Emissions from residential and commercial boilers: Alternative fuels and a modulating-condensing natural gas boiler

In this study, air pollutant emission factors have been developed for emerging fuels and technologies under representative cyclic operation in residential and commercial boilers. The cases explored include 20% biodiesel blends; 100% biodiesel; renewable diesel; and modulating-condensing ("Mod-Con") gas boilers with 100% utility natural gas and 20% hydrogen blends. Three appliances were evaluated: a residential retention-head liquid fuel-fired boiler with pressure atomization and a single firing rate; a residential Mod-Con boiler; and a commercial boiler with a two-stage pressure atomized liquid fuel burner. Measured pollutants included particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NOx), methane (CH₄), volatile organic compounds (VOCs), and hazardous air pollutants (HAPs). Particulates were measured using EPA Methods 5 and 202 to capture filterable and condensable particulates supplemented by real-time PM instrumentation. Results are compared with prior work using conventional fuels and technologies as well as emission factors commonly used in inventories. Findings highlight the importance of burner on-off operation in driving pollutant emissions associated with incomplete combustion; this behavior is not well captured by current emission factors, which are based primarily on steady-state operation and much larger applications.Implications: Residential and commercial boilers contribute substantially to building-sector emissions, but existing emission factors often reflect steady-state operation of larger systems rather than cycling in smaller appliances. This study develops new emission factors for emerging fuels, including renewable diesel, biodiesel blends, and natural gas/hydrogen mixtures, as well as a modern modulating, condensing gas boiler. Results show that burner cycling strongly affects incomplete-combustion pollutants and that condensable particulate matter can dominate total PM, especially in advanced gas systems. These findings support more realistic emissions inventories and regulatory assessments of fuel-switching and decarbonization strategies.