Water Soluble Products yield wt%
ð
Þ¼100 À Crude þ Gas þ Residue
ð
Þ ð 4Þ
where W f is the weight of the biomass (dry basis); W es is the weight of products
soluble in extraction solvents (ether, acetone, acetone, dichloromethane, etc.); W c is
the charged weight (biomass + hydrothermal media); W rp is the weight of reaction
products; and W r is the weight of solid residue, i.e., biochar.
Some of the woody and algal feedstock, their possible hydrothermal pathway,
type of reactor and its specifications, hydrothermal media used, catalyst, and the
extraction solvent used are listed Table 4.
3.1 Effect of Temperature
Temperature plays an important role in the hydrothermal processing irrespective of
biomasses. The temperature for a typical hydrothermal conversion process ranges
between 180 and 700
C, and it mainly depends on the type of feedstock and its
constituent composition (Table 4). Under such given elevated temperatures, it shows
symbiotic effect in the yield of resultant products (biochar, biocrude, and biochar)
from the biomass. The increased temperature depolymerizes the long chain complex
organic molecule of length 800–10,000 units into chemicals of C 15+ to simple gases
like H 2 , CH 4 , and CO 2 . Such conditions not only improved the reaction rate but also
created variation in reaction mechanisms. Tungal and Shende [34] explained the
effect of temperature on pine sawdust under carbonization and liquefaction conditions, where the biocrude yield increased from 24.04 to 30.5% at 1:10 biomass
loading and 90 min reaction time [34]. The tar formed during the hydrothermal
conversion process is due to the impact of temperature under the subcritical condition, and its formation is extremely lowered under supercritical conditions. The two
reasons that controlled this reaction are (1) reduced dielectric constant which led to
free-radical reactions to leaving away with gaseous products and (2) under supercritical region tarry material which behaves like a solvent [51]. From studies it is
observed that when operating temperature is elevated beyond 500–600
C, the
depolymerization reaction occurs resulting in dissociation of complex bonds into
simpler gaseous molecules [35]. Figure 3 shows the effect of temperature
(225–600
C) on the yield of different hydrothermal products. This effect was also
confirmed by a recent study by Alper et.al [38] on hydrothermal liquefaction of
spruce wood at 250–300
C; the yield of biocrude increased slightly from 3.9 to
6.6 wt%. But beyond 300–350
C, the yield progressively decreased to 4.9 wt% due
to the vaporization of biocrude at this temperature range [38]. Similar phenomena
was observed beyond the supercritical condition (>374
C, 219 bar) in the case of
algal biomass Posidonia oceanica due to the stimulation of steam-reforming and
methanation reactions reducing the liquid and solid residue while increasing the
gaseous products [50].
210
C. D. Venkatachalam et al.
ð
Þ¼100 À Crude þ Gas þ Residue
ð
Þ ð 4Þ
where W f is the weight of the biomass (dry basis); W es is the weight of products
soluble in extraction solvents (ether, acetone, acetone, dichloromethane, etc.); W c is
the charged weight (biomass + hydrothermal media); W rp is the weight of reaction
products; and W r is the weight of solid residue, i.e., biochar.
Some of the woody and algal feedstock, their possible hydrothermal pathway,
type of reactor and its specifications, hydrothermal media used, catalyst, and the
extraction solvent used are listed Table 4.
3.1 Effect of Temperature
Temperature plays an important role in the hydrothermal processing irrespective of
biomasses. The temperature for a typical hydrothermal conversion process ranges
between 180 and 700
C, and it mainly depends on the type of feedstock and its
constituent composition (Table 4). Under such given elevated temperatures, it shows
symbiotic effect in the yield of resultant products (biochar, biocrude, and biochar)
from the biomass. The increased temperature depolymerizes the long chain complex
organic molecule of length 800–10,000 units into chemicals of C 15+ to simple gases
like H 2 , CH 4 , and CO 2 . Such conditions not only improved the reaction rate but also
created variation in reaction mechanisms. Tungal and Shende [34] explained the
effect of temperature on pine sawdust under carbonization and liquefaction conditions, where the biocrude yield increased from 24.04 to 30.5% at 1:10 biomass
loading and 90 min reaction time [34]. The tar formed during the hydrothermal
conversion process is due to the impact of temperature under the subcritical condition, and its formation is extremely lowered under supercritical conditions. The two
reasons that controlled this reaction are (1) reduced dielectric constant which led to
free-radical reactions to leaving away with gaseous products and (2) under supercritical region tarry material which behaves like a solvent [51]. From studies it is
observed that when operating temperature is elevated beyond 500–600
C, the
depolymerization reaction occurs resulting in dissociation of complex bonds into
simpler gaseous molecules [35]. Figure 3 shows the effect of temperature
(225–600
C) on the yield of different hydrothermal products. This effect was also
confirmed by a recent study by Alper et.al [38] on hydrothermal liquefaction of
spruce wood at 250–300
C; the yield of biocrude increased slightly from 3.9 to
6.6 wt%. But beyond 300–350
C, the yield progressively decreased to 4.9 wt% due
to the vaporization of biocrude at this temperature range [38]. Similar phenomena
was observed beyond the supercritical condition (>374
C, 219 bar) in the case of
algal biomass Posidonia oceanica due to the stimulation of steam-reforming and
methanation reactions reducing the liquid and solid residue while increasing the
gaseous products [50].
210
C. D. Venkatachalam et al.