Combustion technology for low emissions gas turbines: Selected phenomena beyond NOx

Since recent reviews cover the issues in NOx formation under gas-turbine conditions, and since regulations essentially dictate use of the premixed mode of combustion for minimum NOx, this review concentrates on phenomena that can arise in premixed combustion. Specifically, 1) the initial unmixedness in a fuel-air premixer has been shown to make overall lean mixtures autoignite sooner than might be expected based on the overall fuel-air ratio, because the richer portions of the mixture lead the process; 2) combustion pressure oscillations caused by the interplay between acoustic waves and unsteady heat release in a one-dimensional system can be calculated in good accordance with measured data, and set the stage for multi-dimensional CFD; 3) carbon deposition arising from the flow of liquid fuel over metal surfaces such as found in fuel injectors and swirl cups has been described as a function of temperature and of surface composition; and 4) quenching and subsequent emissions of carbon monoxide can be minimized by preservation of a boundary-layer rather than an impingement type of flow over combustor liners.