Emissions Characteristics of Liquid-Fueled Pilot Stabilized Lean Premixed Flames in a Tubular Premixer-Combustor

Abstract

Global emissions of NOx in a liquid-fueled lean-premixed tubular combustor with a tubular premixer operating under atmospheric pressure are studied experimentally. The effects of equivalence ratio, premixer length, residence time, fuel type, and fuel atomization and dispersion characteristics on NOx emissions are studied. Measurements of exhaust species concentrations are used as the primary indicator of the effectiveness of premixing-prevaporization upstream of the combustor. Qualitative levels of prevaporization-premixing are determined from Mie-scattering signals measured at the exit of the premixer. Emission measurements show that the equivalence ratio is the dominant operating parameter, with premixing length and residence time being less significant within the present operating range. Ultra low NOx operation (<10 ppmv @ 15 percent 02 ) is feasible for equivalence ratios less than 0.5. More significantly, small drops persist beyond the premixer even for very long premixers, and Mie-scattering measurements show considerable spatial inhomogeneity, while allowing ultralow NOx operation. One-dimensional evaporation calculations for single drop trajectories confirm that complete evaporation for typical drop size distributions is not possible with reasonable premixer lengths under atmospheric pressure. Fuel dispersion is found to be the most critical parameter for high combustion efficiency, and adverse effects of poor fuel dispersion cannot be overcome by using longer premixers.