Non-premixed ammonia combustion in porous media to promote thermal cracking of NH$_3$: A low-emission burner concept

This study investigates spatially-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 ≤ 𝛷$_𝑔$ ≤ 1.3) and thermal loads (0.25 ≤ 𝑃 ≤ 0.76 MW m$^{−2}$). Temperatures inside the porous matrix and global emissions of NO$_x$, NH$_3$, N$_2$O, and H$_2$ are measured and compared between non-premixed and premixed operation. Results show that NH$_3$/air flames are stably anchored in high-porosity foams (15 PPI) without external preheating. N$_2$O emissions remain below 40 ppmv (normalized to 15 % O$_2$), and non-premixed operation reduces NO$_x$ by one order of magnitude compared to premixed operation, while maintaining low NH$_3$ slip. Low NH$_3$ and NO$_x$ 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 NH$_3$, 111 ppmv NO$_x$, 𝑃 = 0.25 MW m$^{−2}$, 𝛷$_𝑔$ = 0.8), NO$_𝑥$ is reduced by 90 % compared to premixed operation (2 ppmv NH$_3$, 1148 ppmv NO$_x$) at identical conditions. ... mehrComplementary volume-averaged simulations (VAS) reproduce these trends and indicate that H$_2$ formed in NH$_3$-rich zones promotes NO$_x$ reduction. These findings demonstrate that spatially-distributed, non-premixed combustion in PMBs enables stable, low-emission ammonia combustion and may further reduce NH$_3$ slip through improved downstream mixing and residence time control.