Investigation Of The Mechanisms And Characteristics Of MILD Combustion For Gas Turbine Application

Moderate or Intense Low-oxygen Dilution (MILD) combustion, characterized by low pollutant emissions, enhanced combustion stability, improved pattern factor and high fuel flexibility, is a suitable choice for the future gas turbine combustion technologies. This paper aims to study the thermodynamics, chemical kinetics and fluid mechanics of MILD combustion under gas turbine relevant conditions and to evaluate the combustion behavior in a MILD combustor. The main work is drawn as follows:1. Thermodynamics and chemical kineticsA chemical reactor network model was established, and the model was verified by the the experimental results. The effects of exhaust gas temperature, air preheat, pressure and fuel type on the thermodynamics of MILD combustion were probed into. The critical gas recirculation ratio needed for the realization of MILD combustion were obtained for the B, E+and F class gas turbine operating conditions. The effect of gas recirculation ratio on the ignition delay time of MILD combustion was also examined. The reaction rates of MILD combustion and traditional diffusion flame were compared. It is demonstrated that elevated gas recirculation ratio benefits the increase of MILD mixture tempearture, the decrease of temperatrure increment during the combustion process and the drop of reaction rate, however, resulting in the suppression of ignition delay time. It is thus proposed that the gas recirculation ratio should be controlled and the flame should be lifted from the burner in the case of the thermodynamics of MILD combustion is fulfilled.2. Fluid mechanicsAn axially staged MILD combustor was built to study the effects of mixing approach and gas recirculation ratio on the MILD combustion of CH4and to evaluate the effect of fuel injection velocity on the MILD combustion of10MJ/Nm3syngas. The results acquired from CH4MILD combustion reveales that the secondary air and fuel mixing with the hot flue gas from the gas generation zone separately before air/fuel direct interaction promotes the establishment of MILD scheme. Increased gas recirculation ratio causes the delayed air/fuel interaction, resulting in the decrease of maximum OH*intensity, the widespread of OH*distribution and the suppression of NO production from the MILD combustion zone. However, excessively high gas recirculation ratio vitiates combustion stability. For the10MJ/Nm3syngas MILD combustion, it is reflected that increased secondary fuel injection velocity favors the rapid mixing between the secondary fuel and the high temperature and low oxygen concentration oxidizer, resulting in the increase of flame lift-off distance, the spatially distribution of reaction zone and the elimination of NOX production. However, extremely high secondary fuel injection velocity causes the growth of pressure drop and CO emissions. The secondary fuel injection velocity limited to199-299m/s facilitates the establishment of MILD scheme.3. Design of MILD combustorThe non-reactive and reactive simulations were performed to investigate the effecs of combustor length-to-width ratio, burner arrangement and air-to-fuel-momentum flux ratio on the mixing behavior and combustion performance. It is shown that the decay of air injection is mainly affected by the combustor length-to-width ratio and, the decay length of air injection is almost the same as combustor length at combustor length-to-width ratio of1.1, which is the best choice for the organization of flow field in the full use of limited combustor volume. The fuel injectors positioned away from the combustor axis is beneficial for the complete oxidation of fuel species. Increased distance between air and fuel injectors can postpone the confluence of air and fuel stream. However, in consideration of the complete combustion, the centerline of air injector should be somewhat offset from the combustor axis. Decreased air-to-fuel-momentum flux ratio benefits the delay of air/fuel confluence, the distribution of reaction zone and reduction of reaction temperature and CO emissions. In general, lower air-to-fuel-momentum flux ratio facilitates the realization of MILD combustion.4. Combustion performance of MILD combustorExperiments were conducted on the MILD combustor to study the effecs of air-to-fuel-momentum flux ratio, equivalence ratio, air preheat and fuel type on the MILD combustion of syngas. It is revealed that lower air-to-fuel-momentum flux ratio causes the downstream movement of main reaction zone and the suppression of CO emissions. On the other hand, increased euiqvalence ratio benefits the delay of ignition, the reduction of reaction temperature and the growth of reaction zone volume. The MILD scheme can be established for syngas under lean operating conditions. In addition, air preheat promotes the rise of NOx production and mitigation of CO generation. The MILD combustion can be achived for syngas even under air preheating condition since the thermodynamics, chemical kinetics and fluid mechanics of MILD combustion was fulfilled. In the application of MILD combutor to syngas fuels with various calorific values, it is observed that the increased fuel thermal input causes the increase of reaction zone volume whilst the NOx emissions maintaines at a low level. Basically, the MILD combustor is fuel flexible.