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Phytotechnology with Biomass Production
10.4 Biomethane and Biohydrogen Production
Biomethane and biohydrogen production is a renewable and sustainable technological process for Miscanthus biomass by anaerobic digestion (Figure 10.1).
Compared to grain such as maize, Miscanthus-based biogas presents more
economical and environmental advantages (Wagner et al., 2019).
Traditionally, Miscanthus biomass is first subjected to mechanical chopping and inoculated with anaerobic sludge performing SSF at thermophilic
and mesophilic conditions (Klimiuk et al., 2010). The sewage or wastewater
sludges, crop silages, and animal manures are commonly utilized as inoculum, which is composed of acetogenic bacteria and methanogens (Guo et al.,
2010; Kiesel & Lewandowski, 2017; Klimiuk et al., 2010). During fermentation, biomacromolecules (cellulose and hemicellulose) are hydrolyzed to
monomeric sugars (hexose and pentose) and then digested to organic acids
and hydrogen by homoacetogens. The acetic acid and hydrogen generated
as critical intermediates are rapidly consumed and transformed into CH 4
by methanogens (Guo et al., 2010). The potential of Miscanthus biomass for
methane production has been reported to differ according to biomass harvest time, genotype, and plant fractions (Mangold et al., 2019; Schmidt et al.,
2018; Wahid et al., 2015). This variation is associated with the compositional
differences in the starting biomass, which can be mainly reflected by lignin
incrustation that slowed down the enzymatic hydrolysis efficiency of polysaccharides (Klimiuk et al., 2010). In the case of fermentation, operational
conditions, such as slurry pH, pressure, temperature, and microbial strains,
also reflected notable differences in fermentation processes and were directly
correlated with the conversion efficiency of carbohydrates to methane and
hydrogen (Guo et al., 2010).
For biomethane production, physicochemical pretreatment methods have
been often proposed to enhance biomass-to-biogas conversion efficiency by
fractionation and decomposition of recalcitrant structures of Miscanthus
biomass. Similar to bioethanol production, these mainly include size reduction, ensiling, steam explosion, LHW, acid, alkali, aqueous ammonia soaking, hydrogen peroxide, and enzymatic pretreatments (Jurado et al., 2013;
Katukuri et al., 2017; Li et al., 2016; Menardo et al., 2013; Michalska et al.,
2015; Nges et al., 2016; Zhou et al., 2017). However, the effectiveness of the
pretreatment highly varied with pretreatment methods and conditions. For
example, ensiled biomass showed higher methane yield and digestion rate
than unensiled biomass (Mangold et al., 2019). Aqueous ammonia soaking
increased methane yield by 25%–27% (Jurado et al., 2013), and hydrogen peroxide pretreatment increased methane yield by 49% (Katukuri et al., 2017). In
the case of biohydrogen, one-step extrusion-NaOH pretreatment at moderate
temperature resulted in 77% delignification and more than 95% of cellulose
recovery as well as enhanced hydrogen yield (de Vrije et al., 2002). Mild alkali
pretreatment assisted with Caldicellulosiruptor saccharolyticus and Thermotoga