Non-premixed ammonia combustion in porous media to promote thermal cracking of NH3: a low-emission burner concept

This study investigates distributed, non-premixed ammonia/air combustion in porous media burners (PMBs) as a strategy to mitigate NOx emissions and improve flame stability, two key challenges for ammonia as a carbon-free fuel. To the best of our knowledge, this is the first experimental demonstration of this approach. Experiments are conducted over global equivalence ratios (0.7 ≤ Φg ≤ 1.3) and thermal loads (0.25 ≤ P ≤ 0.76 MW m-2). Temperatures inside the porous matrix and global emissions of NOx, NH3, N2O, and H2 are measured and compared between non-premixed and premixed operation. Results show that NH3/air flames are stably anchored in high-porosity foams (15 PPI) without external preheating. N2O emissions remain below 40 ppmv (normalized to 15 % O2), and non-premixed operation reduces NOx by one order of magnitude compared to premixed operation, while maintaining low NH3 slip. Low NH3 and NOx emissions occur only in non-premixed operation, with the most favorable regimes under globally fuel-lean conditions. In the lowest non-premixed case (31 ppmv NH3, 111 ppmv NOx, P = 0.25 MW-2, Φg = 0.8), NOx is reduced by 90 % compared to premixed operation (2 ppmv NH3, 1148 ppmv NOx) at identical conditions. Complementary volume-averaged simulations (VAS) reproduce these trends and indicate that H2 formed in NH3-rich zones promotes NOx reduction. These findings demonstrate that distributed, non-premixed combustion in PMBs enables stable, low-emission ammonia combustion and may further reduce NH3 slip through improved downstream mixing and residence time control.