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chemical pathways during HTL because they are resistant to hydrothermal conditions and chemical treatments [14].
5.2 Microalgae HTL Products
As presented in Fig. 4, HTL utilizes microalgae feedstock and produces a mixture
of gaseous product, biocrude oil, an aqueous phase, and a solid residue. Biocrude oil
has low oxygen content (10–20%) compared to the aqueous phase and solid residue
[8]. It consists of a mixture of straight-chain and aromatic hydrocarbons with varying O/C and H/C ratios, depending upon the nature of the microalgae feedstock. A
typical microalgae-based biocrude oil has a heating value of 35–40 MJ/kg as a
result low O/C and H/C ratios, relative to raw microalgae with a heating value of
15–25 MJ/kg. Biocrude oil can be upgraded to high-quality fuels or can be directly
co-fired with coal. Aviation gasoline, biojet fuel, and renewable diesel are the main
products of biocrude oil upgrade. Unlike petroleum-based diesel or biodiesel
obtained from transesterification of microalgae lipids, renewable diesel can be used
directly in diesel-powered vehicles because it is compatible with diesel engines and
their distribution infrastructure. Upgrading of HTL biocrude oil is also easily
accomplished within existing petroleum refineries. The emission controls within
these refineries are also capable of removing sulfur from HTL fuel products to meet
stringent local, state, and federal fuel requirements (e.g., ULSD). The aqueous
stream coming out of the HTL process contains large amounts of oxygen and nutrients. The chemicals in the nutrients such as nitrates, phosphates, and sulfates can be
recycled back to the microalgae pond as source of nutrient during cultivation [6, 7,
38, 70]. CO 2 can be extracted from the gaseous mixture and recycled back to the
microalgae pond as carbon source [7]. Similar to the aqueous phase, the solid residue (biochar) is high in oxygen and nitrogen which can be utilized as land
amendment.
Biocrude Oil. Biocrude oil is a viscous liquid mixture of hydrocarbons with
varying lengths. This is derived from carbohydrates, fatty acids, amino acids, and
organic acids that are primary end products of the photosynthetic carbon fixation
pathway (i.e., Calvin-Benson cycle). Biocrude oil from microalgae HTL has three
major components: lipids (fatty acids, sterols), proteins (peptides), and insoluble
fraction (algaenans, asphaltenes) [61]. It is dark and chemically similar to petroleum crude oil [46]. Its energy content is lower than petroleum fuel oil (~70–95%)
[38]. The biocrude oil yield reported as mass fraction of the dry input microalgae,
from the HTL at 350 °C of Nannochloropsis sp., is 43 ± 2 [38], while that from
Spirulina platensis is 39.9 [40]. These values are higher than those predicted by Eq.
(2): 27.7 ± 8.3 for Nannochloropsis sp. and 18.0 ± 6.4 for Spirulina platensis [14].
Other studies investigated the effects of homogenous catalyst on biocrude oil yield
from microalgae under hydrothermal conditions [57, 71, 72]. Results ranged from
14.2 to 67 wt%. The typical elemental compositions (wt%) of biocrude oil from
direct catalytic HTL of microalgae are reported as follows: 73.7–76.1% C,
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
chemical pathways during HTL because they are resistant to hydrothermal conditions and chemical treatments [14].
5.2 Microalgae HTL Products
As presented in Fig. 4, HTL utilizes microalgae feedstock and produces a mixture
of gaseous product, biocrude oil, an aqueous phase, and a solid residue. Biocrude oil
has low oxygen content (10–20%) compared to the aqueous phase and solid residue
[8]. It consists of a mixture of straight-chain and aromatic hydrocarbons with varying O/C and H/C ratios, depending upon the nature of the microalgae feedstock. A
typical microalgae-based biocrude oil has a heating value of 35–40 MJ/kg as a
result low O/C and H/C ratios, relative to raw microalgae with a heating value of
15–25 MJ/kg. Biocrude oil can be upgraded to high-quality fuels or can be directly
co-fired with coal. Aviation gasoline, biojet fuel, and renewable diesel are the main
products of biocrude oil upgrade. Unlike petroleum-based diesel or biodiesel
obtained from transesterification of microalgae lipids, renewable diesel can be used
directly in diesel-powered vehicles because it is compatible with diesel engines and
their distribution infrastructure. Upgrading of HTL biocrude oil is also easily
accomplished within existing petroleum refineries. The emission controls within
these refineries are also capable of removing sulfur from HTL fuel products to meet
stringent local, state, and federal fuel requirements (e.g., ULSD). The aqueous
stream coming out of the HTL process contains large amounts of oxygen and nutrients. The chemicals in the nutrients such as nitrates, phosphates, and sulfates can be
recycled back to the microalgae pond as source of nutrient during cultivation [6, 7,
38, 70]. CO 2 can be extracted from the gaseous mixture and recycled back to the
microalgae pond as carbon source [7]. Similar to the aqueous phase, the solid residue (biochar) is high in oxygen and nitrogen which can be utilized as land
amendment.
Biocrude Oil. Biocrude oil is a viscous liquid mixture of hydrocarbons with
varying lengths. This is derived from carbohydrates, fatty acids, amino acids, and
organic acids that are primary end products of the photosynthetic carbon fixation
pathway (i.e., Calvin-Benson cycle). Biocrude oil from microalgae HTL has three
major components: lipids (fatty acids, sterols), proteins (peptides), and insoluble
fraction (algaenans, asphaltenes) [61]. It is dark and chemically similar to petroleum crude oil [46]. Its energy content is lower than petroleum fuel oil (~70–95%)
[38]. The biocrude oil yield reported as mass fraction of the dry input microalgae,
from the HTL at 350 °C of Nannochloropsis sp., is 43 ± 2 [38], while that from
Spirulina platensis is 39.9 [40]. These values are higher than those predicted by Eq.
(2): 27.7 ± 8.3 for Nannochloropsis sp. and 18.0 ± 6.4 for Spirulina platensis [14].
Other studies investigated the effects of homogenous catalyst on biocrude oil yield
from microalgae under hydrothermal conditions [57, 71, 72]. Results ranged from
14.2 to 67 wt%. The typical elemental compositions (wt%) of biocrude oil from
direct catalytic HTL of microalgae are reported as follows: 73.7–76.1% C,
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae