Investigation of mercury emission and its speciation from an oxy-fuel circulating fluidized bed combustor with recycled warm flue gas

•The oxy atmosphere was suitable for mercury oxidation.•NO and SO2 enhanced mercury oxidation under oxy-coal combustion.•Larger H2O concentration under the oxy atmosphere promoted mercury oxidation.

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 O2/CO2 atmosphere especially with recycled warm flue gas in a 50 kWth 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.