Study Of Pd-M Bimetallic Sorbents For The Removal Of Elemental Mercury From The Coal Derived Fuel Gas At Mid-temperature

Gasification is the source of clean coal conversion technology, and thepyrolysis is one of the necessary steps. More elemental Hg and higherconcentration of Hg could be emitted from coal pyrolysis or gasification thanthat from the coal combustion due to the gas phase product in a reducingatmosphere. The elemental mercury has stronger toxicity and it is difficult toremoval comparing with other forms of mercury. Therefore, the high efficientmercury removal technology for coal derived fuel gas has very importantpractical significance in the gas desulphurizing purification at the same time. Ithas been reported that some transition metal (such as Fe, Mn, Cu, Zn and Ce)oxides can be used as the sorbents for the Hg0and H2S removal, but theirefficiencies of Hg0removal were low at high temperatures. The noble metal Pdhas been considered as the good sorbent for mercury removal atmid-temperature, but easy to be poisoned by H2S. The characteristics of eachmetal could be respectively kept to prepare the bimetallic sorbents which can alleviate H2S poisoning of Pd and improve the efficiency of mercury removal atmid-temperature, there will be very good potential advantages. In this study, theHg0removal over bimetallic sorbents has been researched from coal derived fuelgas at mid-temperature.In our preparatory works, the activity of Hg0removal over the Pd/CACå'ŒPd-Fe/CAC sorbents has been studied and the feasibility of the bimetallicsorbents to remove mercury was preliminary confirmed. In this paper, thecoal-based activated carbon (CAC), coconut shell-based activated carbon(CSAC),-Al2O3, MCM-41molecular sieve and Beta molecular sieve werechosen as support, and Pd, Fe, Mn, Cu, Zn and Ce were used as active metal.Twenty-four kinds of supported sorbent were prepared by pore volumeimpregnation. The effects of five kinds of supports, the addition of the secondmetal, the impregnation sequence of metal components, initial concentration ofmercury and environmental temperature were investigated. Combined with theresults of XRD, BET, XPS, XAS, TPR, TEM and Raman, the Hg0removalmechanism over Pd/γ-Al2O3and1Pd5Fe/γ-Al2O3sorbents was explored. Theconclusions are mainly summarized as follows:(1) The addition of the second metal could improve the efficiency of theHg0removal over Pd/CAC sorbents, especially, Fe, followed by Mn. Fe is thebest choice for the second metal considered its low price. But the support caninfluence the second metal to improve the efficiency of the Hg0removal over Pdsorbents and the efficiency of the Hg0removal over Pd/γ-Al2O3sorbents is difficult to improve, especially the second metal Fe added reduce the efficiencyof the Hg0removal over Pd/γ-Al2O3sorbents.(2) The supports had a significant effect on the activity of Hg0removal overPd-based sorbents. As the single metal palladium used as active component, theefficiency of Hg0removal of sorbent with γ-Al2O3as the carrier was the bestthan those with other carriers, followed by the sorbents with MCM-41and Betazeolite as the carriers. And that of of the sorbent with activated carbon as thecarrier was the worst. As the bimetallic palladium and iron used as activecomponent, the efficiency of Hg0removal of sorbent with CAC as the carrierwas the best, followed by the sorbents with CSAC and Beta molecular sieve asthe carriers. While that of the sorbent with γ-Al2O3and MCM-41molecularsieve as the carrier was the worst.(3) The effects of impregnation sequence of metal components on the Hg0removal were significant, and the effecting rules were related to the support andthe second metal component. At250°C, the Hg0removal efficiency of theCePd/γ-Al2O3(simultaneous impregnation) sorbent was the highest among thoseof bimetallic sorbents obtained with other impregnation order and Pd/γ-Al2O3,and it can maintain for480min under the efficiency of above98%. The analysisresults indicated that the impregnation order mainly affect the dispersion of themetal components for the sorbents loaded with Ce and Pd. When Ce and Pdwere impregnated simultaneously, the dispersion of CeO2and PdO on theγ-Al2O3support was the most uniform. The Hg0removal efficiencies of the bimetallic sorbents with the diffirent impregnation order of Fe and Pd weremuch lower than that of Pd/γ-Al2O3. The Hg0removal efficiency ofZn-Pd/γ-Al2O3(impregnation of Zn before Pd) sorbent was similar to that ofPd/γ-Al2O3sorbent and much higher than that of Pd-Zn/γ-Al2O3andZnPd/γ-Al2O3sorbents.(4) The Hg0removal mechanism of1Pd5Fe/-Al2O3sorbent inN2-H2S-Hg-CO-H2atmosphere could be describe as below: PdO could reactwith H2to form Pd2O and the Pd2O be further reduced to Pd0; PdO could reactwith CO to produce Pd0. Pd-Hg amalgam could be generated from the reactionof Pd0and Hg0. The efficiency of the Hg0removal is reduced because thepoisoning effect of H2S in the coal derived fuel gas could react with PdO toform PdS; Fe2O3in the sorbents could react with H2S to produce FeSxand Sad,and Sadcould react with the generated Pd0to form PdS.