(103d) Screening of Sorbents for Mercury Capture Using Ab initio Computations

Coal is a major source of energy, currently supplying 25% of the world's energy needs and producing 40% of its electricity1. One promising technology to improve the efficiency of energy extraction from coal is the integrated gasification combined cycle (IGCC) plant. While this type of plant can operate with 40% higher efficiency than traditional pulverized coal-fired plants2, pollution is still an issue. In addition to the strictly regulated ?criteria? pollutants, the U.S. Environmental Protection Agency (EPA) lists 188 ?hazardous? pollutants, many of which are found in trace amounts in coal. Among these, the EPA has designated mercury as especially harmful. According to EPA rules from March 2005, mercury emission from coal-fired power plants will ultimately reduce from 48 to 15 tons a year, an approximately 70% reduction3. These measures, coupled with the expected rise in the use of gasification in the United States during the next 20 years, has brought the need for a low-cost mercury removal technique applicable to IGCC4.

In this work, we show how ab initio calculations can aid in the screening and discovery of new mercury sorbents. Our work tests the feasibility of using a pure metal substrate as a sorbent when the relevant mechanism of capture is binary amalgamation. We compute the binary amalgam formation energy for 87 different metal-mercury amalgams for 35 unique metals. While these computations reveal the strongest mercury compound-forming metals, reaction energy alone does not predict success in a real syngas stream due to the possibility of competing reactions. Therefore, we additionally present the results of 94 binary metal oxide formation energies to evaluate the possibility of metal oxidation competing with amalgam formation in the stream.

Using our results, we are able to evaluate the most promising metal candidates for use as mercury sorbents in a gas stream. Comparison with the literature confirms that our predictions match known mercury sorbents. In addition, we show trends in capture behavior over the periodic table, explaining how the correlation between mercury amalgam formation energy and binary oxide formation energy make it particularly difficult to design pure metal sorbent materials. Finally, we present a path for future work in this area.

References

(1) Powell, C. A.; Morreale, B. D. MRS Bull 2008, 33, 309-315.

(2) Nexant, I. 2006, EPA-430/R-06/006.

(3) Padak, B.; Brunetti, M.; Lewis, A.; Wilcox, J. Environ. Prog. 2006, 25, 319-326.