NOx emissions in combustion of lean premixed mixtures injected into hot burned gas

A two-stage combustor concept for gas turbine applications is proposed to achieve ultralow NOx emissions and complete combustion over a wide range of operation. This combustor consists of a first stage that burns a lean premixed mixture of a fixed equivalence ratio, and a second stage where a lean mixture is injected into the hot burned gas from the first stage. The airflow rates to these two stages and the fuel flow rate to the first-stage are fixed, and only the secondary fuel flow rate is modulated to cover the range of fuel-air ratios required by the gas turbine. The first stage equivalence ratio is selected depending on the combustor inlet air temperature, so that the mixture can be burned to near completion. This concept is superior to single-stage lean premixed combustion because control of the fuel flow rate is much easier and more reliable than control of combustion air, which has variable geometry. The constant mixture flow rate to the first stage over a wide operating range is better for flame stabilization. To asses the potential for low NOx emission and complete combustion of the above-mentioned concept, NOx formation in the combustion of a premixed lean mixture injected into burned gas was investigated at atmospheric pressure and inlet air temperatures of 300 and 800 degreesC over a wide range of overall equivalence ratios. The effects of NOx emissions and combustion efficiency of the second-stage airflow rate relative to the first-stage air flow rate and equivalence ratios of mixtures relative to the first and second stages were investigated. Furthermore the effects of modes of mixture injections were studied. This was done by conducting die mixture injection from holes concentrated at an axial position and from holes distributed over a finite distance along the combustion chamber.