80
Water for Energy and Fuel Production
(190°C–250°C), copper–zinc oxide catalysts are used and called low-temperature
shift (LTS) catalysts. The third type of catalysts employs cobalt and molybdenum sulfides as active ingredients, and they are sulfur tolerant and used to treat
“sour gas”-containing sulfur. They are therefore called sour gas catalysts. Finally,
medium-temperature shift catalysts operate between 275°C and 350°C, and they
are copper–zinc catalysts modified with iron oxide. Besides these four types of
catalysts, Pt and Gold catalysts have been intensely investigated and promoters
such as Cu and Al 2 O 3 have been added to the conventional iron and chromium
oxide HTS catalysts.
As discussed earlier, the water–gas shift reaction is moderately exothermic and
equilibrium controlled. The equilibrium constant first sharply decreases with an
increase in the temperature above around 190°C and the levels of around 480°C.
The rate expression is as follows [18]:
4477.8
K p = exp
− 4.33
(4.71)
T
where:
T is expressed in kelvin
Thus, high forward conversion of water–gas shift reaction is favored at low temperature, and it is essentially unaffected by the total pressure. At high temperature, reverse water–gas shift reaction dominates. The reaction is reversible and the
forward reaction rate is strongly inhibited by the reaction products: CO 2 and H 2 .
Low CO level can be obtained by maintaining the reactor temperature at around
200°C. At low temperature, however, condensation of water and its contact with
the catalyst should be avoided. The equilibrium carbon monoxide concentration
is also affected by the steam-to-gas ratio. Higher steam-to-gas ratio lowers the
product CO concentration and increases the hydrogen and carbon dioxide production rates. Since the water–gas shift reaction is always present and equilibrium
controlled, in any steam reforming process a substantial amount of carbon dioxide is present in the reaction mixture. The presence of carbon dioxide also forces
the “dry reforming” reaction between hydrocarbons and carbon dioxide. Thus,
in any autothermal reactors, it is more than likely that steam reforming reaction,
partial oxidation, water–gas shift reaction, and dry reforming reaction all occur
simultaneously.
4.5.2.8 Bio-Oil
Catalytic steam reforming of bio-oil at 750°C–850°C over a nickel-based catalyst is
a two-step process that includes the shift reaction [24–26]:
Bio-oil + H 2 O → CO + H 2
(4.72)
CO + H 2 O → CO 2 + H 2
(4.73)
Water for Energy and Fuel Production
(190°C–250°C), copper–zinc oxide catalysts are used and called low-temperature
shift (LTS) catalysts. The third type of catalysts employs cobalt and molybdenum sulfides as active ingredients, and they are sulfur tolerant and used to treat
“sour gas”-containing sulfur. They are therefore called sour gas catalysts. Finally,
medium-temperature shift catalysts operate between 275°C and 350°C, and they
are copper–zinc catalysts modified with iron oxide. Besides these four types of
catalysts, Pt and Gold catalysts have been intensely investigated and promoters
such as Cu and Al 2 O 3 have been added to the conventional iron and chromium
oxide HTS catalysts.
As discussed earlier, the water–gas shift reaction is moderately exothermic and
equilibrium controlled. The equilibrium constant first sharply decreases with an
increase in the temperature above around 190°C and the levels of around 480°C.
The rate expression is as follows [18]:
4477.8
K p = exp
− 4.33
(4.71)
T
where:
T is expressed in kelvin
Thus, high forward conversion of water–gas shift reaction is favored at low temperature, and it is essentially unaffected by the total pressure. At high temperature, reverse water–gas shift reaction dominates. The reaction is reversible and the
forward reaction rate is strongly inhibited by the reaction products: CO 2 and H 2 .
Low CO level can be obtained by maintaining the reactor temperature at around
200°C. At low temperature, however, condensation of water and its contact with
the catalyst should be avoided. The equilibrium carbon monoxide concentration
is also affected by the steam-to-gas ratio. Higher steam-to-gas ratio lowers the
product CO concentration and increases the hydrogen and carbon dioxide production rates. Since the water–gas shift reaction is always present and equilibrium
controlled, in any steam reforming process a substantial amount of carbon dioxide is present in the reaction mixture. The presence of carbon dioxide also forces
the “dry reforming” reaction between hydrocarbons and carbon dioxide. Thus,
in any autothermal reactors, it is more than likely that steam reforming reaction,
partial oxidation, water–gas shift reaction, and dry reforming reaction all occur
simultaneously.
4.5.2.8 Bio-Oil
Catalytic steam reforming of bio-oil at 750°C–850°C over a nickel-based catalyst is
a two-step process that includes the shift reaction [24–26]:
Bio-oil + H 2 O → CO + H 2
(4.72)
CO + H 2 O → CO 2 + H 2
(4.73)