161
5.3 Reaction Pathways
The production of biocrude oil from the hydrothermal liquefaction of microalgae
biomass has been recently touted as an attractive route to utilizing microalgae’s
embodied energy into a more effective energy carrier with low coke formation [78,
79]. HTL’s chemistry and process conditions (200–380 °C) [80, 81] allow the recycle elements such as O and N into the aqueous and gaseous product streams while
extracting C and H from the hydrocarbons embedded in the microalgae biomass
[82]. The process is initialized by hydrolyzing lipids, proteins, and carbohydrates
into smaller, unstable, and reactive molecules [35]. The decomposition and depolymerization typically occur for a noticeably short period of time, while the repolymerization to produce biocrude oil takes much longer [59]. Water plays an essential
role in cleaving the macromolecule bonds and permits the adequate separation of
the biocrude oil from other phases after HTL reaction.
HTL of microalgae involves three subsequent phases. The first phase is the
hydrolysis of the major components (lipids, proteins, carbohydrates) into their
building blocks (monomers). Hydrolysis results into lipids turning into 1 mole of
glycerol and 3 moles of fatty acids, proteins into different amino acids or peptides,
and carbohydrates into monosaccharides or polysaccharides (reducing and nonreducing sugars). The second phase is the parallel reaction of the monomers which
includes dehydration and cracking. Finally, the third phase is the cross-reaction of
the intermediate products such as Maillard reaction between amino acids and reducing sugars, decarboxylation of fatty acids, and decarboxylation of amino acids [83,
84]. Lipids generate straight-chain and cyclic hydrocarbons (hydroxyl and carboxyl
groups), while proteins and carbohydrates produce heteroatoms [84]. These heteroatoms are N- and O-containing compounds [61, 85–88].
A typical reaction network for microalgae HTL is as follows: Glycerol from
lipids is converted into acrolein and 1,3-dioxan-5-ol. The fatty acids, on the other
hand, are converted into alkanes and alkenes via decarboxylation and into fatty acid
amides if reacted with NH 3 [87, 89]. The amino acids from proteins can undergo
three routes: (1) decarboxylation to form ketones and amines, which in turn can be
converted into alkanes; (2) deamination to form ammonia and organic acids, which
in turn can be converted into alkanes; and (3) nucleophilic reaction to form cyclic
amines [61, 62]. The sugars from carbohydrates are converted into
5- hydroxymethylfural, lactic acid, acetic acid, formic acid, and pyruvaldehyde.
These intermediates can then react with an amide to form amides [10]. A reducing
sugar can also undergo dimerization or Maillard reaction with NH 3 to form a slew
of N- and O-containing compounds such as pyrazine, cyclic oxygenates, indole,
quinoline, pyrazine derivatives, and phenol derivatives [90, 91].
In general, the formation of biocrude oil via HTL from microalgae primarily
involves deoxygenation and denitrogenation [82], with deoxygenation occurring via
decarboxylation and dehydration [92]. Biocrude oil has abundant hydrocarbons,
phenols, free fatty acids, and N- and O-containing heteroatoms [38, 39]. Upgrading
the biocrude oil improves its C/H ratio, HHV, viscosity, and alkane fraction.
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae
5.3 Reaction Pathways
The production of biocrude oil from the hydrothermal liquefaction of microalgae
biomass has been recently touted as an attractive route to utilizing microalgae’s
embodied energy into a more effective energy carrier with low coke formation [78,
79]. HTL’s chemistry and process conditions (200–380 °C) [80, 81] allow the recycle elements such as O and N into the aqueous and gaseous product streams while
extracting C and H from the hydrocarbons embedded in the microalgae biomass
[82]. The process is initialized by hydrolyzing lipids, proteins, and carbohydrates
into smaller, unstable, and reactive molecules [35]. The decomposition and depolymerization typically occur for a noticeably short period of time, while the repolymerization to produce biocrude oil takes much longer [59]. Water plays an essential
role in cleaving the macromolecule bonds and permits the adequate separation of
the biocrude oil from other phases after HTL reaction.
HTL of microalgae involves three subsequent phases. The first phase is the
hydrolysis of the major components (lipids, proteins, carbohydrates) into their
building blocks (monomers). Hydrolysis results into lipids turning into 1 mole of
glycerol and 3 moles of fatty acids, proteins into different amino acids or peptides,
and carbohydrates into monosaccharides or polysaccharides (reducing and nonreducing sugars). The second phase is the parallel reaction of the monomers which
includes dehydration and cracking. Finally, the third phase is the cross-reaction of
the intermediate products such as Maillard reaction between amino acids and reducing sugars, decarboxylation of fatty acids, and decarboxylation of amino acids [83,
84]. Lipids generate straight-chain and cyclic hydrocarbons (hydroxyl and carboxyl
groups), while proteins and carbohydrates produce heteroatoms [84]. These heteroatoms are N- and O-containing compounds [61, 85–88].
A typical reaction network for microalgae HTL is as follows: Glycerol from
lipids is converted into acrolein and 1,3-dioxan-5-ol. The fatty acids, on the other
hand, are converted into alkanes and alkenes via decarboxylation and into fatty acid
amides if reacted with NH 3 [87, 89]. The amino acids from proteins can undergo
three routes: (1) decarboxylation to form ketones and amines, which in turn can be
converted into alkanes; (2) deamination to form ammonia and organic acids, which
in turn can be converted into alkanes; and (3) nucleophilic reaction to form cyclic
amines [61, 62]. The sugars from carbohydrates are converted into
5- hydroxymethylfural, lactic acid, acetic acid, formic acid, and pyruvaldehyde.
These intermediates can then react with an amide to form amides [10]. A reducing
sugar can also undergo dimerization or Maillard reaction with NH 3 to form a slew
of N- and O-containing compounds such as pyrazine, cyclic oxygenates, indole,
quinoline, pyrazine derivatives, and phenol derivatives [90, 91].
In general, the formation of biocrude oil via HTL from microalgae primarily
involves deoxygenation and denitrogenation [82], with deoxygenation occurring via
decarboxylation and dehydration [92]. Biocrude oil has abundant hydrocarbons,
phenols, free fatty acids, and N- and O-containing heteroatoms [38, 39]. Upgrading
the biocrude oil improves its C/H ratio, HHV, viscosity, and alkane fraction.
Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae