| Coal gasification is the pivotal component of advanced and efficient coal utilization technology, which effectively improved the efficiency of energy and had mitigated environmental effects. The heavy metal mercury evaporated from gasification process is mainly in the form of gaseous elemental mercury. Relatively there is a lack of research on the mercury gasification process and removal from syngas.As a traditional desulfurizer, zinc oxide sorbent has been widely applied in the field of coal gasification desulfurization. Nano-ZnO which was synthesized by a homogeneous precipitation and Zn-Ti composite metal oxide based sorbents which were prepared by an impregnated method were applied for the adsorption of gas-phase elemental mercury in nitrogen and simulated gas atmosphere. The performance of sorbents and influencing factors for mercury removal were studied. The adsorption process of H2S on sorbent surface and chemical reaction mechanism of H2S and Hg0were analysised in this paper.Firstly, the mercury removal efficiency of the nano-ZnO was relatively poor in nitrogen atmosphere, which was mainly in the physical adsorption. The presence of H2S promoted the Hg0removal by nano-ZnO observably. The reason was considered like that:H2S was oxidized to adsorbed active elemental sulfur (Sad) by the surface oxygen on the surface of nano-ZnO, The Sad reacted with Hg0to form stable HgS, contributed to Hg0removal. The presence of CO and H2slightly promoted the Hg0removal by the promotion of nano-ZnO desulfurization. Mercury removal by nano-ZnO was suppressed when temperature increased, which may hindered the reaction between Sad and Hg0. the mercury removal efficiency reached to86.7%when0.04%of H2S was added at80℃.Then, nano-ZnO sorbent was modified by Na2S and elemental sulfur to further explore the adsorption and reaction process of Hg0and H2S on the sorbent surface. The results showed that mercury removal efficiency of nano-ZnO sorbent modified by elemental sulfur was certainly improved in a nitrogen atmosphere. The process of H2S adsorpted on the surface of the sorbents was as follows:firstly, S0which was formated from oxidation or decomposition of H2S had the catalytic ability to adsorption of H2S. H2S adsorpted on the surface became numerous and decomposed to form a long chain of polysulphide. When the long chain fractured, different number of S atoms arranged to Sn, which included the formation of surface active sulfur. S and Na2S supported on the surface of sorbents improved the efficiency of mercury removal by promoting adsorption and oxidation of H2S on the sorbent surface.The reaction of Hg and S hampered by elevated temperature led to a reduction in the amount of HgS generation. Optimum temperature of mercury removal reached to150℃by the modified sorbents, the mercury removal efficiency were91.1%and88.1%respectively.Finally, Zn-Ti based sorbents were prepared by ZnO loaded on the surface of TiO2to enhance the adsorption and reaction of H2S. The Experimental results showed that Zn-Ti based sorbents were more effective on mercury removal than single ZnO sorbents. Desorption experiments verified that the way of mercury removal was to form HgS when H2S added. HgS became unstable and decomposed at260℃, resulting in poor efficiency of mercury removal by sorbents at elevated temperature. Simultaneously desulfurization was enhanced with the increase of temperature so H2S and ZnO reacted directly resulted in a reduction of H2S catalytic reaction. Because of the competitive adsorption, the presence of H2O and NH3had a prohibitive effect on Hg0removal. HCl played a significant promoting role in mercury removal and had little affected by elevated temperature. Effects of H2S and HCl on mercury removal over Zn-based sorbents needed to further study. The efficiency of mercury removal were95.2%and91.2%at150℃and200℃by Zn-Ti1sorbents respectively. |
PDF Full Text Request