Preparation Of 1Pd5Fe₂O₃/Al₂O₃ Sorbent By Co-precipition And Its Behaviour Of Mercury Capture From Coal-drived Fuel Gas At Mid-temperature

Coal gasification is the leading technology of the cleaning coal conversion. Higher concentration of elemental mercury can be emitted from coal gasification than that from coal combustion. Elemental mercury is considered as a global threat to human health and environmental because of its toxicity, volatility, durability and bioaccumulation, and it is more difficult to be removed for its high equilibrium vapor pressure, low solubility in water and low melting point. The research on the sorbent for efficient removal of Hg from coal gas is urgently needed. The noble metal Pd has the very excellent ability of removing Hg0 from coal-based fuel gas at mid-temperature, but susceptible to hydrogen sulfide poisoning. Fe₂O₃ has the high adsorption ability for H₂ S and Hg, but its activity of removing mercury is poorer at the high temperature. Therefore, the designed of palladium-iron bimetallic adsorbent can not only remove Hg and H₂ S at the same time, but also have a higher mercury active temperature window, simplifying gas purification process, saving the cost of operation and equipment. Accordingly, a series of 1Pd5Fe₂O₃/Al₂O₃ sorbents of the dual active components were prepared based on the existing research work, and the performance of the Hg0 removal from coal derived fuel gas at mid-temperature over the bimetallic sorbents were studied in this paper.In our preparatory works, multiple series of bimetallic sorbents were prepared by pore volume impregnation method, and the effects of second metal addition on the properties of palladium sorbents to remove the mercury were investigated. It has been found that the effect of transition metal addition is affected by the carrier type of sorbents and preparation technology, and the efficiency of the mercury removal over Pd/γ-Al₂O₃ sorbents was reduced because of the second metal Fe added. Based on this, keeping the proportion of metal components unchanged, coprecipitation method was adopted to prepare 1Pd5Fe₂O₃/Al₂O₃ sorbent in this paper. The performances of the sorbent removing Hg and H₂ S were investigated in the N2-CO-H₂-H₂S-Hg atmosphere（simulated coal gas） at 200 oC using a laboratory-scale fixed-bed reactor. The effects of precipitant type, pH of the precipitation system, and calcination temperature on the physicochemical properties of sorbents were studied. Moreover, the removing Hg0 performances of sorbents from different preparation methods were also compared. Combined with the results of X-ray diffraction（XRD）, N2 adsorption, SEM, TEM, Raman and X-ray photoelectron spectroscopy（XPS）, the mechanism of Hg0 removal over PdO-Fe₂O₃/γ-Al₂O₃ sorbents was explored. The main conclusions are summarized as follows:（1） Comparing the Hg removal activity of the sorbents prepared by four different methods, we can find that the order of their mercury removal activity is coprecipitation method ≈ deposition precipitation method > volume impregnation method > fractional precipitation method.（2） The precipitation pH value mainly affected the content of the active components of the sorbents. With the increase of pH value, the iron oxide loading rate increased, while the loading rate of Pd decreased. The calcination temperature mainly influenced the physical structure of the sorbents, the specific surface area decreases with the increase of the calcination temperature, the pore size and the total pore volume become larger.（3） When using ammonia as precipitant, the best preparation condition is that pH=10 and the calcination temperature was 600 oC. With the ammonium carbonate as precipitator, the best preparation conditions is that p H=8 and the calcination temperature was 700 oC; the mercury removal efficiency can be maintained at 95% in more than 480 min and the mercury capacity up to 41.4 μg/g.（4） The Hg0 removal mechanism of PdO-Fe₂O₃/Al₂O₃ sorbent is considered that PdO could be reduced to Pd0 by H₂ and CO gas and then elemental mercury could react with Pd0 to produce Pd-Hg amalgam; Fe₂O₃ could reacts with H₂ S in the coal derived fuel gas to generate FeSx and Sad（adsorbed active surface sulfur）, HgS could be formed from the reaction of Hg0 and Sad.