In Brazil, the production of bioethanol is based on sugar cane as feedstock, in the US and Europe, bioethanol is produced primarily from starch (cereal and corn).
The European market in biofuels is expected to increase considerably in the near future. Within the next decade, a wider use of renewable transportation fuels is stipulated by current EU policies. Therefore, a growing interest in the use of alternative feedstocks for ethanol production, including lignocellulosic materials is observed.
Renewable feedstocks, containing lignocelluloses (e.g. wood, hay, corn stover and straw) are also potential materials for producing alcohol. In renewable feedstocks (e.g. old wood, straw, grass), the majority of nongrain carbohydrate is available in the form of cellulose and hemicellulose, which primarily have to be broken down into sugars for subsequently being fermented by yeast cells.
Cellulose, an organic compound, is the main component of the primary cell wall of plants. This polysaccharide consists of a linear chain of hundreds to thousands of glucose units.
Hemicellulose can be any of several heteropolymers, existing in almost all plant cell walls along with cellulose. Cellulose is more resistant to hydrolysis, due to its crystalline and strong organization. However, hemicellulose can be easily hydrolyzed by dilute acid or base. Another method for hydrolysis (saccharification) of cellulosic material is the use of special enzymes – so-called cellulases.
After hydrolysis (saccharification), the obtained sugar solution can be fermented to alcohol with yeast cells.
As the process of biological and enzymatic sugar production from lignocellulosic materials is a naturally very slowly occurring process, it can be sped up through a process, called "Steam Explosion".
The "Steam Explosion" procedure has already been developed in the 1980s. The principle of this technology is, that lignocellulosic material (e.g. straw) is heated up to 180 °C under high pressure. After a period of time, the pressure is suddenly relaxed. This leads to a disruption of the cellulosic filaments, which then become available for enzymatic hydrolysis into basic sugar subunits. Finally, the obtained sugar solution (after hydrolysis) can be fermented into alcohol (bioethanol), just as in conventional procedures for bioethanol production of 1stgeneration.
Figure 1: Schematic demonstration of the 2nd generation bioethanol production process, using lignocellulosic material.
Because of being the most abundant reproducible resource on Earth, lignocellulosic biomass (e.g. corn stover, straw), wood and energy crops are attractive feedstocks (materials) for bioethanol production. Additionally, those feedstocks are of
- high yields
- low costs
- good suitability for low quality land
- low environmental impacts.
Lignocellulosic biomass chemically consists of three basic polymers:
- Cellulose (C6H10O5)x
- Hemicelluloses (e.g. xylan (C5H8O4)m
- Lignin [C9H10O3- (OCH3)0.9−1.7]n (in trunk, foliage and bark)
Cellulose – a homopolysaccharide, which is composed of β-d-glucopyranose units, linked together by (1→4)-glycosidic bonds – consists of approximately 40 – 50 wt% of dry wood and provides wood´s strength.
The cellulose molecules are linear composed, the β-d-glucopyranose chain units are in a chair conformation. Additionally, the substituents HO−2, HO−3, and CH2OH are oriented equatorially. After removal of water from each glucose-molecule (glucose anhydride), long cellulose chains containing 5,000 – 10,000 glucose units are formed (namely cellobiose units).
Figure 2: Chemical structure of cellulose.
Hemicellulose – also known as polyose – accounts for 25 – 35 % of the dry wood mass. Hemicellulose is a composition of numerous polymerized monosaccharides (e.g. glucose, mannose, galactose, xylose, arabinose, 4-O-methyl glucuronic acid and galacturonic acid residues). However, xylose is the predominant included pentose sugar (C5-sugar).
Figure 3: Chemical structure of hemicellulose.
Lignin is a highly branched, substituted, mononuclear aromatic polymer in the cell walls of certain biomass, which is often bound to adjacent cellulose fibers, forming a lignocellulosic complex. This complex – and also lignin alone – are mostly resistant to conversion, using microorganisms, as well as chemical reagents.
Figure 4: Chemical structure of lignin.