Overall the staged concept was found to show very promising results not only with natural gas but also with natural gas enriched with propane or hydrogen. CFD analysis of turbulent flame speed, turbulence and strain rates support the hypotheses of lifted off flame. Comparing flame photos in the visible and ultraviolet (UV) range suggest that the flame might be lifted off for the lobe mixer, leading to insufficient time for carbon monoxide (CO) burnout. However atmospheric combustion tests showed lower emissions for the jet-in-cross-flow configuration. The lobe mixer showed the best mixing quality and hence was expected to also be the best variant in terms of combustion. With CFD simulations the quality of mixing of second stage fresh gas with first stage exhaust gas was assessed. The investigated geometries were a simple annular gap, a jet-in-cross-flow configuration and a lobe mixer. Three injection geometries have been studied by means of computational fluid dynamics (CFD) simulations and atmospheric tests. At low gas turbine load an upstream stage (first stage) provides stable combustion at low emissions while at higher loads the downstream stage (second stage) is started to supplement the power. With the objective of improving this situation a staged combustion system has been investigated. Conventional gas turbines suffer from higher emissions at low load operation. Gas turbine power plants with high load flexibility are particularly suitable to compensate power fluctuations of wind and solar plants.
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