Author's School

School of Engineering & Applied Science

Author's Department/Program

Energy, Environmental and Chemical Engineering

Language

English (en)

Date of Award

Summer 8-26-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Daniel E Giammar

Abstract

Geologic carbon sequestration: GCS) has been proposed as a means of mitigating the impacts of carbon dioxide: CO2) emissions from fossil fuel combustion on climate change. Mineral trapping, one of the trapping mechanisms of GCS, is of great importance because it has a potentially high sequestration capacity and provides very long-term sequestration. Forsterite: Mg2SiO4), a magnesium-rich silicate mineral, was studied with respect to its dissolution rates and its release of dissolved magnesium for subsequent precipitation of Mg-carbonate minerals. This study was conducted at conditions relative to GCS. Under different geological conditions, the thermodynamics and kinetics of both dissolution and precipitation reactions can vary. The overall fate of injected CO2 into porous media can be influenced by diffusive transport of aqueous species in addition to chemical reactions.

The rates and mechanisms of forsterite dissolution were studied under different temperatures, CO2 pressures, and salinities that were relevant to GCS. After an initially rapid dissolution period, the dissolution rate declined significantly, an effect that is attributed to the formation of a Si-rich layer at the forsterite surface. The initial dissolution rate increased with increasing temperature and increasing CO2 pressure. The effect of CO2 was through its influence on the pH. The dissolution rate was enhanced by NaCl, which may have been due to its inhibition of the formation of a silica-rich surface layer.

The dissolution of a partially weathered olivine from an Indian source: Mg1.84Fe0.16SiO4) was also studied at conditions relevant to both in situ and ex situ mineral carbonation. The release of magnesium to solution increased with increasing temperature and initial olivine concentration. The declining dissolution rate over time was also attributed to the formation of a Si-rich layer on olivine surface. The dissolution of the naturally weathered olivine was very similar to that of purer olivine at conditions relevant to mineral carbonation.

Experiments were performed to determine the effects of saturation conditions and different initial mineral substrates on magnesite precipitation from water-scCO2 solutions. The critical saturation index necessary for initiating magnesite precipitation at 100 degree C and 100 bar PCO2 was approximately 2. Precipitation was fastest when solutions were seeded with magnesite to remove nucleation as a rate-limiting step. Relative to mineral-free solutions, forsterite did not accelerate magnesite nucleation. The precipitation process did not reach equilibrium within 10 days.

At ambient PCO2, the influences of temperature, solution composition, and the presence of a solid substrate on the nucleation and precipitation of magnesium carbonate minerals were examined. At 25 degree C and 60 degree C the precipitates were hydrated magnesium carbonate minerals: nesquehonite or hydromagnesite), and at 100 degree C the solid phase was identified as brucite. Although magnesite: MgCO3) was predicted to be the most thermodynamically stable magnesium carbonate phase, no magnesite precipitated and instead metastable magnesium carbonate phases formed.

The effects of diffusive transport on both silicate mineral dissolution and carbonate mineral precipitation were studied by integrating bench-scale experiments and a mathematical model that coupled chemical reactions and diffusive transport. Simulations and experiments were performed for a tubular reactor packed with forsterite powder. The diffusivities of Mg2+ and dissolved inorganic carbon were included for quantifying rates of solute transport. The forsterite dissolution rate is a function of the pH, and the model calculated the pH at each location and time point based on the reaction rates and the transport of magnesium and inorganic carbon along the length of the tube. These simulations and experiments are relevant to diffusion-limited zones of GCS sites, and they suggest that diffusion-limitations can lead to local environmental conditions that can result in much different reaction rates and magnesite precipitation. For conditions of 100 degree C and 100 bar PCO2, magnesite precipitation was both predicted and observed to occur after five days at a location about 1 cm below the interface of the forsterite packed bed with a well-mixed CO2-rich aqueous solution.

Comments

Permanent URL: http://dx.doi.org/10.7936/K7ZG6QB6

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