Pore-resolved simulations of ammonia combustion in porous inert media

Liquid ammonia is considered to be a suitable hydrogen carrier for a future carbon-free energy infrastructure,
but NH3 can also be used as a fuel for direct combustion. However, NH3 exhibits poor
combustion characteristics in conventional burners due to its low heating value, difficult flame stabilisation,
and comparatively high pollutant emissions. Porous media combustion (PMC) is utilised in this
work to improve the combustion characteristics of ammonia. In a first step, parameter studies through
volume-averaged simulations (VAS) are conducted to pre-select suitable burner configurations and operating
conditions. This narrows down the range of suitable configurations for effective flame stabilisation
and low NOx formation. Then, direct pore-level simulations (DPLS) of these geometries are performed.
Flame propagation and stabilisation in the DPLS are influenced by the foam surface temperature, geometry
and local acceleration of the flow. Ammonia dehydrogenation leads to H2 production, but has
minimal effect on the combustion characteristics. DPLS results show considerable discrepancies with
VAS due to the simplified modelling of the porous structure in VAS and the absence of heat conduction
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in the solid in the present DPLS.