Study On Catalytic Oxidation And Adsorption Techniques For Elemental Mercury In Flue Gas

Mercury emission control is not only an urgent need of the prevention and control of heavy metal pollution in China,but also faces the big challenge of implementing the Minamata Convention on Mercury.The removal of elemental mercury?Hg⁰?is the key issue of mercury emission control.Currently,catalytic oxidation and adsorption are the major emission control techniques for Hg⁰ removal.This study aims to develop new catalysts with high Hg⁰ oxidation efficiency and good sulfurresistance performance,as well as cheap Hg⁰ adsorbent with high adsorption capacity,to meet the need from various emission sectors.We first studied the Hg⁰ oxidation mechanism of Ce O₂?5?-WO₃?9?/Ti O₂catalyst and the impact of various flue gas components on the Hg⁰ oxidation efficiency by using the in-situ FT-IR,XPS,etc.The oxidation performance and sulfur/hydrothermal resistance of the catalyst were further improved through Cu O doping and flame synthesis.To reduce the cost of Hg⁰ adsorbent and increase the adsorption capacity,,we prepared the activated coke by pyrolysis of waste tires,cotton straw,residue of traditional Chinese medicine and municipal waste as adsorbent precursors,screened the performance of the prepared activated coke,and further improved the adsorption capacity by halogen chemical modification.Main conclusions are as follows:?1?The addition of NO and HCl promotes the mercury oxidation efficiency while SO₂ and NH₃ restrain it.The-NO₂ and Cl^*active sites formed by NO and HCl on the catalyst surface implies that the reaction of NO,HCl and Hg⁰ follow Eley-Ridcal and Languir-Hinshelwood mechanisms,respectively.SO₂ could compete with the adsorbed mercury on the catalyst,resulting in the release of adsorbed mercury into flue gas in the form of Hg⁰.In the presence of SO₂,the formation of Ce?SO₄?₂,Ce₂?SO₄?₃ or NH₄HSO₄inhibits the oxygen absorption and release of Ce⁴⁺/Ce³⁺and forms the active adsorption oxygen,which is the main reason for the catalyst activity reduction via SO₂ poision.?2?Cu O doping can significantly improve the merucry oxidation efficiency and the sulfur resistance of the Ce O₂?5?--WO₃?9?/Ti O₂ catalyst.The reason can be ascribed to the formation of bioxide-reduction cycle structure?Ce-O-Cu?via Cu O doping,which provides more O sites compared with Ce-O-W.The Cu O is easier to react with SO₂ since it is more alkaline than Ce O₂,which helps to maintain the oxidation activity of Ce O₂.?3?The mercury oxidation efficiency of flame synthesis catalyst?Cu O?10?-Ce O₂?10?-WO₃?9?/Ti O₂?reaches 99.2%at 350 ^oC and 87.2-97.7%at low temperature window?100-200 ^oC?,solving the problem of low activity for traditional vanadium tungsten titanium catalyst.After the hydrothermal treatment of 700 ^oC+10%H₂O,the mercury oxidation efficiency of Cu O?10?-Ce O₂?10?-WO₃?9?/Ti O₂still maintain at 95.1%.The active sites maintain high capacity of oxidation adsorpiton and O release,reuslting in high hydrothermal resistance.?4?The adsorption efficiency of active coke derived from cotton straw increased to98.5%?100%via halide-impregnation by NH₄Cl,NH₄Br and NH₄I.The proportion of C-X functional group is the reason for the difference in adsorption efficiency of active coke.The reaction mechanism of I and Br modified active coke reacted with Hg⁰ followed the endothermic reaction;the reaction mechanism of Cl modified active coke reacted with Hg⁰ followed the exothermic reaction.