Hydrothermal Processes in Subcritical Water
125
reveal its potential technical applications as functionalized carbonaceous materials.
Libra et al. [15] and others [30–40] show that with the use of proper starting materials and appropriate catalyst/template-assisted treatment methods, various types of
nanospheres, nanocables, nanofibers, microcables, submicrotubes, and porous structures can be created from HTC coal.
5.2.5 ProCeSS ConSiderATionS
The HTC process generates more water than carbon dioxide. Water acts as a solvent
and a reactant, and therefore carries a significant amount of inorganics and organics,
many of which can be valuable chemicals. The solids in water can cause problems
upon precipitation due to condensation or polymerization reactions. The wastewater
of the HTC process can be processed with aerobic or anaerobic treatment to lower
its total organic content. Some inorganics in water may be good nutrients for soil.
The gases coming out of the HTC process mainly contain carbon dioxide with minor
CO, CH 4 , and H 2 as well as traces of C m H n . The dissolution of carbon dioxide in the
liquid phase may affect the critical temperature condition in the process. An increase
in temperature generally decreases CO and increases H 2 and CH 4 . The progress of
the HTC process can be monitored by following the production of carbon dioxide.
The decarboxylation of feedstock is a major reaction during the HTC process
[15,18,19,21]. While its high rate produces high heating value HTC coal, it also
reduces carbon efficiency. Thus, the process should be optimized based on the end
use of the final product.
In the HTC process, the ratio of biomass to water should be kept as high as possible to enhance polymerization. Less water will also give less energy loss and less
pumping costs for the total throughput. The feedstock can be submerged in the water
by mechanical compacting device to take advantage of the best reaction conditions.
The residence time should be as large as possible to get the complete reaction to
occur and minimize the loss or organics in the wastewater. A recirculation of water
is one way to achieve this objective.
The reaction conditions should take advantages of the possible effects of organic
acids; they may give faster polymerization and higher ash content of the produced
HTC coal. While higher temperature accelerates the process and gives higher carbon
content in the HTC product, high pressure required to achieve high temperature may
be expensive. Pressure–temperature relationship should be optimized based on the
intended use of the end product. Since hydrolysis is a diffusion-controlled reaction,
small particle size of the feedstock may be beneficial. This, however, increases the
energy demand and the investment cost [15,18,19,21].
5.3 hydrOthermal liQUeFaCtiOn
Just like the HTC process, HTL is a wet pyrolysis process in which complex organic
(particularly carbohydrate base) molecules from biowaste (manure and food processing waste), lignocellulose (crop residue), algae, and others are converted to crude oil
type and other liquid fuel products as well as chemicals. To some extent, it mimics the
natural geological process which is thought to be involved in the production of fossil
125
reveal its potential technical applications as functionalized carbonaceous materials.
Libra et al. [15] and others [30–40] show that with the use of proper starting materials and appropriate catalyst/template-assisted treatment methods, various types of
nanospheres, nanocables, nanofibers, microcables, submicrotubes, and porous structures can be created from HTC coal.
5.2.5 ProCeSS ConSiderATionS
The HTC process generates more water than carbon dioxide. Water acts as a solvent
and a reactant, and therefore carries a significant amount of inorganics and organics,
many of which can be valuable chemicals. The solids in water can cause problems
upon precipitation due to condensation or polymerization reactions. The wastewater
of the HTC process can be processed with aerobic or anaerobic treatment to lower
its total organic content. Some inorganics in water may be good nutrients for soil.
The gases coming out of the HTC process mainly contain carbon dioxide with minor
CO, CH 4 , and H 2 as well as traces of C m H n . The dissolution of carbon dioxide in the
liquid phase may affect the critical temperature condition in the process. An increase
in temperature generally decreases CO and increases H 2 and CH 4 . The progress of
the HTC process can be monitored by following the production of carbon dioxide.
The decarboxylation of feedstock is a major reaction during the HTC process
[15,18,19,21]. While its high rate produces high heating value HTC coal, it also
reduces carbon efficiency. Thus, the process should be optimized based on the end
use of the final product.
In the HTC process, the ratio of biomass to water should be kept as high as possible to enhance polymerization. Less water will also give less energy loss and less
pumping costs for the total throughput. The feedstock can be submerged in the water
by mechanical compacting device to take advantage of the best reaction conditions.
The residence time should be as large as possible to get the complete reaction to
occur and minimize the loss or organics in the wastewater. A recirculation of water
is one way to achieve this objective.
The reaction conditions should take advantages of the possible effects of organic
acids; they may give faster polymerization and higher ash content of the produced
HTC coal. While higher temperature accelerates the process and gives higher carbon
content in the HTC product, high pressure required to achieve high temperature may
be expensive. Pressure–temperature relationship should be optimized based on the
intended use of the end product. Since hydrolysis is a diffusion-controlled reaction,
small particle size of the feedstock may be beneficial. This, however, increases the
energy demand and the investment cost [15,18,19,21].
5.3 hydrOthermal liQUeFaCtiOn
Just like the HTC process, HTL is a wet pyrolysis process in which complex organic
(particularly carbohydrate base) molecules from biowaste (manure and food processing waste), lignocellulose (crop residue), algae, and others are converted to crude oil
type and other liquid fuel products as well as chemicals. To some extent, it mimics the
natural geological process which is thought to be involved in the production of fossil