Practical Combustion Methods Used in Industries - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 231

4.2 ENERGY CONSERVATION
4.2.1 B ASIC A PPROACH
By heating combustion air with high temperature gas that has completed heat transfer
to the material to be heated, the volume of fuel consumed can be reduced by an
amount equal to the recovered heat equivalent of the amount of energy required for
the heating. Although it is easily determined from the balance of input and output
of the heat, heating the combustion air increases the adiabatic flame temperature
and improves the heating efficiency proportionally. Consequently, heating within a
specified range becomes possible even if the fuel supply volume is reduced.
In an actual heating application, in-furnace temperature is maintained at a level
corresponding to a specified heating velocity (production rate) because a low in-
furnace temperature causes underheating and a high in-furnace temperature causes
overheating. The volume of heat held by the combustion gas in excess of the in-
furnace gas is the key to heating the object and simultaneously maintaining the
temperature of the in-furnace gas temperature at a uniform level. Failure to maintain
the in-furnace gas temperature means an interruption of normal system operation.
In other words, the heating potential of the combustion gas is determined not merely
from the heat balance calculated on the basis of the total heat input including
recovered heat, but from the amount of heat in excess of the in-furnace gas temper-
ature. For example, it is impossible to heat an object to a temperature higher than
the adiabatic flame temperature, no matter how much fuel is fed into the furnace. It
is also very difficult to heat the object to a temperature close to the adiabatic flame
temperature. However, if an adiabatic flame temperature higher than the required
in-furnace gas temperature is realized with the combustion air heated using the same
fuel, the type of heating mentioned above can be realized.
4.2.2 E FFECT OF I MPROVEMENT
(1) A fuel saving effect by returning recovered heat to the heating system by
preheating air and (2) an enhanced heat-transfer acceleration effect by increased
adiabatic flame temperature can be expected from high-temperature-air combustion
as shown in Table 4.4 . The former can be categorized as an energy-saving effect
resulting from the increased heating potential (heating capacity in excess of the in-
furnace gas temperature); the latter is aimed at realizing an increased heat transfer
rate by utilizing the advantageous condition whereby formation of an extensive high
temperature field is readily realized if the adiabatic flame temperature is increased
by preheating the high temperature air. It is also possible to lower the maximum
flame temperature by averaging the in-furnace gas temperature field with the total
amount of transferred heat fixed at a certain level. This characteristic is important
in connection with the maintenance of the heating equipment. If the same maximum
temperature is allowed, the amount of recovered heat returned to the heating system
can be substantially increased. The following part of this section discusses the effects
of items 1 and 2 above, with the in-furnace conditions varied into two different
cases. The first case is for a furnace where the in-furnace temperature is maintained