Extending low-NOx operating range of a lean premixed-prevaporized gas turbine combustor by reaction of secondary mixtures injected into primary stage burned gas

Reactions of lean to ultra-lean premixed mixtures injected into the hot burned gas that was produced in the lean-burn primary stage can be successfully used for extending the ultra-low NOx operating range of a gas turbine combustor. A tubular lean-lean two-stage model combustor for a simple cycle 200 kW gas turbine was designed based on this concept. Two secondary mixture injection tubes with a flow deflector at the exit, extending into the primary combustion zone, were used to inject lean premixed-prevaporized kerosene-air mixtures of various fuel concentrations into the volumes of hot burned gas produced by the lean-burn primary burners. The emissions and combustion characteristics were evaluated at 600 K inlet air temperature and 0.8 MPa pressure for two sizes of mixture injection tubes. With increasing the secondary fuel flow rate while maintaining the primary mixture equivalence ratio fixed, the NOx concentration remained at the level achieved for no secondary fuel (i.e., single stage combustion) for a while before it gradually increased. A still further increase in the secondary mixture equivalence ratio beyond the primary mixture equivalence ratio finally resulted in a steep increase in NOx emissions due to enhanced thermal NOx formation. It was shown that at proper air splits, high combustion efficiency was achieved while maintaining the NOx emissions in a single digit over a very wide range of overall equivalence ratios. It was found that combustion oscillation hardly occurred in the lean-lean two-stage combustion provided that the primary mixture was lean enough, though severe combustion oscillation occurred at greater overall equivalence ratios (typically > 0.8) in the single stage combustion. (c) 2004 Published by Elsevier Inc. on behalf of The Combustion Institute.