Environmental Engineering Reference
In-Depth Information
2000
ξ
=0.0 ,
ζ
=0.1
h
m
1800
1600
T
f
max
( at
α
= 1.0 )
1400
T
( at
α
= 1.0 )
1200
out
1000
800
T
f
max
( at
α
= 0.5 )
T
( at
α
= 0.5)
out
600
400
0
1
2
3
4
5
6
Gas recirculation ratio, R ( m / m )
rec
in
FIGURE 2.26
Combustion gas temperature in the model furnace affected by gas recirculation
and its heat loss.
Figure 2.26
shows the change of these temperatures plotted as a function of the
recirculation ratio
R
in the range of 0.0 to 6.0, and when the heat loss coefficient,
α, is changed in two conditions, 0.5 and 1.0. Thermal efficiency was also calculated
as shown in
Figure 2.27
with the same condition as the case of
Figure 2.26
.
A large
amount of recirculation gas does not affect the thermal efficiency so strongly if the
heat loss from the recirculating flue gas is relatively small, i.e., less than around
10% loss corresponding to α larger than 0.9.
Heat recirculation by means of preheating the combustion air or fuel results
directly in increasing the maximum flame temperature.
Figure 2.28
describes the
effect of the temperature increase and existing restrictions due to the temperature
limit of furnace materials and combustion stability. Available conditions in actual
furnace operation may hence be restricted between the “over temperature zone” and
“blow-off zone.” If we take a specific temperature to distinguish those two zones,
for example, 2150˚C for the maximum and 1350˚C for the minimum, heat recircu-
lation brings about an expansion of the available operating range when shown as a
function of gas recirculation ratio
R
.
2.2.4.3.2 Heat and Gas Recirculation
Equations 2.14 and 2.15 give the maximum flame temperature when the temperature
at the heat exchanger exit,
T
in
1
, and gas recirculation ratio,
R
, are given. The maximum
flame temperature,
T
fmax
, was calculated and plotted as a function of
T
in
1
and the gas
recirculation ratio
R
with the condition of ξ
h
less than 0.9 indicated as a heat exchanger
limit. The contour lines of
T
fmax
from 1350 to 2150˚C, divided into eight zones, is
shown in
Figure 2.29
.
It clearly demonstrates that increasing the gas recirculation ratio
without any change of
T
fmax
is possible if
T
in
1
can be sufficiently controlled.
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