Environmental Engineering Reference
In-Depth Information
models for HiTAC, we are still at the primitive stage in examining their applicability
in all applications with full confidence. Further research is required before we can
recommend numerical models for HiTAC. However, some trials have been conducted
that allow us to simulate the important characteristics of HiTAC. This is accom-
plished by combining the existing numerical models and methods. The following
examples provide the limits of practical application to numerically simulate the
technology utilizing the experience gained so far.
3.4.1
N
ITRIC
O
XIDE
E
MISSION
The NO concentration usually observed in furnaces is relatively low and the
kinetics of its formation reaction is much slower than the main hydrocarbon
oxidation rate. Therefore, it is possible to assume that the reactions involved in
the NO chemistry can be decoupled from the main combustion reaction mecha-
nism. Because of the slow formation of NO, equilibrium calculations are not
suitable to calculate realistic NO concentrations. Kinetic calculations are essential
to obtain reasonable results. Therefore, the mean NO concentration is obtained by
solving its transport equation based on the flow field and combustion solution
from the main combustion simulations.
˜
˜
Y
Y
x
˜
˜
NO
NO
ρ
u
=
ρ
D
+
S
(3.26)
i
NO
x
i
x
i
i
where the source term,
S
NO
, is determined from different NO mechanisms. Three
chemical-kinetic mechanisms for the NO formation/depletion, i.e., thermal NO,
prompt NO, and NO reburning, are included in this study.
3.4.1.1
Thermal NO
The formation of thermal NO is determined by the extended Zel'dovich mechanism.
+← →
k
1
ON
N+NO
(R3)
+← →
k
2
NO
O+NO
(R4)
+← →
k
3
NOH
N+NO
(R5)
Based on the quasi-steady-state assumption for N atom concentration, the net
rate of NO formation via the above reactions can be determined by
[
]
[ ]
[][]
2
[
]
d
NO
kk
k
NO
NO
kNO
O
[]
[]
−
T
=
2
k
ON
1
12
1+
-1
(3.27)
[
+
[
]
1
2
dt
k
k
k
H
1222
22
3
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