Effect of Lean Primary-Zone Operation on Emissions and Stability of Non-Premixed Combustors

Lean operation in the primary combustion zone of a gas turbine combustor is advantageous from NOx reduction point of view. The present work deals with the influence of lean primary zone operation on combustion performance, NOx emissions, and flame stability of a model gas turbine combustor with simplex atomizer. Air distribution is varied to operate the primary combustion zone from stoichiometric to leaner operating conditions (phi(pri)=1.04-0.61), at fixed fuel flow rate and overall air-fuel ratio. Combustor performance is quantified with the help of gas temperature profile at exit and species concentrations at the end of primary zone. It is found that a primary zone equivalence ratio of 0.79 is optimal for high combustor efficiency and low NOx emissions, with a stable flame. When the primary zone equivalence ratio approaches the lean blowout (LBO) limit (phi(pri)similar to 0.5), intermittent low-frequency, high-amplitude pressure oscillations arise, especially at higher primary zone air flow rates. Inclusion of a metallic ring in the primary combustion zone widens the equivalence ratio for flame stability, with a small reduction in combustion efficiency. It also reduces the noise level and suppresses the intermittent high-amplitude oscillations close to LBO.

Automated summary

The study found that an equivalence ratio of 0.79 in the primary combustion zone of a gas turbine combustor is optimal for high efficiency and low NOx emissions while maintaining flame stability. However, approaching the lean blowout limit can lead to intermittent low-frequency, high-amplitude pressure oscillations.