and biofuels, (iii) medium-value-added products (bulk compound) plus biofuels, and
finally, (iv) coupled to other processes in the context of circular economy. Illustrative
works about these categories are shown in Table 5 and Fig. 3.
4.1 Strategy I: Biofuel Production Only
(Low-Value Compound)
Biofuels are required at low price and in large volume; however, among all
products obtained from microalgae, biofuels have the lowest cost and their price is
always compared with fossil alternatives; moreover, the energy balance of the
production process must be positive. As was shown in Sect. 2 and Table 5, most of
the biofuels can be conceptually obtained from microalgae, but biodiesel is one of
the most interesting alternatives as liquid fuel for transportation (Amaro et al. 2011;
Mallick et al. 2016; Mondal et al. 2017). Nonetheless, it has been demonstrated that
currently microalgae biodiesel is not cost competitive when compared with fossil
diesel (Chisti 2007). Therefore, the idea of using the residual biomass to obtain
more energy was introduced looking for a better revenue, by minimizing wastes to
complete biomass use in the biorefinery concept (Collet et al. 2011; Maurya et al.
2016). In the strategy of biofuel only, there are three scenarios to obtain more
energy: (i) the wastes can be burned to generate heat and electricity, (ii) they can be
converted to biogas using anaerobic digestion or to ethanol using fermentation from
carbohydrate, and (iii) they can be thermochemically processed to obtain other
biofuels (Brownbridge et al. 2014). But in the structure of the biorefinery with
bioprocessing only, selection will depend on the stoichiometric composition of the
cell; lipid content in the cell can vary from 15 to 80% depending on the strain,
leaving over a huge amount of wastes. Residual deoiled biomass composition is
rich in carbohydrates or proteins, and its composition defines the potential application. Residual biomass with high C/N ratio is beneficial for the production of
biomethane, bioethanol, and biohydrogen, while a low C/N ratio means high protein content, which may be beneficial for its use as fertilizer, fermentation medium
for microorganisms, or feed supplement for animals and fish (Maurya et al. 2016).
Biogas production from residual deoiled biomass was suggested to produce heat or
electricity and contribute to the energy balance (Collet et al. 2011; Ward 2015).
Laurens et al. (2017b) established that fractionation approach to microalgal biomass
can create three different potential fuel streams, which allow a 35% reduction in the
overall minimum fuel selling price as compared to the biodiesel-only strategy. As
can be seen from Table 5, the general strategy is focused on exploiting lipids for
biodiesel production and there is a preference for biogas production first from
residual biomass and then from bioethanol. One strategy attempts to diversify its
production of biofuels with hydrogen. In this approach, ethanol is obtained from
fermentation of the released carbohydrates, biodiesel, or jet fuel from the lipid
fraction through hydrotreating and isomerization and finally mixed alcohols
5 Microalgae Biorefineries for Energy …
121
finally, (iv) coupled to other processes in the context of circular economy. Illustrative
works about these categories are shown in Table 5 and Fig. 3.
4.1 Strategy I: Biofuel Production Only
(Low-Value Compound)
Biofuels are required at low price and in large volume; however, among all
products obtained from microalgae, biofuels have the lowest cost and their price is
always compared with fossil alternatives; moreover, the energy balance of the
production process must be positive. As was shown in Sect. 2 and Table 5, most of
the biofuels can be conceptually obtained from microalgae, but biodiesel is one of
the most interesting alternatives as liquid fuel for transportation (Amaro et al. 2011;
Mallick et al. 2016; Mondal et al. 2017). Nonetheless, it has been demonstrated that
currently microalgae biodiesel is not cost competitive when compared with fossil
diesel (Chisti 2007). Therefore, the idea of using the residual biomass to obtain
more energy was introduced looking for a better revenue, by minimizing wastes to
complete biomass use in the biorefinery concept (Collet et al. 2011; Maurya et al.
2016). In the strategy of biofuel only, there are three scenarios to obtain more
energy: (i) the wastes can be burned to generate heat and electricity, (ii) they can be
converted to biogas using anaerobic digestion or to ethanol using fermentation from
carbohydrate, and (iii) they can be thermochemically processed to obtain other
biofuels (Brownbridge et al. 2014). But in the structure of the biorefinery with
bioprocessing only, selection will depend on the stoichiometric composition of the
cell; lipid content in the cell can vary from 15 to 80% depending on the strain,
leaving over a huge amount of wastes. Residual deoiled biomass composition is
rich in carbohydrates or proteins, and its composition defines the potential application. Residual biomass with high C/N ratio is beneficial for the production of
biomethane, bioethanol, and biohydrogen, while a low C/N ratio means high protein content, which may be beneficial for its use as fertilizer, fermentation medium
for microorganisms, or feed supplement for animals and fish (Maurya et al. 2016).
Biogas production from residual deoiled biomass was suggested to produce heat or
electricity and contribute to the energy balance (Collet et al. 2011; Ward 2015).
Laurens et al. (2017b) established that fractionation approach to microalgal biomass
can create three different potential fuel streams, which allow a 35% reduction in the
overall minimum fuel selling price as compared to the biodiesel-only strategy. As
can be seen from Table 5, the general strategy is focused on exploiting lipids for
biodiesel production and there is a preference for biogas production first from
residual biomass and then from bioethanol. One strategy attempts to diversify its
production of biofuels with hydrogen. In this approach, ethanol is obtained from
fermentation of the released carbohydrates, biodiesel, or jet fuel from the lipid
fraction through hydrotreating and isomerization and finally mixed alcohols
5 Microalgae Biorefineries for Energy …
121