Environmental Engineering Reference
In-Depth Information
model, even when temperatures are different. For this reason, the single use of this
model is not appropriate for simulating HiTAC, and it should be combined with the
chemical reaction rate model in which reaction characteristics of HiTAC are taken
into account.
of a furnace, the reaction rates at those points will be considered the same by this
3.1.2.4
Problems in Temperature Calculation
Among the numerous step reactions forming the complicated comb
ustion reaction
scheme, it is in chain terminating reactions where radicals and monatomic molecules
combine to form stable species. In the temperature range above 2000 K, the endo-
thermic reverse reactions of the terminal reactions become significant. These endo-
thermic reactions are called thermal dissociation. However, as the temperature falls,
the forward reactions still proceed. When the mixture ratio is close to stoichiometric,
the combustion temperature in real flames tends to decrease considerably more, due
to the thermal dissociation, than the theoretical combustion temperature based on
the assumption of complete combustion.
Since most combustion models such as one-step global reaction model do not
predict precise concentrations of intermediate species in the flame, as stated before,
they are incapable of estimating the correct combustion temperature caused by the
thermal dissociation in the high-temperature range. Since it is important in the
numerical simulations of practical furnaces to estimate the heat transfer rate to the
material to be heated as precisely as possible, the accuracy of flame temperature
greatly influences the radiation heat transfer rate, which is proportional to the flame
temperature to the fourth power. Therefore, it is necessary to work out some means
to raise the accuracy of temperature prediction, even when we use a combustion
model that does not deal with intermediate species.
3.2
COMBUSTION MODEL FOR HIGH TEMPERATURE
AIR COMBUSTION
3.2.1
C
H
T
EMPERATURE
A
IR
C
OMBUSTION
HARACTERISTICS
OF
IGH
When highly preheated air (1100 to 1600 K ) is used, combustion by autoignition
takes place immediately after the mixing of fuel and air; thus stabilizing flames by
a pilot flame or a flame holder is not required. Combustion occurs with any type of
conventional burners using highly preheated air, but in a HiTAC furnace combustion
takes place as a lifted flame relatively free in space. The incoming preheated air is
first diluted with burned gas recirculating inside the furnace, then it makes contact
with the fuel. Flames formed in such a flow field have totally different features from
those of conventional burner flames. The flame has low luminosity, and almost
transparent reaction zones resembling a spreading mist in the furnace rather than a
flame. In this situation NO
x
emission and flame temperature are extremely low
despite the high-temperature preheated air.
The above characteristics have not been seen with ordinary combustion using
ambient temperature air, because they are not reflected in the combustion models
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