Environmental Engineering Reference
In-Depth Information
auto-ignition. In contrast, if a fuel of lo
w calorific value is adopted, the combustible
domain disappears at ambient temperature and reappears when the mixture is pre-
heated over a certain temperature level, as shown in the figure exhibiting conceptual
trends.
Most of the previous research in this field has been aimed at burning ultra-lean
mixtures or low calorific value fuels produced in chemical processes or vented from
coal mines. In those cases the resultant maximum flame temperature with heat
recirculation was not crucial to the tolerance of materials used in the system because
of the low calorific value. Further, scientific studies on heat recirculating combustion
have been mostly carried out on relatively small-scale premixed flames. However,
for large-scale industrial use, diffusion or non-premixed combustion is more common
because of its controllability and safety. Heat recirculating combustion in diffusion
or non-premixed combustion can be achieved by heating combustion air with the
recycled heat from burned products. The temperature of combustion air in an adia-
batic system can theoretically be raised to almost the same temperature as the exhaust
stream by regenerative heat exchangers. In practice, the regenerative combustion
system for implementing high temperature air combustion is the system shown in
Figure 1.3
,
where a heat recirculating method (by use of honeycomb-type regener-
ators) is applied to a heating furnace. A pair of burners, operating alternately, is used
as a unit and the flow path for air ejection in each burner is filled with ceramic
Pilot burner A
Pilot burner B
Spark Plug A
Spark Plug B
Air
Air
Fuel
Fuel
open
close
Flow and combustion
Heat transfer
Fuel
Slab
Fuel
open
close
Burner A
Burner B
4-way switching valve
Insulation
Air
Exhaust gas
Ceramic Honeycomb
FIGURE 1.3
Schematic diagram of HiTAC furnace operated with high frequency alternating
flow regenerators.
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