The characteristics of both particulate formation and ash deposition play an important role in retrofitting the conventional air-fired coal power plant into the recycled oxy-fuel plant. In this paper, an intensively comparative study was performed with a 25 kW quasi one-dimensional down-fired pulverized coal combustor for clarifying the differences between air combustion and recycled oxy-fuel combustion of bituminous coal. In oxy-fuel mode, the volume recycle ratio of flue gas (dry basis) was kept at 77.8%, and then the oxygen concentration at 30% to provide a similar heat flux output to the air mode. A nitrogen-aspirated, isokinetic particulate sampling probe, followed by either electric low-pressure impactor (ELPI) or scanning mobility particle spectroscopy (SMPS), was introduced to measure fine particulates, while an air-cooled deposition probe was designed to collect ash deposit samples. The results indicated that, under similar furnace temperature profiles, the oxy-fuel combustion leads to the higher fine particulate formation, finer bulk ash particle formation and lower ash deposition. The aerodynamic factor, instead of the chemical composition related to fine particulates, determines the difference in the ash deposition behavior in two combustion modes. (C) 2012 Elsevier Ltd. All rights reserved.