| OCR Text |
Show the surface to incandescence. A typical impingement burner can achieve combustion intensities of up to 400 MBtu/hr-ft2 at temperatures up to 2500°F (Reference 1). However, since heat transfer to the refractory occurs by flame impingement, the surface may not radiate uniformly. At high throughputs, the flame can extend out from the burner surface, creating higher temperature zones and limiting proximity to the load if overheating and flame impingement on process tubes are to be avoided. Further, due to the large thermal mass of the refractory, it is not possible to respond quickly to changes in process conditions. The other type of radiant burner, the porous surface type, represents a method of generating high radiant heat flux with improved energy conversion efficiency and uniformity. Figure 1 shows a schematic representation of a porous surface burner. In these systems, premixed gaseous fuel and air flow through a porous media and combust on the surface. The combustion reactions occur in intimate contact with this surface, normally heating it to incandescence at 1500°F to 2200°F, with NOx emissions typically under 20-25 ppm. Under normal operating conditions, the surface glows incandescently to release 20 to 30 percent of the heat of combustion as direct radiant energy. c Materials used in these burners are highly porous matrices of ceramic or metal fibers. Since the mixture flows uniformly through all parts of the matrix, the combustion and, hence, radiant output are uniform over the entire burner surface. In addition, high porosity gives the burner a low thermal mass with the capability to respond quickly to rapidly changing process conditions. The burner surface ' can be made in a variety of shapes, so that the heat transfer can be matched precisely to a load. Combustion occurs only on the very outer surface of the burner with no attached flame. The interior of the porous material remains at the temperature of the incoming reactants, allowing stability over a wide range of operating conditions. ", \::-,,~. "'1' -', " '\ ' -/1 Figure 1. ',. / , ,,, ....... . . - ; . . .... . -,.., , ;- -. - ' - . , ' . " • 1 ~ ;./ . , . ~I ...... '~....... / . .. _, " .". :-""-" "2.'" . -'1 -) .,,'<:: ' - ~~ I, '" "~ ,',' .- --, :. - .. p '~'\\' /". I ••• :::-, ';-~ RADIANT ', - : / Z- /" HEAT I :. -- , FLUX SENSIBLE HEAT OF COMBUSTION PRODUCTS Radiant and Sensible Heat Flux from Porous Surface Radiant Burner Porous surface radiant burners are presently supplied by Alzeta and other manufacturers for a variety of industrial applications including refinery process heaters, process air heaters, paper processing dryers, heat transfer fluid heaters and vaporizers, and 2 |
