Practical Combustion Methods Used in Industries - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 235

1.0

Effect of preheated air
sensible heat

0.5

Effect of improved heat
transfer by high-temperature
air combustion

T g = 1773 K

CH

4

Air ratio 1.0

0.0

300 800 1300 1800

Preheated air temperature, K

FIGURE 4.15 Effect of improved heat transfer by preheated air combustion.

heat to the total energy input — in other words, the aforementioned fuel saving
effect and apparent calorific value improvement. In this case, the fuel-saving effect
is more than double the apparent calorific value improvement effect. This implies
that estimations on the basis of recovered heat bear little relation to the real effect
indicator. With the preheated air sensible heat at 1100 K or higher, the volume of
fuel supply will be reduced by half.
Heating Control Applied on a Zone-by-Zone Basis — The same fuel saving
effect as mentioned above is applied here, as well. In addition, zone-by-zone heating
control in high temperature air combustion can accelerate heat transfer as described
below by utilizing the advantageous condition whereby formation of an extensive
high temperature field is realized if the adiabatic flame temperature is increased.
The dominant heat transfer inside a furnace takes the form of radiation heat
transfer and the heat transfer rate can be defined as a function of the fourth power
of gas temperature. The heat transfer rate increases to a considerable extent at high
temperatures. Accordingly, high temperature air combustion has the potential to
obtain a high heat transfer rate in excess of that resulting from an increase in the
flame temperature by increasing the preheated air temperature. Heating potential is
now combined with the time-based capacity, or the heat transfer capacity per unit
time. It should be noted that the increased amount in the enthalpy corresponding to
the increased temperature of the flame is evaluated as the fuel saving effect described
above. An effective method of utilizing the potential of the high heat transfer rate
is, for example, to divide the inner space of a furnace into zones to control heating.
The total heat transfer rate can be further increased by heating a ( T g + T g ) zone

T g ) zone at different temperatures. The rate achieved will be greater
than that for a single furnace space which is heated at T g uniformly. For example,
a furnace at an in-furnace temperature of 1473 K is divided into a zone of

and a ( T g -