IntroductionAmongthe potential bioenergy resources, lignocellulosic biomass has been identi?edas a cheap and effective feed-stock for the production of biofuels such asbioethanol, biobutanol, and biogas. Lignocellulosic biomass is available about180million tons per year from agriculture and other source. Environmental,long-term economic and national security concerns have motivated research overthe last 25 years into renewable, domestic sources of fuels and chemicals nowmostly derived from petroleum. While recent improvements in single-stagehydrolysis/fermentation may lead to decreased processing costs and increasedfermentation product yields, the limiting step remains the conversion of rawbiomass into a more usable resource.

Lignocellulose, essentially the cell wallmaterial of woody plants, is a porous micro-structured composite mainlyconsisting of cellulose, hemicellulose and lignin. It has been projected thatlignocellulosic biomass has the potential to be a large scale, low-cost andsustainable feedstock for renewable fuels and chemicals. Various types oflignocellulosic biomass such as corn stover, rice straw, wood, are abundantlyavailable as agricultural wastes.

They may be used as renewable feedstocks for?a? biorefinery that? produces? biofuels? and? chemicals. Some?energy crops suchas switch grass may be planted on poor lands that are vacant. Unfortunately,lignocellulosic biomass is very recalcitrant. Without proper pretreatment, it?isdifficult to release fermentable?sugars such as glucose using enzymehydrolysis. The pretreatment step is often the most expensive part of alignocellulosic ethanol process. It is critical to improve this step in orderto make lignocellulosic ethanol economically competitive.

The pretreatment stepis responsible for a signi?cant portion of the energy consumption and cost ofthe biofuel production process and improvements are required. According to theRenewable 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 productionlevel in 2000. This industry forms an infra- structure from which future growthin cellulosic sub- strates utilization may occur.

Demand for fuel ethanol isexpected to increase. In addition to ethanol, forty chemicals and chemicalfeedstocks have been identi?ed as potential products from renewable plantbiomass. The goal behind pretreatment is to break the lignin seal and disruptthe crystalline structure of cellulose. Pre- treatment has been viewed as oneof the most expensive processing steps in cellulosic biomass-to-fermentablesugars conversion with costs as high as 30¢/gallon ethanol produced.

Pretreatment also has great potential for improvement of e?ciency and lowering of costthrough research and development. An ideal pretreatment ful?ls many technicaland economic requisites:(1)Free as much as carbohydrates, preferably asmonomers; (2) provides accessibility to the cellulose by enzymes; (3) minimizessugar degradation; (4) minimizes lignin solubilization; (5) is environmentalfriendly; (6) is low energy demanding; (7) is based on a simple and robusttechnology. 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 (Weilet al., 1997; Zeng et al., 2007), and biological (Keller et al., 2003; Sousa et al., 2009), and are reviewedextensively elsewhere (Mora-Pale et al.,2011; Sousa et al., 2009).

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