The production of N-containing bulk chemicals from biomass is at a later stage of devel­opment than oxygenated chemicals. Genetically modified plants may produce elevated levels of 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 and aspartic acid (see Figure 7). Other nitrogen-based chemicals could be produced by using pro­tein waste streams from bioethanol and biodiesel production chains. Aspartic acid is an amino acid that can be produced by reaction of ammonia with fumaric acid, which can be theoretically 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

 

Condensation

 

Caprolactam

 

Lysine

 

3-Aminotelrahydrofuran

 

Aspartic anhydride

 

Fermentation

 

2-amino-1,4-bulanediol

 

Aspartic acid

 

Fermentation

 

( 11UCONC

 

ulutanc acid

 

4-aminonutanol

 

Glutamic acid

 

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

large potential to be converted into a wide spectrum of N-containing chemicals (aspartic anhydride, 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 block for five carbon polymers. The building block and its derivatives have the potential to build similar polymers but with new functionality to derivatives of the petrochemicals derived from maleic anhydride (Werpy and Petersen, 2004). These polymers could include polyesters and polyamides. The major technical hurdles for the development of glutamic acid as a build­ing block include the development of very low-cost fermentation routes. There are currently several fermentation routes for the production of sodium glutamate. One of the major challenges for the development of a low-cost fermentation is to develop an organism that can produce glutamic acid as the free acid.

In general, there is a midterm potential for production of acrylic acid and other N-containing bulk chemicals like acrylonitrile, acrylamide, and caprolactam. The production of N-based chemicals from biomass is expected to become competitive in the market when large quantities of proteins (as a byproduct of biofuel production chains) will be available at affordable prices.

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

FIGURE 2 World average composition of the above ground standing biomass.

feedstock is that, unlike biomass, it is very low in oxygen content. The most important chemical products 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 limited number of bulk chemicals from which a wide spectrum of secondary commodity chemicals are produced. These commodity chemicals have many applications in almost all the sectors of our society as textiles, plastics, resins, food and feed additives, and others. The bulk chemicals from 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 biomass components 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 com­pared to petroleum, biomass generally has less hydrogen, more oxygen, and a lower fraction of carbon. The compositional variety in biomass feedstocks is both an advantage and a
drawback. An advantage is that biorefineries can make more classes of products than can petroleum refineries and can rely on a wider range of raw materials. A drawback is that a relatively larger range of processing technologies is needed, and most of these technologies are still at a precommercial stage (Dale and Kim, 2006). Another difference with petroleum resources concerns the seasonal changes which biomass suppliers have to face, since harvesting is usually not possible throughout the year. A switch from crude oil to biomass may require a change in the capacity of chemical industries, with a requirement to generate the materials and chemicals in a seasonal time frame. Alternatively, biomass may have to be stabilized prior to long-term storage in order to ensure continuous, year-round operation of the biorefinery (Clark et al., 2009).

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

In the previous section, the possibility to replace existing bulk chemicals from fossil refin­ery 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 technologi­cal barriers exist, especially in the production of aromatics. Rather than a head-to-head sub­stitution of petrochemicals with biochemicals, biomass resources can be used to produce platform chemicals which better reflect the initial biomass composition and are easier to be achieved. At the same time, the products must ensure to meet the same functional properties expected by the consumers. The head-to-head substitution of petrochemicals with biochemicals is consistently disadvantaged by the presence of large quantity of oxygen in the biomass feedstock. Future product trees should accommodate the native oxygen content of biomass to reduce the need for deoxygenation. These considerations imply the need for a radical shift from petroleum-based to biomass-based chemical engineering aiming at new value chains with a new range of oxygenated products, novel production routes, and integrated biorefineries built from intensified unit operations which operate at moderate conditions (Marquardt et al., 2010).

1.2 Basic Elemental Conversions in Biomass Processing

In order to be used for production of biofuels and chemicals, biomass needs to be depolymerized and deoxygenated. Deoxygenation is required because the presence of O in biofuels reduces the heat content of molecules and usually gives them highpolarity, which hinders blending with existing fossil fuels (Lange, 2007). Chemical applications may require muchless deoxygenation, since the presence of O often provides valuable physical and chem­ical properties to the product. Biomass feedstocks usually have an amount of carbon which must be retained throughout the value chain, few hydrogen, which must be added, and too much oxygen, which must be rejected along with other undesirable elements (such as nitro­gen and sulfur). Hydrogen is usually added as water (H2O), even if this implicates an addition of extra oxygen, which must be rejected. The addition of hydrogen as H2 is more attractive and efficient (using proper metal catalysts) but underprivileged by the fact that elemental hydrogen is not present in nature and energy must be invested to produce it. Oxygen is rejected either as CO2 or H2O. In both cases, there are elemental issues: in the first case every mole of oxygen removes half a mole of carbon (thus reducing carbon efficiency), while in the second case 1 mol of oxygen removes 2 moles of hydrogen (which, contrarily, needs to be added). It would be most desirable to reject oxygen as O2, but this is not a typical output of any biomass conversion process. The other undesired elements, sulfur and nitrogen, are usually rejected in their oxide forms (SO2 and NO2, respectively), thus contributing to rejec­tion of excess oxygen.

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

Reducing conditions

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

1. PRINCIPLES OF BIOREFINING

products from the decomposition of lignin via high temperature thermal processes (Haveren et al., 2008). This "cracking" of lignin results in a complex mixture of polyhydroxylated and alkylated phenol compounds, where the abundance and type of products are influenced by reaction conditions. Clearly, improved separation techniques for aromatic lignin monomers must be achieved.

Biomass can be converted to chemicals through thermochemical or (bio-)chemical pro­cesses. The most promising thermochemical process is direct gasification of biomass, where the whole feedstock is kept at high temperature (>700 °C) with low oxygen levels to produce syngas, 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 structures synthesized by nature, since they destroy the original biomass structure, which should be rather used to the maximum possible extent (Marquardt et al., 2010). Contrarily, the target of (bio-)chemical processes is to access the rich molecular structure already available in biomass without significant degradation of the basic components. For this purpose, the pretreatment step of lignocellulosic biomass is particularly important, since the three main biomass 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 be selectively depolymerized. Next, the resulting molecular structures need to be isolated and catalytically re-functionalized into target molecules. Such an advanced strategy offers the chance to establish conversion processes with higher carbon efficiency and lower entropic losses compared to conventional thermochemical processes. Although conceptually attrac­tive, its implementation requires the tailoring of the industrial value chains to the nature of the bio-based raw materials. Preserving the natural molecular structures in the raw materials requires a shift from gas-phase reactions at high temperatures, prevalent in petro­leum-based chemical engineering, to liquid-phase reactions at lower temperatures. Likewise, low-temperature separation technologies should be favored over classical distillation if possible. Higher viscosities of the process media and the management of large amounts of water are inevitable side effects offering their own challenges (Marquardt et al., 2010).

Figure 10 shows the selected new bulk chemicals and derivatives which can be produced from biomass. A total of 13 intermediates are identified as potential bulk chemicals from which a wide spectrum of products can be obtained. They are specified according to the num­ber 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.

1.3 Lignin

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

advantage of lignin’s polymer and polyelectrolyte properties. These are primarily applications targeted at dispersants, emulsifiers, binders, and sequestrants. Generally, lignin is used in these applications with little or no modification other than sulfonation or thio hydroxymethylation. These uses mainly represent relatively low value and limited volume growth applications. An economic study shows that when lignin is used for purposes other than power, the overall revenue improvement of a biorefinery concept is between $12 and $35 billion (Holladay et al., 2007). However, as will be shown hereinafter, significant technology developments are required to capture the lignin value benefit.

Besides the immediate opportunities for heat and power production, the specific types of products 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)

Besides its uses as transportation biofuel, ethanol also has interesting applications as bulk chemical from which C2 derivatives can be produced. In particular, ethanol can be converted via dehydration to ethene, one of the bulk petrochemicals, which has a world production of 107 million tons/year. Once produced from bioethanol, ethene can be then used for the pro­duction of other important chemicals like 1,2-dichloroethane (world production of 20 million tons/year), vinyl chloride, butadiene, and others.

6.2.1 C3 Bulk Chemicals

Acetone is an important chemical compound with a market 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 next 5-10 years (Bos et al., 2010). Acetone can be a valuable bulk chemical for the production of propene, whose production from fossil refinery is large (50 million tons/year) due to its wide applications (mainly as polypropylene).

Lactic acid is a promising bulk chemical which can lead to many derivatives (in particular polymers), thanks to two reactive sites, the carboxylic group and the hydroxyl group. The pro­duction of lactic acid from biomass (fermentation of sugars) is already established in the mar­ket, 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. Important derivatives which can be produced from lactic acid are acrylic acid via dehydration (current global market of 2 million tons/year) and 1,2-propanediol by reduction (1.5 million tons/year).

3- HPA has the potential to be a key bulk chemical for deriving both commodity and specialty chemicals. The basic chemistry of 3-HPA is not represented by a current petrochemically derived technology (Werpy and Petersen, 2004). Its production from bio­mass depends on the development of low-cost fermentation routes, since this conversion pathway should in principle have the same yields of that leading to lactic acid. The potential derivatives are similar to those produced from lactic acid, since they have identical reactive sites. In both cases, the development of new catalysts able to directly reduce the carboxylic acid 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 dehy­dration of 3-HPA to acrylic acid and acrylamide will require the development of new acid catalyst systems that afford high selectivity (Werpy and Petersen, 2004).

Gasification produces syngas, a mixture of H2, CO, CH4, and other light gases. Technology to produce methanol 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 further converted to green gasoline via the methanol to gasoline process or to olefins via the methanol to olefins process (Cui et al., 2006; Lee, 1995). Because of the high degree of technology devel­opment in methanol and DME catalysts and processes, this conversion pathway is extremely promising. The main drawback for this technology is the purification of biomass-derived syngas, which is capital intensive, and demonstration that gasification can proceed smoothly with 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 biomass is still at a precommercial stage, due to the expensive purification of syngas streams and the need for catalyst and process improvements able to reduce unwanted side-products such as methane and higher molecular weight products such as waxes. Syngas can also be converted to mixed alcohols (like ethanol and other alcohol chemicals), but this technology has not been commercialized yet. Major challenges concern the catalyst and process improvements needed to increase 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 (and natural gas), there is continuing controversy over gasification economics at the scale needed for the lignocellulosic biorefinery.