Lignocelluloses are often a major or sometimes the sole components of different waste streams from various industries, forestry, agriculture and municipalities. Hydrolysis of these materials is the first step for either digestion to biogas (methane) or fermentation to ethanol. However, enzymatic hydrolysis of lignocelluloses with no pretreatment is usually not so effective because of high stability of the materials to enzymatic or bacterial attacks. The present work is dedicated to reviewing the methods that have been studied for pretreatment of lignocellulosic wastes for conversion to ethanol or biogas. Effective parameters in pretreatment of lignocelluloses, such as crystallinity, accessible surface area, and protection by lignin and hemicellulose are described first. Then, several pretreatment methods are discussed and their effects on improvement in ethanol and/or biogas production are described. They include milling, irradiation, microwave, steam explosion, ammonia fiber explosion (AFEX), supercritical CO2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv processes, wet oxidation, ozonolysis, dilute- and concentrated-acid hydrolyses, and biological pretreatments.

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Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review.

Poplar wood was treated with peracetic acid, KOH, and ball milling to produce 147 model lignocelluloses with a broad spectrum of lignin contents, acetyl contents, and crystallinity indices (CrIs), respectively. An empirical model was identified that describes the roles of these three properties in enzymatic hydrolysis. Lignin content and CrI have the greatest impact on biomass digestibility, whereas acetyl content has a minor impact. The digestibility of several lime-treated biomass samples agreed with the empirical model. Lime treatment removes all acetyl groups and a moderate amount of lignin and increases CrI slightly; lignin removal is the dominant benefit from lime treatment.

Fundamental factors affecting biomass enzymatic reactivity.

Production of ethanol from sugar (hexoses) and starchis carried out by a well developed technology. Production of ethanol from lignocellulosic materials is more difficult, requires several process steps, and is as yet in the development stage.

Production of ethanol from lignocellulosic materials: State of the art

Corn stover is emerging as a viable feedstock for producing bioethanol from renewable resources. Dilute-acid pretreatment of corn stover can solubilize a significant portion of the hemicellulosic component and enhance the enzymatic digestibility of the remaining cellulose for fermentation into ethanol. In this study, dilute H2SO4 pretreatment of corn stover was performed in a steam explosion reactor at 160°C, 180°C, and 190°C, approx 1 wt % H2SO4/ and 70-s to 840-s residence times. The combined severity (Log10 [R0] — pH), an expression relating pH, temperature, and residence time of pretreatment, ranged from 1.8 to 2.4. Soluble xylose yields varied from 63 to 77% of theoretical from pretreatments of corn stover at 160 and 180°C. However, yields >90% of theoretical were found with dilute-acid pretreatments at 190°C. A narrower range of higher combined severities was required for pretreatment to obtain high soluble xylose yields when the moisture content of the acid-impregnated feedstock was increased from 55 to 63 wt%. Simultaneous saccharification and fermentation (SSF) of washed solids from corn stover pretreated at 190°C, using an enzyme loading of 15 filter paper units (FPU)/ g of cellulose, gave ethanol yields in excess of 85%. Similar SSF ethanol yields were found using washed solid residues from 160 and 180°C pretreatments at similar combined severities but required a higher enzyme loading of approx 25 FPU/g of cellulose.

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Effects of Temperature and Moisture on Dilute-Acid Steam Explosion Pretreatment of Corn Stover and Cellulase Enzyme Digestibility

Publisher Summary This chapter describes physicochemical and biological treatments for enzymatic or microbial conversion of LignocelIulosic Biomass. Microbial utilization of the inexhaustible lignocellulosic biomass for the production of industrial chemicals, liquid fuels, protein-rich food and feed, and the preparation of cellulose polymers is an attractive approach to help meet energy and food demands. Lignocelluloses contain cellulose, hemicellulose, and lignin as the major components. Lignocellulosic raw materials by the virtue of their structure are relatively refractory to direct bioconversion. Cellulose in lignocellulosic substrates has the regions of a highly resistant crystalline structure and the lignin surrounding cellulose forms a physical barrier that allows only limited sites available for enzymatic attack. The use of whole P . chrysosporium cells and lignolytic enzymes in the pulp and paper industry appears to hold promise in processes designed for biopulping and biobleaching and in decolorizing bleach plant effluents. This can have a favorable impact on the world economy, sparing scarce energy sources and alleviating the current environmental problems with waste effluents.

Physicochemical and Biological Treatments for Enzymatic/Microbial Conversion of Lignocellulosic Biomass

Twelve white-rot fungi were grown in solid-state culture on lemon grass (Cymbopogon citratus) and citronella (Cymbopogon winterianus) bagasse. The two lignocellulosic substrates had 11% permanganate lignin and a holocellulose fraction of 58%. After 5 to 6 weeks at 20°C, nine fungi produced a solid residue from lemon grass with a higher in vitro dry matter enzyme digestibility than the original bagasse; seven did the same for citronella. The best fungus for both substrates was Bondarzewia berkeleyi; it increased the in vitro dry matter enzyme digestibility to 22 and 24% for lemon grass and citronella, respectively. The increases were correlated with weight loss and lignin loss. All fungi decreased lignin contents: 36% of the original value for lemon grass and 28% for citronella. Practically all fungi showed a preference for hemicellulose over cellulose.

Biodelignification of lemon grass and citronella bagasse by white-rot fungi.

The operation of packed bed fermenters for ethanol production using the EX-FERM technique with two cycles of 24 h each is described. Twelve Saccharomyces strains were tested with sugarcane particles which had been previously dried and stored as the substrate. All the strains showed acceptable sugar consumption, in some cases above 97%, and ethanol yield coefficients. Sugar consumption values differed, significantly among yeast strains for both cycles. Specific initial rates of ethanol production for the strains ranged from 0.75 to 0.82 g/g· h. Sugar extraction from the particles influenced the first 4 h period of each cycle; final sugar extraction was about 96%. The initial yeast biomass figures were low, within 2–4 g/l, and the final distribution of yeast between solids and circulating liquid varied according to the yeast strain employed. Hydrolysis of sucrose into its components was demonstrated for selected yeast strains during the EX-FERM concurrent extraction and fermentation. The results of the present study support the validity of the operation of packed bed fermenters in cycles for the EX-FERM technique, and suggest employment of smaller cane particles.

EX-FERM ethanol production using chipped sugarcane in packed bed fermenters

Twelve white-rot fungi were grown in solid state culture on sugarcane chips previously fermented by yeast employing the EX-FERM process. The lignocellulosic sugarcane residue had 12.5% permanganate lignin and 81.3% holocellulose. After 5 to 6 weeks at 20° C, all fungi produced a solid residue which had a lower in vitro dry matter enzymatic digestibility than the original bagasse, with the exception of Coriolus versicolor which showed a slight increase of 0.6 units. Four fungi produced a residue with higher soluble solids than the original sample. Lignin losses were rather similar for all fungi tested, an average value of 38.64% of the original value was obtained. About the same amount of hemicellulose was degreaded, 32.22%. Most fungi showed a preference for hemicellulose hydrolysis over cellulose degradation. The two fungi that showed greater cellulolytic activity were Sporotrichum pulverulentum and Dichomitus squalens. No appreciable dry matter losses were detected for Agrocybe aergerita and Flammulina velutipes.

White-rot fungal growth on sugarcane lignocellulosic residue

Effects of some physical and chemical pretreatments on the composition, enzymatic hydrolysis and digestibility of lignocellulosic sugar cane residue

Effects of some physical and chemical pretreatments on the composition and enzymatic hydrolysis and digestibility of lemon grass and citronella bagasse

S. de Cabrera

Papers

Biodelignification of lemon grass and citronella bagasse by white-rot fungi.

EX-FERM ethanol production using chipped sugarcane in packed bed fermenters

White-rot fungal growth on sugarcane lignocellulosic residue

Effects of some physical and chemical pretreatments on the composition, enzymatic hydrolysis and digestibility of lignocellulosic sugar cane residue

Effects of some physical and chemical pretreatments on the composition and enzymatic hydrolysis and digestibility of lemon grass and citronella bagasse