Fuels containing large amounts of hydrogen have combustion properties highly depending on composition, in particular hydrogen concentration, and operating conditions such as pressure. A thorough understanding of strained
laminar flames is a prerequisite to achieve improved knowledge of more complex system involving hydrogen-rich alternative fuels.
This paper reports a numerical investigation of syngas counter-flow diffusion flame structure and emissions over a wide range of operating conditions (H2/CO ratio between 0.4 and 2.4 and ambient pressure from 1 to 10 atm). Special attention is focused on optimal operating conditions in regard to NOx emissions and NOx reactions pathways.
Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism adopted is GRI-Mech 3.0.
Computational results showed that flame structure and emissions are impacted by syngas composition and ambient pressure. The maximum flame temperature exhibits a peak at an intermediate scalar dissipation rate for a given value of H2/CO ratio. For values of strain rate lower than the intermediate value, flame structure is influenced by combined effects of adiabatic temperature and radiation heat loss, whereas only adiabatic temperature effect prevails at higher values of strain rate. The flame temperature increases more the syngas is H2-rich for strain rates values below the intermediate value. The opposite behavior is noticed at strain rate values higher than the intermediate value. NOx formation is closely related to flame temperature. Hydrogen-rich syngas flames produce more NOx at lower strain rates while NOx levels increase towards hydrogen-lean syngas flames at higher strain rates. Zeldovich route is found to be the main NOx formation route and its contribution to the NOx production continually increases with H2 content and pressure.
