Abstract
Cellulosic bioethanol production has been fraught with challenges, including fluctuations in feedstock supply, handling costs, pretreatment, enzymes, and other logistical problems. Most studies of lignocellulosic ethanol production have focused on a single type of biomass; however, full utilization of various lignocellulosic biomass sources might enhance bioethanol production and the economic feasibility of the biorefinery. The goal of this study was to evaluate the effectiveness of popping pretreatment on saccharification and fermentation for individual and mixed biomass. We then compared separate hydrolysis and fermentation (SHF) with simultaneous saccharification and fermentation (SSF) processing, with the aim of optimizing production of bioethanol from biomass. Saccharification efficiencies were increased significantly in all the popping-pretreated compared to the non-pretreated individual and mixed biomass. The SSF was superior compared to SHF processing. Our results indicated that the saccharification efficiencies of both individual and mixed biomass were improved after popping pretreatment; in particular, the production of bioethanol from mixed biomass was identified as a suitable approach for more extensive application.
Introduction
The energy industry is constantly looking for renewable and environmentally friendly energy resources. Among the suggested solutions, bioethanol has recently emerged as an effective solution to address the concerns arising from limited fossil fuels and the effects of greenhouse gas emissions. The industrial-scale production of lignocellulosic bioethanol from lignocellulosic biomass has been investigated for many years (Energy, 2015). Biomass available from agricultural residue and waste is generated from harvesting and processing cultivated crops. It is cheaper than starch and does not compete with food sources, making it is attractive for utilization in bioconversion.
In tropical countries such as Vietnam, coffee (Coffea canephora), cassava (Manihot esculenta), and coconut (Cocos nucifera) are common crops. Huge amount of residues is generated after harvesting of coffee, cassava, and coconut, but only small amounts of their residue yields are used for handicrafts or fertilizer production, while the remainder is mostly burned or considered waste, becoming a source of pollution (Prata and Oliveira, 2007, Mussatto et al., 2011, Esquivel and Jiménez, 2012, Ferraz et al., 2012, Nuwamanya et al., 2012, Wi et al., 2015). According to the Food and Agriculture Organization (FAO) of the United Nations (FAO, statistics, updated to 2013), in each year, 1.5, 9.8, and 1.3 million tons of coffee bean, cassava starch, and coconut, respectively, are produced in Vietnam. For every kg of coffee bean, cassava starch, and coconut produced, approximately 1.0, 0.4, and 0.4 kg of CH, CS, and CC, respectively, are generated (Esquivel and Jiménez, 2012, Nuwamanya et al., 2012).
The operational costs of producing ethanol from biomass are increased due to the necessary addition of a pretreatment step. Pretreatment destroys recalcitrant structures consisting of cellulose, hemicelluloses, and lignin to improve accessibility of poly-carbohydrate components to enzymes (Hendriks and Zeeman, 2009, Sarkar et al., 2012). One promising pretreatment method involves popping (Choi et al., 2012, Wi et al., 2011, Wi et al., 2015), which is based on a non-chemical physical pretreatment principle and allows a simple system to be used to achieve greater saccharification efficiency, with lower environmental impact compared with conventional methods (Wi et al., 2013).
A consistent, stable supply of sustainable feedstock from a variety of sources is required to support large-scale lignocellulosic bioethanol production. Unfortunately, mixed biomass feedstock has rarely been the subject of experimental studies due to differences in composition and density among different types of feedstock (Sokhansanj and Hess, 2009, Tumuluru et al., 2011). One approach that has been studied extensively involves the formation of a pellet after grinding and densification (Panwar et al., 2010), which is then pretreated under harsh conditions to change the distribution of the lignin and reduce biomass recalcitrance (Rijal et al., 2012). This process consumes a large amount of energy. In addition to providing diversified sources of biomass, the use of mixed biomass might be an important alternative when constructing a lignocellulosic bioethanol production facility. In this study, we examined a popping pretreatment method for individual and mixed biomass samples prepared from leftover solid CH, CS, and CC wastes under identical conditions, and performed bioethanol conversion.
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