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Co2 Storage As Carbonate Minerals

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Co2 Storage As Carbonate Minerals CO2 STORAGE AS CARBONATE MINERALS Report Number PH3/17 February 2000 This document has been prepared for the Executive Committee of the Programme. It is not a publication of the Operating Agent, International Energy Agency or its Secretariat. Title: CO2 storage as carbonate minerals Reference number: PH3/17 Date issued: February 2000 Other remarks: Introduction This study is an assessment of above-ground processes that would bind CO2 as a mineral carbonate. For example, a number of people have suggested converting naturally occurring mineral oxides to carbonates by reaction with CO2 captured at a power station. The mineral carbonate would be stored indefinitely. If carbonate storage were to be used, it could be a safe store for CO2 captured during the generation of power from fossil fuels. Such a process would have the attraction of ‘locking-up’ CO2 in a chemical that occurs naturally and is widely distributed, thus avoiding many of the environmental and safety concerns expressed about other storage options. Potentially suitable minerals for CO2 sequestration are, in general, complex silicates that originated deep within the earth’s mantle. There are huge quantities of such rocks and, over geological time- scales, they naturally ‘weather’ to a carbonate by reaction with atmospheric CO2. However, the carbonation reaction is extremely slow at normal temperatures and pressures. In addition, the mass of rock required is several times the mass of CO2 stored. In principle, these disadvantages could be minimised by use of large-scale processes similar to those used in mineral extraction industries where large quantities of materials are routinely processed to recover a relatively small quantity of product. The scientific basis for processes in which CO2 is stored in carbonate minerals is assessed in the study. The practical implications are also assessed. Storage opportunities based on carbonate minerals are identified and their potential application described. Six potential processing routes are identified and assessed in outline. A ‘favoured’ option is selected for detailed evaluation. The overall objective of this study is to assess the potential of mineral carbonates as a long-term store for fossil fuel derived CO2, and to calibrate this against other CO2 storage options. Approach adopted The study was done in several stages. Initially, an extensive literature review was undertaken to identify and classify processes suggested as being applicable to the sequestration of CO2 as a mineral carbonate. The next stage in the study was a geological survey of the availability and type of potential source rocks. This was followed by a scientific critique of the required reactions. Following the above, a short-list of potential process routes was prepared. Processes on the short-list were assessed in outline with the objective of identifying a ‘favoured’ process thought most likely to have potential as a sequestration option. This outline assessment was done using similar principles to the assessment methodology developed to assess novel power generation systems (see report PH3/16). The process were evaluated in terms of their ability to sequester the 3 million tonnes per year of CO2 emitted from a modern coal-fired power station capable of producing 500MWe. i Two processes were assessed in detail. It was evident from the outline assessment that most of the processes were unlikely to be viable. One process was selected as being more likely to be viable than the others, but key thermodynamic factors that determine the design data for this process are not well documented or are not known. Therefore a better understood process known to have major drawbacks, but with much in common with the favoured process, was also selected for detailed examination; this process was used as a baseline for the assumptions necessary to evaluate the favoured process. The study was undertaken by CSMA Consultants, of Redruth, Cornwall, England. Results and discussion Mineral carbonates can be formed from a variety of common minerals. Magnesium carbonate is the most commonly suggested carbonate to sequester CO2 because it is stable and suitable source rocks are widely available (see section 2 of the main report). Magnesium is generally favoured over calcium as the metallic cation because magnesium-rich rocks contain more reactive material per tonne than their calcium equivalents. On a geological time-scale these oxides and carbonates are not stable. Weathering and erosion slowly convert exposed mineral oxides into carbonates by absorbing atmospheric CO2, the carbonates slowly form sedimentary deposits, over aeons the sedimentary deposits enter the earth’s mantle, and the carbon is eventually recirculated as CO2 emission from volcanic activity. However, this geological cycle does not invalidate sequestration of anthropogenic CO2 as a mineral carbonate because the quantities and timescale required are orders of magnitude smaller. There is a concern that mineral carbonates are sparingly soluble in rain water which, even if unaffected by sulphur and nitrogen oxides, is mildly acidic. The susceptibility and rate of dissolution of the sequestration product would depend on its form and would need to be tested. It is possible that storage areas would need to be sealed to prevent water ingress. initial screening Table S1 lists the processing options identified in a literature review. Each process is described in section 5 of the report where the processes are assessed in outline to determine their likely viability. As can be seen in table S1, many of the processes are similar to each other in relying on the formation of a hydroxide which is reacted with CO2 to produce a carbonate. This is because the thermodynamic equilibrium of most of these reactions is favoured by low temperatures - the high temperatures that are often necessary to get an adequate rate of reaction drive the equilibrium in the wrong direction. The chemistry is explained in section 4 of the main report. The equilibrium for the carbonation reaction with the hydroxide is relatively insensitive to temperature, therefore the reaction can be speeded up to viable rates by heating. Table S1: Processes considered. Process Raw Material Raw material CO2 reacts with: CO2 stored No. reacts with: as: 1 Mg silicate rock HCl Mg(OH)2 MgCO3 2 Mg-rich brine deposit H2O Mg(OH)2 MgCO3 3a Mg silicate rock MgCl2 Mg(OH)2 MgCO3 3b Mg silicate rock CO2 Mg3Si2O5(OH)4 MgCO3 4 Ca silicate rock HCl Ca(OH)2 CaCO3 5 MgCO3 + NaCl CO2 MgCO3 + NaCl NaHCO3 ii A brief outline of each process follows. process 1 In process 1 a magnesium silicate rock is dissolved in hydrochloric acid (HCl); magnesium hydroxide is produced via intermediate stages in which H2O and HCl are evaporated; the Mg(OH)2 which is a fine powder reacts with CO2 to form magnesium carbonate (MgCO3). This process uses a widely available feedstock and standard technology and does not have any major unassailable environmental problems. This process is evaluated in depth in the report (see section 7) because it is well documented and represents a marker for the other potential mineral carbonate sequestration approaches. It is not viable as a CO2 sequestration option. As demonstrated in the body of the report the dehydration and crystallisation stages of the process require more than 4 times the energy generated by the power station producing the CO2. Overall the process produces more CO2 than it sequesters. process 2 The process relies on a naturally occurring Mg-rich brine; such brines are found in large continental evaporitic basins but are rare. The processing route is a modification of process 1, also involving an energy-intensive dehydration step. Huge volumes of hydrochloric acid are generated and have to be disposed of. This process is rejected as being unlikely to be viable or gain wide acceptance. process 3a In an effort to avoid the energy-intensive dehydration stage of processes 1 and 2 it has been proposed that a magnesium chloride melt could be used as a solvent for the synthesis of Mg(OH)2. The process still requires a major dehydration step equivalent to about half the amount of evaporation in process 1. The net CO2 savings of the overall process are likely to be small. process 3b This method attempts to avoid the energy-intensive steps of the above processes by direct carbonation. Although the method is kinetically unproven it was decided that it was likely to be the most attractive of the processing options identified and was therefore examined in detail (see later). process 4 This process uses a calcium silicate rock which is dissolved in hydrochloric acid. The calcium hydroxide produced is dissolved in water and then reacted with CO2 to produce calcium carbonate. Hugh volumes of water are involved as the solubility of Ca(OH)2 is very low; the tailings lagoons required by the process would be enormous. Further, the concentration of Ca in silicate rocks is generally considerably lower than for Mg thus requiring more feedstock for a given sequestration capability. The process was deemed unsuitable for further investigation. process 5 1 This process is based on the dissolution of MgCO3 from dolomite. There is a net sequestration of CO2 because for every mole of MgCO3 reacted there are two moles of sodium bicarbonate (NaHCO3) produced. The basic chemistry and thermodynamics involved are proven. Extremely large quantities of material are involved; for example, the quantity of solids entering the reaction stage is about 8 times as large as for the equivalent step in process 1. The process would need to be located near the sea and an additional source of large quantities of salt (NaCl). In principle the NaHCO3 could be stored dry but it is quite soluble in water and care would be needed to ensure that is did not find its way back into the 1 Dolomite is a mixture of calcium carbonate (54%) and magnesium carbonate (46%).
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