Today, more than ever before, the necessity of producing low emissions gas turbines is of paramount importance. Reducing the emission of Nitrogen Oxides in particular has been of primary focus. In this vein, there has been a continuing trend towards a higher degree of fuel-air premixing prior to combustion for the sake of reducing NOx emission. The various methods that have been developed for this purpose are not without their flaws however. Some may decrease NOx while increasing CO and UHC emissions. Some may decrease NOx while sacrificing thermal efficiency. As such there is still a search for a technology that can combine the strengths of all the methods without any of their weaknesses. Flameless oxidation is a technology that has been developed during the last two decades and shows promise for being an alternative method of achieving low NOx emissions. Thus far, it has been successfully applied to industrial furnaces. The patented combustion principle of flameless oxidation (FLOX®), developed by WS Wärmeprozesstechnik is based on high internal flue gas recirculation which leads to a dilution of the combustion zone. The high recirculation ratios are achieved by the momentum of the air and fuel jets entering the combustion chamber.
The work contained herein is twofold; in the first part atmospheric experiments are conducted on a cylindrical, lean, non-premixed combustor. Various combustor design parameters such as fuel-air injection configuration, oxidizer pre-heating, and steam injection are investigated with respect to flame temperature, volume and emission formation. Based on this work, a novel combustor design is developed, which is capable of burning methane as a stand-alone combustor, or part of a sequential system that contains water as an inlet constituent.
In the second part of the work, the combustion chamber is modeled as a chemical reactor network in order to select a reduced chemical mechanism for numerical calculations. The physical chamber was meshed and CFD simulations were performed under matching boundary conditions to that which experimental measurements were taken using the selected reduced mechanism. CFD results for a basic case are validated using the experimental results and show excellent agreement with the NOx, and temperature measurements. The CFD is used to gain further insight into the effect of water injection on methane combustion. It is shown that steam effectively reduces the NOx production by dilution of the flame as well as by a chemical route wherein the concentration of the OH radical pool changes and affects the chemistry of the NOx formation. It has been demonstrated that while pre-heating does indeed enhance mixing inside the chamber and thus promotes flameless combustion, it also increases NOx production and at sufficient levels of H2O dilution, preheating is unnecessary. A combination of preheating and H2O vitiate allows for predictions of ultra-low pollutant levels and smooth temperature profiles at lower recirculation values prescribed by other authors. A novel combustor design and injector configuration has been designed that predicts characteristic emissions and properties of flameless combustion.
