Effects of Ambient Conditions and Fuel Composition on Combustion Stability

Recent regulations on NOx emissions are promoting the use of lean premix (LPM) combustion for industrial gas turbines. LPM combustors avoid locally stoichiometric combustion by premixing fuel and air upstream of the reaction region, thereby eliminating the high temperatures that produce thermal NOx. Unfortunately, this style of combustor is prone to combustion oscillation. Significant pressure fluctuations can occur when variations in heat release periodically couple to acoustic modes in the combustion chamber. These oscillations must be controlled because resulting vibration can shorten the life of engine hardware. Laboratory and engine field testing have shown that instability regimes can vary with environmental conditions. These observations prompted this study of the effects of ambient conditions and fuel composition on combustion stability. Tests are conducted on a subscale combustor burning natural gas, propane, and some hydrogen/hydrocarbon mixtures. A premix, swirl-stabilized fuel nozzle typical of industrial gas turbines is used. Experimental and numerical results describe how stability regions may shift as inlet air temperature, humidity, and fuel composition are altered. Results appear to indicate that shifting instability regimes are primarily caused by changes in reaction rate.