Renewable Energy 36 (2011) 2771e2775
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Construction and demolition lignocellulosic wastes to bioethanol Vahid Jafari a, Sara Rahim Labafzadeh a, Azam Jeihanipour a, b, Keikhosro Karimi a, c, *, Mohammad J. Taherzadeh a a
School of Engineering, University of Borås, Borås, Sweden Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden c Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2011 Accepted 25 April 2011 Available online 11 May 2011
This work deals with conversion of four construction and demolition (C&D) lignocellulosic wastes including OSB, chipboard, plywood, and wallpaper to ethanol by separate enzymatic hydrolysis and fermentation (SHF). Similar to other lignocelluloses, the wastes were resistant to the enzymatic hydrolysis, in which only up to 7% of their cellulose was hydrolyzed. Therefore, the lignocellulosic wastes were treated with phosphoric acid, sodium hydroxide, or N-methylmorpholine-N-oxide (NMMO), which resulted in improving the subsequent enzymatic hydrolysis to 38.2e94.6% of the theoretical yield. The best performance was obtained after pretreatment by concentrated phosphoric acid, followed by NMMO. The pretreated and hydrolyzed C&D wastes were then successfully fermented by baker’s yeast to ethanol with 70.5e84.2% of the theoretical yields. The results indicate the possibility of producing 160 ml ethanol from each kg of the C&D wastes at the best conditions. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Construction engineered wood waste Wallpaper Ethanol Pretreatment Phosphoric acid NMMO
1. Introduction Construction and demolition (C&D) wastes are one of the major waste streams in the world, which contain lignocelluloses in addition to cement, glass, etc. According to the National Association of Home Builders, approximately 1.7 tons of lignocellulosic wastes are generated during the construction of a 180-m2 home in the USA [1]. The Engineered Wood Association (APA) has reported that at least 30 percent of this lignocellulosic waste from residential buildings is from structural engineered wood products in the USA [2], used in all parts of the constructions. Engineered wood wastes consist of a number of different materials, including Oriented Strand Board (OSB) which is made of strands, wafers sliced from small-width, and round wood logs, and attached with a binder; chipboard (particle board) which is mainly in the form of separate particles combined with an artificial resin or other suitable binder and attached together under heat and pressure; and plywood in which thin sheets of wood (veneer) are joined together by addition of an adhesive. Approximately 10% of engineered wood products’ contents are different types of glues including UF (urea formaldehyde), MUPF (melamine urea phenol formaldehyde) and PMDI
* Corresponding author. Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. Tel.: þ983113915623; fax: þ983113912677. E-mail address:
[email protected] (K. Karimi). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.04.028
(polymeric diphenylmethane-4,40 -diisocyanate). Wallpapers are also among the building wastes. These materials include synthetic and/or natural fibers with cellulose as a carrier, which is covered by the colors including pigments and filling materials such as crayon, kaolin, titanium dioxide, and other additives used for e.g. surface treatment. Considering the cellulose content of these types of building wastes, they can be converted to ethanol instead of landfilling. Ethanol is nowadays produced from sugar-based materials such as sugarcane juice or starch-based materials such as corn or wheat. On the other hand, intensive research in the last three decades dedicated to produce ethanol from wood residuals, straw, and other lignocellulosic materials [3e5]. However, only a few publications were detected on ethanol production from C&D lignocellulosic wastes [6,7]. Lignocellulosic materials can be converted to ethanol by a process including pretreatment, hydrolysis of cellulose and hemicellulose to monomer sugars, fermentation of the sugars to ethanol, and finally distillation and purification of ethanol. Compared to the starch- or sugar-based materials, conversion of lignocelluloses is much more complicated due to their resistance to enzymatic attacks. Lignified and crystalline structure of wood-based wastes makes them inaccessible to enzyme. Therefore, pretreatment is an important stage for cellulose bioconversion processes, whose objectives are to separate lignin and hemicellulose from
2772
V. Jafari et al. / Renewable Energy 36 (2011) 2771e2775
cellulose, reduce the crystallinity of cellulose, and enhance the porosity of the materials so that the material will be prepared for further enzymatic degradation. Pretreatment should be efficient in obtaining these goals, avoiding degradation or loss of carbohydrate, and avoiding formation of inhibitory by-products for the subsequent hydrolysis and fermentation. There are several pretreatment methods developed for lignocelluloses, including physical, chemical, and biological methods [8e10]. Among these methods, pretreatment with alkali, acids, and cellulosic solvents are shown to be promising methods in improving enzymatic hydrolysis of different lignocelluloses [11]. One of the most efficient and promising method is concentrated phosphoric acid pretreatment that resulted in about 97% hydrolysis yield by isolation of lignin, acetic acid, and hemicelluloses [12]. Among the alkali pretreatments, sodium hydroxide was shown to be very efficient in improving high crystalline cellulosic materials [13]. This alkali pretreatment resulted in almost complete enzymatic conversion (99.1% of theoretical yield) of high crystalline cellulose (cotton and denim) to glucose. N-methylmorpholine-N-oxide (NMMO) is a nonderivatizing and environmentally friendly solvent of cellulose. The pretreatment of spruce wood with NMMO resulted in improving its ethanol yield from 6.8% to 89% by decreasing the cellulose crystallinity [14]. The purpose of this study was to evaluate whether C&D lignocellulosic wastes consisting of OSB, chipboard, plywood, and wallpaper are suitable raw materials for ethanol production via enzymatic hydrolysis and fermentation. Three promising pretreatment methods using sodium hydroxide, concentrated phosphoric acid, and NMMO were applied to improve the yield of ethanol from the wastes. 2. Materials and methods 2.1. Substrates The C&D wastes used in this study were three types of engineered wood products: OSB (oriented strand board), chipboard, and plywood, obtained from a local shop in Borås, Sweden, and wallpaper provided by Eco-Boråstapeter AB (Borås, Sweden). The substrates were milled and screened to obtain particles less than 1.0 mm in diameter. The compositions (carbohydrate and lignin fractions) of these species were characterized before and after the chemical pretreatments [15]. The total solid content of the pretreated and untreated samples were determined after 105 C drying for 24 h. 2.2. The pretreatment procedures 2.2.1. Concentrated phosphoric acid and acetone The pretreatment of 10 g substrate was carried out using 80 ml 85.9% phosphoric acid in a flask and shaker incubator at 50 C for 30 min [12]. Then, 400 ml acetone was added to the solid/liquid slurry and centrifuged at 4000 rpm and room temperature for 10 min. The centrifuged solids were washed with distilled water and centrifuged for three times. The pretreated substrates were then filtered with filter paper and further washed with hot distilled water until pH 7 obtained. 2.2.2. Sodium hydroxide In this pretreatment, 10 g of the substrate was mixed with 190 g 12% w/w sodium hydroxide and stirred for 10 min. The mixture was then cooled to 0 C and blended periodically for 3 h. The supernatant was then separated from the cellulosic materials with centrifugation, and washed with hot distilled water until neutral pH was obtained [16].
2.2.3. N-Methylmorpholine-N-oxide (NMMO) The commercial NMMO solution with 50% w/w water content (BASF, Ludwigshafen, Germany) was supplemented by 0.6 g/L propylgallate as an antioxidant to stabilize the mixture of cellulose and NMMO. The cellulose solvent was obtained by concentrating this solution to 85% NMMO using heating and stirring under vacuum. Then, 190 g of the NMMO solvent was mixed with 10 g of the waste species to obtain 5% (w/w) suspension in a 500 ml Erlenmeyer flask. The flasks were heated in an oil bath at 130 C for 1 h and mixed by glass rod every 15 min to dissolve the cellulose. After that, 300 ml boiling distilled water was rapidly added in order to stop the pretreatment and regenerate the cellulose. The suspension was then filtered under vacuum and washed until a clear filtrate was obtained [14,17]. 2.3. Enzymes and yeast strain Cellulase (SIGMA, C2730) and b-glucosidase (SIGMA, G0395) enzymes were used for enzymatic hydrolysis of the substrates. The activity of cellulase was determined [18] as 80 FPU/ml, whereas the activity of b-glucosidase was reported by the supplier as 2.52 IU/mg solid. Yeast strain Saccharomyces cerevisiae CCUG 53310 (Culture Collection, University of Göteborg, Sweden) was used for the cultivations. The yeast strain was maintained on agar plates containing (g/L): yeast extract 10, D-glucose 20, agar 20, and peptone 10. The yeast was cultivated in medium including 50 g/L glucose as carbon and energy source supplemented with nutrients (g/L): yeast extract, 5; (NH4)2SO4, 7.5; MgSO4.7H2O, 0.75; KH2PO4, 3.5; CaCl2.2H2O, 1. The pH was adjusted to 4.8 0.1, and then 250 ml prepared medium in 1000-ml cotton-plugged flasks were autoclaved and incubated in shaker incubators for 30 h at 30 C [19]. 2.4. Hydrolysis and fermentation Separate hydrolysis and fermentation (SHF) was performed. Enzymatic hydrolysis of the 5% w/v pretreated substrate without further drying was carried out with enzyme loading of 20 FPU cellulase and 50 IU b-glucosidase per gram of substrate at 45 C and pH 4.8 in 50 mM sodium citrate buffer for 96 h. The hydrolyzates were then supplemented with required nutrients [19], inoculated with 9 1 g/L S. cerevisiae, and anaerobically fermented at 30 C for 24 h. In anaerobic fermentation, the flasks were equipped with a loop-trap containing water to prevent air entrance while allowing the gas to exit. All data represent the average of duplicated experiments. 2.5. Analytical methods Carbohydrate and lignin contents of pretreated and untreated materials were analyzed according to NREL procedure [15], and the sugar analyses were carried out using a HPLC (Waters, Milford, USA) on an ion-exchange column (Aminex HPX-87P, Bio-Rad, USA) with ultra-pure water as eluent with flow rate 0.6 ml/min at 85 C. The hydrolyzes and fermentation profiles were examined by analysis of glucose and ethanol concentrations, using the HPLC on an ion-exchange column (Aminex HPX-87H, Bio-Rad) operated at 60 C with 0.6 ml/min flow rate and eluent of 5 mM sulfuric acid. 3. Results The C&D wastes i.e. OSB, chipboard, plywood, and wallpaper were first analyzed for carbohydrate and lignin contents, followed by hydrolysis of the materials without any pretreatment. It resulted in low yield of hydrolysis, and therefore, three different pretreatments were applied. Then, the pretreated materials were hydrolyzed and fermented to ethanol by S. cerevisiae. The results are described here:
V. Jafari et al. / Renewable Energy 36 (2011) 2771e2775 Table 1 Chemical composition of untreated substrates (weight percent of dry matter). Raw material
Glucan (%)
Mannan (%)
Xylan (%)
Lignin (%)
Total solid (%)
OSB Plywood Chipboard Wallpaper
42.2 43.9 43.0 42.5
16.1 12.6 14.0 10.8
5.2 4.6 6.8 5.3
31.6 33.2 29.4 19.8
95.1 94.3 93.2 78.4
3.1. Characterization of the raw materials Carbohydrate and lignin fractions of the OSB, chipboard, plywood, and wallpaper waste species were analyzed and the results are presented in Table 1. Glucan was the main constituent of all the materials, so these species of C&D waste could be promising alternatives as feedstock for ethanol production. The second most important sugar in all the materials was mannan, which probably indicates a high portion of softwoods in the C&D wastes. The other carbohydrate available in the wastes was xylose with 4.6e6.8%. More than 29% of the wastes were lignin in OSB, chipboard, and plywood, while less than 20% lignin was detected in the wallpaper. The sums of the four dominant components (glucan, mannan, xylan, lignin) were 95.1, 94.3, 93.2, and 78.4% in OSB, chipboard, plywood, and wallpaper, respectively. The extractives and adhesives present in the wastes were not analyzed and might be part of the undetected materials. The largest loss of material was observed for wallpaper, which could be due to its high quantity of color and surface treatment chemicals.
2773
The total recovery of the C&D wastes after pretreatment with phosphoric acid was 72e76% of the initial materials. High proportions of xylose, 28.3e50.0%, were lost during the treatment by phosphoric acid, while more than 80% of glucan in the wastes was recovered. A part of the lignin was also lost during the treatment by phosphoric acid. Sodium hydroxide had a better performance in term of total recovery, which was about 82e88% of the initial materials used. More than 89% of glucan was recovered after NaOH pretreatment. The recovery of mannan was also more than 92%; However, the recovery of xylose was lower than that of lignin except for plywood. Most of the wood wastes components (91e94%) were recovered in treatment by NMMO. However, the loss in carbohydrates depended on the raw materials. While more than 91% recovery of xylose was observed for OSB, plywood, and wallpaper, 20% of xylose in the chipboards was lost. 3.4. Enzymatic hydrolysis of treated wastes
The untreated waste species (OSB, chipboard, plywood, and wallpaper) were enzymatically hydrolyzed by adding 20 FPU/g cellulase and 50 IU/g b-glucosidase at 45 C and pH 4.8 in 50 mM sodium citrate buffers for 96 h. This resulted in hydrolysis of only 5.6%, 6.7%, 6.8%, and 3.8% of glucan of OSB, chipboard, plywood, and wallpaper, respectively. In order to improve these hydrolyses, the materials were subjected to pretreatment.
The treated waste species were hydrolyzed by addition of 20 FPU/g cellulase and 50 IU/g b-glucosidase at 45 C and pH 4.8 in 50 mM sodium citrate buffer for 96 h. The results are summarized in Table 3. It shows drastic improvements of hydrolysis yields after different pretreatments, compared to the hydrolysis of untreated samples. The average standard deviation for the hydrolysis results was less than 5.0%. The pretreatment by phosphoric acid improved the yield of enzymatic hydrolysis of OSB, chipboard, plywood, and wallpaper to 87.0e93.5% (Table 3). The pretreatment with sodium hydroxide resulted in more than 5.5-fold improvements in the subsequent hydrolyses, but it was not as effective as the acid and resulted in 38.2e73.4% hydrolysis of the glucan to glucose for the C&D wastes. The pretreatment with NMMO showed better results than NaOH for OSB, chipboard, and plywood, and the enzymatic hydrolysis was improved to 50.8e58.5% of the theoretical yield of glucan (Table 3). Contrary to the engineered wood wastes, NaOH pretreatment was more effective for wallpaper than NMMO; i.e. 73.4% conversion was observed in NaOH pretreatment compared to 43.5% conversion which was observed for NMMO-treated wallpaper.
3.3. Pretreatment of C&D wastes
3.5. Ethanol production from the building wastes
The C&D waste species were treated with three pretreatment methods including alkali (12% sodium hydroxide or NaOH), cellulose solvent (85% NMMO), and cellulose-dissolving acid (85.9% phosphoric acid). After the pretreatments, the compositions of carbohydrates and lignin in the materials were analyzed, and the recovery of each studied component is presented in Table 2. The average standard deviation for the results was less than 3.0%.
Production of ethanol from treated C&D waste was also investigated. The pretreated wastes were hydrolyzed and anaerobically fermented by S. cerevisiae for 24 h. The results are presented in Table 4. These results show that the enzymatic hydrolyzates can be efficiently converted to ethanol by S. cerevisiae. The yields of ethanol were higher than 80% for OSB, chipboard, and plywood. The yield of ethanol was relatively lower for wallpaper.
3.2. Enzymatic hydrolysis of the raw materials
Table 2 Carbohydrate and lignin recovery (%) after treatment. Material
Pretreatment process
Glucan (%)
Mannan (%)
Xylan (%)
Lignin (%)
Total recovery (%)
OSB
Phosphoric NaOH NMMO Phosphoric NaOH NMMO Phosphoric NaOH NMMO Phosphoric NaOH NMMO
82.0 91.7 98.8 83.5 89.1 95.4 80.2 93.2 98.4 85.4 91.1 97.7
55.9 92.5 92.5 63.6 93.6 99.3 50.8 96.8 98.4 58.4 97.3 90.8
53.8 75.0 94.2 50.0 63.2 80.0 71.7 87.0 91.3 57.0 74.0 100.0
74.7 79.4 90.5 78.9 85.4 91.9 85.8 86.4 96.1 83.4 85.0 91.0
72.3 83.1 91.0 76.1 88.2 94.4 75.6 87.5 94.2 75.3 82.0 92.1
Chipboard
Plywood
Wallpaper
acid
acid
acid
acid
2774
V. Jafari et al. / Renewable Energy 36 (2011) 2771e2775
Table 3 Yield of enzymatic digestibility of pretreated waste materials, as percentage of theoretical yield. Pretreatment
Untreated
Material\Hydrolysis time
24 h
48 h
96 h
Phosphoric Acid 24 h
48 h
96 h
NaOH 24 h
48 h
96 h
NMMO 24 h
48 h
96 h
OSB Chipboard Plywood Wallpaper
4.6 5.6 5.9 2.1
4.9 6.3 6.5 2.7
5.6 6.7 6.8 3.8
74.1 67.1 87.6 88.1
85.6 76.6 90.1 92.4
87.0 87.7 94.6 93.5
33.2 35.1 23.9 61.8
40.7 47.2 32.3 69.3
47.6 48.3 38.2 73.4
40.2 47.2 33.6 11.8
49.9 57.4 36.7 29.8
58.5 64.8 50.8 43.5
The enzymatic hydrolyses were performed using 50 g/L pretreated samples, 20 FPU/g cellulase and 50 IU/g b-glucosidase at 45 C.
4. Discussion At least one third of the wood wastes from construction and demolition wastes are structural engineered wood products [2]. The wallpaper waste produced during its manufacturing is also a considerable portion of wastes disposed to landfill or combusted in incinerators. Developing a beneficial and environmentally friendly method for conversion of these kinds of wastes to ethanol was the main objective of the current study. The results imply that the engineered woods can be beneficially converted to ethanol rather than going to landfill, with up to 94% hydrolysis yield and 0.42 g ethanol/g glucose. In other words, about 160 ml ethanol can be produced from each kg of the C&D wastes at the best conditions. In ethanol production from lignocelluloses, a pretreatment method is necessary to enhance the subsequent enzymatic hydrolysis. Beside the inherent resistance property of the original wood for feasible enzymatic hydrolysis rate, the presence of adhesives, colors, and method of wood processing in building wastes and wallpaper may make these materials more complicated and resistant to bioconversion. Inhibitory effects on both the enzymes and yeast might be possible. These could be the reasons for lower yields of hydrolysis of the untreated materials. The reason for lower hydrolysis yields of wallpaper might be presence of higher amounts of colors and surface treatment reagents. Phosphoric acid is able to disrupt lignocelluloses’ structure at concentration of 85.9% and eliminate the resistance of hemicelluloses and lignin in different lignocellulosic materials. In the case of construction and demolition wastes, some impurities such as gypsum, lime, color, and glue may react with phosphoric acid and might create some problems in recirculation of the acid. More investigation is necessary in order to deal with problems arising from these additives. Despite good results of the pretreatment by the acid, the high viscosity of phosphoric acid raises also the problem of mixing, especially in wallpaper, because of their pulpy structure. The remarkable changes in the hydrolysis rates of the cellulosic substances after phosphoric acid pretreatment might be due to the changes in accessibility of macromolecule structures, but not due to acid hydrolysis [20]. Easy recycling of phosphoric acid and its reconcentration is one of the main advantages of this method [12]. The composition of pretreated samples showed that sodium hydroxide slightly removed lignin and hemicelluloses at the applied conditions, but not as much as phosphoric acid. This finding
supports previous research performed by Zhao et al. [16] on alkaline pretreatment of spruce at different concentrations and temperatures. It also resulted that bioconversion efficiency of cellulose-to-glucose was enhanced up to 73% for C&D lignoellulosic wastes when using sodium hydroxide as pretreatment chemical. Working at low temperature under atmospheric pressure is an advantage of this process, which promotes dissolution of carbohydrates [16]. NaOH is also known to be very effective on pure cellulose [13], but not so effective on softwoods [21]. It should notice that the waste woods mainly consisted of softwood. Contrary to sodium hydroxide and phosphoric acid, no considerable change in cellulose, hemicelluloses, and lignin content of NMMO-treated samples was observed, although the weight loss could be due to the dissolution of components as well as inefficient filtration. NMMO improved the enzymatic hydrolysis of employed substrates at least 7.5-fold compared to untreated samples. The reason is that this pretreatment process gives higher fraction of b-glucosidic bonds accessible to cellulase due to the mostly amorphous structure [11]. The advantages of the NMMO pretreatment are that cellulose can be dissolved without any derivatization and the used NMMO can be recovered and reused almost completely [11,22]. Kuo and Lee [22] reported that the recycled NMMO was as effective as fresh NMMO in enhancing bioconversion of bagasse to ethanol. The hydrolysis yield of NMMO-treated wallpaper was less than that of engineered wood wastes. The rich-lignin content in engineered wood wastes plays the role of antioxidant in stabilizing the lignocellulose/NMMO mixture, due to the high antioxidant capability of lignin. This might be reason for higher efficiency of NMMO pretreatment on the substrate containing lignin. The comparison between the hydrolysis and fermentation results of treated and untreated wastes indicated that the pretreatment is a key process for efficient bioconversion of the building wastes. Therefore, optimization of the pretreatment seems to be necessary for optimization of bioconversion of the wastes. Furthermore, the construction and demolition wastes contain a variety of chemicals, e.g. adhesives, glues, and resins, which may arise some problem in pretreatment (especially in recycling of the treating agents, e.g. NaOH, phosphoric acid, and NMMO), enzymatic hydrolysis, and fermentation of these materials. More investigations are necessary to find the effects of these chemicals on the ethanol production process from the wastes. 5. Conclusion
Table 4 Ethanol yield from pretreated waste materials after 4 days enzymatic hydrolysis and one day fermentation. Pretreatment methods
Phosphoric acid NaOH NMMO
Ethanol yield (% of theoretical yield) OSB
Chipboard
Plywood
Wallpaper
82.2 83.1 84.2
80.2 84.1 78.3
84.3 80.4 85.0
76.3 70.5 74.4
Construction and demolition lignocellulosic wastes seem to be a potential raw material for ethanol production via enzymatic hydrolysis and fermentation. However, enzymatic hydrolysis of the wastes without pretreatment resulted in less than 7% of the theoretical yield. Pretreatment with concentrated phosphoric acid is an efficient method for improvement of ethanol production from building wastes, resulted in 87e95% hydrolysis of the cellulose. However, NMMO resulted in a better recovery of the lignocelluloses after the pretreatment.
V. Jafari et al. / Renewable Energy 36 (2011) 2771e2775
References [1] Center NR. Residential construction waste management demonstration and evaluation, first report, Prepared for U.S. Available from. Environmental Protection Agency, http://www.toolbase.org/PDF/CaseStudies/resi_constr_ waste_manage_demo_eval.pdf; 1995. [2] Gaskin J. Potential environmental risks of onsite beneficial reuse of ground engineered wood wastes from residential construction. Available from. College of Agricultural & Environmental Sciences, http://www.toolbase.org/ PDF/CaseStudies/resi_constr_waste_manage_demo_eval.pdf; 2004. [3] Gonzalez-Garcıa S, Gasol CM, Gabarrell X, Rieradevall J, Moreira MT, Feijoo G. Environmental profile of ethanol from poplar biomass as transport fuel in Southern Europe. Renew Energ 2010;35:1014e23. [4] Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, et al. The path forward for biofuels and biomaterials. Science 2006;311:484e9. [5] Taherzadeh MJ, Karimi K. Enzymatic-based hydrolysis processes for ethanol from lignocellulosic materials: a review. BioResources 2007;2:707e38. [6] O. Naoyuki. Pretreatment method of lignocellulose from waste building materials and production method of ethanol. Japanese patent 2006075007 (2006). [7] Tsutomu I, Tomoko S, Masanobu N, Kengo M, Shuji H, Kinji Sh. Alkali pretreatment for producing bioethanol fuel from lignocellulosics. Part 2. Bioethanol production from waste and recycled materials. Jpn Tappi J 2009; 63:581e91. [8] Eggeman T, Elander RT. Process and economic analysis of pretreatment technologies. Bioresour Technol 2005;96:2019e25. [9] Taherzadeh MJ, Karimi K. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. Int J Mol Sci 2008;9:1621e51. [10] McMillan JD. Bioethanol production: status and prospects. Renew Energ 1997; 10:295e302. [11] Kuo C-H, Lee C-K. Enhancement of enzymatic saccharification of cellulose by cellulose dissolution pretreatments. Carbohydr Polymer 2009;77:41e6.
2775
[12] Zhang YHP, Ding SY, Mielenz JR, Cui JB, Elander RT, Laser M, et al. Fractionating recalcitrant lignocellulose at modest reaction conditions. Biotechnol Bioeng 2007;97:214e23. [13] Jeihanipour A, Taherzadeh MJ. Ethanol production from cotton-based waste textiles. Bioresour Technol 2009;100:1007e10. [14]. Shafiei M, Karimi K, Taherzadeh MJ. Pretreatment of spruce and oak by N-methylmorpholine-N-oxide (NMMO) for efficient conversion of their cellulose to ethanol. Bioresour Technol 2010;101:4914e8. [15] Ruiz R, Ehrman T. Determination of carbohydrates in biomass by high performance liquid chromatography, Laboratory Analytical procedure, National Renewable energy laboratory. Lap 1996;002. [16] Zhao Y, Wang Y, Zhu JY, Ragauskas A, Deng Y. Enhanced enzymatic hydrolysis of spruce by alkaline pretreatment at low temperature. Biotechnol Bioeng 2008;99:1320e8. [17] Jeihanipour A, Karimi K, Taherzadeh MJ. Enhancement of ethanol and biogas production from high-crystalline cellulose by different modes of NMO pretreatment. Biotechnol Bioeng; 2009. [18] Adney B, Baker J. Measurement of cellulase activities, Laboratory Analytical procedure, National Renewable energy Laboratory. Lap 1996;006. [19] Karimi K, Emtiazi G, Taherzadeh MJ. Ethanol production from dilute-acid pretreated rice straw by simultaneous saccharification and fermentation with Mucor indicus, Rhizopus oryzae, and Saccharomyces cerevisiae. Enzym Microb Technol 2006;40:138e44. [20] Zhang YHP, Lynd LR. Toward an aggregated understanding of enzymatic hydrolysis of cellulose: noncomplexed cellulase systems. Biotechnol Bioeng 2004;88:797e824. [21]. Mirahmadi K, Kabir MM, Jeihanipour A, Karimi K, MJ Taherzadeh. Alkali pretreatment of spruce and birch to improve bioethanol and biogas production. BioResources 2010;5:928e38. [22] Kuo C-H, Lee C-K. Enhanced enzymatic hydrolysis of sugarcane bagasse by Nmethylmorpholine-N-oxide pretreatment. Bioresour Technol 2009;100: 866e71.
