| Mercury pollution is highly toxic, persistent, enriched and global. In October 2013, China signed the Minamata Convention aimed at controling and reducing the mercury emission in global scope, which brought unprecedented huge pressure to limit mercury to our country, forced the industry involving mercury upgrading. The coal industry is the largest source of mercury in China, efficient and clean coal gasification technology become an important research direction, but the mercury will be released into the gas in the form of elemental mercury in coal gasification process. It is difficult to oxide the elemental mercury to oxidized mercury for CO and H2. At present, the study on the removal of elemental mercury in the gas is still relatively few, and the developed adsorbents had problems such as the mercury removal efficiency is low, the mechanism is not clear and so on. Therefore, exploratory study on mercury adsorption mechanism in reduction gas has important practical significance for the development of coal gasification technology.The adsorbents based cerium with special Ce4+/Ce3+redox is conductive to oxidize the elemental mercury in reducing atmosphere, thus the mercury removal efficiency can improve greatly. Based on this characteristic, this paper studied the adsorption mechanism when elemental mercury adsorpted on the metal oxide adsorbent in coal gas atmosphere from the experimental and theoretical simulation perspectives. Firstly, CeO2 adsorbent was prepared by homogeneous precipitation method, and the CeO2 adsorbent with 4% and 8% mass fraction Pd was prepared by deposition precipitation method. The physical structure of the adsorbent surface was depth analysis by BET、SEM、XRD and other testing methods, which found that the specific surface area of pure CeO2 was 34.80 cm2/g-1, the total pore volume was 0.029 g-1/cm3, the average pore diameter of 3.12 nm. With the increase of the mass fraction of the doped Pd, the specific surface area increases, the total pore volume increases, and the average pore size decreases gradually. Pd uniform distribution on the CeO2 surface, the dispersion is better, the structure is developed, the crystal structure is complete. Due to this structure, the doping of Pd is conducive to the physical adsorption of gas molecules.Then, in the atmosphere of simulated coal gas, the effect of the temperature and the doping ratio of Pd on the adsorption of mercury on the surface of CeO2 adsorbent was studied. The results showed that with the increase of temperature, the mercury adsorption efficiency was first increase and then decrease, and the adsorption efficiency was highest at 200 ℃, the adsorption efficiency was lowest at 150℃. Through the analysis, it is found that the temperature increase is mainly to inhibit the physical adsorption and strengthen the chemical adsorption. When the temperature is higher than 200 ℃, the adsorption efficiency decreases with the increase of temperature, as a result of the amount of S gradually decreased, which reacted with Hg0 to generate HgS. With the increase of Pd mass fraction, the maximum removal efficiency of Hg0 increased from 25.9% to 95.66%. Compared with the effect of pure CeO2 adsorption, the adsorption effect was obviously enhanced, and the maximum adsorption efficiency increased gradually, which showed that the doping of Pd could effectively improve the adsorption efficiency of CeO2.Finally, the elemental mercury adsorbed on the three different surface (100,110,111) and different adsorption sites (Ce top, Ce-O, O top, O-O) of CeO2 was calculated by the VASP software based on the density functional theory. The results showed that the adsorption capacity of mercury on the CeO2 surfaces was weak, and the mercury atoms were far away from the CeO2 surface. The results were consistent with experimental results in this paper. The adsorption of Hg on different adsorption sites of Pd-CeO2 (111) surface was calculated. The results showed that the Hg atom has a strong chemical adsorption with the adsorption substrate, and a new chemical bond formed between the atoms, which further shows that the doping of Pd on the surface of CeO2 is beneficial to improve the adsorption efficiency of Hg. The adsorption process of Hg on Pd-CeO2 (111) surface in H2S was studied. The S-H bond of H2S molecules was broken and eventually transformed into S atoms adsorpted on Pd-CeO2 (111) surface, Hg atoms and S atoms on the Pd-CeO2 (111) surface had a strong chemical adsorption in the Z direction, which indicated that the adsorption efficiency of mercury on Pd-CeO2 (111) surface was improved due to the formation of HgS. |
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