Experimental and numerical study of pollutant emissions from a domestic condensing boiler fed with natural gas enriched with H2

Hydrogen is recognized as a promising resource for decarbonizing not only the industrial sector, but also the domestic heating systems. Through the partial substitution of natural gas with hydrogen, domestic combustion- based conversion systems can potentially offer improved efficiency, reduced carbon emissions, and cleaner combustion, i.e., lower levels of particulate matter. However, hydrogen exhibits properties that are significantly different from natural gas: (i) because of its higher laminar flame speed, hydrogen is more susceptible to flashback, which may pose significant concerns from the safety point of view; (ii) because of its higher adiabatic temperature, NOx emissions are expected to increase. Thus, experimental and numerical investigations are needed to better understand how the addition of hydrogen to the fuel mixture modifies the combustion process and how to mitigate/control the higher propensity to flashback and NOx formation within domestic devices. In this study, we investigated experimentally and numerically the performances and the emissions of a domestic condensing boiler with a stainless steel coil heat exchanger equipped with a perforated cylindrical burner fed with mixtures of H2 2-enriched natural gas and air, at several power levels (15, 24, and 30 kW), in a wide range of dilution ratios (from 1.16 to 1.4). 3D numerical simulations, including a detailed kinetic mechanism and conjugate heat transfer between the gaseous phase and the burner plate, were carried out with satisfactory agreement with the experimental data. The experimental results demonstrated the ability of the investigated device to properly work with fuel mixtures including up to 35% (molar basis) of hydrogen. The numerical simulations were repeated by considering pure hydrogen as a fuel in more diluted conditions (with dilution ratios from 1.4 to 2) and the same heat exchanger with a modified perforated burner to prevent the occurrence of flashback phenomena. The numerical results suggested the possibility to (partially) replace natural gas with hydrogen in domestic boilers with minimal modifications to existing perforated burners.