Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 11

Recirculation Ratio

0 0.5 1 1.5 2 345

OO

Re = 1.2 10 3
Re = 4.2 10 3

1800

air

fuel

1600

Stable Region
(combustible by auto- and forced-ignition)

1400

auto-ignition limit

1200

1000

forced-ignition limit

800 Noncombustible Region

Blow-off (forced-ignition)

600

Unstable Point (CO < 100 ppm)

Auto-ignition Temperature
(diluted by N 2 or CO 2 )

400

20 15 10 5 0

O 2 concentration, %

FIGURE 1.4 auto-ignition limits and blow-off limits of propane in a preheated air or a diluted
air with nitrogen.

temperature of the fuel. Actually, very diluted air, whose oxygen concentration is as
low as 3%, can sustain combustion when it is preheated up to 1200 K.
Hasegawa et al. 14 discussed the individual and multiple influences of heat and
gas recycling. In Chapter 2 , Figure 2.29 shows contours of the maximum flame
temperature on the combined effect of preheated air temperature and recycling rate
of burned gases, where R is the gas recycling rate. A combination of highly preheated
air and high recycling rate of burned product generates relatively low maximum
flame temperature. One can understand that the stoichiometric flame temperature in
very diluted air, where mass fraction of oxygen is far below the value in normal
atmospheric air, is not as high as is usually expected. This is the key for HiTAC
when it is applied to practical combustion systems. Keeping the global equivalence
ratio constant, the flame temperature in the furnace can be varied or regulated by
combining the preheated air temperature and the recycling rate of burned gases.
The concept of the HiTAC is illustrated in Figure 1.5 , compared with that of a
conventional furnace combustion. Extremely high temperature flames are usually