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Mining Engineering Department, Karadeniz Technical University, Trabzon, Turkey
Received Date: September 10, 2015; Accepted Date: September 12, 2015; Published Date: September 21, 2015
Citation: Aydiner K (2015) Carbon-dioxide Storage in Geological Media. J Powder Metall Min 4:e135.doi:10.4172/2168-9806.1000e135
Copyright: © 2015 Aydiner K. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Each day more CO2 is discharged into atmosphere and the projections do not refer to the considerable decreases for the emissions in the near future, as the fossil fuel have been expected to remain the main energy resource for its approximately 150 years of life even by the more conservative estimations. This case leads to two alternatives as i) accomplishing lower carbon emissions or ii) switching to cleaner energy resources not emitting CO2. However, currently these two alternatives are far from the reality since there is any successful solution by each of these alternatives. So the most promising solution is to capture, sequester and store CO2 in a sink area or media.
CO2 can be stored in geological formations either under the ocean floor or in the terrestrial areas. Storage under the ocean floor does not be seen as applicable since the transport and the storage processes are extremely costly and difficult. In the terrestrial areas CO2 can be stored in low rank coal deposits, saline aquifers depleted oil reservoirs and deep massive rock masses. In these media storage is performed basically as the injection of CO2 in a supercritical fluid form. The success of these type in-situ geological storage methods is dependent on an impermeable layer preventing upright movement of gas. Otherwise, as a result of the upright movement, it transforms again its gas form.
As a geological storage alternative the mineral carbonation method has been considered as a promising method in CO2 storage. After the storage process CO2 is transformed into different minerals. The method can principally be considered as a dissolution process and as a result of the chemical reactions between CO2 and the minerals containing Mg and Ca, geologically stable and environmentally safe carbonate minerals (calcite and magnesite) are formed. The method can be applied as in-situ and ex-situ. One of a couple of pioneering work (CarbFix Project) have been conducted at Iceland. As an in-situ storage process CO2 charged water is injected deep into basalt rock. However, in-situ processes has a basic disadvantage as the distribution of CO2 in whole body of rock underground is difficult and requires extensive logging.
Ex-situ storage of CO2 requires to process rock outside the main rock. Researches on this type of storage method are on the lab scale currently. After a comminution process the rock material is processed in a pressure and temperature controlled acid-water solution. CO2 is injected into a process medium at a suitable pressure and flow rate. CO2 interacting with the minerals in the solution performs carbonation process. This is an exothermic reaction and as a result thermodynamically stable carbonates are formed. High rates of Mg and Ca containing minerals carbonated as a result of CO2 effect. Main reaction is as follows.
CaO + CO2 → CaCO3 + 179 kJ/mol
MgO + CO2 → MgCO3 + 118 kJ/mol
Pure Ca and Mg oxides are not abundant in nature. However, high amount of minerals containing chemical combinations of these oxides are found. Peridotite, serpentine and dunite rocks contain high rates of Mg oxides. Amounts of these rock groups exceed far beyond the total reserves of the fossil fuels.
Industrial carbonation processes can be performed as gas-solid, gas-liquid or a combination these two. Gas-solid process base the direct contact of CO2 to fine grained minerals and has advantage over the solution process as it requires low energy. Gas-solid processes occur at room temperature and by lower process speeds.
Higher process speeds can be obtained by gas-liquid processes. Speed of the process is dependent mainly on the dissolution rate of the Mg under high pressure and temperature. By most used solution method, the fine grained Ca and Mg containing silicate minerals taken into a solution using suitable reactive and then CO2 is injected into solution. Acceptable process rates are possible. Output of this process depends basically on the temperature and the pressure of process medium. Experiments showed that the higher temperatures resulted in the higher dissolution rates of Ca or Mg.
As they offer more stable and environmentally safe alternative of storage for CO2, more attention and effort needs to be directed to the mineral carbonation (either in-situ or ex-situ). Both methods need more research in the future. Efficient distribution/injection of the CO2 in deep rock mass is the most important challenge for in-situ carbonation method. Main barrier over the current ex-situ carbonation method can be seen as the efficiency and the cost of the process. Therefore, the research efforts focused mainly on the process kinetics (reactives, pressure, temperature etc.) and the material properties (type of rock, mineral content, grain size etc.), to define the most effective process.--
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