Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 5

xchanger that also has short switching time. It has
been proved, through extensive research and development efforts to date, that not
only carbon dioxide but also nitrogen oxide can be substantially reduced. This type
of technology, utilizing high temperature air, has attracted significant attention for
the past several decades. For example, since the 1960s, fundamental research has
been implemented in the field of special combustion systems, such as a supersonic
combustion ram-jet engine, called SCRAM-jet. SCRAM-jet is now attracting atten-
tion in the aerospace field, and substantial results have been obtained. However, the
technological concept intending to apply high temperature air combustion to con-
ventional industrial furnaces is innovative, and one can say that it is a novel com-
bustion technology because it can contribute to further reduction in energy consump-
tion and promote environmental preservation.
In general, by implementing regenerative combustion, the heat generated by
combustion is not uselessly discharged but can be effectively recovered. Therefore,
remarkable energy savings (about 30%) and prevention of global warming have been
realized. In contrast, it is known that NO

using a heat-storage-type heat e

emission increases with an increase in

x

temperature of the air used for combustion; this has even been shown with numerical
simulation on a flame formed in the field of a simple flow. This fact has also been
experimentally confirmed in conventional combustion systems, and is now widely
acknowledged among researchers and engineers in the field of combustion technol-
ogy. From the point of view of reducing the emission of pollutants (especially NO

many researchers and engineers had doubts on the possibility of utilizing high
temperature air combustion. They believed that the technology has not been devel-
oped in either the industrial sector or the academic sector. Thus, in the industrial
sector, finding a way to achieve a balance between energy savings and reducing
emission of a pollutant has been a challenge for many years.
In the meantime, entering the 1990s, unexpectedly reduced values of NO

),

x

were

x

measured in an experimental furnace when high-cycle alternating regenerative com-
bustion technology was applied. The NO alues were reduced further when the

velocity of the airflow injected into the furnace was increased. A concentration of
very low NO

v

x

as confirmed experimentally when the air of a
temperature of 1350˚C was injected with the velocity of 90 m/s. In addition, because
of the temperature rise of combustion air, combustion in a low oxygen concentration
atmosphere became possible. The flame was transparent and colorless. This colorless
and transparent flame had not been observed previously. In contrast, most flames
exhibit a local high temperature as seen in all existing industrial furnaces. Also
observed was the fact that combustion proceeds across a wide region in the furnace.
The temperature in the furnace is close to the limit of its operation, and the temper-
ature distribution in the furnace is almost uniform. These characteristics revealed
that an almost ideal heating furnace can be constructed. Thus, this high temperature
air combustion technology has suddenly become the focus of a great deal of attention
as a novel combustion technology not only in the industrial sector but also in the
academic sector.
The development of this high temperature air combustion technology, especially
the development of low NO

, at most 80 ppm, w

x

urner technology for natural gas combustion, has been
conducted with leadership from the industrial sector. At the beginning of the 1990s,

b

x