Environmental Engineering Reference
In-Depth Information
4mm
4mm
Fuel
Fuel
Air
Air
Temperature,
O
2
,
CH
4
Temperature,
O
2
,
CH
4
O
2
21%
CH
4
Temperature
100%
O
2
3%
Temperature
CH
4
20%
-2
-1
0
1
2
-2
-1
0
1
2
position
position
mm
mm
(B) Ordinary flame front
(A) HiTAC flame front
diluted with N
2
FIGURE 1.8
Thickness of reaction zone and temperature rise.
1.2.3
H
EAT
T
RANSFER
IN
H
IGH
T
EMPERATURE
A
IR
C
OMBUSTION
Heat transfer in furnaces depends not only on the internal temperature distribution
but also on the physical properties of combustion gases and of furnace walls.
Although the heat transfer rate is augmented with the increase of furnace tempera-
ture, there is a maximum operating temperature due to the maximum temperature
limit of furnace materials used, such as fire bricks. Also, temperature fluctuations
in turbulent combustion, because a peak temperature at an instant, sometimes dete-
riorates the surface of the material being heated or the insulation on the wall.
Therefore, we must achieve effective heat transfer under these limitations.
An earlier section explained that temperature fluctuations in HiTAC are much
smaller than in ordinary combustion. This fact allows us to raise the operating
temperature because a small instantaneous peak temperature does not exceed the
limit. So, it is possible to increase the combustion load for the same size furnace.
Or, we can reduce the furnace size by adopting HiTAC for the same combustion rate.
If combustion air can be preheated to a high temperature at the entry of a furnace
using the recovered sensible heat, it can save some quantity of energy in heating the
materials to the specified temperature. The heat capacity of the material being heated
does not change because of the operating systems or because of the heat input to
the furnace. However, the efficiency of heating, the heating time, and the uniformity
of temperature rise of the material being heated do change depending on the local
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