Biofuels from Lignocellulosic Biomass - Sustainable Biotechnology: Sources of Renewable Energy - page 36

complex than for corn-based ethanol production. Hydrolysates of lignocellulosic
biomass typically contain significant amounts of pentoses (e.g. xylose and arabi-
nose). These C5 sugars are not readily fermented to ethanol by the commonly-used
yeast ( Saccharomyces cerevisiae ). Efficiently converting both glucose and pentoses
(xylose and arabinose) into ethanol or other biofuels and at reasonably high con-
centrations (8-12%) is another challenge for the fermentation microorganisms.

6.2 Pretreatment Methods

Many pretreatment processes have been tested for the capability to facilitate lig-
nocellulosic biomass component separation and to aid in subsequent access for
the hydrolytic enzymes [25, 26]. The more extensively studied methods are listed
in Table 4, which includes AFEX (ammonia fiber explosion) and ARP (ammonia
recycle percolation) [27, 28], lime [29], organosolv [30], liquid hot water, ionic
liquid [31], dilute acid and steam explosion [32, 33], and enzyme treatment [34].
Additional information on pretreatments is available from Taherzadeh and Karimi
[35] and Jorgensen, Kristensen, and Felby [27].

Ta b l e 4 Features of some pretreatment processes

Digestibility
of cellulose
(%)

Xylose
yield (%) Pretreatment Conditions and solid load Effects on biomass

Dilute acid 0.5-2% H 2 SO 4

140-210 ◦ C, 1-30 min;
≤ 40% solid

Remove and hydrolyze
hemicelluloses,
redistribute lignin,
form furfural and
HMF

75-90 <85

Steam explosion 170-250 ◦ C, 1-20 min;
≥

45-65 90

50% solid

Liquid hot water 190-260 ◦ C min-h, 1-10%
solid

Remove of some
hemicelluloses, lignin

80-98 80-90

AFEX and ARP Anhydrous or 15%
ammonia, 90 or 170 ◦ C;
>50% solid

Remove lignin, partially
hydrolyze
hemicelluloses and
cellulose, decrystalize
cellulose

>90 >80-90

Lime ∼ 0.1 g CaO/g biomass,

55 ◦ C a few weeks,
20-40% solid

Remove lignin >90 >90

Alkaline
peroxide

1-7.5% H 2 O 2 , pH 11.5,
30-85 ◦ C 45 min-24 h;
15% solid

Solublize and oxidize
lignin

>90 >95

Organosolv Methanol, ethanol, acetone

etc. +<0.1 M acid as
catalyst, 160-200 ◦ C, h;
25% solid

Remove of lignin and
some hemicelluloses

>70 90-100