Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 71

1. Flame extinction limit expressed by flame strain rate increases exponen-

tially as the air preheat temperature becomes higher. But there is a limit
to the preheated air temperature and, above the limit, flame extinction
does not take place. This critical air preheat temperature is about 1320 K
for propane.

2. Flame temperature becomes the adiabatic flame temperature when the air

is preheated. This leads to the capability of the lean combustion even
below the lower flammability limit. Thus, NO x reduction is made possible
using the high-temperature low-oxygen air as the combustion air.

3. Under the condition of constant air preheat temperature, both NO x con-

centration and flame temperature decrease with an increase of the flame
strain rate.

4. When the flame strain rate increases, the diffusion layer becomes thinner.

Flame temperature decreases with the increase of the strain rate.

5. NO x emissions become nearly constant in a flame temperature range from

1300 to 1625 K. This weak temperature dependency is inherent in the
prompt NO mechanism. Accordingly, in the low temperature range the
prompt NO becomes predominant.

6. The overall chemical reaction rate in the high temperature air combustion

can be expressed as the Arrhenius type. The activation energy for propane
in the high temperature air combustion is estimated as 380 kJ/kmol.

2.3.2 B URNING V ELOCITY

Flames obtained by feeding fuel into air preheated to about 1000˚C by recirculation
of hot combustion gases are known not only to be efficient but also to show low
NO emission. 15-17 Large fuel load, the amount of fuel burned per unit of volume and
time, combined with simultaneous low emission of NO, is a desirable property of
a flame in industrial devices. A high performance combustion process is required,
especially in cases where the composition of the combustion gases is restricted to
certain values by the properties of the material to be heated such that the permissible
range of equivalence ratios is narrow.
Here, flat flames of methane with preheated and diluted air are treated theoret-
ically to elucidate the mechanism by which preheated and diluted air leads to high
efficiency and low NO emission flames. Since the calculations are done on flat
flames, the fuel load is substituted with the fuel flux, the amount of fuel burned per
unit time per unit area of the flame.

2.3.2.1 Simulation Model

In a flat flame, the fuel is not injected into a flow of hot air, but all the mixture is
preheated and diluted. Dilution and preheating are obtained by assuming recircula-
tion of products of complete combustion into the unburned fuel mixture, and then
this is set to an initial high temperature at the burner.