Fundamental limits of NO formation in fuel-rich premixed methane-air flames

Increasingly stringent regulations on pollutant emission are the driving force for designers of natural-gas-fired combustion systems to find ways of controlling NOx formation. To achieve significant emissions reduction, more insight is needed into the mechanisms of NO formation. Martijn van Essen’s thesis provides insight into the effects of two NOx control strategies, flue-gas recirculation (FGR) and burner stabilization, on NO formation. Since many combustion systems operate under fuel-rich (oxygen deficient) conditions, the focus of the thesis was on fuel-rich premixed methane-air flames. The experiments were performed at reduced pressures, to facilitate the measurements of the distributions of temperature and concentrations of key species (CH, OH and NO) using the optical technique laser-induced fluorescence (LIF). Towards this end, a low-pressure flame cell was constructed, and protocols for quantifying the LIF measurements were implemented. The experimental results were compared with detailed calculations based on a chemical mechanism widely used in combustion. The experimental results demonstrate that both burner stabilization and FGR are promising techniques for lowering NO emissions in fuel-rich flames. A comparison with detailed calculations allowed tracing the mechanistic origins of the observed reduction in NO formation rate. However, due to the existence of multiple formation mechanisms, more research is needed in order to improve predictive power of the flame models, particularly under very rich conditions. The quantitative results in this thesis are ideally suited for improving these models