OCR Text |
Show - 5 - Variations in air/fuel mixing had a noticeable effect on the heat release profiles in the furnace, however, changes in overall furnace efficiency were small (3]. To ~xplain the observed trends, classical jet penetration theory was applied and is discussed in the context of the experiments below. Jet penetration In glass melting furnaces, the flames are typically unswirled jet flames with air/fuel mixing dominated by turbulent diffusion. In the specific case of an under port fired situation, the gas jet mixes with or penetrates through the combustion air flow to a degree dependent on the gas injection angle and velocity relative to the combustion air. If the gas jet penetrates through the combustion air flow, the residual gas burns in the oxygen lean external recirculation zone (ERZ). The residual gas, burning in this low oxygen region, would be expected to produce a thermo-chemical environment less conducive to high rate s of thermal NOx formation. The results show that the NOx emission s for the 20/20 burner are lower than the 20/12 or the 12/12 case. A phenomenological explanation for this is that the component of the gas velocity in the vertical direction is largest f or the 20/20 case and thus the gas jet has the highest momentum in this direction compared to the other cases. This would suggest t hat -the gas jet can more easily penetrate through the combustion air in this case . The effect of the gas injection angle on t he NOx e mi ssions is illustrated in Figure 4 for different inJection velocities and furnace operating conditions. From jet penetration theories and experiments [3, 6) in a cross flow, a penetration formula can be derived which is dependent on: nozzle diameter, gas veloci ty , air veloci ty, res pect~ ve injection angles and the densi ties of the jet and the surroundings; Lfl = 200 do 28 (1/V g )1/2 ( 1 ) r P s 1 -0.45 (V g )0.36 0.94 0.35 0.35 Yp=0.86 - J sin(oincl) cos(oincl) f (2 ) l P j (V a )0.85 Lfl is the flame length (m); Yp is the perpendicular d is tance that the air stream is penetrated by the gas stream (m) _ Y is not calculated for the entire gas flame length, rather a frac~ion of the flame length, f, has been used. This fraction, f, is set at the point of highest combustion rate (3, 5] in the flame and, therefore, highest potential for NOx formation For the calculations, a fraction f of 0.4 is selected as representative of this point. Thus, if Yp is less than 0.3 m, which is the estimated thickness of the combustion air flow from the air duct, a relatively higher NOx production would be expected. Calculations wi th (2) have shown that the baseline penetration depth was 0.25 m (20/12, 125 m/s, 1100 o C, 2% 02). Over the whole range of cases studied the penetration depths varied from 0.14 m for the 12/12, 75 mis, 1300 0 C configuration up to 0.5 m for the |