Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 57

2000

T f max

1500

T out

1000

T in 1

T f max = 1600 C

500

ζ = 0.1

m

α = 0.95

0

0 1 2 3 4 5 6

Gas recirculation ratio, R ( m / m )
rec in

FIGURE 2.31 Superimposed contour map of the maximum combustion gas temperature and
the temperature affected by gas recirculation and heat recirculation.

is remarkably small compared with conventional furnaces. Both T fmax and ∆ T distri-
butions were then superimposed in Figure 2.31 . Here the contour lines are shown of
∆ T for every 50˚C in the range from 100 to 300˚C. The results show that an increase
of both heat recirculation by air preheat and gas recirculation results in decrease of
flame temperature and also decrease of the temperature difference between the flame
and the flame gas.

2.2.4.3.3 Thermal Efficiency
The operation to increase heat and gas recirculation while maintaining the maximum
flame temperature can be performed as shown in Figure 2.32 , where T fmax

, T out and
T in 1 are indicated in Figure 2.31 . T in 1 has to be increased to compensate the T fmax
decay resulting from the effect of dilution due to the combustion gas recirculation.
The temperature difference, ∆ T , becomes noticeably smaller and T out becomes grad-
ually closer to T fmax . The thermal efficiency in this condition was calculated and the

result is shown in Figure 2.33 . I t revealed that this operation can increase the thermal
efficiency as well as improving the temperature difference, ∆ T .

2.2.4.4 Discussion

The method of additional enthalpy combustion has been generally interpreted to be
an effective way to enhance the thermal field in furnaces for increasing the heat
transfer rate. The excessive increase of the maximum flame temperature, however,
is a major problem causing overheating of materials in the furnaces. The result
obtained here, as shown in Figure 2.33, revealed that high temperature additional