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3 Advanced Technologies (Biological and Thermochemical) …
3.1.3 Hydrothermal Liquefaction
Hydrothermal Liquefaction (HTL) is a thermochemical process for the conversion of
biomass into bio-fuels in the presence of water as a reaction medium. This process
operates at high pressure (50–200 bar) but relatively mild temperature (<400 °C)
and uses water as the main solvent, mostly in subcritical or near-critical conditions
(the critical point of water being T = 374 °C and P = 221 bar) [3, 33, 38]. Water at
hydrothermal conditions (under elevated temperature and high pressure) has unique
properties. For instance, the dielectric constant of water that represents its polarity
decreases significantly at elevated pressure and temperature compared to that of
the ambient water. This increases the solubility of hydrophobic organic materials
such as free fatty acids that are normally more soluble in non-polar solvents [39, 40].
Hot-compressed water also has low viscosity and fewer and weaker hydrogen bonds.
As discussed earlier, pyrolysis requires a drying process for wet feedstock, which
makes it costly for processing some wet biomass and residues such as kitchen
waste, wastewater sludge, manure, newly harvested forestry/agricultural residues or
microalgae, food processing waste, etc. HTL becomes a cost-effective and promising
solution to handling the bio-feedstocks with high moisture content as the wet feedstock can be directly used without the need for costly dewatering and drying operations [24]. Another advantage of hydrothermal liquefaction is that the HTL bio-crude
oils have more desirable properties compared to pyrolysis bio-oils. For example, they
have a lower oxygen content (<15–20%) and moisture content (<5–10%) compared
to pyrolysis oils that have around 30–40% oxygen and 20–30% moisture contents,
and hence higher HHV values than pyrolysis oils [41].
Hydrothermal liquefaction of biomass involves a series of reactions, producing
HTL bio-crude oil (or simply bio-crude or bio-oil), Water-Soluble Products (WSP),
solid residue or biochar, and non-condensable gases as the main products. The main
reactions involved can be classified as follows [40–42]:
– Biomass undergoes hydrolysis and de-polymerization reactions of biomass
components (cellulose, hemicellulose and lignin) leading to a mixture of
monomers and unit structures
– Decomposition/degradation of the produced monomers into reactive intermediates.
– Re-polymerization, condensation, cyclization and water-reforming/gasification
reactions of the reactive intermediates/fragments to form final liquid/solid/gaseous
products.
HTL of biomass/wastes has been widely investigated in batch or continuous
reactors for the production of renewable bio-oils. Batch reactors are usually stirred
reactors equipped with mixers and heater. Most of the HTL studies are performed
in batch reactors, while continuous flow operations are more desirable for largescale applications in order to promote energy utilization efficiency and improve the
economic feasibility. The main challenge for using a continuous flow reactor for
3 Advanced Technologies (Biological and Thermochemical) …
3.1.3 Hydrothermal Liquefaction
Hydrothermal Liquefaction (HTL) is a thermochemical process for the conversion of
biomass into bio-fuels in the presence of water as a reaction medium. This process
operates at high pressure (50–200 bar) but relatively mild temperature (<400 °C)
and uses water as the main solvent, mostly in subcritical or near-critical conditions
(the critical point of water being T = 374 °C and P = 221 bar) [3, 33, 38]. Water at
hydrothermal conditions (under elevated temperature and high pressure) has unique
properties. For instance, the dielectric constant of water that represents its polarity
decreases significantly at elevated pressure and temperature compared to that of
the ambient water. This increases the solubility of hydrophobic organic materials
such as free fatty acids that are normally more soluble in non-polar solvents [39, 40].
Hot-compressed water also has low viscosity and fewer and weaker hydrogen bonds.
As discussed earlier, pyrolysis requires a drying process for wet feedstock, which
makes it costly for processing some wet biomass and residues such as kitchen
waste, wastewater sludge, manure, newly harvested forestry/agricultural residues or
microalgae, food processing waste, etc. HTL becomes a cost-effective and promising
solution to handling the bio-feedstocks with high moisture content as the wet feedstock can be directly used without the need for costly dewatering and drying operations [24]. Another advantage of hydrothermal liquefaction is that the HTL bio-crude
oils have more desirable properties compared to pyrolysis bio-oils. For example, they
have a lower oxygen content (<15–20%) and moisture content (<5–10%) compared
to pyrolysis oils that have around 30–40% oxygen and 20–30% moisture contents,
and hence higher HHV values than pyrolysis oils [41].
Hydrothermal liquefaction of biomass involves a series of reactions, producing
HTL bio-crude oil (or simply bio-crude or bio-oil), Water-Soluble Products (WSP),
solid residue or biochar, and non-condensable gases as the main products. The main
reactions involved can be classified as follows [40–42]:
– Biomass undergoes hydrolysis and de-polymerization reactions of biomass
components (cellulose, hemicellulose and lignin) leading to a mixture of
monomers and unit structures
– Decomposition/degradation of the produced monomers into reactive intermediates.
– Re-polymerization, condensation, cyclization and water-reforming/gasification
reactions of the reactive intermediates/fragments to form final liquid/solid/gaseous
products.
HTL of biomass/wastes has been widely investigated in batch or continuous
reactors for the production of renewable bio-oils. Batch reactors are usually stirred
reactors equipped with mixers and heater. Most of the HTL studies are performed
in batch reactors, while continuous flow operations are more desirable for largescale applications in order to promote energy utilization efficiency and improve the
economic feasibility. The main challenge for using a continuous flow reactor for