Page 310 - Biofuels for a More Sustainable Future

276 Biofuels for a More Sustainable Future

2.2 Biomass conversion to biofuel
Biomass conversion is a critical stage in the BSC to convert biomass feed-
stocks to biofuels and/or biochemicals. Biomass conversion happens in
biorefineries, which are analogous to petroleum refineries that typically
produce multiple fuels and chemicals from petroleum crude. According
to IEA Bioenergy Task 42, biorefining is “sustainable processing of bio-
mass into a spectrum of marketable products and energy” (Cherubini,
2010; Cherubinietal., 2007). There is a wide range of technologies to
convert different types of biomass feedstocks into building blocks and then
into value-added products. Those technologies can be categorized into
two main types: biochemical and thermochemical (Cherubini, 2010; An
et al., 2011b).
Common biochemical processes in biofuel production include fermen-
tation and anaerobic digestion. Fermentation process employs microorgan-
isms to convert sugars and starch into recoverable products (e.g., ethanol).
For sugar-based (e.g., sugarcane and sweet sorghum) and starch-based bio-
mass (e.g., corn grain), minimum pretreatment is needed for size reduction
and extraction. For lignocellulosic biomass (e.g., corn stover, woody bio-
mass), pretreatment and enzymatic hydrolysis are necessary to obtain
sugars (Cherubini, 2010; Humbird et al., 2017). Fermentation time and
temperature can vary based on different microorganisms used (Dutta
et al., 2011). After distillation, bioethanol usually can be either for further
production (e.g., polyethylene, Mohsenzadeh et al., 2017)or senttobe
blended with conventional gasoline [e.g., E15 (Romano and Zhang,
2008)]. Choosing the bioethanol pathway can influence BSC design in
several aspects. On the supplier side, as sugar- and starch-based biomass
are feedstocks in the food industry, biomass supply and price can be in
equilibrium with the food market, which may bring in more uncertainties
(Bai et al., 2012). On the end-use side, as bioethanol can be blended with
gasoline, the market demand and price can relate to vehicle fuel market
(Wang et al., 2013).
Anaerobic digestion breaks down biodegradable biomass by bacteria in
anaerobic ambient (Cherubini, 2010; Sharma et al., 2013). The digestion
temperature usually ranges from 30 to 65°C. One common product of these
processes is biogas that can be upgraded to biomethane, an alternative to nat-
ural gas (Romano and Zhang, 2008). Biogas production can affect the BSC
design mainly in two aspects. The transportation network and supply chain
needs to be carefully designed for decentralized (e.g., farm-based) or