Mai Uibu

CO2 mineral sequestration in oil-shale wastes from Estonian power production

In the Republic of Estonia, local low-grade carbonaceous fossil fuel--Estonian oil-shale--is used as a primary energy source. Combustion of oil-shale is characterized by a high specific carbon emission factor (CEF). In Estonia, the power sector is the largest CO(2) emitter and is also a source of huge amounts of waste ash. Oil-shale has been burned by pulverized firing (PF) since 1959 and in circulating fluidized-bed combustors (CFBCs) since 2004-2005. Depending on the combustion technology, the ash contains a total of up to 30% free Ca-Mg oxides. In consequence, some amount of emitted CO(2) is bound by alkaline transportation water and by the ash during hydraulic transportation and open-air deposition. The goal of this study was to investigate the possibility of improving the extent of CO(2) capture using additional chemical and technological means, in particular the treatment of aqueous ash suspensions with model flue gases containing 10-15% CO(2). The results indicated that both types of ash (PF and CFBC) could be used as sorbents for CO(2) mineral sequestration. The amount of CO(2) captured averaged 60-65% of the carbonaceous CO(2) and 10-11% of the total CO(2) emissions.

Developments in CO2 mineral carbonation of oil shale ash

Solid waste and atmospheric emissions originating from power production are serious problems worldwide. In the Republic of Estonia, the energy sector is predominantly based on combustion of a low-grade carbonaceous fossil fuel: Estonian oil shale. Depending on the combustion technology, oil shale ash contains 10-25% free lime. To transport the ash to wet open-air deposits, a hydraulic system is used in which 10(7)-10(8) cubic meters of Ca(2+)-ion-saturated alkaline water (pH level 12-13) is recycled between the plant and sedimentation ponds. The goals of the current work were to design an ash-water suspension carbonation process in a continuous mode laboratory-scale plant and to search for potential means of intensifying the water neutralization process. The carbonation process was optimized by cascading reactor columns in which the pH progressed from alkaline to almost neutral. The amount of CO(2) captured from flue gases can reach 1-1.2 million ton at the 2007 production level of the SC Narva Power Plants. Laboratory-scale neutralization experiments were carried out to compare two reactor designs. Sedimentation of PCC particles of rhombohedral crystalline structure was demonstrated and their main characteristics were determined. A new method providing 50x greater specific intensity is also discussed.

Waste oil shale ash as a novel source of calcium for precipitated calcium carbonate: carbonation mechanism, modeling, and product characterization.

In this paper, a method for converting lime-containing oil shale waste ash into precipitated calcium carbonate (PCC), a valuable commodity is elucidated. The mechanism of ash leachates carbonation was experimentally investigated in a stirred semi-batch barboter-type reactor by varying the CO(2) partial pressure, gas flow rate, and agitation intensity. A consistent set of model equations and physical-chemical parameters is proposed to describe the CaCO(3) precipitation process from oil shale ash leachates of complex composition. The model enables the simulation of reactive species (Ca(2+), CaCO(3), SO(4)(2-), CaSO(4), OH(-), CO(2), HCO(3)(-), H(+), CO(3)(2-)) concentration profiles in the liquid, gas, and solid phases as well as prediction of the PCC formation rate. The presence of CaSO(4) in the product may also be evaluated and used to assess the purity of the PCC product. A detailed characterization of the PCC precipitates crystallized from oil shale ash leachates is also provided. High brightness PCC (containing up to ∼ 96% CaCO(3)) with mean particle sizes ranging from 4 to 10 μm and controllable morphology (such as rhombohedral calcite or coexisting calcite and spherical vaterite phases) was obtained under the conditions studied.

Organic acid catalyzed synthesis of 5-methylresorcinol based organic aerogels in acetonitrile

A method of preparing 5-methylresorcinol and formaldehyde based organic aerogels in non-aqueous media with a benzoic acid derivative as a catalyst is being proposed in this paper. Here acetonitrile is used as a solvent that allows direct drying with carbon dioxide over the supercritical state without the need for a solvent exchange. The acidic properties of 2,6-dihydroxy-4-methyl benzoic acid promote the reaction of sol–gel polymerization, and at the same time it takes part in the reaction as a co-monomer and influences the nanostructure of the material. The evolution of the polymer was monitored using nuclear magnetic resonance spectroscopy and the structure of the resulting organic aerogels depending on the molar ratio of 5-methylresorcinol to 2,6-dihydroxy-4-methyl benzoic acid was studied by nitrogen adsorption–desorption measurements, scanning electron microscopy and infrared spectrometry.

Assessing the frost resistance of illite-based ceramics through the resonant frequency of free vibration and internal damping

Experimental samples with porosity ranging from 6 % to 50 % were prepared by sacrificing template method from illite-rich clay. The frost resistance was assessed by measuring the shift in resonant frequency of free flexural vibration of prismatic samples and by the change of internal damping due to freeze-thaw cycles. Resonant frequency was determined by the impulse excitation technique and internal damping of the material was evaluated from the width of resonant peak. Samples were saturated with water and subjected to freeze-thaw cycles in the temperature range from –22 to 20 °C. Resonant frequency and internal damping were measured after 50, 100, 200, and 300 freeze-thaw cycles. Resonant frequency drops with increasing number of freeze-thaw cycles, and the effect is more pronounced in the case of samples with higher porosity. Internal damping of material was not found to be a suitable quantity for assessing the frost damage.