study, Khampuang et al. [42] observed that liquefaction performed for 60 min at
340
C with a 1:1 (v/v) ethanol/water with addition of 10 wt% of NaOH resulted in
oil yield of 50.2–57.2%; rather with no catalyst it was 49.0%. The oil yield with
KOH addition (51.4–57.5%) was slightly higher than that with NaOH (50.5–56.4%)
[42]. Similarly, alkali especially potassium in the form of K 2 CO 3 exhibited a
progressive effect on yield of biocrude. The hydrothermal treatment of cotton
cocoon shell showed a catalytic activity which is as follows:
KOH > NaOH > K 2 CO 3 > Na 2 CO 3 [43]. The alkali catalysts weaken the C-C
bond, thereby decreasing the activation energy. It also triggers biomass swelling and
increase in the surface area exposed, enhancing the retro-aldol cleavage. These alkali
catalysts have the tendency to promote water-gas shift reaction during the hydrothermal conversion, thus favoring H 2 and CO 2 formation from CO. The resulting
hydrogen gas in turn acts as a reducing agent, increasing the HHV and quality of the
biocrude.
Heterogeneous Catalysts Heterogeneous catalysts have been mostly used in
hydrothermal gasification process and low-temperature applications, where they
enhance the quality of the biocrude obtained. de Caprariis et al. [37] investigated
that HTL of oak wood with addition of Fe as a catalyst increases the quality and
quantity of biocrude. Fe acts as a sacrificial catalyst, which in turn undergoes surface
oxidation into Fe 2 O 3 , thereby reducing the acids and aldehydes formed during the
process. Gasification is crucial to a certain extent for biocrudes containing higher
percentages of oxygenated compounds. Few other studies show that by doping
oxides like Al 2 O 3 with active substance like KF, Pd, and Pt and using it as a catalyst,
the yield of the biocrude increased significantly [38, 46]. However, extensive
gasification will reduce the biocrude yield. Some of the other heterogeneous catalysts reported for hydrothermal conversion of lignocellulosic and algal biomass
include Ni doped over SiO 2 , TiO 2 , and zeolite [44, 48].
4 Biomass to Fuel
The useful products obtained from the hydrothermal processing are biochar,
biocrude, and biogas, which have their own significance. These products were
always energy intensified compared to their feedstock. This was confirmed by
elemental analysis of the feedstock and the resulting products. The biochar obtained
is converted into pellets and used for various applications, whereas the biocrude and
biogas are further purified to get fine chemicals. The main parameter that is to be
noted for any fuel is its heating value. The energy density of products can be
compared by calculating the HHV using the united formula by Channiwala and
Parikh [59] that can be used for both feedstock and the resulting products [59]:
216
C. D. Venkatachalam et al.
340
C with a 1:1 (v/v) ethanol/water with addition of 10 wt% of NaOH resulted in
oil yield of 50.2–57.2%; rather with no catalyst it was 49.0%. The oil yield with
KOH addition (51.4–57.5%) was slightly higher than that with NaOH (50.5–56.4%)
[42]. Similarly, alkali especially potassium in the form of K 2 CO 3 exhibited a
progressive effect on yield of biocrude. The hydrothermal treatment of cotton
cocoon shell showed a catalytic activity which is as follows:
KOH > NaOH > K 2 CO 3 > Na 2 CO 3 [43]. The alkali catalysts weaken the C-C
bond, thereby decreasing the activation energy. It also triggers biomass swelling and
increase in the surface area exposed, enhancing the retro-aldol cleavage. These alkali
catalysts have the tendency to promote water-gas shift reaction during the hydrothermal conversion, thus favoring H 2 and CO 2 formation from CO. The resulting
hydrogen gas in turn acts as a reducing agent, increasing the HHV and quality of the
biocrude.
Heterogeneous Catalysts Heterogeneous catalysts have been mostly used in
hydrothermal gasification process and low-temperature applications, where they
enhance the quality of the biocrude obtained. de Caprariis et al. [37] investigated
that HTL of oak wood with addition of Fe as a catalyst increases the quality and
quantity of biocrude. Fe acts as a sacrificial catalyst, which in turn undergoes surface
oxidation into Fe 2 O 3 , thereby reducing the acids and aldehydes formed during the
process. Gasification is crucial to a certain extent for biocrudes containing higher
percentages of oxygenated compounds. Few other studies show that by doping
oxides like Al 2 O 3 with active substance like KF, Pd, and Pt and using it as a catalyst,
the yield of the biocrude increased significantly [38, 46]. However, extensive
gasification will reduce the biocrude yield. Some of the other heterogeneous catalysts reported for hydrothermal conversion of lignocellulosic and algal biomass
include Ni doped over SiO 2 , TiO 2 , and zeolite [44, 48].
4 Biomass to Fuel
The useful products obtained from the hydrothermal processing are biochar,
biocrude, and biogas, which have their own significance. These products were
always energy intensified compared to their feedstock. This was confirmed by
elemental analysis of the feedstock and the resulting products. The biochar obtained
is converted into pellets and used for various applications, whereas the biocrude and
biogas are further purified to get fine chemicals. The main parameter that is to be
noted for any fuel is its heating value. The energy density of products can be
compared by calculating the HHV using the united formula by Channiwala and
Parikh [59] that can be used for both feedstock and the resulting products [59]:
216
C. D. Venkatachalam et al.