Environmental Engineering Reference
In-Depth Information
Enthalpy of gas at adiabatic
flame temperature
Heating potential
Enthalpy of exhaust gas
Preheated air temperature
FIGURE 4.13
Heating potential of combustion gas.
the enthalpy of the in-furnace gas can actually contribute to the heating during
operation. The heating potential of this combustion gas is equal to the enthalpy of
the gas between the adiabatic flame temperature and the in-furnace gas temperature
disregarding the heat loss in heating.
Figures 4.13
and
4.14
shows the conceptual
definition of the heating potential. In the case of an ideal adiabatic system without
heat extraction, the in-furnace gas temperature is the adiabatic flame temperature
T
ad
and the adiabatic enthaply of flame gas is the enthalpy
H
ad
equivalent to the
temperature. In the case of heating with heat extraction, the in-furnace temperature
is the temperature
T
g
determined by the heat extraction amount and the exhaust gas
enthalpy is the enthalpy
H
g
equivalent to the temperature.
The difference between
H
ad
and
H
g
is generated by heat extraction, and is
proportional to the volume of heat extraction. Disregarding the fixed heat-loss
amount of the heating equipment, the volume of heat extraction is basically the
amount of heat transferred to the material to be heated. In other words, the difference
between
H
ad
and
H
g
w
as used for the heating.
It is assumed that heating is executed without preheating the air and the process
is referential. We can see by what margin the fuel supply requirement for the process
is reduced by introducing an air-preheating step. Assume that the enthalpy of the
combustion gas per unit volume of fuel supply, which is equivalent to the in-furnace
gas temperature, is
H
g
. Heating is executed without preheating the air to satisfy the
condition
H
0
>
H
g
. With the acceleration maintained at a fixed level, the following
equation can be established:
(
H
0
-
H
g
)
V
0
= (
H
0
+
H
a
-
H
g
)
V
a
(4.2)
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