Fuel Variability Effects on Turbulent, Lean Premixed Flames at High Pressures

Fuel flexibility will be a key issue for the operation of future stationary gas turbines because of the increasing amount of off-spec natural gas qualities from new resources and upcoming new fuels derived from biomass which will be more important in the near future. The performance of gas turbines in terms of flame stability and low emission combustion must be at least maintained also with these new fuels. Therefore, the impact of fuel variation on combustion characteristics must be known for the combustor design. This paper addresses the effect of hydrogen and propane addition on flame characteristics like lean blowout (LBO), emissions (NO_x, CO), flame positions and turbulent flame speeds for flames at gas turbine relevant conditions. Hydrogen enriched fuels are typical constituents of gasification fuels such as those obtained from biomass, while propane is considered a typical higher hydrocarbon present in off-spec natural gas. Turbulent, lean premixed flames of different fuels (methane, methane/hydrogen and methane/propane) have been studied in a generic, axis-symmetric, high-pressure gas turbine combustor. Flame stabilization is achieved aerodynamically via a recirculation zone induced by the combustor geometry with sudden expansion. Turbulence at the combustor inlet is generated using a turbulence grid (perforated plate). LBO limits are detected using the global OH chemiluminescence flame signal collected with a photo-multiplier and a data acquisition system together with the exhaust gas temperature measured with a thermocouple. The species concentrations (CO₂, O₂, CO, NO_x) are measured by exhaust gas analyzers. Flame front positions and turbulent flame speeds are determined with Laser Induced Fluorescence measurements of the OH radical (OH-PLIF). Flame characteristics will be presented for the following fuel/air mixtures at a mixture preheating temperature of 673 K: pure methane, H₂-enriched flames containing up to 50% hydrogen by volume, methane/propane mixtures containing up to 50% propane by volume. LBO limits, NO_x emissions will be presented for different pressures. Most probable flame front positions and turbulent flame speeds are presented at a pressure of 5 bars for fuel mixtures between pure methane and 50% of each additive (propane and hydrogen). Experiments have revealed that a premixed mixture of 50% hydrogen and 50% methane, by volume, can significantly extend the lean blowout limit by up to 22% compared to pure methane. Because of a 120 K lower flame temperature a drastic reduction of the NO_x emission (about 57%) is observed. Addition of hydrogen also significantly decreases the flame position (50%), changes the shape of the flame front and because of a higher global reaction rate increases the turbulent flame speed. Experiments with different methane/propane mixtures showed an increase (approximately 25–30%) of the NO_x concentration at a propane content of 50%. Additionally, the flame stabilizes closer to the combustor inlet for higher propane contents.