工业燃煤锅炉烟气循环方式下飞灰的汞吸附特性

为探究燃煤锅炉烟气循环方式下飞灰汞的吸附特性，利用固定床汞吸附装置，对立式煤粉沉降炉不同模拟烟气循环工况条件下形成的飞灰汞吸附特性进行了研究，重点考察了烟气循环比例、煤种、关键燃烧气体组分等因素的影响.结果表明:①烟气循环工况条件下形成的飞灰汞吸附能力明显优于非烟气循环工况（空气燃烧）条件下形成的飞灰，并且随烟气循环比例的增加，飞灰汞吸附量呈逐渐增加趋势.褐煤在净烟气循环比例为60%、40%、20%的条件下形成的飞灰汞吸附量分别是非烟气循环条件下的3.0、2.3和1.6倍.②烟气循环引起燃烧气体氛围中ρ（SO₂）与ρ（NO）的变化会影响飞灰的物化特性及其汞吸附性能.随燃烧氛围中ρ（SO₂）的提高，飞灰汞吸附量先增后减.烟气循环比例为40%且燃烧气体氛围中ρ（NO）为803 mg/m3条件下，ρ（SO₂）为2 857 mg/m3时褐煤与烟煤燃烧形成的飞灰汞吸附量较高（分别为0.45和0.75 μg/g），分别较ρ（SO₂）为1 428和4 286 mg/m3时提高了25%~300%和53%~78%.随燃烧氛围中ρ（NO）的提高，飞灰汞吸附量呈逐渐增加趋势.烟气循环比例为40%且燃烧气体氛围中ρ（SO₂）为2 857 mg/m3条件下，ρ（NO）为1 205 mg/m3时褐煤和烟煤燃烧形成的飞灰汞吸附量较高，分别较ρ（NO）为803和402 mg/m3时提高了1.2~3.6和1.1~1.6倍.③飞灰中UBC（未燃尽碳）、CaO、MgO及Fe₂O₃可促进飞灰对汞的吸附.与褐煤飞灰相比，烟煤飞灰表现出更优的汞吸附性能，与UBC、CaO、MgO及Fe₂O₃在飞灰中的含量存在一定的正相关性.研究显示，烟气循环方式下飞灰汞吸附特性发生明显变化，煤质的合理选择、烟气循环比例、循环气体成分及浓度参数的优化控制可显著改善燃煤锅炉烟气中汞的排放控制效果. 

Mercury Adsorption Characteristics of Fly Ash Formed in Industrial Coal-Fired Boilers in Flue Gas Circulation Mode

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.