Simulation of a syngas counter-flow diffusion flame structure and NO emissions in the pressure range 1–10 atm

•Optimal operation conditions for syngas combustion in regard to NOx emissions.•Wide range of operating conditions (H2/CO ratio, strain rate, pressure).•Zeldovich route is the main NO formation route.•Hydrogen-rich syngas flames produce more NO at low strain rates.•Hydrogen-lean syngas flames produce more NO at high strain rates.

This paper reports a numerical investigation of syngas flame structure and NO reaction pathways over a wide range of operating conditions (H2/CO ratio between 0.4 and 2.4, scalar dissipation rate from equilibrium to extinction and ambient pressure from 1 to 10 atm) in mixture fraction space. An analysis of optimal operation conditions for syngas combustion in regard to NO index emissions is also provided.Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism adopted is GRI-Mech 3.0.The computational predictions showed that flame temperature exhibits a peak at an intermediate scalar dissipation rate for a given value of H2/CO ratio. From hydrogen-lean syngas to hydrogen-rich syngas fuels, maximum flame temperature increases for scalar dissipation rate values lower than the intermediate value whereas decreases at higher values. Zeldovich route is found to be the main NO formation route and its contribution to the NO production continually increases with the increase of hydrogen content and pressure. Hydrogen-rich syngas flames produce more NO at lower scalar dissipation rates while NO levels increase towards hydrogen-lean syngas flames at higher scalar dissipation rates.