CARBON-MONOXIDE EMISSIONS IN LEAN PREMIXED COMBUSTION

Lean premixed combustion (LPC) is being used to meet NOx regulations on stationary gas-turbines and is also being considered for supersonic aircraft. For example, sub-10 ppm NOx levels (<1 g NOx/kg of fuel) have been demonstrated in stationary applications. Emission of carbon monoxide is one of the issues limiting the ultimate NOx reduction attainable by LPC. Carbon monoxide (CO) produced in the flame stabilization zone can survive the flame and must be oxidized in the turbulent postflame gas, introducing a much greater dependence on aerodynamics than is the case for NOx (in lean premixed flames). This article assesses three physical mechanisms that might quench CO oxidation reactions: 1) boundary-layer flow of nonequilibrium postflame gas over a cooled wall; 2) microscale mixing of nonequilibrium postflame gas with dilution jets; and 3) interaction between postflame CO burnout and the acoustic waves that can accompany premixed combustion. Analyses in this article suggest that in all three cases the CO will generally burn out, but also indicate circumstances that could lead to quenching.