Flue gas recirculation threshold: A novel promising concept for NOX control in quasi-mild combustion of syngas fuels.

Despite advancements in alternative fuel combustion that have effectively eliminated carbon emissions, NOX emissions remain a significant challenge. This article explores the characteristics of various combustion regimes, with a particular focus on CH4/H2/CO syngas fuels. The findings indicate that the most practical method to achieve exceptionally low NOX emissions in industrial settings is by adopting quasi-mild combustion (QMC) regimes or operating under conditions closely aligned with this regime. Therefore, a new concept titled "FGR-threshold" is introduced as a specific flue gas recirculation (FGR) for each fuel composition, indicating the boundary point of transition between combustion regimes. It is shown that the required FGR-threshold for achieving QMC regime increases in syngas fuels compared to pure methane. Closer inspection of developed regime maps reveals that by decreasing the CH4 fraction and replacing it with H2 or CO, the FGR-threshold for achieving QMC increases up to 15% for H2-enriched and 5% for CO-enriched fuels compared to pure methane. The ternary syngas mixtures exhibit FGR-threshold values positioned between these two limits. Research has demonstrated that to reduce NOX emissions to environmentally acceptable levels, a certain proportion of the FGR-threshold can be applied, regardless of the fuel type or preheating level. For all syngas fuels 50% in excess of the FGR-threshold can guarantee more than 90% reduction in NOX emissions, limiting concentrations to below 200 ppm. This finding accentuates the environmental benefits of FGR-threshold and its role for industrial applications requiring stringent NOX control.