Fuel/air mixing and nitrogen oxide formation in a lean premixed gas turbine combustor

Lean premixed combustion of natural gas is proving to be a viable and attractive concept for dry NO_x control in gas turbine systems. These combustors have the potential of meeting sub-10 ppm NO_x emissions levels without after-treatment of the combustion products. The lean premixed combustor can also simultaneously produce low CO and UHC emission levels.; A combustion facility capable of simulating some critical aspects of industrial lean premixed gas turbines was designed and constructed for this research. A dual-annular counter-rotating swirler (DACRS) was used to mix gaseous fuel and combustion air immediately upstream of a sudden-expansion combustion chamber. The facility was designed and constructed with an optically accessible combustion tunnel for laser diagnostic measurements of the fuel/air mixing and combustion processes.; A phase-Doppler particle analyzer was used to measure the axial and swirl velocity components of the fresh reactants near the exit of the DACRS. Planar laser-induced fluorescence was used to acquire spatially and temporally resolved quantitative images of the fuel distribution downstream of the premixer under non-firing conditions. Temporal unmixedness was found to generate as much as 92% of the variation in the local equivalence ratio. The best mixing was achieved at the state combining the highest inlet pressure, highest combustion air flow rate, and lowest equivalence ratio. Of these parameters, the combustion air flow rate was shown to be the most influential in affecting the fuel/air premixing.; Engine-out emissions concentrations at the leanest operating conditions showed simultaneous NO_x, CO and UHC all at sub-10 ppm levels. The combustion air flow rate and combustor inlet pressure both had little impact on the amount of NO_x generated.; A chemical reactor design tool model was developed to predict engine-out NO_x concentrations for lean premixed combustors. The CRM model only requires information regarding the reactor loading, the combustor inlet state, and the combustor geometry. Model improvements were obtained by including the effects of reactant unmixedness based on the fuel/air distribution measurements recorded during the non-reacting mixing studies.