The combustion process of both pure NH3 and a NH3/H2 fuel blends is here analyzed using two kinetics processors, i.e., Chemkin-Pro-and CANTERA: detailed kinetic mechanisms have been tested and compared in terms of laminar flame speed and ignition delay time (IDT) with the aim to identifying the most suitable ones for the evaluation of NOx emissions. The generic swirl burner being used in Cardiff University's Gas Turbine Research Center has been considered as validation test case. In addition, this paper presents an experimental campaign followed by a computational fluid dynamics (CFD) approach for the assessment of NOx emission using axisymmetric Reynolds-Averaged Navier–Stokes (RANS) simulations, leading to a significant reduction of the computational time. Different pressures and mass flow rates are evaluated to understand correlations of NOx formation for pollutants reduction purpose. A direct comparison between experimental and numerical results is carried out in terms of flow field, flame shape, and NOx emissions. Results show that the increase in pressure from 1.1 bar to 2 bar results in reduction of NOx emissions from 2515 ppmv to 885 ppmv, also indicating guidelines for using a simplified RANS analysis, which leads to improved computational efficiency, allowing wide sensitivity and optimization analysis to support the design development of an industrial combustion system.

Ammonia combustion is gaining interest as a feasible alternative to traditional fossil fuels because of to the low environmental impact and as hydrogen and energy carrier. This study used Computational Fluid Dynamics (CFD) simulations to compare various turbulence models for premixed ammonia/hydrogen combustion in a swirl-stabilized burner. The primary aim was to identify the best turbulence model for accurately predicting the flow dynamics, combustion behaviour, and emissions profiles of ammonia/hydrogen fuel blends. The turbulence models evaluated were Large Eddy Simulation (LES), Realizable k- $$\epsilon$$ ϵ , Renormalization Group (RNG) k- $$\epsilon$$ ϵ , k- $$\omega$$ ω SST, and Reynolds Stress Model (RSM). On the LES side, a further comparison of two subgrid models (Smagorinsky-Lilly and WALE) was investigated. The Flamelet Generated Manifold (FGM) method was utilized with a detailed chemistry scheme taking into consideration all $$\hbox {NO}_x$$ NO x reactions. To improve the prediction of $$\hbox {NO}_x$$ NO x emissions, additional scalar transport equations for NO and $$\hbox {NO}_2$$ NO 2 were included. This methodology aimed to be a balance between computational efficiency and the accuracy expected of detailed chemistry models. Validation was done with a swirl burner from Cardiff University’s Gas Turbine Research Centre. Results showed that all turbulence models accurately captured flame characteristics in terms of exhaust temperature and axial velocity with minor differences in the recirculation zones, where only the RSM model can predict the velocity trend as the LES simulation while other RANS models differ by at least 7 m/s. The temperature reached by the LES resulted 100 K higher than the other models in the flame zone. LES simulation can predict the emission value with an error of less than 10 $$\%$$ % . Moreover, the error related to emissions derived from the RANS simulations was not negligible, underestimating $$\hbox {NO}_x$$ NO x emissions by about 35 $$\%$$ % . However, RSM model produced results that were closer to those derived from the high-fidelity LES when compared to the others RANS models, particularly in terms of flame thickness and emissions. It was concluded that it is mandatory to perform an unsteady analysis to reach reasonable results.

Interest in using renewably produced, partially cracked ammonia in gas turbines is gaining traction, but challenges relating to emissions of NO_x and unburned ammonia remain. The present work progresses existing research on using hydrogen stratification to reduce NO_x from ammonia/hydrogen flames by experimentally and numerically investigating the effects of also injecting nitrogen from the cracking process. It additionally assesses the NO_x reduction capability of a recently developed novel swirl burner by adding hydrogen to the stratified flow to maintain the diffusive equivalence ratio at two high NO production conditions, slightly lean and stoichiometric. At slightly globally rich conditions, maintaining the diffusive equivalence ratio at 0.9 resulted in an order of magnitude reduction in NO emissions with only a 33% increase in unburned NH₃, compared to a fully premixed flame with the same fuel and air flow rates. This stratified configuration was found to increase consumption of NO by NH₂, likely due to flame morphology effects, while NO production from OH and HNO pathways was reduced. The reduced OH intensity was posited as the cause for increased NH₃ emission. A strong emissions sensitivity to diffusive equivalence ratio was found, as the case with a stoichiometric diffusive equivalence ratio did not show such marked improvements over its corresponding premixed condition. Both stratified and premixed flames were found to be stable, however stratification has potential to trigger instabilities at different frequencies to premixed.