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Water for Energy and Fuel Production
a more expensive process variable, it does not affect the HTC process as significantly
as the temperature. While both dehydration and decarboxylation reactions are
suppressed with an increase in pressure, this, however, does not significantly affect
the overall HTC process. An increase in pressure facilitates (1) the removal of
extractable, (2) solubilization of compressed gases and physical compacting of biocoal, and (3) hydrogen ion transfer and condensation polymerization between solids
allowing the use of higher biomass/water ratio.
Although initial extraction and hydrolysis are rapid reactions, overall HTC process
is a slow-limiting process. The diffusion-controlled transport mechanisms during
biomass degradation and condensation polymerization govern the overall rate of
reaction. Due to its slow nature, HTC coal yield increases with an increase in residence time.
Finally, feedstock characteristics such as chemical composition, volatile and noncombustible fractions, moisture content, particle size, and energy content significantly affect conversion efficiency and char characteristics. These effects are well
described in the studies mentioned earlier [2,15,18–40].
5.2.3 ComPAriSon oF hTC And dry PyrolySiS ProCeSS
HTC process produce a high amount of solids, more water-soluble organics, and
fewer gases that mainly contain CO 2 [2,15,18,19,22]. The chemical structure of
hydrochar more closely resembles coal than charcoal in terms of elemental composition and types of chemical bonds and their relative quantities. As discussed
earlier, HTC hydrochar has higher H/C and O/C ratios than the char coming from
dry pyrolysis (Figure 5.2). Thus, the ratio of decarboxylation to dehydration reaction is higher in HTC than in dry pyrolysis. Figure 5.2 shows that even though feed
composition of animal-derived biomass is different from that of plant materials,
the final hydrochar products coming from these materials have similar elemental
compositions. The aromatic structure of hydrochar product is substantially different
from that of char from dry pyrolysis. These and other structural differences indicate
that different reaction mechanisms govern these two processes. Radical mechanism
pathways taken in dry pyrolysis are completely suppressed in hot water in favor of
ionic reactions.
HTC primarily starts out with hydrolysis reactions of biomacromolecules resulting in the production of oligosaccharides, hexoses, pentoses, and fragments of lignin
[2,15,18,19,22]. These intermediates in the aqueous phase follow completely different reaction pathways than those in thermally driven dry pyrolysis. For example, in
dry pyrolysis of glucose, the major product is anhydrous glucose, which appears only
in a very small amount in the HTC process. In the HTC process, HMF is a crucial
intermediate, which provides a standard platform for many subsequent reactions.
Hydrolysis in the HTC process completely disintegrates the physical structure of
biomass. This is not the case for dry pyrolysis. The degree of hydrolysis, however,
depends on the temperature and process design [2,15,18,19,22]. Final H/C ratios
produced by HTC of lignin and cellulose are different. This is a result of different
reaction paths of these two processes.
Water for Energy and Fuel Production
a more expensive process variable, it does not affect the HTC process as significantly
as the temperature. While both dehydration and decarboxylation reactions are
suppressed with an increase in pressure, this, however, does not significantly affect
the overall HTC process. An increase in pressure facilitates (1) the removal of
extractable, (2) solubilization of compressed gases and physical compacting of biocoal, and (3) hydrogen ion transfer and condensation polymerization between solids
allowing the use of higher biomass/water ratio.
Although initial extraction and hydrolysis are rapid reactions, overall HTC process
is a slow-limiting process. The diffusion-controlled transport mechanisms during
biomass degradation and condensation polymerization govern the overall rate of
reaction. Due to its slow nature, HTC coal yield increases with an increase in residence time.
Finally, feedstock characteristics such as chemical composition, volatile and noncombustible fractions, moisture content, particle size, and energy content significantly affect conversion efficiency and char characteristics. These effects are well
described in the studies mentioned earlier [2,15,18–40].
5.2.3 ComPAriSon oF hTC And dry PyrolySiS ProCeSS
HTC process produce a high amount of solids, more water-soluble organics, and
fewer gases that mainly contain CO 2 [2,15,18,19,22]. The chemical structure of
hydrochar more closely resembles coal than charcoal in terms of elemental composition and types of chemical bonds and their relative quantities. As discussed
earlier, HTC hydrochar has higher H/C and O/C ratios than the char coming from
dry pyrolysis (Figure 5.2). Thus, the ratio of decarboxylation to dehydration reaction is higher in HTC than in dry pyrolysis. Figure 5.2 shows that even though feed
composition of animal-derived biomass is different from that of plant materials,
the final hydrochar products coming from these materials have similar elemental
compositions. The aromatic structure of hydrochar product is substantially different
from that of char from dry pyrolysis. These and other structural differences indicate
that different reaction mechanisms govern these two processes. Radical mechanism
pathways taken in dry pyrolysis are completely suppressed in hot water in favor of
ionic reactions.
HTC primarily starts out with hydrolysis reactions of biomacromolecules resulting in the production of oligosaccharides, hexoses, pentoses, and fragments of lignin
[2,15,18,19,22]. These intermediates in the aqueous phase follow completely different reaction pathways than those in thermally driven dry pyrolysis. For example, in
dry pyrolysis of glucose, the major product is anhydrous glucose, which appears only
in a very small amount in the HTC process. In the HTC process, HMF is a crucial
intermediate, which provides a standard platform for many subsequent reactions.
Hydrolysis in the HTC process completely disintegrates the physical structure of
biomass. This is not the case for dry pyrolysis. The degree of hydrolysis, however,
depends on the temperature and process design [2,15,18,19,22]. Final H/C ratios
produced by HTC of lignin and cellulose are different. This is a result of different
reaction paths of these two processes.