Stringent environmental standards have motivated combustion engineers to develop combustion techniques for achieving low levels of pollutant emissions from gas turbine combustors. One example is the lean premixed (LP) combustion system, where the fuel and air are premixed upstream of the combustor to avoid the formation of stoichiometric regions. The combustion zone is operated with excess air to reduce the flame temperature; consequently, thermal NOx is virtually eliminated [1]. Another example is the RQL concept. In this system the combustion is started in a fuel-rich zone (minimizing NOx formation due to the lack of availability of oxygen as well as low temperature) followed by rapid cooling because of air jets and a lean burning zone which further oxidizes the
remaining UHC and CO [2]. There are some improvements in LP and RQL original concepts to produce combustion devices with extremely low NOx emissions. For example: Lean Pre-mixed Burner with Spatially Periodic Recirculation of Combustion Products [3]; Double Swirler Lean Premixing Pre-Vaporizing Burner [4]; Rich-Burn,Quick-Mix, Lean Burn Trapped Vortex Combustor [5]; Rich-Catalytic Lean-Burn Combustion