(25d) Effect of Flue Gas Contaminants on CO2 Sequestration In Saline Formation

A great deal of concern has been expressed with regard to global climate change and its link to growing atmospheric concentrations of carbon dioxide. To decrease the impact of anthropogenic CO₂ on global climate, several strategies are under development that will potentially remove CO₂ from the atmosphere or decrease CO₂ emission. One such strategy involves the capture of CO₂ from large point sources (such as fossil fuel-fired power plants) and the long-term storage of CO₂ underground. Carbon dioxide may be sequestered in various geological formations, including depleted oil and gas reservoirs, unmineable coal seams, basalt formations, and deep saline aquifers.  Deep saline aquifers have been reported to have the most estimated capacity for CO₂ sequestration.  Most currently sequestration studies in saline formation are focused on pure CO₂.  However, the impacts of contaminants such as O₂, SOx and NOx, typical of flue gas streams in post-composition systems, are often neglected. This study examines the impact of flue gas contaminants on saline formation under CO₂ sequestration conditions.    

Most CO₂ captured from power plants, especially coal-fueled power plants, will inevitably contain some level of various contaminant species that will react chemically differently than pure CO₂ in saline formation.  For example, if SO₂ is present in the gas stream, sulfurous/sulfuric acid will from in the presence of water in the reservoir. This will further reduce the pH of the brine in the system.  Mineral species in the formation rock, such as K-feldspar, calcite, kaolinite, etc., that are stable in the native environment but not stable at low pH will dissolve, and new product mineral phases possibly precipitate as the pH drops in the system.  Therefore, there is a need to investigate the impact of flue gas contaminants on saline formation under CO₂ sequestration conditions.   

In this study, both simulation and experimental works were conducted.  The simulation work was conducted via use of Geochemist workbench.   We simulated the chemical reactions in the Mt. Simon sandstone environment with exposure to CO₂ mixed with other gas species under conditions that mimic sequestration. (Experimental conditions were as follows: solid material - Mt. Simon sandstone; liquid  - Illinois Basin brine; T and P - 50 C, 1500 psi; gas composition - 1% SO₂, 4% O₂, 95% CO₂).  The simulated experimental conditions include various Mt. Simon/Ill brine ratios from 3, 5 and 15 in a one liter vessel both with and without SO₂ and O₂.    Preliminary results showed that, as expected, the addition of SO₂ and O₂ affected pH, the concentrations of cations in the aqueous phase, and the oxidation state of iron.  Fluid pH was acidic in all simulations, although though it was markedly lower in the simulations with SO₂ and O₂. Increasing the proportion of solid material in the system buffered the system and increased pH.  Ca and Mg ions responded differently to the addition of SO₂ and O₂ and showed a strong dependence on pH in the mixed gas system.  Aqueous ferrous iron and siderite were predicted by the CO₂-alone simulations.

Selected conditions were examined experimentally in an autoclave reactor.  Reactions were conducted in a closed, well mixed reactor with  CO₂ or CO₂/O₂/SO₂ flue gas product mix at the identical conditions (50 °C, 1,500 psi, 300 R.P.M. and 7 days of reaction time). The results were compared with the simulation predictions.