| Mercury from coal-fired flue gas is considered as a global pollutant,endangering human health and the ecological environment.Chemical looping combustion is a promising type of combustion technology with internal separation of CO2 that can achieve no external energy consumption.Iron-based oxygen carriers have the potential to catalyze the oxidation and adsorption of mercury while realizing the role of lattice oxygen transport.The research on the migration and change of mercury during the chemical looping combustion process has become a hot research topic.This paper first simulates coal gasification and chemical looping combustion through chemical thermodynamic calculations,and then optimizes the preparation and use of a magnetic Co core iron-titanium-based oxygen carrier with a core-shell structure,and finally develops an experimental study on the occurrence and migration and transformation of mercury in chemical looping combustion process.The effects of reaction temperature,gasification medium,flue gas composition,cycle times,and amount of oxygen carrier on the release and migration of mercury in coal under chemical looping combustion are obtained,which provides theoretical and experimental basis for the clean combustion of coal chemical looping.Based on chemical thermodynamics method,the paper uses HSC Chemistry 6.0calculation software to carry out the simulation study on the mercury release and migration law by the reaction temperature,gasification medium and flue gas components in the three modes of coal gasification,chemical looping combustion and chemical looping gasification.The thermal calculation results show that the oxygen carrier mainly reacts with HCl to promote the oxidation of mercury in chemical looping combustion.The release of mercury in coal gasification and chemical looping combustion conversion is affected by the reaction temperature.Almost all mercury present in coal will be converted into Hg0(g),and only a small amount of mercury exists in the form of Hg2+(g).The addition of oxygen carrier promotes the release of mercury and the generation of Hg O(g)in coal.The content of HCl in fuel reactor is higher than that of Cl2 and Cl.Under the action of HCl,the oxygen carrier provides lattice oxygen to promote the conversion of Hg0(g)to Hg2+(g)to generate Hg Cl2.The amount of Hg Cl2 produced increased with the increase of chlorine content,and the element of chlorine increased the occurrence range of Hg Cl2.In chemical looping combustion,the production of SO2 and NO in flue gas components is more than that of coal gasification,and the production of CO is lower.The H2O in gasification medium will react with Cl2 to form HOCl and HCl,which are less oxidizing,to inhibit the oxidation of mercury.The mercury release and form distribution of chemical looping gasification process are similar to chemical looping combustion,but it will inhibit the oxidation of mercury because of the stronger reducing atmosphere.The paper uses precipitation method to prepare Co core,sol-gel method to prepare Ti O2intermediate layer,impregnation method to prepare Fe2O3,and finally prepares magnetic Co core iron-titanium-based oxygen carrier.X-ray powder diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),vibrating sample magnetometer(VSM)and other analytical testing methods are used to characterize the physical and chemical properties of the oxygen carrier and optimize the preparation method.The results show that the Co core prepared by this process is spherical crystals,and the thickness of the Ti O2 coating layer is about 300 nm.The average particle size of the final magnetic Co core iron-titanium-based oxygen carrier is about 2.5?m with good dispersion,and the saturation magnetization is 18.28 emu/g.Based on the high-temperature tube furnace experimental system,the magnetic Co core iron-titanium-based oxygen carrier was used to conduct experimental research on the release,transformation and migration distribution of mercury in fuel and air reactor was investigated during coal chemical looping combustion.The effects of several parameters,such as temperature in flue reactor,flue gas composition,gasification medium,the number of cycles and chemical looping gasification conditions were investigated.The experimental results show that the mercury release concentration in coal under chemical looping combustion presents a unimodal characteristic and increases with the increase of temperature,and the mercury release reaches its peak at about 120 s.The oxygen carrier has chemical adsorption of Hg0 and Hg2+,and the mercury release rate increases monotonously with the increase of temperature.The best mercury removal effect is at 800?,when the mercury precipitation rate is 40%,indicating that the magnetic Co core iron-titanium-based carrier is beneficial to reduce mercury emissions in coal.Temperature is the decisive factor affecting the release and distribution of mercury in two reactors.About 90.7%-97.6%of the mercury is released in fuel reactor.The proportion of Hg0 in the total gaseous mercury under all working conditions is higher than 60%.In chemical looping combustion,the CO-based reducing atmosphere inhibits the oxidation of mercury.The low concentration of NO in flue gas components is beneficial to the oxidation of mercury,and SO2 mainly inhibits the oxidation of mercury by affecting the generation of chlorine.In addition,the oxygen carrier can also directly affect the catalytic oxidation of mercury by providing lattice oxygen or indirectly by affecting other flue gas components.The addition of H2O to the gasification medium promotes the release of mercury and inhibits the oxidation of mercury.The oxygen carrier was superparamagnetic in 20 cycles,the saturation magnetization decreased by 1.2 and 3.66 emu/g after 10 and 20 cycles respectively,and the magnetic separation recovery rate was 92.7%and 81.46%,indicating that the oxygen carrier has good magnetic separation and recovery effect which can achieve the purpose of separation and recycling with ash.The oxygen carrier still has a good mercury removal effect within 10cycles,and the mercury release rate is 48.4%.After 20 cycles,the mercury removal effect is significantly reduced.The number of cycles will also have a certain impact on the distribution of mercury forms and the impact is not significant within 10 cycles.The mechanism of mercury release and conversion under chemical looping gasification is different from chemical looping combustion.Since the adsorption of mercury is weakened and the gas composition changes,the mercury removal effect of the oxygen carrier becomes worse.The mercury precipitation rate at 800°C is 58.9%.Due to the stronger reducing atmosphere and the weaker catalytic oxidation ability of oxygen carrier to mercury,the oxidation of mercury is inhibited and the final relative content of Hg0 is higher than that of chemical looping combustion. |
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