Review (Alvira et al 2010). A large number of

Review of Literature :-

Lignocellulosic Biomass—————–

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The widely used lignocellulosic
materials as feedstock to produce ethanol are:

1)  Agricultural residues (sugarcane bagasse
(SCB), rice straw and wheat straw),

2) Energy crops (fast-growing
trees and grasses),

3) Forestry wastes (dead trees
and tree branches),

4) Municipal solid wastes
(household garbage and paper wastes)

5) Industrial wastes (rice mill
wastewater and paper and pulp effluent).

 

Among the feedstock, SCB has
several advantages compared to other materials. SCB is produced as a part of
the sugar production process, so it does not require a separate harvest. It is
also physically ground as part of the juice extraction process (Fox et al 1987). Furthermore, SCB is
cheap, readily available, and has high fermentable sugar value (Martin et al 2002).

PRETREATMENT OF LIGNOCELLULOSIC
BIOMASS:-

Current pretreatment research is
focused on identifying, evaluating, developing and demonstrating promising approaches
that primarily support the subsequent enzymatic hydrolysis of the treated
biomass with lower enzyme dosages and shorter bioconversion times (Alvira et al 2010). A large number of
pretreatment approaches have been investigated on a wide variety of feedstock
types and there are several recent review articles, which provide a general
overview of the field (Hendriks and
Zeeman 2009, Taherzadeh and Karimi, 2008, Yang and Wyman 2008). The purpose
of the pretreatment is to remove lignin and hemicellulose, reduce cellulose
crystallinity, and increase the porosity of the materials. Pretreatment must
meet the following requirements: (1) improve the formation of sugars or the
ability to subsequently form sugars by enzymatic hydrolysis; (2) avoid the
degradation or loss of carbohydrate; (3) avoid the formation of byproducts
inhibitory to the subsequent hydrolysis and fermentation processes; and (4) be
cost effective. Physical, physico-chemical, chemical, and biological processes have
been used for pretreatment of lignocellulosic materials (Sun et al 2002). Pretreatment results must be balanced against their
impact on the cost of the downstream processing steps and the trade-off between
operating costs, capital costs, and biomass costs (Wyman 1995, Palmqvist and Hahn-Hagerdal 2000)

The pretreatment removes lignin
and alters the composition of lignocellulosic materials, which increases the
digestibility of polysaccharides (Soderstrom
et al 2003). The available pretreatment methods are based on biological/
physical/ chemical/physico-chemical principles.

Acid Pretreatment :-

The pretreatment of lignocellulose
with acids at ambient temperature enhances the digestibility of the lignocellulosic
materials. Schell et al (2003)
pretreated corn stover at 20% (w/w) solid concentration over a range of
conditions, encompassing a residence time of 3–12 min, temperature of 165–
195ºC, and H2SO4 concentration of 0.5–1.4% (w/w). The pretreated solids were
tested, using the simultaneous saccharification and fermentation (SSF) process,
to measure the reactivity of their cellulose component to enzymatic digestion
by cellulase. The cellulose conversion obtained in SSF was 80–87% for most of
the digestible pretreated solids. Xu et
al (2009) utilized acetic acid for the pretreatment of raw corn stover. The
highest glucan recovery reported was 97.42%, when 15 g acetic acid/kg of
biomass was employed. The highest xylan recovery of 81.82% was observed, when
10 g acetic acid/kg raw corn stover was used during the pretreatment. Saha et al (2005) pretreated wheat
straw using 0.75% v/v of H2SO4 at 121°C for 1 h, and the reported
saccharification yield was 74%. Cara et
al (2007) reported 76.5%  of
hydrolysis yield from olive tree biomass, when it was pretreated with 1.4% H2SO4
at 210°C. Dilute acid pretreatment is performed by soaking the material in dilute
acid solution and then heating to temperatures between 140°C and 200°C from
several minutes up to an hour based on the biomass. Sulphuric acid below 4 wt%
concentrations has been of most interest because it is inexpensive and
effective. In acid pretreatment, part of hemicellulose is hydrolysed to monomer
sugars. Solubilized hemicelluloses (oligomers) are subjected to hydrolytic
reactions producing monomers, furfural, HMF and other (volatile) products in
acidic environments. The solubilized lignins will condensate and precipitate in
acidic environment, this decreases the enzymatic digestibility (Liu and Wyman 2003). The advantage of
acid pretreatment is the solubilization of hemicellulose and making the
cellulose more easily accessible for the enzymes and the disadvantage is the
formation of volatile degradation products. Strong acid pretreatment for the
ethanol production is not attractive, because there is a risk on the formation
of inhibitors. Dilute acid pretreatment is considered as one of the promising pretreatment
methods; because secondary reactions during the pretreatment can be prohibited
in dilute acid pretreatment. Dilute acid pretreatment along with steam
explosion are the most widely studied methods. The National Renewable Energy
Laboratory (NREL) of US Department of Energy, which currently is developing
ethanol production technologies from biomass, has preferred the dilute acid
pretreatment for the design of its process alternatives (Aden et al 2002, Wooley et al 1999). Yang and Wyman (2004) studied effect of xylan and lignin removal by
acid flow through pretreatment in corn stover and concluded that only little
lignin is dissolved by acid pretreatment but at the same time it increases the
susceptibility to enzymes. Baggase, corn stover, rice straw and hulls, wheat
straw are some of the biomass gave high yield on hydrolysis by dilute acid
pretreatment (Lynd et al 2002, Martinez
et al 2000, RodrÕ´guez-Chong et al
2004, Saha et al 2005a,b, Schell et al 2003. Xiao and Clarkson (1997)
showed that the addition of nitric acid during acid pretreatment has a
tremendous effect on the solubilization of lignin of newspaper. Hamelinck et al (2005) reported the
efficiency of dilute acid hydrolysis has about 35% from biomass to ethanol and
improvements in pretreatment efficiency by process combinations can bring the
ethanol efficiency to 48%. Although dilute acid pretreatment can significantly improve
the cellulose hydrolysis, its cost is higher than steam explosion or AFEX and
neutralization of pH is necessary for the downstream enzymatic hydrolysis or
fermentation process (Sun et al 2002).
Xu et al (2009) studied four different
pretreatments with and without addition of low concentration organic acids on
corn stover at 195°Cfor 15 min and reported that the pretreatment with acetic
and lactic acid yielded the highest glucan recovery of 95.66%. Simultaneous
saccharification and fermentation (SSF) of water-insoluble solids (WIS) showed
that a high ethanol yield of 88.7% of the theoretical based on glucose in the
raw material in acetic acid pretreatment.

 Alkali Pretreatment:-

Alkali pretreatment improves
cellulose hydrolysis, and effectively removes lignin. This process exhibits
lesser hemicellulose and cellulose loss than acid or hydrothermal processes (Carvalheiro et al 2008). Alkali pretreatment
is performed at temperatures ranging from 30 to 121°C, and the treatment time
ranges from seconds to days. This method was reported to cause less sugar
degradation than acid pretreatment, and was more effective on soft wood
residues than hard wood materials (Kumar
et al 2009). Nevertheless, the possible loss of fermentable sugars must be
considered to optimize the operating parameters. Sodium, potassium, calcium and
ammonium hydroxides are suitable for alkaline pretreatment of lignocellulosic
biomass. The addition of NaOH causes swelling which increases the internal
surface of cellulose and decreases the degree of polymerization, which provokes
lignin structure disruption (Taherzadeh
and Karimi 2008). The digestibility of hardwood by NaOH ranges from 14 to
55%, and the reduction in the lignin content varies from 20 to 55% (Kumar et al 2009). Millet et al (1976)
reported that dilute NaOH pretreatment decreased the lignin content from 55 to
20%, and increased the digestibility of hardwood from 14 to 55%. However, no
effect of dilute NaOH pretreatment on softwoods with lignin content greater
than 26% was observed. Silverstein et al
(2007) reported 65.63% lignin reduction and 60.8% cellulose conversion for
cotton stalks, treated with 2% NaOH for 90 min at 121°C and15 psi. Peng et al (2009) evaluated the sequential
treatments of dewaxed SCB with 1 and 3% NaOH aqueous solutions. The results
showed 25.1% hemicellulose yield, which accounts for 74.9% of the original
hemicellulose. Alkaline pretreatment of chopped rice straw with 2% NaOH and 20%
solid loading at 85°C for 1 h decreased the lignin content by 36% (Zhang and Cai 2008). The separated and
fully exposed microfibrils showed an increase in the external surface area and
porosity, and this facilitates the enzymatic hydrolysis. The main effect of
NaOH pretreatment on lignocellulosic biomass is the breakage of the ester bonds
that cross link lignin and xylan (Tarkov and Feist 1969). Akhtar et al (2001) pretreated wheat straw, rice straw and SCB with
2% NaOH, with the intention of improving enzymatic hydrolysis. As a result of
pretreatment, 33%, 25.5% and 35.5% hydrolysis was achieved, respectively. Alkaline
pretreatment is based on the effects of the addition of dilute bases on the
biomass. The effect of alkaline pretreatment depends on the lignin content of
the materials. Alkali pretreatments increase cellulose digestibility and they
are more effective for lignin solubilization, exhibiting minor cellulose and
hemicelluloses solubilization than acid or hydrothermal processes (Carvalheiro et al 2008). The mechanism
of alkaline pretreatment is believed to be saponification of intermolecular
ester bonds cross-linking xylan hemicelluloses. The porosity of the lignocellulosic
materials increases with the removal of crosslinks. Dilute NaOH treatment of
lignocellulosic materials caused swelling, leading to an increase in internal
surface area, a decrease in crystallinity, separation of structural linkages
between lignin and carbohydrates and distruption of the lignin structure (Sun et al 2002). Sodium, potassium,
calcium and ammonium hydroxides are suitable alkaline agents for pretreatment,
among which sodium hydroxide has been studied the most (Kumar et al 2009). Compared with acid pretreatment, alkali
pretreatment appears to be the most effective method in breaking the ester
bonds between lignin, hemicelluloses and cellulose, and avoiding fragmentation
of the hemicelluloses polymers (Gasper
et al 2007). Alkaline pretreatment of chopped rice straw with 2% NaOH with
20% solid loading at 85°C for 1 hr decreased the lignin by 36% (Zhang and Cai 2008). NaOH has been
reported to increase hardwood digestibility from 14% to 55% by reducing lignin content
from 24–55% to 20% (Kumar et al 2009a). According to Bjerre et al (1996), NaOH pretreatment was highly effective for the
straws with relatively low lignin content of 10 – 18%. Alkali extraction can
also cause solubilization, redistribution and condensation of lignin and
modifications in the crystalline state of the cellulose. These effects can
lower or counteract the positive effects of lignin removal and cellulose
swelling (Gregg and Saddler 1996).
The monomeric forms of hemicelluloses are probably easily degradable to other
(volatile) compounds and for example furfural, which leads to losses of
digestible substrate for the ethanol process (Bobleter 1994). Ca(OH)2, also known as lime, has been widely
studied. Lime pretreatment removes amorphous substances such as lignin, which
increases the crystallinity index. Lignin removal increases enzyme
effectiveness by reducing non-productive adsorption sites for enzymes and by
increasing cellulose accessibility (Kim
and Holtzapple 2006). Lime also removes acetyl groups from hemicelluloses
reducing steric hindrance of enzymes and enhancing cellulose digestibility
(Mosier et al 2005). Pretreatment with lime increases pH and provides a low-cost
alternative for lignin removal (Chang et
al 1998). Typical lime loadings are 0.1 g Ca(OH)2/g biomass. A minimum of about
5 g H2O/g biomass is required. Lime pretreatment can be performed at a variety
of temperatures, ranging from 25 to 130 °C, and the corresponding treatment
time ranges from weeks (25 °C) to hours (130 °C). An advantage of using
temperatures below 100 °C is that a pressure vessel is not required, allowing
for the possibility of simply pretreating a pile of biomass without the need
for a vessel. Regardless of the temperature, lime treatment remove approximately
33% of lignin and 100% of acetyl groups. For low-lignin herbaceous materials
(e.g., switchgrass), this level of pretreatment is sufficient to render the
biomass digestible (Chang et al 1997,
Gandi et al 1997, Kaar and Holtzapple 2000). For high-lignin woody
materials (e.g.,poplar wood), additional lignin removal is required and can be
accomplished by adding either oxygen or air to the lime pretreatment system.
The combined action of alkali and oxygen solubilizes significant portions of
the lignin (~80%), which renders even recalcitrant biomass digestible. Oxygen
can be added at high pressures (~15 atm) and high temperatures (~160 °C),
resulting in a relatively rapid reaction (~6 h) (Chang et al., 2001).
Alternatively, 1-atm air can be percolated through a 55°C pile for reaction
times of about 1 month. The action of lime is slower than ammonia and other
more expensive bases, but its low cost and safe handling makes it attractive. Lime
has been proven successfully at temperatures from 85–150°C and for 3–13 h with
corn stover (Kim and Holtzapple 2006).
Oxidative lime pretreatment of poplar (Chang et al 2001) at 150°C for 6 h
removed 77.5% of the lignin from the wood chips and improved the yield of
glucose from enzymatic hydrolysis from 7% (untreated) to 77% (treated) compared
to the untreated and pretreated poplar wood. Pretreatment with lime has lower
cost and less safety requirements compared to NaOH or KOH pretreatments and can
be easily recovered from hydrolysate by reaction with CO2 (Mosier et al 2005). An important aspect of alkali pretreatment is
that the biomass on itself consumes some of the alkali. The residual alkali
concentration after the alkali consumption by the biomass is the alkali concentration
left over for the reaction. Pavlostathis
and Gossett (1985) found during their experiments an alkali consumption of
approximately 3 g NaOH/100 g TS. Lime works remarkably better than sodium
hydroxide.

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