Performance analysis of domestic boilers fuelled with hydrogen-enriched natural gas blends and pure hydrogen

In response to the growing urgency of climate change, many corporations and institutions are actively working to reduce greenhouse gas emissions through sustainable technologies and policies. One promising approach is the replacement of natural gas with hydrogen in residential and industrial energy applications. Currently, hydrogen is being blended in small quantities into existing natural gas networks worldwide, which affects the performance of appliances originally designed for natural gas due to changes in gas properties. This study investigates the effects of hydrogen addition on domestic boilers designed for natural gas, analysing combustion characteristics as hydrogen and oxygen content vary. Stoichiometric, lean, and incomplete combustion conditions were considered, with a focus on pollutant formation in incomplete combustion and mitigation through lean combustion with excess air. Key parameters, including oxygen levels, carbon monoxide from incomplete combustion, air and exhaust gas temperatures, water content, and relative humidity, were analysed for domestic boilers fuelled with hydrogen-enriched natural gas and pure hydrogen. Analytical combustion models were developed to estimate condensable water mass, heating value ratios, saturation temperature, exhaust gas specific heat, energy fraction of condensation, and condensation efficiency. Findings indicate that water vapour, whether in air or exhaust, significantly impacts boiler performance. Optimal efficiency was achieved at maximum water vapour molar fractions, which corresponded to stoichiometric hydrogen combustion with high humidity-applicable to both conventional and condensing boilers. Pollutant emissions formation was minimized with higher excess air in lean combustion, which also reduced the water vapour fraction. Hydrogen addition further reduced pollutants, as pure hydrogen combustion produces only water. For optimal economic and environmental outcomes, the study recommends pure hydrogen combustion under lean conditions with minimal excess air, ensuring higher performance and reduced emissions compared to natural gas. The findings emphasize the potential benefits of hydrogen as a clean alternative for residential heating while addressing the practical constraints and trade-offs associated with its adoption in domestic boiler applications. The study provides valuable insights into optimizing hydrogen combustion for both conventional and condensing boiler systems under varying operating conditions.