# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 3 Results and Discussion
### Emissions
However, since the physical phenomena (turbulence, combustion, chemical kinetics) are strongly coupled, and there are always experimental measurement uncertainties; especially in regions of steep gradients, it is not feasible to ascertain the main reason for the observed differences. These are most likely due to the combination of different sources of uncertainty. The marginal impact or a slight increase of the CO emissions from 0% to 20% aligns with the findings of other authors. [PERSON] et al. [59] conducted experiments using four different burners while maintaining the thermal power and excess air coefficient unchanged. Their results indicated that, although CO emissions varied with the burner type, they were largely unaffected by hydrogen molar fractions between 0% and 30%. Similarly, [PERSON] et al. [20], in experiments with a low dry emission burner, observed that CO emissions remained nearly unchanged, at constant thermal power, from 0% to 26% H\({}_{2}\) (vol.). Figure 13 presents the computed emissions of O\({}_{2}\), CO\({}_{2}\), CO, and NO on a volumetric, dry basis for \(\alpha\) = 50%, as a function of the excess air coefficient and hydrogen content in the fuel. The influence of the fuel composition on emissions was discussed earlier. For a given fuel composition, the O\({}_{2}\) emissions increase as the excess air coefficient rises, while CO and CO\({}_{2}\) emissions decrease, as expected. This occurs because a higher excess air coefficient provides more oxygen for fuel combustion, leading to lower CO emissions. Additionally, CO\({}_{2}\) emissions decrease due to dilution effects. The CO emissions remain high for \(\lambda\) = 1.3 and 1.5, but they drop below 100 ppm when the hydrogen content in the fuel reaches 60% or more. NO emissions increase with the excess air coefficient for a given fuel composition, particularly at high hydrogen content. In fact, for \(\alpha\) = 50%, the adiabatic flame temperature in the reaction zone near the burner, based on the local equivalence ratio, rises as \(\lambda\) increases from 1.3 to 1.7, as previously discussed. The higher temperatures, which can be observed in Figure 10, enhance thermal NO formation, resulting in increased NO emissions. These results indicate that secondary air is particularly beneficial for pure hydrogen flames, as there are no CO emissions and NO emissions are relatively low despite an increase in NO emissions with a higher excess air coefficient. For \(\mathrm{NG/H_{2}}\) mixtures, CO emissions are high and increase with the primary air fraction. However, this effect is mitigated by increasing the excess air coefficient. Thus, there is potential to optimize the combustor design to reduce CO emissions while using secondary air. Possible improvements include reducing the diameter of the secondary air injection holes to enhance the jet momentum and penetration and lengthening of the top-mounted plenum, and, consequently, the overall combustor length, to increase the residence time of secondary air, thereby allowing for complete fuel combustion. Additional \(\mathrm{NO_{x}}\)-reduction strategies, including exhaust gas recirculation, lean combustion, and MILD combustion, warrant further investigation. The EDC model is based on the assumptions that the reaction rates are controlled by turbulence-chemistry interaction and reactions occur primarily in fine turbulence structures. Accordingly, the model may not capture the fast chemical kinetics of hydrogen combustion well and, therefore, the accuracy may decrease when the molar fraction of hydrogen in the fuel is high. This issue should be investigated in future work, even though the EDC model has been used by several authors to simulate hydrogen flames, e.g., [60; 61].