REDUCTION OF NOx EMISSION BY AIR STAGING IN A SWIRLED NATURAL GAS BURNER

The present work describes the experimental investigation on NOx emissions of partially premixed flames characterised by air staging through radial injection of premixed jets into a annular cross-flow of swirling air. The influence of the level of premix has been studied by monitoring the NOx and CO emissions and by measuring the isothermal flow pattern by particle image velocimetry (PIV). Three main parameters were varied independently: the fuel flow rate (thermal power), the overall equivalence ratio Φg and the swirl strength S. NOx emissions increase with increasing partial premixing and reach a maximum value at a premix equivalence ratio Φ p ≈ 5, followed by a dramatic decrease as Φ p approaches values ≈ 3. The benefits of air staging are more significant at medium swirl intensities (S ≈ 1) and moderately lean global conditions (Φ g ≥ 0.8) since otherwise flame blow off occurs. In the most favourable conditions, single digit NO x levels were reached without penalizing combustion performances or increasing CO emissions. Such conditions may be conveniently imposed by adjusting the swirl strength, the equivalence ratio of the premixed jets and the global equivalence ratio. A preliminary analysis, based on a characteristic mixing length deduced for isothermal jets discharging into uniform cross-flow, has been used to better understand the results and their implications. PIV measurements of the isothermal flow field were used to confirm such analysis and to correlate emissions with the turbulent mixing of the premixed first stage with the secondary swirling air. PIV maps have been compared with the corresponding flame photographs and the structure and dynamics of the different combustion regimes have been qualitatively interpreted and correlated with the emission levels.