First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 209
Solid phase:
-Cellulose
-Hemicellulose
-Some lignin
Liquid phase:
-Lignin
-Some hemicellulose
Liquid phase:
-Hemicellulose
-Some lignin
Solid phase:
-cellulose
-lignin
Alkali
pretreatment
Acid
pretreatment
Figure 6.13Separation of lignocellulosic biomass components in acidic and alkaline pretreatment
conditions.
and hemicellulose is distilled during alkaline pretreatment (Hamelincket al., 2005).
Kyeonget al. (2011) has investigated the effect of sodium hydroxide pretreatment on
canola straw. The results showed that in pretreatment condition with 7.9% sodium
hydroxide concentration, 5.5 h of reaction time, and 68.4∘C of reaction temperature,
the maximum glucose yield, which can be recovered by enzymatic hydrolysis at the
optimum conditions was 95.7%. An increase of the surface area and pore size in
pretreated canola straw by sodium hydroxide pretreatment was observed by scanning
election microscope (Daraei Garmakhanyet al., 2014a,b). Saha and Cotta (2006)
studied the ethanol production from alkaline peroxide pretreated and enzymatically
saccharified wheat straw. The results showed that maximum yield of monomeric sugars
from wheat straw (8.6% w/v) was achieved by alkaline peroxide pretreatment (21.5%
H 2 O 2 v/v, pH 11.5, 35∘C, 24 h) and enzymatic saccharification (45∘C, pH 5, 120 h) by
three commercial enzyme preparations (cellulase,β-glucosidase, and xylanase) and
using 0.16 ml of each enzyme preparation per gram of straw was 672±4mgg−^1 (96.7%
yield). During the pretreatment, no measurable quantities of furfural and hydrox-
ymethyl furfural were produced. The concentration of ethanol (per liter) from alkaline
peroxide pretreated and enzymatic saccharified wheat straw (66 g) by recombinant
Escherichia colistrain FBR5 at pH 6.5 and 37∘C in 48 h was 18.9±0.9 g with a yield of
0.46 g/g of available sugars (0.29 g/g straw). The ethanol concentration (per liter) was
15 ±0.1 g with a yield of 0.23 g/g of straw in the case of SSF by theE. colistrain at pH 6
and 37∘C in 48 h. Alkaline pretreatment disrupted the cellulose–lignin bond (Sun and
Cheng, 2002; Merino and Cherry, 2007).
Alkaline pretreatment has several defects such as
- high cost of chemicals (NaOH has a higher cost than H 2 SO 4 )
- higher concentration of alkaline materials than acids
- problems of recovery
- recovery of chemicals increases the cost of the process
- requires washing of biomass after pretreatment (extra cost of waste water filtering).
