hydrogen production from microalgae biomass (Buitrón et al. 2017; Sambusiti et al.
2015; Xia et al. 2015). Recent results show a clear potential of microalgae as
feedstock for DF, achieving molar yields up to 3 mol H 2 /mol sugar, which represents 75% of the maximum theoretical yield (Nayak et al. 2014). Such values are
obtained only with other carbohydrate-rich substrates operated under thermophilic
conditions or with a reduced hydrogen partial pressure. In DF, the highest yields are
produced with the simplest carbohydrate molecules (Quemeneur et al. 2011);
hence, carbohydrates must be released to be assimilated for hydrogen production
when microalgae are used as substrate (Nguyen et al. 2010). Hydrolysis for cell
wall disruption is a usual method to obtain fermentable sugars (Günerken et al.
2015).
Therefore, the major constraint to the use of microalgae for DF is related to the
hydrolysate quality in terms of reducing sugar concentration and the pre-treatment
efficiencies. Methane production is a frequent concern in DF systems because
methanogenic microorganisms can be presented in the inoculum used. For instance,
using wet untreated biomass, Kumar et al. (2016) produced methane rather than H 2
because of an inefficient inoculum heat pre-treatment.
As in other technologies for biofuel production using microalgae biomass, the
suitable DF application depends on its insertion into an integrated scheme. The final
by-product of DF is a mixture of volatile fatty acids and solvents, depending on the
operational conditions and the microorganisms present.
3.3.4 Chemical transformation
Transesterification
Microalgae biodiesel is generally produced through the extraction and further
transesterification of algal oil. Transesterification is the reaction of triglycerides
(TAGs) with alcohol or methanol, in the presence of a catalyst that produces
glycerol and fatty acid methyl esters (FAME or biodiesel) derived from TAGs. The
complete biomass conversion depends on lipid profile, oil impurities, catalyst
nature, temperature, and time. Transesterification can be catalyzed by acids, alkalis,
or lipase enzymes (Chisti 2007). Recently, Lemões et al. (2016) have studied direct
wet-transesterification using ethanol with yields similar to those obtained from
extracted lipids. Furthermore, contributions to sustainability are claimed based on
savings related to the unnecessary dewatering of the microalgae biomass, and the
use of ethanol as renewable feedstock. Other innovation is transesterification in
supercritical conditions, a catalyst-free chemical reaction that enables the full
transformation of TAG (Ngamprasertsith and Sawangkeaw 2011), dramatically
accelerated under supercritical conditions.
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2015; Xia et al. 2015). Recent results show a clear potential of microalgae as
feedstock for DF, achieving molar yields up to 3 mol H 2 /mol sugar, which represents 75% of the maximum theoretical yield (Nayak et al. 2014). Such values are
obtained only with other carbohydrate-rich substrates operated under thermophilic
conditions or with a reduced hydrogen partial pressure. In DF, the highest yields are
produced with the simplest carbohydrate molecules (Quemeneur et al. 2011);
hence, carbohydrates must be released to be assimilated for hydrogen production
when microalgae are used as substrate (Nguyen et al. 2010). Hydrolysis for cell
wall disruption is a usual method to obtain fermentable sugars (Günerken et al.
2015).
Therefore, the major constraint to the use of microalgae for DF is related to the
hydrolysate quality in terms of reducing sugar concentration and the pre-treatment
efficiencies. Methane production is a frequent concern in DF systems because
methanogenic microorganisms can be presented in the inoculum used. For instance,
using wet untreated biomass, Kumar et al. (2016) produced methane rather than H 2
because of an inefficient inoculum heat pre-treatment.
As in other technologies for biofuel production using microalgae biomass, the
suitable DF application depends on its insertion into an integrated scheme. The final
by-product of DF is a mixture of volatile fatty acids and solvents, depending on the
operational conditions and the microorganisms present.
3.3.4 Chemical transformation
Transesterification
Microalgae biodiesel is generally produced through the extraction and further
transesterification of algal oil. Transesterification is the reaction of triglycerides
(TAGs) with alcohol or methanol, in the presence of a catalyst that produces
glycerol and fatty acid methyl esters (FAME or biodiesel) derived from TAGs. The
complete biomass conversion depends on lipid profile, oil impurities, catalyst
nature, temperature, and time. Transesterification can be catalyzed by acids, alkalis,
or lipase enzymes (Chisti 2007). Recently, Lemões et al. (2016) have studied direct
wet-transesterification using ethanol with yields similar to those obtained from
extracted lipids. Furthermore, contributions to sustainability are claimed based on
savings related to the unnecessary dewatering of the microalgae biomass, and the
use of ethanol as renewable feedstock. Other innovation is transesterification in
supercritical conditions, a catalyst-free chemical reaction that enables the full
transformation of TAG (Ngamprasertsith and Sawangkeaw 2011), dramatically
accelerated under supercritical conditions.
5 Microalgae Biorefineries for Energy …
113