other organic solvents like hexane, methanol, acetic acid, acetone, etc. [19]. At this
range the solvent behaves like a compressible fluid which generates high solvation
environment, thus making the long chain hydrocarbons dissociate into the shorter
chain [20]. A wide variety of raw feedstocks can be utilized for HTL conversion like
lignocellulosic biomass, algal biomass, solid wastes that contain organic matters, etc.
The primary product from HTL is the biocrude that should be further moved for
upgradation. Along with the primary product, other by-products like hydrochar,
aqueous, and biogas are produced. HTL finds its own place in attracting the research
group because of its integrated characteristics of solvents like high ion product and
low dielectric constant [21]. The major advantages of HTL than other thermochemical conversions are the following: effort for preconditioning of feed is not mandatory; it operates at moderate temperature; and heating rates can be slower than
pyrolysis [22].
2.3 Hydrothermal Gasification
The other way of depolymerizing biomass is by hydrothermal gasification in which
water reacts with biomass to produce gaseous hydrocarbon. Based on the operating
conditions, they are classified into supercritical water gasification, near-critical water
gasification, and low-temperature aqueous-phase reforming [23]. For low temperature HTG the sugar-like fructose or sugar alcohols like galactitol, sorbitol APR is
considered as raw material. For high-temperature SCWG, raw materials like glucose,
mannose, and fructose can be considered as good feedstocks [24]. To understand the
background of HTG, the behaviors of solvents at high temperature and high pressure
should be studied. The operating temperature for gasification ranges between
350 and 700
C which yields the gaseous products like H 2 , CH 4 , and CO 2
[25]. Unlike other hydrothermal conversions, HTG produces hydrocarbon gas that
has high HHV and negligible percentage of tar that is considered as advantage over
other hydrothermal processes.
2.4 Conversion Chemistry
It is necessary to understand the behavior and mechanism by which the lignocellulosic and algal biomass degrades in the subcritical and supercritical environment. In
case of lignocellulosic biomass, the major constituents present are cellulose, hemicellulose, and lignin, which influence the yield and composition of the products
formed. Likewise for algal biomass, the degradation of carbohydrates, proteins, and
lipids is to be studied. Due to different structures and properties of the major
constituents, the depolymerization reaction pathways are different in the hydrothermal environment as given in Fig. 2.
Hydrothermal Conversion of Biomass into Fuel and Fine Chemicals
207
range the solvent behaves like a compressible fluid which generates high solvation
environment, thus making the long chain hydrocarbons dissociate into the shorter
chain [20]. A wide variety of raw feedstocks can be utilized for HTL conversion like
lignocellulosic biomass, algal biomass, solid wastes that contain organic matters, etc.
The primary product from HTL is the biocrude that should be further moved for
upgradation. Along with the primary product, other by-products like hydrochar,
aqueous, and biogas are produced. HTL finds its own place in attracting the research
group because of its integrated characteristics of solvents like high ion product and
low dielectric constant [21]. The major advantages of HTL than other thermochemical conversions are the following: effort for preconditioning of feed is not mandatory; it operates at moderate temperature; and heating rates can be slower than
pyrolysis [22].
2.3 Hydrothermal Gasification
The other way of depolymerizing biomass is by hydrothermal gasification in which
water reacts with biomass to produce gaseous hydrocarbon. Based on the operating
conditions, they are classified into supercritical water gasification, near-critical water
gasification, and low-temperature aqueous-phase reforming [23]. For low temperature HTG the sugar-like fructose or sugar alcohols like galactitol, sorbitol APR is
considered as raw material. For high-temperature SCWG, raw materials like glucose,
mannose, and fructose can be considered as good feedstocks [24]. To understand the
background of HTG, the behaviors of solvents at high temperature and high pressure
should be studied. The operating temperature for gasification ranges between
350 and 700
C which yields the gaseous products like H 2 , CH 4 , and CO 2
[25]. Unlike other hydrothermal conversions, HTG produces hydrocarbon gas that
has high HHV and negligible percentage of tar that is considered as advantage over
other hydrothermal processes.
2.4 Conversion Chemistry
It is necessary to understand the behavior and mechanism by which the lignocellulosic and algal biomass degrades in the subcritical and supercritical environment. In
case of lignocellulosic biomass, the major constituents present are cellulose, hemicellulose, and lignin, which influence the yield and composition of the products
formed. Likewise for algal biomass, the degradation of carbohydrates, proteins, and
lipids is to be studied. Due to different structures and properties of the major
constituents, the depolymerization reaction pathways are different in the hydrothermal environment as given in Fig. 2.
Hydrothermal Conversion of Biomass into Fuel and Fine Chemicals
207