Modeling of Minimum NOx in Staged-Combustion Architectures at Elevated Temperatures

Abstract

Current combined cycle plants can achieve cycle efficiencies of up to 62%. Further increases in cycle efficiency are achievable through higher turbine inlet temperatures. Such high temperatures pose a NOx-abatement limitation with current combustor architectures due to increased thermal NO formation. Staged combustion shows the potential for not only NOx-abatement at high firing temperatures, but also for enhanced turn-down performance. Staged combustion devices have been explored in the literature previously, as well as implemented commercially. The objective of this work is to identify the theoretical minimum NOx levels attainable with fuel-staging for a given firing temperature and combustor residence time, at the same time identifying the corresponding optimum staged combustor configurations. In other words, its objective is to understand, for an idealized case, the fundamental physical limits of this approach. A reactor model of an axially-staged combustor is incorporated in ANSYS Chemkin. The main burner is modeled as a conventional lean-premixed DLN-type combustor using a laminar flame calculation, while the axial fuel staging calculation for the minimum NOx is modeled as a perfect mixer followed by a constant-pressure batch reactor of fixed residence time. Optimum NOx levels are found by constraining CO emissions to a fixed percentage above equilibrium levels. Parametric studies are performed for a range of firing temperatures to find the optimum main combustor equivalence ratio (or fuel staging ratio) and secondary stage residence time, while fuel and air preheat temperatures and combustor pressure remain fixed. With the assumption of instantaneous mixing of secondary fuel and main combustor products, staging provides drastically reduced minimum NOx levels (∼ 1 ppm corrected to 15% excess air at 1975 K firing temperature) compared to conventional premixed architectures while providing significantly enhanced part-load CO performance. Moreover, the minimum theoretical NOx is relatively insensitive to firing temperature and overall combustor residence time over a practical range of values.