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

The present study investigates the potential of porous media burners to improve the combustion characteristics of unconventional fuels, with a focus on ammonia (NH3). The unique heat recirculation characteristics of porous media extend the flammability limits of premixed flames, counteracting the inherently poor combustion properties of NH3. The present 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. Infrared radiometry was employed to determine the solid matrix temperature using a narrow band-pass filter to avoid interference of gaseous species. The solid temperatures in the preheat region and the combustion zone revealed the flame front propagation behavior as influenced by variations in the mixture equivalence ratio, H2/NH3 ratio, and the mass flow rate, highlighting the geometric advantages provided by the radial porous media burner. The measured exhaust gas temperatures and the ceramic foam temperatures were correlated to provide insight into flame dynamics. Emissions of NO, N2O, along with NH3 and H2 slip, were analyzed in 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.