Elsevier

Bioresource Technology

Volume 99, Issue 18, December 2008, Pages 8856-8863
Bioresource Technology

Pretreatment of woody and herbaceous biomass for enzymatic saccharification using sulfuric acid-free ethanol cooking

https://doi.org/10.1016/j.biortech.2008.04.049Get rights and content

Abstract

A sulfuric acid-free ethanol cooking (SFEC) treatment was developed to achieve complete saccharification of the cellulosic component of eucalyptus and baggase flour, thereby avoiding the problems associated with the use of strong acid catalysts. Cutter-milled flours were exposed to an ethanol (EtOH)/water/acetic acid mixture in an autoclave. Enzymatic hydrolysis experiments of the pretreated samples demonstrated that almost complete conversion of the cellulosic components to glucose was achieved under optimal conditions. A large-scale trial revealed that there was little consumption of in-feed EtOH during SFEC; therefore, it is considered that most part EtOH used can be essentially recovered and reused. Field emission scanning electron microscopy showed that SFEC induced the formation of pores ranging in size from approximately 10 to several 100 nm. It can be assumed that the porous surface was due to the partial removals of lignin and hemicellulose, which improved the accessibility of the enzyme onto the substrate.

Introduction

The cost of ethanol (EtOH) production from starch and sucrose for use as a vehicle fuel is ultimately high because the cultivation of grain and sugar crops involves the consumption of substantial amounts of petroleum stocks. Consequently, it has been suggested that the large-scale use of EtOH as a fuel will require the utilization of cellulosic feedstock (Arato et al., 2005, Duff and Murray, 1996, Farrell et al., 2006, Grohmann et al., 1985, Millet et al., 1975, Mosier et al., 2005, Sun and Cheng, 2002). Moreover, in non-agripower countries such as Japan, converting farmland into areas for biofuel-crop production is not a realistic approach due to the low food self-sufficiency rate in these countries. In comparison with grasses or agricultural residues, woody biomass essentially has an advantage in high volume per unit of land area. Therefore, future energy security is dependent on the utilization of woody biomass.

The saccharification of cellulosic has been studied for 120 years (Saeman, 1945, Sherrard and Kressman, 1945). To date, cellulose hydrolysis has been accomplished by using either acids or enzymes. Enzymatic hydrolysis is an attractive option in the conversion of lignocellulosic biomass to glucose since it produces yields superior to those produced by acid-catalyzed hydrolysis. The development of an effective pretreatment method for lignocellulosic biomass is the most important aspect of the enzymatic saccharification process.

Various pretreatment methods have been developed, including physical (Endo et al., 2006, Mandels et al., 1974, Muraki et al., 1982) and chemical methods (Aziz and Sarkanen, 1989, Chum et al., 1988, Chum et al., 1990a, Chum et al., 1990b, Grohmann et al., 1986, Knappert et al., 1980, Pan et al., 2005, Pan et al., 2005, Shimizu and Usami, 1978 and a combination of these two methods (Grethlein and Converse, 1991, Kurabi et al., 2005, Maekawa, 1992, Matsumura et al., 1977, Nakayama and Okamura, 1989, Sassner et al., 2005). In particular, dilute sulfuric acid-based chemical pretreatment (Mosier et al., 2005) is the most popular pretreatment method. This pretreatment process pioneered by Grethlein and Converse (1991) and Knappert et al. (1980) can break down the lignin–hemicellulose shield in agricultural residues and woody biomass and is usually accompanied by the degradation of hemicellulose. The acid-catalyzed treatment has been widely assessed from the viewpoints of efficiency improvement (Grohmann et al., 1985, Grohmann et al., 1986), kinetic analysis (Converse et al., 1989), economy (Hinman et al., 1992), etc. This mainstream process, however, might have some undesirable effects. For example, the formation of aldehydes such as furfural due to the degradation of the produced monosaccharide is essentially inevitable; this in turn lowers the conversion yield of polysaccharides and inhibits the EtOH-fermentation process. As an additional problem, sulfuric acid might corrode the reaction vessels. According to an autoclave manufacturer, the corrosion rate of stainless steel (SUS304 and SUS316) is ⩾1.25 mm/year even when ⩽0.25% sulfuric acid is used at boiling point; on the other hand, when ⩽20% acetic acid (AA) is used, the rate is ⩽0.125 mm/year at boiling point. The recovery of the spent acid also complicates the downstream processing steps.

Recently, we have conducted a combined sulfuric acid-free EtOH cooking (SFEC) and pulverization process in order to achieve the complete saccharification of the total cellulosic portion of woody biomass (Teramoto et al., 2008). The boiling point of EtOH is lower than that of water, which facilitates its ready separation from a mixed aqueous solvent. Moreover, EtOH can be effectively recovered from used biomass; therefore, a process in which only biomass is utilized as the raw material could be realized, even if EtOH is initially consumed. Experimentally, eucalyptus wood chips were used as raw materials and exposed to an EtOH/water/AA mixed solvent in an autoclave. The SFEC treatment is similar to organosolv pulping (Aziz and Sarkanen, 1989, Chum et al., 1988, Chum et al., 1990a, Chum et al., 1990b, Pan et al., 2005, Pan et al., 2005, Pan et al., 2005). However, organosolv pulping is generally conducted using a strong acid catalyst in order to accomplish the intensive removal of the lignin component, whereas the SFEC system that uses AA does not assume complete lignin removal. The SFEC system can avoid the above mentioned problems originating from the use of a strong acid catalyst. Even though autocatalyzed and acetic acid-catalyzed ethanol pulping has been performed previously (Girard and Van Heiningen, 2000, Oliet et al., 2001), the process was not conducted as a pretreatment of woody biomass for enzymatic saccharification.

In a previous study, the SFEC-treated solid products were pulverized by ball-milling in order to improve the enzymatic digestibility (Teramoto et al., 2008). Thereupon, it was demonstrated that enzymatic hydrolysis experiments attained 100% conversion of cellulosic components to glucose under optimal conditions. Morphological observations revealed that SFEC treatment induced the formation of pores ranging in size from approximately 10 to several 100 nm. It could be assumed that the porous surface was due to the partial removal of the lignin and hemicellulose fractions, which improved the accessibility of enzyme onto the substrate. However, as one might be concerned, we recognize that ball-milling is energy-intensive and expensive. Consequently, in the present study, we performed an economical pulverization process, namely, cutter-milling, instead of ball-milling, and this rough grinding was incorporated prior to the SFEC treatment. A number of SFEC experiments were carried out, mainly for eucalyptus pre-ground flour. A schematic representation of the process is illustrated in Fig. 1. Structural analysis of the pretreated eucalyptus samples was also carried out using wide-angle X-ray diffractometry, particle-size measurement, and field emission scanning electron microscopy (FE-SEM).

Section snippets

Samples and solvents

Eucalyptus wood chips (major axis, 25–50 mm; minor axis, 10–20 mm; thickness, 2–5 mm) were purchased from Oji Paper Co., Ltd., Tokyo, Japan. The eucalyptus chips comprised material from six species (main component: Eucalyptus globulus). The eucalyptus chips were cutter-milled to pass through either a 2- or 0.2-mm pore-size sieve. Bagasse was provided from Japan Planning Organization, Inc., Tokyo, Japan, and cutter-milled to pass through a 0.2-mm pore-size sieve. The flour samples were stored at 20 

Results and discussion

The dry basis contents of holocellulose, α-cellulose, and Klason lignin in the original materials are 76.3, 42.2, and 28.1 wt% for eucalyptus and 63.2, 30.7, and 22.8 wt% for baggase, respectively. The hemicellulose content was determined to be 34.1 (eucalyptus) and 32.5 (baggase) wt%, respectively, by subtracting the α-cellulose content from the holocellulose content.

A number of SFEC experiments with eucalyptus flour (particle size <0.2 mm) were carried out in EtOH:water using a liquor:wood ratio

Conclusions

Our results indicate that the SFEC pretreatment of cutter-milled eucalyptus and bagasse flour can readily improve their enzymatic digestibility, thereby avoiding the problems associated with the use of strong acid catalysts. Significant savings in pulverization energy can thus be realized on comparison with the combined SFEC and ball-milling pretreatment proposed previously. The degradation of pentosan and hexosan in this cooking process was low. A large-scale trial revealed that there was

Acknowledgements

The authors are greatly thankful to Ms. Mariko Morimoto-Ago and Prof. Kunihiko Okajima of Tokushima Bunri University for providing the FE-SEM observation facilities. The authors are deeply grateful to Ms. Naomi Kadotani (AIST), Ms. Manami Asano (AIST), and Ms. Noriko Tanaka (AIST) for their assistance with the experiments.

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