Development Of Membrane Delivery Catalytic Oxidation System And For The Conversion Of Elemental Mercury From Flue Gas

The pollution by atmospheric mercury emitted from coal combustion has beenbecoming more serious, and the development of mercury emission control technology hasbeen paid more and more attention in the environmental field. Elemental mercury （Hg⁰） isone of the main forms in coal combustion flue gas, which is difficult to be captured withthe existing air pollution control devices. At present, the favored method for Hg⁰removalis to oxidize it to its oxidized form (Hg²⁺) by means of some proper catalysts. However,the traditional catalytic oxidation mode （TCO） has encountered some obviousdisadvantages on the oxidation of trace level Hg⁰in flue gas, including the low removalefficiencies of Hg⁰, high operating cost of oxidants and potential secondary pollutionproblems and so on.In order to overcome the above disadvantages, a novel MDCO_s technology was putforward for the oxidation of the trace level Hg⁰in flue gas. The MDCO_s system employeda porous ceramic membrane as principal body, and the catalytic material of MnO_xwasimpregnated on it. By means of the MDCO_s system, the adjustable delivery of thereagents for Hg⁰oxidation （e.g. HCl）, adsorption-enrichmen and catalytic oxidation ofHg⁰can be well integrated.Firstly, the Mn/Al₂O₃membrane catalytic system was built with MnO_xas maincatalyst/adsorbent and HCl as oxidant precursor respectively. And then the conversionprocess of Hg⁰to Hg²⁺in the presence/absence of SO₂, NO and O₂were studied.Simultaneously, the Deacon process of HCl （the conversion of HCl to Cl₂）was discussedin detailed by studying its delivery, conversion, adsorption and adsorption-enrichment.Moreover, the above results were compared to that in the TCO mode.Further, to improve the conversion efficiency of trace level Hg⁰in the Mn/Al₂O₃membrane catalytic system, and decrease the consumption of HCl, the Ru-Ti/Al₂O₃membrane catalytic system was built by means of loading RuOxon the ceramic pipe after doping it with TiOx. The Deacon process and conversion of Hg⁰were studied under thetemperature of300℃and various flue gasambiences. To obtain higher sufficient activityfor Hg⁰conversion under the temperature as low as150℃, the multi-metal oxide catalystswere employed to form the Mn-Mo-Ru membrane catalytic system. And then, theconversion of Hg⁰and the Deacon process of HCl were all discussed. Finally, theactivation reactions of HCl by Deacon process were discussed over the catalysts with Ru.The kinetic equations were setup and the key factors that influenced the conversion of Hg⁰were also determined. The main conclusions were summarized as follows:（1） The Hg⁰removal efficiency of81.8%and93.6%were achieved with1ppmv and5ppmv of HCl under300℃in the Mn/Al₂O₃membrane catalytic system, respectively. TheHg⁰removal efficiency of97.4%was achieved with only3ppmv of HCl in theRu-Ti/Al₂O₃membrane catalytic system. And under low temperature of150℃, over90%of Hg⁰removal efficiency could be achieved with only8ppmv of HCl in the Mn-Mo-Rumembrane catalytic system.Compared with the TCO mode, the MDCO_s system had not only more high removalefficiency of trace level Hg⁰, but also significant economic superiority. Moreover, theMDCO_s system exhibited excellent sulfur-tolerance to low concentration SO₂withinsignificant effect for the conversion of Hg⁰. However, over1000ppmv of SO₂, forexample,1500ppmv of SO₂, had obvious inhabitation for the conversion of Hg⁰.（2） Because the Deacon process of HCl had a crucial effect on the conversion of Hg⁰,the activation of HCl in the MDCO_s system was detailedly studied. The results of theDeacon process of HCl show that low conversion efficiency of HCl of only32.6%wasachieved in the Mn/Al₂O₃membrane catalytic system under300℃. The performance ofRu-Ti/Al₂O₃membrane catalytic system was better than that of Mn/Al₂O₃catalyst, withthe Deacon efficiency of60.3%under300℃. Under low temperature of150℃, the abovetwo membrane catalytic system had lower Deacon efficiency of less than10%. However,the efficiency of23.0%could be reached with the Mn-Mo-Ru membrane catalytic system.Moreover, the amount of escaped HCl was low with only0.5mg/m3. The high Deaconefficiency of HCl effectively validated the high conversion of Hg⁰in the MDCO_s system.（3） The conversion mechanism of Hg⁰over the Mn/Al₂O₃membrane catalytic systemshowed that Hg⁰was oxidized by the lattice oxygen on the surface of the catalysts, andthen converted to HgO in the absence of oxidants. The speciation of HgCl₂was dominant,which resulted from the combination of Hg⁰with activated chlorine under the ambience of HCl. Among the production of Ru-Ti/Al₂O₃and Mn-Mo-Ru membrane catalytic systems,over90%were converted to HgCl₂through the combination of Hg⁰with activatedchlorine. However, the speciation of HgO was not dominant in the presence of oxidants.（4） The XPS results show that the speciation of Mn, Mo, Ru and Ti on the freshcatalysts were mainly Mn（IV） and Mn（III）, Mo（Ⅵ） and Mo（IV）, Ru（Ⅳ） and Ti（Ⅳ）,respectively. The above speciation had no obvious change during the process of theconversion of Hg⁰to Hg²⁺and the process of the Deacon reaction. During the processabove, part of Mn（IV） and Mo（Ⅵ） could be regenerated by the oxidation of Mn（III） toMn（IV） and the oxidation of Mo（Ⅳ） to Mo（Ⅵ） under the ambience of oxygen,respectively. And the regeneration process of Mn（IV） and Mo（Ⅵ） kept the stability of thecatalysts, and meanwhile, improved the life and catalytic activity of the catalysts.（5） The characterizations and kinetic analysis showed that the conversion of Hg⁰mainly occurred via a Mars-Maessen mechanism. The produced amount of Hg²⁺in theMDCO_s system would be affected by the concentration of Hg⁰, balance concentration ofHCl in the flue gas, content of oxygen, BET of the catalysts, especially reactiontemperature and the speciation and content of transition metals. In addition, the masstransfer process of HCl in the MDCO_s system was also determined to be the rate limitingstep for the conversion of Hg⁰.In a word, the innovations and characteristics of the article included mainly:1) toimprove the removal efficiency of trace level Hg⁰in flue gas, a MDCO_s technology wasput forward, which employed ceramic membrane as catalytic carrier, and the transitionmetals of Mn, Mo, Ru and Ti as catalytic materials. The MDCO_s system had obviousinnovation over the system structure by integrating the release and adsorption-enrichmentof oxidants, and catalytic oxidation for Hg⁰.2) The MDCO_s not only could achieved theexcellent conversion of Hg⁰, but also had better resistance ability against SO₂under thetemperature of300℃and150℃. Moreover, compared with the TCO mode, the MDCO_ssystem could significantly decrease the dosage and escaped amount of oxidants, inhibit thepoison for catalysts resulted from the lots of dusts in the flue gas, and improve theactivation of catalysts under the low temperature.3) The high Deacon efficiencies wereachieved with the Mn-Mo-Ru membrane catalytic system under the ambience of HCl andO₂and the temperature of300℃.