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Hydrothermal Processes in Subcritical Water
5.2.2 eFFeCTS oF oPerATing CondiTionS
A number of operating conditions such as (1) the water and solid concentrations,
(2) the feed slurry pH value, (3) the reaction temperature and pressure, (4) the reaction
residence time, and (5) the nature of feedstock affect the product distributions. These
effects are well described in the literature [2,15,18–40]. Here we briefly summarize
these literature results.
HTC process and formation of hydrochar requires the presence of water. It is
known that the biomass above the water surface does not carbonize, although only
small amount of water is necessary. The process of carbonization is accelerated by
water because of its active role as a solvent, a reactant, and a catalyst during various steps of biomass degradation and subsequent condensation and aromatization
processes. Water helps thermally driven pyrolysis. The initial step of hydrolysis is
very important and the role of water for this step increases with an increase in temperature. Water facilitates the condensation polymerization of active intermediate
species and also dissolves numerous compounds formed during the HTC process.
The amount of water can also affect the transport of fragments from the influence of
reactive centers. Generally, very low concentration of biomass in water may result
in very low production of precipitated carbonized solids since most biomass may be
dissolved. However, excessive biomass may result in some unreacted organic materials. Generally, increase in feed solid concentration increases the monomer concentration in the liquid phase. The key is to optimize the effect of residence time–solids
concentration interplay on the extent of polymerization reaction. For each feedstock,
there will be an optimum solids concentration to achieve the highest yield of carbonized solids.
It is known that during HTC, pH drops due to the formation of acetic, formic,
lactic, and levulinic acids [15,18,19,21]. It is also known that natural coalification requires a neutral-to-weak acidic environment [15,18,19,21]. The effects of
the nature and quantity of acids and bases on the product characteristics are well
described in many of the studies mentioned earlier. In general, high pH values
result in the product with a high H/C ratio, hydrolysis reactions are favored by the
acidic conditions, and a weak acidic condition improves the overall rate of HTC
[2,15,18–40].
Depending on the nature of the feedstock, an exothermal effect during hydrothermal operation can occur at as low as 100°C temperature (e.g., for peat). Temperature
is the most important parameter in the HTC process. It is known that hydrolysis
of glucose with subsequent dehydration may take several seconds at 270°C, but it
will take up to several hours at 150°C. The rate of polymerization is also temperature dependent. The temperature has a definite influence on the nature of biomass
that can be hydrolyzed. Hemicellulose hydrolyzes at 180°C and lignin at 200°C, but
cellulose hydrolysis requires 220°C [2,15,18,19,21]. Pyrolytic reactions also become
more important at higher temperatures.
The temperature also affects all the physical and chemical properties that are
important in the HTC process. The viscosity of water is also decreased by twofold
when the temperature is increased from the room temperature to around 350°C.
Lower viscosity helps the water penetrate in porous biomass media. While pressure is
Hydrothermal Processes in Subcritical Water
5.2.2 eFFeCTS oF oPerATing CondiTionS
A number of operating conditions such as (1) the water and solid concentrations,
(2) the feed slurry pH value, (3) the reaction temperature and pressure, (4) the reaction
residence time, and (5) the nature of feedstock affect the product distributions. These
effects are well described in the literature [2,15,18–40]. Here we briefly summarize
these literature results.
HTC process and formation of hydrochar requires the presence of water. It is
known that the biomass above the water surface does not carbonize, although only
small amount of water is necessary. The process of carbonization is accelerated by
water because of its active role as a solvent, a reactant, and a catalyst during various steps of biomass degradation and subsequent condensation and aromatization
processes. Water helps thermally driven pyrolysis. The initial step of hydrolysis is
very important and the role of water for this step increases with an increase in temperature. Water facilitates the condensation polymerization of active intermediate
species and also dissolves numerous compounds formed during the HTC process.
The amount of water can also affect the transport of fragments from the influence of
reactive centers. Generally, very low concentration of biomass in water may result
in very low production of precipitated carbonized solids since most biomass may be
dissolved. However, excessive biomass may result in some unreacted organic materials. Generally, increase in feed solid concentration increases the monomer concentration in the liquid phase. The key is to optimize the effect of residence time–solids
concentration interplay on the extent of polymerization reaction. For each feedstock,
there will be an optimum solids concentration to achieve the highest yield of carbonized solids.
It is known that during HTC, pH drops due to the formation of acetic, formic,
lactic, and levulinic acids [15,18,19,21]. It is also known that natural coalification requires a neutral-to-weak acidic environment [15,18,19,21]. The effects of
the nature and quantity of acids and bases on the product characteristics are well
described in many of the studies mentioned earlier. In general, high pH values
result in the product with a high H/C ratio, hydrolysis reactions are favored by the
acidic conditions, and a weak acidic condition improves the overall rate of HTC
[2,15,18–40].
Depending on the nature of the feedstock, an exothermal effect during hydrothermal operation can occur at as low as 100°C temperature (e.g., for peat). Temperature
is the most important parameter in the HTC process. It is known that hydrolysis
of glucose with subsequent dehydration may take several seconds at 270°C, but it
will take up to several hours at 150°C. The rate of polymerization is also temperature dependent. The temperature has a definite influence on the nature of biomass
that can be hydrolyzed. Hemicellulose hydrolyzes at 180°C and lignin at 200°C, but
cellulose hydrolysis requires 220°C [2,15,18,19,21]. Pyrolytic reactions also become
more important at higher temperatures.
The temperature also affects all the physical and chemical properties that are
important in the HTC process. The viscosity of water is also decreased by twofold
when the temperature is increased from the room temperature to around 350°C.
Lower viscosity helps the water penetrate in porous biomass media. While pressure is