Avoiding emissions of the greenhouse gas CO2 to the atmosphere by its safe and permanent storage is required for all options within the carbon dioxide capture and storage (CCS) scheme. Only mineral carbonation allows for a sequestration process, where the carbon is rapidly converted to its chemically most stable form, a solid carbonate. The process involves the pre-treatment of a magnesium or calcium oxide bearing material, its carbonation (Mg/Ca-leaching followed by carbonate precipitation) and subsequent processing steps downstream. The products can be used given a market exists or deposited without the need of monitoring programs (IPCC, 2005). The latter is critical in the case of geological storage, i.e. when captured CO2 is not processed in a carbonation plant, but sent to suitable storage reservoirs (depleted oil and gas fields, saline aquifers). Any onshore geosequestration project might face the opposition of local residents, under whose premises the CO2 plume would spread. Public perception, regulatory issues, well bore integrity, risk of induced seismicity. Such barriers will slow down the deployment of subsurface CO2 storage and will in turn help alternative sequestration techniques to gain momentum. Within the CCS value chain, capturing CO2 from flue gases and the carbonation step are per se energy intensive and thus expensive unit operations. Our approach is to avoid the costly capture step by contacting cooled flue gas with the raw mineral during wet-grinding and dissolution (Mg-leaching), followed by Mg-carbonate precipitation at increased temperature, where carbonate solubility is low (Verduyn et al., 2009). Figure 1 below visualizes this concept in comparison with the traditional CCS approach outlined above.
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Figure 1: (A) CCS scheme including the separate capture and storage of CO2, either in a reservoir or mineralization plant. (B) The concept of direct flue gas mineralization for combined capture and storage (after Verduyn et al., 2009).
Direct flue gas carbonation needs to address two trade offs: First, although silicate dissolution would be faster at higher temperature, the latter has to be moderate in order to simultaneously capture CO2, i.e. dissolving it into the aqueous solution. Second, more CO2 would dissolve at higher CO2 partial pressure, but co-pressurization of about 90% nitrogen (the bulk of the flue gas) increases both capex and opex. A precise knowledge of the Mg-leaching and precipitation kinetics is key to finding the optimum temperature and pressure conditions.
Direct flue gas mineralization using activated serpentine
During the last years, we conclusively investigated the dissolution behavior of the Mg-silicate olivine (e.g. Hänchen et al, 2006/2007; Prigiobbe et al. 2009) and the precipitation regime of Mg-carbonates (Hänchen et al., 2008). Resources of pure olivine, however, are limited. Worldwide the most abundant Mg-silicates are serpentines. Therefore, for future work, we intend to build up on our experience with olivine, but change the feed silicate. Serpentines contain crystal water, which can be freed via thermal treatment (roasting), which destabilizes the crystal lattice and thus accelerates the dissolution kinetics. In 2010, we received a batch of already thermally pretreated lizardite-type serpentine from Shell in Amsterdam, The Netherlands. Using this material, we like to answer the following questions:
Werner M., Verduyn M., van Mossel G., Mazzotti M. - 2011. Direct flue gas CO2 mineralization using activated serpentine: Exploring the reaction kinetics by experiments and population balance modelling. Energy Procedia 4, 2043-2049. DOI
Prigiobbe V., Costa G., Hänchen M., Baciocchi R., Mazzotti M. - 2009. The effect of CO2 and salinity on olivine dissolution kinetics at 120°C. Chem. Eng. Sci. 64, 3510-3515. DOI
Hänchen M., Prigiobbe V., Baciocchi R., Mazzotti M. - 2008. Precipitation in the Mg-carbonate system - effects of temperature and CO2 pressure. Chem. Eng. Sci. 63, 1012-1028. DOI
Hänchen M., Krevor S., Lackner K., Mazzotti M. - 2007. Validation of a population balance model for olivine dissolution. Chem. Eng. Sci. 62, 6412-6422. DOI
Hänchen M., Prigiobbe V., Storti G., Seward T., Mazzotti M. - 2006. Dissolution kinetics of fosteritic olivine at 90-150°C including effects of the presence of CO2. Geochim. et Cosmochim. Acta 70, 4403-4416. DOI
Verduyn M., Boerringter H., Oudwater R., van Mossel G. - 2009. A Novel Process Concept for CO2 Mineralization; Technical Opportunities and Challenges. In: 5th Trondheim Conference on CO2 Capture, Transport and Storage, Trondheim, Norway.
IPCC - 2005. Special Report on CCS. Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L. (Eds.) Cambridge University Press, UK. pp. 431.
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