Abstract A 37 kW down fired pilot scale laboratory combustor, specially modified to allow recycle of both dirty and cleaned flue gas, was used to investigate aerosol formation under air- and oxy-coal combustion conditions. The focus was on emission of soot or black carbon, formation of fine and ultra-fine particulate matter, and partitioning and vaporization of sodium in the mineral matter. Whereas previous research has addressed some of these issues using fresh, once through CO 2 as the O 2 diluent, this paper describes results using actual recycled flue gas, which is closer to industrial practice. Specific emphasis is on determining the effects of various extents of recycled flue gas cleanup on soot and ash aerosol formation characteristics. Without clean up, recycle can create flue gas with contaminant concentrations approximately four times greater than those in air combustion, and this is important in determining the effects of retrofit on soot and ash aerosol properties. Tests were conducted for three coals over air and a wide range of oxy-fired conditions at 27–32% inlet O 2 with a constant 3% O 2 in the exhaust. Results suggest that changes in the combustion atmosphere have only subtle effects on sodium partitioning chemistry, whereas soot mass emissions can be significantly diminished upon being recycled into the furnace along with dirty moist flue gas, in addition to being decreased relative to air-firing by the high CO 2 environment present under oxy-fired conditions. Ultrafine particle concentrations may increase within the boiler despite flue gas treatment with fabric filters, which may fail to remove the finest of them. Sodium vaporization under oxy-combustion appears to follow similar behavior as under air combustion, suggesting that initial fouling mechanisms are also similar.