Turbulent flame speed measurement of NH3/H2/air and CH4/air flames and a numerical case study of NO emission in a constant volume combustion chamber (C.V.C.C.)

Abstract

Ammonia combustion is a meaningful method to retrieve stored amounts of excess variable renewable energy. The practical combustors like engines and gas turbines fired by the common hydrocarbon fuels need to be operated by the carbon-free like ammonia or ammonia/hydrogen blends shortly. Thus, in this study, the turbulent flame speed (ST) and NO emission potentials of the NH3/H2 blend (NH3/H2 = 40/60, 50/50, and 60/40 vol%) are investigated in comparison with the methane/air flames in a constant volume combustion vessel. The effects of Lewis number (Le), turbulent intensity (u′=0.78–2.34 m/s), and pressures (1, 5 bar) on normalized ST/SL are considered and validated against literature proposed correlations. Flame morphologies show that the turbulent flame wrinkling characteristics are mainly dominated by the turbulent intensity (u′/SL), turbulent length scale (lT/lF) at different pressures, and differential-diffusion term (Le). The differential-diffusion and turbulent stretch, jointly determine the ST/SL of NH3/H2/air and CH4/air mixtures. The scaling parameter ReT,flame/Le2 can describe the self-similar propagation characteristics of ST/SL of spherical flame and consider the differential-diffusion effect. The further Damköhler number, Da modification on the general turbulent flame speed correlation proposed as: ST/SL ∼ ADaB·(ReT,flame/Le2)0.5 unifies different hydrogen content cases by considering the turbulent stretch effect. When literature experimental data are also correlated together, the scaling law becomes: ST/SL ∼ Re0.5/Le ∼ (u'/SL)0.5(lT/lF)0.5Le-1 with equal weighting factors of u'/SL and lT/lF terms, this follows the Damköhler's second hypothesis at highly turbulent regimes. The impact of turbulent intensity and residence time on NO emissions in the flue gas of NH3/H2/air and CH4/air flames are independent of turbulent intensity due to the long residence time. Simulated results show that residence time has contrary effects on NO emissions of NH3/H2/air and CH4/air flames due to the contrary role of the thermal-NOx pathway. Practical utilization of NH3/H2/air in gas turbine combustors could increase the flame residence time by increasing the back-flow zone in the post-flame zone and swirl number to suppress the NOx emissions.