GPPS-2017-16 A Kinetics Study on the NOx Emissions of Axially Staged Combustion System for Gas Turbine Applications

Axially-staged combustor has great NOx-abatement potential for advanced gas turbines of high turbine inlet temperature in future. A chemical reactor network (CRN) model is established for the axially-staged combustor. The effects of important combustor parameters on NOx emissions under operating conditions of advanced gas turbine were simulated using the CRN model with CHEMKIN. Simulation results are first presented for perfectly mixed combustion system to study the effect of the first stage temperature, residence time split, and flow rate split on NOx emissions. It is found that staged combustion can significantly reduce NOx emissions compared with single stage combustion under lean premixed conditions. It is also shown that (a) the first stage temperature and residence time split have greater impact on NOx emissions than flow rate split and (b) NOx emissions can be reduced significantly by shortening residence time of high temperature stage or decreasing the first stage temperature. Then, the effect of unmixedness at the inlet of the second stage is introduced to model the axially-stage combustor. It is worth noting that the mixing between fuel and air which are injected into the second stage chamber has greater impact on NOx emissions than the mixing between the exhaust gas from upstream and the fresh-mixture jet of the second stage. INTRODUCTION Demands for higher-efficiency gas turbines are pushing the turbine inlet temperature (TIT) to material limit, which can result in significant increase in NOx emissions that conflict with the increasingly-strict environmental regulations. For example, the MHI J-class gas turbine has a NOx emission of 25 ppm at a TIT of 1600 oC, exceeding the emissions limits of most of the developed countries. The continued interest in advancing the efficiency of gas turbines will likely keep driving the increase in the TIT, and hence the increase in NOx emissions. Lieuwen et al. [1] predicted that at future gas turbine operating conditions, significant NOx emissions may be produced if conventional lean premixed combustion system is to be used. Therefore, it is necessary to develop advanced combustion technologies to meet the regulatory requirements. Staged combustion is a promising technology that can significantly reduce the NOx emissions while offering flexibility in load-variation of the combustor. Figure 1 illustrates the basic concept of an axially-staged combustor. The combustor consists of two combustion stages. The compressor discharge air is split into primarily two parts: the first part is fed into the first stage (stage 1), while the other part is fed into the second stage (stage 2). Fuel is also split into two parts, and fed into the two stages separately after mixing with the air of each stage. Staged combustion offers the flexibility of varying the combustor load by adjusting the fuel split into the two stages, while operating both of the stages at lean premixed combustion mode to meet emission standards. At low load,most (or all) of the fuel is fed into stage 1, while above a certain load more fuel is supplied to stage 2. Fuel Stage 1 Stage 2 Air Figure 1: Axially-staged combustor concept. Tinlet: the inlet temperature, Toutlet: the outlet temperature, : the residence time, Q: the mass flow. Therefore, staged combustion has gained interest for the research and development of future advanced gas turbines. Sattelmayer et al. [2] stated that the NOx reduction potential of staged combustion is low unless mixing between the stages takes place before the secondary combustion. Hayashi and Himada [3] proposed a two-stage combustor concept for achieving stable combustion with ultra-low NOx emissions for a wide range of operating conditions at atmospheric pressure. In their study, the air-flow of air in both the stages and the fuel-flow rate in stage 1 were fixed. Then, only the secondary fuel flow rate were changed to cover the range of fuel-air ratios utilized in a gas turbine. Hayashi et al. [4] extended their previous work and investigated the NOx