Effect of air preheat temperature and oxygen concentration on flame structure and emission

The structure of turbulent diffusion flames with highly preheated combustio n air (air preheat temperature in excess of 1150 degrees C) has been obtain ed using a specially designed regenerative combustion furnace. Propane gas was used as the fuel. Data have been obtained on the global flame features, spectral emission characteristics, spatial distribution of OH, CH, and C-2 species, and pollutant emission from the flames. The results have been obt ained for various degrees of air preheat temperatures and O-2 concentration in the air The color of the flame was found to change from yellow to blue to bluish-green to green over the range of conditions examined. In some cas es a hybrid color flame was also observed The recorded images of the flame photographs were analyzed using color-analyzing software. The results show that thermal and chemical flame behavior strongly depends on the air prehea t temperature and oxygen content in the air. The flame color was observed t o be bluish-green or green at very high nil preheat temperatures and low-ox ygen concentration. However, at high-oxygen concentration, the flame color was yellow. The flame volume was found to increase with increase in air-pre heat temperature and decrease in oxygen concentration The flame length show ed a similar behavior. The concentrations of OH, CH, and C-2 increased with an increase in air preheat temperatures. These species exhibited a two-sta ge combustion behavior at low-oxygen concentration and single-stage combust ion behavior at high-oxygen concentration in the air. Stable flames were ob tained for remarkably low equivalence ratios, which would not be possible w ith normal combustion air. Pollutant emission, including CO2 and NOx, was m uch lower with highly preheated combustion air at low O-2 concentration tha n with normal air The results also suggest uniform flow and flame thermal c haracteristics with conditioned, highly preheated air. Highly preheated air combustion provides much higher heat flux than normal air, which suggests direct energy savings and a reduction of CO2 to the environment. Colorless oxidation of fuel has been observed under certain conditions.