Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 49

2.2.4 F LAME WITH H EAT AND C OMBUSTION P RODUCTS
R ECIRCULATION

A further improved heating method can be obtained by combining heat recirculation
with hot combustion products recirculation in a furnace, which decreases the max-
imum flame temperature and creates a more uniform profile of combustion gas
temperature in the furnace. In this section, the advantages of this heating method
are described and are compared, by using a simple heat balance calculation, to the
conventional additional enthalpy combustion method, which does not employ hot
combustion products recirculation.

2.2.4.1 Improved Heating Method

2.2.4.1.1 Heat and Combustion Product Recirculation
To avoid localized overheating of the load, there has been a restriction of heat input
and amount of heat recirculation for additional enthalpy combustion using fuel with
high heating value. It is, however, possible to overcome this thermal restriction by
controlling the following two parameters in the combustion process, which influence
adiabatic flame temperature. They are (1) initial temperature of the air and fuel and
(2) vitiation of the inlet air and fuel when combustion occurs under the condition
of constant air ratio, constant fuel heating value, and atmospheric pressure in a
furnace. Thus, if there is a way to control the flow rate of combustion products to
vitiate the inlet air and fuel, it is possible to increase the amount of heat recirculation,
particularly for high heating value fuels, over the restriction level without raising
flame temperature. This heating method combining heat recirculation with high
temperature combustion product recirculation in the furnace, schematically shown
in Figure 2.24 , can significantly improve the uniformity of the temperature profile
formed in the furnace when compared to periodical alternate additional enthalpy
combustion.

(1- α ) Q rec

Gas Recirculation

m rec

Q loss

m m

α Q

Q rec

Air Preheater

T max

m exh

m in

Q f

Q cold

Q out

Q exh

Q in1 Q in2

Furnace

Mixer

Q m

Q ev

Heat Recirculation

FIGURE 2.24 Schematic diagram of model furnace system with heat recirculation and gas
recirculation.