Finite-Rate Entrainment Effects On Nitrogen Oxide (Nox) Emissions In Staged Combustors

Axially staged combustors offer the potential for achieving low NOx emissions from gas turbines at the elevated temperatures required to achieve at least 65% combined cycle efficiencies. While there is evidence that premixing between the main burner products and secondary stream reduces NOx, it is important to characterize the specific requirements needed to achieve low emissions at gas turbine conditions. This study examines the sensitivity of NOx to finite-rate, large-scale entrainment of the main and secondary streams under the simplifying assumption of infinitely-fast small-scale mixing. Essentially, this allows us to isolate the effects of limited large-scale entrainment rates by using a homogeneous/uniform condition for the entrained and reacting gases. We use a reduced-order reactor network model to examine a generic staged-combustor whose inputs are physical time scales that embody the finite-rate entrainment characteristics. With simulations conducted over a large parameter space at typical operating conditions (25 atm, 650 K air), the results show that, even when small-scale mixing is infinitely fast and the reaction zone is uniform, entrainment significantly affects NOx emissions due to its influence on the equivalence ratio - and thus the temperature and time - at which the entrained mixture ignites. Fuel-air staging is shown to be vital to NOx reduction in most practical cases where entrainment times exceed roughly 1 ms and the secondary stream finishes entraining before the main burner products. A constrained NOx minimization indicates that using the leanest possible main burner and re-routing as much air as possible to the secondary stage is key to low NOx with finite-rate entrainment; for example, less than 10 ppm NOx can be achieved if the secondary stage contains 20% of the total mass flow. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.