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Call number: LD2668 .T4 1958 S49

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This thesis has addressed several gaps in the knowledge with regards to the calcium looping cycle for carbon dioxide capture, including identification of engineering challenges associated with the scale-up of the technology to pilot scale and beyond; assessment of changes in sorbent morphology during the pilot scale capture process; and partitioning of elemental impurities in the limestone between the solid and gaseous phase during the carbon dioxide capture process. Hydrodynamic investigations identified the optimum superficial velocities required for the reactor in order to optimise solids entrainment and flux, and to minimise gas bypassing. Estimations made in determining how particle attrition would affect minimum fluidisation velocity confirmed a decrease of approximately 0.09m/s for every 5 % reduction in particle size. Amendments made to the exhaust diameter and position, and the loop seals, improved the pressure balance of the system thus enhancing solids transfer. Reactor and process modifications, including modification of carbonator temperature, and maintenance of temperature above 420°C in standpipes resulted in improved carbon dioxide capture %. Increasing bed inventory had a positive effect of carbon dioxide capture % due to an increased Ca looping ratio. Steam addition also benefitted the carbonation process, due to improved sorbent morphology and therefore carbon dioxide diffusion into the sorbent. Sulfur dioxide was considered to have a detrimental effect on carbon dioxide capture due to pore pluggage, although burner- derived steam had a positive effect in maintaining capture %. Gaseous elemental emissions from the process were low for all elements, but concentrations of elements in the solid sorbent phase were influenced by bed inventory, implying that filtering systems may be required in industry for the large masses of sorbent required. Concentrations of elements in the sorbent were identified to be below levels typical of cement, with the exception of Ni, implying that there is potential for spent sorbent to be used in the cement industry with adequate mitigation measures in place.

A primary concern in the geologic storage of anthropogenic carbon dioxide is the leakage of buoyant CO₂ plumes into shallower formations, aquifers, or the surface. Man-made wells drilled through these formations present a potential leakage pathway for this CO₂ as the cement binding the well to the earth develops fractures or debonded microannuli form over time. Typically, wells with poor cementing or suspected leaks are subject to a cement-squeeze, in which new cement is injected to eliminate the leakage pathway. However, small fractures or leakage pathways are often difficult for oilfield cement to repair, as the cement dispersion is potentially screened out from dispersing fluid and cannot enter the fracture. Therefore a low-viscosity sealant is desired that can enter these leakage pathways easily and provide a robust seal. A class of poly(acrylic acid) polymers known commercially as Carbopol® are pH-sensitive microgels and swell/thicken upon neutralization with alkali cement components. These polymer dispersions are tested for ease of placement into cement fractures and subsequent development of resistance to displacement. Laboratory experiments involved injecting various unswollen polymer microgel dispersion into constructed cement fractures while measuring injection pressure and the pH of the polymer effluent to quantify the chemical reactions taking place and the induced viscosity changes. Fractures were constructed in order to allow for visual inspection of the polymer microgel swelling during and after injection, qualitatively useful in determining the polymer’s efficiency at blocking cement fractures. It was determined that polymer microgels undergo syneresis in the presence of calcium cations that are dissolved from minerals present in cement. The syneresis causes the polymer to collapse onto the cement fracture face and expelled water is left to fill the rest of the fracture, providing little to no resistance to subsequent flow. However, the syneresed polymer does show some potential in blocking or partially blocking small aperture fractures and is not entirely detrimental to fracture blockage in small amounts. An acid pre-flush prior to polymer injection has been seen to favorably reduce the amount of calcium and therefore extent of syneresis, allowing swollen polymer microgels to remain intact and block fluid flow. / text

The factors which affect the engineering properties of calcium carbonate cemented soil are examined. The influence of calcium carbonate content, molding moisture content, and confining pressure on the strength characteristics of two types of soil is investigated in two distinct phases of the research. Type A soil, obtained from the University of Arizona Campbell Avenue Farm in Tucson, was used for the artificially cemented specimen stage. It is composed of sand and silt-size particles with some clay and is virtually free of calcium carbonate in its natural state. Sierrita soil, obtained from the Twin Buttes Open Pit Mine south of Tucson, was used for the reconstituted sample stage. It is naturally cemented with calcium carbonate and is composed mainly of sand, gravel, a small amount of silt, and occasional large-sized (boulder and cobble) particles. Specimens for triaxial compression testing were compacted for each phase of the study under carefully controlled conditions. Three test series were carried out on Type A soil artificially cemented with calcium carbonate. Three percentages (0%, 15%, and 30%) on a dry weight basis of the soil were used. Two molding water contents, one dry and one wet of optimum moisture content, were established for each test series. Unconsolidated undrained triaxial compression tests were carried out on oven-dried specimens at three different confining pressures to obtain shear strength parameters. The fabric characteristics of selected specimens were then defined by viewing them under a scanning electron microscope. The results indicate that the strength of the calcium carbonate cemented soil depends on the distribution and not necessarily the content of the cementing agent within the soil mass. Visual examination of the various microstructures of the artificially cemented soil confirmed the hypothesis that strength gain occurs when the calcium carbonate particles are concentrated at the points of contact between soil grains. Visual examination of the fabric of the naturally cemented Sierrita soil showed the microstructure to be highly compressed with weathered calcium carbonate particles dominating the soil structure. The calcium carbonate content was found to range from 14 to 23%. Because of sampling difficulties, an in situ cohesion value for the Sierrita soil could not be obtained from conventional laboratory tests. Therefore, the value was obtained by back analysis of the stability of actual slopes existing at Twin Buttes Mine. Slope stability analyses using Bishop's Modified Method with a search routine based on the Simplex Method of Nelder and Mead were performed. Stability analyses were also performed using strength properties of artificially cemented Type A soil. These analyses showed the relationships among cohesion, friction angle, safety factor, and calcium carbonate content for a specified slope geometry.

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The thermodynamic properties of chloride, nitrate and nitrite AFm hydrates have been determined. Investigations of solid solutions and thermodynamic calculations on the influence of these anions on mineralogical changes in cement paste were performed and compared with experiments. To calculate volume changes, densities of principal crystalline phases occurring in cement were critically assessed and tabulated, in some cases with addition of new data. Database was obtained by calculating densities from crystallographic data and unit cell contents. In hydrated cements, anion sites in AFm phase are potentially occupied by OH, SO₄ and CO₃ ions. C1, NO₃ or NO₂ ions readily displace hydroxide, sulfate and carbonate in the AFm structures. Nitrates and nitrites do not have ability to displace chloride from the Friedel’s salt (C1-AFm) though. The binding power of AFm for nitrite/nitrate/chloride was calculated and confirmed experimentally at 25°C. It was observed that presence of chloride, nitrate or nitrite alters the AFm/Aft balance and thereby affect the specific volume of paste solids. It was found that the success of nitrite as a corrosion inhibitor for protection of embedded steel arises from its “smart” behaviour. AFm normally stores and sequesters nitrite. If chloride ingress occurs in service, the AFm undergoes ion exchange, gaining chloride and forming Friedel’s salt, while releasing soluble nitrite ions to the pore fluid. As a result, the aqueous ratio of [NO₂^-]/[C1^-] increases and remains within the passivation range for steel.