Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 59

enthalpy combustion can be effective under the condition of lowering the adiabatic
flame temperature by gas recirculation. This improved heating method of combining
high temperature additional enthalpy combustion with gas recirculation makes it
possible to achieve a uniform temperature profile in furnaces while holding the
maximum flame temperature constant.
A profile of thermal efficiency affected by gas recirculation ratio R and maximum
flame temperature T fmax w as calculated to determine the optimized operating condi-
tion between thermal efficiency and NO x emission. NO x emission is in principle
supposed to have a strong relationship with the peak flame temperature. A small
amount of cold gas recirculation is actually one way to achieve low NO x level even
though it frequently causes unstable combustion, which results from a flame tem-
perature decrease. It is assumed for this discussion that NO x level is determined
mainly by the maximum flame temperature.
The contour profile of the thermal efficiency obtained is shown in Figure 2.33
based on the assumption that the maximum heat exchange coefficient of inlet air
and fuel, ξ h , which was defined in Equation 2.2, can be less than 0.9. The lines of
constant thermal efficiency in Figure 2.33 demonstrate that there is a trade-off point
in terms of thermal efficiency maximization and NO x emission minimization on the
line of the heat exchanger limit line corresponding to ξ h = 0.9. The lowest T fmax is
found on that heat exchanger limit line for a given thermal efficiency. For example,
when a design thermal efficiency was fixed at 0.7, the lowest maximum flame
temperature, 1730˚C, was found on the boundary line of the heat exchanger limit
with gas recirculation R = 4.5. If flexibility in operating temperatures is more
important than minimizing NO x emissions, then it is possible to maintain thermal
efficiency by varying the gas recirculation ratio while changing the operating tem-
peratures. Using the previous thermal efficiency as an example, 0.7 can be attained
with the maximum flame temperature in the range from 1730 to 2150˚C by varying
the gas recirculation ratio. If the maximum flame temperature was fixed at 1800˚C,
it could be attained with any thermal efficiency in the range from 0.45 to 0.75 by
controlling the gas recirculation ratio. Thus, maximum thermal efficiency can be
obtained while minimizing the NO x emission level or constant flame temperature
can be maintained. The same profile was plotted with a three-dimensional plot in
Figure 2.34 as a reference.
High temperature additional enthalpy combustion, combined with high tem-
perature combustion gas recirculation taking place in the above-mentioned con-
dition, is unique compared with normal combustion because the temperature
increase has to be small due to the heat capacity increase. This unique combustion,
recognized currently as high temperature air combustion, has completely different
characteristics from a conventional operating method concerning flammability
limits, combustion stability, combustion noise, and ignition process beyond the
autoignition temperature.
This improved heating method using high temperature air combustion can pro-
vide several advantages such as control of the maximum flame temperature in
responding to the properties of heated materials within the available zone and in
principle very low NO x levels.