GT2009-59409 COMBUSTION STABILITY AND EMISSIONS IN A LEAN PREMIXED INDUSTRIAL GAS TURBINE BURNER DUE TO CHANGES IN THE FUEL PROFILE

ABSTRACT This paper deals with an experimental investigation of dry low emission (DLE) burners for industrial gas turbines. Changes in the fuel profile, pressure drop over the burner and external pilot flame stabilization have been investigated regarding combustion stability and emissions. This has been achieved by parallel experimental work in a water rig and a newly commissioned atmospheric combustion test rig. Some verifying tests in a high pressure rig were also conducted. The work in the water rig has been directed towards evaluating different fuel profiles at the burner exit due to changes in the fuel outlet geometry. Variations of the fuel outlet geometry were achieved by altering the effective area of the hardware configuration of the fuel outlet ports or by moving or adding fuel outlet ports. A few of the tested configurations in the water rig was chosen for further evaluation by atmospheric combustion tests with respect to combustion stability and emissions. A more general study on combustion stability and emissions was also performed for different burners, burner configurations and variations in pressure drop over the burner. The pressure drop over the burner in the test corresponds very well to the pressure drop measured over a single burner in an annular combustion chamber of an industrial gas turbine at different loads. The combustion was monitored by a high speed video camera equipped with an image intensifier. Simultaneously the dynamic pressure was measured by a piezoelectric pressure transducer, making it possible to know when each image was taken relative to the pressure. Results for different hardware configurations will be shown considering the frequency response from the flame and the dynamic pressure as well as the characteristic combustion instability close to lean blowout. INTRODUCTION Considerable effort has been spent during the last few decades to develop lean premixed combustors for industrial gas turbines and aero turbines. The aim of these designs is to offer low pollutant and high efficiency combustion processes. Combustion instabilities can be a major problem in the design of high performance combustion systems and can occur in combustion systems over a broad size range. Combustion instabilities have been observed in propulsion systems, power plants and in different industrial processes. A significant challenge in the design and operation of modern industrial, commercial gas turbines is to eliminate or prevent such combustion instabilities. Combustion-driven oscillations can disrupt the combustion air supply which may result in smoke and other pollutant formation and/or lower efficiency of the combustion. The oscillations can produce large-amplitude flexing of structural elements with consequent early fatigue failure. Locally increased heat transfer due to oscillations may cause a higher rate of material deterioration. In many cases the combustion process is unable to sustain large combustion instabilities resulting in a partial or total flame blow-off. In a gas turbine with an annular combustion chamber a very low frequency oscillation can appear close to the lean blowout (LBO) limit for one or a number of the burners. At LBO, burners operate in a very unstable mode at the same time other burners may operate at higher equivalence ratio causing large fluctuations in the turbine inlet temperature, possibly poor combustion and higher NOx. Spontaneous oscillations in a combustion system can occur due to a number of reasons, e.g. acoustic instabilities, shock instabilities, fluid dynamic instabilities within chambers, and system instabilitie