Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 6

ork at Tokyo Gas led to the development of an advanced fuel direct injection
(FDI) concept.

the w

The concept utilizes the idea of discrete injection of fuel gas into
hot combustion products. During the same time at NFK a lot of efforts were made
to reduce NO

4

The company
tested several configurations and designs for air and fuel gas ports. The best results,
corresponding to low NO

emission using the process of additional enthalp y.

5

x

emissions, were achie ved when the injection ports for

x

fuel and air were positioned apart.
Developments in other countries traveled different routes. Rather than developing
new combustion processes, the work focused on burner designs. The flameless oxida-
tion (FLOX) burner, developed in Germany in 1991 to 1992, utilizes a number of air
jets, which entrain combustion products before mixing with the fuel. In 1991, the

6

IFRF designed a series of natural gas burners in SCALING 400 studies. Substantial

7

NO as achieved when either 80 or 100% of the fuel gas was provided

through the individual injectors located on the burner circumference. Furthermore, in
this case, NO

reduction w

x

ustion
air. When the burner was fired with 80 or 100% fuel staging, the combustion mode
resembled the FDI system. During the tests, it was observed that the radiation heat
flux of the staged flame were approximately 20% higher than those of the baseline
(unstaged) flame. This, at first, appeared as a surprising result. However, it was later
confirmed in other experiments in which, despite the lower flame temperatures with
the staged flames, the higher flame volume caused an increase of the heat flux. The
most recent developments include a Canadian Gas Research Institute (CGRI)

emissions were no longer dependent on preheating of the comb

x

b urner

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that also features discrete injections of fuel gas and air.
Based on the observations of combustion regime in high performance industrial
furnaces, which will be shown in later chapters, it could be concluded that the
key for high temperature air combustion technology (HiTAC) is the dilution of
fuel and combustion air by burned gases in the furnace. Specifically, combustion
in an atmosphere of low oxygen concentration is the key. This situation can be
realized by the recirculation flow induced by high velocity air injection. These
clarifications in terms of the true HiTAC mechanism have provided a framework
within which the technology could be utilized for specific applications for energy
conservation and pollution reduction.

1.2 INNOVATION OF HIGH TEMPERATURE AIR

COMBUSTION

1.2.1 F C

UNDAMENTALS OF OMBUSTION

1.2.1.1 Heat Recirculating Combustion

Preheating of comb ustible mixture by recycled heat from flue gases has been con-

sidered an effective technology not only for combustion of low calorific fuels but
also for fuel conservation. It is called heat-recirculating combustion in which reac-
tants are heated prior to the flame zone by heat transfer from burned products without
mixing of the two streams.

The temperature histories of premixed combustion in a
one-dimensional adiabatic system are schematically compared in Figure 1.1

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for

1,2