Numerical analysis of ammonia dehydrogenation in porous media combustion

Porous media combustion (PMC) is utilised in this work to improve the combustion characteristics of ammonia. Specifically, the effect of heat recirculation in the solid matrix on dehydrogenation and thus pollutant formation is studied in a two-zone porous burner operated with pure ammonia-air at equivalence ratios $\phi=0.8,0.9,1.0$ by performing direct pore-level simulations (DPLS) with state-of-the-art ammonia combustion chemistry, detailed diffusion and conjugate heat transfer.
The selected geometric and thermal properties of the tailored triply-periodic minimal surface (TPMS) $\rm Al_2O_3$-gyroid and SiSiC-diamond zones stabilise the flame near the interface between the two zones, leading to preheating of the incoming gases and enhanced $\rm NH_3$ dehydrogenation upstream of the flame front.
Downstream of the $\rm H_2$ production zone, combustion is predominantly driven by $\rm H_2$ because of the high ($\mathcal{F}>80\%$) ammonia conversion ratio. Hydrogen chemistry and pollutant formation are closely coupled. Ammonia and the H radical are found to primarily contribute to $\rm H_2$ formation. With increasing equivalence ratios, the $\rm H_2$ production zone expands whereas the $\rm NO$ production zone contracts and shifts downstream towards the completely burnt gas. ... mehrThe Stagni and NUIG kinetic mechanisms show qualitative agreement, while a maximum difference of $17\%$ in path fluxes is observed. Moreover, results from accompanying volume-averaged simulations (VAS) show good qualitative agreement with the DPLS but over predict the solid and gas temperatures by 150K. The performance of PMC is also compared with a freely-propagating $\rm NH_3$-air flame. The enhanced $\rm NH_3$ dehydrogenation from the heat recirculation in the porous medium is found to decrease the net $\rm NO_x$ production in PMC, where the $\rm N_2H_2$ de-$\rm NO_x$ pathway is more active due to the early dehydrogenation. Moreover, $\rm NH_3$ dehydrogenation is observed to be insensitive to small increases in heat recirculation, e.g. due to radiation in the selected burner configuration.