BIOETHANOL PRODUCTION FROM RESIDUAL LIGNOCELLULOSIC MATERIALS: A REVIEW - PART 1

Abstract

Lignocellulosic materials (LCM) are produced in large quantities and without clear application and their use as raw material for bioethanol production shows economic and ecologic benefits. LCM are composed mainly of three polymers: cellulose made up of glucose units, hemicellulose made up of several sugars (as xylose or arabinose), and lignin made up of phenylpropane units, interconnected in a strong structure. Pretreatment is an important step for bioethanol production from LCM, causing the solubilisation of hemicellulosic fraction (leading to the recovery of hemicellulose-derived saccharides) in order to obtain a solid phase enriched in cellulose and more susceptible to enzymatic attack. This study provides a comparative data regarding the chemical composition of various LCM used for bioethanol production, as well as different pretreatment technologies for improving the enzymatic hydrolysis of LCM. [PUBLICATION ABSTRACT]

Headnote

Lignocellulosic materials (LCM) are produced in large quantities and without clear application and their use as raw material for bioethanol production shows economic and ecologic benefits. LCM are composed mainly of three polymers: cellulose made up of glucose units, hemicellulose made up of several sugars (as xylose or arabinose), and lignin made up of phenylpropane units, interconnected in a strong structure. Pretreatment is an important step for bioethanol production from LCM, causing the solubilisation of hemicellulosic fraction (leading to the recovery of hemicellulose-derived saccharides) in order to obtain a solid phase enriched in cellulose and more susceptible to enzymatic attack. This study provides a comparative data regarding the chemical composition of various LCM used for bioethanol production, as well as different pretreatment technologies for improving the enzymatic hydrolysis of LCM.

Keywords: bioethanol, lignocellulosic material, pretreatment

Introduction

The control of European energy consumption and the increased use of energy from renewable sources are important components of measures needed to reduce greenhouse gas emission and to meet the Kyoto Protocol of the United Nation Framework Convention on Climate Change. Improving technological process, the use of energy efficiency technologies and the use of energy from renewable sources in transportation are the most effective measures to reduce dependency from fossil fuels. The European Council of March 2007 established a mandatory target of a 20% share of energy from renewable sources in energy consumption by 2020 and a mandatory 10% minimum target for the share of biofuels in transport petrol and diesel consumption by 2020. Member States may encourage the use of biofuels made from wastes, residues, non-food cellulosic material and lignocellulosic material (LCM) (EU-Directive 2009/28/EC).

LCM can be classified into six main groups: crop residues (corn stover, corn cobs, rice husks, barley husks, rye straw, oat straw, rice straw, wheat straw, corn stalks, cotton stalks, soya stalks, sugarcane bagasse), hardwoods (eucalyptus, acacia, poplar, black locust), softwoods (salix, spruce, pine), cellulose wastes (newspaper, waste office paper, recycled paper sludge), herbaceous biomass (alfalfa stems, switch grass), and municipal solid wastes (MSW) (Sánchez, & Cardona, 2008).

The joint U.S. Department of Energy and U.S. Department of Agriculture billion ton study (Biomass as Feedstock, 2005) found that in the United States about 368 million dry tons of sustainably removable biomass could be produced on forestlands and about 998 million dry tons could come from agricultural lands. Forestlands include 52 million dry tons of fuel wood harvested from forests, 145 million dry tons of residues from wood processing, 47 million dry tons of urban wood residues, 64 million dry tons of residues from logging, and 60 million dry tons of biomass from fuel treatment operations. Agricultural lands in the United States could produce nearly 1 billion dry tons of biomass annually, including 428 million dry tons of primary crop residues, 377 million dry tons of perennial energy crops and the rest of biomass from agricultural lands (Foust et al. 2006).

Based on the Pan-European natural resources database and Geographic Information System for European Union, land productivity potentials (Fischer et al., 2010a) and land use scenarios (Fischer et al., 2010b) were elaborated resulting in 42.7 million hectares of energy crops, such as herbaceous and woody lignocellulosic feedstock, and 166 million dry tons of agricultural residues available for biofuel production in EU27, Switzerland and Norway by 2030 (Gnansounou, 2010). In Romania and Bulgaria, the analysis suggests that almost 8 million hectares of arable land for bioenergy feedstock production could be released by 2030 without influencing the food production (Fischer et al., 2010b).

Physico-chemical and chemical processes have been used for pretreatment of lignocellulosic materials. Physico-chemical methods include: steam explosion (autohydrolysis), liquid hot water (LHW), CO2 explosion and AFEX (Ammonia fiber/ freeze explosion). Chemical methods are based on: acid pretreatment, alkaline pretreatment, wet oxidation, ozonolysis and organosolv.

This review is focused on pretreatment methods for bioethanol production from lignocellulosic materials because they represent the largest amount of residues remaining after harvesting, with great potential, both economic and industrial, in order to produce bioethanol as an alternative fuel source.

Lignocellulosic material

LCM structure consists of three basic polymers: cellulose, hemicellulose and lignin found in stalks, stems and leaves (Demirbas, 2005; Arin & Demirbas, 2004). Cellulose is a homopolysaccharide composed of β-D-glucopyranose units linked together by (1 [arrow right] 4)-glycosidic bonds. Glucose anhydride, formed by removing water from glucose, is polymerized into long chains of cellulose containing 5000-10000 glucose units. The basic unit of the cellulose polymer consists of two anhydride glucose units called cellobiose units. Hemicellulose is a mixture of various polymerized monosaccharide such as glucose, mannose, galactose, xylose, arabinose, 4-O-methyl glucuronic acid and galacturonic acid (Mohan et al., 2006). Lignin is a mononuclear aromatic polymer located in the cell walls of biomass and is connected to cellulose fibers (Yaman, 2004; Balat et al., 2008).

LCM can vary in composition and moisture content according to: region, fertilization practices, harvesting, storage and storage time (Table 1).

A recent study of the National Renewable Energy Laboratory (NREL) together with the U.S. Department of Energy (DOE) presents the composition of corn stover from a variety of commercial hybrid varieties using a rapid compositional analysis method known as NIR/PLS (Near-Infrared Spectroscopy/Projection-to-Latent-Structures modeling) (Templeton et al., 2009). This study evaluated the compositions of 508 corn stover samples (Table 2) collected from 47 regions of 8 corn growing countries, after 2001, 2002 and 2003 harvesting. Another recent study of NREL (National Renewable Energy Laboratory) shows a comparison between the average composition of 9 corn stover samples obtained in 2012 from two lots, compared to 2011, obtained from one sample from overall distribution (Humbird et al., 2011).

Biochemical process

The main biochemical processes consisting in transformation of LCM into bioethanol (Figure 1) are (Wooley et al., 1999; Wooley et al., 2000; Aden et al., 2002; Aden, 2008; Humbird & Aden, 2009; Humbird et al., 2011):

* Pretreatment. LCM is treated at higher temperatures for a short time to release hemicellulosic sugars to enzymatic hydrolysis (Figure 2).

* Simultaneous saccharification and fermentation (SSF). Enzymatic hydrolysis is initiated in a continuous bioreactor using enzymes and then the biomass is inoculated with fermenting microorganisms. At this stage, most of the cellulose and xylose are converted into bioethanol.

1. Pretreatment

The first step in the conversion of LCM into bioethanol is reducing the size by pretreatment (Graf & Koehler, 2000). The goal of any pretreatment technology is to alter or remove structural and compositional obstacles for hydrolysis in order to improve the rate of enzymatic hydrolysis and the increasing yields of fermenting sugars from cellulose and hemicellulose (Mosier et al., 2005a). This is an important tool for cellulose conversion processes, changing the structure of cellulosic biomass, making it more accessible for enzymes that convert polysaccharide into fermentable sugars (Patel et al., 2007; Balat et al., 2008).

Pretreatment (Figure 3) must meet the following requirements (Silverstein, 2004): * Improving the ability to form carbohydrates by hydrolysis;

* Avoiding degradation or loss of carbohydrates;

*Avoiding the formation of byproducts that inhibit hydrolysis and fermentation process.

Pretreatment can be done in various ways, such as: steam explosion (Brownell & Saddler, 1987; Zhang et al., 2007); liquid hot water; CO2 explosion; AFEX (Alizadeh et al., 2005; Teymouri et al., 2004; Teymouri et al., 2005; Indacoechea et al., 2006); acid pretreatment (Martín et al., 2007a); alkaline pretreatment (Silverstein et al., 2007; Champagne, 2007); wet oxidation; ozonolysis; organosolv.

1.1. Physico-chemical pretreatment

Steam explosion pretreatment

In this process which takes place at high pressure, is introduced steam at high temperature into a closed room which contains LCM. After 1-5 min, the steam is released from the matrix chains causing breakage and separation of fibers with minimal loss of material (Mabee et al., 2006; Balat et al., 2008).

Another type of steam explosion pretreatment refers to a technique in which LCM is rapidly heated with high pressure steam without adding any chemicals. Mixture of biomass/steam is kept for a period of time to promote hydrolysis of hemicellulose and the process is completed by decompression. Steam explosion involves chemical effects because acetic acid is generated by hydrolysis of the acetyl groups associated with hemicellulose and can catalyze the hydrolysis and degradation of glucose or xylose (Mosier et al., 2005a).

Liquid hot water pretreatment (LHW)

LHW is one of the hydrothermal pretreatment methods applied for pretreatment of LCM (Taherzadeh et al., 2008) that benefit from the use of no chemical agents other than water, being more environmentally friendly technologies (Garrote et al., 1999). The objective of LHW pretreatment is to solubilize mainly the hemicellulose, to make the cellulose more susceptible to enzymatic attack and to avoid the formation of inhibitors, such as hydroxymethylfurfural (HMF) and furfural (Alvira et al., 2010). The LHW pretreatment involves saturation of lignocellulosic biomass with water, followed by heat treatment of the slurry at temperatures between 120 and 200°C for 5-15 min under pressure (Kim et al., 2009). The pH is restricted to 4-7 to minimize formation of monomeric sugars and sugar decomposition products (Mosier et al., 2005a; Mosier et al., 2005b). This hydrothermal pretreatment is easy to perform: there is a low usage of energy; the process is without the difficult steps of handling and recovery of chemicals; equipment corrosion can be excluded (Cybulska et al., 2010). The process is already applied to lignocellulosic biomass such as corn stover (Mosier et al., 2005c), sugarcane bagasse (Laser et al., 2002) and wheat straw (Pérez et al., 2008).

CO2 explosion pretreatment

This method is based on the use of CO2 as a supercritical fluid. The supercritical CO2 as well as its use in extraction as solvent is capable of enhancing the enzymatic hydrolysis of LCM. This process can remove the lignin and increase the digestibility of the substrate. Co-solvents such as ethanol, acetic acid and water improve the delignification of LCM. CO2 forms carbonic acid in aqueous solutions, favoring hydrolysis of the polymers. CO2 molecules are similar in size to those of water and ammonia, which allow them to penetrate in the same way the pores of LCM. After the release of CO2 under pressure, there is a change in the structure of cellulose and hemicellulose, this allows enzymes to have accessibility to the substrate surface. The main advantages of this treatment are: availability of technology at relatively low cost; it does not produce toxic degradation products; easy recovery after extraction; the process is considered environmentally friendly (Taherzadeh & Karimi, 2008).

AFEX pretreatment (Ammonia fiber/ freeze explosion)

AFEX pretreatment involves the injection of liquid ammonia (Hamelinck et al., 2005). AFEX is a process in which LCM with moisture content of 15-30 % are placed in a pressure container with liquid ammonia. This system does not release any sugars but allows hemicellulose and cellulose to be attacked by enzymes and be reduced to soluble sugars (Dale & Moelhman, 2000; Balat et al., 2008).

2. Chemical pretreatment

Acid pretreatment

The pretreatment process (Table 3) converts most of the hemicellulosic sugars from LCM into soluble sugars (glucose, xylose, arabinose, mannose) by hydrolysis reactions. Acetyl groups from hemicellulose are released as acetic acid. Sugar degradation products such as furfural and HMF can also be formed in the pretreatment (Humbird et al., 2011). The aim of this process is to obtain high yields of sugars from LCM (Lee, 2005). There are several types of acid pretreatment: the use of sulfuric acid (Parajó et al., 1993) hydrochloric acid (Kurakake et al., 2005), peracetic acid (Teixeira et al., 1999), nitric acid (Brink, 1993), or phosphoric acid (Hussein et al., 2001). Dilute acid pretreatment is one of the most studied and widely used (Karimi et al., 2006; Dale & Moelhman, 2000; Tucker et al., 2003; Chung et al., 2005; Kim et al., 2005; Agbogbo & Wenger, 2006).

There are two types of dilute acid pretreatment processes: at moderate temperatures (T < 160 °C) and higher temperatures (T > 160 °C) (Silverstein et al., 2007). Dilute acid pretreatments at moderate temperatures are using sulfuric acid or phosphoric acid to convert LCM, including hemicellulosic fraction, into soluble sugars, followed by enzymatic hydrolysis of cellulosic fraction to glucose (Um et al., 2002). Higher pretreatment temperatures have a higher yield of xylose recovery and increased enzymatic digestibility of cellulosic residues (Tucker et al., 2003).

Alkaline pretreatment

Alkaline pretreatment uses temperatures and low pressures compared to other technologies. This process can remove lignin without having large effects on other components (McMillan, 1997). NaOH treatment increases internal surface of LCM, decreases the degree of crystallinity and breaks the lignin structure (Li et al., 2004). Alkaline pretreatment reduces lignin and hemicellulose content of biomass, increasing the surface and allowing penetration of water molecules in the inner layer as well as breaking connections between hemicellulose and lignin (Lee, 2005). This treatment removes acetyl and uronic acid from hemicellulose, which slows the enzyme accessibility of hemicellulose and cellulose surface (Ramirez, 2005; Balat et al., 2008).

Wet oxidation pretreatment

LCM is subjected to the action of water at elevated temperatures in the presence of oxygen (Schmidt & Thomsen, 1998). This treatment has the advantage of not generating the degradation products, such as furfural and HMF (Klinke et al., 2002; Varga et al., 2003). The presence of oxygen makes this process more expensive but allows working at lower temperatures than the autohydrolysis.

Ozonolysis pretreatment

Ozone can be used for the degradation of lignin and hemicellulose in many LCM. This treatment has a delignification effect on the feedstock and practically does not affect the cellulose. The main advantages of the ozonolysis process are: high efficiency in removing lignin; it does not produce toxic products which could affect the following processes; reactions are carried out under conditions of ambient temperature and atmospheric pressure (Vidal & Molinier, 1988).

Organosolv pretreatment

Organosolv pretreatment represents an alternative method for the delignification of LCM. This method is using an organic or aqueous organic solvent with addition of a catalyst (HCl or H2SO4) to break the internal lignin and hemicellulose bonds. Among the organic solvents used in the process are: methanol, ethanol, acetone and ethylene-glycol (Sun & Cheng, 2002).

Conclusions

Pretreatment process is an important step for bioethanol production from LCM, changing the structure of cellulosic biomass, making it more accessible for enzymes in order to improve the rate of enzymatic hydrolysis and the increasing yields of fermenting sugars from cellulose and hemicellulose. The main advantages of pretreatment methods for LCM are: steam explosion causes hemicellulose degradation and lignin transformation; liquid hot water, CO2 explosion and wet oxidation do not cause formation of inhibitory compounds; AFEX increases accessible surface area; acid pretreatment hydrolyzes hemicellulose to xylose and other sugars; alkaline pretreatment removes hemicelluloses and lignin; ozonolysis reduces lignin content; organosolv hydrolyzes lignin and hemicellulose. LCM is economically attractive for obtaining bioethanol as an alternative fuel source.

Acknowledgments

The work of Cristian-Teodor Buruiana was supported by Project SOP HRD - TOP ACADEMIC 76822/2010

References

References

Aden, A. 2008. Biochemical Production of Ethanol from Corn Stover: 2007 State of Technology Model, Technical Report, NREL/TP-510-43205.

Aden, A., Ruth, M., Ibsen, K., Jechura, J., Neeves, K., Sheehan, J., & Wallace B. (National Renewable Energy Laboratory) & Montague, L., Slayton, A., Lukas, J. (Harris Group). 2002. Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover, Technical Report, NREL/TP-510-32438.

Agbogbo, F., & Wenger, K. 2006. Effect of pretreatment chemicals on xylose fermentation by Pichia stipitis. Biotechnol Lett., 28, 2065-9.

Alizadeh, H., Teymouri, F., Gilbert, T.I., & Dale, B.E. 2005. Pretreatment of switchgrass by ammonia fiber explosion (AFEX). Applied Biochemistry and Biotechnology - Part A. Enzyme Engineering and Biotechnology, 124(1-3), 1133-1141.

Allen, S.G., Schulman, D., Lichwa, J., Antal M.J., J., Laser, M., & Lynd, L.R. 2001. A comparison between hot liquid water and steam fractionation of corn fiber. Industrial and Engineering Chemistry Research, 40(13), 2934-2941.

Alvira, P., Tomás-Pejó, E., Ballesteros, M., & Negro, M.J. 2010. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresource Technology, 101, 4851-4861.

Araque, E., Parra, C., Freer, J., Contreras, D., Rodríguez, J., Mendonça, R. & Baeza, J. 2007. Evaluation of organosolv pretreatment for the conversion of Pinus radiata D. don to ethanol. Enzyme and Microbial Technology, 43(2), 214-219.

Arin, G., & Demirbas, A. 2004. Mathematical modeling the relations of pyrolytic products from lignocellulosic materials. Energy Sources, 26, 1023-32.

Balan, V., Da Costa Sousa, L., Chundawat, S.P.S., Marshall, D., Sharma, L.N., Chambliss, C.K., & Dale, B.E. 2009. Enzymatic digestibility and pretreatment degradation products of AFEX-treated hardwoods (Populus nigra). Biotechnology Progress, 25(2), 365-375.

Balat, M., Balat, H., & Oz, C. 2008. Progress in bioethanol processing. Progress in Energy and Combustion Science, 34, 551-573.

Ballesteros, M., Oliva, J. M., Negro, M. J., Manzanares, P., & Ballesteros, I. 2004. Ethanol from lignocellulosic materials by a simultaneous saccharification and fermentation process (SFS) with Kluyveromyces marxianus CECT 10875. Process Biochemistry, 39(12), 1843-1848.

Biomass as Feedstock for a Bioenergy and Bioproducts Industry. 2005. The Technical Feasibility of a Billion-Ton Annual Supply. A Joint Study Sponsored by U.S.DOE and U.S.D.A.

Brink, D. L. 1993. Method of treating biomass material. US Patent 5221357.

Brosse, N., Sannigrahi, P., & Ragauskas, A. 2009. Pretreatment of miscanthus giganteus using the ethanol organosolv process for ethanol production. Industrial and Engineering Chemistry Research, 48(18), 8328-8334.

Brownell, H.H., & Saddler, J.N. 1987. Steam pretreatment of lignocellulosic material for enhanced enzymatic hydrolysis. Biotechnol Bioeng, 29, 228-35.

Cara, C., Romero, I., Oliva, J. M., Sáez, F., & Castro, E. 2007. Liquid hot water pretreatment of olive tree pruning residues. Applied Biochemistry and Biotechnology, 137-140(1-12), 379-394.

Champagne, P. 2007. Feasibility of producing bio-ethanol from waste residues: a Canadian perspective, resources. Conserv Recycl, 50, 211-30.

Chung, Y.C., Bakalinsky, A., & Penner, M.H. 2005. Enzymatic saccharification and fermentation of xylose-optimized dilute acid-treated lignocellulosics. Appl. Biochem. Biotechnol., 124, 947-61.

Cuevas, M., Sánchez, S., Bravo, V., Cruz, N., & García, J.F. 2009. Fermentation of enzymatic hydrolysates from olive stones by pachysolen tannophilus. Journal of Chemical Technology and Biotechnology, 84(3), 461-467.

Cybulska, I., Lei, H., & Julson, J. 2010. Hydrothermal Pretreatment and Enzymatic Hydrolysis of Prairie Cord Grass. Energy Fuels, 24, 718-727.

Dale, M.C., & Moelhman, M. 2000. Enzymatic simultaneous saccharification and fermentation (SSF) of biomass to ethanol in a pilot 1301 Multistage continuous reactor separator. Ninth Biennial bioenergy conference, Buffalo, New York, 15-19.

Demirbas, A. 2005. Estimating of structural composition of wood and non-wood biomass samples. Energy Sources, 27, 761-7.

Díaz, M.J., Cara, C., Ruiz, E., Romero, I., Moya, M., & Castro, E. 2010. Hydrothermal pretreatment of rapeseed straw. Bioresource Technology, 101(7), 2428-2435.

EU-Directive 2009/28/EC of The European Parliament and of The Council of 23 April 2009 on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. Official Journal of the European Union, 16-62.

Ferreira-Leitão, V., Cruz Perrone, C., Rodrigues, J., Machado Franke, A.P., Macrelli, S., & Zacchi, G. 2010. An approach to the utilisation of CO2 as impregnating agent in steam pretreatment of sugar cane bagasse and leaves for ethanol production. Biotechnology for Biofuels, 3(1).

Fischer, G., Prieler, S., van Velthuizen, H., Berndes, G., Faaij, A., Londo, M., & de Wit, M. 2010b. Biofuel production potentials in Europe: Sustainable use of cultivated land and pastures, Part II: Land use scenarios. Biomass and Bioenergy, 34, 173-187.

Fischer, G., Prieler, S., van Velthuizen, H., Lensink, S.M., Londo, M., & de Wit, M. 2010a. Biofuel production potentials in Europe: Sustainable use of cultivated land and pastures. Part I: Land productivity potentials. Biomass and Bioenergy, 34, 159-172.

Foust, T., et al. 2006. 30X30 A Scenario for Supplying 30% of 2004 Motor Gasoline with Ethanol by 2030. A Production Scenario for Cellulosic Ethanol from Corn Stover. Developed by the National Laboratories. A Study Sponsored by U.S.DOE and EERE.

Foust, T.D., Aden, A., Dutta, A., & Phillips, S. 2009. An economic and environmental comparison of a biochemical and a thermochemical lignocellulosic ethanol conversion processes, Springer Science+Business Media B.V., Cellulose, 16, 547-565.

Gao, M., Xu, F., Li, S., Ji, X., Chen, S., & Zhang, D. 2010. Effect of SC-CO2 pretreatment in increasing rice straw biomass conversion. Biosystems Engineering, 106(4), 470-475.

Garrote, G., Domínguez, H., Parajó, J.C., 1999. Hydrothermal processing of lignocellulosic materials. Holz Roh Werkst. 57, 191-202.

Garrote, G., Kabel, M.A., Schols, H.A., Falqué, E., Domínguez, H., & Parajó, J.C. 2007. Effects of Eucalyptus globulus Wood Autohydrolysis Conditions on the Reaction Products. J. Agric. Food Chem., 55, 9006-9013.

Gnansounou, E. 2010. Production and use of lignocellulosic bioethanol in Europe: Current situation and perspectives. Bioresource Technology, 101, 4842-4850.

González-García, S., Moreira, M.T., & Feijoo, G. 2010. Comparative environmental performance of lignocellulosic ethanol from different feedstocks. Renewable and Sustainable Energy Reviews, 14, 2077-2085.

Graf, A., & Koehler, T. 2000. Oregon cellulose-ethanol study: an evaluation of the potential for ethanol production in Oregon using cellulose based feedstocks. Salem, Oregon, USA: Oregon Dept. of Energy, 96.

Gullón, B., Yáñez, R., J.L. Alonso, J.L., & Parajó, J.C. 2010. Production of oligosaccharides and sugars from rye straw: A kinetic approach. Bioresource Technology, 101, 6676-6684.

Hamelinck, C.N., van Hooijdonk, G., & Faaij, A.P.C. 2005. Ethanol from lignocellulosic biomass: techno-economic performance in short-, middle- and long-term. Biomass Bioenergy, 28, 384-410.

Humbird, D., & Aden, A. 2009. Biochemical Production of Ethanol from Corn Stover: 2008 State of Technology Model, Technical Report, NREL/TP-510-46214.

Humbird, D., Davis, R., Tao, L., Kinchin, C., Hsu, D., & Aden, A. (National Renewable Energy Laboratory) and Schoen, P, Lukas, J., Olthof, B.,Worley, M., Sexton, D., Dudgeon, D. (Harris Group Inc.). 2011. Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol - Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover, Technical Report, NREL/TP-5100-47764.

Hussein, M.Z.B., Rahman, M.B.B.A, Yahaya, A.H.J, Hin, T.Y.Y., & Ahmad, N. 2001. Oil palm trunk as a raw material for activated carbon production. J. Porous Mater, 8, 327-34.

Indacoechea, I., Bolado, S., Garcia-Cubero, M.T., & Diez, R. 2006. Pretreatment processes of lignocellulosic material for bioethanol conversion: ozonolysis. 17th International congress of chemical and process engineering, Chisa, Prague.

Karimi, K., Emtiazi, G., & Taherzadeh, M.J. 2006. Ethanol production from dilute-acid pretreated rice straw by simultaneous saccharification and fermentation with Mucor indicus, Rhizopus oryzae, and Saccharomyces cerevisiae. Enzyme and Microbial Technology, 40, 138-44.

Kim, K.H. 2005. Two-stage dilute acid-catalyzed hydrolytic conversion of softwood sawdust into sugars fermentable by ethanologenic microorganisms. J. Sci. Food Agric., 85, 2461-7.

Kim, S. B., & Moon, N. K. 2003. Enzymatic digestibility of used newspaper treated with aqueous ammonia-hydrogen peroxide solution. Applied Biochemistry and Biotechnology - Part A. Enzyme Engineering and Biotechnology, 106(1-3), 365-374.

Kim, Y., Mosier, N.S., & Ladisch, M.R. 2009. Enzymatic digestion of liquid hot water pretreated hybrid poplar. Biotechnology Progress, 25(2), 340-348.

Klinke, H.B., Ahring, B.K., Schmidt, A.S., & Thomsen, A.B. 2002. Characterization of degradation products from alkaline wet oxidation of wheat straw. Bioresource Technology, 82(1), 15-26.

Kurakake, M., Ouchi, K., Kisaka, W., & Komaki, T. 2005. Production of L-arabinose and xylose from corn hull and bagasse. J Appl Glycosci, 52, 281-5.

Laser, M., Schulman, D., Allen, S.G., Lichwa, J., Antal, M.J., & Lynd, L.R., 2002. A comparison of liquid hot water and steam pretreatments of sugar cane bagasse for conversion to ethanol. Bioresour Technol, 81, 33-44.

Lee, Y.J. 2005. Oxidation of sugarcane bagasse using a combination of hypochlorite and peroxide. Master's Thesis, Department of Food Science, Graduate Faculty of the Louisiana State University & Agricultural and Mechanical College.

Li, Y., Ruan, R., Chen, P.L., Liu, Z., Pan, X., Lin, X., et al. 2004. Enzymatic hydrolysis of corn stover pretreated by combined dilute alkaline treatment and homogenization. Trans ASAE, 47, 821-5.

Mabee, W.E., Gregg, D.J., Arato, C., Berlin, A., Bura, R., Gilkes, N., et al. 2006. Updates on softwood-to-ethanol process development. Appl. Biochem. Biotechnol., 129-132, 55-70.

Martín, C., Alriksson, B., Sjode, A., Nilvebrant, N.O., & Jonsson, L.J. 2007a. Dilute sulfuric acid pretreatment of agricultural and agro-industrial residues for ethanol production. Appl Biochem Biotechnol, 137-140, 339-352.

Martín, C., Klinke, H.B., & Thomsen, A.B. 2007b. Wet oxidation as a pretreatment method for enhancing the enzymatic convertibility of sugarcane bagasse. Enzyme and Microbial Technology, 40(3), 426-432.

Martín, C., Thomsen, M.H., Hauggaard-Nielsen, H., & Belinda Thomsen, A. 2008. Wet oxidation pretreatment, enzymatic hydrolysis and simultaneous saccharification and fermentation of clover-ryegrass mixtures. Bioresource Techonology, 99(18), 8777-8782.

McMillan, J.D. 1997. Bioethanol production: status and prospects. Renew Energy, 10, 295-302.

Mesa, L., González, E., Cara, C., Ruiz, E., Castro, E., & Mussatto, S.I. 2010. An approach to optimization of enzymatic hydrolysis from sugarcane bagasse based on organosolv pretreatment. Journal of Chemical Technology and Biotechnology, 85(8), 1092-1098.

Mohan, D, Pittman, C.U, & Steele, P.H. 2006. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuels, 20, 848-89.

Mosier, N., Hendrickson, R., Ho, N., Sedlak, M., & Ladisch, M.R., 2005c. Optimization of pH controlled liquid hot water pretreatment of corn stover. Bioresour Technol, 96, 1986-1993.

Mosier, N.S., Hendrickson, R., Brewer, M., Ho, N.W.Y., Sedlak, M., Dreshel, R., Welch, G., Dien, B.S., Aden, A., & Ladisch, M.R. 2005b. Industrial scale-up of pH-controlled liquid hot water pretreatment of corn fiber for fuel ethanol production. Appl Biochem Biotechnol, 125, 77-85.

Mosier, N.S., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M., & Ladisch, M.R. 2005a. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresource Tech, 96, 673-686.

Muñoz, C., Mendonça, R., Baeza, J., Berlin, A., Saddler, J., & Freer, J. 2007. Bioethanol production from bio-organosolv pulps of Pinus radiata and Acacia dealbata. Journal of Chemical Technology and Biotechnology, 82(8), 767-774.

Nigam, P.S., Gupta, N., & Anthwal, A. 2009. Pre-treatment of agro-industrial residues. In: Nigam, P.S., Pandey, A., eds. Biotechnology for agro-industrial residues utilization. 1 ed. Netherlands, Springer, 13-33.

Pan, X., Gilkes, N., Kadla, J., Pye, K., Saka, S., Gregg, D., Ehara, K., Xie, D., Lam, D., & Saddler, J. 2006. Bioconversion of hybrid poplar to ethanol and co-products using an organosolv fractionation process: optimization of process yields. Biotechnology and Bioengineering, 94(5), 851-861.

Pan, X., Xie, D., Yu, R.W., & Saddler, J.N. 2008. The bioconversion of mountain pine beetle killed lodgepole pine to fuel ethanol using the organosolv process. Biotechnology and Bioengineering, 101(1), 39-48.

Parajó, J.C., Garrote, G., Cruz, J.M., & Dominguez, H. 2004. Production of xylooligosaccharides by autohydrolysis of lignocellulosic materials. Trends in Food Science & Technology, 15, 115-120.

Parajó, J.C., Vazquez, D., Alonso, J.L., Santos, V., & Dominguez, H. 1993. Prehydrolysis of Eucalyptus wood with dilute sulphuric acid: operation at atmospheric pressure. Holz als Roh-und Werkstoff, 51, 357-63.

Park, N., Kim, H. -., Koo, B. -., Yeo, H., & Choi, I. -. 2010. Organosolv pretreatment with various catalysts for enhancing enzymatic hydrolysis of pitch pine (Pinus rigida). Bioresource Technology, 101(18), 7046-7053.

Patel, S.J., Onkarappa, R, & Ks, S. 2007. Funkal pretreatment studies on rice husk and bagasse for ethanol production. Electron. J. Environ. Agric. Food Chem., 6, 1921-6.

Pedersen, M., & Meyer, A.S. 2009. Influence of substrate particle size and wet oxidation on physical surface structures and enzymatic hydrolysis of wheat straw. Biotechnology Progress, 25(2), 399-408.

Pérez, J.A., Ballesteros, I., Ballesteros, M., Sáez, F., Negro, M.J., & Manzanares, P. 2008. Optimizing liquid hot water pretreatment conditions to enhance sugar recovery from wheat straw for fuel-ethanol production. Fuel, 87(17-18), 3640-3647.

Ramirez, S.E. 2005. Long-term lime pretreatment of poplar wood. Master's thesis, Texas A&M University.

Saha, B.C., Iten, L.B., Cotta, M.A., & Wu, Y.V. 2005. Dilute acid pretreatment, enzymatic saccharification and fermentation of wheat straw to ethanol. Process Biochemistry, 40(12), 3693-3700.

Sánchez, O.J., & Cardona, C.A. 2008. Trends in biotechnological production of fuel ethanol from different feedstocks. Bioresource Technology, 99, 5270-5295.

Sannigrahi, P., Miller, S.J., & Ragauskas, A.J. 2010. Effects of organosolv pretreatment and enzymatic hydrolysis on cellulose structure and crystallinity in loblolly pine. Carbohydrate Research, 345(7), 965-970.

Sassner, P., Galbe, M., & Zacchi, G. 2008. Techno-economic evaluation of bioethanol production from three different lignocellulosic materials. Biomass and Bioenergy, 32, 422-430.

Schmidt, A.S., & Thomsen, A.B. 1998. Optimization of wet oxidation pretreatment of wheat straw. Bioresource Technology, 64(2), 139-151.

Silverstein, R.A. 2004. A Comparison of chemical pretreatment methods for converting cotton stalks to ethanol. Master's Thesis (adv: R. Sharma), Biological and Agricultural Engineering, North Carolina State University.

Silverstein, R.A., Chen, Y., Sharma-Shivappa, R.R., Boyette, M.D., & Osborne, J. 2007. A comparison of chemical pretreatment methods for improving saccharification of cotton stalks. Bioresource Technol., 98, 3000-11.

Sun, Y., & Cheng, J. 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresource Technology, 83, 1-11.

Taherzadeh, M.J., & Karimi, K. 2008. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: A review. International Journal of Molecular Sciences, 9(9), 1621-1651.

Teixeira, L.C., Linden, J.C., & Schroeder, H.A. 1999. Optimizing peracetic acid pretreatment conditions for improved simultaneous saccharification and co-fermentation (SSCF) of sugar cane bagasse to ethanol fuel. Renew Energy, 16, 1070-3.

Templeton, D.W., Sluiter, A.D., Hayward, T.K., Hames, B.R., & Thomas, S.R. 2009. Assessing Corn Stover Composition and Sources of Variability via NIRS. Cellulose, 16, 621-639.

Teymouri, F., Laureano-Perez, L., Alizadeh, H., & Dale, B.E. 2004. Ammonia fiber explosion treatment of corn stover. Appl Biochem Biotechnol, 115, 951-63.

Teymouri, F., Laureano-Perez, L., Alizadeh, H., & Dale, B.E. 2005. Optimization of the ammonia fiber explosion (AFEX) treatment parameters for enzymatic hydrolysis of corn stover. Bioresource Technology, 96, 2014-8.

Tucker, M.P., Kim, K.H, Newman, M.M., & Nguyen, Q.A. 2003. Effects of temperature and moisture on dilute-acid steam explosion pretreatment of corn Stover and cellulase enzyme digestibility. Appl Biochem Biotechnol, 105, 165-78.

Um, B.H., Karim, M.N., & Henk, L.L. 2002. Modeling of dilute acid pretreatment and enzymatic hydrolysis of corn stover. In: Session 1-feedstock production, genetic modification and processing, 24th symposium on biotechnology for fuels and chemicals, Gatlinburg, Tennessee, USA.

Varga, E., Schmidt, A.S., Réczey, K., & Thomsen, A.B. 2003. Pretreatment of corn stover using wet oxidation to enhance enzymatic digestibility. Applied Biochemistry and Biotechnology - Part A. Enzyme Engineering and Biotechnology, 104(1), 37-50.

Vidal, P.F., & Molinier, J. 1988. Ozonolysis of lignin - improvement of in vitro digestibility of poplar sawdust. Biomass, 16(1), 1-17.

Wooley, R., Ruth, M., Sheehan, J., & Ibsen, K. (Biotechnology Center for Fuels and Chemicals) and Majdeski, H., Galvez, A. (Delta-T Corporation). 1999. Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current, Dilute Acid Prehydrolysis and Enzymatic Hydrolysis Current and Futuristic Scenarios. Technical Report, NREL/TP-580-26157.

Wooley, R., Sheehan, J., Aden, A., & Riley, C. 2000. Life Cycle Energy and Environmental Impacts of Bioethanol Made from Corn Stover, Biofuels Program, K Milestone Completion Report.

Xu, J., Thomsen, M.H., & Thomsen, A.B. 2010. Investigation of acetic acid-catalyzed hydrothermal pretreatment on corn stover. Applied Microbiology and Biotechnology, 86(2), 509-516.

Yaman, S. 2004. Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Convers Manage., 45, 651-71.

Zhang, L., Wang, T., Jiao, S., Hao, C., & Mao, Z. 2007. Effect of steam-explosion on biodegradation of lignin in wheat straw. 2007 ASAE Annual Meeting, Paper number 077076, Minneapolis, Minnesota, 17-20.