Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 25

T alf

3000

T

ad

2000

1000

300

0.0 0.5 1.0

H 2 mole fraction

FIGURE 1.13 Adiabatic flame temperature and adiabatic limit flame temperature of H 2 -air
mixture.

with an ideal regenerator is better. The combustion process of both cases is isenthal-
pic combustion. Figure 1.14c , in contrast, includes the isothermal process of high
temperature air combustion. We define the thermodynamic efficiency as the ratio
between the work and the enthalpy difference at inlet and exit, as shown by the
following equation:

η = W /( H I - H E )

Figure 1.15 shows the relationship between the thermodynamic efficiency and
the maximum temperature during the combustion process. The point η ad , indicated
by an open circle on any curve, corresponds to the efficiency of the ordinary
combustion without any heat loss, expressed by I - F - E in Figure 1.12 . The lower
broken branch of the curve shows the drop in efficiency of the ordinary combustion
with a heat loss, I - F c - E in Figure 1.12, and the upper real branch of the curve
corresponds to the efficiency increase due to preheating by heat recirculation, I - H -
F r - E in Figure 1.12. It is reasonable that we can get higher efficiency if the maximum
temperature is raised by preheating, I - C - F rc - E . However, it should be noted here that
the combination of higher preheating and a leaner mixture produces the higher
thermodynamic efficiency, when we keep the maximum temperature constant.
The relationship between the thermodynamic efficiency of isothermal com-
bustion and the maximum temperature during the combustion process, I - C - F c - E
is shown in Figure 1.16 . The same curve of isenthalpic combustion for φ = 0.1 in
Figure 1.15 is also drawn in the figure for the convenience of comparison. The