Results are presented on the thermal characteristics of propane-air diffusion flames using high temperature
combustion air. Global flame characteristics are presented using several different gaseous fuels. A specially designed
regenerative combustion test furnace facility, built by Nippon Furnace Kogyo, Japan, has been used to preheat the
combustion air to elevated temperatures. A heat recovery system equipped with a honeycomb type regenerator could
preheat the combustion air to temperatures of about 1300oC. The difference in temperature between the furnace and
incoming combustion air could be 50oC or less. The oxygen concentration in combustion air was varied from 21% by
volume (normal air) to 2%. The flame signatures were found to be very different with high temperature combustion air (in
excess of 1000oC) and with different fuels than near room temperature combustion air. The flames with highly preheated
combustion air were much more stable and homogeneous (both temporally and spatially) as compared to roomtemperature
combustion air. Stable flames were obtained at remarkably low equivalence ratios, which would not be
possible with normal temperature air. The global flame features showed flame color to change from yellow to blue,
bluish-green and green over the range of conditions examined using propane as the fuel. In some cases hybrid color flame
was also observed. Under certain conditions flameless or colorless oxidation of the fuel has also been observed for some
fuels. Some fuelss provided purple color flame under similar operational conditions. Information on the flame spectral
emission characteristics, spatial distribution of OH, CH and C2 species and emission of pollutants has been obtained. Low
levels of NOx along with negligible amounts of CO and HC were obtained with high temperature combustion air.
Experimental results have been complemented with numerical simulations. Calculated results showed flame features
similar to those obtained experimentally. The thermal and chemical behavior of high-temperature air combustion flames
depends on fuel property, preheat temperature and oxygen concentration of air. High temperature air flames provide much
higher heat flux than normal air, which helps to save energy and the subsequent reduction of greenhouse gases to the
environment. The high temperature air combustion technology provides significant energy savings (up to about 60%),
downsizing of the equipment (about 30%) and pollutants reduction (about 25%). Energy savings translate to reduction of
CO2 and other greenhouse gases to the environment. The challenges and opportunities with high temperature-air
combustion technology are also described.