Environmental Engineering Reference
In-Depth Information
1.5
Average gas temperature 1473K
Temperature of material to be
heated 1273K
1.4
1.3
1.2
1.1
Average gas temperature 1473K
Temperature of material to be
heated 300K
1.0
0
100
200
300
400
500
Inter-zone temperature, gap/2 K
FIGURE 4.16 Example of heat transfer acceleration effect by zone-to-zone heating control
method.
(1473 +
T g
and the velocity of heat transfer to the material to be heated under the above
conditions, disregarding heat loss, is shown in Figure 4.16 .
The ordinate axis indicates the heat transfer rate in the unit of transfer velocity
ratio obtained by normalizing the heat transfer rate under each condition, by the
heat transfer rate when the in-furnace gas temperature is kept at a uniform level at
1473 K. The figure shows that the heat transfer rate of the material to be heated
with a temperature of 1273 K in the range of
T g ) K and another zone of (1473 -
T g ) K. The relationship between
T g ~ 200 K (1673 and 1273 K zones)
is about 25% higher than in the furnace at 1473 K. This method results in a higher
heat transfer and a higher heating velocity (production rate) for the same heat input
per unit time. Further, it is possible to lower the average in-furnace temperature with
the heat transfer rate maintained at a fixed level, which means a proportional reduc-
tion of heat input to the lowered temperature.
As described above, formation of a stable high temperature field is a prerequisite
to dividing a furnace space into a high temperature zone and a low temperature
zone, which are easily realized in high temperature air combustion.
4.3 POLLUTION REDUCTION
4.3.1 B ASIC C ONCEPT OF L OW NO X C OMBUSTION
NO x generation is basically a function of temperature, oxygen concentration, and
residence time. In a typical heating furnace, residence time is usually long enough
to enable the generation of NO x and the basic measures for low NO x combustion
 
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