160
8.6–11.1% H, 8.3–11.1% O, and 3.9–6.3% N with a higher heating value of 34.5 to
40.1 MJ/kg [14, 36]. The N content leads to the formation of NO x emission during
combustion. Although sulfur content is below detection limit, it ranges between 0.5
and 1% [38, 45]. Microalgal biocrude consists of cyclic N and O compounds (e.g.,
pyrrolidinedione, piperidinedione), cyclic oxygenates (e.g., phenol), and cyclic
nitrogenates (e.g., pyrimidine, pyrazine) derived from protein derivatives (peptides
and/or cyclic peptides) and carbohydrates [38, 39, 73].
Aqueous Phase. The aqueous phase is an organic-rich HTL fraction consisting
mainly of phosphate, ammonium, acetate, K
+
, Na
+
, and Mg
2+
[31, 72]. This phase
can be recycled and fed into the growth culture medium because of its high nitrogen
content [53, 74]. Because this aqueous phase is rich in organics, recycling it supplies the necessary carbon source needed by certain heterotrophic microalgae
strains. Alternatively, this phase can also be gasified at supercritical conditions (i.e.,
>647.1 K, >22.1 MPa), in combination with the solid residue to generate a hydrogen-rich and methane-rich fuel gas (syngas). In a 2018 study, a modeling platform
was developed for the combined gasification of three species of microalgae
(Nannochloropsis oculate, Fucus serratus, and Scenedesmus almeriensis) combined with tar [75]. The model handled variations in O 2 which improved performance and efficiency, utilized steam that produced hydrogen, and employed CO 2
absorber that recycled carbon and generated high-quality syngas. Microalgae’s protein and carbohydrate fraction contributed to the production of H 2 , while the fatty
acid component led to the production of CH 4 [76].
Solid Residue. The solid outcome in microalgae HTL is an oxygen-rich residue
with high ash content but exceedingly small percent of hydrogen, nitrogen, and
sulfur. It is also known as char which can be used as fertilizer/soil amendment due
to its nutrient load or as catalyst. Figure 4 revealed that solid residue yield is about
5 wt% of the total Spirulina platensis feedstock with the following elemental composition: 11.82% C, 2.41% H, 1.61% N, 84.16% O, and 0.61% S [40]. Investigations
from other studies reported yields below 10% [25, 49, 72]. Char serves as feedstock
for other thermochemical process such as gasification and carbonization (both pyrolytic and hydrothermal) and combustion.
Gaseous Phase. The gaseous phase consists of CO 2 , CH 4 , H 2 , CO, and C 2 and
represents approximately 20% of microalgae feedstock [36, 45, 38]. Gaseous products increased by 20% accompanied by a decrease in biocrude oil yield if reaction
temperature is increased from 350 °C to 380 °C [38]. CO 2 and H 2 (662 mmol/mol
and 297 mmol/mol, respectively) are the main components of the gaseous phase,
and CH 4 , N 2 , C 2 H 4 , and C 2 H 6 are the minor components [38]. Changing the residence time from 60 to 120 min increased the gaseous phase by 48% [40]. CO occurs
at very low concentration because [75] it is a highly reactive and unstable radical
that easily forms CO 2 via the water-gas shift reaction ([4, 77] oxygen removal is
achieved via decarboxylation (i.e., removes carboxyl group and releases CO 2 ) and
not decarbonylation (releases CO) [36]. CO 2 can be recycled into the growth
medium as an inorganic carbon source for the microalgae, while H 2 can be used in
the upgrade of the generated biocrude oil.
E. P. Resurreccion and S. Kumar
8.6–11.1% H, 8.3–11.1% O, and 3.9–6.3% N with a higher heating value of 34.5 to
40.1 MJ/kg [14, 36]. The N content leads to the formation of NO x emission during
combustion. Although sulfur content is below detection limit, it ranges between 0.5
and 1% [38, 45]. Microalgal biocrude consists of cyclic N and O compounds (e.g.,
pyrrolidinedione, piperidinedione), cyclic oxygenates (e.g., phenol), and cyclic
nitrogenates (e.g., pyrimidine, pyrazine) derived from protein derivatives (peptides
and/or cyclic peptides) and carbohydrates [38, 39, 73].
Aqueous Phase. The aqueous phase is an organic-rich HTL fraction consisting
mainly of phosphate, ammonium, acetate, K
+
, Na
+
, and Mg
2+
[31, 72]. This phase
can be recycled and fed into the growth culture medium because of its high nitrogen
content [53, 74]. Because this aqueous phase is rich in organics, recycling it supplies the necessary carbon source needed by certain heterotrophic microalgae
strains. Alternatively, this phase can also be gasified at supercritical conditions (i.e.,
>647.1 K, >22.1 MPa), in combination with the solid residue to generate a hydrogen-rich and methane-rich fuel gas (syngas). In a 2018 study, a modeling platform
was developed for the combined gasification of three species of microalgae
(Nannochloropsis oculate, Fucus serratus, and Scenedesmus almeriensis) combined with tar [75]. The model handled variations in O 2 which improved performance and efficiency, utilized steam that produced hydrogen, and employed CO 2
absorber that recycled carbon and generated high-quality syngas. Microalgae’s protein and carbohydrate fraction contributed to the production of H 2 , while the fatty
acid component led to the production of CH 4 [76].
Solid Residue. The solid outcome in microalgae HTL is an oxygen-rich residue
with high ash content but exceedingly small percent of hydrogen, nitrogen, and
sulfur. It is also known as char which can be used as fertilizer/soil amendment due
to its nutrient load or as catalyst. Figure 4 revealed that solid residue yield is about
5 wt% of the total Spirulina platensis feedstock with the following elemental composition: 11.82% C, 2.41% H, 1.61% N, 84.16% O, and 0.61% S [40]. Investigations
from other studies reported yields below 10% [25, 49, 72]. Char serves as feedstock
for other thermochemical process such as gasification and carbonization (both pyrolytic and hydrothermal) and combustion.
Gaseous Phase. The gaseous phase consists of CO 2 , CH 4 , H 2 , CO, and C 2 and
represents approximately 20% of microalgae feedstock [36, 45, 38]. Gaseous products increased by 20% accompanied by a decrease in biocrude oil yield if reaction
temperature is increased from 350 °C to 380 °C [38]. CO 2 and H 2 (662 mmol/mol
and 297 mmol/mol, respectively) are the main components of the gaseous phase,
and CH 4 , N 2 , C 2 H 4 , and C 2 H 6 are the minor components [38]. Changing the residence time from 60 to 120 min increased the gaseous phase by 48% [40]. CO occurs
at very low concentration because [75] it is a highly reactive and unstable radical
that easily forms CO 2 via the water-gas shift reaction ([4, 77] oxygen removal is
achieved via decarboxylation (i.e., removes carboxyl group and releases CO 2 ) and
not decarbonylation (releases CO) [36]. CO 2 can be recycled into the growth
medium as an inorganic carbon source for the microalgae, while H 2 can be used in
the upgrade of the generated biocrude oil.
E. P. Resurreccion and S. Kumar