depends on the properties like fixed carbon, calorific value,
moisture content, volatile matter and residual; ash and alkali
contents play important roles (McKendry 2002). The energy
produced from a selected biomass vary according to its
properties and the conversion technology applied.
Two broad categories of techniques involved in the
conversion process of biomass feedstock into biofuels and
valuable chemicals are: (1) biochemical conversion technologies, including aerobic and anaerobic degradation,
enzymatic hydrolysis and fermentation, performed by bacterial enzymes and microorganism; (2) thermochemical
technologies, which depend on thermal conversion of waste
biomass feedstocks into biofuels and commodity chemicals.
The thermochemical process technology is the oldest one
useful for different purposes, such as cooking, heating, and
formation of coke, compared to the biochemical conversion
technology. Combustion, gasification, pyrolysis, and
hydrothermal liquefaction are the main thermochemical
conversion technologies (Elliott 2011).
2 Hot Compressed Water as Green Process
Medium
Water is benign, ecologically acceptable, readily available
and is low-cost solvent. Recently, water in vicinity of its
critical point attained significant research interest, particularly as a green medium with low polarity and much lower
dielectric constant for reforming organic waste products and
other by-products, being important from an ecological point
(Bubalo et al. 2015). The principle of the sub- and supercritical water processes is to heat up wet biomass below and
over critical points of water, respectively, to decompose
organic wastes into valuable bio-based chemical compounds. Sub- and supercritical water treatment technologies
are attaining importance as a way of high-valued materials
recovery.
Both sub- and supercritical water methodologies are
inexpensive, efficient and employ green solvent (water) for
biomass processing; therefore, these techniques are considered as “green” processing techniques for economic transformation of biomass feedstocks to bio-oil and renewable
chemicals (Vogel 2012). In addition to their “green,” nature,
they are also tunable solvents, so their physicochemical
properties can be easily changed by changing the reaction
conditions to control reaction kinetics and reaction products.
Furthermore, the power of sub- and supercritical water can
also be varied by varying the reaction pressure and temperature. This feature provides some benefits over the conventional solvents used under normal conditions.
2.1 Hot Compressed Water
Chemistry/Properties
The term “hot compressed water” (HCW) refers to the water,
heated over 200 °C temperatures at extreme pressure.
The HCW is a general term for both super- and subcritical
water when water is heated over 200 °C and sufficiently high
pressure whose physical properties could be optimized and
controlled by changing temperature, pressure and density in a
broad range (Kruse and Dinjus 2007). The sub- and supercritical water possess different physical and chemical properties such as reduced densities, high compressibility (like
gases) and catalytic activity around the critical point in
comparison to their properties at normal conditions (Reddy
et al. 2015). Tester et al. reported an enormous change in
solvation behavior of water from highly polar to non-polar
that occurs on increasing temperature from 25 to 350 °C at
30 MPa pressure (Fig. 1a) (Peterson et al. 2008). Specifically, the dielectric constant of water shows a sharp decrease
from about 80 to less than 2 on increasing the temperature
from 25 °C to 450 °C. However, the ionic (K w [H 3 O
+
]
[OH
− ]) by-products of water steadily increases first from
10
−14 to 10
−11 below 350 °C and then decreases to five-fold
on rising the reaction temperature to 500 °C (Fig. 1a). Furthermore, the thermal conductance of sub- and supercritical
water is apparently higher than one at ambient pressure steam
(Cheng and Ye 2014). As the state of water changes from
liquid to gas (steam), its thermal conductance also decreases
significantly. Consequently, it is possible to gain most of
energy from sub- and supercritical water by preheating
incoming water, thereafter cooling the processed water.
Additionally, the high compressibility (like gases) of HCW
due to reduced densities in comparison to water at ambient
conditions leads to a significant effect of solutes on the
properties of HCW, which is of utmost importance for
chemical reaction pathways. HCW also possesses high catalytic activity, attributed to the variations in ion dissociation
constants with temperature. As shown in Fig. 1b, phase
diagram of water at various temperature and pressure is referenced to highlight the different reaction pathways based on
the co-existence behavior of liquid and vapor phases.
The HTL pathways in general occur between about 200 and
370 °C temperature and 4–20 MPa pressure range. However,
higher temperatures up to 500 °C or above favor hydrothermal gasification and thermolysis processes (Kruse 2008).
2.1.1 Properties of HCW in Subcritical Range
The term “subcritical water” denotes to water, which
remains in liquid state at the temperature ranges from its
boiling range and critical point. Subcritical water
24
K. Sharma et al.
moisture content, volatile matter and residual; ash and alkali
contents play important roles (McKendry 2002). The energy
produced from a selected biomass vary according to its
properties and the conversion technology applied.
Two broad categories of techniques involved in the
conversion process of biomass feedstock into biofuels and
valuable chemicals are: (1) biochemical conversion technologies, including aerobic and anaerobic degradation,
enzymatic hydrolysis and fermentation, performed by bacterial enzymes and microorganism; (2) thermochemical
technologies, which depend on thermal conversion of waste
biomass feedstocks into biofuels and commodity chemicals.
The thermochemical process technology is the oldest one
useful for different purposes, such as cooking, heating, and
formation of coke, compared to the biochemical conversion
technology. Combustion, gasification, pyrolysis, and
hydrothermal liquefaction are the main thermochemical
conversion technologies (Elliott 2011).
2 Hot Compressed Water as Green Process
Medium
Water is benign, ecologically acceptable, readily available
and is low-cost solvent. Recently, water in vicinity of its
critical point attained significant research interest, particularly as a green medium with low polarity and much lower
dielectric constant for reforming organic waste products and
other by-products, being important from an ecological point
(Bubalo et al. 2015). The principle of the sub- and supercritical water processes is to heat up wet biomass below and
over critical points of water, respectively, to decompose
organic wastes into valuable bio-based chemical compounds. Sub- and supercritical water treatment technologies
are attaining importance as a way of high-valued materials
recovery.
Both sub- and supercritical water methodologies are
inexpensive, efficient and employ green solvent (water) for
biomass processing; therefore, these techniques are considered as “green” processing techniques for economic transformation of biomass feedstocks to bio-oil and renewable
chemicals (Vogel 2012). In addition to their “green,” nature,
they are also tunable solvents, so their physicochemical
properties can be easily changed by changing the reaction
conditions to control reaction kinetics and reaction products.
Furthermore, the power of sub- and supercritical water can
also be varied by varying the reaction pressure and temperature. This feature provides some benefits over the conventional solvents used under normal conditions.
2.1 Hot Compressed Water
Chemistry/Properties
The term “hot compressed water” (HCW) refers to the water,
heated over 200 °C temperatures at extreme pressure.
The HCW is a general term for both super- and subcritical
water when water is heated over 200 °C and sufficiently high
pressure whose physical properties could be optimized and
controlled by changing temperature, pressure and density in a
broad range (Kruse and Dinjus 2007). The sub- and supercritical water possess different physical and chemical properties such as reduced densities, high compressibility (like
gases) and catalytic activity around the critical point in
comparison to their properties at normal conditions (Reddy
et al. 2015). Tester et al. reported an enormous change in
solvation behavior of water from highly polar to non-polar
that occurs on increasing temperature from 25 to 350 °C at
30 MPa pressure (Fig. 1a) (Peterson et al. 2008). Specifically, the dielectric constant of water shows a sharp decrease
from about 80 to less than 2 on increasing the temperature
from 25 °C to 450 °C. However, the ionic (K w [H 3 O
+
]
[OH
− ]) by-products of water steadily increases first from
10
−14 to 10
−11 below 350 °C and then decreases to five-fold
on rising the reaction temperature to 500 °C (Fig. 1a). Furthermore, the thermal conductance of sub- and supercritical
water is apparently higher than one at ambient pressure steam
(Cheng and Ye 2014). As the state of water changes from
liquid to gas (steam), its thermal conductance also decreases
significantly. Consequently, it is possible to gain most of
energy from sub- and supercritical water by preheating
incoming water, thereafter cooling the processed water.
Additionally, the high compressibility (like gases) of HCW
due to reduced densities in comparison to water at ambient
conditions leads to a significant effect of solutes on the
properties of HCW, which is of utmost importance for
chemical reaction pathways. HCW also possesses high catalytic activity, attributed to the variations in ion dissociation
constants with temperature. As shown in Fig. 1b, phase
diagram of water at various temperature and pressure is referenced to highlight the different reaction pathways based on
the co-existence behavior of liquid and vapor phases.
The HTL pathways in general occur between about 200 and
370 °C temperature and 4–20 MPa pressure range. However,
higher temperatures up to 500 °C or above favor hydrothermal gasification and thermolysis processes (Kruse 2008).
2.1.1 Properties of HCW in Subcritical Range
The term “subcritical water” denotes to water, which
remains in liquid state at the temperature ranges from its
boiling range and critical point. Subcritical water
24
K. Sharma et al.