D5e.2 Fluid dynamics modelling results on gas flaring

In this work package CFD and experiments are used to determine whether flame-regulated flares can be used to lower hydrogen flaring emissions. Special focus is on the emissions of NOx and unburnt hydrogen for regulated flares with different fuel flows, fuel hydrogen content and additional air in the burning mixture. For this a burner was converted to a flare. The experiments which were performed with this set-up are discussed in report D5E.1. The current report is focused on the CFD simulations performed with regards to this work.

In total 8 simulation cases are computed, which vary in fuel flow, hydrogen content of the fuel and additional air added to the burning mixture. These simulations use an unstructured mesh of the domain, which includes the burner geometry, a chimney and a portion of the surrounding air. RANS k- ε is used as a turbulence model, which inherently means that all simulations are steady. Consequently, the resulting flow fields are of averaged parameters. Furthermore, the simulations include full chemistry with 9 species and 22 reactions, radiation and turbulence-chemistry interaction.

The main conclusion of the simulations is that dilution of the flame leads to a reduction in both the amount of unburnt hydrogen escaping and production of NOx. This dilution can be due to introducing additional air in the burning mixture or lowering the hydrogen content. Important to note here is that the dilution should not be done to such a degree that the a well-developed flame is not maintained anymore. Then the hydrogen largely just escapes. The fact that flame dilution is so effective in lowering emissions shows that flame-regulated flares are a viable strategy for reducing the environmental impact of hydrogen flaring.

Furthermore, the simulations show a strong correlation between temperatures and NOx emissions, a phenomenon already well established. It shows that reducing the burning temperatures should be a priority when designing flares for hydrogen service.