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Miscanthus Biomass for Energy
10.3 Bioethanol Production
Several studies have reported using various pretreatment steps to reduce
the recalcitrance of Miscanthus biomass for the valorization of macromolecules (Brosse et al., 2012; Ge et al., 2016). Miscanthus biomass is characterized by complex components, composing of cellulose, hemicellulose, and
lignin (Zub & Brancourt-Hulmel, 2010). Therefore, an optimal combination
of various pretreatment strategies is essential for efficient fractionation and
further bioethanol production. The processing methods include mechanical treatment to improve the biomass maneuverability, thermochemical
steps for the disruption and solubilization of unproductive compounds,
and subsequent enzymatic hydrolysis and fermentation (Figure 10.1). The
effectiveness of such processing procedures on the chemical composition, sugar recoveries, inhibitor formation, lignin removal, and bioethanol
production performances is summarized and discussed in the following
subsections.
10.3.1 Physicochemical Pretreatment
The reduction of particle size by mechanical chopping, grinding, or milling
is often an initial pretreatment step of the solid starting feedstocks to facilitate subsequent thermochemical or enzymatic hydrolysis and fermentation
disrupting their structural regularity and reducing the degree of crystallinity and polymerization (Hendriks & Zeeman, 2009). Generally, small particle
sizes are preferred for efficient enzymatic hydrolysis due to their specific
surface area, i.e., sugar accessibility. The particle size distribution is directly
associated with chemical composition, sugar recoveries, delignification, and
fermentation performances (Khullar et al., 2013). Generally, mechanical treatment by itself is incapable of disrupting and depolymerizing lignin that seals
cellulose and hemicellulose tightly (Sun et al., 2016). Many thermochemical
methods to conduct after preliminary size reduction have been investigated,
including either the utilization of concentrated and dilute acid and alkali
(Alam et al., 2019; Scordia et al., 2013; Si et al., 2015; Yoo et al., 2016; Zhao
et al., 2020b), liquid hot water (LHW) and steam explosion (Li et al., 2013; Yeh
et al., 2016), organosolv and ionic liquids (Brosse et al., 2009; Dash & Mohanty,
2019; Kim et al., 2018), or a combination of processing (Auxenfans et al., 2014;
Rodríguez et al., 2011; Wang et al., 2010; Zhu et al., 2015). However, depending on the pretreatment conditions, various components might be formed as
inhibitors that limit enzymatic activity.
Dilute acid (organic and inorganic acids) and alkali (metal hydroxide and
aqueous ammonia) pretreatments have been extensively explored to enhance
the enzymatic digestibility of Miscanthus biomass (Alam et al., 2019; Ji et al.,
2015; Si et al., 2015; Vanderghem et al., 2012; Yoo et al., 2016). Since glucosidic bonds of cellulose and hemicellulose are susceptible to acid, a high