| The development of advanced coal conversion technology based on the coal gasification is a very potential technology to achieve efficient and clean use of coal. Mercury is normally released in the form of Hg0during the gasification process and it is significantly harmful to human and environmental health due to its extreme toxicity, persistence, long-range transport and ability to bioaccumulate. When developing advanced and efficient coal conversion technology, one of the urgent problems which need to be solved is to achieve the efficient and economical removal of elemental mercury (Hg0) from syngas. The syngas belongs to the strong reducing atmosphere due to the the presence of H2, CO and other reducing gas. Furthermore, Hg0is in the high temperature environment, so it is difficult to be removed by the existing air pollution eontrol device. To date, the study of Hg0capture from syngas is still in the initial stage and the developed mercury sorbents have the defects of inefficiency and narrow temperature window. Therefore, the mechanism study on the cpture of elemental mercury from syngas would be of great scientific and practical values.On the basis of the above analysis, CeO2-TiO2(CeTi) sorbents with large oxygen storage capacity were employed to capture Hg0from syngas. The Ce0.2Ti sorbents exhibited superior Hg0removal activity with Hg0removal efficiency higher than80%from80to150℃. H2S and HC1were significantly effective for Hg0capture when they were employed separately. However, the combination of H2S and HC1exerted a prohibitive effect on Hg0removal. H2and CO had a negligible effect on Hg removal. In the presence of H2S, NH3prohibited the Hg0removal because of the consumption of the surface oxygen. Experimental and density functional theory (DFT) studies demonstrated that Hg0removal over CeTi sorbents follows the Eley-Rideal mechanism, in which active surface sulfur reacts with gas-phase Hg0. Efficient removal of Hg0was achieved over CeTi sorbents, which plays an extremely important role in mercury emission control in an extremely reducing environment.According to the formation of a Pd-Hg amalgam, Pd/Al2O3sorbents were developed to capture Hg0in syngas at high temperatures. More than90%of mercury was captured by8Pd/Al2O3at270℃due to the formation of Pd-Hg amalgam, meeting the requirement of high temperature mercury removal. H2and CO promoted the Hg0capture due to the reduction of PdO to Pd metal. Hg0capture was significantly inhibited with the presence of H2S because of the consumption of the active palladium. HC1also found to be limitedly promotional on Hg0capture ascribed to the catalytic oxidation of Hg0to HgCl2.CeO2was indroduced into Pd/Al2O3to improve the ability of resisting H2S poisoning from syngas. As a result, the addition of CeO2enhanced the Hg0removal activity due to the consumption of H2S. The combination of Pd and CeO2resulted in a great synergy for Hg0removal at high temperatures. The optimal mass ratio of CeO2/Al2O3was1.0, which was attributed to the excellent sulfur removal activtiy. Therefore, it was promising for Pd-Ce/Al sorbents to capture Hg0and H2S simultaneously.CeTi sorbents exhibited high regeneration activity, which coulds reach even exceed the capacity of fresh sorbents after5regeneration cycle. The regeneration activity of PCxA and Pd/Al2O3sorbents was similar. Following the first regeneration cycle, the removal efficiency decreased slightly due to the incomplete mercury evolution at400℃and degradation of the loaded palladium. More importantly, it could remain constant thereafter over5regeneration cycles, which indicated they are prospective candidates for Hg0control from syngas. This excellent regeneration performance played an extremely important role in reducing the application cost of cerium and palladium based sorbents and realizing their large-scale application. |
PDF Full Text Request