| OCR Text |
Show The most important result that shows in the above figures is that the peak gas temperature location is stabilized in the space between the fiber mat and metal screen. The peak gas temperature, which generally can be identified as the intensive combustion reaction zone, occupied just a narrow space with a temperature of 2500°F and more. Upstream, the temperature was reduced to approximately 2000°F at the fiber mat, and downstream, the temperature at the location of the metal screen is less than 1800°F, at all desired operating ranges. This indicates that the design has succeeded in increasing the burner operation temperature 400°F more than the metal temperature limitation. Through a wide range of firing rates and excess air levels, the natural gas/air mixture forms a stable combustion reaction with a uniform radiation surface from the tested burner. The only limitation that affects the burner operation is the temperature restriction of the metal alloy. Even though the peak chemical reaction zone proved to be controlled between the fiber mat and the metal screen, by increasing the firing rate, the combustion intensified. The temperature of combustion gas, the fiber mat and metal screen will increase gradually. It was found that when the burner operates over 200 KBtu/hr/ft2 with the excess air rate of 10%, the fiber mat will be over the temperature restriction of 2100°F, but this difficulty can be overcome by increasing the diluted air level. It w a s found the burner can be operated with radiant mode over a very wide range of excess air rate. Figure 7 shows the flame stability as the function of firing rate and excess air rate. O n increasing the excess air rate, an unstable blue flame appeared and a portion of the flame lifted up from the fiber mat surface. As a result of this, the radiant surface starts to lose its intensity. The graphic in Figure 7 shows just this criteria. Further increasing the excess air rate will result in the blue flame moving to the top of the metal screen, caused the burner to become non-radiant, and the flame tends to lift off the burner. However, this limitation is far from the desired operating zone. There is no difficulty in stabilizing the flame and operating the burner with high intensity radiant surface when increasing the excess air rate up to 6 0 %. Two major pollutant emissions, CO and NOx, were measured during the burner evaluation. Figures 8 and 9 show the measured C O and N O x emission, respectively, as the function of firing rate for three excess air rates. The results indicate that the concentrations of C O and N O x emission are strongly dependent of the excess air rates, but are affected less by the firing rate for relative lean combustion. On increasing the excess air rate with constant firing rate, both N O x and C O emissions are reduced. When the firing rate increases, the C O emission is reduced while N O x is increased. This is because, at the relative lean equivalence ratio, the combustion reaction tends to be more complete, thus reducing the C O emission. Upon reducing the equivalence ratio by increasing the excess air rate, there will be more diluted air to quench the flame temperature, which narrows the high temperature chemical reaction zone. A narrow high temperature zone will physically provide shorter resistance time for the N O x formation. W h e n increasing the firing rate, the combustion reaction 7 IV-25 |
