Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 13

the burner shows the maximum temperature in the furnace, and most of the nitric
oxides emitted from the furnace are formed there. However, combustion in this
region is essential to sustain the combustion in the furnace, and whole flame cannot
exist if extinction occurs in this portion.
Figure 1.6a shows conceptual temperature histories along streamlines passing
through and by the flame zone. The former experiences near-stoichiometric flame
temperature, which is slightly below the theoretical adiabatic temperature, T ad , and
the latter rises only as mixing progresses. Turbulent mixing between the two pro-
duces large temperature fluctuations that can usually be observed in ordinary turbu-
lent flames. If combustion air is preheated, it forces up all temperature profiles to
some extent, with the same degree of fluctuations exceeding the theoretical adiabatic
temperature of ordinary combustion T ad , as shown in Figure 1.6b . This is a well-
known feature of the preheated air combustion (PAC) achieved by use of a recuper-
ator. However, once the preheating temperature exceeds the auto-ignition tempera-
ture of the fuel, HiTAC becomes possible. Then, as shown in Figure 1.6c , the
temperature history along the streamline passing through the reaction zone shows a
relatively mild temperature rise due to slow heat release in a low oxygen concen-
tration atmosphere compared with those in previous cases. The other extreme indi-
cates a temperature profile along the streamline outside the reaction zone, and it
rises only with the progress of mixing between preheated combustion air and burned
gas recirculating in the furnace. Accordingly, temperature fluctuations generated
between these two are very small compared with the previous cases. In spite of the
use of highly preheated air, the mean temperature as well as the instantaneous peak
temperature is considerably lower in HiTAC than in ordinary combustion.
1.2.2 P RINCIPLE OF C OMBUSTION C ONTROL FOR CO 2 AND NO X
R EDUCTION
1.2.2.1 Carbon Dioxide
As long as hydrocarbon fuels are used, CO 2 will be emitted in proportion to the
carbon content of the fuel, unless it is artificially fixed or removed. The only way
for combustion engineers to suppress CO 2 emission from combustion devices is
energy conservation by raising the thermal efficiency of the device. In that sense,
HiTAC is one of the most attractive technologies for use in the furnace industry.
The basic concept is easy to understand. If all of the heat loss and waste from a
heating furnace can be eliminated, for example, all the heat generated from fuel will
be transferred to the material being heated in the furnace. Therefore, the high
efficiency of waste heat recovery is one of the most promising measures available
to suppress CO 2 emission, that is, to reduce greenhouse gases. The higher the
efficiency of the regenerator, the less CO 2 emitted.
Although available heat in exhaust gases can be transferred efficiently to the
incoming cold combustion air using an infinitely long heat exchanger, the actual heat
transfer rate is limited by the geometry of heat exchangers. Accordingly, the maximum
obtainable temperature of combustion air depends not only on the tolerance of materials
used, such as heat-resistance alloys and refractory, but also on heat losses of the system.