OCR Text |
Show ture history in the combustor may be achievable. This suggests the potential to achieve superior NOx control in the combustion process. Since the thermal-NOx reactions are highly temperature dependent, controlling the temperature history through the flame zone is equivalent to controlling the NOx emissions. In singlestage PM burners, control of the temperature profile is constrained by the equivalence ratio of fuel and air and the ability of the porous media to transfer heat from the surface. Even wi th high radiant heat transfer from the surface, the temperature profile exhibits a peak flame temperature that can set the lower limit of NOx formation. The present investigation looks at staging as a technique to produce a uniform temperature profile which does not exhibit this peak flame characteristic and hence results in lower NOx formation when compared to a single-stage PM burner at the same equivalence ratio. The possibility that this type of control could be achieved in porous burners is one of the major factors motivating the present research. EXPERIMENTAL SET-UP The experimental apparatus used in this investigation consisted of an experimental burner, fuel/air control instrumentation, and exhaust gas analysis instrumentation. A brief description of this apparatus is presented below. Burner Description A detailed sketch of the experimental two-stage burner is shown in Figure 1. The cylindrical burner was oriented with its axis vertical, such that the flow of gases was upward. The burner is comprised of the following components: 1) burner core, 2) sidewalls, 3) first-stage flame holder, and 4) second-stage mixture inlet. The burner core was constructed from 4.6 cm diameter by 2.5 cm long porous ceramic cylinders. These reticulated ceramic cylinders, made of partially stabilized zirconia (PSZ), have the appearance of a sponge. They can be obtained in a number of pore sizes ranging from 10 pores per inch (ppi) to 65 ppi; however, 10 ppi was used for this investigation. The average porosity of the ceramic media varies from 84 to 87%, while the thermal conductivity for the 10 ppi ceramic is approximately 1 W/m-K (3). These ceramic cylinders were inserted into 5.0 cm diameter quartz tubing which served as the burner sidewalls. The quartz sidewalls contained the flow of gases in the ceramic matrix and provided radial heat losses via radiation. -3- |