Subcritical water hydrolysis pretreatment of sugarcane bagasse to produce second generation ethanol
Graphical abstract
Introduction
Fossil fuels are the most important source of energy worldwide, raising deep environmental issues as the consumption of these resources generates greenhouse gases, potentially causing irreversible damage to the planet. To prevent this and reduce global dependency on fossil fuels, alternative sources of energy are being developed. An alternative is the use of agricultural residues from plant sources that have a lignocellulosic fraction capable of converting to fermentable sugars and producing biofuels [1].
Indeed, sugarcane bagasse is a residue generated from sugarcane production that may be appropriate for 2 G ethanol production [2]. Agricultural waste is generally unsuitable for fermentation because the lignocellulosic complex – the structural component of a plant composed of hemicellulose, cellulose, and lignin – is too stable for mechanical decomposition or even biological depolymerization [3]. This prevents yeast (Saccharomyces cerevisiae), which requires simple sugars as a substrate, from digesting the material and producing ethanol. Converting lignocellulosic biomass (LB) into simple sugars is the long-standing bottleneck preventing the cost-effective production of 2 G biofuels, such as ethanol.
LB can be made appropriate for fermentation after a pre-treatment step [4]. Pre-treatment of LB is a challenge because the process must breakdown biomass efficiently, avoiding the formation of sub-products that inhibit fermentation [5,6]. Different pre-treatment technologies have been tested, such as acidic, alkaline, enzymatic, and chemical hydrolysis with ferric chloride and surfactants [[7], [8], [9], [10], [11], [12], [13], [14], [15]]. Dilute acid hydrolysis is the closest to commercialization further, and, this pre-treatment contributes a significant fraction of the total costs of producing 2 G ethanol [16]. An environmentally friendly alternative that may avoid these problems is SWH. Subcritical water takes place when water is heated to temperatures higher than the standard boiling point (100 °C), under pressure high enough to keep water in the liquid state. In this state, water changes its physicochemical properties, high density, and low viscosity, enabling efficient penetration in the cellulosic structure, decreased dielectric constant, and increased organic compound solubility [[17], [18], [19]]. Unlike acid treatment, not only SWH use produces no chemical wastes, but it also makes the water-based process more environmentally innocuous.
Previous work described the SWH treatment from different LB in order to obtain fermentable sugars, for instance, Lachos-Perez et al. [20] obtained fermentable sugars yield of 15.5 % from sugarcane bagasse by SWH, besides Prado et al. [1] reported fermentable sugars yield of 23.1 % from the same raw material in SWH; otherwise, Maravić et al. [21] observed the SWH effect on beet pulp by SWH obtaining fermentable sugars yield of 15.84 %; Abaide et al. [22] reached fermentable sugars yield of 33.5 % from SWH of rice straw; among other works [[23], [24], [25], [26], [27]]. However, there are few works related to the potential production of 2 G ethanol from SWH treatment. Wang et al. [28] evaluated an integrated system for producing ethanol and methane from SWH of corn stover and, the authors obtained 2.7 and 6.94 g L−1 ethanol 2 G.
As the operational conditions required to breakdown the LB are severe, degradation of fermentable sugars occurs together with the hydrolysis process. This degradation carries out to the formation of organic acids, such as 5-Hydroxymethylfurfural (5-HMF) and furfural (FF), among others, which are toxic to fermentative yeasts and thereby inhibit fermentation [5,6,29]. Flow-through SWH has been proposed as a method to reduce the degradation of sugars to produce degradation products [30] and to reduce the time residence by sugars in the reactor zone by continuously removing them as soluble products [31]. The challenge is that flow-through conditions result in the production of dilute hydrolyzate mixtures that may not be suitable for direct fermentation [32]
The objective of this work was to examine the use of flow-through SWH as a pre-treatment for the production of 2 G ethanol from sugarcane bagasse. Because dilute sugar concentration is a potential problem with flow-through pre-treatment, two strategies were employed to increase the level of fermentable sugars in the hydrolyzate: 1) partial removal of water using a rotary evaporator, simulating the flash evaporation approach advocated by Archambault-Léger et al. [32] and 2) enzymatic hydrolysis of the hydrolyzate (to convert oligomers into fermentable sugars [33]) and/or the solids recovered after flow-through hydrolysis treatment. Experimental evaluation of the evaporative concentration approach is necessary since evaporation may increase the concentrations of inhibitors as well as fermentable sugars, offsetting the anticipated benefits. Accordingly, the resulting hydrolyzates were fermented using either S. cerevisiae SA-1 on its own or a combination of S. cerevisiae SA-1 and the pentose metabolizing yeast, S. stipitis NRRLY-7124 to determine 2 G ethanol production potential. The results of this study provide the basis for future work to develop SWH as an environmentally benign approach for the conversion of sugarcane bagasse into ethanol.
Section snippets
Raw materials
Dry sugarcane bagasse (Saccharum officinarum Linnaeus) was supplied by the National Biorenewables Laboratory (LNBR) of the National Center for Energy and Materials Research (CNPEM) located in the city of Campinas in the state of São Paulo, Brazil. Initially, the material was ground using a knife mill (Marconi, model MA 340, Piracicaba, SP, Brazil). Particles were selected by size using Tyler series sieves and a magnetic type vibratory stirrer (Bertel, model N.1868, Caieiras, SP, Brazil).
Characterization of the raw material
Table 1 shows the characterization of sugarcane bagasse. Protein content (1.5 ± 0.1 %) and total extractives (6.1 ± 0.5 %) were similar to values reported by Lachos-Perez et al. [20]. The content of soluble and insoluble lignin was 2.40 ± 0.5 % and 19.70 ± 0.7 %, respectively, similar to those reported Ávila, Forte and Goldbeck [11]. Cellulose and hemicellulose were found to be 33 ± 1 % and 28 ± 1 %, respectively. Santo et al. [45] reported similar values of 38.3 % and 20.1 %, with the
Conclusion
Flow-through SWH has the potential for converting the hemicellulose content of sugarcane bagasse into simple sugars while minimizing the formation of degradation products and co-producing a cellulose-rich solid fraction that is suitable for enzymatic hydrolysis to produce glucose. The maximum xylose concentration was obtained under the following operating conditions: temperature 200 °C, a flow rate 5.0 mL min–1, pressure 15 MPa, and a collection time of 19 min (Experiment 2). Several
Declaration of Competing Interest
None.
Acknowledgments
The authors acknowledge the financial support of the Higher Level Personnel Improvement Coordination (CAPES)– Brazil-Finance Code 001 and process: 88887.370357/2019-00. The authors acknowledge the financial support of the São Paulo – FAPESP (2018/05999-0; 2018/14938-4; 2018/14582-5; 2018/23835-4, 2019/08542-3). T. Forster-Carneiro thanks CNPq for the productivity grant (302473/2019-0). "Research supported by the Pilot Plant for Process Development of CTBE – Brazilian Bioethanol Science and
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