275
Fuel Production by Supercritical Water
the reactivity of biomass gasification in SCW. The last two are important for the
reforming under supercritical conditions. These and other studies found that the
yields of H 2 O and CO increased with increasing water density. Yields of H 2 were
4 times better with NaOH and 1.5 times better with ZrO 2 compared to the reaction
without a catalyst. Supercritical fluids gave increased pore accessibility, enhanced
catalyst ability to coking, and increased desired product selectivity. While hightemperature SCW gasification produces hydrogen and carbon dioxide, Sinag et al.
[152] showed that a combination of two technologies—SCW and hydropyrolysis on
glucose in the presence of K 2 CO 3 —produces phenols, furfurals, organic acids, aldehydes, and gases. xu and Antal [20,21], Antal and xu (2012, pers. comm.), and Antal
et al. [144] studied gasification of 7.69 wt% digested sewage sludge in SCW and
obtained gas that largely contained H 2 , CO 2 , a smaller amount of CH 4 , and a trace of
CO. Other waste materials show a similar behavior.
Kong et al. [151] briefly summarized the reported work for the catalytic hydrothermal gasification of various types of biomass in SCW. They showed that in the
literature, catalytic hydrothermal gasification in SCW has been examined for glucose,
organic wastewater, cellulose, soft and hard wood, grass, lignin, sawdust, rice straw,
alkylphenols, corn, potato starch gels, potato waste, glycerol, cellobiose, bagasse,
sewage sludge, catchetol, vaniline, glycine, and many others. In all cases, the major
products were hydrogen and methane depending on the operating conditions. The
catalysts examined included Ni, Ru, Rh, Pd, Pt on alumina, NaOH, KOH, Na 2 CO 3 ,
K 2 CO 3 , ZrO 2 , activated carbon, and Ni on carbon. The preference was given to the
disposable or cheap catalysts or to the reforming catalysts if the objective was to carry
out reforming along with gasification. The results show that except at low temperatures, the main product in all cases was hydrogen. Catalytic operations decrease the
productions of char and tar and increase the production of hydrogen. Carbon and base
catalysts play important roles in the increased gas yields and hydrogen production.
Tanksale et al. [7] provided an extensive review of various catalytic and other processes to produce hydrogen from biomass. Supercritical gasification in water was one
of these processes. Azadi and Farnood [5] reviewed heterogeneous catalysts for subcritical water and SCW gasification of biomass and wastes. The review provided an
extensive information of carbon conversion and hydrogen and methane productions
in sub- and supercritical conditions for a variety of biomass by various commercially
available and laboratory-made catalysts that included supported and skeletal metal
catalysts, activated carbon, metal wires, and other innovative catalysts.
The generation of hydrogen from waste has long-term and strategic implications
since hydrogen is the purest form of energy and is very useful for product upgrading, fuel cell, and many other applications. Hydrogen can be produced from waste
via numerous high-temperature technologies such as conventional or fast pyrolysis
(e.g., olive husk, tea waste, crop straw, etc.), high-temperature or steam gasification
(e.g., bio-nutshell, black liquor, wood waste, etc.), supercritical fluid extraction (e.g.,
swine manure, orange peel waste, crop grain residue, petroleum-based plastic waste,
etc.), SCW gasification (e.g., all types of organic waste, agricultural and forestry
waste, etc.) as well as low-temperature technologies such as anaerobic digestion and
fermentation (e.g., manure slurry, agricultural residue, MSW, tofu wastewater, starch
of food waste, etc.). For high-temperature technologies, SCW gasification generates
Fuel Production by Supercritical Water
the reactivity of biomass gasification in SCW. The last two are important for the
reforming under supercritical conditions. These and other studies found that the
yields of H 2 O and CO increased with increasing water density. Yields of H 2 were
4 times better with NaOH and 1.5 times better with ZrO 2 compared to the reaction
without a catalyst. Supercritical fluids gave increased pore accessibility, enhanced
catalyst ability to coking, and increased desired product selectivity. While hightemperature SCW gasification produces hydrogen and carbon dioxide, Sinag et al.
[152] showed that a combination of two technologies—SCW and hydropyrolysis on
glucose in the presence of K 2 CO 3 —produces phenols, furfurals, organic acids, aldehydes, and gases. xu and Antal [20,21], Antal and xu (2012, pers. comm.), and Antal
et al. [144] studied gasification of 7.69 wt% digested sewage sludge in SCW and
obtained gas that largely contained H 2 , CO 2 , a smaller amount of CH 4 , and a trace of
CO. Other waste materials show a similar behavior.
Kong et al. [151] briefly summarized the reported work for the catalytic hydrothermal gasification of various types of biomass in SCW. They showed that in the
literature, catalytic hydrothermal gasification in SCW has been examined for glucose,
organic wastewater, cellulose, soft and hard wood, grass, lignin, sawdust, rice straw,
alkylphenols, corn, potato starch gels, potato waste, glycerol, cellobiose, bagasse,
sewage sludge, catchetol, vaniline, glycine, and many others. In all cases, the major
products were hydrogen and methane depending on the operating conditions. The
catalysts examined included Ni, Ru, Rh, Pd, Pt on alumina, NaOH, KOH, Na 2 CO 3 ,
K 2 CO 3 , ZrO 2 , activated carbon, and Ni on carbon. The preference was given to the
disposable or cheap catalysts or to the reforming catalysts if the objective was to carry
out reforming along with gasification. The results show that except at low temperatures, the main product in all cases was hydrogen. Catalytic operations decrease the
productions of char and tar and increase the production of hydrogen. Carbon and base
catalysts play important roles in the increased gas yields and hydrogen production.
Tanksale et al. [7] provided an extensive review of various catalytic and other processes to produce hydrogen from biomass. Supercritical gasification in water was one
of these processes. Azadi and Farnood [5] reviewed heterogeneous catalysts for subcritical water and SCW gasification of biomass and wastes. The review provided an
extensive information of carbon conversion and hydrogen and methane productions
in sub- and supercritical conditions for a variety of biomass by various commercially
available and laboratory-made catalysts that included supported and skeletal metal
catalysts, activated carbon, metal wires, and other innovative catalysts.
The generation of hydrogen from waste has long-term and strategic implications
since hydrogen is the purest form of energy and is very useful for product upgrading, fuel cell, and many other applications. Hydrogen can be produced from waste
via numerous high-temperature technologies such as conventional or fast pyrolysis
(e.g., olive husk, tea waste, crop straw, etc.), high-temperature or steam gasification
(e.g., bio-nutshell, black liquor, wood waste, etc.), supercritical fluid extraction (e.g.,
swine manure, orange peel waste, crop grain residue, petroleum-based plastic waste,
etc.), SCW gasification (e.g., all types of organic waste, agricultural and forestry
waste, etc.) as well as low-temperature technologies such as anaerobic digestion and
fermentation (e.g., manure slurry, agricultural residue, MSW, tofu wastewater, starch
of food waste, etc.). For high-temperature technologies, SCW gasification generates