An analytic model for the effects of nitrogen dilution and premixing characteristics on NOx formation in turbulent premixed hydrogen flames

Abstract

Advanced hydrogen gas turbine is a promising technology to achieve near-zero emission of carbon dioxide and higher cycle efficiency. With the increased firing temperature and pressure ratio, nitrogen reinjection combined with dry premixed combustion is promising to achieve the challenging low NOx emission. In this study, the effects of nitrogen dilution and fuel/air premixing characteristics on the flame characteristics and NOx emission are investigated first through simulating one-dimensional premixed flames with a 13-species and 39-reaction mechanism at the elevated engine operation conditions. The variation of flame thicknesses and laminar flame speeds with nitrogen dilution is investigated. The NOx formation is characterized by the flame-front NOx and the constant NOx formation rates in the post-flame region. It is shown that the flame-front NOx is an order of 1 ppm and does not change significantly (within 20%) with nitrogen dilution. In contrast, the NOx formation rates in the post-flame region decrease monotonically with nitrogen dilution due to the decrease of oxygen concentration. A detailed analysis of NOx formation reveals that the N2O pathway is significant and it can account for at least 20% of the NOx formation in the post-flame region. Then an analytic model considering both the extended Zeldovich mechanism and the N2O pathway is constructed by assuming the involved radicals being in chemical equilibrium. The model can be employed to efficiently estimate the NOx formation in fully premixed hydrogen gas turbines. Next, the effects of fuel/air premixing characteristics on the mean NOx formation rate in the post-flame region are quantified by reconstructing the PDF of mixture fraction. It is shown that without the nitrogen dilution, the NOx formation rate increases dramatically with fuel/air unmixedness due to the existence of local hot spots. Nitrogen dilution can dramatically reduce the NOx formation rate at the same level of unmixedness through reducing the local hot spots. Moreover, nitrogen dilution reduces the sensitivity of the NOx formation rate to fuel/air unmixedness, which greatly alleviates the mixing requirement for the premixing nozzles in gas turbines. Finally, a model for the estimation of NOx emission is constructed, which builds the connection between NOx emission, nitrogen dilution, unmixedness and flow residence time in combustors.