A Study on Combustion Characteristics of Radial Inward Flow Porous Media Burners for Ammonia-Hydrogen Flames

Abstract The present study investigates the potential of porous media burners to enhance the combustion characteristics of ammonia (NH3), attributable to the structure's heat recirculation, which extends the flammability limits of premixed flames. This study establishes a radial inward flow porous media burner with dynamic flame stabilization, featuring a compact ceramic foam made of silicon carbide. The foam's Voronoi-based lattice porous structure includes a continuously graded topology, with a linear void fraction increasing from the outer to inner radial direction. The burner flammability limits were evaluated with up to 30% hydrogen (H2) by volume added to the fuel stream. The stability range for pure NH3 operation was 0.8≤ϕ≤1.3, which was extended to 0.55≤ϕ≤1.5 for 30/70 H2/NH3 mixtures. Revealing the flame front propagation behavior under various operating conditions, infrared radiometry was employed to determine the solid matrix temperature using a narrow bandpass filter to avoid interference from gaseous species. The exhaust gas and ceramic foam temperatures were correlated to provide insight into flame dynamics. Emissions of NO, N2O, along with NH3 and H2 slip, were analyzed for various mixture compositions and equivalence ratios. Conditions which minimized NO emissions included (a) near lean-limit NH3/H2 combustion, (b) rich conditions of ϕ≥1.2, and (c) pure NH3 at near-stoichiometric conditions with low combustion temperature due to low mass flux. The measured pollutant emissions were compared with a perfectly stirred reactor model, yielding good agreement.