Environmental Engineering Reference
In-Depth Information
CO
+
H O
CO
+
2
H
(R6)
2
2
The reaction rate is given for each of the four reactions as follows:
[
]
=−
d
CH
E
RT
[
] [
]
a
b
4
d
AT
CH
H O
2
exp
(3.8)
4
dt
[
]
=−
d
CH
E
RT
[
] [
]
a
b
4
d
AT
CH
O
2
exp
(3.9)
4
dt
[]
=−
d
H
E
RT
[][]
a
b
2
d
AT
HO
2
exp
(3.10)
2
dt
[
]
=−
d
CO
E
RT
]
[
]
b
[
a
d
AT
CO
H O
2
exp
(3.11)
dt
where [ ] indicates molar concentration mol/cm
3
. The reaction rate constants for
This reaction mechanism is divided into two parts: the first half is composed of
reactions to turn methane into intermediate compounds and the second half repre-
sents combustion of intermediates including comparatively slow reaction of CO
oxidization. Since the first-half reactions depend on concentrations of methane and
oxygen, they are not much different from the one-step global reaction. The difference
lies in that the intermediates, CO and H
2
, are also generated from reverse reactions
in the four-step reaction model. This model is quite different from the one-step
global reaction model in the later steps of combustion.
3.2.4.3
Four-Step Reaction Model (Srivatsa)
This model was originally used for simulating gas turbine combustors and is appli-
cable to combustion under an extreme condition where the characteristic time of
mixing is very short, nearly as short as that of chemical reactions, i.e., a combustion
with a low Damköhler number. To bring low NO
x
combustion into effect using high
temperature combustion air, it is necessary to mix preheated air with burned product
inside the furnace by a high-momentum jet. Since the Damköhler number of com-
bustion is considered to be comparatively low under these conditions, this model is
suitable for the present purpose in comparison with other models.
8
2CH
2CH
+
H
(R7)
4
3
2
2CH
+→
O
2CO
+
3H
(R8)
3
2
2
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