The which currently is developing ethanol production

The pretreatment removes lignin and alters the composition of lignocellulosic materials, which increases the digestibility of polysaccharides (Soderstrom et al 2003).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.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.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, RodrO guez-Chong et al 2004, Saha et al 2005a,b, Schell et al 2003.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%.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).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%.Akhtar et al (2001) pretreated wheat straw, rice straw and SCB with 2% NaOH, with the intention of improving enzymatic hydrolysis.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).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).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).According to Bjerre et al (1996), NaOH pretreatment was highly effective for the straws with relatively low lignin content of 10 18%.Pretreatment with lime increases pH and provides a low-cost alternative for lignin removal (Chang et al 1998).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).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).


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