| Coal gasification is a potential coal utilization technology, which receives widespread attention with the characteristic of being clean and high efficient. But in the gasification process, mercury will evaporate mainly in the form of Hg0. It has been demonstrated that reducing environment is not suitable for Hg oxidation. At present, there is little research available on mercury removal under reducing atmosphere.Actived coke has been widely used in the field of flue gas desulfurization. The mercury removal capacity is also as good as that of active carbon. Moreover, it has advantages of abundant raw material and reasonable price, making it be an excellent adsorbent or support material, In this work, activated coke was prepared from lignite, and then it was modified with manganese acetate by the impregnation method (Mn-AC). In order to improve the mercury removal efficiency of sorbent at high temperature, the mercury removal mechanism in simulated syngas was investigated, and the influences of temperature and gas composition were considered.In N2atmosphere, the mercury removal of AC is mainly by physical adsorption. After H2S was added, the mercury removal efficiency increased from46.6%to95.8%. Based on the analysis of XPS and FT-IR, H2S is easily oxidized into elemental sulfur (Sad) by the aid of active oxygen, C-O functional groups, then Sad provides active absorption sites forHg0to form stable HgS. With the temperature increasing, the oxidation effect of functional groups gets weaker and weaker, and the amount of active oxygen becomes insufficient, causing the decrease of mercury adsorption sites.MnxOy was introduced into AC to improve its ability of resisting high temperature. As a result, the mercury adsorption capacity of sorbent improved remarkably after modified, the average Hg removal efficiency was84.3%under the simulated syngas atmosphere at200℃. After adsorption, a large scale of S2-and S0was generated on the surface of Mn-AC, meaning that Sad and HgS were produced in the mercury adsorption process. Once modified, the number of active oxygen on the surface of sorbent grew obviously, and the lattice oxygen could convert to active oxygen in hypoxia state, ensuring enough Sad to form stably.In the simulated syngas, CO, H2, and NH3are unfavorable for the formation of the Sad, and all three of them inhibit mercury removal. HCl promotes the mercury removal by forming active Cl*, but the formation of Cl*consumes the surface oxygen. Thus, HCl has a negligible effect on Hg removal when H2S exists.To analyse the influence of temperature, the sorbent was placed at different temperature to remove mercury. It turned out that Hg0capture was significantly inhibited at higher temperature, which can be attributed to that:1. The reaction equilibrium constant K decreases with the temperature increasing, namely, the reaction between Hg0and Sad is weakened.2. HgS and HgO, products generated in the process of mercury removal, completely decompose at300℃.3. H2shows greater reduction capacity, reducing the amount of Sad to adsorb mercury.The characteristic of mercury desorption of Mn-AC was investigated in N2atmosphere. The result showed that the mercury removal efficiency of sorbent decreased little in the heat regeneration experiment. After the first and second desorption, the mercury removal efficiency stayed at90.9%and89.2%separately. While its desulfurization showed a dramatically decreased tendency. The reason why this phenomenon occurred is as follow:lattice oxygen is consumed in the process of H2S and Hg removal, resulting in the active sites of H2S removal decreasing. Therefore, oxidation treatment is necessary after the heat regeneration. Only in this way can the sorbent regain the ability to removal H2S and Hg simultaneously. |
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