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Geologic sequestration of CO₂ generated by coal-fired power plants is a critical component of Carbon Capture and Storage (CCS) (Pacala and Socolow, 2004). In addition to CO₂, coal combustion generates SO_x, NO_x, and other constituents.  Purity requirements for CO₂ injected into a geologic reservoir are being debated worldwide and have yet to be established (Gale, 2009).  Conventional CO₂ separation technologies that can be retrofitted to existing power plants yield CO₂ that is greater than 90% pure (Rao and Rubin, 2002).  However, these technologies impose large parasitic energy costs of 30 to 40% of the net power plant output.  In addition, even just a few tenths of a percent of the common impurities (e.g., SO_x and NO_x) will influence the geochemistry of a water-rock system.  Novel technologies such as oxy-fuel combustion and integrated gasification combined cycle (IGCC) power plants dramatically reduce parasitic energy costs but also produce higher levels of impurities in the combustion products.  In either case, any industrial process is susceptible to off-normal occurrences and accidents that may inadvertently introduce impurities into the CO₂ that is injected into the geologic storage reservoir. 

Geologic sequestration of CO₂ that contains impurities is known as co-sequestration, a term first used with reference to co-injected CO₂ and H₂S (Williams, 2002).  The geochemical effects of co-injected impurities on a geologic storage reservoir and its caprock are largely unknown.  The essential problem facing geologic carbon-sulfur co-sequestration is that SO₂, the most abundant constituent in SO_x, is very reactive in water-rock systems.  We are undertaking a series of hydrothermal experiments to evaluate CO₂-SO₂-brine-rock reactions and processes in saline reservoirs.  Our results will help to determine the viability of co-sequestering sulfur with carbon and bring the management and storage of carbon emissions closer to a practical reality. 

Our experiments emulate actual carbonate and siliclastic formations, the Madison Limestone and Weber Sandstone, respectively.  Both formations are viable sequestration targets (Surdam and Zhao, 2007).  In southwest Wyoming these formations also house natural accumulations of CO₂.  These two "natural analogs" have stored supercritical CO₂ for geologic time scales.  Experimental work developed in the context of these naturally occurring fluid-rock systems helps clarify long-term storage behavior of reactive carbon and sulfur in anthropogenic systems. 

The Weber Sandstone is an anhydrite and dolomite cemented, pyrite-bearing, arkosic sandstone housing a Na-SO₄ brine (I = 0.4 molal, pH = 8.0).  The Madison Limestone is a dolostone that also contains calcite, anhydrite, and accessory pyrite and silicate minerals.  It houses a Na-Cl-SO₄^2- brine (I = 0.5 molal).  Hydrothermal experiments emulate both formations by reacting synthetic rock and brine at in-situ conditions (110°C and 25 MPa) for approximately 2000 hours.  Supercritical CO₂ containing 500 ppm SO₂ is then injected and the experiments continued for an additional 500 to 1100 hours.  Parallel experiments are performed without SO₂ to provide a basis of understanding for the interaction of SO₂ with supercritical CO₂-brine-rock systems. 

Synthesis and interpretation of the Madison Limestone experiments are nearly complete and represent the culmination of one MS student's graduate research.  The results of the Weber Sandstone experiments are being processed and represent a significant component of a second MS student's graduate research.  PRF funds are being leveraged with other funding sources for this work.  The following are highlights of this research.

1)    The dolomite-calcite-anhydrite mineral assemblage and reaction textures produced in supercritical CO₂-brine-rock experiments are consistent with mineral assemblages and textures present in the natural CO₂ reservoir of the Madison Limestone on the Moxa Arch.  These results are consistent with a thermodynamic assessment for the Madison Limestone-brine system performed by Kaszuba et al. (2011). 

2)    Injection of supercritical CO₂ into the Madison Limestone experiments decreases pH, increases Eh, and drives reaction pathways along the pyrite-anhydrite saturation boundary of an Eh-pH diagram.  The bulk mineralogy present in the experiment, and by analogy in the natural system, does not change in response to emplacement of supercritical CO₂.  Minerals are instead dissolved, mobilized and re-precipitated.  Mineral dissolution and re-precipitation textures observed in the Madison Limestone in southwest Wyoming could be records of the emplacement of CO₂ as opposed to infiltration of aqueous fluids or diagenetic changes.

3)    Published geochemical modeling studies hypothesize that co-injected SO₂ will disproportionate into H₂S and H₂SO₄, decreasing pH in formation waters at least one unit more than acidification by CO₂ alone.  Injection of supercritical CO₂ into Madison Limestone and Weber Sandstone experiments decreases in-situ pH between 2.5 and 3.5 units.  Co-injection of supercritical CO₂ + SO₂ yields pH values that closely track SO₂-absent experiments, never differing by more than 0.1 pH units.  Thus, supercritical CO₂ and reservoir rock determine pH, co-injected SO₂ does not generate extreme pH.

4)    Anhydrite precipitates in response to injection of supercritical CO₂ ± SO₂ and provides a mineral trap for sulfur in a carbon-sulfur co-sequestration scenario.  Anhydrite precipitation decreases aqueous sulfate activity, ultimately leading to carbonate re-precipitation and thus mineralization of carbon. 

5)    Equilibrium laboratory experiments can predict the long-term fate of reactive carbon in a natural carbon dioxide reservoir as well as a sequestration scenario, even if equilibrium is not achieved on the laboratory scale.

References

Gale, J., 2009. Impure thoughts. Int. J. Greenhouse Gas Control 3, 1-2.

Kaszuba, J. P., Navarre-Sitchler, A., Thyne, G., Chopping, C., and Meuzelaard, T., 2011. Supercritical carbon dioxide and sulfur in the Madison Limestone:  A natural analogue in Southwest Wyoming for geologic carbon-sulfur co-sequestration. Earth Planet. Sci. Lett. 309, 131–140.

Pacala, S., and Socolow, R., 2004. Stabilization wedges: Solving the climate problem for the next 50 years with current technologies. Science 305, 968-972.

Rao, A.B., and Rubin, E.S., 2002. A technical, economic, and environmental assessment of amine-based CO₂ capture technology for power plant greenhouse gas control. Environ. Sci. Technol. 36, 4467-4475.

Surdam, R.C., and Zhao, Z., 2007.  The Rock Springs Uplift: An outstanding geological CO₂ sequestration site in southwest Wyoming.  Wyoming Geological Survey, WSGS-2007-CGRD-02.

Williams, R.H., 2002. Major roles for fossil fuels in an environmentally constrained world, Sustainability in Energy Production and Utilization in Brazil: The Next Twenty Years: Universidade Estadual de Campinas (Unicamp), Campinas, Sao Paulo, Brazil.