Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 15

Regenerators for heat-recirculating combustion appeared first in Europe and
studies on heat-recirculating combustion were reviewed by Weinberg. 9 The typical
regenerative heat exchanger for preheating combustion air was a bed packed with
ceramic balls.
The size varies depending on the cycle time between 30 s to several minutes,
which determines the amount of heat storage. A typical regenerator at that time
produced preheated air exceeding 1273 K, generally within 300 K below the furnace
temperature. 15 Ho wever, if the hot gas flow escaping through a shortcut of minimum
pressure drop occurs in the bed, it gives rise to uneven temperature distribution in
a cross section, resulting in inefficiency of the regenerator.
In contrast, the volume of a honeycomb-type regenerator of necessary heat
capacity can be minimized because of its large surface area-to-volume ratio. As a
result, direct installation of a regenerator into a burner becomes possible, forming
a thermal dam at the exit of the furnace as illustrated in Figure 1.3 . Temperature
distribution in a ceramic honeycomb is quasi one-dimensional, because ceramic
honeycombs assure the uniformity of temperature in a cross section. There is an
example in which combustion air of 1570 K was actually obtained by this type of
regenerative system for the mean furnace gas temperature of 1623 K, that is, an
approximately 50 K difference. Almost constant temperature of combustion air, less
than 50 K variation during a cycle, was realized by the use of regenerators with
small heat capacity, and about 40% fuel saving, and hence CO 2 reduction, was
achieved. 16
1.2.2.2 Nitric Oxides
During the last quarter of a century, extensive experiments and detailed chemical
kinetic calculations have been carried out to clarify the formation mechanisms of
nitric oxides. As a result, the temperature rise in combustion air has been recognized
as one of the influencing factors on nitric oxides emission from combustion systems
because it often causes higher flame temperatures, where most nitric oxides are
rapidly formed. The influence of inlet air temperature on nitric oxides emission from
a prototype furnace is shown in Figure 1.7 , demonstrating an exponential increase
of nitric oxide emission with temperature rise of combustion air. These characteristics
have been widely taken among combustion engineers as common knowledge of
nitric oxides emission from combustion devices.
Clearly a large amount of nitric oxide is formed with the increase in flame
temperature. Consider what will happen when preheated air higher than the auto-
ignition temperature of a fuel is used. If reaction between fuel and preheated
normal air at near-stoichiometric ratio occurs, the flame temperature must be
extremely high. Therefore, a large quantity of nitric oxides may be emitted when
non-premixed combustion takes place with high temperature pure air in furnaces.
If this happens, the reduction of nitric oxides in HiTAC seems unpromising. There
is one more important dependency factor that needs to be discussed. This is the
oxygen concentration in the reaction zone where local combustion reactions take
place. Combustion always occurs in near-stoichiometric mixture even though the
mixture is non-premixed. However, when referring to stoichiometric ratio, few