Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 24

T alf

100

L

φ =0.1

H w

50

A

F rc

C

H

F r

0

T

I F ad

F c

E

-50

300 1000 2000 3000

Temperature, K

FIGURE 1.12 Enthalpy-temperature diagram for H 2 -O 2 mixture (φ = 0.1).

T alf , higher temperature above this point is unrealistic. Accordingly, isenthalpic

combustion, no matter how it is preheated or not, is generally associated with a
temperature rise until it reaches the adiabatic limit temperature, T alf .

In contrast, if the mixture preheated to the point C is introduced into a large
quantity of burned product such as a well-stirred reactor and the heat subtraction in
proportion to the heat release rate is assumed, we can consider an isothermal
combustion burning at the preheated temperature. In this case, the condition at the
reactor exit is indicated by the point F c on the Hp -curve, and no temperature rise
occurs during the combustion process. Further, the same quantity of heat subtracted
from the reactor can be utilized effectively for heating. Therefore, we can consider
that the idealized extreme of high temperature air combustion is an isothermal
combustion.
Figure 1.13 shows the relationship between the adiabatic flame temperature
(broken line) and the adiabatic limit temperature (solid line) in terms of fuel con-
centration in a hydrogen-air mixture. The change in adiabatic limit temperature
seems much less compared with that in the adiabatic flame temperature, which
strongly depends on the equivalence ratio of the mixture. Therefore, we understand
that a considerably high temperature can be realized by preheating even in lean
combustion.
Now, we introduce the thermodynamically ideal engine into the analysis of
combustion process in furnaces to evaluate the effective energy in various types of
combustion explained above, where the effective heat is taken as the heat transform-
able to the work. An ordinary furnace operated without heat recirculation, modeled
in Figure 1.14a a nd Figure 1.14b , shows the heat recirculation furnace equipped