| OCR Text |
Show ,I 4 . Infrared energy can be focused, reflected, or polarized to meet specific applications. 5 . Intensive radiant heat removal directly from the combustion zone provides an effective approach to reduce NOx emissions. 6. Relatively low capital and operating costs are expected because of high efficiency. Four types of combustion-heat transfer modes have been adopted for radiant heat transmission from co~bustion gases, as shown in Figure 1. Mode (a) in Figure 1 shows a sketch of the non-permeable radiant burnerheater in which the non-permeable radiation component is heated by heat convection from the combustion gases. A variety of radiant tubes and walls have been developed based on this concept. Because of difficulties with the further enhancement of convective heat transfer between the hot gases and the radiant walls, temperatures at the radiant surface are usually restricted below l8000 F. Mode (b), shown in Figure l, is a schematic permeable radiant wall in which hot gases flow through and heat the permeable wall resulting in high surface temperatures. This concept has been suggested to enhance energy conversion (1,2) and heat transfer in the gas duct (3). Using a pair of porous walls that are oppositely placed at the inlet and outlet of a gas-fired combustor for internal radiant heat regeneration has been proved. Super adiabatic combustion temperatures have been achieved in the combustion chamber (4). High-temperature porous materials are the maj or technical b~rrier for the development of this technology toward wide applications. Mode (c), shown in Figure 1, is called a surface-combustion radiant burner in which the gaseous fuel/oxidant mixture passes through the porous medium and then combusts entirely in the downstream gas phase in the vicinity of the porous media surface. The radiating surface is heated by conduction from hot gases and the flowing gases within the porous phase are also preheated. The surface can be heated up to 18000 to 22000 F by controlling the flame close to the surface as much as possible. Focusing on improvement of the burner material to achieve higher surface temperatures and prevent the risk of flashback, many types of surface-combustion radiant burners have been developed, such as "ACOTECH" by Shell Oil Co. (5), "PYROCORE" by Alzeta Corp. (6), and the fibermat burner in Japan (7). Since a significant amount of combustion heat is removed through the radiating surface, and very lean premixed flames can be stabilized above the surface due to preheating the combustible mixture, NOx emissions from the radiant burners can be significantly reduced (8). Regardless of the popularity of the surface-combustion mode described above, recent studies show the possibility of -a different mode of operation called porous-combustion mode or reactor mode (9), as shown in Mode (d) in Figure 1. In this mode, combustion is carried out completely within the porous media. Therefore, higher combustion intensities can be achieved with this mode than with the conventional surface-combustion mode. In addition, higher conversions to radiant energy would be possible due to higher radiating surface temperatures. As a result of high combustion intensity and high heat |
