Emissions characterization of residential wood-fired hydronic heater technologies

Residential wood-fired hydronic heaters (RWHHs) can negatively impact the local ambient air quality and thus are an environmental concern in wood burning areas of the U. S. Only a few studies have been conducted which characterize the emissions from RWHHs. To address the lack of emissions data, a study was conducted on four appliances of differing design using multiple fuel types to determine their thermal, boiler, and combustion efficiency as well as the emissions of carbon dioxide (CO2), carbon monoxide (CO), total hydrocarbons (THC), nitrous oxide (N2O), methane (CH4), total particulate matter (PM) mass, and particle number as well as particle size distribution (PSD). Three of these appliances were fired with split-log cordwood with the fourth unit using hardwood pellets. The measured thermal efficiencies for the appliances tested varied from 22 to 44% and the combustion efficiencies from 81 to 98%. Depending on appliance and fuel type, the emission factors ranged from about 1300 to 1800 g kg−1 dry fuel for CO2, 8–190 g kg−1 dry fuel for CO, <1–54 g kg−1 dry fuel for THC and 6–120 mg kg−1 for N2O. For the particle phase pollutants, the PM mass emission factors ranged from 0.31 to 47 g kg−1 dry fuel and the PM number emission factors from 8.5 × 1010 to 2.4 × 1014 particles kg−1 dry fuel, also depending on the appliance and fuel tested. The PSD for all four appliances indicated a well established accumulation mode with evidence of a nucleation mode present for Appliances A and B. The average median aerodynamic particle diameters observed for the four appliances ranged from 84 to 187 nm while burning red oak or pellets. In general, the pellet-burning appliance had the highest overall operating efficiency and lowest emissions of the four units tested.

► Four residential wood-fired hydronic heaters were tested with multiple fuels.
► Combustion efficiency as well as gas and particle emissions were determined.
► The emissions varied widely depending on the technology and fuel utilized.
► The pellet-burning appliance had the highest efficiency and lowest emissions.