Combustion characteristics of nonpremixed ammonia-hydrogen/air coaxial flames at elevated temperature and pressure in a model combustor

To facilitate the use of ammonia (NH3) combustion in industrial burners, the combustion characteristics of nonpremixed NH3-hydrogen (H2)-nitrogen (N2)/air coaxial flames at elevated inlet gas temperature (Tin) and chamber pressure (P) in a model combustor are studied under varying hydrogen mole fraction (xh,f), fuel-equivalence ratio (ϕ) and fuel Reynolds number (Ref) conditions. Increasing P and Tin each exert opposite effects on the combustion characteristics of the nonpremixed flames. Increasing P shrinks fuel-lean limits, decreases nitrogen oxides (NOx) emissions, increases unburned NH3 emissions and shifts the main reaction zone upstream and thus local extinction to occur near the flame base. Meanwhile, increasing Tin at given Ref expands the fuel-lean limits in terms of xh,f, increases NOx emissions, decreases unburned NH3 emissions and causes local extinction to primarily occur near a breakpoint due to the increased flow velocity. When P and Tin increase simultaneously, NOx and NH3 emissions exhibit the same trends as a function of ϕ as observed when each parameter is varied individually. Conditions with relatively low and nearly 1:1 NOx and unburned NH3 emissions are identified (approximately 490 ppm at ϕ = 0.6, xh,f = 0.50, P = 4.0 bar and Tin = 600 K), providing favorable conditions for the application of a selective catalytic reduction (SCR) system.