Combustion of lean prevaporized fuel–air mixtures mixed with hot burned gas for low-NO x emissions over an extended range of fuel–air ratios

Abstract

Reaction of lean to ultra-lean mixtures supported by high-temperature burned gas can resolve the dilemma between complete combustion versus ultra-low NO x emissions in lean premixed gas turbine combustors. The combustion characteristics and NO x emissions in “lean–lean” two-stage combustion were investigated for premixed–prevaporized kerosene–air mixtures using a co-axial flow configuration. Secondary prevaporized kerosene–air mixtures of lean to ultra-lean compositions were injected into the stream of hot burned gas prepared by the combustion of lean prevaporized kerosene–air mixtures stabilized on an annular perforated flame holder in the primary stage. The progress of mixing and reactions of the secondary mixture jets of prevaporized kerosene–air mixture injected into the co-axial primary hot burned gas flow were investigated by spatial gas sampling and direct photography. The effects of the ratio of secondary to primary air flow rates and equivalence ratios of the primary and secondary mixtures on the emissions from the two-stage combustor were studied. Imparting swirl to the secondary mixture jets resulted in an enhancement of the mixing of the jets with the primary burned gas. It was also shown that the use of reaction of lean to ultra-lean secondary mixtures supported by the hot burned gas from the primary stage is much advantageous in extending the operating range of ultra-low NO x emissions of the 10 ppm level and complete combustion as compared with other approaches such as fuel staging and variable geometry. The proposed ultra-low NO x combustion concept has a potential of suppressing combustion instabilities that is often experienced with lean premixed combustion.