4
Advanced Geochemical Methods for Sequestering Carbon

Emissions of CO2 from the use of fossil energy may be controlled by capturing CO2 in energy production facilities and then injecting the CO2 into deep sedimentary formations. The capture and storage of CO2 poses two principal difficulties: (1) the capture of CO2 from combustion products is energy-intensive, expensive, and likely applicable only to large-scale stationary processes; and (2) the buoyancy of gas-phase CO2 in reservoirs poses inherent risks of leakage.

In contrast, CO2 is naturally captured directly from the atmosphere by its reaction with silicate minerals to form carbonates as rocks are weathered. Unfortunately, this process, while thermodynamically favored, is very slow. If such weathering processes could be artificially accelerated, it might be possible to manage the CO2 produced by fossil fuels while avoiding some of the difficulties of conventional CO2 capture and storage. Geochemical immobilization can effectively eliminate the risk of CO2 leakage. In addition, the use of geochemical processes allows the direct capture of CO2 from the air, thus potentially lowering the cost of managing emissions from dispersed sources.

Research is needed in the following broad areas:

  • Assessment of the reactivity, abundance, and economic availability of suitable alkaline minerals and rocks;

  • Development of processes that can accelerate the carbonation reactions; and

  • Assessment of the capacity, cost, and environmental impact of these processes.

CHEMICALLY ENHANCED WEATHERING

It has been found that the rate of the very slow natural carbonation reactions of magnesium silicate minerals can be greatly accelerated by high-temperature pretreatment. Reduction to ultrafine particle sizes also helps. Both of these options are energy-intensive, however, and probably not practical. The minerals react readily with mineral acids, but the resulting salts no longer can react with CO2. It may be possible, however, to find reagents that can convert only a small portion of the rock, so as to leave a porous structure, which can then react with CO2.

Research Areas

Research is needed on chemical methods to accelerate the natural weathering process by which minerals form carbonates.

Following are specific questions that need to be addressed:

  • Could low-cost chemical methods be used to “pretreat” silicate minerals in order to facilitate the removal of metal ions and speed up the carbonation reactions?



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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report 4 Advanced Geochemical Methods for Sequestering Carbon Emissions of CO2 from the use of fossil energy may be controlled by capturing CO2 in energy production facilities and then injecting the CO2 into deep sedimentary formations. The capture and storage of CO2 poses two principal difficulties: (1) the capture of CO2 from combustion products is energy-intensive, expensive, and likely applicable only to large-scale stationary processes; and (2) the buoyancy of gas-phase CO2 in reservoirs poses inherent risks of leakage. In contrast, CO2 is naturally captured directly from the atmosphere by its reaction with silicate minerals to form carbonates as rocks are weathered. Unfortunately, this process, while thermodynamically favored, is very slow. If such weathering processes could be artificially accelerated, it might be possible to manage the CO2 produced by fossil fuels while avoiding some of the difficulties of conventional CO2 capture and storage. Geochemical immobilization can effectively eliminate the risk of CO2 leakage. In addition, the use of geochemical processes allows the direct capture of CO2 from the air, thus potentially lowering the cost of managing emissions from dispersed sources. Research is needed in the following broad areas: Assessment of the reactivity, abundance, and economic availability of suitable alkaline minerals and rocks; Development of processes that can accelerate the carbonation reactions; and Assessment of the capacity, cost, and environmental impact of these processes. CHEMICALLY ENHANCED WEATHERING It has been found that the rate of the very slow natural carbonation reactions of magnesium silicate minerals can be greatly accelerated by high-temperature pretreatment. Reduction to ultrafine particle sizes also helps. Both of these options are energy-intensive, however, and probably not practical. The minerals react readily with mineral acids, but the resulting salts no longer can react with CO2. It may be possible, however, to find reagents that can convert only a small portion of the rock, so as to leave a porous structure, which can then react with CO2. Research Areas Research is needed on chemical methods to accelerate the natural weathering process by which minerals form carbonates. Following are specific questions that need to be addressed: Could low-cost chemical methods be used to “pretreat” silicate minerals in order to facilitate the removal of metal ions and speed up the carbonation reactions?

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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report Can chemistry help maintain carbonates in solution so they do not immediately precipitate on surfaces, which slow or stop subsequent carbonation reactions? BIOLOGICALLY MEDIATED ENHANCEMENT OF WEATHERING PROCESSES Biological processes might be used to accelerate the weathering process. This acceleration might, for example, be implemented in large leaching piles such as those used for biologically mediated extraction of copper from ores. Microbes may be able to enhance the physical weathering of magnesium (Mg)- and calcium (Ca)-containing silicate minerals, as well as to enhance the liberation of the metal ions from their mineral form. Strong acids could be used to achieve this same goal, but in this case large amounts of acid would be required to liberate significant quantities of the metal ions, energy would need to be added to recover the acid, and the carbonation reaction would no longer be spontaneous. Microbiological systems may be able to circumvent these limitations by selectively producing acid in quantities large enough only to break up the minerals while providing an important mechanism to enhance metal extraction. In this context, biological processes would not leach the bulk of the alkaline source rock but would be used to mechanically decompose rock particles, increasing surface area and thereby increasing the rate at which subsequent processing could dissolve rock and make carbonates. Biological systems can play an important role in significantly enhancing the natural rate of weathering, which occurs on extremely long time scales (approximately 105 to 106 years). They could speed up these processes by breaking up mineral particles and increasing the reactive surface area. This is possible if the microbes or bacteria act very specifically (for example, producing acid) in key locations to help break apart the mineral structure. These bacteria could further speed the process by increasing the liberation of metal ions from the silicate minerals. Producing a carbonate mineral could be achieved by then combining these systems with some form of CO2 (such as aqueous, supercritical, or gaseous). A key consideration with respect to the use of biological systems to enhance the rate of weathering is the potential leverage available to the microbes. Because of the large quantities of silicate minerals necessary to sequester industrially interesting quantities of CO2, it is crucial that each microbe be able to help liberate several orders of magnitude more metal ions than the amount of acid produced. Research Areas The aim of research will be to identify the role that bacteria and microbes can play in enhancing the weathering process of serpentine (or similar) minerals to extract suitable metal ions (Mg2+, Ca2+) to neutralize carbonic acid. It will be necessary to understand the scale, biochemistry, and kinetics of each of these processes and reactions. Following are some specific questions that need to be addressed: Can biological systems extract energy from the overall exothermic and spontaneous carbonation reaction? Are other energy sources and nutrients required for the microbes?

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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report What are the mechanics by which bacteria and microbes could influence the weathering of silicate minerals? Do they tend to help break up larger mineral structures? In nitrogen-containing minerals, can nitrifying bacteria cause weathering? What is the potential increase in reactivity of the weathered rock? Are there biological mechanisms for extracting and/or liberating free or complexed metal ions from the silicate (i.e., serpentine) minerals? MICROBIAL FIXATION OF CO2 TO FORM CARBONATE MINERALS Cyanobacteria are known to use their photosynthetic energy to generate small quantities of calcium carbonate (CaCO3). In the Green Lake in Fayetteville, New York, species of synecococcus have been shown to deposit significant amounts of CaCO3 at the bottom of the lake in summer months. As noted, silicate minerals could be utilized as the source of Mg2+ or Ca2+ ions. It is not known whether any microbes are able to utilize Mg2+ to form magnesium carbonate (MgCO3). Selective techniques could be used for developing cyanobacterial strains that will be able to effectively utilize free Mg2+ ions to form MgCO3. The carbonate formation ability of biological systems is not well understood. Further understanding of this process can help enhance the carbonation reaction once a suitable source of alkalinity is present. Microbial carbonate formation can be coupled to biologically mediated metal ion extraction from silicate minerals to provide one possible pathway for CO2 immobilization. Potential benefits to microbial carbonation reactions include increasing carbonation reaction rates as well as increasing the presence of chemical factors that could help solubilize the carbonate after formation, so that the microbes can keep the surface of the silicate minerals free for further weathering reactions. These reactions could be carried out in industrial-scale bioreactors, large leaching piles, or in situ underground. Research Areas Research is needed to understand the biochemistry and regulatory aspects involved in the carbonation reaction in bacteria and microbes and how it might help us increase the rate and/or lower the cost of forming magnesium carbonates. Following are specific questions that need to be addressed: What is the role of CaCO3 formation in microbes? Can cyanobacteria that can utilize Mg2+ to form MgCO3 be found, isolated, and engineered? What are the characteristics of the genes and enzymes involved in this process, and what are the factors that influence their regulation? What chemical means are utilized to keep carbonate minerals in solution to prevent blocking or inactivating the surface area with carbonates? What is known about side reactions?

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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report FORMATION OF IRON CARBONATES There are various basic minerals containing iron in the +2 oxidation state that can react with CO2 to form iron carbonate (FeCO3). Possible options include oxides, silicates, and perhaps sulfides. Compared to magnesium silicate minerals, these minerals have the disadvantage of being unstable relative to oxidation, since the +3 oxidation state does not form a stable carbonate. However, under specific conditions, they could provide an important energy source by means of redox reactions for bacteria, and may have different, possibly faster, weathering properties. Under the conditions in which they are typically encountered, they have been stable for millions of years. The option of iron carbonation has not been studied nearly as extensively as that of magnesium carbonation, and further research needs to be performed to understand the advantages and disadvantages of these iron carbonation processes. Options for utilizing the base iron minerals are similar to those for magnesium bases except that oxygen must be avoided and a fairly narrow pH range is required for formation of the carbonate. Research Areas Research is needed to understand the chemistry involved in the carbonation reaction in iron and microbes and how it might help increase the rate and/or lower the cost of forming iron carbonates. Following are specific questions that need to be addressed: What conditions are required to enhance the thermodynamics and kinetics for forming stable iron carbonates? What role can reduced iron play in providing energy for bacteria to speed the weathering and carbonation reactions of iron- and non-iron-containing minerals? ENHANCED DISSOLUTION OF CO2 IN BRINES A major concern about the stability of supercritical CO2 storage in deep aquifers relates to its buoyancy. Less dense than water, CO2 will float under the top seal atop the water in an aquifer and could migrate upward if the top seal is not completely impermeable. The stability of sequestered CO2 in saline aquifers is much more certain if the CO2 is dissolved in the liquid or precipitates as a carbonate in the formation. Brines with dissolved CO2 have a greater density than that of CO2-free brines, and this greater density significantly reduces CO2 buoyancy and leakage problems from subsurface CO2 disposal. Dissolution disposal would facilitate the selection of aquifers as disposal sites, since the requirement for an impermeable top seal could potentially be relaxed, thereby expanding the number of possible disposal reservoirs. Over very long time scales, the CO2 injected into very large saline aquifers with enough capacity will eventually dissolve into the brine and pose significantly reduced risk of CO2 release. As with natural weathering, the natural dissolution of CO2 in brines is too slow to be a practical storage solution on the large scale necessary. The capacity of the brine in an aquifer to dissolve CO2 is much greater if the pH of the brine is at the high end of the naturally occurring range. Such brines also open up options for enhancing the dissolution rate. They may also enhance the formation of carbonate

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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report minerals. If the pH of a brine is high enough, it may even be utilized as an absorbent for CO2 that would be disposed of and needs no regeneration. Research Areas Research is needed to understand the capacity issues, engineering approaches, and physical and chemical conditions for enhancing the dissolution of CO2 in brines. Possible system concepts and research areas include the following: Brine is pumped from an aquifer to the surface to mix with moderately high-pressure CO2 and is reinjected at another point in the aquifer. The CO2 is envisioned to dissolve either in the aquifer or on the way down in the reinjecting well. Less CO2 pressure would be required than that needed for direct injection of the CO2. Supercritical CO2 is pumped directly into an aquifer where the dissolution process is enhanced by means of an in situ contactor or other chemical or physical means. Regarding absorbent or adsorbent transport of CO2 to brines—CO2 could be carried into deep aquifers by means of a low-cost disposable or regenerable chemical adsorbent or absorbent. At depth, the CO2 would be transferred to the brine. If regenerable, the CO2-lean carrier would be recovered back at the surface in a loop process. If the pH of the brine is sufficiently high, it may be possible to use the brine as a first-stage absorbent for low-partial-pressure CO2 in a flue gas stream, either in a conventional low-pressure-drop contactor or in a novel high-residence-time contactor. Cleanup with a conventional absorbent may be required to recover residual CO2. The physical and chemical processes that could enhance dissolution of CO2 into brines need to be identified. The conditions and the brine compositions that are best for dissolution and/or mineralization need to be discovered. Studies need to be performed on acid recovery and potential by-products. Soils Natural weathering rates are limited by, among other factors, the exposed surface area of chemically reactive rock. Weathering reactions might be accelerated simply by adding suitable alkaline rocks in powdered form to agricultural soils. This treatment would be particularly applicable in acidic soils. Breakdown of the source rock might be accelerated by biological mechanisms and by the high partial pressure of CO2 in soils. Once the alkaline rock is dissolved, the metal ions might either form carbonates that remained with the soil or leach from the soils with run-off. In the case of run-off, the net effect would be to add alkalinity to the ocean, thereby removing CO2 from the atmosphere.

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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report Research Areas Research is needed to understand and develop methods to treat soils to accelerate natural weathering reactions and other processes favorable to removal of CO2 from the atmosphere. Research is needed in the following areas: To assess the weathering rates of suitable alkaline minerals in soils. Such research should enable crude prediction of how weathering rates depend on the size and pretreatment of the base rock, on the kind of soil, and on the means of application of the alkaline minerals. To increase understanding of the fate of metal ions leached into soils. To assess the impacts of adding small amounts of base to soils.