BACKGROUND This work evaluated the mechanism of H2O on mercury oxidation and adsorption by activated carbon in simulated flue gas in a fixed-bed reactor. The effect of the addition of SO2 on the mercury removal was also discussed. Mercury desorption tests were conducted in a setup for classifying mercury forms adsorbed on activated carbon by the temperature-programmed desorption (TPD) method. RESULTS The results indicated that H2O could inhibit Hg-0 removal through blocking the microporous channels of activated carbon. Additionally, the enriched H2O could provide the electrons to inhibit Hg-0 oxidation. While medium H2O concentrations around 15% could dominate the competitive adsorption in O-2/CO2, high and low H2O concentrations of 23% and 8%, respectivly, could not dominate the competition. CONCLUSION With the addition of SO2 in O-2/CO2, Hg-0 removal efficiency decreased with H2O increased from 8% to 23%. Furthermore, Hg-0 adsorption proportion were all larger than 87% with the addition of SO2 with H2O in O-2/CO2, which indicated that Hg-0 adsorption was the main contributor to Hg-0 removal performance. These results are useful for the control of mercury in O-2/CO2 combustion technology. (c) 2018 Society of Chemical Industry