(6df) Evaluating Issues in Environmental Adsorption: Mercury Oxidation and Shale Pore Characteristics

The research presented here will focus on recent work focused around the field of environmental adsorption. The topics are: 1) identification of adsorbed mercury species in applications related to coal-fired flue gases and 2) evaluation of analytic adsorption techniques for the determination of pore characteristics in shale gas. Both of these topics have implications in the field of clean energy: coal-fired power remains the primary source of electric energy in the United States, China and India and the primary anthropogenic source of emitted mercury, while the production of natural gas from non-traditional sources, such has black shale, has outpaced the information available about these sources and the long-term implications of the their development.

1.) Mercury exists in various forms in the flue gases of coal combustion, from its elemental state (Hg⁰) at the boiler exit, to various oxidized forms (Hg²⁺) as it interacts with fly ash particle surfaces as the flue gas cools. The exact species of the oxidized mercury is, at this point, an outstanding question. Thermodynamic calculations suggest that the favored form in coal combustion flue gas is HgCl₂, but HgO and HgBr₂ could exist under certain conditions, such as the introduction of Br into the combustion process or in cases when coal is burned and contains trace amounts of Br.

An electron ionization quadrupole mass spectrometer (EI-QMS) has been modified for the direct measurement of elemental and oxidized Hg at industrial relevant concentrations (< 15 μg m^-3 or 2 ppbv) from a methane combustion flue gas. Ultimately, the extent of homogeneous and heterogeneous oxidation may be measured, with the Hg species directly identified through mass spectrometry. In addition, the oxidized species created upon exposure to surfaces present in coal combustion, such as fly ash and activated carbon, are identified through the use of packed-bed experiments. A variety of sorbents have been evaluated, with the MS used to track any desorbed species. This information is critical for evaluating the extent of reaction on a sorbent surface and the potential for desorption of various oxidized Hg species. By investigating the desorbed species as a function of temperature provides indication of the relative strength of adsorption of the various Hg species and subsequently their relative stabilities on the various sorbents tested. These types of investigations will also increase the fundamental understanding of the surface reaction of Hg on a variety of sorbents and provide further understanding that can lead to advances in Hg capture.

2.) The rapid ascent of shale sourced gas has created a need for analytical evaluation of shale, in order to determine permeability and gas-in-place. Researchers have focused on the adsorption properties of shale, in order to evaluate potential gas-in-place and understand how gas transports from micropores to macropores to fractures. The literature, to this point, has been unfocused, and pointed towards evaluating as many shale samples from the broadest perspective as possible. However, analytical adsorption approaches appropriate for investigating homogenous samples, such as activated carbon or zeolitic material, has limitations when applied to highly heterogeneous samples. Shale, which has high variability at all scales, from the formation level to the microscopic level, is a particularly difficult challenge.

This work focuses on creating a standard analytical method for the evaluation of gas shale, focused on low-pressure adsorption techniques. The outgas temperature will be evaluated to determine effects on adsorption isotherms, with emphasis on potential changes in pore volume. The samples will be evaluated with 3 probe gases (N₂ at 77 K, Ar at 87 K and CO₂ at 273 K). Isotherms will be evaluated with DFT techniques to determine pore size distribution, while using information from Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), elemental analysis and total and inorganic carbon contents to support the gas adsorption observations. The thorough analytical approach presented in this work will present a method that researchers can use to properly evaluate pore characteristics of shale gas, which can be used to support research in flow models and gas-in-place estimates.