133
Hydrothermal Processes in Subcritical Water
types of wastewaters (including wastewater from paper and pulp industries), glucose,
glycine, dairy manure, poultry litter, and so on. Some of these studies are briefly
described below.
A study by Minowa et al. [71–74] using glucose and glycine as model compounds
of carbohydrates and proteins indicated that a significant oil production started at
temperatures >250°C and increased with the temperature. This and other studies have shown that fatty acids and lipid are the main reactants in HTL process.
Below 300°C, aliphatic compounds are the major source of bio-oil. Protein is widely
involved in HTL reaction possibly by peptide bond splitting and amino acid conversion dehydration. Within the range of 300°C–450°C, the protein conversion reaction
intensifies and the peptide bond begins to react. Saccharide reaction mainly belongs
to the splitting of branched chains and the group transfer while considerable dehydration and cyclization of the main chain still appear to be dominant. The decomposition of an individual cellulosic biomass differs based on its structure. Decomposition
is easier in hemicelluloses due to amorphous structure. Cellulose is little crystalline to decompose due to beta(1–4)-glycosidic linkages and relatively intermediate
degree of polymerization (500–10,000). The major products of holocellulose degradation include cellohexaose, cellopentaose, cellotriose, cellobiose, fructose, glucose,
erythrose, glycolaldehyde, glyceraldehyde, pyruvaldehyde, and furfurals [71–74].
5.3.3.2 lignocellulose
The presence of liquid water as solvent is essential for HTL of lignocellulose feedstock.
Water in this case acts as a solvent and reactant along with its role as a vehicle for
biomass and a carrier for the catalyst. Furthermore, water is simple to use, inexpensive,
and environmentally benign.
Lignocellulose is the largest segment of the total biomass and contains a significant amount of lignin along with cellulose. It is the lignin component along with
crystalline cellulose that is difficult to convert to bio-oil in the HTL process. In the
absence of a catalyst, lignin produces very little bio-oil and ends up as a solids residue in the HTL process. While water is an excellent medium for the intermediate
hydrolysis of cellulose and other higher molecular-weight carbohydrates to watersoluble sugars, it is not as effective for hydrolysis of heavily aromatic and multiring
aromatic structures. The breakdown of lignin requires high temperature or the presence of a catalyst. Within lignocellulosic substances, softwood gives much lower
yield than hardwood because of the difference in their lignin contents. Zhang [45]
and Akhtar and Amin [41], among others, have given an extensive review of HTL of
a variety of lignocellulosic biomass such as various energy crops, herbaceous products, forestry and other agricultural wastes, and various crop oil wastes. Midgett [42]
examined the HTL process for materials such as tallow seed, switchgrass, and pine
dust. Zhong and Wei [80] studied the effect of temperature on four different types of
woods and concluded that the bio-oil yield was affected by both the temperature and
the lignin content of the wood.
An interesting study was carried out by Sugano et al. [48] in which they examined the effectiveness of black liquor, paper regeneration wastewater, and the water
on HTL of herbaceous eucalyptus biomass. Like softwood, this material contains
lignin. The study showed that black liquor gave very low oil yield and high yield of
Hydrothermal Processes in Subcritical Water
types of wastewaters (including wastewater from paper and pulp industries), glucose,
glycine, dairy manure, poultry litter, and so on. Some of these studies are briefly
described below.
A study by Minowa et al. [71–74] using glucose and glycine as model compounds
of carbohydrates and proteins indicated that a significant oil production started at
temperatures >250°C and increased with the temperature. This and other studies have shown that fatty acids and lipid are the main reactants in HTL process.
Below 300°C, aliphatic compounds are the major source of bio-oil. Protein is widely
involved in HTL reaction possibly by peptide bond splitting and amino acid conversion dehydration. Within the range of 300°C–450°C, the protein conversion reaction
intensifies and the peptide bond begins to react. Saccharide reaction mainly belongs
to the splitting of branched chains and the group transfer while considerable dehydration and cyclization of the main chain still appear to be dominant. The decomposition of an individual cellulosic biomass differs based on its structure. Decomposition
is easier in hemicelluloses due to amorphous structure. Cellulose is little crystalline to decompose due to beta(1–4)-glycosidic linkages and relatively intermediate
degree of polymerization (500–10,000). The major products of holocellulose degradation include cellohexaose, cellopentaose, cellotriose, cellobiose, fructose, glucose,
erythrose, glycolaldehyde, glyceraldehyde, pyruvaldehyde, and furfurals [71–74].
5.3.3.2 lignocellulose
The presence of liquid water as solvent is essential for HTL of lignocellulose feedstock.
Water in this case acts as a solvent and reactant along with its role as a vehicle for
biomass and a carrier for the catalyst. Furthermore, water is simple to use, inexpensive,
and environmentally benign.
Lignocellulose is the largest segment of the total biomass and contains a significant amount of lignin along with cellulose. It is the lignin component along with
crystalline cellulose that is difficult to convert to bio-oil in the HTL process. In the
absence of a catalyst, lignin produces very little bio-oil and ends up as a solids residue in the HTL process. While water is an excellent medium for the intermediate
hydrolysis of cellulose and other higher molecular-weight carbohydrates to watersoluble sugars, it is not as effective for hydrolysis of heavily aromatic and multiring
aromatic structures. The breakdown of lignin requires high temperature or the presence of a catalyst. Within lignocellulosic substances, softwood gives much lower
yield than hardwood because of the difference in their lignin contents. Zhang [45]
and Akhtar and Amin [41], among others, have given an extensive review of HTL of
a variety of lignocellulosic biomass such as various energy crops, herbaceous products, forestry and other agricultural wastes, and various crop oil wastes. Midgett [42]
examined the HTL process for materials such as tallow seed, switchgrass, and pine
dust. Zhong and Wei [80] studied the effect of temperature on four different types of
woods and concluded that the bio-oil yield was affected by both the temperature and
the lignin content of the wood.
An interesting study was carried out by Sugano et al. [48] in which they examined the effectiveness of black liquor, paper regeneration wastewater, and the water
on HTL of herbaceous eucalyptus biomass. Like softwood, this material contains
lignin. The study showed that black liquor gave very low oil yield and high yield of