MILD Combustion Chamber

The stringent regulations on exhaust emissions for gas turbines require the development of new combustion technologies in order to achieve low emissions combined with high combustion intensities.

Combustion with very high dilution or exhaust gas recirculation and simultaneous preheating can substantially reduce combustion peak temperatures and thereby NOx, while simultaneously showing very good stability behavior and very low noise and CO emissions.

Different technologies based on this general principle have been developed, such as Flameless Oxidation (FLOX) [1], High Temperature Air Combustion (HiTAC) [2], Colorless Distributed Combustion (CDC) [3] and moderate and intense low oxygen dilution combustion (MILD) [4]. These technologies have been successfully applied to furnace systems under atmospheric pressures in order to reduce NOx emissions [5]. However, this technology, here termed MILD combustion, is also an auspicious concept for reducing NOx and CO emissions in stationary gas turbines. The successful application of MILD combustion requires fast mixing of exhaust gas recirculation (EGR), fresh air and fuel in the combustion chamber before the combustion process begins. The recirculation of hot exhaust gas into fresh air and fuel stream increases the temperatures of the mixture over its auto ignition temperature and thereby causing a simultaneous ignition at different locations. Due to avoidance of small reaction zones and locally high peak temperatures, homogenous combustion results in very low NOx emissions. Additionally, the volumetric combustion zone is comparable to the model of a well-stirred reactor leading to very good flame stabilities and low CO emissions. Compared to furnace systems, the conditions for MILD combustion in gas turbine applications are different due to increased thermal intensities and the missing heat transfer resulting in higher exhaust gas recirculation (EGR) temperatures. The higher temperatures result in increased reactivity and to enable the MILD combustion mode, faster mixing has to be achieved. In this study, a new design of a MILD combustion chamber is presented based on a reverse flow configuration combined with concentric arrangement of air and fuel nozzle, Fig. 1.

For this chamber design, the pressure is firstly kept constant to 1 bar to set a reference point. Secondly, pressure is increased up to 5 bar. The results show that increasing pressure makes the low NOx-CO-emission point shift to a leaner part (lower adiabatic temperatures) and become thinner, Fig. 2. For deeper insight to the combustion process, OH*-chemiluminescence measurements as well as CFD-simulations are performed, [6].

As previously described, to achieve a flameless combustion regime, fast mixing of exhaust gas recirculation with fresh air and fuel in the combustion chamber is needed. In this work the active control of flameless oxidation combustion is investigated. The underlying idea remains in the improvement of mixing between fresh and burned gases using an inlet air pulsating unit. The pulsation should increase the entrainment of burned gases and then enhance the low emission process. For this purpose a setup for the pulsation of the air flow is introduced. It offers the possibility to actively control the frequency as well as the shape of pulsation. In order to gain deeper insight of the mixing and emission formation process, mixture fraction and velocity field in the cold flow are investigated. Tomographic images are shown in Fig.3. For equivalence ratios where NO_x emission are observed in standard non-pulsated conditions, results under atmospheric pressure show a drastic decreased of these emission when using the pulsating unit, Fig.4. CO emissions are maintained at a very low level so that flame extinction is not observed. For equivalence ratio of 0..57, where the biggest decrease in NOx emission is observed, OH-chemiluminescence images show a larger distributed flame over the combustion chamber when pulsation is activated compared to non-pulsated flow, confirming the hypothesis of mixing enhancing between fresh and burned gases.