agronomy Article Alternative Lime Pretreatment of Corn Stover for Second-Generation Bioethanol Production Iria Fírvida † , Pablo G. del Río † , Patricia Gullón , Beatriz Gullón *, Gil Garrote and Aloia Romaní   Citation: Fírvida, I.; del Río, P.G.; Gullón, P.; Gullón, B.; Garrote, G.; Romaní, A. Alternative Lime Pretreatment of Corn Stover for Second-Generation Bioethanol Production. Agronomy 2021, 11, 155. https://doi.org/10.3390/ agronomy11010155 Received: 15 December 2020 Accepted: 12 January 2021 Published: 15 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Chemical Engineering, Faculty of Science, Campus Ourense, Universidade de Vigo, As Lagoas, 32004 Ourense, Spain; beagullon@gmail.com (I.F.); pdelrio@uvigo.es (P.G.d.R.); pgullon@uvigo.es (P.G.); gil@uvigo.es (G.G.); aloia@uvigo.es (A.R.) * Correspondence: bgullon@uvigo.es † These authors contributed equally to this work. Abstract: In this work, a delignification process, using lime (Ca(OH) 2 ) as an alternative alkali, was evaluated to improve enzymatic saccharification of corn stover cellulose, with the final goal of obtaining second-generation bioethanol. For that, an experimental design was conducted in order to assay the effect of temperature, lime loading, and time on the corn stover fractionation and enzymatic susceptibility of cellulose. Under conditions evaluated, lime pretreatment was selective for the recovery of cellulose (average of 91%) and xylan (average of 75.3%) in the solid phase. In addition, operating in mild conditions, a delignification up to 40% was also attained. On the other hand, a maximal cellulose-to-glucose conversion (CGC MAX ) of 89.5% was achieved using the solid, resulting from the treatment carried out at 90 ◦ C for 5 h and lime loading of 0.4 g of Ca(OH) 2 /g of corn stover. Finally, under selected conditions of pretreatment, 28.7 g/L (or 3.6% v/v) of bioethanol was produced (corresponding to 72.4% of ethanol conversion) by simultaneous saccharification and fermentation. Hence, the process, based on an alternative alkali proposed in this work, allowed the successful production of biofuel from the important and abundant agro-industrial residue of corn stover. Keywords: corn stover; biorefinery; lime pretreatment; delignification; bioethanol 1. Introduction A sustainable future relies on an increased share of eco-friendly energy, particularly in developing countries. Consequently, biofuels are considered important substitutes for oil-based combustibles, solving the depletion issue of non-renewable sources and decreasing the ecological problems related to them [1,2]. One of the most employed biofuels is bioethanol, which is an alternative for gasoline in vehicles and can be obtained from sources of sugar (namely sucrose and starch) via fermentation. The ethanol obtained from different food-related sources, such as sugarcane, sugar beet, or maize (sources of sucrose and starch, respectively) is commonly known as first-generation bioethanol [3–5]. Among sugars sources, corn or maize (scientific name: Zea mays L.) is considered a yearly crop, which is generated worldwide in enormous amounts to obtain feed, food, and material for applications in the industry [6]. However, the use of crops as feedstock may lead to some conflicts, augmenting the demand and price of foods [7–9]. In this way, the use of alternative, ubiquitous, and renewable sources is fundamental for the production of biofuels under a more profitable and sustainable point of view [10–14]. Lignocellulosic materials (LCMs) fulfill these characteristics, including attractive choices for the partial replacement of fossil fuels, like remarkable availability, ubiquity, enhancement of local economy resulting from cultivation, carbon neutrality, and appropriateness for the manufacture of LCM-derived ethanol [15,16]. Regarding the different LCMs, corn stover is an agricultural residue, a by-product from the grain harvesting of maize, and is considered an appealing feedstock for the manufacture of advanced biofuels or second-generation ethanol, due to its availability in Agronomy 2021, 11, 155. https://doi.org/10.3390/agronomy11010155 https://www.mdpi.com/journal/agronomy