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Hydrothermal Processes in Subcritical Water
The literature has shown that HTC is more energy efficient as a pretreatment
process than dry pyrolysis for wood combustion when the water content is >50%
[2,15,18–40]. Generally, when the water content of feed slurry is >50%–70%, wet
pyrolysis is preferred over dry pyrolysis. For such a slurry, dry pyrolysis will only
be capable of producing charred material. However, HTC process can be used for
a slurry containing 70%–90% water or even higher. The amount of external heat
required will depend on the process design. Generally, HTC process with a slurry
containing >40% water will have an energetic advantage over dry pyrolysis. HTC
process needs to manage the required pressure and its effect on the safety and material issues. The energy from hot water also needs to be recovered. In general, energetic requirements to run the process and its auxiliary equipment are higher for a
hydrothermal system than for dry pyrolysis.
5.2.4 ProduCT ChArACTeriSTiCS And uSAgeS
HTC process largely produces hydrochar (solid) and liquid with dissolved nutrients.
The fate of heavy metals and organic chemicals (present in the original feedstock
or created by the reactions) is generally not known and must be traced during the
process. Wet pyrolysis cannot destroy heavy metals. Since they have a toxic risk
potential, their fate needs to be followed. If they accumulate in the solid char, which
is subsequently used as soil nutrient, they can affect the food chain. Generally, except
for zinc, heavy metal concentrations in char do not exceed the allowable limits
[15,18,19,21,30–40]. Also, heavy metal contents of hydrochar obtained by wet pyrolysis are less than those obtained in biochar from dry pyrolysis [15,18,19,21,30–40].
Just like for heavy metals, a systematic knowledge of the fate of organic compounds
during the HTC process is not well understood. Unlike heavy metals, new organic
compounds can be formed during condensation, polymerization, and aromatization
reactions. The fate of compounds such as polychlorinated biphenyl (PCB) and hexachlorobenzene (HCB) needs to be particularly followed along with all other organic
chemicals during the HTC process [15,18,19,21,30–40].
For both animal manures and sewage sludge, hydrochar retains a significant
level of calcium, potassium, and phosphorus. pH affects the mobility and sorption
capability of the nutrients, particularly for the case of phosphorus. In the HTC
process, dissolution of water-soluble minerals can be significant [15,18,19,21,30–40];
however, the nutrient content will also depend on the technique for dewatering the
solid conversion product. The ratio between evaporation and dewatering governs
the amount of plant nutrients that will be adsorbed or retained at the hydrochar
interface. Nutrient retention should be an important parameter in the detailed
process design.
Generally, an increase in temperature decreases the hydrochar yield and increases
the yield of liquids and gases such as CO 2 , CO, and H 2 . An increase in temperature
also decreases the H/C and O/C ratios in the hydrochar. The maximum allowable
yields (which may be obtained at very large residence time) for various feedstock
are illustrated by Libra et al. [15] and others [18,19,21]. A lower biomass solid concentration (i.e., high water concentration) generally gives a lower hydrochar yield.
Hydrothermal Processes in Subcritical Water
The literature has shown that HTC is more energy efficient as a pretreatment
process than dry pyrolysis for wood combustion when the water content is >50%
[2,15,18–40]. Generally, when the water content of feed slurry is >50%–70%, wet
pyrolysis is preferred over dry pyrolysis. For such a slurry, dry pyrolysis will only
be capable of producing charred material. However, HTC process can be used for
a slurry containing 70%–90% water or even higher. The amount of external heat
required will depend on the process design. Generally, HTC process with a slurry
containing >40% water will have an energetic advantage over dry pyrolysis. HTC
process needs to manage the required pressure and its effect on the safety and material issues. The energy from hot water also needs to be recovered. In general, energetic requirements to run the process and its auxiliary equipment are higher for a
hydrothermal system than for dry pyrolysis.
5.2.4 ProduCT ChArACTeriSTiCS And uSAgeS
HTC process largely produces hydrochar (solid) and liquid with dissolved nutrients.
The fate of heavy metals and organic chemicals (present in the original feedstock
or created by the reactions) is generally not known and must be traced during the
process. Wet pyrolysis cannot destroy heavy metals. Since they have a toxic risk
potential, their fate needs to be followed. If they accumulate in the solid char, which
is subsequently used as soil nutrient, they can affect the food chain. Generally, except
for zinc, heavy metal concentrations in char do not exceed the allowable limits
[15,18,19,21,30–40]. Also, heavy metal contents of hydrochar obtained by wet pyrolysis are less than those obtained in biochar from dry pyrolysis [15,18,19,21,30–40].
Just like for heavy metals, a systematic knowledge of the fate of organic compounds
during the HTC process is not well understood. Unlike heavy metals, new organic
compounds can be formed during condensation, polymerization, and aromatization
reactions. The fate of compounds such as polychlorinated biphenyl (PCB) and hexachlorobenzene (HCB) needs to be particularly followed along with all other organic
chemicals during the HTC process [15,18,19,21,30–40].
For both animal manures and sewage sludge, hydrochar retains a significant
level of calcium, potassium, and phosphorus. pH affects the mobility and sorption
capability of the nutrients, particularly for the case of phosphorus. In the HTC
process, dissolution of water-soluble minerals can be significant [15,18,19,21,30–40];
however, the nutrient content will also depend on the technique for dewatering the
solid conversion product. The ratio between evaporation and dewatering governs
the amount of plant nutrients that will be adsorbed or retained at the hydrochar
interface. Nutrient retention should be an important parameter in the detailed
process design.
Generally, an increase in temperature decreases the hydrochar yield and increases
the yield of liquids and gases such as CO 2 , CO, and H 2 . An increase in temperature
also decreases the H/C and O/C ratios in the hydrochar. The maximum allowable
yields (which may be obtained at very large residence time) for various feedstock
are illustrated by Libra et al. [15] and others [18,19,21]. A lower biomass solid concentration (i.e., high water concentration) generally gives a lower hydrochar yield.