Environmental Engineering Reference
In-Depth Information
Fuel
Highly-
Preheated
Air
Exhaust
(a) HiTAC
Exhaust
Ambient Air
Fuel
Ambient Air
(b) Ordinary combustion
B
Burnt Gas
A
Ambient Air
H
Highly Preheated Air
BH
Mixture of
B
and
H
BA
Mixture of
B
and
A
Combustion
FIGURE 1.5
Mixing and combustion in furnace.
generated in furnaces, if direct combustion between fuel and high temperature fresh
air occurs. As a result of the modified furnace geometry, not only extinction of base
flames occurs by the shear motion of high velocity inlet air but also the dilution of
air with burned gas (BH) must occur prior to combustion by separating fuel and air
inlets. Note that those are the conditions in which ordinary combustion cannot be
sustained with ambient temperature air. In addition, the fuel injected separately into
the furnace also entrains burned gas in the furnace, and some changes in the fuel,
such as pyrolysis, decomposition, and vaporization of liquid fuel, if any, during this
preparation period. Weak combustion reactions may occur between fuel and
entrained product (B*F) and the main combustion follows in the mixing zone of
fuel and diluted air with a large amount of burned gas (B*F*BH). The change of
flame due to a low concentration of oxygen caused by the high rate of recycling of
burned gas probably yields a broadened reaction zone, where relatively slow reac-
tions may be taking place. In established combustion without preheated air, direct
combustion between fuel and fresh air (F*A) occurs in the near-field of the burner.
Thereafter, some combustion in diluted condition with burned gases may follow in
the downstream portion of the flame because of the entrainment of recirculated
burned gas by the incoming combustion air. Combustion (F*A) in the vicinity of
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