Practical Combustion Methods Used in Industries - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 237

are to lower the peak temperature of the flame and to reduce the oxygen concentra-
tion. However, since an excessively low oxygen concentration in a combustion field
results in unstable combustion, the most typical method of low NO x combustion
employs a reduction of oxygen concentration while increasing the air preheating
temperature to secure stable combustion. This method meets the needs of both energy
saving and low NO x combustion.
Figure 4.17 shows the basic concept of low NO x combustion. Superlow NO x
combustion can be achieved with a flame that forms a flat temperature distribution
by controlling the peak temperature of the flame instead of using a conventional
flame that forms a temperature distribution with a local high temperature spot (hot
spot). High temperature air combustion agrees well with this concept, with reinforced
in-furnace recirculation, which lowers the oxygen concentration in the combustion
reaction field, and stabilized combustion, which results from the use of extremely
hot combustion air.
Figure 4.18 shows the results of a combustion analysis using CFD in accordance
with the basic concept described above. The abscissa axis indicates the distance
from the burner, which is nondimensionalized by the width of the furnace (4 m).
The ordinate axis indicates the temperature. In a conventional combustion method,
a conspicuous high temperature peak is present in the temperature distribution and
the oxygen concentrations at this peak point at 1600 and 1200 K are both 7%. The
temperature distribution of high temperature air combustion in which recirculation
of in-furnace gas is reinforced and the mixture of fuel and combustion air is con-
trolled to be entirely flat although there is a peak. The oxygen concentration at the
peak point is 1%, far below the level in a conventional combustion method. The
peak temperature of the high temperature air combustion at 1600 K is lower than

2000

1800

1600

1400

1200

burner end

Distance from burner

FIGURE 4.17 Basic concept of low NO x combustion.