Study On The Capture Of Trace Element Selenium In Coal-fired Flue Gas By Mineral Adsorbents

Selenium released in the process of coal combustion,due to its highly toxic and volatile,has attracted more and more attention.Selenium is often in the gas phase or concentrated in fine particles in the flue gas,which is difficult to be captured by the existing equipment.Therefore,the absorption of selenium in flue gas by mineral adsorbents has become one of the research hotspots.Firstly,thermodynamic equilibrium simulation was carried out for the morphology distribution of Se with temperature change.The results show that Se basically exists in the form of SeO₂（g）in oxidizing atmosphere and will not be affected by S.The morphology distribution of Se is more complex under reductive atmosphere,the element S can be combined with the element Se to form Se S（g）.The adsorption performances and leaching toxicities of Ca O,Mg O,γ-Al₂O₃ and calcium-based minerals on SeO₂ were investigated in a two-stage vertical tube furnace.The results show that,（1）The adsorption efficiency of metal oxides on SeO₂ meets:Ca O>γ-Al₂O₃>Mg O,leaching toxicity meets:γ-Al₂O₃>Mg O>Ca O.The adsorption efficiencies of Ca O,Mg O,γ-Al₂O₃ reach the highest at 700℃,and the adsorption form of selenium is Se（IV）.（2）The adsorption mechanism of calcite and dolomite is the reaction between Ca O generated by decomposition and SeO₂.The adsorption efficiency below 700℃is lower than Ca O;Surface physical adsorption was the main adsorption product,and leachable selenium was the main adsorption product at 500℃.The hole structure is flourishing and the adsorption efficiency is higher than Ca O at 900℃.The adsorption property of calcite is higher than that of dolomite.（3）The F-Ca O obtained by calcite calcination forms a uniformly dispersed monolithic structure,and its adsorption performance is better than that of Ca O.SeO₂ adsorbed on the surface of F-Ca O by chemical reaction may physically adsorb more SeO₂ molecules,forming multi-layer adsorption.The changes of adsorption efficiency and leaching characteristic of F-Ca O on SeO₂ under different temperatures and concentrations in O₂/SO₂/SO₂+O₂ atmosphere were investigated,and the competitive adsorption mechanism and adsorption priority between SO₂ and SeO₂ were explored.The results show that,（1）The selenium element in O₂ atmosphere is still Se（IV）.At700-800℃,O₂ can improve the adsorption performance,but enhance the leaching toxicity of the products.（2）In SO₂ atmosphere,SO₂ reacts with Ca O to form Ca SO₃,which can be transformed into Ca SO₄,Ca S or S⁰.Ca SO₄ is formed in SO₂+O₂ atmosphere.（3）On the one hand,SO₂ will occupy the active sites,destroy the pore structure and inhibit the adsorption of SeO₂.On the other hand,the dense product layer can prevent the SeO₂ molecules adsorbed from separating again.Two factors affect the SeO₂ adsorption process together.（4）The adsorption capacity of SeO₂ under SO₂/SO₂+O₂ atmosphere meets:SeO₂ first>absorption at the same time>SeO₂ later≈pure N₂.The leaching toxicity meets:SeO₂ later>absorption at the same time>SeO₂ first>pure N₂.Both SO₂ and SeO₂ have no obvious effect on the adsorption products of the other side.SO₂ is more likely to react with F-Ca O.The influence of CO₂ atmosphere on the adsorption efficiency and leaching characteristic of F-Ca O on SeO₂ under different temperatures,concentrations and adsorption time was explored.And the competitive adsorption mechanism and adsorption priority between CO₂ and SeO₂ were investigated.The results show that,（1）At 600-700℃,the carbonation reaction of F-Ca O in the atmosphere of CO₂ causes the formation of the massive structure,which binds to each other and inhibits the adsorption of SeO₂.The thermal stability of the product decreases slightly and the leaching toxicity increases.（2）It is difficult for carbonation to occur at 800℃or above,so it shows a lower inhibition effect on the adsorption of SeO₂.（3）The adsorption capacity of SeO₂ under CO₂ atmosphere meets:SeO₂ first≈pure N₂>absorption at the same time≈SeO₂ later.The leaching toxicity meets:absorption at the same time>SeO₂ later>SeO₂first≈pure N₂.SeO₂ can replace the CO₂ adsorbed by F-CaO.