119
Hydrothermal Processes in Subcritical Water
nature of these mechanisms and their relative significance during the course of reaction
primarily depend on the nature of the feedstock.
During the HTC process, biomass components are hydrolyzed to produce a large
amount of monomers and oligomers [15]. Simultaneously, water-soluble extractables are also produced. These monomers, oligomers, and extractables then further
undergo dehydration, decarboxylation, and condensation reactions. Many intermediates such as 5-hydroxymethylfurfural (HMF) are very reactive and some of them
have high chemical values. These intermediates further undergo polymerization to
produce humic acids, bitumen, and insoluble solids, some of which precipitate as
HTC coal or hydrochar. Some components of biomass (e.g., crystalline cellulose) or
oligomer cellulose do not hydrolyze under these reaction conditions. More details on
the reaction mechanisms during the HTC process are given in the published literature [15,18,22–27,30–32].
Carbonization of biomass has a number of advantages over biological treatment.
First, it takes only few hours as opposed to days and months taken by the biological process allowing more compact reactor design. While toxic feedstock cannot
be converted biochemically, high temperature and chemical reactivity of hydrothermal environment can destroy pathogens and potential organic contaminants such
as pharmaceutically active compounds [33–40]. HTC also produces useful gas, liquid, and solid products that can be further utilized, which contribute to greenhouse
gas (GHG) and climate change mitigations, odor reduction, and soil amelioration
[33–40]. The discussion in Sections 5.2.1 through 5.2.5 closely follows an excellent
review by Libra et al. [15] and others [2,18–40].
5.2.1 reACTion meChAniSmS
As mentioned earlier, HTC is accompanied by a series of chemical reactions such
as hydrolysis, dehydration, decarboxylation, polymerization, and aromatization
[14,15,18,21]. These reaction mechanisms are briefly described below.
Hydrolysis reactions during the HTC process mainly break down ether and ester
bonds resulting in a wide range of products that include saccharides of cellulose
and phenolic fragments of lignin. Along with other degradation mechanisms mentioned later, the intermediate products are also further hydrolyzed such as HMF
converted to levulinic acid and formic acid. This transformation is further discussed in Chapter 7 on biofine process. Hemicellulose is hydrolyzed around 180°C
and cellulose is hydrolyzed above ~200°C. The detailed mechanism of cellulose
hydrolysis is given by Peterson et al. [5]. While initial hydrolysis reactions are
favored by the alkaline conditions, further degradation of glucose is accelerated
by the acidic conditions. In a pH range of 3–7, the rate of reaction is largely independent of H + and OH − concentrations. The hydrolysis of lignin occurs around
200°C and produces highly active low-molecular-weight substances. Some of these
substances go through condensation reactions and precipitate from the solutions.
In general, hydrolysis reactions are fast and transport limited. The structures produced from hemicellulose and lignin interact with each other resulting in high
solubility of aromatic structures. At high temperatures, condensation reactions are
likely to occur.
Hydrothermal Processes in Subcritical Water
nature of these mechanisms and their relative significance during the course of reaction
primarily depend on the nature of the feedstock.
During the HTC process, biomass components are hydrolyzed to produce a large
amount of monomers and oligomers [15]. Simultaneously, water-soluble extractables are also produced. These monomers, oligomers, and extractables then further
undergo dehydration, decarboxylation, and condensation reactions. Many intermediates such as 5-hydroxymethylfurfural (HMF) are very reactive and some of them
have high chemical values. These intermediates further undergo polymerization to
produce humic acids, bitumen, and insoluble solids, some of which precipitate as
HTC coal or hydrochar. Some components of biomass (e.g., crystalline cellulose) or
oligomer cellulose do not hydrolyze under these reaction conditions. More details on
the reaction mechanisms during the HTC process are given in the published literature [15,18,22–27,30–32].
Carbonization of biomass has a number of advantages over biological treatment.
First, it takes only few hours as opposed to days and months taken by the biological process allowing more compact reactor design. While toxic feedstock cannot
be converted biochemically, high temperature and chemical reactivity of hydrothermal environment can destroy pathogens and potential organic contaminants such
as pharmaceutically active compounds [33–40]. HTC also produces useful gas, liquid, and solid products that can be further utilized, which contribute to greenhouse
gas (GHG) and climate change mitigations, odor reduction, and soil amelioration
[33–40]. The discussion in Sections 5.2.1 through 5.2.5 closely follows an excellent
review by Libra et al. [15] and others [2,18–40].
5.2.1 reACTion meChAniSmS
As mentioned earlier, HTC is accompanied by a series of chemical reactions such
as hydrolysis, dehydration, decarboxylation, polymerization, and aromatization
[14,15,18,21]. These reaction mechanisms are briefly described below.
Hydrolysis reactions during the HTC process mainly break down ether and ester
bonds resulting in a wide range of products that include saccharides of cellulose
and phenolic fragments of lignin. Along with other degradation mechanisms mentioned later, the intermediate products are also further hydrolyzed such as HMF
converted to levulinic acid and formic acid. This transformation is further discussed in Chapter 7 on biofine process. Hemicellulose is hydrolyzed around 180°C
and cellulose is hydrolyzed above ~200°C. The detailed mechanism of cellulose
hydrolysis is given by Peterson et al. [5]. While initial hydrolysis reactions are
favored by the alkaline conditions, further degradation of glucose is accelerated
by the acidic conditions. In a pH range of 3–7, the rate of reaction is largely independent of H + and OH − concentrations. The hydrolysis of lignin occurs around
200°C and produces highly active low-molecular-weight substances. Some of these
substances go through condensation reactions and precipitate from the solutions.
In general, hydrolysis reactions are fast and transport limited. The structures produced from hemicellulose and lignin interact with each other resulting in high
solubility of aromatic structures. At high temperatures, condensation reactions are
likely to occur.