Applying heat flux method to laminar burning velocity measurements of NH3/CH4/air at elevated pressures and kinetic modeling study

Abstract

Combustion of ammonia (NH3) blended fuels under elevated pressure conditions is critical for adopting this non-carbon fuel in the energy system for decarbonization. In the present work, laminar burning velocities of ammonia/methane(CH4)/air mixtures were measured using the heat-flux method at the pressure from 1 to 5 atm with the mixture equivalence ratios ranging from 0.6 to 1.6 and the mole fraction of NH3 ranging from 0 to 1.0. The relatively completed results obtained at elevated pressures were then used for validating and modifying the kinetic mechanisms (CEU-NH3-Mech 1.0) leading to a new version (CEU-NH3-Mech-1.1). Experimental results of NH3/H2/air in the present work, NH3/H2/CO/air mixtures measured on the same setup and reported in our previous works were also considered in the development of the kinetic mechanism. It was found that the CEU-NH3−Mech-1.1 can predict well the laminar flame speed, ignition delay time and species concentration in the ammonia oxidation at high temperatures for both NH3/CH4/air and NH3/H2/CO/air mixtures in a wide range of equivalence ratios and elevated pressures, including oxygen-enriched combustion conditions. The present experimental results also show that the value of pressure exponent (β) varies with the mole fraction of ammonia and behaves differently for the mixtures of ammonia blending into CH4 and H2. The kinetic and sensitivity analyses show that the sensitive reactions for β are weakly correlated to those for the laminar burning velocity, indicating that β can also work as a potential parameter for validating kinetic mechanisms. Ammonia content in the NH3/CH4/air mixtures determines the pressure exponent variation at over-rich equivalence ratios and reaction pathway variation in the post-flame zone. This work also clarifies the utilization of ammonia containing fuels in rich-lean combustion strategies.