56
Water for Energy and Fuel Production
Equation 4.25 is the famous Boudouard reaction. Coke can be formed from CO, CO 2
as well as CH 4 . Coke can also be formed from ethylene through the polymerization
reaction as
C H
2 4 → Polymers → Coke
(4.29)
The coke deposition at a sustained level should be avoided because it leads to several
undesirable side reactions, loss in catalyst activity, and poor heat transfer between
the catalyst and the gas phase. If the coke deposition becomes very extensive, it can
block the open surface area causing an excessive pressure drop within the reactor
and it can also cause localized “hot spots” that can induce “runaway” conditions
for the reactor. Coke formation can be minimized by the use of an excess steam.
The catalyst can also be regenerated periodically, by burning off the deposited coke
through the oxidation reactions.
4.3 dry reForming
While steam reforming has been extensively used to produce hydrogen, it is generally not a desirable process to make syngas of a diverse composition that may be
needed for the downstream conversion of syngas to a variety of fuels and chemicals
by Fischer–Tropsch (FT) and other processes [57–62]. To generate syngas of different composition, steam reforming is often coupled with dry reforming and partial
oxidation. A combination of steam reforming, dry reforming, and partial oxidation
is called “tri-reforming.” Since both dry reforming and tri-reforming have strategic
fuel values, they are briefly described in this and the following section.
Depending on the reaction conditions, steam reforming and water–gas shift reaction can also be accompanied by “dry reforming” reaction in which carbon dioxide
produced from reverse water–gas shift reaction can react with hydrocarbons according to the following reactions:
(4.30)
and
(4.31)
Here, the second “dry reforming” reaction is illustrated for methane. This reaction was first studied by Fischer and Tropsch in 1928. It is briefly covered here
because of its close alignment with the steam reforming reaction. While the kinetic
mechanisms for dry reforming and steam reforming reactions on conventional
catalysts are very similar, generally steam reforming is faster and dry reforming requires higher temperature and is accompanied by more coke formation than
steam reforming. While dry reforming provides a mechanism for chemical use of
greenhouse gas “carbon dioxide,” it is not the solution for the complete removal of
carbon dioxide due to stoichiometry of various reactions occurring simultaneously.
CO
H
CO H O
kJ/mol
2
K
2
2
298
0
41 2
+
+
=
→
∆H
.
CH
CO
CO
H
kJ/mol
4
K
+
→
+
=
2
2
298
0
2
2
247 4
∆H
.
Water for Energy and Fuel Production
Equation 4.25 is the famous Boudouard reaction. Coke can be formed from CO, CO 2
as well as CH 4 . Coke can also be formed from ethylene through the polymerization
reaction as
C H
2 4 → Polymers → Coke
(4.29)
The coke deposition at a sustained level should be avoided because it leads to several
undesirable side reactions, loss in catalyst activity, and poor heat transfer between
the catalyst and the gas phase. If the coke deposition becomes very extensive, it can
block the open surface area causing an excessive pressure drop within the reactor
and it can also cause localized “hot spots” that can induce “runaway” conditions
for the reactor. Coke formation can be minimized by the use of an excess steam.
The catalyst can also be regenerated periodically, by burning off the deposited coke
through the oxidation reactions.
4.3 dry reForming
While steam reforming has been extensively used to produce hydrogen, it is generally not a desirable process to make syngas of a diverse composition that may be
needed for the downstream conversion of syngas to a variety of fuels and chemicals
by Fischer–Tropsch (FT) and other processes [57–62]. To generate syngas of different composition, steam reforming is often coupled with dry reforming and partial
oxidation. A combination of steam reforming, dry reforming, and partial oxidation
is called “tri-reforming.” Since both dry reforming and tri-reforming have strategic
fuel values, they are briefly described in this and the following section.
Depending on the reaction conditions, steam reforming and water–gas shift reaction can also be accompanied by “dry reforming” reaction in which carbon dioxide
produced from reverse water–gas shift reaction can react with hydrocarbons according to the following reactions:
(4.30)
and
(4.31)
Here, the second “dry reforming” reaction is illustrated for methane. This reaction was first studied by Fischer and Tropsch in 1928. It is briefly covered here
because of its close alignment with the steam reforming reaction. While the kinetic
mechanisms for dry reforming and steam reforming reactions on conventional
catalysts are very similar, generally steam reforming is faster and dry reforming requires higher temperature and is accompanied by more coke formation than
steam reforming. While dry reforming provides a mechanism for chemical use of
greenhouse gas “carbon dioxide,” it is not the solution for the complete removal of
carbon dioxide due to stoichiometry of various reactions occurring simultaneously.
CO
H
CO H O
kJ/mol
2
K
2
2
298
0
41 2
+
+
=
→
∆H
.
CH
CO
CO
H
kJ/mol
4
K
+
→
+
=
2
2
298
0
2
2
247 4
∆H
.