CO2 sequestration via carbonation of widely available low-cost minerals, such as olivine, can permanently dispose of CO2 in an environmentally benign and a geologically stable form. We report the results of studies of the mechanisms that limit aqueous olivine carbonation reactivity under the optimum sequestration reaction conditions observed to date: 1 M NaCl + 0.64 M NaHCO3 at Te 185 degrees C and P(CO2) approximately equal to 135 bar. A reaction limiting silica-rich passivating layer (PL) forms on the feedstock grains, slowing carbonate formation and raising process cost. The morphology and composition of the passivating layers are investigated using scanning and transmission electron microscopy and atomic level modeling. Postreaction analysis of feedstock particles, recovered from stirred autoclave experiments at 1500 rpm, provides unequivocal evidence of local mechanical removal (chipping) of PL material, suggesting particle abrasion. This is corroborated by our observation that carbonation increases dramatically with solid particle concentration in stirred experiments. Multiphase hydrodynamic calculations are combined with experimentto better understand the associated slurry-flow effects. Large-scale atomic-level simulations of the reaction zone suggest that the PL possesses a "glassy" but highly defective SiO2 structure that can permit diffusion of key reactants. Mitigating passivating layer effectiveness is critical to enhancing carbonation and lowering sequestration process cost.

Storage of CO2 through mineral sequestration using olivine has been shown to produce environmentally benign carbonates. However, due to the formation of a rate-limiting reaction product layer, the rate of reaction is insufficient for large-scale applications. We report the results of altering the reactant solution composition and the resultant reaction mechanism to enhance the reaction rate. The products were analyzed for total carbon content with thermal decomposition analysis, product phase compositions with Debye-Scherrer X-ray powder diffraction (XRD), surface morphology with scanning electron microscopy (SEM), and composition with energy dispersive X-ray spectroscopy (EDXS). Carbon analysis showed that an increase in bicarbonate ion activity increased the olivine to carbonate conversion rate. The fastest conversion rate, 63% conversion in one hour, occurred in a solution of 5.5 M KHCO3. Additionally, SEM confirmed that when the bicarbonate ion activity was increased, magnesium carbonate product particles significantly increased in both number density and size and the rate passivating-reaction layer exfoliation was augmented.

To investigate the effects of carbonated water injection (CWI) on dolomite porous media, a set of two coreflooding experiments were performed on outcrop samples. The results allowed the identification of the effects of dissolution on porosity and permeability on outcrop dolomite samples. Experiments were carried out at 70 °C, injection pressures of 8500 and 7500 psi, and a constant injection flow rate of 2 cm3/min. The experimental setup was assembled and commissioned arranging two coreholders connected in series, each one containing a rock sample, to improve result acquisition. The injected fluid was carbonated water made from synthetic sea water (salinity of 38 kppm) saturated with 21.5% of total solubility in CO2 X-ray Computed Tomography (CT) provided image acquisition allowing the evaluation of porosity evolution throughout the tests. Experiments showed a tendency for porosity increase for the sample in the first coreholder, possibly associated with rock dissolution. The spatial porosity profile from the CT scan for the first coreholder showed porosity variation in the first centimeters of the rock sample inlet. In the second coreholder, porosity remained unchanged during the entire test, indicating that the dissolution effect promoted by carbonated water injection was most notable in the first sample when compared with the second. The amount of dissolved moles afforded the behavior regarding dissolution phenomena. Permeability behavior was analyzed through the pressure drop values from the sample for the entire experiment, registered by pressure transducers installed in each coreholder. Results showed consistent permeability behavior for the first coreholder samples, despite the tendency of the porosity to increase. The samples placed in the second coreholder presented lower permeability because minerals dissolved in the first sample were transported to the second sample and likely precipitated and so related to the porous media blockage.

Carbon dioxide emissions from fossil fueled power plants are of concern due to their potential to induce global climate change [1]. Mineral carbonation, i.e. reaction of the anthropogenic CO 2 in a purpose-built processing plant to produce a geologically stable carbonate is a possible method to reduce emissions. Understanding the carbonation reaction mechanism is essential for process control. We are investigating the nanoscale reaction mechanism in two magnesium silicates: San Carlos olivine and serpentine. We showed earlier that carbonation begins by formation of a reaction layer on olivine surfaces, in our case on [001] single crystal surfaces [2]. We are using the standard USDOE Albany Research Center reaction conditions: isothermal reaction experiments for various times up to 8 hrs in 1 M NaCl and 0.64 M Na HCO 3 aqueous solution at 185C under 2200 psi CO 2 pressure[3], but without rapid stirring, to preserve reaction layer morphology. The overall carbonation reaction is:Mg 2 SiO 4 + 2CO 2 = 2MgCO 3 + SiO 2 .(1) Several microscopy techniques were required to provide sufficient insight to understand the mechanism: standard and stereographic surface imaging, HREM and STEM imaging, EELS and EDS nanospectroscopy, and FFT optical diffraction. When the specimens are removed from the reaction vessel it was necessary to clean them, to prevent NaCl/NaHCO 3 crystal formation on their surfaces. The cleaning method affected the surface morphology. After gently rinsing reacted crystals in pure water the surface appeared rumpled, as shown in fig. 1. Stereographic SEM images showed that the light grain-boundary-like features are raised ridges, curling above the featureless surface regions. EDS spectra showed this surface to have a larger Si/Mg ratio than olivine. The larger particles are reaction products or debris left from cleaning, not important for this discussion. Specimens that had gone through the same reaction cycle but that were cleaned by sonication in water had the surface morphology shown in fig.2, where the raised ridges are removed. Stereographic images from these surfaces showed that the dark grain-boundary-like features were grooves in the surface; EDS nanospectra showed that the composition at the bottom of the grooves was olivine, but the "islands" between the grooves had higher Si/Mg composition ratio than olivine. Cross section specimens were made for HREM, to examine this reaction or passivation layer (PL) on the olivine surface. Frequent cracking of the layer during preparation indicated that significant residual stress was present. The interface between the olivine substrate and the PL is shown in fig.3. The PL is amorphous and its interface with olivine is nearly flat on the nanoscale. NanoEELS showed the PL to have an [O]/[Si] slightly less than 2; the LL peak occurred at 22.8±0.4eV and the Si-L edge shape both agree with earlier results for SiO 2 [4]. There was a dilute distribution of nanoparticles in the PL which were shown to be MgCO 3 by FFT from the HREM images. The average particle size was...

Coal can support a large fraction of global energy demands for centuries to come, if the environmental problems associated with CO 2 emissions can be overcome. Unlike other candidate technologies, which propose long-term storage (e.g., ocean and geological sequestration), mineral sequestration permanently disposes of CO 2 as geologically stable mineral carbonates. Only benign, naturally occurring materials are formed, eliminating long-term storage and liability issues. Serpentine carbonation is a leading mineral sequestration process candidate, which offers large scale, permanent sequestration. Deposits exceed those needed to carbonate all the CO 2 that could be generated from global coal reserves, and mining and milling costs are reasonable (~$4 to $5/ton). Carbonation is exothermic, providing exciting low-cost process potential. The remaining goal is to develop an economically viable process. An essential step in this development is increasing the carbonation reaction rate and degree of completion, without substantially impacting other process costs. Recently, the Albany Research Center (ARC) has accelerated serpentine carbonation, which occurs naturally over geological time, to near completion in less than an hour. While reaction rates for natural serpentine have been found to be too slow for practical application, both heat and mechanical (attrition grinding) pretreatment were found to substantially enhance carbonation reactivity. Unfortunately, these processes are too energy intensive to be cost-effective in their present form.In this project we explored the potential that utilizing power plant waste heat (e.g., available up to ~200-250 o C) during mechanical activation (i.e., thermomechanical activation) offers to enhance serpentine mineral carbonation, while reducing pretreatment energy consumption and process cost. This project was carried out in collaboration with the Albany Research Center (ARC) to maximize the insight into the potential thermomechanical activation offers.Lizardite was selected as the model serpentine material for investigation, due to the relative structural simplicity of its lamellar structure when compared with the corrugated and spiral structures of antigorite and chrysotile, respectively. Hot-ground materials were prepared as a function of grinding temperature, time, and intensity. Carbonation reactivity was explored using the standard ARC serpentine carbonation test (155 o C, 150 atm CO 2 , and 1 hr.). The product feedstock and carbonation materials were investigated via a battery of techniques, including Xray powder diffraction, electron microscopy, thermogravimetric and differential thermal, BET, elemental, and infrared analysis. The incorporation of low-level heat with moderate mechanical activation (i.e., thermomechanical activation) was found to be able to substantially enhance serpentine carbonation reactivity in comparison with moderate mechanical activation alone. Increases in the extent of carbonation of over 70% have been observed in this feasibility study, indi...