This study evaluates mercury speciation and the effects of HCl, NO, steam and SO2 on mercury emission and its speciation from the combustion of Chinese bituminous coal and American bituminous coal under O-2/CO2 atmosphere especially with recycled warm flue gas in a 50 kW(th) circulating fluidized bed. Compared to air coal combustion, higher gaseous mercury concentration and higher percentage of Hg2+ under oxy-coal combustion were observed. Moreover, HCl, NO, SO2 and steam could cause the percentage of Hg2+ to increase. Compared to the air atmosphere, larger steam concentration under the oxy atmosphere played a significant role in promoting mercury oxidation. HCl promoted mercury oxidation via O and Cl radicals under oxy-combustion. These results are very significant for alternative methods of Hg removal under oxy-fuel combustion. (C) 2016 Elsevier B.V. All rights reserved.