Numerical analysis of ammonia-air flame stabilisation in porous media

The hydrogen carrier ammonia is a potential replacement for carbon-based fuels. Ammonia can be stored and transported with minor modification in the existing infrastructure, thus providing a potential storage solution for H2 as a fuel. Direct combustion of NH3 is attractive for energy conversion. However, low laminar burning velocity, high NOx emissions, and high toxicity make combustion of NH3 challenging. A viable solution is found in porous media combustion (PMC), where heat recirculation within the solid matrix improves flame stability. Heat transfer from the reaction zone to the upstream NH3-air mixture can accelerate H2 production from dehydrogenation of ammonia and simultaneously reduce NOx emissions. The physics of PMC can be investigated in detail by performing direct pore-level simulations (DPLS) with complex combustion kinetics and detailed transport models. The objective of this work is to investigate the NH3 dehydrogenation in PMC and its effect on flame stabilisation. Given the high computational cost of DPLS, the solid phase is not resolved in this work. The thermal effects of the solid matrix are implemented as a temperature boundary condition and the solid temperature data is extracted from reduced-order volume-averaged simulations (VAS). ... mehrIn-house solvers are used for DPLS and VAS. The two-zone porous burner comprises of an upstream distributor to laminarise the flow and a downstream 15 PPI (pore per inch) SiSiC porous layer. In the computational domain for DPLS, the fuel/air distributor is resolved as channels and the porous geometry is extruded using the snapyHexMesh tool in OpenFOAM. Three operating conditions for equivalence ratios 𝜙=0.9,1.0,1.1 and a burner thermal load of 𝑃=0.25 MW/m2 are analysed. The flame structure and production rate of H2 for 𝜙=0.9 are shown in Fig. 1. The ratio 𝑐=𝑌𝐻2𝑂/𝑌𝐻2𝑂,burnt defines the progress variable, where 𝑌 is the mass fraction. High solid temperatures near the channel outlets and selected geometrical properties of two zones cause the flame to stabilise near the interface between the distributor and the porous layer. Individual flames are visible over each channel and flame penetration is governed by flame-wall interaction as well as local geometry of the porous structure. The consumption of NH3 is accompanied by H2 production. It can also be observed that the downstream combustion process (𝑐>0.93) is dominated by H2 produced from ammonia dehydrogenation, which expands the flame front as a consequence.