(481c) Impact of Alkalinity Sources On the Life-Cycle Energy Efficiency of Mineral Carbonation Technologies

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 ûC 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:

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

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

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

á not
all alkalinity sources benefit from high reaction temperatures;

á 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.