Article pubs.acs.org/EF
Bioethanol Production from Hydrothermally Pretreated and Delignified Corn Stover by Fed-Batch Simultaneous Saccharification and Fermentation Cristian-Teodor Buruiana,† Camelia Vizireanu,† Gil Garrote,*,‡,§ and Juan Carlos Parajó‡,§ †
Department of Food Science, Food Engineering and Applied Biotechnology, Faculty of Food Science and Engineering, ’’Dunarea de Jos’’ University of Galati, 111 Domneasca Street, 800201 Galati, Romania ‡ Department of Chemical Engineering, Faculty of Science, University of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Ourense, Spain § CITI (Centro de Investigación, Transferencia e Innovación), University of Vigo, Tecnopole, San Cibrao das Viñas, 32900 Ourense, Ourense, Spain S Supporting Information *
ABSTRACT: Bioethanol was manufactured from corn stover after consecutive stages of hydrothermal pretreatment (to solubilize hemicelluloses) and delignification. Hydrothermal pretreatment was carried out by heating aqueous suspensions of corn stover up to reach 180−200 °C, and the resulting solids were delignified with ethanol−water solutions (containing 30−70 wt % of ethanol) at 180−200 °C. The chemical processing was optimized using the Response Surface Methodology. Delignified solids were employed as substrates for fed-batch Simultaneous Saccharification and Fermentation (SSF) under selected conditions (liquid to solid ratio = 4 g/g, enzyme to substrate ratio = 10 FPU/g substrate). The maximum ethanol concentration (67.5 g ethanol/L, corresponding to 89.1% glucan conversion into ethanol) was obtained using samples pretreated hydrothermally at 180 °C and delignified at 200 °C in a medium containing 30 wt % ethanol.
1. INTRODUCTION The interest in alternative sources for manufacturing transportation fuel is boosted by the increasing prices of fossil fuels and by concerns regarding global warming and greenhouse gas emissions.1,2 Many countries have directed state policies toward the utilization of biomass for both meeting their future energy demands and decreasing their dependence on the supply of fossil fuels.3 Bioethanol, the most widely used transportation biofuel, can be produced from different types of biomass: sucrosecontaining feedstocks, starch-containing materials, and lignocellulosic materials (LCM).1 LCM are mainly composed of cell wall structural components (lignin, cellulose, and hemicelluloses). LCM contain about 15−35% lignin4 and 45−65% polysaccharides (including cellulose and hemicelluloses). Lignin is an aromatic polymer made up of phenylpropane units bonded via covalent bonds to hemicelluloses, conferring rigidity and a high level of compactness to the plant cell wall.5 Cellulose is a linear homopolymer of glucose monomers linked by β-(1→4)-glycosidic bonds, whereas hemicelluloses are heteropolymers made up of a number of sugars and substituents (such as acetyl groups, uronic acids, and esterified phenolic acids). The manufacture of bioethanol from LCM may be carried out using enzymes capable of hydrolyzing polysaccharides to sugars, followed or coupled with fermentation of sugars to ethanol.6 Owing to the poor susceptibility of native LCM to enzymes, the raw materials have to be pretreated before hydrolysis7,8 in order to cause chemical and structural modifications.9−12 © 2014 American Chemical Society
The pretreatment is one of the most important individual steps in processes for bioethanol production from LCM. Recently, the hydrothermal fractionation (or autohydrolysis) has received attention because of its environmentally friendly character: this technology uses just water for LCM processing, avoiding acid recovery or acid precipitation stages and related solid disposal or handling operation.13 Hydrothermal treatments dissolve some LCM components and can be tuned to cause hemicellulose solubilization as the major effect. Under selected conditions, hemicelluloses can be extensively solubilized and converted into sugar oligomers and monosaccharides.14,15 In comparison with other pretreatment technologies, hydrothermal processing shows a number of advantages, including the following: (i) no chemicals different from water and LCM are needed, limiting the environmental impact; (ii) hemicelluloses can be broken down into soluble saccharides at good yields with limited generation of byproducts; (iii) corrosion problems are limited; (iv) no acid neutralization or recirculation is needed; (v) the physicochemical modification of substrates increases the susceptibility to enzymatic hydrolysis and facilitates the further separation of lignin and cellulose.16,17 Organosolv delignification has been employed for improving the susceptibility of native LCM to enzymatic hydrolysis. For example, the Lignol process combines two major stages: ethanol-based organosolv (which removes lignin, hemicelluReceived: November 19, 2013 Revised: January 28, 2014 Published: January 29, 2014 1158
dx.doi.org/10.1021/ef402287q | Energy Fuels 2014, 28, 1158−1165
Energy & Fuels
Article
loses, and extractives from the substrate) and enzymatic saccharification - fermentation of the delignified solids to produce ethanol from the cellulosic fraction.12,18,19 Considered as a solvent for delignification, ethanol has the advantage of being easily recovered by distillation, whereas hemicellulosic saccharides (about half in oligomeric form) are present in the liquid phase.20 Some advantages of organosolv methods can be summarized as follows: (i) efficiency and selectivity, with limited degradation of polysaccharides and lignin precipitation; (ii) environmentally friendly character, with reduced emissions of greenhouse gases; (iii) from an economic perspective, good yields are obtained in facilities with limited investment costs; (iv) organic solvents can be recovered and reused; (v) lignin and polysaccharides can be used as substrates for manufacturing fuels and chemicals.19 Compared to acid hydrolysis, the enzymatic hydrolysis of cellulose to glucose presents low toxicity, limited utility costs, and low corrosion.3 The enzymatic hydrolysis and fermentation steps can be carried simultaneously (according to the Simultaneous Saccharification and Fermentation technology, SSF), which presents the following advantages: (i) reduced end-product inhibition of the hydrolytic enzymes, (ii) reduced chance for contamination; and (iii) reduced capital cost.21,22 SSF allows higher bioethanol yields and requires less enzymes, because the end-product inhibition caused by the glucose formed from cellulose is relieved by the yeast fermentation.23 On the other hand, when SSF is performed in fed-batch mode, the gradual substrate supply results in decreased concentrations of inhibitors, which can be assimilated by microorganisms (at least in part) in a way that their inhibition effects on enzymes are decreased.24 This work deals with the optimization of a biorefinery scheme in which corn stover is first subjected to hydrothermal pretreatment under nonisothermal conditions for recovering valuable hemicellulose-derived compounds in liquid phase, followed by organosolv delignification of the resulting solids with ethanol−water solutions to remove lignin, and further production of second generation bioethanol production by fedbatch SSF of hydrothermally pretreated-delignified solids. The effects of selected variables (severity in hydrothermal stage, temperature in delignification stage, and ethanol concentration in delignification media) on selected operational variables were assessed in selected experiments. Fed-batch SSF of the most favorable pretreated samples was performed in media containing enzymes and Saccharomyces cerevisiae cells.
Table 1. Raw Material Composition and Operational Conditions Employed in the Hydrothermal Pretreatment TA (°C)
raw material
180
190
3.39 3.64 S0 (severity) (dimensionless) Material Balance Data (g ACS/100 g raw material, oven dry basis) YA (hydrothermal pretreatment 76.0 73.0 yield) NVC (nonvolatile compounds) 18.3 23.2 Solid Phase Composition (g/100 g HDCS, oven dry basis)a Gn (glucan) 34.48 40.45 43.60 Xn (xylan) 14.54 15.78 13.78 Arn (arabinan) 2.16 1.92 1.29 AcG (acetyl groups) 1.93 1.49 1.22 KL (Klason lignin) 18.49 22.08 23.40 extractives 12.20 ash 5.00 proteins 4.25 Liquid Phase Composition (g monomer equivalent/L or g/L) gluco-oligomers 1.67 1.78 xylo-oligomers 2.68 5.80 arabinosyl groups in oligomers 0.62 0.94 oligomers acetyl groups in oligomers 0.14 0.38 glucose 0.72 0.67 xylose 0.29 0.27 arabinose 0.14 0.23 acetic acid 1.98 2.14 hydroxymethylfurfural
