Abstract: In order to explore mercury adsorption characteristics of fly ash formed in coal-fired boilers in flue gas circulation mode, mercury adsorption characteristics of fly ash formed in a vertical pulverized coal settling furnace under different simulated flue gas circulation conditions were evaluated by a fixed bed mercury adsorption device. Some key factors, such as flue gas cycle ratio, coal type and flue gas component (e.g., SO₂ and NO) were mainly investigated. The results revealed that the mercury adsorption capacity of fly ash in flue gas circulation mode was obviously higher than that in non-flue gas circulation (i.e., air combustion) mode. With an increase in flue gas circulation, the amount of mercury adsorption by fly ash increased gradually. For fly ash formed from lignite, when clean flue gas circulation ratio was 60%, 40% and 20%, the amount of mercury adsorption was 3.0, 2.3 and 1.6 times that with non-circulation of flue gas, respectively. This study also indicated that the flue gas circulation caused changes in the concentration of SO₂, NO in combustion gas, therefore affecting the physicochemical property of fly ash and its mercury adsorption property. With increase of SO₂ concentration in combustion gas, the amount of mercury adsorbed by fly ash increased to the peak value and then decreased. When flue gas circulation ratio was 40%, NO concentration in combustion gas was 803 mg/m³, and SO₂ concentration in combustion gas was 2857 mg/m³, the amount of mercury adsorbed by fly ash formed from lignite and bituminous was 0.45 and 0.75 μg/g, respectively, which increased by 25%-300% and 53%-78%, respectively, compared with the amount of adsorbed mercury at 1428 mg/m³ and 4286 mg/m³ of SO₂. When NO concentration increased in combustion gas, the amount of mercury adsorbed by fly ash gradually increased. When flue gas circulation ration was 40%, SO₂ concentration in combustion gas was 2857 mg/m³, and NO concentration in combustion gas was 1205 mg/m³, the amount of mercury adsorbed by fly ash formed from lignite and bituminous was the highest, which is 1.2-3.6 and 1.1-1.6 times higher than those at NO concentrations of 803 mg/m³ and 402 mg/m³, respectively. It was also found that unburned carbon, CaO, MgO and Fe₂O₃ in fly ash promoted mercury adsorption. Compared with fly ash formed from lignite, the fly ash formed from bituminous showed a better mercury adsorption performance, which is positively correlated with the content of the above-mentioned substances in the fly ash. This study shows that the mercury adsorption characteristics of fly ash vary significantly in different flue gas circulation mode, and emission control effect of mercury in flue gas from coal-fired boilers can be significantly improved by reasonable selection of coal types, optimal control of the flue gas circulation ratio, circulating gas compositions and their concentration parameters.