Supercritical fluids (SCFs; conditions where the sol­vent is both above the critical temperature and critical pressure of the chemical) show unique properties that are different from those of either gases or liquids under standard conditions. SCFs have liquidlike densities and gaslike transport properties of diffusivity and viscosity. So, SCFs have the ability to penetrate the crystalline structure of lignocellulosic biomass overcoming the mass transfer limitations encountered with other pre­treatments. Another important advantage is the fact that SCFs have tunable properties such as partition coef­ficients and solubility. Small changes in temperature or pressure close to critical point can result in up to 100-fold changes in solubility, which can simplify sepa­ration. Supercritical carbon dioxide (CO2) with a critical temperature (Tc) of 31 °C and a critical pressure (Pc) of

7.4 MPa, as well as supercritical water has been used for biomass pretreatment. REAC fuels and Renmatix are examples of companies employing this kind of tech­nology (Table 17.6).

Other technologies such as gamma rays, ozonolysis, biological pretreatment (mainly with fungi) are still in an earlier phase and currently face challenges in scaling up and commercialization (Agbor et al., 2011; Alvira et al., 2010).

Summary of Lignocellulosic Biomass Pretreatments

Recently technoeconomic comparisons of some of the different pretreatment technologies have been done using identical feedstocks, and analytical methods to generate comparable data (Wyman et al., 2005, 2011; Eggeman and Elander, 2005). The results indicated that no clear winning pretreatment technology could be identified and that further optimization potential is available in the pretreatment methods. It is also clear that the optimal pretreatment technology is very much substrate dependent further hampering the surfacing of a predominant technology (Table 17.7). The effect of pH on solubilization of the different lignocellulosic

components was nicely illustrated by Garlock et al.

(2011) as depicted in Figure 17.1. Table 17.6 summarizes the effect of various pretreatment methods on the chemi­cal composition and chemical/physical structure of lignocellulosic biomass. It can be concluded that at the moment there is no clearly winning technology also because each subsequent conversion process (e. g. fermentative, chemocatalytic) has its own set of require­ments. Therefore, a wide range of technologies are currently in the progress of being scaled-up. In Table 17.7 an overview of currently worldwide devel­oped demonstration and pilot plant facilities is pre­sented for production of bioethanol and other chemicals.

LIGNOCELLULOSIC
BIOREFINERIES—CLASSIFICATION

Biorefineries can be classified on the basis of a num­ber of their key characteristics. Major feedstocks include perennial grasses, starch crops (e. g. wheat and maize), sugar crops (e. g. beet and cane), lignocel — lulosic crops (e. g. managed forest, short rotation coppice, and switchgrass), lignocellulosic residues (e. g. stover and straw), oil crops (e. g. palm and oilseed rape), aquatic biomass (e. g. algae and seaweeds), and organic residues (e. g. industrial, commercial and post­consumer waste).

These feedstocks can be processed to a range of bio­refinery streams termed platforms. These platforms include single carbon molecules such as biogas and syngas, five — and six-carbon carbohydrates from starch, sucrose or cellulose; a mixed five — and six-carbon carbo-hydrates stream derived from hemicelluloses, lignin, oils (plant-based or algal); organic solutions from grasses; and pyrolytic liquids. These primary plat­forms can be converted to a wide range of marketable products using combinations of thermal, biological and chemical processes (Table 17.8).

Knowledge of a biorefinery’s feedstock, platform and product allows it to be classified in a systematic manner (Cherubini et al., 2009). The classification of biorefineries enables the comparisons of biorefinery systems, im­proves the understanding of global biorefinery develop­ment and allows the identification of technology gaps.

FIGURE 17.1 Cell wall model showing the general effect of pH on solu­bilization of hemicellulose and lignin. (A) Untreated cell wall and (B) cell wall during pretreatment. Cellulose can also be degraded under extremely acidic condi­tions; however, that is not portrayed in this diagram. Source: Designed by Garlock et al., 2011 based on figures from Mosier et al., 2005 and Pedersen and Meyer, 2010. (For color version of this figure, the reader is referred to the online version of this book.)

TABLE 17.8 Biomass-Derived Chemical Building Blocks

Derived Chemical Building cont’d

An overview of current feedstocks, platforms and prod­ucts is given in Figure 17.2.

C6 AND C6/C5 SUGAR PLATFORM

Six-carbon sugar platforms can be accessed from su­crose or through the hydrolysis of starch or cellulose to give glucose. Glucose serves as feedstock for (biological) fermentation processes providing access to a variety of important chemical building blocks. Glucose can also be converted by chemical processing to useful chemical building blocks.

Mixed six — and five-carbon platforms are produced from the hydrolysis of hemicelluloses. The fermentation of these carbohydrate streams can in theory produce the same products as six-carbon sugar streams; however, technical, biological and economic barriers need to be overcome before these opportunities can be exploited. Chemical manipulation of these streams can provide a range of useful molecules.

Fermentation Products

The number of chemical building blocks accessible through fermentation is considerable. Fermentation has been used extensively by the chemical industry to produce a number of products with chemical produc­tion through fermentation starting around the turn of the twentieth century. Around 8 million tons of fermen­tation products are currently produced annually (Bakker et al., 2010).

• Fermentation-derived fine chemicals are largely manufactured from starch and sugar (wheat, corn, sugarcane, etc.)

• The global market for fermentation-derived fine chemicals in 2009 was $16 billion and is forecast to increase to $22 billion by 2013 (Frost and Sullivan,

2011) .

• The market is broken down as follows:

Modern biotechnology is allowing industry to target new and previously abandoned fermentation products and improve the economics of products with commer­cial potential. Coupled with increasing fossil feedstock costs, cost reductions in the production of traditional fermentation products such as ethanol and lactic acid will allow derivative products to capture new or

increased market shares. Improving cost structures will also allow previously abandoned products such as butanol to reenter the market. Many see the future abundant availability of carbohydrates derived from lignocellulosic biomass as the main driver. However, carbohydrate costs are increasing strongly in recent years and its use for nonfood products is under pres­sure even in China. Fermentation also gives the indus­try access to new chemical building blocks previously inaccessible due to cost constraints. The development of cost-effective fermentation processes to succinic, ita — conic and glutamic acids promises the potential for novel chemical development.

Chemical Transformation Products

Six — and five-carbon carbohydrates can undergo selec­tive dehydration, hydrogenation and oxidation reactions to give useful products, such as sorbitol, furfural,
glucaric acid, HMF and levulinic acid. Over 1 million tons of sorbitol are produced per year as a food ingre­dient, personal care ingredient (e. g. toothpaste) and for industrial use (ERRMA, 2011; Vlachos et al., 2010).