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Show AFRC - 1996 Int. Symposium Baltimore 1000 U 800 o ~ ~ 1",- "" ~'" ". , ", \..'.,.. . , Dd\, , '\, .. ~I~ ~ ~_Io. _ .. 21 41 fue l fract i on ("wet") Stable combustion ...... ..., ..... "', '" ""'''''' ..... • 61 (m3fm3) September 30 - October 2 1996 Page 6 of 14 "', ' . 81 101 • 02 = 121 Q 02 = 111 • 02 = 161 • 02 = 191 -"-calculated (Su = 0 .3 m/s) I Figure 2: boundary of stability of combustion: Experiments: mixture velocity = 1.1 mIse Modeling: laminar flame speed (Su) = 0.30 mIse The experimental data suggest that, within the range considered, the effect of the initial oxygen concentration on the flame stability is relatively small. This was confirmed by model calculations on the laminar flame velocity of (hot) methane/air/flue-gas mixtures at air factors between 1.1 and 1.3. Figure 3 (next page) is basically the same figure as Figure 2, but presents as well the adiabatic flame temperature, as calculated using an enthalpy balance. The figure indicates that, maintaining a stable flame, significantly lower flame temperatures may be achieved when a "hot" gas/air/flue-gas mixture is burnt. It is of practical interest to note that a significant reduction in flame temperature may be achieved by a limited increase in mixture temperature. For instance, in the experimental facility used, it is possible to bum a gas/air mixture at room temperature at a flame temperature of approximately 1650 °C. In contrast, a mixture with an initial temperature of 300 °c may be burnt at a flame temperature of approximately 1420 °C. The NOx production (g/GJ) of a natural gas flame decreases by approximately a factor 2 when the flame temperature decreases by 100°C. Thus, this observation indicates that a significant NOx reduction could be obtained by the combustion of mixtures at an elevated temperature. Although this seems counter-intuitive, one must keep in mind that, to maintain a stable flame, two parameters are varied at the same time. Flue-gases are being added to lower the |
