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Miscanthus Biomass for Energy
(Zhu et al., 2015), dilute acid presoaking coupled with wet explosion (Sørensen
et al., 2008), dilute acid assisted with ionic liquids (Auxenfans et al., 2014),
alkaline peroxide and electrolyzed water (Wang et al., 2010), ammonia with
ionic liquids (Rodríguez et al., 2011), and aqueous ammonia with electron
beam irradiation (Yang et al., 2015b) have been proposed to boost sugar and
ethanol yields of Miscanthus biomass. Although some of these approaches,
when combined, are potentially efficient for removing lignin and hemicellulose, complicated procedures need extra capital investment and operating
costs. Besides, research only demonstrates their feasibility in the laboratory,
more industrial or life cycle assessments coupled with detailed process economics are required before commercialization.
10.3.2 Enzymatic Hydrolysis and Fermentation
Apart from chemical compositions caused by external and intrinsic elements and pretreatment methods as discussed previously, solid and enzyme
loading, surfactant addition, and applied microorganisms are also responsible for enzymatic hydrolysis and fermentation performances of biomass
(Vanderghem et al., 2012).
Low solid loading would be beneficial for shortening enzymatic hydrolysis and fermentation duration and reaching high ethanol yield but causing
low ethanol titer, which could be unable to meet the minimal requirement
(around 40 g L −1 ) for commercial ethanol distillation. Simultaneous saccharification and fermentation (SSF) of Miscanthus biomass was commonly
conducted at solid loading less than 15% with the maximum ethanol titer
less than 30 g L −1 (Cha et al., 2015a; Scordia et al., 2013; Yoo et al., 2016). Given
60% of cellulose in pretreated Miscanthus biomass and 90% of glucose-toethanol conversion efficiency, the lowest solid loading for SSF should be
greater than 13% to achieve 40 g L −1 of ethanol concentration. Increasing
solid loading within certain limits would theoretically enhance ethanol
concentration. High solid loading with advantages of high ethanol titer
and less water consumption is preferred from cost-efficiency and environmental standpoints (Chen et al., 2016). However, the decrease in glucan-toethanol yield is inevitable due to hydrophobic absorption between lignin
and cellulase inhibiting enzymatic absorption and insufficient mixing
(Kristensen et al., 2009). Several improvement strategies, such as surfactant
addition (Alam et al., 2019), size reduction (Khullar et al., 2013), and thermotolerant microbial strains (Cha et al., 2015a), have been explored to enhance
enzymatic saccharification and microbial digestion at high solid loading.
However, initial studies aimed only to ferment glucose derived from cellulose using hexose-consuming microbial strains such as Saccharomyces
cerevisiae. Recently, modified strains from genetic engineering capable of
digesting pentose and hexose simultaneously and pentose-metabolizing
bacterial strains of Escherichia coli have been developed to ferment the
potential sugars in biomass sufficiently.