In regard to gasification for power generation, the removal of mercury by sorbents at elevated temperatures preserves the higher thermal efficiency of the integrated gasification combined cycle system. Unfortunately, most sorbents display poor capacity for elemental mercury at elevated temperatures. Previous experience with sorbents in flue gas has allowed for judicious selection of potential high-temperature candidate sorbents. The capacities of many sorbents for elemental mercury from nitrogen, as well as from four different simulated fuel gases at temperatures of 204−371 °C, have been determined. The simulated fuel gas compositions contain varying concentrations of carbon monoxide, hydrogen, carbon dioxide, moisture, and hydrogen sulfide. Promising high-temperature sorbent candidates have been identified. Palladium sorbents seem to be the most promising for high-temperature capture of mercury and other trace elements from fuel gases. A collaborative research and development agreement has been initiated between the Department of Energy's National Energy Technology Laboratory (NETL) and Johnson Matthey for optimization of the sorbents for trace element capture from high-temperature fuel gas. Future directions for mercury sorbent development for fuel gas application will be discussed.

Coal gasification with subsequent cleanup of the resulting fuel gas is a way to reduce the impact of mercury and arsenic in the environment during power generation and on downstream catalytic processes in chemical production. The interactions of mercury and arsenic with Pd/Al 2 0 3 model thin film sorbents and Pd/Al 2 Oj powders have been studied to determine the relative affinities of palladium for mercury and arsenic, and how they are affected by temperature and the presence of hydrogen sulfide in the fuel gas. The implications of the results on strategies for capturing the toxic metals using a sorbent bed are discussed.

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The dispersion and location of Pd in alumina-supported sorbents prepared by different methods was found to influence the performance of the sorbents in the removal of mercury, arsine, and hydrogen selenide from a simulated fuel gas. When Pd is well dispersed in the pores of the support, contact interaction with the support is maximized, Pd is less susceptible to poisoning by sulfur, and the sorbent has better long-term activity for adsorption of arsine and hydrogen selenide, but poorer adsorption capacity for Hg. As the contact interaction between Pd and the support is lessened the Pd becomes more susceptible to poisoning by sulfur, resulting in higher capacity for Hg, but poorer long-term performance for adsorption of arsenic and selenium.

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The adsorption of arsine by copper−palladium alloys was studied using a high-throughput composition spread alloy film (CSAF) sample library. A Cu x Pd 1−x CSAF coupon that spanned the complete alloy composition space (x = 0−1) was prepared by an evaporative deposition technique. The coupon was exposed to AsH 3 in a N 2 background at 288 °C in a small flow reactor. Arsenic uptake was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy (SEM-EDX), and micro X-ray diffraction (μ-XRD). Pd-and Cu-rich alloy compositions exhibited large surface concentrations of As after exposure to AsH 3 . In the Cu-rich alloy, composition and structure measurements suggest the formation of a Cu 3 As phase. Arsenic uptake at Pd-rich alloy compositions is consistent with Pd 2 As or Pd 8 As 3 phases; structural results suggest Pd 2 As with a hexagonal structure. In contrast, over a wide range of intermediate compositions (x Cu ≈ 0.20−0.75), little As uptake was observed. These results contribute to a basis for rational design of sorbents for the capture of arsenic from fluid streams, and to an understanding of the stability of palladium−copper alloy membranes employed for hydrogen separation from coal-derived syngas. This work illustrates the application of high-throughput approaches based on CSAF sample libraries that can be applied to a wide variety of materials development and optimization challenges.

The surface composition of a series of Pd/alumina sorbents has been characterized to better understand the factors influencing their ability to adsorb mercury from fuel gas. Both a temperature effect and a dispersion effect were found. Maximum adsorption of Hg occurred at the -lowest temperature tested, 204°C, and decreased with increasing temperatures. Maximum adsorption of Hg on a per-atom basis of Pd is observed at low loadings of Pd ( < S.5% Pd) due to better dispersion of Pd at those loadings; a change in its partitioning occurs at higher loadings. The presence of H2S "in the fuel gas acts to promote the adsorption of Hg through its association with Hg in the Pd lattice.