Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 3

by using some type of mechanism has been sought. The evaluations of the combus-

tion of ultra-lean mixtures have varied depending on the positions of the evaluators.
However, research on the combustion of ultra-lean mixtures at the combustion limit
has been a challenge to combustion engineers and attractive to researchers in the
field. Because ultra-lean mixtures do not burn under normal pressure and temperature
conditions, it becomes necessary for the mixture to accelerate the reactions by
methods such as: (1) catalytic oxidation using a catalyst, (2) injection of active
particles such as high energy radicals, (3) preheating, or (4) increase of pressure.
Among these, except for oxidation with catalyst, interest has focused on a method
of preheating the ultra-lean mixture using a heat recirculation method. The heat
recirculation method mentioned here is proposed as a method of preheating the
unburned mixture without using external energy. The method provides a means of
recirculating the heat from the high temperature side (burned gas) back to the
unburned mixture side using an appropriate heat exchange method. The preheating
gives additional enthalpy to the unburned mixture without dilution by the combustion
products. The combustion system utilizing heat recirculation, in fact, has been
commonly used in industrial combustion units for many years. The methods of
recovering waste heat and preheating combustion air using various types of heat
exchangers have been employed primarily to improve thermal efficiency and stable
combustion. Here, it should be pointed out that the high temperature air combustion
technology is an extension of the above means. Therefore, from an industrial per-
spective, the heat recirculation combustion method used for the combustion of ultra-
lean mixtures is not, in principle, a totally new method of combustion.
In 1971, Weinberg

ge perceived the fact that preheating the
mixture using the heat recirculation method can attain stable combustion of an ultra-
lean mixture and expand the flammablilty limits of the mixture. He proposed the
concept of additional enthalpy combustion for ultra-lean mixtures. A special feature
of this method is that a heat source for preheating is not necessary to maintain
combustion except at the time of start-up. Furthermore, the temperature of the final
exhaust gas does not necessarily become high. The novelty of Weinberg's idea lies
in maintaining sustained combustion without any assistance from an external heat
source, for example, by applying the heat recirculation method used in conventional
industrial furnaces for ultra-lean mixtures. In this case, the success of applying the
heat recirculation method depends on (1) whether or not a heat exchange method is
appropriate and (2) how much of the heat loss from a combustion unit can be reduced.
Although various heat exchange methods have been proposed, the major heat recir-
culation methods studied to date can be broadly classified into (1) indirect (external)
heat recirculation methods and (2) direct (internal) heat recirculation methods.
The indirect (external) heat recirculation methods are methods of circulating
heat at the exterior of a flame zone without essentially changing the structure of the
flame as represented by the double spiral type burner proposed by Lloyd and
Weinberg,

of Imperial Colle

1,2

, a so-called Swiss Roll burner. The indirect (external) heat
recirculation methods can further be classified into methods that mainly utilize
conduction only and methods that also actively utilize radiation for the heat feedback.
For the latter methods, a combustor was proposed wherein a porous solid wall is
positioned to enclose the combustion chamber, because the porous solid wall has

namely

1,2