A Low Emissions, Lean Premixed Burner

A novel method of aerodynamically stabilizing lean premixed natural gas flames on a conventional burner was investigated. Flames were anchored in a wake of hot combustion products downstream of a small ring placed in the exit plane of a conventional burner. This approach to bluff body stabilization significantly increased the overall stability regime of the burner, permitting stable lean premixed combustion down to equivalence ratios of 0.55. The most influential parameter affecting stability of a ring stabilized flame was determined to be the gap size between the outer edge of the ring and the inner edge of the burner. As the gap size was increased from 0.8 mm to 3.2 mm, stability improved. The lower flame temperatures associated with lean premixed combustion resulted in substantially reduced emissions, particularly NOx emissions which were virtually eliminated. CO emissions were reduced at equivalence ratios down to 0.75 due primarily to the excess air available for combustion. At equivalence ratios below 0.75 temperatures in the exhaust gas dropped to the point where conversion of CO to CO2 was inhibited and CO concentrations increased. Unburned hydrocarbons increased at very low equivalence ratios as the flames became more upright and reactant leakage through the gap was more apparent. Although increasing the gap size improved flame stability, emission of unburned hydrocarbons also increased, thus an optimum configuration will exist for each set of operating constraints. Simultaneous measurements of NOx, CO, and hydrocarbon emissions suggested that a 0.8 mm gap size operating at an equivalence ratio near 0.8 was optimum. At this optimum, measured pollutant emissions met and surpassed the most strict standards in North America.