# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 3 Results and Discussion
### Emissions  
The computed emissions of O\({}_{2}\), CO\({}_{2}\), CO, and NO on a volumetric and dry basis are shown in Figure 12 for \(\lambda=1.3\) as a function of the primary air fraction and hydrogen content in the fuel. The measured CO and CO\({}_{2}\) emissions for two different primary air fractions, \(\alpha=50\%\) and \(\alpha=66.7\%\), are also plotted in Figure 12. The CO\({}_{2}\), CO, and NO emissions are corrected to \(3\%\) O\({}_{2}\). The O\({}_{2}\) molar fraction increases slightly, while that of CO\({}_{2}\) decreases with increasing hydrogen content in the fuel, as expected from a single step, global and complete chemical reaction. These trends become more pronounced for higher hydrogen content in the fuel. The O\({}_{2}\) emissions are similar for \(\alpha=50\%\) and \(66.6\%\) but slightly lower for \(\alpha=100\%\), suggesting incomplete combustion in the former cases. This is consistent with the high CO emissions observed for \(\alpha=50\%\) and \(66.6\%\), which can be attributed to the limited penetration of the secondary air jets. This restricted penetration reduces the residence time in the high-temperature oxidation zone and diminishes mixing efficiency, both of which are critical for the complete oxidation of CO. The NO emissions are lowest for \(\alpha=50\%\) and show little variation between \(\alpha=66.7\%\) and \(\alpha=100\%\). For a given fuel composition, secondary air helps to reduce temperatures in the primary reaction zone, downstream of the burner, where most fuel is burned (see Figures 7 and 9 for \(\alpha=100\%\)). This temperature reduction leads to lower NO emissions, particularly for \(\alpha=50\%\). As the hydrogen molar fraction in the fuel increases from \(0\%\) to \(80\%\), NO emissions rise, with a more pronounced increase for pure hydrogen flames. This trend is linked to temperature levels, which remain relatively low in the former case (see Figures 7 and 9), significantly limiting NO formation via the thermal route. As a result, NO emissions are much lower than in pure hydrogen flames. Figure 11: Influence of the excess air coefficient and hydrogen content in the fuel on the axial temperature profile. The reduction in CO emissions with increasing hydrogen content in the fuel has been reported by several authors, e.g., [18; 58]. This reduction is primarily attributed to two factors: the lower amount of carbon in the fuel, which directly reduces CO formation, and the enhanced production of OH radicals due to the higher hydrogen content, which significantly accelerates CO oxidation. In contrast, the NO emissions increase as the molar fraction of hydrogen in the fuel rises, and this trend is stronger when the molar fraction approaches the unity. As the hydrogen molar fraction increases, the adiabatic flame temperature rises, promoting NO formation through the thermal mechanism. Additionally, the increased concentration of O, H, and OH radicals with hydrogen enrichment further enhances NO production through the same pathway. As a result, in the absence of mitigating factors, an increase in NO\({}_{\rm x}\) emissions is expected. This trend has been widely observed in both experimental and computational studies across various flame and burner configurations, as well as different combustion systems (see, e.g., [15; 18; 24]). The predicted CO\({}_{2}\) emissions for natural gas were underestimated compared to the experimental data, while those of CO were overpredicted. A reverse trend was observed  
Figure 12: Molar fractions of the species at the exit for \(\lambda=1.3\) as a function of the primary air fraction and hydrogen content in the fuel (open symbols—predictions, solid symbols—measurements). for 60% and 80% H\({}_{2}\), while accurate predictions were obtained for 20% and 40% H\({}_{2}\) at \(\alpha\) = 50%. The CO emissions were underestimated for 20% and 40% H\({}_{2}\) at \(\alpha\) = 66.7%. Several reasons may explain the discrepancies between the measurements and the predictions.