Environmental Engineering Reference
In-Depth Information
Without heat extraction: Adiabatic system
Air
Enthalpy of gas at adiabatic
flame temperature:
H
ad
Adiabatic flame temperature:
T
ad
Fuel
Heating potential =
H
ad -
H
g
With heat extraction: Heating
Air
In-furnace temperature:
T
g
Enthalpy of exhaust gas:
H
g
Fuel
FIGURE 4.14
Conceptual definition of heating potential.
where
V
0
and
V
a
are fuel supply amount when no air preheating is involved, and (
H
0
-
H
g
) and (
H
0
+
H
a
-
H
g
) are both heating potentials. This represents the actual
phenomenon in progress. The energy saving effect of combustion with air preheating
is now obtained from
v
=
v
0
-
v
a
. Under these conditions,
the percentage volume of fuel supply obtained by normalizing the fuel supply
requirements at respective preheated air temperatures by the fuel supply amount for
the heating without air preheating is found to be the reciprocal of the heating potential
rate. Further, with
H
0
>
H
g
> 0, the following equation can be established:
V
=
V
0
-
V
a
in place of
HH
HHH
−
+−
V
V
H
HH
0
g
a
=
<
0
=
a
(4.3)
+
0
0
a
g
0
a
0
Thus, it may be proved that the effect is greater than a mere reduction of fuel
supply volume by the preheated air sensible temperature. Although the heating
potential increases only by the preheated air sensible temperature, the heating poten-
tial rate increases greatly. The larger the enthalpy of the in-furnace gas (or the higher
the in-furnace gas temperature), the greater the effect becomes. The left side of the
inequality represents the effect of fuel saving from combustion with air preheating,
and the right side represents the apparent effect of an increased calorific value with
the preheated air sensible heat considered to be an increase in the latent heat of
the fuel.
Figure 4.15
shows the relationship between the preheated air temperature and
the fuel supply requirement when CH
4
is burned at an air ratio of 1.0 and at an in-
furnace gas temperature of 1773 K. The solid line in the figure shows the fuel supply
rate and the dotted line indicates the ratio of the latent heat of fuel to the total energy
input, which includes the preheated air sensible heat. These values are calculated,
respectively, by
V
a
/
V
0
and
v
a
/
v
0
in Equation 4.3 above. The difference between the
ordinate axis 1.0 and the solid lines vs. the ordinate axis 1.0 and the dotted line,
respectively, indicate the rate of fuel saving and the ratio of preheated air sensible
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