OCR Text |
Show as high as 500 MBtu/h- ft2 (1.6 Mlrl/m2) of firing intensi ty was achieved in an earlier study (21) . Therefore, 10 to 1 turndown ratio for this porous burner is reasonably expected. • Higher emissivity of the radiating surface can improve flame stabilization of the porous combustor operating at a lower level of excess air because more heat removal from the surface is beneficial to prevent flashback . RADIANT HEAT OUTPUT Because the major purpose of the present study was to explore flame stabilization and combustion emissions of the porous -combustion burner, radiant heat output was not directly measured. However, heat radiation from the burner radiating surface to the open environment can be wellestimated based on the measured surface temperatures and data of the surface emissivities provided by the material manufacturers. The calculated radiant heat flux is verified based on an overall heat balance of the firing system. Figure 6 shows the measured surface temperature, calculated radiant heat output versus air/fuel ratio at a firing rate of 264 MBtu/h-ft2 (833 kW/m2). Since the flame location in the porous bed is closer to the radiating surface at higher excess air operation, the surface temperature is almost a constant in the operating range, resulting in quite equal' radiant heat output, as shown in Figure 6. In this typical condi tion, radiant heat output as high as 80 MBtu/h- ft2 or 30% of the overall energy input was achieved. 3000 ~100 - ~ N +J ~ - Q ~80 ..c u.. - 0 0 0 -0 ":::-3 0 ~ +J en Q) - b- ---n - rr- - -A- ~60 ~ L- ~ :::3 +J +J a 2000- - :::3 L- - -- --- - FGT 0... Q) .-- - +J 0... - -40 :::3 E 0 Q) +J I- - FIRING INTENSI1Y: 264 MBtu/h.ft2 ... c a - ~20 ""0 0 RADIANT OUTPUT a Ct:: - fj. SURFACE TEMPERATURE I- 1000 , I , I , I , 1.0 1.2 1.4 1.6 1.8 Stoichiometric Ratio Figure 6. Effect of air/fuel ratio on radiant heat output. |