroethane (world production of 20 milliontons/year), vinyl chloride, butadiene, and others.

Acetone is an important chemical compoundwith amarket volume of 3 million tons/year.As already mentioned, it is possible to produce acetone via the ABE fermentation process.This process is widely studied and is expected to be competitive in the market within the next5-10 years (Bos et al., 2010). Acetone can be a valuable bulk chemical for the production ofpropene, whose production from fossil refinery is large (50 million tons/year) due to its wideapplications (mainly as polypropylene).

Lactic acid is a promising bulk chemical which can lead to many derivatives (in particularpolymers), thanks to two reactive sites, the carboxylic group and the hydroxyl group. The pro-duction of lactic acid from biomass (fermentation of sugars) is already established in the mar-ket, with an annual production around 0.26 million tons and a 10% annual growth (Jem et al.,2010). Major applications are in the food sector, industrial uses, and personal care. Importantderivatives which can be produced from lactic acid are acrylic acid via dehydration (currentglobal market of 2million tons/year) and 1,2-propanediol by reduction (1.5million tons/year).

3-HPA has the potential to be a key bulk chemical for deriving both commodity andspecialty chemicals. The basic chemistry of 3-HPA is not represented by a currentpetrochemically derived technology (Werpy and Petersen, 2004). Its production from bio-mass depends on the development of low-cost fermentation routes, since this conversionpathway should in principle have the same yields of that leading to lactic acid. The potentialderivatives are similar to those produced from lactic acid, since they have identical reactivesites. In both cases, the development of new catalysts able to directly reduce the carboxylicacid groups to alcohols is required. The esterification of the carboxylic group to an ester,and then reduce the ester, is technically easier, but the process is more expensive. The dehy-dration of 3-HPA to acrylic acid and acrylamide will require the development of new acidcatalyst systems that afford high selectivity (Werpy and Petersen, 2004).

In fossil refinery, succinic acid is currently produced from butane/butadiene via maleicacid and has a production volume of 30-50 kilotons/year (Bos et al., 2010). This process isrelatively expensive and the existing market for succinic acid is limited. However, if a moreeconomic production route could be established, it has a potential market of hundredsto thousands tons, thanks to its many possible derivatives (Sauer et al., 2008). Succinic acidcan be efficiently produced from fermentation of sugars, on condition that low-cost fermen-tation routes are established. The basic chemistry of succinic acid is similar to that ofthe petrochemically derived maleic acid/anhydride. These compounds can be convertedvia hydrogenation/reduction to butanediol, tetrahydrofuran, and gamma-butyrolactone.

In the case of succinic acid, the technical challenge is the development of catalysts that wouldnot be affected by impurities in the fermentation. Noteworthy is the possibility to producepyrrolidinones, so addressing a large solvent market (Werpy and Petersen, 2004).

Furfural is the starting material for industrial production of furan compounds and todayit is completely produced from biomass feedstocks rich in C5 sugars. The market volume is0.2-0.3 million ton/year. It is obtained from hydrolysis of C5 sugars along with other degra-dation products. Removal of these impurities is expensive and industrial uses of furfural willbenefit of an optimization of the furfural production process (Patel, 2006). Many valuablechemicals can be derived from furfural (e.g., maleic anhydride, furfuryl alcohol, etc.), andthe chemistry for the conversions is well developed (Kamm et al., 2006b).

Itaconic acid has a chemistry similar to the fossil-derived chemicals maleic acid andmaleicanhydride, which are used as monomers in the production of acrylate-based polymers andthermoset resins in oil refinery (Bos et al., 2010). Itaconic acid is currently produced via fungalfermentation and is used primarily as a specialty monomer. The major applications includethe use as a copolymer with acrylic acid and in styrene-butadiene systems. The major techni-cal hurdles for the development of itaconic acid as a bulk chemical include the developmentof very low-cost fermentation routes. The primary elements of improved fermentationinclude increasing the fermentation rate, improving the final titer, and potentially increasingthe yield from sugar. Besides important chemical derivatives, itaconic acid can also undergopolymerization, but the properties of polyitaconic polymers need to be ascertained in orderto evaluate its use as a polymer (Werpy and Petersen, 2004).

Xylitol is commercially produced from hydrogenation of xylose, the most abundant C5sugar in hemicellulose. At the moment, there is limited commercial production of xylitol,but once a cheaper production route is established a large potential for production of ethyleneglycol and 1,2-propanediol via hydrogenation is expected.

Another promising C5 bulk chemical is LA. It is produced from dehydration by means ofacid treatment of C6 sugars like glucose and fructose. LA is one of the most important build-ing blocks available from carbohydrates and has attracted interest from a number of largechemical industry firms: it has frequently been suggested as a starting material for a widenumber of compounds (Bozell et al., 2000; Hayes et al., 2006; Kamm et al., 2006b; Werpyand Petersen, 2004). The technical barriers for this option include improvement of the processfor LA production itself, even if the LA yield is already at 70% (Hayes et al., 2006). The familyof chemical compounds available from LA is quite broad, and addresses a number of largevolume chemical markets. Besides chemicals, LA shows promising efficiency in the conver-sion to methyltetrahydrofuran and ethyl levulinate, two fuel additives which can be blendedup to 20% with gasoline and diesel (without requiring any modification of the engine).

Sorbitol is produced by catalytic hydrogenation of glucose on a large industrial scale(1.1 million tons/year; Patel, 2006). Besides the food industry, it can be used for productionof surfactants and polyurethanes. Sorbitol has potential for the production of isosorbide atlow costs (if higher yields are achieved through optimization of process conditions anddehydration catalysts). Isosorbide is a very effective monomer for raising the glass transition

temperature of polymers. The major applications are as a copolymer with PET for the use inbottle production. Hydrogenolysis of sorbitol leads to glycols, while direct polymerizationforms polyesters for the resin market, whose characteristics need to be properly tested.

2,5-FDCA is formed by an oxidative dehydration of glucose, where side reactions still needto be minimized. FDCA has a large potential as a replacement for terephthalic acid, a widelyused component in various polyesters, such as polyethylene terephthalate (PET) andpolybutyleneterephthalate (PBT). This bulk chemical has high versatility in production ofderivatives through simple chemical reactions: selective reduction leads to partially or fullyhydrogenated products (with applications as new polyesters), combination with diaminesproduces new nylons, etc. (Werpy and Petersen, 2004). Like the other sugar-derived products,the primary technical barriers to production and use of FDCA include development of effec-tive and selective dehydration processes for sugars.

Glucaric acid is the product of catalytic oxidation (with nitric acid, which should bereplaced by oxygen) of glucose. Glucaric acid can serve as starting point for the productionof a wide range of products with applicability in high volume markets, like new nylons (e.g.,polyhydroxypolyamides) or new surfactants.

7 NEXT RESEARCH OUTLOOK

The success of the chemical industry in biomass conversion to chemical products is highlydependent on the development of new catalysts. Since the original molecular structure of bio-mass components is supposed to be preserved, the focus of catalysis researchwill have to shiftfrom building functional structures out of simple building blocks to the re-functionalizationof complex molecular structures (Marquardt et al., 2010). A crucial role is played by the nextresearch achievements for basic chemical reactions like dehydration, condensation,hydrogenation, and so on, which require high selectivity to be implemented at commercialscale. Enzymatic or whole-cell biocatalysts are often high-performance alternatives resultingin high selectivity and yield (Stephanopoulos, 2007). Hybrid catalysts, combining enzymeswith chemocatalysts in a complex molecular or nanoparticulate structure, constitute evenmore sophisticated options (Marquardt et al., 2010). In particular, the specific developmentsneeded in the main conversion reactions are:

• Hydrogenation/reduction: this reaction is generally used to add hydrogen, e.g. to an acidfunctional group to form alcohols. Research developments should ensure the possibility tooperate at milder conditions (pressure, temperature, etc.) giving high selectivity, by meansof the improvements in catalyst performances. Catalysts should also improve theirtolerance to inhibitory compounds and lifetime.

• Oxidation: this reaction oxidizes carbon and converts alcohols into acid functional groups.In future biorefineries, mineral oxidants like sulfuric acid and nitric acid should bereplaced by air, molecular oxygen, dilute hydrogen peroxide, and others. Tolerance toinhibitory components of biomass processing streams should also be enhanced.

• Dehydration: this reaction removes oxygen from the substrate and it is fundamental forbiomass processing. It requires improvements in the selectivity, needed to avoid sidereactions. New heterogeneous catalysts (solid acid catalysts) are preferred over liquidcatalysts.

217 NEXT RESEARCH OUTLOOK

• Fermentation: fermentation processes convert sugars into valuable products. In general, animprovement of microbial biocatalysts to reduce acetic acid coproducts and increaseyields is needed. Lower costs to recover the products are necessary to scale-up.

• Polymerization: it is usually done through esterification to produce innovative polymers,whose applications need to be tested. Issues of selectivity and control of molecular weightand properties are still open.

The combination of new catalysts and new substrates offers innovative and largely unex-plored opportunities to establish novel production pathways and novel innovative productswith particular properties which must be still explored (Vennestr�m et al., 2010). The flexi-bility in tailoring the value chain, from feedstocks to the desired products (or vice versa), com-bined with the several possible uses of side streams, may lead to different options. Theseoptions must be systematically evaluated and screened to identify those with the bestperformances, including carbon efficiency, energy consumption, environmental impacts,and production cost. Ideally, such an evaluation should precede laboratory experiments incatalysis and production processes, in order to specifically focus research activities on themost promising alternatives.

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