157
time, catalyst amount, and microalgae’s ash content, although the extent to which
each of these factors affects HTL mechanism has not been fully understood.
Microalgae subjected to hot compressed liquid water near its critical point (374 °C,
22.1 MPa) is subjected to two competing reactions: hydrolysis (depolymerization or
decomposition) into smaller units that are highly reactive and repolymerization of
the units to form bio-oil, gas, aqueous phase, and solid residue [25, 38, 55]. Varying
the process condition such as increasing reaction time or temperature may lead to
increase in solid fraction yield and a decrease in the biocrude oil yield [56–58].
Increasing the reaction time decreases the viscosity of the biocrude oil which in turn
lowers its concentration in the liquid phase [49].
A 2011 study compared the individual HTL characteristics of the three components of microalgae (lipids, proteins, and carbohydrates) with the HTL characteristics of various microalgae strains [39]. The study proposed that the biocrude oil
conversion yield for the HTL of any microalgal strain is 55–80 w/w% in lipids,
11–18 w/w% in proteins, and 6–15 w/w% in carbohydrates, all of which are much
lower compared to experiments performed by authors who utilized microalgae with
very low lipid content [59, 60]. Another study [61] showed that at reaction temperatures below 250 °C, HTL of lipids and algaenans significantly contributes to the
composition of the resulting biocrude. Conversely, at reaction temperatures between
300 and 375 °C, HTL of proteins and carbohydrates produces the majority of the
components in the biocrude, with an elevated level of nitrogen due to the decomposition of amines in proteins.
A generalized equation was empirically developed by Biller and Ross [39] to
estimate the percent biocrude oil yield (w/w) from the HTL of Nannochloropsis
oculata and Chlorella vulgaris with known data on lipid, protein, and carbohydrate
contents and yields via Eq. (2). However, the equation cannot be applied to other
microalgal strain because microalgal components in other strains do not behave
independently during HTL and cannot be linearly added to produce a single biocrude oil yield value. Instead, their interactions are characterized via cross-linking
mechanisms [61]:
BioY LipC LipY
ProC ProY
CarC CarY
= (
)(
)+ (
)(
)+ (
)(
)
(2)
where
Bio Y = Biocrude oil yield (w/w %)
Lip C = Lipid content (w/w %)
Lip Y = Lipid yield (w/w %)
Pro C = Protein content (w/w %)
Pro Y = Protein yield (w/w %)
Car C = Carbohydrate content (w/w %)
Car Y = Carbohydrate yield (w/w %)
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
time, catalyst amount, and microalgae’s ash content, although the extent to which
each of these factors affects HTL mechanism has not been fully understood.
Microalgae subjected to hot compressed liquid water near its critical point (374 °C,
22.1 MPa) is subjected to two competing reactions: hydrolysis (depolymerization or
decomposition) into smaller units that are highly reactive and repolymerization of
the units to form bio-oil, gas, aqueous phase, and solid residue [25, 38, 55]. Varying
the process condition such as increasing reaction time or temperature may lead to
increase in solid fraction yield and a decrease in the biocrude oil yield [56–58].
Increasing the reaction time decreases the viscosity of the biocrude oil which in turn
lowers its concentration in the liquid phase [49].
A 2011 study compared the individual HTL characteristics of the three components of microalgae (lipids, proteins, and carbohydrates) with the HTL characteristics of various microalgae strains [39]. The study proposed that the biocrude oil
conversion yield for the HTL of any microalgal strain is 55–80 w/w% in lipids,
11–18 w/w% in proteins, and 6–15 w/w% in carbohydrates, all of which are much
lower compared to experiments performed by authors who utilized microalgae with
very low lipid content [59, 60]. Another study [61] showed that at reaction temperatures below 250 °C, HTL of lipids and algaenans significantly contributes to the
composition of the resulting biocrude. Conversely, at reaction temperatures between
300 and 375 °C, HTL of proteins and carbohydrates produces the majority of the
components in the biocrude, with an elevated level of nitrogen due to the decomposition of amines in proteins.
A generalized equation was empirically developed by Biller and Ross [39] to
estimate the percent biocrude oil yield (w/w) from the HTL of Nannochloropsis
oculata and Chlorella vulgaris with known data on lipid, protein, and carbohydrate
contents and yields via Eq. (2). However, the equation cannot be applied to other
microalgal strain because microalgal components in other strains do not behave
independently during HTL and cannot be linearly added to produce a single biocrude oil yield value. Instead, their interactions are characterized via cross-linking
mechanisms [61]:
BioY LipC LipY
ProC ProY
CarC CarY
= (
)(
)+ (
)(
)+ (
)(
)
(2)
where
Bio Y = Biocrude oil yield (w/w %)
Lip C = Lipid content (w/w %)
Lip Y = Lipid yield (w/w %)
Pro C = Protein content (w/w %)
Pro Y = Protein yield (w/w %)
Car C = Carbohydrate content (w/w %)
Car Y = Carbohydrate yield (w/w %)
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae