Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 16

1600

Preheated air combustion
(High Temp. Flame)

1400

1200

1000

800

600

400

Conventional
combustion

200 HiTAC

0

400 600 800 1000 1200 1400 1600

Air preheat temperature, K

FIGURE 1.7 Preheated air temperature on NO x emission.

people refer to the actual concentration of oxygen because utilization of atmo-
spheric air as the oxidizer in practical combustion systems is far too common to
be noted. Thus, we must pay special attention to the influence of oxygen concen-
tration on nitric oxide formation.
Suppose that we use highly diluted air with inert gas such as nitrogen for
combustion. Figure 1.8 shows numerically predicted turbulent diffusion flames
between methane and air or their dilution together with their temperature and species
concentration profiles across the flame. The ordinary methane-air flame has a typical
thin reaction zone called a flamelet, where rapid combustion reactions produce a
steep concentration gradient in methane and oxygen as well as a sharp and high-
temperature rise due to the heat release in the flamelet. In contrast, the diffusion
flame between diluted methane (40%) in nitrogen and diluted air in nitrogen, in
which actual oxygen concentration is 8%, shows a broadened reaction zone associ-
ated with a lower peak temperature in spite of the preheating of both flows up to
1273 K. The unexpected low flame temperature is caused by a combination of high
preheating of reactants and the dilution by a large amount of inert gas. Because the
local reaction rate becomes small in diluted circumstances, increased volume of the
reaction zone results from burning fuel at the same rate as ordinary combustion,
hence the same total heat release rate in the furnace. Therefore, the reaction and
heat release zone of combustion in the dilution with plenty of burned gases may
become widely distributed compared with that of ordinary combustion and yield a
mild temperature rise locally. These facts must be taken into account when attempt-
ing to suppress nitric oxides emission from practical HiTAC systems.