Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition source energy, air/fuel preheat, and equivalence ratio on the particulate matter emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests. Increasing the residence time of spray droplets in the hot combustion zone by increasing the swirl number promotes the burn out of solid residues Decreasing the mean diameter of fuel droplets by increasing the atomizing air flow rate has a similar effect. Ignition source energy, air/liquid fuel preheat, and equivalence ratio have either a weak or ambiguous effect on the measured particulate emissions. The residual material collected from the exhaust is composed of both carbonaceous matter and fly ash. However, the majority of particulate matter consists of ash, even at relatively poor combustion conditions. These results suggest that it is possible to provide enough oxygen availability and residence time for droplets to undergo nearly complete burnout during combustion. Under such conditions, the total particulate matter emissions could be further mitigated by reducing the inherent ash content in the fuel.