In this study, co-combustion of Bursa-Orhaneli lignite and woodchips mixtures containing 10%, 30% and 50% by wt. of woodchips was studied in a circulating fluidized bed combustor in order to investigate the effect of excess air ratio (k) on the flue gas emissions. The combustor has an inside diameter of 108 mm and a height of 6 m. The temperature of the combustor is kept at 850 degrees C during the combustion tests. During the combustion tests, CO, O-2, NO, and SO2 emissions in the flue gas was continuously measured and recorded by ABB-AO 2000 flue gas analyzer. The results of the tests showed that increasing excess air had a cooling effect on the combustor, but at the same time it also provided smoother temperature profile along the combustor. In order to get minimum flue gas emissions, the optimum excess air ratios for the co-combustion tests of fuel mixture including 10%, 30% and 50% by wt. of woodchips were determined to be 1.18, 1.32 and 1.41, respectively. While woodchips addition to the lignite made CO emissions worse, it did not change NO emission. CO and NO emissions were below the limits but SO2 emission was above the limit for the case of 10% and 30% woodchips co-combustion at optimum excess air ratio. In the case of 50% woodchips co-combustion, SO2 and NO emissions were under the limits but CO emission was a little above the limit at optimum excess air ratio. (C) 2013 Elsevier Ltd. All rights reserved.