XXXIV Meeting of the Italian Section of the Combustion Institute 1 AIR STAGING IN A SWIRLED NATURAL GAS BURNER WITH RADIAL INJECTION F. Cozzi, A. Coghe aldo.coghe@polimi.it Department of Energy, Politecnico di Milano, Milano, Italy Abstract The present work describes the experimental investigation on NO x emissions of partially premixed flames characterised by air staging through radial injection of premixed jets into a co-flow swirling air. NO x emissions were found to increase with increasing partial premixing and reach a maximum value at Φ p ≈ 5, followed by a dramatic decrease as Φ p approaches values ≈ 3. In the most favourable conditions, single digit NO x levels were reached without penalizing combustion performances or increasing CO emissions. The effects of staging on the amount of NO 2 found at the exhaust were also examined and the results corroborate the idea that NO 2 is formed in the turbulent mixing region, favoured by the increasing cooling rate of the combustion gas by entrainment of cold unburned gas. PIV measurements of the isothermal flow field are used to confirm the analysis and to better understand how the emissions are related to the turbulent mixing of the premixed first stage with the secondary swirling air. Introduction Staged combustion has been suggested as an effective way to reduce nitrogen oxides in gas turbine combustors, particularly when burning low calorific value (LCV) gas derived from agricultural wastes [1-7] or coal-derived gas from integrated gasification combined cycle plant. Staged combustion can achieve lower NO x emissions by staging the injection of either air or fuel in the near burner region. Mixture formation can be improved by burning in a swirling flow and employing the recirculation region to control fuel-air mixing and promote flame stabilization [8-10]. The primary objective of our research project is to investigate air staging obtained through radial injection of premixed jets into a cross-flow swirling air. Staged combustion has been already explored by testing two different methods of fuel injection [11] and the results indicate that appreciable benefits of air staged combustion in swirling flames can be obtained by transverse injection of the premixed first stage in the secondary swirling cross-flow. Experimental set-up and procedures The pilot-scale air-staged swirl burner used in the present investigation has been already described [11]. It is composed of two concentric pipes separately supplied by the fuel mixture and a surrounding swirling co-flow of air. The fuel is supplied along the inner pipe (L/d ≈ 35, 8 mm i.d. and 15 mm o.d.) into an injector which