138
Water for Energy and Fuel Production
dictated by the thermodynamic equilibrium at a given temperature and pressure.
In general, in the subcritical region, more methane is produced compared to hydrogen. The partial pressure of water can also affect the gas composition. Higher partial pressure and lower biomass concentration can result in more steam reforming
producing more hydrogen. An appropriate catalyst (such as nickel) can also reform
methane to produce more hydrogen. The catalyst can also help to reduce the gasification temperature while maintaining useful kinetics. Lower temperature and pressure
help in lowering the capital costs for the equipment as well as lowering the possible
corrosion effect on the reactor walls, thus allowing the use of less costly alloys for
the reactor vessel.
The catalysts for biomass gasification under subcritical conditions are discussed
in an excellent review by Elliot [9]. His analysis is briefly described below.
Elliot et al. [102–105] examined the subcritical gasification of biomass feedstock
that included cellulose, lignin, hollocellulose (cellulose and hemicellulose), and a
Douglas fir wood flour using nickel catalyst and added sodium carbonate cocatalyst.
The results showed that at 350°C, the catalyst gave 42% of carbon fed compared to
15% of carbon fed in the absence of catalyst. Both hydrogen and methane concentrations were higher for the catalytic operations compared to those without catalyst. The
carbon monoxide concentration was close to zero in the presence of catalyst. With
regard to the activity of alkali additions, the activity follows the order: Cs > K > Na.
The study by Elliot et al. [102–105] also indicated that conventional support for
nickel, namely, alumina (other than alpha-alumina), silica, various ceramic supports, minerals such as kieselguhr and other silica-alumina, were unstable in a hot
liquid water environment due to mechanisms such as dissolution, phase transition,
and hydrolysis. They reported useful supports such as carbon, monoclinic zirconia
or titania, and alpha-alumina.
Elliot evaluated the base metal catalysis, noble metal catalysis, and activated carbon catalysis for HTG. His important conclusions are summarized as follows:
1. Of all the base metal catalysts examined [102–105], such as nickel, magnesium, tungsten, molybdenum, zinc, chromium, cobalt, rhenium, tin, and
lead, nickel was found to be the most active and stable catalyst. Various supports such as kieselguhr, silica-alumina, alpha-alumina, alumina-magnesia
in spinel form, and carbon examined in the literature [6–11,91,102–105]
gave a varying degree of success. The most useful promoters were ruthenium, copper, silver, and tin impregnated at 1 wt%.
2. For noble metal catalysis, while some conflicting results are reported by
various investigators [6–11,91,95–105], in general, platinum, palladium,
and silver showed minor activities to HTG at 350°C; iridium had some
activities but the best activities were shown by ruthenium and rhodium.
Rutile form of titania and carbon supports was found to be effective. Vogel
et al. [96] and Vogal and Hildebrand [97] found ruthenium doping on nickel
catalyst on carbon to be effective for HTG.
3. While activated carbon and charcoal were found to be the effective catalysts
by some investigators [6–11,91,95–105], these results were mostly obtained
under supercritical conditions.
Water for Energy and Fuel Production
dictated by the thermodynamic equilibrium at a given temperature and pressure.
In general, in the subcritical region, more methane is produced compared to hydrogen. The partial pressure of water can also affect the gas composition. Higher partial pressure and lower biomass concentration can result in more steam reforming
producing more hydrogen. An appropriate catalyst (such as nickel) can also reform
methane to produce more hydrogen. The catalyst can also help to reduce the gasification temperature while maintaining useful kinetics. Lower temperature and pressure
help in lowering the capital costs for the equipment as well as lowering the possible
corrosion effect on the reactor walls, thus allowing the use of less costly alloys for
the reactor vessel.
The catalysts for biomass gasification under subcritical conditions are discussed
in an excellent review by Elliot [9]. His analysis is briefly described below.
Elliot et al. [102–105] examined the subcritical gasification of biomass feedstock
that included cellulose, lignin, hollocellulose (cellulose and hemicellulose), and a
Douglas fir wood flour using nickel catalyst and added sodium carbonate cocatalyst.
The results showed that at 350°C, the catalyst gave 42% of carbon fed compared to
15% of carbon fed in the absence of catalyst. Both hydrogen and methane concentrations were higher for the catalytic operations compared to those without catalyst. The
carbon monoxide concentration was close to zero in the presence of catalyst. With
regard to the activity of alkali additions, the activity follows the order: Cs > K > Na.
The study by Elliot et al. [102–105] also indicated that conventional support for
nickel, namely, alumina (other than alpha-alumina), silica, various ceramic supports, minerals such as kieselguhr and other silica-alumina, were unstable in a hot
liquid water environment due to mechanisms such as dissolution, phase transition,
and hydrolysis. They reported useful supports such as carbon, monoclinic zirconia
or titania, and alpha-alumina.
Elliot evaluated the base metal catalysis, noble metal catalysis, and activated carbon catalysis for HTG. His important conclusions are summarized as follows:
1. Of all the base metal catalysts examined [102–105], such as nickel, magnesium, tungsten, molybdenum, zinc, chromium, cobalt, rhenium, tin, and
lead, nickel was found to be the most active and stable catalyst. Various supports such as kieselguhr, silica-alumina, alpha-alumina, alumina-magnesia
in spinel form, and carbon examined in the literature [6–11,91,102–105]
gave a varying degree of success. The most useful promoters were ruthenium, copper, silver, and tin impregnated at 1 wt%.
2. For noble metal catalysis, while some conflicting results are reported by
various investigators [6–11,91,95–105], in general, platinum, palladium,
and silver showed minor activities to HTG at 350°C; iridium had some
activities but the best activities were shown by ruthenium and rhodium.
Rutile form of titania and carbon supports was found to be effective. Vogel
et al. [96] and Vogal and Hildebrand [97] found ruthenium doping on nickel
catalyst on carbon to be effective for HTG.
3. While activated carbon and charcoal were found to be the effective catalysts
by some investigators [6–11,91,95–105], these results were mostly obtained
under supercritical conditions.