158
5.1 Microalgae Components
Lipids. Lipids are nonpolar compounds that are hydrophobic, usually occurring in
the form of triglycerides (TAGs), an aliphatic compound consisting of a glycerol
backbone attached to three fatty acids [62]. Upon hydrolysis, TAGs easily decompose to produce 1 mole of glycerol (by-product) and 3 moles of free fatty acids
(FFAs) (main product) in hot compressed water without catalyst [18]. The glycerol
produced is then converted via HTL into methanol, acetaldehyde, propionaldehyde,
acrolein, allyl alcohol, ethanol, formaldehyde, and gas products (CO, CO 2 , and H 2 )
[62, 63]. The FFAs can be converted into long-chain hydrocarbons although they
are thermally stable [14]. At ambient temperature, lipids are typically insoluble in
solvents, but as temperature increases, they become more polar and soluble. The
solubility of lipids in water is enhanced if the water’s dielectric constant is within
subcritical conditions (e.g., 350 °C, 20 MPa) [17]. The glycerol produced during
HTL is a mixture of fatty acids, methanol, and salts that is soluble in water [64]. The
higher the lipid content of a microalgae species, the higher the biocrude oil yield
from HTL.
Proteins. Proteins consist of several monomer units of amino acids joined by
peptide bonds. The majority of the nitrogen in microalgae is found in proteins. The
HTL process inadvertently incorporates most of the nitrogen in proteins to the biocrude oil, therefore affecting the biocrude oil’s properties. Proteins are easily hydrolyzable to produce low yields of amino acids [65] because the peptide bonds in
proteins are more stable than the glycosidic bonds in glucose/starch. Amino acid
yields during HTL are also low (~10%) since they can further undergo (1) decarboxylation to produce carbonic acid and amines and (2) deamination to produce
ammonia and organic acids [14, 17]. The degradation products of an amino acid
include CO 2 , CO, CH 4 , hydrogen, short-chain hydrocarbons (alkanes, alkenes),
alcohols (≤ C5), aldehydes, amides, and carboxylic acids [12]. These products can
repolymerize to form long-chain hydrocarbons and aromatic compounds such as
phenols or nitrogen heterocyclics such as indole or pyrrole [14, 49, 66, 67]. The
amino acid (activation energy: 148–166 kJ/mol) decomposition is a first-order
kinetic reaction described by the Arrhenius equation [68]. Amino acids degrade
very quickly, usually decomposing more than 70% within 20–30 s at around
350 °C [69].
Carbohydrates. Carbohydrates are a collective term for monosaccharides, oligosaccharides, and polysaccharides. It can exist as cellulose, hemicellulose, and lignin. In the case of microalgae subjected to hydrothermal conditions, its carbohydrate
content are not converted directly into biocrude oil. Rather, HTL degrades the carbohydrates into polar compounds such as organic acids (i.e., acetic, formic, lactic),
aldehydes, benzenes, and alcohols [17, 18, 39]. The aldehyde and benzene fractions
are then converted to long-chain hydrocarbons forming the biocrude oil [14].
Algaenans. Algaenans are recalcitrant biopolymers existing in the cell walls of
green microalgae. They consist of hydrocarbons with variable lengths and alkylaromatics [61]. No study has been done to analyze their degradation behavior and
E. P. Resurreccion and S. Kumar
5.1 Microalgae Components
Lipids. Lipids are nonpolar compounds that are hydrophobic, usually occurring in
the form of triglycerides (TAGs), an aliphatic compound consisting of a glycerol
backbone attached to three fatty acids [62]. Upon hydrolysis, TAGs easily decompose to produce 1 mole of glycerol (by-product) and 3 moles of free fatty acids
(FFAs) (main product) in hot compressed water without catalyst [18]. The glycerol
produced is then converted via HTL into methanol, acetaldehyde, propionaldehyde,
acrolein, allyl alcohol, ethanol, formaldehyde, and gas products (CO, CO 2 , and H 2 )
[62, 63]. The FFAs can be converted into long-chain hydrocarbons although they
are thermally stable [14]. At ambient temperature, lipids are typically insoluble in
solvents, but as temperature increases, they become more polar and soluble. The
solubility of lipids in water is enhanced if the water’s dielectric constant is within
subcritical conditions (e.g., 350 °C, 20 MPa) [17]. The glycerol produced during
HTL is a mixture of fatty acids, methanol, and salts that is soluble in water [64]. The
higher the lipid content of a microalgae species, the higher the biocrude oil yield
from HTL.
Proteins. Proteins consist of several monomer units of amino acids joined by
peptide bonds. The majority of the nitrogen in microalgae is found in proteins. The
HTL process inadvertently incorporates most of the nitrogen in proteins to the biocrude oil, therefore affecting the biocrude oil’s properties. Proteins are easily hydrolyzable to produce low yields of amino acids [65] because the peptide bonds in
proteins are more stable than the glycosidic bonds in glucose/starch. Amino acid
yields during HTL are also low (~10%) since they can further undergo (1) decarboxylation to produce carbonic acid and amines and (2) deamination to produce
ammonia and organic acids [14, 17]. The degradation products of an amino acid
include CO 2 , CO, CH 4 , hydrogen, short-chain hydrocarbons (alkanes, alkenes),
alcohols (≤ C5), aldehydes, amides, and carboxylic acids [12]. These products can
repolymerize to form long-chain hydrocarbons and aromatic compounds such as
phenols or nitrogen heterocyclics such as indole or pyrrole [14, 49, 66, 67]. The
amino acid (activation energy: 148–166 kJ/mol) decomposition is a first-order
kinetic reaction described by the Arrhenius equation [68]. Amino acids degrade
very quickly, usually decomposing more than 70% within 20–30 s at around
350 °C [69].
Carbohydrates. Carbohydrates are a collective term for monosaccharides, oligosaccharides, and polysaccharides. It can exist as cellulose, hemicellulose, and lignin. In the case of microalgae subjected to hydrothermal conditions, its carbohydrate
content are not converted directly into biocrude oil. Rather, HTL degrades the carbohydrates into polar compounds such as organic acids (i.e., acetic, formic, lactic),
aldehydes, benzenes, and alcohols [17, 18, 39]. The aldehyde and benzene fractions
are then converted to long-chain hydrocarbons forming the biocrude oil [14].
Algaenans. Algaenans are recalcitrant biopolymers existing in the cell walls of
green microalgae. They consist of hydrocarbons with variable lengths and alkylaromatics [61]. No study has been done to analyze their degradation behavior and
E. P. Resurreccion and S. Kumar