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Water for Energy and Fuel Production
5.1.1 ProPerTieS oF WATer AT high TemPerATure And PreSSure
Water at room temperature and pressure is a rather benign, polar substance with
low diffusivity, high dielectric constant (i.e., low permittivity), and low dissociation constant. These properties do not allow any meaningful reactions with organic
molecules. As the temperature and pressure increase, the dielectric constant quickly
decreases, the shared electron by oxygen and hydrogen atoms tends to circulate more
evenly, and electronegativity of the oxygen molecule is reduced (i.e., less polar).
When the temperature of the water increases from 25°C to 300°C, the dielectric
constant decreases from 78.85 to 19.66, resulting in water molecules to become fairly
nonpolar. This nonpolarity increases the affinity of water for organic hydrocarbons.
As the temperature and pressure increase, the dissociation constant also significantly increases. The dissociation constant for water at 300°C is about 500 times
higher than that at room temperature. An increase in pressure also increases the
ionization of water. At room temperature and pressure, low dissociation constant
allows H + and OH − ions in hydrolysis or dissociation in equilibrium balance and the
rate of acid- or base-catalyzed reaction rates is low. High dissociation constant at
higher temperature and pressure facilitates more acid–base-catalyzed reactions [5].
For the above two reasons, water becomes a good solvent for typically nonpolar
and hydrophobic hydrocarbons at high temperature and pressure. Water at 300°C
possesses the properties of acetone at 25°C. The increased solubility of organics
in water at high temperature enhances the possibilities of ionic reactions and the
contacts of dissociated H + with hydrocarbons, thereby accelerating the activities of
hydrolysis. These dramatic changes in physical and chemical properties thus allow
various organic reactions to take place in the water [5,14]. In addition, water has the
ability to carry out condensation, cleavage, and hydrolysis reactions and to affect
the selective ionic chemistry, which are more compatible with the organic reactions.
Thus, water becomes a medium similar to organic hydrocarbons in which different
types of organic reactions can freely occur. As the temperature increases and crosses
the critical temperature, water and organic hydrocarbons become more homogeneous in carrying out the various types of organic reactions. The role of water under
supercritical conditions is discussed in Chapter 10.
High-temperature water, due to its properties, can act as a reactant and a catalyst
for a second pathway to cascade organic molecular transformation of biomass (and to
some extent coal) that leads to refined biomass, oil, or synthetic gaseous fuel. Water
can cause organic material from biomass to disintegrate and reform by the addition
of H + to open carbon bond into fragments that can then be converted to different
types of hydrocarbons. Thus, hot water can be a catalyst for a series of ionic reactions.
Hydrothermal operation differentiates itself from dry pyrolysis in that degradation of
biomass in dry pyrolysis is caused by thermal forces, whereas the disintegration of
biomass in water can occur due to acid- or base-catalyzed reactions. Water can act as a
base to nibble certain organic molecules, and once the reaction conditions are changed,
it can act as an acid promoting different sets of reactions. Thus, basic mechanisms for
changes in hydrothermal operations (i.e., wet pyrolysis) are different from those occurring in the dry pyrolysis, and this difference is largely caused by the changes in the
physical and chemical properties of water as its temperature increases [15].
Water for Energy and Fuel Production
5.1.1 ProPerTieS oF WATer AT high TemPerATure And PreSSure
Water at room temperature and pressure is a rather benign, polar substance with
low diffusivity, high dielectric constant (i.e., low permittivity), and low dissociation constant. These properties do not allow any meaningful reactions with organic
molecules. As the temperature and pressure increase, the dielectric constant quickly
decreases, the shared electron by oxygen and hydrogen atoms tends to circulate more
evenly, and electronegativity of the oxygen molecule is reduced (i.e., less polar).
When the temperature of the water increases from 25°C to 300°C, the dielectric
constant decreases from 78.85 to 19.66, resulting in water molecules to become fairly
nonpolar. This nonpolarity increases the affinity of water for organic hydrocarbons.
As the temperature and pressure increase, the dissociation constant also significantly increases. The dissociation constant for water at 300°C is about 500 times
higher than that at room temperature. An increase in pressure also increases the
ionization of water. At room temperature and pressure, low dissociation constant
allows H + and OH − ions in hydrolysis or dissociation in equilibrium balance and the
rate of acid- or base-catalyzed reaction rates is low. High dissociation constant at
higher temperature and pressure facilitates more acid–base-catalyzed reactions [5].
For the above two reasons, water becomes a good solvent for typically nonpolar
and hydrophobic hydrocarbons at high temperature and pressure. Water at 300°C
possesses the properties of acetone at 25°C. The increased solubility of organics
in water at high temperature enhances the possibilities of ionic reactions and the
contacts of dissociated H + with hydrocarbons, thereby accelerating the activities of
hydrolysis. These dramatic changes in physical and chemical properties thus allow
various organic reactions to take place in the water [5,14]. In addition, water has the
ability to carry out condensation, cleavage, and hydrolysis reactions and to affect
the selective ionic chemistry, which are more compatible with the organic reactions.
Thus, water becomes a medium similar to organic hydrocarbons in which different
types of organic reactions can freely occur. As the temperature increases and crosses
the critical temperature, water and organic hydrocarbons become more homogeneous in carrying out the various types of organic reactions. The role of water under
supercritical conditions is discussed in Chapter 10.
High-temperature water, due to its properties, can act as a reactant and a catalyst
for a second pathway to cascade organic molecular transformation of biomass (and to
some extent coal) that leads to refined biomass, oil, or synthetic gaseous fuel. Water
can cause organic material from biomass to disintegrate and reform by the addition
of H + to open carbon bond into fragments that can then be converted to different
types of hydrocarbons. Thus, hot water can be a catalyst for a series of ionic reactions.
Hydrothermal operation differentiates itself from dry pyrolysis in that degradation of
biomass in dry pyrolysis is caused by thermal forces, whereas the disintegration of
biomass in water can occur due to acid- or base-catalyzed reactions. Water can act as a
base to nibble certain organic molecules, and once the reaction conditions are changed,
it can act as an acid promoting different sets of reactions. Thus, basic mechanisms for
changes in hydrothermal operations (i.e., wet pyrolysis) are different from those occurring in the dry pyrolysis, and this difference is largely caused by the changes in the
physical and chemical properties of water as its temperature increases [15].