Low NO x emission from radially stratified natural gas-air turbulent diffusion flames

Abstract

Research oriented to establish principles of design and operation of an ultra-low-NO x burner is discussed. Gradual admixing of the burner air with the centrally injected fuel is achieved by radial stratification of the flame. This radial fuel stratification is brought about by a combination of the swirling air flow and strong positive radial density gradients in the flame. A burner capable of varying the radial distributions of the burner airflow and recirculated flue gas, and of their swirl velocities has been used to realize the idea of radial fuel stratification in experimental flames using the flame tunnel of the MIT Combustion Research Facility. Distributions of gas velocities, temperatures and major chemical species concentrations determined in one megawatt (thermal) input natural gas-air turbulent diffusion flames show high fuel concentrations on the axis of the flames and demonstrate that fuel stratification is effective in reducing NO x emission without increasing trace emissions of combustibles. The effects of stratification are manifested in two ways: firstly, they improve combustion air staging by increasing fuel residence time in the pyrolysis flame zone, and secondly, permit increased amounts of flue gas to be recirculated through the burner without an impairment of the flame stability. The NO x emission at 3% O 2 reached with radial flame stratification but without flue gas recirculation was 70 ppm (56 ppm CO), and 15 ppm (<10 ppm CO) was achieved when 32% of the flue gas was recirculated through the burner.