| OCR Text |
Show ( \ -. gas/air mixture to the surface, the fine reticulate structure also acts as thermal insulation and therefore helps to define the combustion zone. Radiation feedback from the combustion process acts to preheat the gas/air mixture further improving efficiency (1). The top, more open porosity layer acts to adsorb energy from the flame and to emit radiant heat to the process load. The flame is contained within this zone. The dual structure is shown in Fig. 3. The dual layer structure is designed to provide optimum performance in terms of radiant efficiency. Pore size, layer thickness and density all contribute to burner performance. Decreasing the pore size and increasing top layer thickness or density will result in excess radiation back into the tile and flashback can occur. On the other hand if the top layer is too thin, the density too low, or the pore size too open less heat will be adsorbed from the flame, radiation efficiency will decrease and flame lift off can occur. Table 1 shows the effect of BTU input and density on infrared efficiency and surface temperature. Surface temperature was determined using a two color pyrometer and IR efficiency calculated assuming an emittance of 0.9 using the Stephan-Boltzman equation. Surface combustion burners have been shown to have low NOx emissions. The transfer of energy from the flame to the solid and the subsequent infrared radiation effectively cools the combustion process, thereby minimizing the generation of NOx species. Table 2 shows the effect of input and excess air on NO emissions for an Infracel Dual Burner. The usual effect of increasing NO emissions with increasing input and decreasing excess air can be seen. However, under all conditions tested, the emissions were less than 20 ppm. These measurements were made by the Alzeta Corporation using a Teco 10, chemiluminscent analyzer. In addition to the combustion performance of an infrared radiant burner, material performance is critical to assuring longevity in industrial environments. The properties of primary concern are strength, thermal shock resistance and temperature use limit. Table 3 shows mechanical properties of commercially available materials used by Hi-Tech Ceramics. Lithia-alumina-silicate (LAS) is known for its low thermal expansion and high thermal shock resistance. Earlier work has shown that the application of an SiC based coating (HE) lChen, V-K., R.D. Matthews, and J.R. Howell (1987), "The Effect of Radiation on the Structure of a Premixed Flame within a Highly Porous Media", ASME HTD-Vol 81, pp. 35-42. |
