Screening of metal oxides and metal sulfides as sorbents for elemental mercury at elevated temperatures

Abstract

A technical challenge for the implementation of Integrated Gasification Combined Cycle (IGCC) coal power plants is determining a method to efficiently capture the pollutants present in coal-derived syngas. One such method is to capture the pollutants at elevated temperatures to decrease the parasitic energy costs associated with their capture. Elemental Hg is difficult to capture because it is present in low concentrations and is relatively inert. In order to overcome technical and economic impediments to investigating potential materials experimentally, here we present an alternative approach in which we screen these materials using density functional theory. We compute the thermochemistry for 23 materials reacting with Hg to evaluate their efficacy for Hg capture. We also compute the thermochemistry of these materials reacting with H2, a major component of syngas. Using these calculations we were able to obtain estimates for the thermochemistry of 21 metal oxide and metal sulfide compounds, 17 of them containing Hg, where no experimental data are available. We predict several sorbent materials such as BaO2, CrO2, and Na2O2 to be effective for Hg capture at elevated temperatures but unusable in syngas because of their reactivity with H2. These materials may instead be effective for Hg capture from flue gas. We also predict that the selectivity of K2S2 for Hg over H2 is favorable, even at the IGCC conditions. Finally, we experimentally evaluate the Hg adsorption ability of K2S2, BaO2, and CrO2 in order to test the theoretical predictions. CrO2 was also evaluated for its potential to H2 reduction. CrO2 is shown to be an effective high-temperature sorbent for Hg in an inert carrier gas but is not stable in H2 streams, which agrees well with our theoretical calculations.