Thermal Characteristics of Gaseous Fuel Flames Using High Temperature Air

Abstract

Recent advances on high temperature air combustion (HiTAC) have demonstrated significant energy savings, higher and uniform thermal field, lower pollution, and smaller size of the equipment for a range of furnace applications. The HiTAC technology has evolved from the conception of excess enthalpy combustion (EEC) to high and ultra-high preheated air combustion. In the HiTAC method, combined heat regeneration and low oxygen methods are utilized to enlarge and control the flame thermal behavior. This technology has shown promise for much wider applications in various process and power industries, energy conversion, and waste to clean fuel conversion. For each application the flow, thermal, and chemical behavior of HiTAC flames must be carefully tailored to satisfy the specific needs. Qualitative and quantitative results are presented on several gas-air diffusion flames using high-temperature combustion air. A specially designed regenerative combustion test furnace facility, built by Nippon Furnace Kogyo, Japan, was used to preheat the combustion air to elevated temperatures. The flames with highly preheated combustion air were significantly more stable and homogeneous (both temporally and spatially) as compared to the flames with room-temperature combustion air. The global flame features showed the flame color to change from yellow to blue to bluish-green to green over the range of conditions examined. In some cases hybrid and purple color flame was also observed. Under certain conditions flameless or colorless oxidation of the fuel has also been demonstrated. Information on global flame features, 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 levels of CO and HC have been obtained using high-temperature combustion air. The thermal and chemical behavior of high-temperature air combustion flames depends on fuel property, preheat temperature, and oxygen concentration of air. Waste heat from a furnace in high-temperature air combustion technology is retrieved and introduced back into the furnace using regenerator. These features help save energy, which subsequently also reduce the emission of CO2 (greenhouse gas) to the environment. Flames with high temperature air provide significantly higher and uniform heat flux than normal air, which reduces the equipment size or increases the process material throughput for same size of the equipment. The high-temperature air combustion technology can provide significant energy savings (up to about 60%), downsizing of the equipment (about 30%), and pollution reduction (about 25%). Fuel energy savings directly translates to a reduction of CO2 and other greenhouse gases to the environment.