Influence of Water Injection on Heat Release Distribution, Lean Blowout and Emissions of a Premixed Swirl Flame in a Tubular Combustor

Abstract

Water injection is often used to control NOx emissions and/or to increase the power output from non-premixed combustion of gaseous and liquid fuels to increase the power output. In non-premixed flames the drop of NOx generation due to water injection is associated with a strong effect on the heat release distribution, which often leads to a loss of combustion stability, if the amount of injected water exceeds a water to fuel ratio in the order of unity. Since premixed combustion dominates in large gas turbines for power generation fueled with natural gas, water injection for emission reduction is usually not an issue. However, the increasing share of fluctuating electricity from renewables stimulates R&D on the improvement of the operational flexibility of combined cycle power plants. One aspect in that context is power augmentation by injection of liquid water in premixed combustors without drawbacks regarding emissions and flame stability. An atmospheric single burner combustor test rig has been set up at the Lehrstuhl für Thermodynamik at Technische Universität München in order to investigate the effect on gas turbine combustion. For all investigations presented in the paper natural gas has been used as fuel. The test rig was operated at different preheating temperatures (T=573 … 673K) and different equivalent ratios (Φ=0.4 … 1.0). In contrast to approaches where steam is injected into the combustion chamber liquid water was used. Water injection has been realized using two-fluid nozzles with air as atomizing gas. The influence of varying amounts of water and atomizing air on blowout limits, flame shape and emissions was investigated by emission and OH*-chemiluminescence measurements. The results of the experimental study will be presented and they will be compared to experimental data of gas turbine combustors with steam injection taken from literature. At constant equivalence ratios a significant reduction of the NOx emissions was found for increasing amounts of injected water, whereas for measurements at constant adiabatic flame temperature reduction in NOx emissions was very small. In comparison to literature data for steam injected lean premixed flames a smaller operational window of the investigated configuration was detected. The effect of liquid water injection on the flame shape is similar compared to the effect of steam injection found in literature.