Experimental Study On Hg Emission And Fly Ash Adsorption In Flue Gas Recirculation Of Low Load Coal-fired Boiler

Mercury emission from coal combustion is the main anthropogenic source of mercury pollution in the atmospheric environment.Industrial coal-fired boilers consume a large amount of coal every year.Because most of them are concentrated in cities and surrounding areas,the quality of coal is generally poor,and the level of pollutant control technology and technology are backward.Therefore,the emission and control of mercury in flue gas is worthy of attention and research.With the increasingly strict atmospheric emission standards of boilers,flue gas circulating denitrification as an economic and effective denitrification technology has been applied and promoted in some coal-fired industrial boilers,especially for low load industrial coal-fired boilers.However,the flue gas circulation of low-load coal-fired boiler causes the change of flue gas atmosphere and fly ash physicochemical characteristics,especially the generation of mercury and the removal of fly ash.Therefore,it is of great significance to study the generation characteristics of mercury and the law of fly ash adsorption and removal under the flue gas circulation mode of low-load coal-fired boiler.In this work,a pulverized coal settling furnace was built to study the influence of combustion flue gas composition on the emission of mercury,and mercury species were studied at the meanwhile。The variation of physicochemical properties of fly ash caused by flue gas circulation and the influence of mercury adsorption on fly ash was studied on a fixed bed mercury adsorption device.The results show that:（1）Reaction temperature and coal type are important factors that affect mercury emission.With the increase of flue gas circulation ratio,the oxygen content of flue gas decreased,the Hg_gas^tot concentration in flue gas decreased,and the Hg²⁺/Hg_gas^tot ratio decreased.Different proportion of flue gas purification results in different concentration of flue gas pollutants,which has an important influence on mercury oxidation.When the proportion of flue gas circulation is low,the increase of SO₂ concentration caused by flue gas circulation can promote the increase of Hg²⁺/Hg_gas^tot.However,with the increase of cycle ratio,SO₂ has a restraining effect.Due to flue gas circulation,the accumulation of NO in flue gas can obviously promote the increase of Hg²⁺/Hg_gas^tot ratio of flue gas,which depends on the O₂content.The increase of H₂O content in the circulating flue gas is beneficial to the oxidation of mercury,and the effect of H₂O on the oxidation of mercury is weak under the condition of low oxygen.Due to the increase of SO₂ content in combustion atmosphere caused by flue gas circulation,the concentration of Hg_gas^tot in flue gas is obviously promoted,and the components such as NO,CO₂ and H₂O have no obvious effect on Hg_gas^tot in flue gas.（2）Flue gas circulation causes the change of flue gas atmosphere and fly ash physicochemical properties,which has an important influence on the mercury adsorption performance of fly ash.The mercury adsorption capacity of fly ash formed in the clean flue gas cycle is significantly better than that in the air atmosphere combustion.With the increase of flue gas circulation ratio,the mercury adsorption capacity of fly ash increased gradually.Compared with lignite,bitumite fly ash shows better mercury adsorption performance,and has a positive correlation with unburned rate of fly ash.With the same proportion of flue gas cycle and different proportion of flue gas pollutant purification,the mercury adsorption performance of fly ash formed by combustion is obviously different.The increase of SO₂concentration in combustion atmosphere has a complex effect on the mercury adsorption performance of fly ash.It is affected by SO₂ concentration and coal type,and in most cases,it shows inhibition effect.In the experiments of lignite and bituminous coal,the NO concentration in combustion atmosphere has a positive correlation with the adsorption performance of fly ash.Under the experimental conditions,SO₂ in the simulated flue gas can inhibit the adsorption of mercury by fly ash,while NO and HCl can obviously promote the adsorption of mercury by fly ash.The content of O₂ in the atmosphere coupled with the above factors has a great influence on the adsorption performance of fly ash.When the oxygen content of flue gas is low,the inhibition of SO₂ on the adsorption capacity of fly ash is deepened,and the promotion of NO and HCl on the adsorption capacity of fly ash is weakened,which has a negative impact on the adsorption capacity of fly ash.