More than 100-h steady warm flue gas recycle operation was carried out on a 50 kW(th) oxy-fuel circulating fluidized bed (CFB) combustor burning three kinds of fuel. The results demonstrate the good safety benefit of oxy-CFB operation, especially in the oxy-transition process. A slightly higher oxygen concentration, ranging from 22.2% to 23.4% for different fuels in oxy-fuel operation, can bring equivalent or higher carbon burnout than air combustion. SO2 concentration in ppm unit is higher in flue gas while the SO2 emission in mg/MJ unit is lower than air combustion. The desulfurization efficiency of limestone can reach 80% in oxy-fuel combustion in this test. The higher Ca utilization rate burning coal in oxy-fuel combustion than that in air combustion may be associated with the high moisture content in the flue gas. Fuel nitrogen conversion ratio in oxy-fuel is much lower than in air combustion, and it looks like higher volatile content in fuel leads to a bigger reduction of NO in the recycle flue gas. (C) 2013 Elsevier Ltd. All rights reserved.