Methane emissions from small residential wood combustion appliances: Experimental emission factors and warming potential

Methane emission factors (g/GJ) were determined testing residential heating biomass appliances (6-11 kW) under real-world operating conditions. User behavior for manually load appliances was simulated following a loading scheme starting from the cold start conditions, followed by two nominal batches and a final batch either with the nominal load of the appliance or by over loading the firebox (closing the air valves) and lasted until burn out. The results were analyzed both on batch-per-batch basis and for total combustion cycle from cold start to burn out in order to determine the critical situations causing high methane emissions. For comparison two automatic pellet appliances (8-25 kW) were also tested. Emission factors (EFs) for these automatic appliances are more than an order of magnitude lower with respect to batch-working room heaters. For the latter the average EFs ranged from 142 g/GJ to 238 g/GJ and showed both batch-to-batch and inter-appliance variability; however, many of the observed differences were not statistically significant. The results highlighted the importance of the user behavior to avoid high methane emissions. The climate relevance of methane emission levels has been assessed using global warming potential (GWP) taken from the literature, comparing CO2equivalent emissions with that of N2O and other near-term climate forcers (CO, NOx, VOC, black carbon) emitted by the same appliances. The results show that the warming impact of CH4 is lower than that of BC and CO (compounds emitted in relevant levels in small appliances burning wood), but is still an important portion of the CO2 avoided for the substitution of fossil fuels with biomass. Although the uncertainties associated with GWP are large and EFs are based on a limited number of appliances and fuel types, the results show that in the short term (i.e., 20-year period) CO2eq for all the non-CO2 forcers offset the CO2 benefits of biomass use.