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Show have verified the major advantages of the surface combustor-heater concept: high heattransfer rates, high combustion intensity and efficiency, low pressure drop, as well as ultra-low NOx and CO emissions. If successfully developed and commercialized, the surface combustor-heater concept can provide the basis for development of advanced and more efficient process heaters, boilers, water heaters, thermal fluid heaters, and air heaters with ultralow pollutant emissions for industrial, commercial, and residential applications. The purpose of this paper is to describe the surface combustor-heater concept and present the preliminary experimental results obtained on the bench-scale surface combustorheater. DISCUSSION The Concept In a conventional combustor, the presence of the large number of cold heat-exchange surfaces within the combustion zone would quench the flame, thereby producing high CO and total hydrocarbons (THC) emissions. In the surface combustor-heater, because the combustion reaction takes place in many pores which do not directly contact the cooling surfaces, the surfaces could be located downstream or even within the combustion zone to increase heat-transfer rate to the heat exchanger, and still provide high efficiency combustion. In general, the combustor-heater has four zones as shown in Figure 2. 1 . Mixture Distribution Zone - Cooled Distribution Grate. A gaseous fuel/air mixture is distributed under the porous matrix by the cooled distribution grate. The cooled grate also prevents flashback at low firing-rate operation (high turndown ratio). 2. Preheating Zone - Lower Part of the Porous Matrix. The mixture exiting the grate is preheated to ignition temperature. 3. Combustion and Heat-Transfer Zone - Bottom Row(s) of the Embedded Tubes Above the Preheating Zone. Combustion occurs within the porous bed, and heat extraction by the embedded tubes takes place. Combustion is controlled by the heat extraction. 4. Heat-Transfer Zone - Upper Row(s) of the Embedded Tubes Above the Combustion Zone. Combustion is completed in this zone, and the only heat transfer taking place is from the postburn gases and the hot particles to the tubes. As shown in Figure 2, a natural gas/air mixture is supplied at the bottom of the porous matrix. Initially, combustion occurs above the bed surface, but as the bed is heated by the hot products of combustion, the flame front moves deeper into the bed until it is stabilized somewhere in the bed above the distribution grate, with complete combustion occurring inside 4 INSTITUTE o F GAS TEe H N 0 LOG Y |
