Sorption and Oxidation of Mercury in Flue Coal-Fired Power Plants

Sunday, November 7, 2010
Hall 1 (Salt Palace Convention Center)
Erdem Sasmaz, Energy Resources Engineering, Stanford University, Stanford, CA

Activated carbon (AC) and flue gas desulfurization (FGD) processes are often incorporated to capture trace elements (TEs), such mercury (Hg), selenium (Se) and arsenic (As). Although these processes work fairly well for oxidized Hg capture for combustion applications, they fail for gasification fuel gases since gasification conditions require Hg capture at an elevated temperature, at which point activated carbon-based sorbents to break down. Hence, worldwide there exist no control technologies for TE species on current coal gasification plants used for energy generation. Also, elemental Hg proceeds through combustion flue gas uncaptured due to its low reactivity. Lastly, TE capture via FGD and activated carbon decrease coal's sustainability by compromising the recycling of fly ash and FGD waste byproducts. Recycling coal-combustion byproducts (CCBs) increases coal's sustainability through minimizing waste associated with the process. Typical CCBs are fly ash from loss on ignition and spent gypsum, which is obtained from calcium (Ca)-based sorbents in SO2 scrubbers. Traditionally, fly ash is used in concrete and asphalt manufacturing and spent gypsum is recycled through wallboard and stucco production. An unintended consequence of the recycling process is the existence of TEs in both byproducts. As the gypsum demand increases, it is essential to limit Hg capture in FGD processes to prevent the possible leaching of Hg within the high-temperature recycling processes of gypsum. AC that is used for Hg capture is collected in electrostatic precipitator units along with the fly ash for recycling processes. However, within the concrete manufacturing process the addition of spent activated carbon prevents concrete from meeting the freeze-thaw requirements, which compromises its functionality. Hence, the primary goal of this study is to design a multipollutant sorbent material that will be effective in capturing the TEs at elevated temperatures of both flue and fuel gases of coal-based energy production and to understand heterogeneous chemical pathway of Hg both considering AC and calcium oxide (CaO) sorbents. Results of this study will benefit the planet through the reduction of harmful emissions, and the effective recycling of fly ash and FGD by allowing developed and developing countries to prosper economically from sales associated with the manufacture of coal combustion byproducts, and environmentally through the prevention of waste materials filling landfills.

Although the adsorption mechanism of Hg is complicated, a fundamental understanding can be achieved by investigating both energetic and electronic interactions of Hg on CaO and noble metals surfaces using first principles quantum mechanical calculations. Preliminary theoretical investigations have been carried out and it has been found that mercury strongly binds to CaO surfaces as Hg2+ and forms a stable complex on the surface. SO2 also binds strongly to CaO surfaces with chemisorption being the likely adsorption mechanism. Both of these results indicate that both Hg and sulfur (S) bind to Ca-based sorbents in wet FGD systems. Palladium (Pd) alloys and overlays have also been investigated as a replacement for Ca and organic-based mercury sorbents, and the results show that the addition of a small amount of gold (Au) and silver (Ag) is able to enhance the sorbent reactivity. It is important to note that Hg binding is sensitive to the specific local concentration of Au and Ag atoms and it would be difficult to optimize the structure for fabrication due to the invariance of the atomic configuration. Further preliminary studies revealed that the overlay substrate takes the lattice constant of the underlying metal which leads to an enhancement of the Hg binding compared to the Pd alloys of Pd deposited on another metal having a larger lattice spacing. The use of overlays removes the dependence of the random atomic arrangement, making the fabrication process easier, increasing the potential for these novel materials to be scaled up. Both of these CaO and noble metal, i.e. Pd, Au, Ag, studies have been completed and published in several manuscripts, i.e., Journal of Physical Chemistry C and Main Group Chemistry.1-3

During my PhD in the department of Energy Resources Engineering at Stanford University under the supervision of Prof. Jennifer Wilcox at Stanford University, I also designed a burner and fixed bed reactor to investigate Hg adsorption and possible oxidation on noble metals and AC. Quantum chemistry modeling tools guided the experimental research by helping to understand the physical mechanism and reducing the time and cost typically required for material fabrication. AC samples functionalized with halogens and sulfur are tested and the effects of functional groups and active sites on the binding of Hg are considered. Well-known AC samples assist in understanding the adsorption and oxidation mechanisms of Hg on the sorbent surface. In addition, Pd-Au sorbents have been fabricated by Johnson Matthey Technology Centre in Reading, UK according to the first principles quantum mechanical results. These sorbents are tested in the packed bed reactor (PBR) and the mechanism of Hg binding is investigated. To simulate the flue gas condition, methane is combusted in a laminar flow tubular burner and mixed with mercury and chlorine to simulate the flue gas environment. Mercury adsorbed sorbent materials are characterized using X-ray Photoelectron Spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy. Characterization experiments before and after the tests are necessary to identify the oxidation state of Hg on the carbon surface. The innovative nature of the project is evident as it is the first attempt to use molecular modeling techniques to propose a mechanism for the adsorption of the TEs onto Ca-based sorbents and Pd alloys and overlays. To further increase the technical merit of the proposed research, the adsorption strengths of these novel sorbents with TEs can be experimentally tested against qualitative model interpretations in order to further validate the model simulations.

As a PhD student and a researcher, I would like to take a part to improve clean energy technologies by combining first-principle computational tools with experiments. I have a strong background in Thermodynamics, Fluid Mechanics, Mass and Heat Transfer, Reaction Kinetics, Surface Science, Catalysis and Spectroscopy. I believe that the knowledge I gained from my education combined with the laboratory experience has adequately prepared me to train for a faculty position in an engineering program focused on energy.

References 1. Sasmaz, E.; Wilcox, J. Phys. Chem. C, 2008, 112, 42. 2. Aboud, S.; Sasmaz, E.; Wilcox, J. Main Group Chem. 2008, 7, 205. 3. Sasmaz, E.; Aboud, S.; Wilcox, J. Phys. Chem C 2009, 113, 7813.

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