Development and Evaluation of Nanoscale Sorbents for Mercury Capture from Warm Fuel Gas. Final Report: September 27, 2004 through May 31, 2006

摘要

Several different types of nanocrystalline metal oxide sorbents were synthesized and evaluated for capture of mercury (Hg) from coal-gasifier warm fuel gas. Detailed experimental studies were carried out to understand the fundamental mechanism of interaction between mercury and nanocrystalline sorbents over a range of fuel gas conditions. The metal oxide sorbents evaluated in this work included those prepared by GTIs subcontractor NanoScale Materials, Inc. (NanoScale) as well as those prepared in-house. These sorbents were evaluated for mercury capture in GTIs Mercury Sorbent Testing System. Initial experiments were focused on sorbent evaluation for mercury capture in N2 stream over the temperature range 423533 K. These exploratory studies demonstrated that NanoActive Cr2O3 along with its supported form was the most active of the sorbent evaluated. The capture of Hg decreased with temperature, which suggested that physical adsorption was the dominant mechanism of Hg capture. Desorption studies on spent sorbents indicated that a major portion of Hg was attached to the sorbent by strong bonds, which suggested that Hg was oxidized by the O atoms of the metal oxides, thus forming a strong HgO bond with the oxide. Initial screening studies also indicated that sulfided form of CuO/alumina was the most active for Hg capture, therefore was selected for detailed evaluation in simulated fuel gas (SFG). It was found that such supported CuO sorbents had high Hg-sorption capacity in the presence of H2, provided the gas also contained H2S. Exposure of supported CuO sorbent to H2S results in the formation of CuS, which is an active sorbent for Hg capture. Sulfur atom in CuS forms a bond with Hg that results into its capture. Although thermodynamically CuS is predicted to form unreactive Cu2S form when exposed to H2, it is hypothesized that Cu atoms in such supported sorbents are in dispersed form, with two Cu atoms separated by a distance longer than required to form a Cu2S molecule. Thus CuS remains in the stable reactive form as long as H2S is present in the gas phase. It was also found that the captured Hg on such supported sorbents could be easily released when the spent sorbent is exposed to a H2-containing stream that is free of Hg and H2S. Based on this mechanism, a novel regenerative process has been proposed to remove Hg from fuel gas at high temperature. Limited multicyclic studies carried out on the supported Cu sorbents showed their potential to capture Hg from SFG in a regenerative manner. This study has demonstrated that supported nanocrystalline Cu-based sorbents have potential to capture mercury from coal syngas over multiple absorption/regeneration cycles. Further studies are recommended to evaluate their potential to remove arsenic and selenium from coal fuel gas.