Environ. Sci. Technol. 2009, 43, 561–564
CO2 Storage in Shallow Underground and Surface Coal Mines: Challenges and Opportunities VYACHESLAV N. ROMANOV* U.S. Department of Energy, Pittsburgh and Parsons Corporation, South Park, Pennsylvania TERRY E. ACKMAN, YEE SOONG, AND ROBERT L. KLEINMAN U.S. Department of Energy, Pittsburgh, Pennsylvania
ISTOCKPHOTO
Effective storage of CO2 requires a better understanding of coal and minerals such as clays to develop new sorbent materials and sequestration technologies.
electricity generation and for a quarter of world energy consumption, it has been perceived until recently as an unwelcome guest “from the era and pages of Charles Dickens” by environmentalists and legislators (1). For coal power generation to be properly considered, CO2 and other greenhouse gas (GHG) generation and deposition must be addressed to assuage global climate change concerns. The ongoing development of a “pathway to stabilization” of CO2 emissions championed by the U.S. Department of Energy (DOE) (2) is an integral part of the global response to these challenges. Capturing and sequestering CO2 emissions is one of the principal modes of carbon management. The current strategies are geared toward implementation of various sequestration options, including terrestrial (via improved management of forests, range-, wet-, and agricultural lands), geological sequestration, and advanced biological and chemical approaches (2, 3). One of the carbon storage opportunities is sequestration in deep (>1.5 km) coal seams that are not suitable for mining. In this option, CO2 injected into a coal bed becomes adsorbed onto the coal’s surface and is immobilized. The main difficulty of this method is maintaining injectivity as the coal matrix imbibes CO2 and swells (4). Furthermore, delivery of the captured GHG emission from the point of power generation to the remote sequestration site involves dealing with logistical problems and relatively high transportation costs. However, there are numerous shallow (1500 m long. Similarly, shallower coal is more economical both for mining and for storage of bulk quantities of (potentially sorbent) rock materials. Yet no one 562
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 3, 2009
FIGURE 3. Clay (top) and coal (bottom) abundance maps. The rasterized swelling clays map (18) has been digitized using the R2V software package (19). The vectorized digital data set for the coal fields (20) has been provided by the U.S. Geological Survey. has suggested carbon sequestration in coal seams at relatively shallow depthssthe whole intent of deep injection is that unmined coal has greater adsorption capacity (Figure 2).
Trapping mechanisms Validation of the available sequestration options relies on understanding of the corresponding trapping mechanisms. In the case of sorption, GHGs may be secured by a combination of factors: surface area; adsorption affinity; pore structure; solubility; permeability; chemical reactivity; and the kinetics of mineral trapping. The importance of a specific trapping mechanism depends not only on the sorbent properties but also on the ambient conditions such as pressure and temperature. The contribution of the various factors should be evaluated with due diligence before committing to major sequestration field projects. Certain fine-grained materials with well-developed microporous structures could behave similarly to or perhaps better than coal in adsorbing GHGs. For example, the large specific surface area of clays due to their microporous structure (g100-150 g/m2) indicates a vast potential for CO2 and possibly other GHG sequestration (8). In fact, the total surface area of expandable clay, both external and interlamellar, can be several times larger than the values obtained by conventional methods and reported in the literature (9). Coal preparation plants produce large quantities of shale or mudstone refuse, which typically contains various clay minerals: thus coal mines already produce a promising
material that could be optimized for CO2/GHG sequestration. How to further engineer/enhance complex structures such as clays can be gained from advances in nanomaterial studies. Particularly interesting insight comes from clean energy technologies based on the “nano-confinement” of supercritical gases (10, 11). Kaneko et al. (13) describe a system of solid submicroscopic pores in discussing the sorption of H2 in a carbon nanotube (CNT) matrix, and mechanistically speak of “nanospaces”. Therein, enhanced molecular fields due to nano properties can induce chemical reactions that appear to stabilize the adsorbate in a manner akin to the sort of bonds that form under high-pressure conditions. If these unique properties of such nanospaces can be controlled, it could facilitate the development of new chemical engineering techniques to permit the quasi-permanent trapping of GHGs at low(er) ambient pressures and thus improve the prospect for viable sequestration initiatives. Clays are such candidates, with a chemical composition based on phyllosilicate crystalline phases with incorporated metal cations and other impurities, including hydration and structural water (12). This molecular structure makes substitution rather facile and means that clay minerals are seldom homogeneous. A corollary of this structure is that clays can expand for better pore accessibility and then contract to trap small molecules into a “sandwich” between the phyllosilicate micelles, or lamellae. Cations of medium size that would not clog the interstices between the strata can play a significant role in promoting the interlamellar sorption in dry clays (9). Also, small amounts of residual water have been shown (8) to have a profound effect on accessibility to the areas between the lamellae, forming pseudocrystals. Expandable clays, e.g., smectite mixtures, are known to swell by 30-100% at an increasing extent of cation hydration (14). In nonexpandable clays, a few residual water molecules may “prop apart” the regions of crystal overlap and allow sufficiently polarized CO2 to penetrate into these regions by leveraging apart points of weakness, especially at higher pressures (15). Arguably, by varying the residual water content and other ambient conditions such as temperature and pressure, it is possible to induce GHG trapping in the interlamellar regions through a sequence of controlled swelling and shrinking of the sorbent. The trapping of small molecules in clays’ lamellar structure is analogous to the system described by Kaneko et al. We therefore refer to clays’ sorption mechanism as nanotrapping because of the molecular size regime (typically
