274403 Impact of Alkalinity Sources On the Life-Cycle Energy Efficiency of Mineral Carbonation Technologies

Wednesday, October 31, 2012: 1:20 PM
336 (Convention Center )
Abby Kirchofer, Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA, Adam Brandt, Energy Resources Engineering, Stanford University, Stanford, CA, Sam Krevor, Petroleum Engineering, Imperial College, London, England, Valentina Prigiobbe, Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX and Jennifer Wilcox, Department of Energy Resources Engineering, Stanford University, Stanford, CA

Impact of alkalinity sources on the life-cycle energy efficiency of mineral carbonation technologies

Abby Kirchofer1, Adam Brandt2, Sam Krevor3, Valentina Prigiobbe4, and Jennifer Wilcox2

1 Earth, Energy, and Environmental Sciences, Stanford University

2Energy Resources Engineering, Stanford University

3Petroleum Engineering, Imperial College

4Petroleum and Geosystems Engineering, University of Texas at Austin

Mineral carbonation is a Carbon Capture and Storage (CSS) technology where gaseous CO2 is reacted with alkaline materials (such as silicate minerals and alkaline industrial wastes) and converted into stable and environmentally benign carbonate minerals (Metz et al., 2005). Previous studies on mineral carbonation have principally focused on identifying process conditions optimal for enhancing the chemical rates of reactions. No study, however, has yet performed a scheme that optimizes these conditions with respect to engineering and economic consideration for the goal of producing a viable carbonation process.

Here, we present a holistic, transparent life cycle assessment model of aqueous mineral carbonation built using a hybrid process model and economic input-output life cycle assessment approach. We compared the energy efficiency and the net CO2 storage potential of various mineral carbonation processes based on different feedstock material and process schemes on a consistent basis by determining the energy and material balance of each implementation (Kirchofer et al., 2011). In particular, we evaluated the net CO2 storage potential of aqueous mineral carbonation for serpentine, olivine, cement kiln dust, fly ash, and steel slag across a range of reaction conditions and process parameters. A preliminary systematic investigation of the tradeoffs inherent in mineral carbonation processes was conducted and guidelines for the optimization of the life-cycle energy efficiency are provided.

The LCA model allows for the evaluation of the tradeoffs between different reaction enhancement processes while considering the larger lifecycle impacts on energy use and material consumption. All main process stages are included in the tool, and comprehensive system boundaries are applied throughout the model. The process-model core of the mineral carbonation LCA tool includes 8 process stages defined generically to be applicable to a variety of mineral carbonation technologies (see Figure 1). Because our tool aims to compare process schemes that vary significantly in input resource and process design, the model is built at a general, first-order level. The methodology accounts for the following three types of energy consumption: on-site energy consumption, energy of material and energy inputs consumed in the sector of interest (embodied direct energy), and energy of material and energy inputs consumed in all other sectors (embodied indirect energy). Including both on-site and embodied energy allows a full accounting of the total greenhouse gas (GHG) reduction benefits of each process scheme.


Figure 1. Life cycle process model schematic for aqueous mineral carbonation based on the Olivine – 155 C case; line thickness is scaled to the energy and mass fluxes.

The life-cycle assessment of aqueous mineral carbonation suggests that a variety of alkalinity sources and process configurations are capable of net CO2 reductions. The maximum carbonation efficiency, defined as mass percent of CO2 mitigated per CO2 input, was 83% for CKD at ambient temperature and pressure conditions. In order of decreasing efficiency, the maximum carbonation efficiencies for the other alkalinity sources investigated were: olivine, 66%; SS, 64%; FA, 36%; and serpentine, 13%. Within the range of parameters tested, for all cases maximizing the extent reacted of the alkalinity source unambiguously improves process efficiency. In general, maximizing the extent reacted is critical to optimizing the processes because it minimizes the material handling requirements which contribute negatively to the energy budget. In particular, we observed that:

the process efficiency is maximized by increasing extent reacted through the most energetically favorable enhancement measure;

mixing, heating, and grinding are the main energy drivers across all processes;

reuse of carbonate as aggregate does not necessarily improve life-cycle energy efficiency;

not all alkalinity sources benefit from high reaction temperatures;

any steps to increase reaction rates would dramatically improve process efficiency.

Additionally, the CO2 storage potential of mineral carbonation was estimated using the life-cycle assessment results and alkalinity source availability. The annual storage potential for a given alkalinity source was calculated by multiplying its availability (Mt/yr) by the CO2 sequestration efficiency of mineral carbonation of that alkalinity source (t-CO2/t-alkalinity source). For industrial alkalinity sources, availability was based on U.S. production rates (Kelly et al., 2011). For natural alkalinity sources, availability is estimated based on U.S. production rates of a) lime (18 Mt/yr) or b) sand and gravel (760 Mt/yr) (USGS, 2011). The low estimate assumes the maximum sequestration efficiency of the alkalinity source obtained in the current work and the high estimate assumes a sequestration efficiency of 85%. The total CO2 storage potential for the alkalinity sources considered in the U.S. ranges from 1.3% to 23.7% of U.S. CO2 emissions, depending on the assumed availability of natural alkalinity sources and efficiency of the mineral carbonation processes.

References

Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L. A. Meyer (eds.) (2005) IPCC Special Report on Carbon Dioxide Capture and Storage, Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Kirchofer, A., A. R. Brandt, S. Krevor, V. Priogiobbe and J. Wilcox (2011) Impact of alkalinity sources on the life-cycle energy efficiency of mineral carbonation technologies. In preparation.

Kelly, K. E., G. D. Silcox, A. F. Sarofim and D. W. Pershing (2011) International Journal of Greenhouse Gas Control, 5, 1587-1595.

USGS (2011) Mineral Commodity Summaries 2011, United States Geological Survey.


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