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Show N +J '+- ".L:c':."J. +J m :2 STABLE COMBUSTION RADIANT SURFACE: IN POROUS MEDIA 300 0 A WITHOUT COATING • • WITH COATING • 200 OUTER BLUE FLAME 100 FlASHBACK O~~--r-~~~--~~~~~~~~~~--r-'--r~--~ 1.0 1.5 2.0 2.5 Air Ifuel Stoichiometric Ratio Figure 5. Effect of operating parameters on combustion modes in the porous burner. 3.0 mode and the flashback mode, as shown in this figure. features can be concluded based on the results - Several important • Range of firing rate (turndown ratio) for stable porous combustion is as high as 6 to 1, but not limited yet, as shown in the figure. • Range of excess air for stable porous-combustion is dependent on firing rate. • At lower firing rate, combustion can be stabilized in the porous media at a very high level of excess air up to 140%, even not limited yet. The reason is twofold: First, the very lean flame can be held in the very small pores in the porous bed and second, the mixture can be more 'significantly preheated at lower flow rates that result in expansion of the lean flammability. This feature provides a basis of ultra-low NOx emission combustion and low heating value gas combustion in the porous burner. • At higher firing rate, excess air level for stable porous combustion is relatively low, resulting in higher thermal efficiency. Because of the limited capacity of the natural gas rotameter used in this test, the firing rate could not be further increased. It is reasonable to predict that stable porous combustion can be achieved at a much higher firing rate. In fact, |