Experiment And Mechanism Analysis Of Interface Between Mercury In Flue Gas And Fly Ash From Coal-fired Power Plant

Mercury is one of highly toxic heavy metals.Mercury and its compounds can enter into human body in some different ways,such as respiratory tract,skin,alimentary canal and so on,which lead to the abnormal metal-nervous,gingivitis,fremitus,and even death.Combusting coal is classified as the largest single anthropogenic source of mercury air emissions.Since coal-fired power plants consume the largest proportion of all the coal,their mercury emission becomes a crucial factor in determining the degree level of the directly and potential threats for the ecology.Understanding the mecury control techniques regarding coal-fire power plants play a vital role on resolving our national mercury pollution and environmental protect,which highlights its realistic and scoical significance.It should point out that high cost-effictive parameter is regarded as an important target to estimate the sorbent whether it should be developed and applied.Fly ash,as the by-product of coal combustion,might catalyze the oxidation of mercury and contribute to its capture.Compared to the other sorbents in terms of mercury adsorption,there is nothing comparable to the advantage of fly ash in the economic,which would inspire a large motivation to promote the mercury control of coal-fired power plants.Additionally,the application of fly ash substituted for the other mercury-adsorbed sorbents is benefit for high-efficient utilizing resources,energy conservation and emission reduction,building the conserving society.However,for fly ash,the disadvantage,including the low mercury removal efficiency,the complicated compositions and their contents,rapidly give rise to difficulty in its application.So far,the mercury-fly ash interaction is still unknown and needs to be studied more thoroughly.Without the enough experimental and theoretical strategies,there severly lack the modified methods for improving the mercury removal efficiency of fly ash.This becomes the main obstacle for fly ash used in industrial mercury control.The purpose of this dissertation is to analysis the characteristics of fly ash in order to understand the basic principles of mercury adsorbed by fly ash.It can considerably promote a great progressive in the mercury control for coal-fired power plants.As mentioned above,for studying the characteristics of fly ash and mercury retention in fly ash.The mechanical sieving method was adopted to separate five different sizes of particles.In this work,the five fly ashes were characterized by mass distribution,LOI?Loss of Ignition?,surface morphology,elemental distribution,crystal structures and Raman spectrum of carbon.The results showed that the contents of unburned carbon?UBC?in all the fly ashes were very low.The smaller the size of particles were,the larger the UBC content were.The shapes of particles would generally change the ball as decreasing the particle size.Fly ash consisted of the four main elements?C,O,Al and Si?and some minor elements,such as Fe,Ti,Mg,Ca,K,Na,S,and so on.Among them,there existed the strong nagtive correlation between C and O,but the correlation cannot be found between C and Si or Al.Particle sizes had no effect on the feature of tailings in fly ash.All the particle sizes included the one band and D1 Raman peaks.However,the carbons in fly ash were characteristic of the very disordered structure,and this disordered level would be more serious as decreasing the particle sizes.Furthermore,the eight fly ahes from different coal-fired power planted were set as samples and undergone the 300?,450?,600?and 750?treatments.The adsorption experimental results showed that the75-85%total mercury content of fly ash would be decomposed at 300?.There was only less than 3%totoal mercury content for the fly ash after 600?treatment.At any temperature,no linear relationship was observed between carbon and mercury in fly ashes.Meanwhile,there was also no correlation between Cl and mercury in fly ashes.The setup mathematic model showed that mercury retention was relative to carbon,silicon,iron,sulphur,and magnesium in fly ash.The carbon and inorganic in fly ashes determined the amount of mercury retention.For studying mercury adsorption of fly ash.Five fly ashes with different particle sizes were used as the examined samples to perform the mercury adsorption testing.The mechanism was explored by an analysis of interplay btween mercury removal efficiency?MRE?and paticle size,LOI and Raman spectrum of carbon.The results showed that all the fly ashes had the low MREs that are less than 30%.Particle size affected the MRE.There was no linear correlation between LOI and MRE.Some certain inorganic components may have the high mercury capture capacity.The calculated results by the first-principle theory indicated that no clear difference in MRE was found between the sp²and sp³ carbon.HCl gas can considerable improve the MRE of fly ash.When the concentration of HCl gas was 100ppm,the MRE of fly ash can reach highly 82.9%.For SO₂ gas,the MRE depended on the concentration of SO₂ gas.Both large and small concentrations cannot facilitate the mercury removal.When the concentration of SO₂ gas was 500ppm,the MRE was only 5.81%.However,there was a weak growth in the MRE even in the optimized concentration.For NO gas,all the MREs of samples presented a clear decrease under any concentrations.When the concentration was 50ppm,the MRE only decreased to 6.93%.For the mixture gas,the MRE would have a considerable decrease for the fly ash as compared to that in HCl gas,but there was an entire opposite tendency for SO₂ and NO gases.Additionly,mercury retention cannot determine the MRE of fly ashes.UBC in fly ash cannot aslo determine the MRE.There was no any correlation between MRE and porous texture including pore volume,pore size and BET surface area.The Cl concentration in fly ash presented an approximate linear relation.However,some samples also deviated this route.Hence,the MRE of fly ash did not depond on one component,but rather the combined action of many components of fly ashes.In terms of the effect of components on the mercury adsorption of fly ash.The floating method was used to separate the six types of UBC from six fly ashes,respectively.After the mercury adsorption tests,the results showed that the higher the LOI of fly ash was,the higher LOI of separated UBC was.There was on linear relation between MRE and LOI of these separated UBCs.The inorgainic material in the UBC may also affect MRE.It should point out that not all the fly ahes was suitable to be separated to obtain the UBC as the mercury sorbent,because UBC poccesses a strong selectivity for mercury.For the inorgainic components,the Fe-and Mn-oxidate were selected as the typically respectative to evaluate the effect on the mercury adsorption.The Ce and Cu elements were doped into these oxidate to simulate the practical cases because there were many elements in fly ash.In this work,we only focused on the effect of iron content on MRE.Then we fabricated four different Mn-Ce-Fe oxidate.The adsorption tests showed that the MRE increased with iron content.When the more ratio of Mn-Ce-Fe is 5:4:2,the MRE reached 97.01%.While continuing to increase the iron content,the MRE presented a rapid decrease to 39.09%.Hence,iron content affected significantly the MRE.At the same time,the five different mole ratio of Mn-Cu-Fe oxidate were produced by impregnation method and coprecipitation method.The adsorption test were performed for all the samples under120?,150?and 180?.The results showed that in the initial stage,the MRE increased with the iron content.When the iron raito is 3,the MRE rapidly increase to 27.25%and32.03%for samples fabricated by impregnation method and coprecipitation method,respectively.Furthermore,there existed the decrease in the MRE with increasing iron content.The MREs of all the samples increased with temperature,and its magnitudes reached highly 39.13%?150??and 44.36?180??.The results also showed that the Mn-Cu-Fe oxidates have smaller MRE than the Mn-Ce-Fe oxidates.This suggested that the doped Fe and Mn elements considerably affected the MRE.Based on this conclusion,it can be seent that Mn-and Fe-oxidate would show the discreteness in MRE because of the variation of elements and their contents in fly ash,which would bring the great difficulty to analysis the MRE tendency.In the other hand,the fly ashes were impregnated by H₂SO₄ with 30%,60%,and 98%concentration and of Na₂SO₄ solution with the corresponding same SO₄^2-concentration.The adsorption testing indicated that the sulfuration deteriorate the MRE of activated carbon.For the fly ash,the mercury adsorption capacity depended on the competing adsorption between SO₄^2-and organic surface.For studying the mechanism of mercury adsorption.The quantum chemical calculation was used to study the mercury adsorption for carbon and Al₂O₃ as the two typical materials in fly ash,with which the mechanism of mercury adsorption were explored by analyzing the interfacial structure and electronic orbits.All the possible approach of SO₃ molecular in the carbonaceous surface were studied.The results showed that the normal decrease in the MRE of carbonaceous materials mainly resulted from the competing mechanism between SO₃ and C in the carbonaceous surface.Meanwhile,SO₃decreased the activity of its next-near-neighbor carbon atom.It also changed the LUMO and HOMO orbits and increased the gap of LUMO-HOMO.These motions gave rise to the difficulty in chemical process between carbonaceous surface and mercury.Additionally,the effect of sulfuric acid on MRE of carbonaceous surface was also studied.All the possible approach of SO₄ molecular in carbonaceous surface was carried out,and two SO₄molecule with and without charge were proposed to simulate the practical condition.The results showed that the carbonaceous surface with no-charge SO₄ molecular had the larger adsorption energy than the pure carbonaceous surface.However,most of carbonaceous surfaces with the charge SO₄ molecular had smaller adsorption energy than the pure carbonaceous surface.The SO₂ molecule was formed in the adsorption procedure.The influence of SO₄ molecular on MRE of carbonaceous surface was very complicated.Firstly,there was the competing adsorption between mercury and SO₄ or SO₄^2-;secondly,the contentration of SO₄ or SO₄^2-affected the mercury adsorption.The large contentration of SO₄ without charge was benefit for the MRE,but the SO₄^2-showed an oposite tendency.Furthermore,Al₂O₃ was selected to represent the the inorganic materials in fly ash to evaluate the MRE.The Hg/Al₂O₃?100??1×1?model was set up to study the mercury adsorption on the Top and Hollow activate sites by the energy belt,partical density of electron?PDOS?,and density electron different.The results showed that Al atom never interacts with mercury.The PDOS of mercury on the Top active site had no any change before and after mercury adsorption,but there were some changes in the PDOS of mercury adsorbed on the Hollow active site,which orbital hybridization may be occurred between Hg and Al₂O₃?100??1×1?.All the calculated results indicated that oxgen plays the main role on mercury adsorption of Al₂O₃.