Generally, two main routes for the conversion of lignocellulosic biomass can be distin­guished, which can lead to the production of biofuels and other value-added commodity chemicals (Figure 2.1):

The (Bio)Chemical Route: Biochemical conversion makes use of the enzymes of bac­

teria or other microorganisms to break down and convert the biomass. In most cases the microorganisms themselves are used to perform the conversion processes, such as fermen­tation, anaerobic digestion or composting. Sometimes, only the isolated enzymes are used, also known as biocatalysis. Plant monomers can also be further converted chemically.

The Thermochemical Route: Thermochemical conversion includes processes in which

heat and pressure are the dominant mechanisms to convert the biomass into another chemical form.

The bioconversion of lignocellulosic residues to biofuels and biochemicals is more complicated than the bioconversion of sugar or starch-based feedstock. Plant cell walls are naturally resistant to microbial and enzymatic (fungal and bacterial) deconstruction. This recalcitrant nature of the lignocellulosic feedstock (resistance of plant cell walls to deconstruction) therefore poses a significant hurdle in the biochemical route and necessitates

extra pretreatment steps before this lignocellulosic biomass can serve as low-cost feedstock for the production of fuel ethanol and other value-added commodity chemicals. Plant cell walls are comprised of long chains (polymers) of sugars (carbohydrates such as cellulose and hemicellulose), which can be converted into common monomer sugars such as glucose, xylose, and so on, the ideal substrates for chemical, physical, and fermentation processes [2]. However, these polymers are bound together by lignin, which has to be degraded first before the sugar polymers become accessible to hydrolysis by chemical or biological means. Lignin is a complex structure containing aromatic groups linked in a three-dimensional structure that is particularly difficult to biodegrade [3]. Lignins perform an important role in strengthening cell walls by cross-linking polysaccharides, thus providing support to structural elements in the overall plant body. This also helps the plant to resist moisture and biological attack [4]. These same properties, however, constitute one of the drawbacks of using lignocellulosic material in fermentation, as they make lignocellulose resistant to physical, chemical, and biological degradation. The higher the proportion of lignin, the higher the resistance to chemical and enzymatic degradation [5]. Overcoming the recalcitrance of lignocellulosic biomass is a key step in the biochemical production of fuels and chemicals; it is the main goal of the pretreatment.

In the thermochemical conversion route, the recalcitrant nature of the lignocellulosic biomass poses no problems to the technology. However, other limitations of the biomass need to be taken into account in this case: the energy density of biomass is low compared to that of coal, liquid petroleum or petroleum-derived fuels. And most biomass, as received, has a high burden of physically adsorbed moisture, up to 50% by weight [6].