284
Water for Energy and Fuel Production
hydrocarbons, and CO 2 . The best reducing agent for CO 2 is hydrogen. As shown by
Rozovskii et al. [170], the synthesis of methanol from CO and H 2 proceeds not by
their direct interaction, but by the transformation of CO into CO 2 :
Water–gas shift reaction: CO + H 2 O → CO
H
0
2 + H 2 ∆ 298 k = − 41kJ/mol (10.28)
Methanol synthesis: CO 2 + 3H 2 → CH OH + H O ∆H
0
= − 49.3 kJ/mol (10.29)
3
2
298 k
Direct utilization of the last reaction meets opposition because it converts expensive
hydrogen into inexpensive water. Thus, a CO 2 reduction by hydrocarbons is preferred
with lower hydrocarbons and alcohols. While dry reforming of hydrocarbons converts carbon dioxide and hydrocarbons into useful syngas, the tri-reforming allows
the process to produce the syngas with a variety of H 2 /CO ratios.
The H 2 /CO ratio in syngas is very important for its further use for a variety of
chemical products. Syngas can be converted to acetone, acetic acid, and ethylene by
an exothermic reaction, while pure CO can be used for the production of acetic acid,
formic acid, polyurethane, polycarbonates, methyl acrylates, and so on. A H 2 /CO
ratio of about 1 is required for the productions of polycarbonates, oxo alcohol, formaldehyde, iron ore reduction reaction, and so on; a H 2 /CO ratio of about 2 is required
for methanol and Fischer–Tropsch (FT) synthesis, and a H 2 /CO ratio of 3 or higher
is required for ammonia synthesis and hydrogen production.
Tri-reforming also offers some other advantages. Since dry and steam reforming reactions are highly endothermic, a careful integration of these reactions into any process
scheme that internally generates heat (like partial oxidation) is very important in order
to make the overall process energy balance more efficient, thus avoiding the need for
expensive external heating. Both dry and steam reforming reactions require very high
temperatures (>600°C) to reduce the cooking. While steam reduces carbon deposition,
an addition of oxygen provides the necessary heat that can jump-start dry and steam
reforming reactions and maintain the catalyst in a clean and carbon-free state through
oxidation of coke on the catalyst surface. The extent to which oxygenates are added to
the reforming reactions is determined strictly by the process conditions and the catalyst
employed. While the combination of dry reforming and partial oxidation has been studied by a number of investigators [170–176], these studies have been largely restricted
to one or two hydrocarbons. Since dry reforming produces water, the steam reforming
always accompanies dry reforming, making these studies relevant for tri-reforming.
The most extensive study of tri-reforming was carried out by Puolakka et al. [172]
and Puolakka [173]. The study focused on the tri-reforming of five model compounds—
methane, heptanes, n-dodecane, toluene, and ethanol—over a number of different
catalysts. It was reported that 0.25% Rh on ZrO 2 catalyst gave the best results, and its
performance was comparable to the results for commercial Ni catalyst. The five model
compounds were chosen to represent different types of fossil/biofuels. Methane was
chosen to represent natural gas, n-heptane to represent aliphatic component of gasoline,
n-dodecane to represent aliphatic component of biodiesel, toluene to represent aromatic
part of gasoline, and diesel oil and ethanol to represent oxygenated compounds in biofuel.
The use of SCW as reaction medium for conducting the reforming can be an
attractive and novel method. The literature on gasification/reforming under SCW
Water for Energy and Fuel Production
hydrocarbons, and CO 2 . The best reducing agent for CO 2 is hydrogen. As shown by
Rozovskii et al. [170], the synthesis of methanol from CO and H 2 proceeds not by
their direct interaction, but by the transformation of CO into CO 2 :
Water–gas shift reaction: CO + H 2 O → CO
H
0
2 + H 2 ∆ 298 k = − 41kJ/mol (10.28)
Methanol synthesis: CO 2 + 3H 2 → CH OH + H O ∆H
0
= − 49.3 kJ/mol (10.29)
3
2
298 k
Direct utilization of the last reaction meets opposition because it converts expensive
hydrogen into inexpensive water. Thus, a CO 2 reduction by hydrocarbons is preferred
with lower hydrocarbons and alcohols. While dry reforming of hydrocarbons converts carbon dioxide and hydrocarbons into useful syngas, the tri-reforming allows
the process to produce the syngas with a variety of H 2 /CO ratios.
The H 2 /CO ratio in syngas is very important for its further use for a variety of
chemical products. Syngas can be converted to acetone, acetic acid, and ethylene by
an exothermic reaction, while pure CO can be used for the production of acetic acid,
formic acid, polyurethane, polycarbonates, methyl acrylates, and so on. A H 2 /CO
ratio of about 1 is required for the productions of polycarbonates, oxo alcohol, formaldehyde, iron ore reduction reaction, and so on; a H 2 /CO ratio of about 2 is required
for methanol and Fischer–Tropsch (FT) synthesis, and a H 2 /CO ratio of 3 or higher
is required for ammonia synthesis and hydrogen production.
Tri-reforming also offers some other advantages. Since dry and steam reforming reactions are highly endothermic, a careful integration of these reactions into any process
scheme that internally generates heat (like partial oxidation) is very important in order
to make the overall process energy balance more efficient, thus avoiding the need for
expensive external heating. Both dry and steam reforming reactions require very high
temperatures (>600°C) to reduce the cooking. While steam reduces carbon deposition,
an addition of oxygen provides the necessary heat that can jump-start dry and steam
reforming reactions and maintain the catalyst in a clean and carbon-free state through
oxidation of coke on the catalyst surface. The extent to which oxygenates are added to
the reforming reactions is determined strictly by the process conditions and the catalyst
employed. While the combination of dry reforming and partial oxidation has been studied by a number of investigators [170–176], these studies have been largely restricted
to one or two hydrocarbons. Since dry reforming produces water, the steam reforming
always accompanies dry reforming, making these studies relevant for tri-reforming.
The most extensive study of tri-reforming was carried out by Puolakka et al. [172]
and Puolakka [173]. The study focused on the tri-reforming of five model compounds—
methane, heptanes, n-dodecane, toluene, and ethanol—over a number of different
catalysts. It was reported that 0.25% Rh on ZrO 2 catalyst gave the best results, and its
performance was comparable to the results for commercial Ni catalyst. The five model
compounds were chosen to represent different types of fossil/biofuels. Methane was
chosen to represent natural gas, n-heptane to represent aliphatic component of gasoline,
n-dodecane to represent aliphatic component of biodiesel, toluene to represent aromatic
part of gasoline, and diesel oil and ethanol to represent oxygenated compounds in biofuel.
The use of SCW as reaction medium for conducting the reforming can be an
attractive and novel method. The literature on gasification/reforming under SCW