Feeding H2-admixtures to domestic condensing boilers: Numerical simulations of combustion and pollutant formation in multi-hole burners

Hydrogen can be produced through electrolysis from excess wind and solar power. Then, its injection into the natural gas network allows mitigating the challenges posed by the variability and intermittency of renewables, by exploiting the existing infrastructure for storage and distribution. However, the addition of hydrogen to natural gas affects gas properties; hence we need to ensure the safe and efficient operation of existing end-user equipment, such as domestic burners and boilers. This work intends to investigate the effect of H2 addition on the combustion process and pollutant emissions in domestic condensing boilers. To this purpose, 3-dimensional numerical simulations of multi-hole geometries mimicking perforated burners, typically encountered in such appliances, are performed by using detailed kinetics and taking into account different hole-to-hole distances. Indeed, since the burner holes are located very close to each other, the neighbour premixed flames can interact, thus differing from single flame behaviour. The impact of H2 on the operating conditions, i.e. equivalence ratio and thermal load, is preliminarily evaluated to ensure that the numerical model closely emulates the practical situation we expect from the fuel composition change. Anticipation of the reaction zone occurs with H2-admixtures, while the downstream temperature decreases because the boiler naturally shifts towards leaner conditions and smaller thermal loads when adding H2 to natural gas. This behaviour has a positive impact on pollutant emissions as the NO thermal formation route is suppressed in a larger measure than the increase of the NNH-intermediate contribution.