Page 56 - Advances in bioenergy (2016)

to improve hydrolysis yields by rendering them more accessible to attack by cellulases.

Thermal pretreatment, such as steam pretreatment/steam explosion, liquid hot water, ammonia
fiber explosion, and CO explosion, involve heating of feedstock to about 150°C–180°C to
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break down hemicellulose and lignin. At higher temperatures (above 250°C), phenolic
compounds are formed which later retard the fermentation process, so care is taken not to
pretreat the feedstock in severe thermal conditions.

Acid pretreatment, such as dilute acid pretreatment, is performed by soaking the material in
dilute acid solution (0.5–1.5%) and then heating to temperatures between 140°C and 200°C for
a certain time (from several minutes up to an hour). Sometimes, concentrated sulfuric acid is
also utilized for feedstock pretreatment. The hemicellulose is hydrolyzed and the main part is
usually obtained as monomer sugars. 7

Alkaline pretreatment uses bases such as calcium hydroxide (lime), potassium hydroxide,
aqueous ammonia, ammonia hydroxide, and sodium hydroxide in combination with hydrogen
peroxide or others and breaks the bonds between lignin and carbohydrates making the latter

more accessible to enzymatic attack. 8
In the organosolv process, an organic or aqueous–organic solvent mixture with addition of an
inorganic acid catalyst (H SO or HCl) is used to break the internal lignin and hemicellulose
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bonds. Solvents that are used are typically methanol, ethanol, acetone, ethylene glycol,
triethylene glycol, and phenol. 9

Biological pretreatments typically utilize wood degrading fungi (soft, brown, and white rot) to
modify the chemical composition of the lignocellulosic feedstock. Generally, soft and brown-
rot fungi primarily degrade the hemicellulose, while imparting minor modifications to lignin.
White-rot fungi can more actively attack the lignin component. 10

Enzymatic Hydrolysis (Second-Stage Hydrolysis)

Enzymatic hydrolysis takes place at mild conditions of temperature and pressure. As a result,
glucose yields are high, chances for the production of fermentation inhibiting compounds are
less, equipment requirements are not significant, and there is a reduction in the total
environmental impact of the whole process. In the following sections, a detailed description of
the function and structure of the cellulases used in the second-stage hydrolysis is provided,
prior to the fermentation toward ethanol production.

ENZYMATIC BREAKDOWN OF CELLULOSE

Cellulose

Cellulose, that is the most abundant biopolymer on earth, is formed by β-(1,4)-linked D-
glucose units, where adjacent D-glucoses are flipped making cellobiose the fundamental
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repeating unit (Figure 2.1). Polymers of cellulose form robust microfiber structures, which