Controlling Emissions from Gas Turbines—the Importance of Chemical Kinetics and Turbulent Mixing

Abstract

An ideal combustor composed of a fuel-air mixing zone followed by a perfectly stirred reactor (PSR) and a series of plug-flow reactors is studied to determine emission levels of NO_x CO, unburned hydrocarbons and soot. It is concluded that these emissions can be reduced to negligible levels provided the stirred reactor is operated at high mass loading rates (Q = [mdot]/P²V) and mixing is perfectly controlled. Fuel and air must be premixed before entering the PSR and local φ must be less than 1.2. Perfect stirring in the PSR is necessary. A controlled amount of dilution air is instantaneously added at the PSR exit to quench NO formation but not CO and hydrocarbon oxidation. The turbulent mixing process required to obtain this ideal is discussed. Mixing times are related to the time scale for large scale eddy breakup (τ₁, ∼ l/u) which in turn can be related to the rate of viscous energy dissipation (∊∼u³/l). It is shown that to obtain PSR conditions, l must approach L_C , the characteristic chamber dimension, and u must approach U, the mean axial velocity. Under such conditions, the turbulent energy dissipation will be too large. Finally it is concluded that significant gains in emission control can be obtained by reducing l and increasing turbulent intensity.