3.4 Effect of Catalyst
Water acts itself as a catalyst during subcritical or supercritical hydrothermal conditions, and yet the products resulted with mostly oxygenated compounds with low
high heating value (HHV). Thus to reduce the high O/H ratio in the resulting
biocrude, it is required to add more catalysts into the reacting system. The addition
of catalysts improves the gasification efficiency and reduces the solid residue (tar and
char) by suppressing the condensation and repolymerization into stable macromolecules. Alkali salts and hydroxides have been frequently used as homogeneous
catalysts, whereas Ni, Fe, Al, Pd, Pt, and Ti were used as heterogeneous catalysts,
which were utilized to enhance the yield of biocrude.
Homogeneous Catalysts Homogeneous catalysts improve gasification by accelerating the water-gas shift reaction, thereby increasing the liquid product yields. They
raise pH of the mixture to more alkaline, which in turn inhibits the dehydration of the
cellulose, hemicellulose, and lignin; thereby increasing the H/C ratio in the biocrude
and biochar. Alkali addition suppresses tar and biochar formation during the hydrothermal conversion process. It also prevents higher degree of oxygen removal in the
biocrude, and instead of decarboxylation, it might result in the formation of unstable
unsaturated compounds that can be easily polymerized into char and tar.
Küçük and Ağırtaş [41] investigated alkali-catalyzed hydrothermal liquefaction
of common reed in subcritical water environment, where catalytic solvent (ethanol) + 10% NaOH resulted in high conversion of 57.6% at 290
C [41]. In another
Fig. 6 Effect of hydrothermal media on biochar yield
Hydrothermal Conversion of Biomass into Fuel and Fine Chemicals
215
Water acts itself as a catalyst during subcritical or supercritical hydrothermal conditions, and yet the products resulted with mostly oxygenated compounds with low
high heating value (HHV). Thus to reduce the high O/H ratio in the resulting
biocrude, it is required to add more catalysts into the reacting system. The addition
of catalysts improves the gasification efficiency and reduces the solid residue (tar and
char) by suppressing the condensation and repolymerization into stable macromolecules. Alkali salts and hydroxides have been frequently used as homogeneous
catalysts, whereas Ni, Fe, Al, Pd, Pt, and Ti were used as heterogeneous catalysts,
which were utilized to enhance the yield of biocrude.
Homogeneous Catalysts Homogeneous catalysts improve gasification by accelerating the water-gas shift reaction, thereby increasing the liquid product yields. They
raise pH of the mixture to more alkaline, which in turn inhibits the dehydration of the
cellulose, hemicellulose, and lignin; thereby increasing the H/C ratio in the biocrude
and biochar. Alkali addition suppresses tar and biochar formation during the hydrothermal conversion process. It also prevents higher degree of oxygen removal in the
biocrude, and instead of decarboxylation, it might result in the formation of unstable
unsaturated compounds that can be easily polymerized into char and tar.
Küçük and Ağırtaş [41] investigated alkali-catalyzed hydrothermal liquefaction
of common reed in subcritical water environment, where catalytic solvent (ethanol) + 10% NaOH resulted in high conversion of 57.6% at 290
C [41]. In another
Fig. 6 Effect of hydrothermal media on biochar yield
Hydrothermal Conversion of Biomass into Fuel and Fine Chemicals
215