When the pressure and temperature of the water reach a critical point, the
supercritical condition is achieved. The pressure and temperature at this level
correspond to Pc 22.1 MPa and Tc 374
C. At this point, all the physiochemical
characteristics of water such as ion product, viscosity, water density and dielectric
properties will be extremely higher. However, this will equally change the chemical
properties like dipole moment, pH, polarity, etc. which will make water not only
solvent for ionic species but for non-ionic species [38]. The property of water at this
point makes it very attractive and environmentally potential reaction medium for
organic and inorganic compounds. This draws the attention of the researchers
looking for green and environmentally friendly solvents for chemical reactions
without pollution to the atmosphere [39].
4.3 Supercritical Water Gasification of Biomass (SCWG)
The global energy crisis is the driving factor for the development of this technology,
as hydrogen that can be used as fuel can be produced. In recent years, supercritical
water at temperature 374
C and higher pressure of 221 bar is widely employed in
converting biomass to liquid and gaseous fuels, which can be detached easily from
the reacting phase (i.e. water) by simple cooling to ambient temperature [40].
Understanding the influence of this novel technology on biomass is a difficult task
because of the different components available in the biomass structure. However as
stated earlier, hydrogen which is crucial for the development of a bio-economy can
be produced easily with this technology. Despite the prospect of the hydrothermal
technology in hydrogen and other fuel production, the only workable biomass that
can be sustainable for this technology is biomass source that avoided any potential
competition with food cycle. In this context, the best biomass source that can serve
this purpose is the biomass from MSW source.
Almost 90% of the available land biomass structure and cell walls of all terrestrial
plants is made up of lignocellulose. Lignocellulosic biomass consists of 40–55%
cellulose fibres and 15–35% hemicellulose surrounded in 20–40% of lignin. In the
three components of lignocellulosic biomass, i.e. cellulose, hemicellulose and lignin,
celluloses and hemicelluloses are polysaccharides of C6 and C5 monomers
connected by β-(1–4)-glycosidic linkages. The main compounds in the lignin are
polymers of para-hydroxyphenyl (H lignin), guaiacyl (G lignin) and syringyl
(S lignin) alcohol as shown in Fig. 11.
Most of the cellulose in biomass is crystalline, although a small portion is
amorphous. The crystalline cellulose is built up by 5,000–15,000 glucose units
β-1,4 glycosidic and hydrogen bonds, while the amorphous hemicellulose consists
of branched C5- and C6-sugars (xylose, manose, arabinose, glucose and galactose)
linked by β-1,4 glycosidic bonds with a polymerisation degree of 100–1,000. Lignin
is composed of p-hydroxy-propyl-benzene structures, namely, p-coumaryl alcohol,
coniferyl alcohol and sinapyl alcohol, connected by ether and C–C bonds to build a
three-dimensional network, although the proportion of units in lignin varies from
192
A. A. Gado et al.
supercritical condition is achieved. The pressure and temperature at this level
correspond to Pc 22.1 MPa and Tc 374
C. At this point, all the physiochemical
characteristics of water such as ion product, viscosity, water density and dielectric
properties will be extremely higher. However, this will equally change the chemical
properties like dipole moment, pH, polarity, etc. which will make water not only
solvent for ionic species but for non-ionic species [38]. The property of water at this
point makes it very attractive and environmentally potential reaction medium for
organic and inorganic compounds. This draws the attention of the researchers
looking for green and environmentally friendly solvents for chemical reactions
without pollution to the atmosphere [39].
4.3 Supercritical Water Gasification of Biomass (SCWG)
The global energy crisis is the driving factor for the development of this technology,
as hydrogen that can be used as fuel can be produced. In recent years, supercritical
water at temperature 374
C and higher pressure of 221 bar is widely employed in
converting biomass to liquid and gaseous fuels, which can be detached easily from
the reacting phase (i.e. water) by simple cooling to ambient temperature [40].
Understanding the influence of this novel technology on biomass is a difficult task
because of the different components available in the biomass structure. However as
stated earlier, hydrogen which is crucial for the development of a bio-economy can
be produced easily with this technology. Despite the prospect of the hydrothermal
technology in hydrogen and other fuel production, the only workable biomass that
can be sustainable for this technology is biomass source that avoided any potential
competition with food cycle. In this context, the best biomass source that can serve
this purpose is the biomass from MSW source.
Almost 90% of the available land biomass structure and cell walls of all terrestrial
plants is made up of lignocellulose. Lignocellulosic biomass consists of 40–55%
cellulose fibres and 15–35% hemicellulose surrounded in 20–40% of lignin. In the
three components of lignocellulosic biomass, i.e. cellulose, hemicellulose and lignin,
celluloses and hemicelluloses are polysaccharides of C6 and C5 monomers
connected by β-(1–4)-glycosidic linkages. The main compounds in the lignin are
polymers of para-hydroxyphenyl (H lignin), guaiacyl (G lignin) and syringyl
(S lignin) alcohol as shown in Fig. 11.
Most of the cellulose in biomass is crystalline, although a small portion is
amorphous. The crystalline cellulose is built up by 5,000–15,000 glucose units
β-1,4 glycosidic and hydrogen bonds, while the amorphous hemicellulose consists
of branched C5- and C6-sugars (xylose, manose, arabinose, glucose and galactose)
linked by β-1,4 glycosidic bonds with a polymerisation degree of 100–1,000. Lignin
is composed of p-hydroxy-propyl-benzene structures, namely, p-coumaryl alcohol,
coniferyl alcohol and sinapyl alcohol, connected by ether and C–C bonds to build a
three-dimensional network, although the proportion of units in lignin varies from
192
A. A. Gado et al.