Page 58 - Materials Chemistry, Second Edition

44 N. E. Korres

Fig. 3 Biological processes during anaerobic digestion

3.2 Bioethanol Production from Lignocellulosic Materials

Cellulosic ethanol is chemically identical to first-generation bioethanol (i.e.,
CH 3 CH 2 OH), but it is produced from different feedstock via a more complex
process (cellulose hydrolysis).
In contrast to first-generation bioethanol, which is derived from sugar or starch
produced by food crops (e.g., wheat, maize, sugar beet, sugarcane, sweet sor-
ghum), (Fig. 4), cellulosic ethanol may be produced from agricultural residues,
other lignocellulosic materials, or energy crops (EUBIA, European Biofuels
Technology Platform, undated). Agricultural crop residues are lignocellulosic
biomass (non-grain, non-root portion of agricultural crops) that remains in the field
after harvest (Nelson 2007). The most common residues include the stalks, ears,
and cobs from corn (stover) and straw from wheat crop, sugarcane bagasse, barley
hull, wheat barn, rice husks, and rice washing drainage (Singh et al. 2010). Oilseed
crops, e.g., sunflower or soybeans, produce fewer residues than grain crops and in
most cases are not considered for soil sustainability reasons. Residues from other
crops, e.g., cotton and pruning from orchard and vineyards, may be available but
their use, due to their limited amount in most temperate climates, as lignocellulosic
feedstock for bioethanol production is not feasible (Singh et al. 2010).
As mentioned earlier, lignocellulosic materials are abundant in most countries
and they are generally considered to be more sustainable although they need to be
hydrolyzed into simple sugars prior to distillation. This may be achieved using
either acid or enzyme hydrolysis. More specifically, the production process of
bioethanol from lignocellulosic materials consists of the feedstock pre-treatment,
hydrolysis, fermentation, product separation and distillation and post-treatment of
the liquid fraction (Fig. 5) (Balat et al. 2008; Hendriks and Zeeman 2009).
According to Di Nicola et al. (2011), the classic method used in the fermentation
of the hydrolyzed biomass is the separate hydrolysis and fermentation, in which