Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 10

1.2.1.2 Definition of High Temperature Air

The term high temperature air is used throughout this topic. The temperature of the
air indicated often varies depending on the situation where it is used. What is the
definition of high temperature air when furnace combustion is considered? Imagine
that a gaseous fuel at ambient temperature is injected into an air stream. When the
fuel mixes with combustion air, some heat is necessary to initiate combustion, and
a recirculating flow of combustion products behind a flame holder or a pilot flame
is frequently utilized for stabilizing flames in furnaces. However, if combustion air
is sufficiently heated prior to mixing, combustion takes place somewhere down-
stream in the furnace following the mixing of two reactants, even if the flame in the
near field of the fuel jet is blown off by a strong shear motion.
Although the temperature level of preheated air does not seem important when
discussing preheated air combustion, the fact described above is significant in real-
izing advanced low NO x combustion technology, which will be explained later.
Therefore, the auto-ignition temperature of a gaseous fuel with air as the limit of
high temperature, that is, the air temperature at which a gaseous fuel is ignited
automatically in it and in which continuous combustion is sustained, should be called
high. Although the definition of high temperature is not given by a fixed value, it is
now possible to give high temperature air combustion (HiTAC) a clear meaning.
Following the above definition, preheated air combustion (PAC) is defined as com-
bustion with the air of preheated temperature below the auto-ignition limit that has
long been utilized in industry.
Once the combustion air is preheated to higher than the PAC limit, a method to
stabilize a flame is not necessary for furnace combustion. This auto-ignition tem-
perature of a gaseous fuel varies depending on the kind of fuel and concentration
of oxygen of the diluted air. Figure 1.4 shows auto-ignition and combustible limits
for propane in preheated air or diluted air with inert gas. auto-ignition and combus-
tion occur even in an atmosphere of oxygen content as low as 3% when it is preheated
above 1200 K.

1.2.1.3 Heat Recirculation and Exhaust Gas Recirculation

Global excess air ratio or equivalent ratio is one of the combustion parameters that
characterize the operating condition of furnaces. The temperature of combustion
products in adiabatic circumstances can be easily defined by the ratio. However,
heat subtraction by the material being heated in the furnace and the heat loss from
practical systems are influencing factors in defining gas temperature in the furnace.
Therefore, the reduced temperature level of burned gas as well as its recycling flow
rate largely affects flame temperature with gas recirculation.
Exhaust gas recycling, whether it is internal or external, is an effective method
to reduce flame temperatures, and thereby nitric oxides emission. Combustion with
normal ambient air usually becomes unstable when the exhaust gas recycling rate,
defined as the mass ratio between exhaust gas and fresh reactants, exceeds 30%. As
is shown in Figure 1.4, however, a stable combustion domain appears for high rates
of exhaust gas recycling, if combustion air is preheated over the auto-ignition