Introduction

Among
the potential bioenergy resources, lignocellulosic biomass has been identi?ed
as a cheap and effective feed-stock for the production of biofuels such as
bioethanol, biobutanol, and biogas. Lignocellulosic biomass is available about
180million tons per year from agriculture and other source. Environmental,
long-term economic and national security concerns have motivated research over
the last 25 years into renewable, domestic sources of fuels and chemicals now
mostly derived from petroleum. While recent improvements in single-stage
hydrolysis/fermentation may lead to decreased processing costs and increased
fermentation product yields, the limiting step remains the conversion of raw
biomass into a more usable resource. Lignocellulose, essentially the cell wall
material of woody plants, is a porous micro-structured composite mainly
consisting of cellulose, hemicellulose and lignin. It has been projected that
lignocellulosic biomass has the potential to be a large scale, low-cost and
sustainable feedstock for renewable fuels and chemicals. Various types of
lignocellulosic biomass such as corn stover, rice straw, wood, are abundantly
available as agricultural wastes. They may be used as renewable feedstocks for?
a? biorefinery that? produces? biofuels? and? chemicals. Some?energy crops such
as switch grass may be planted on poor lands that are vacant. Unfortunately,
lignocellulosic biomass is very recalcitrant. Without proper pretreatment, it?is
difficult to release fermentable?sugars such as glucose using enzyme
hydrolysis. The pretreatment step is often the most expensive part of a
lignocellulosic ethanol process. It is critical to improve this step in order
to make lignocellulosic ethanol economically competitive. The pretreatment step
is responsible for a signi?cant portion of the energy consumption and cost of
the biofuel production process and improvements are required. According to the
Renewable Fuels Association (2003), the US annual fuel ethanol capacity was 2.9
• 109 US gallons in 2002, an increase of 109 US gallons over the production
level in 2000. This industry forms an infra- structure from which future growth
in cellulosic sub- strates utilization may occur. Demand for fuel ethanol is
expected to increase. In addition to ethanol, forty chemicals and chemical
feedstocks have been identi?ed as potential products from renewable plant
biomass. The goal behind pretreatment is to break the lignin seal and disrupt
the crystalline structure of cellulose. Pre- treatment has been viewed as one
of the most expensive processing steps in cellulosic biomass-to-fermentable
sugars conversion with costs as high as 30¢/gallon ethanol produced.
Pretreatment also has great potential for improvement of e?ciency and lowering of cost
through research and development. An ideal pretreatment ful?ls many technical
and economic requisites:(1)Free as much as carbohydrates, preferably as
monomers; (2) provides accessibility to the cellulose by enzymes; (3) minimizes
sugar degradation; (4) minimizes lignin solubilization; (5) is environmental
friendly; (6) is low energy demanding; (7) is based on a simple and robust
technology. Several pretreatment strategies have been advanced in recent years,
including physical (da Silva et al.,
2010), chemical (Binder and Raines,
2010; Kim and Holtzapple 2005; Li et al., 2008), physicochemical (Kaar et al., 1998; Krishnan et al., 2010;
Lau et al., 2010), hot water (Weil
et al., 1997; Zeng et al., 2007), and biological (Keller et al., 2003; Sousa et al., 2009), and are reviewed
extensively elsewhere (Mora-Pale et al.,
2011; Sousa et al., 2009).

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