The Mechanism Of Magnetic Fiber Material To Capture Weak Magnetic Fly Ash Particles Coupled With Puriperation For Elemental Mercury In Flue Gas

Fine particles of fly ash and trace Hg⁰produced by coal burning in coal-fired power plants are important sources of fine particulate matter(PM_2.5)and mercury in the atmosphere,among which PM_2.5can cause serious damage to human respiratory system and cardiopulmonary function,while mercury is a neurotoxicant,which has a more lasting dam to the environment and human health due to its strong migration,enrichment and concealment.At present,it is a major trend in the field of coal flue gas purification to use bag filter to achieve collaborative purification of various pollutants.Therefore,it is an important topic to study and enrich the collaborative purification mechanism to improve the efficiency of filter material and filter bag to collect fine particles of fly ash,and at the same time to cooperate with fine particles to effectively remove trace mercury in flue gas and reduce its harm to the environment and human health.At present,there are two trapping mechanisms of Brownian diffusion interception and inertial interception for fine fly ash particles by conventional fiber filter materials.However,the Brownian diffusion effect and inertial impact effect of submicron（0.5-1.0μm）fly ash particles are weak,resulting in the low fiber trapping efficiency.Because the fly ash particles of coal-fired power plants contain a small amount of magnetic components,such as Fe₃O₄,α-Fe₂O₃,γ-Fe₂O₃,the fly ash particles generally show weak magnetic characteristics.The magnetic field generated by magnetic fibers can induce the weak magnetic fly ash fine particles to migrate to the fiber surface,and enhance the trapping effect of weak magnetic fly ash fine particles.Therefore,this paper proposes a new method of collecting weak magnetic fly ash particles by magnetic fiber filter material,and studies the interaction mechanism between magnetic fiber magnetic field force and weak magnetic fly ash fine particles.At the same time of magnetic fiber trapping weak magnetic fly ash particles,Hg⁰will unite with the surface carboxylic acid function of the residual carbon in the fly ash particles and be oxidized to Hg²⁺.α-Fe₂O₃andγ-Fe₂O₃in the weak magnetic fly ash particles will be combined with Hg⁰and reduced to Fe O.The chemical adsorption oxygen in Fe₂O₃will directly oxidize Hg⁰to Hg ^O.However,due to the low content of unburned carbon and magnetic beads in fly ash,this catalytic oxidation adsorption effect is limited.On this basis,a magnetic mercury adsorbent Mag Fe Si-Cu⁰which can be recycled was developed in this paper,and the mercury removal performance and copper amalgam adsorption mechanism of its synergistic fly ash were studied.In addition,on the macro scale,adsorbent particles in bag filter are easy to be affected by gravity and inertia force and settle,resulting in limited adsorption of Hg⁰in suspension state and local escape of mercury on the surface of the filter bag.However,magnetic filter bag can induce magnetic adsorbent particles to migrate to the surface of the filter bag through the magnetic field generated by magnetic filter bag,and improve the deposition amount of adsorbent particles on the surface of the filter bag and the retention time of mercury adsorption.In this paper,the magnetic field distribution characteristics of magnetic filter bags were studied,and the mechanism of adsorbent particle migration and deposition and the performance of mercury removal were further studied.Firstly,magnetic cobalt ferrite nanoparticles were loaded on the surface of traditional fibers such as PPS,P84,PTFE and PSA by co-precipitation method and impregnation method.The magnetic filter materials for removing weak magnetic fly ash particles were developed by the preparation of magnetic fibers loaded with cobalt ferrite particles through the needling process of pre-opening,fine opening,cutting and coaling.The physicochemical characteristics of magnetic fiber filter material were analyzed by SEM,FTIR,XRD and VSM characterization.The results showed that the carbonyl group on the surface of magnetic P84 fiber formed rho-density conjugation with the active group-Oh of cobalt ferrite particles,and the maximum remanence and coercivity are 3.75emu and 1600Oe,respectively.Compared with the original P84 filter material,the variation range of acid breaking strength,warp and weft strength and temperature retention of magnetic P84 filter material is not more than 5%.Secondly,the numerical calculation method of CFD-DPM was used to simulate the interception process of weak magnetic fly ash particles captured by magnetic single fiber,and the dynamic characteristics of the particles under the magnetic force from the particle size d_p,particle susceptibility x_p,inlet velocity u and fiber magnetic field strength H_f.The results showed that the magnetic field generated by the single magnetic fiber can form a gravitational zone around the fiber and induce the weak magnetic fly ash particles to migrate to the fiber deposition surface.For the weak magnetic fly ash particles in the diffusion interception control area（d_p≤0.5μm）and the inertial interception control area（dp≥1.0μm）,the diffusion interception coefficient Pe and the inertial impact coefficient St play a leading role,respectively.Taking u=0.14m/s as the critical point,the smaller d_pand larger u in the diffusion interception area,the higher the particle capture rate,and the larger d_pand smaller u in the inertial interception area,the higher the particle capture rate.At the same time,the magnetic susceptibility of particles x_p（0.5%≤x_p≤3.5%）did not significantly promote the capture of weak magnetic fly ash particles in the two control zones,but it had a significant effect on the capture of weak magnetic fly ash particles in the transition zone（0.5≤d_p≤1.0μm）,and the capture rate of weak magnetic particles increased nearly 1.3 times when x_pincreased to 3.5%.Thirdly,while the magnetic fibers collect fine particles of weak magnetic fly ash,the mercury adsorption performance of weak magnetic fly ash with different carbon content was studied.It was found that the mercury adsorption rate of weak magnetic fly ash with carbon content of 12.7%was only 35%.It is difficult to achieve efficient removal of Hg⁰by fly ash alone.In this paper,magnetic adsorbent Mag Fe Si-Cu⁰was developed by loading Cu⁰and Fe₃O₄nanoparticles on the surface of Fe Si by powder metallurgy.The results showed that the adsorption of trace Hg⁰on the magnetic particle surface was controlled by the external diffusion of mercury and the adsorption of surface copper amalgam.The mercury adsorption efficiency decreased with the increase of gas temperature,and was positively correlated with the copper load.Cu⁰in Mag Fe Si-Cu⁰can be directly combined with Hg⁰to form copper amalgam.Lattice oxygen of some active component Cu O was released and Hg⁰was oxidized to Hg O,while Cu²⁺was reduced to Cu⁺.The carbonyl and carboxyl groups on the surface of unburned carbon in the weak magnetic fly ash particles can be used to oxidize Hg⁰into Hg-OM compounds.The chemisorption oxygen ofα-Fe₂O₃andγ-Fe₂O₃directly oxidizes Hg⁰to Hg O.The adsorption of mercury by SO₂and NO in flue gas was not significantly affected.The reaction process of copper amalgam was inhibited by O₂to a certain extent.The magnetic adsorbent particles had good adsorption and regeneration characteristics,and the removal efficiency decreased from 90.1%to 81%after five cycles of adsorption and regeneration.Finally,based on the study of magnetic adsorbent enhancing the mercury removal performance of weak magnetic fly ash particles,the migration and deposition of magnetic adsorbent particles induced by the magnetic bag magnetic field and the effect of this induction mechanism on the mercury removal performance of adsorbent particles were further studied.Firstly,combined with the magnetic filter coercivity and remanent magnetic strength and other characteristic parameters,the spatial magnetic field distribution calculation unit of Comsol permanent magnet was used to obtain the spatial magnetic field intensity distribution characteristics of magnetic filter bag.Based on this,the numerical calculation model of adsorbent particle migration movement under magnetic field force induction was established.The effects of different parameters on the particle suspension velocity,residence time and deposition distribution on the filter bag were discussed.On this basis,the dynamic performance of removing Hg⁰from four filter bags with adsorbent particles including original P84,PTFE,PSA and magnetic P84 was studied and compared.The results showed that the maximum magnetic flux density on the filter bag surface reaches 280Oe after axial magnetization,and the N-S magnetic sensing line distribution features of N pole on the outer surface and S pole on the inner surface were presented.The results of attribution analysis showed that when the magnetic field intensity was 150Oe,the dust concentration was 30g/m³and the particle size was 15μm,the adsorbent particles migrated the longest and the deposition rate on the filter bag surface was the highest.The best mercury removal performance of magnetic particles with magnetic P84 synergies is 76.2%when the filtration wind speed is 0.8m/min,the adsorbent concentration is 35g/m³and the flue gas temperature is 150℃.