2.2 MINERAL CARBONATION SEQUESTRATION
Mineral carbonation involves the reaction of CO2 with a compound that forms thermally stable and poorly solubleat ambient conditions. Such compounds include the elements calcium, magnesium and iron. To date attention has primarily been paid to magnesium silicates (serpentine and olivine) and sometimes calcium silicates (wollastonite), which are widely distributed around the world. Iron appears not to have been investigated previously due to its value as a resource for steelmaking. However, may be somewhat unique in this regard in having large deposits of low-grade magnetite that may be suitable for sequestration purposes.
Mineral carbonation occurs naturally and will ultimately sequester manmade GHG emissions through natural weathering of crustal rocks. However, the timeframe for this process is very large and the objectives of human intervention into the cycle are primarily to speed up the process.
Three key sources of published information have been instrumental in our understanding of mineral sequestration technologies. The IPCC published a Special Report on Carbon dioxide Capture and Storage (2006)59, and the Abo Akademi University in have published a number of technical and literature reviews (Zevenhoven et al. (2008, 2011)).60,61.
Zevenhoven et al. (2011) have presented a comprehensive, up to date literature review on the subject. They note that the basic idea of mineral carbonation is to mimic a naturally occurring process called “weathering”, where calcium (Ca) or magnesium (Mg) from silicate minerals is bound over geologic time with CO2, forming environmentally benign and stable calcium and magnesium carbonates (CaCO3, MgCO3). The raw materials for this process are widely available globally in the form of magnesium silicates (serpentine and olivine) and sometimes calcium silicates (wollastonite). These minerals are found on the east coast of Australia, for example in the Newarea rim.
There are recognised and well-documented advantages of mineral carbonisation:
- very large resources of the required minerals
- no post-storage monitoring needed, and some products may be useful for building products
- exothermic process chemistry
However, major issues that need to be addressed include:
- very slow reaction chemistry
- large material transport and storage requirements
- process costs and economics, including the process conditions and the cost of additives aimed at increasing the reaction kinetics
The major sources of calcium and magnesium minerals of interest for carbonation occur in igneous deposits originating from oceanic plates. Minerals of interest, reaction energy and the quantity required to sequester a unit weight of CO2, assuming complete reaction of the mineral, is shown in Table 2.1.
|Mineral||Formula||Products of complete reaction with CO2||Mineral requirement (kg/kg CO2)|
|Mg Olivine||Mg2SiO4||SiO2 + 2MgCO3||1.6|
|Mg Serpentine||Mg3Si2O5(OH)4||2SiO2 + 3MgCO3 + 2H2O||2.1|
|Wollastonite||CaSiO3||CaCO3 + SiO3||2.6|
|Basalt||varies||MgCO3, CaCO3, FeCO3||4.9|
|Magnetite||Fe3O4||Fe2O3 + FeCO3||5.3|
The global reaction chemistry of the key minerals with CO2 is illustrated by equations (4) to (6) below:
|Mg3Si2O5(OH)4 (s) +3CO2 (g) -> 3MgCO3 (s) + 2SiO2 (s) +2H2O (l)||~ −1.4 MJ/kg CO2||(4)|
|Mg2Si2O4 (s) +2CO2 (g) -> 2MgCO3 (s) +SiO2 (s)||~ −1.4 MJ/kg CO||(5)|
|CaSiO3 (s) + CO2 (g) -> CaCO3 (s) + SiO2 (s)||~ −2.3 MJ/kg CO2||(6)|
Zevenhoven et al. (2011) also note that industrial by-products from steelmaking slags could also be utilised as a raw material for the carbonation process, since these contain both MgO and CaO. The available tonnage of these materials is not as high as the natural mineral deposits, but could amount to the fixing of a few 100 Mt/a of CO2 worldwide.
The mineral mass necessary to bind unit mass of CO2 as carbonate are in the range 1.8–3 t mineral/t CO2 for relatively pure minerals. This means that for a 500MW unit emitting 3.4 Mt/a of CO2, around 6 – 10 Mt of mined minerals per year would be required. This is a scale comparable with large mines in Australia for other minerals and coal for each 0.5 GW of generating capacity.
Experiments on Carbonation
The base reaction rate (natural weathering) of metal oxide-bearing ore is extremely slow. By simply crushing ore to particle sizes of 1mm and suspending in aqueous solution 100% dissolution can be achieved within around 2000 years (Hangx 2009)63.
Various approaches have been adopted to try to improve the reaction kinetics, leading to the development of multi-stage process proposals now. Sipila et al. (2008) provide a table showing the state of the art in this regard, where it is shown that the three main routes are:
- Direct Carbonation
- Indirect Carbonation, and:
- Other Routes
These routes and further sub-routes comprise at least twelve different proposed methods for increasing the carbonation reaction rate.
Direct Carbonation refers to processes where the reaction occurs within the mineral matrix and Indirect Carbonation to where the metal ion (Ca2+ or Mg2+) is released from the matrix as a preliminary stage prior to carbonation.
Studies at the Los Alamos National Laboratory in the USA (1997, 2002)64,65, found that gas-solid contacting at elevated temperature and pressure (500°C, 340 bar) gave 25% conversion after two hours with 0.1mm sized serpentine particles. Further work in Finland (Zevenhoven66) showed considerably lower reaction kinetics for the gas-solid contacting case than a direct aqueous process developed by the Albany Research Center (ARC) in the USA using NaHCO3 and NaCl at 150 bar and 155°C for serpentine (O’Connor et al. 2004, 2005)67,68, This later research program achieved the benchmark reaction rates for mineral carbonation using an aqueous phase ex-situ direct activated cation approach which achieved up to 80% conversion of metal oxides to carbonates within 1.5 hours at costs of US$54-$78 per tCO2 (in 1995).
Depending on the type of Table 2.2 below gives the optimal carbonation conditions, (Gerdemann et al. 2007)69. It is important in this table to note the high pressures that are apparently required to achieve reasonable reaction times and conversion efficiencies. By comparison, the pressure to which CO2 must be compressed to achieve supercritical conditions at atmospheric temperature for pipeline transportation to geological storage is around 75 bar. These aggressive process conditions suggest relatively high technical and economic risk.used for the direct carbonation process, different process conditions can be applied.
|Mineral||Temperature (°C)||Pressure of CO2 (bar)||Additive Solution||Carbonation after one hour (%)|
|olivine||185||150||0.64M NaHCO3 1M NaCl||49.5%|
|serpentine||155||115||0.64M NaHCO3 1M NaCl||73.5%|
It should be noted that the (expensive) additives in Table 2.2 cannot be recycled and reused when employed in the direct carbonation methods.
Sipila et al. (2008) state that the direct aqueous mineral-carbonation route appears to be the most promising CO2 mineralisation alternative to date. However, although high carbonation degrees and acceptable rates have been achieved in the process, it is still too expensive to be applied on a larger scale. In 2008 the cost ranged from 40–80 Euro per tonne CO2 (~$50–$100 AUD/tCO2 at current exchange rates.
Mg2+ and Ca2+ cations do not exist as the ideal oxides or hydroxides in large deposits. In practice these cations are more commonly found as less-reactive silicates (olivine, serpentine) in ultramafic rocks at concentrations less than 50% (IPCC 2005).
If the process of mineral carbonation is divided into several steps it is characterised as indirect carbonation. In this case the reactive component is first extracted from the mineral as oxide or hydroxide and then, in another step, reacted with CO2 to form the desired carbonates. The first step can be carried out at atmospheric pressure and this is then followed by the carbonation step at elevated temperature and pressure (>500°C and >20 bar). Alternatively weak acids or bases can be used to form compounds such as MgCl in conjunction with Si chelators such as EDTA and citric acid to prevent the reverse reaction (called the ‘Activated cation’ process by the 70.2011)
It has been found in indirect carbonation that the kinetics of the reaction with MgO is slower than the kinetics of the reaction with Mg(OH)2. However, a three step process (MgO to Mg(OH)2 to MgCO3) still has slow kinetics, even at 525°C and 45 bar (Sipila et al. (2008)). These authors contend that research on high pressure gas-solid contacting in fluidised beds has the potential to improve this method of carbonation relative to the liquid-based techniques.
A pH swing process has been proposed in 71). In this process the gas is contacted by ammonia (NH3) with calcium chloride (CaCl2) to form ammonium carbonate (NH4)2CO3 and CaCl2. This solution is sent to a precipitator, where calcium carbonate is precipitated out of solution at low pH, leaving ammonium chloride. The pH is then raised in another vessel using 2CaO. SiO2 to re-form the NH3 and CaCl2 for recycling. In the Japanese experiments, the loss of NH3 was considerable.(Yogo et al. (2005)
Acetic acid has also been used to convert the minerals into Ca2+ ions prior to precipitation of the calcium carbonate (Kakizawa et al. (2001)72).
The two primary goals of current research are:
- To increase the rate of the carbonation reaction at less severe process conditions (temperature and pressure) in order to reduce the size and cost of the process vessels, and:
- To reduce the operating costs of the process. Of these process costs the two most urgent factors are the power plant parasitic load and improving the recycle rate of the chemical additives to avoid costly additive consumption.
The challenge for mineral sequestration is to find an optimal process that uses a (financial- and carbon-) cost-effective level of technology to achieve an acceptable reaction rate.
Transport and disposal
Two of the compelling features of mineral sequestration are the large reserves of reactant in common ultramafic rock and the permanence of the carbonated waste. These features are also a major challenge for carbonation processes. CO2 must be physically combined with reactant, so in a high- throughput process either the CO2 stream must be transported to the site of reaction, or the mineral must be transported to the site of CO2 emissions. Alternatively, if a chemical such as ammonia could be used to contact the flue gases to remove CO2, then this component could be circulated between the gas source and the mineral (see case studies below).
Australian coal and gas power stations produced slightly over 200 Mt of CO2 in 200973, which with an consumption rate value of 3 t mineral/t CO2, translates to around 600Mt of mineral reactant, plus overburden and 900Mt of product in order to sequester all the CO2 from these sources. This is an extremely large operation in terms of logistics — for example, Australia’s iron ore production in 2008 was around 350 Mt/year.
Sourcing, transporting and pre-treating this volume of minerals is a major economic and engineering challenge. Some of the product may be saleable, but the volumes produced are expected to very quickly overwhelm the market for these products so disposal is also very likely to be essential. This could possibly be achieved in the space left by the mining operations, although the mass and volume of waste carbonate is higher then the mined material.
In addition to the materials handling challenges mineral sequestration must also have community support. Natural forms of chrysotile (mineral from which asbestos may be extracted) can be found in serpentine rocks. Technology proponents advised that these occurrences are generally localised and may be avoided by selective mining, or handled using practices standard in mining operations that process similar materials. On the other hand the mineral sequestration processes typically destroy 100% of asbestos contained within reactants even if full rock dissolution is not achieved (IPCC Special Report on CSS 2005). This may present an opportunity for mineral sequestration to be used to remediate asbestos wastes including abandoned mines.
Case Studies on Mineral Carbonation Sequestration
The Calera process is a propriety process developed and marketed by the Calera company of USA74. A demonstration plant has been constructed at the gas fired Moss Landing power plant while a plant previously proposed for Yallourn in has now been cancelled due to the unavailability of brines of suitable quality and quantity. The Moss Landing plant has been demonstrated to capture flue gas CO2 from a 10MW power generator at 90% efficiency.
US Patent 7887694 provides information of a range of embodiments of the technology75. Broadly the technology envisages the introduction of brines containing alkaline earth metal (calcium and/or magnesium) ions to a reactor where it is contacted with CO2 containing gas. The CO2 dissolves in the water to produce carbonate and bicarbonate ions, resulting in a decrease in the pH of the solution. The solution pH is then raised through the introduction of alkalinity to the point where the alkali metals are precipitated as carbonates. Additional cycles of introduction of CO2, followed by additional alkalinity, may be applied to cause further carbonate precipitation. The carbonates produced are suitable for cement manufacture.
The technology requires the availability of a very large quantity of brines containing alkaline earth metal ions (Ca, Mg). These may be industrial waste water, mineralised spring water or sea water. This concept bypasses the rate-limiting step in mineral sequestration which is the dissolution of the alkaline earth ions from the parent minerals.
The source of alkalinity to initiate precipitation may be calcium or magnesium oxide, sodium or potassium hydroxides, alkaline flyash or by electrolytic decomposition of sodium chloride by application of a Calera propriety electrolytic cell. Proprietary precipitation enhancing agents may also be employed.
One issue with the technology is the quantity and quality of brine that must be processed to provide sufficient alkaline earth metals to sequester industrial quantities of CO2. For example, seawater contains approximately 1270 ppm magnesium and 400 ppm calcium ions. Complete depletion of these ions would consume 2.7kg of CO2 per tonne of water. A plant capable of sequestering 5 Mt/a of CO2 will then require a minimum of 1851 Mt/a (5 Mt/day) of seawater. Brines of higher Mg and Ca content would be more suitable for the process, but their local availability in the quantities required is in question.
Integrated Carbon Sequestration Pty. Ltd. (ICS)
The ICS Process is a mineral carbonation process invented by Hunwick that is being developed by a company he formed, Integrated Carbon Sequestration Pty Ltd. It is patented in Australia76, and patents have been received or are pending in other countries and regions around the world. It secures the permanent storage of carbon dioxide by reacting the gas with ultramafic rocks notably serpentinite and olivine, and other metal silicate rocks. It differs from other processes in that it avoids the need to handle pressurised, pure carbon dioxide: silicate rock is converted directly to carbonate (plus silica) by reacting it with the solution used to scrub the gas from the flue gases of the host power station (or other point source). Also, all rock-handling including emplacement of the carbonated product may be restricted to the mine area, with an interconnect to the power station and mine in the form of liquid pipelines. By avoiding having to strip pure carbon dioxide from a capture solution, compress the gas to supercritical pressures and heat-treat the rock, capital and operating costs are reduced and the parasitic energy demands are said to be reduced.
The ICS process has advanced beyond the proof-of-concept stage, with CSIRO having undertaken extensive experimental tests in a 300 ml autoclave studying the effects of temperature and pressure, backed up by detailed process simulation studies of various process configurations using the ASPEN package.77
CSIRO has also advised the task force that work of a more basic nature is being undertaken, funded by CSIRO78. These studies are attempting to get a better understanding of the mechanisms involved and the development of insight into ways to improve the process.
GreenMag Group have an alliance with the Priority Research Centre for Energy at the University of Newcastle, NSW. GreenMag Group are pursuing research and development in collaboration with the University to develop a global reference facility at laboratory, pilot and demonstration scales to sequester CO2 using magnesium silicate deposits that exist in the New England area of NSW. A presentation on this approach was given by Marcus St. John Dawe at the National CCS Week conference in Melbourne in 2010.79 The following information was provided publicly at this presentation:
- The process is commercially secret, and two provisional patents on the technology have been lodged.
- The economics is site-specific and the upper Hunter Valley in NSW is the best opportunity in Australia, since serpentine materials are available in the region. However, the process is relevant for all deposits of serpentine globally, which are widespread.
- Most of the carbonated product would be placed back into the mined rock area after processing, with some used to make new building products.
- $3.04M had been awarded from the NSW Clean Coal Council, and matching funds from the Federal Government have also been approved pending the final contribution from an industry partner of $3.04M, which is now being sought. This will fund a pilot scale facility at the University at a new adjacent campus at the former BHP Billiton research laboratories.
- A pre-feasibility study has shown that the price of carbon dioxide required to make the mineral carbonation process viable is $70/t CO2 including a 25% energy penalty. The aim is to reduce this to $40/t CO2, including a 15% energy penalty. This latter target depends on new technology development.
- It is hoped by GreenMag Group that a demonstration scale project (100,000 t/year CO2) can be developed by 2016 for a cost of $85M. This could lead to a full scale 2 Mt/year CO2 commercial plant for a cost of $1 to $2 billion by 2020.
The task force has discussed the project further with GreenMag Group and the principal academic researchers at the University of Newcastle as well as inspect the laboratory-scale laboratory facilities at the university, which are excellent. The discussions and further information obtained by the Task Force are commercial-in-confidence. However, an open public report on a design and financial analysis of a proposed unimproved mineral carbonation process has been carried out independently by Rayson et al. at the University of Newcastle (2008).80 The Task Force has utilised this public information to undertake a financial analysis (see Section 5 of this report).
Orica is a large Australian company and is interested in mineral carbonation from the point of view of supporting a cleaner coal industry and reducing 81,82 and has several innovative ideas that could be explored scientifically and in terms of chemical engineering. These include linking various carbon-intensive resource industries such as iron and steel and cement with a mineral carbonation process.emissions. Supply of explosives for the rock mining operation is a further motivation. The company is technologically advanced and has significant resources to apply to the problem, including ongoing alliances with the University of Sydney, Columbia University, University of Arizona and CSIRO. Orica has reviewed the information from the USA Albany Research Centre (ARC — mentioned in the literature review above) and believes that the efficiency of this process can be improved at several different levels. Orica has published in the field, including patent applications,
It has been reported that Alcoa at Kwinana in 83. At Kwinana, Alcoa is sequestering 70,000 t CO2 per year using all the residue “red mud” from the refinery. This amounts to 30t “red mud”/tCO2, which is about ten times the rate of serpentine rock usage envisaged by the proponents of the other mineral carbonation processes, above. At this rate of usage, about 300,000 tCO2 per year could be sequestered by Alcoa’s “red mud” waste streams in Australia.has developed a process to utilise the waste product “red mud” from alumina production to sequester pure CO2 from its own operations
59 “IPCC Special Report on Carbon Dioxide Capture and Storage”, 2006, pp320–337.
60 Sipila J., Teir S., Zevenhoven R., “Carbon dioxide sequestration by mineral carbonation: Literature Review Update”, Faculty of Technology, Abo Akademi University, Finland, 2008.
61 Zevenhoven, R., J. Fagerlund, and J.K. Songok, “CO2 mineral sequestration: developments toward large-scale application.” Greenhouse Gases: Science and Technology, 2011. 1(1): p. 48–57.
62 Penner L., O’Connor W., Dahlin D., Gerdemann S., Rush G., “Mineral Carbonation: Energy Costs of Pretreatment Options and Insights Gained from Flow Loop Reaction Studies”, 3th Annual Conference on Carbon Capture and Sequestration, Va, USA, 2004.
63 Hangx, S.J.T., C.J. Spiers, “Coastal spreading of olivine to control atmospheric CO2 concentrations: A critical analysis of viability.” International journal of greenhouse gas control, 2009. 3(6): p. 757.
64 Lackner, K.S., “Progress on binding CO2 in mineral substrates.” Energy Conversion and Management, 1997. 38: p. S259.
65 Lackner, K.S., “Carbonate chemistry for sequestering fossil carbon.” Annual review of energy and the environment, 2002. 27(1): p. 193.
66 Zevenhoven, R., “Mineral carbonation for long-term CO2 storage: an exergy analysis.” International journal of applied thermodynamics, 2010. 7(1): p. 23.
67 O’Connor, W. K., D. C. Dahlin, G. E. Rush, S. J. Gerdemann and L. R. Penner (2004). “Energy and economic considerations for ex-situ and aqueous mineral carbonation.” Report for U.S. Department of Energy
68 O’Connor W., Dahlin D., Rush G., Gerdemann S., Penner L., Nilsen R., “Aqueous mineral carbonation: Mineral availability, pretreatment, reaction parametrics, and process studies”, DOE/ARC-TR-04-002, 2005.
69 Gerdemann, S.J., W.K. O’Connor, D.C. Dahlin, L.R. Penner and H. Rush, “Ex situ aqueous mineral carbonation.” Environmental science & technology, 2007. 41(7): p. 2587.
70 Baird J., Clinton Foundation, Personal Communication, Melbourne, 2011.
71 Yogo K., Eikou T., Tateaki Y., “Method for fixing carbon dioxide”, Japan Patent JP2005097072, 14.4.2005
72 Kakizawa, M., A. Yamasaki, and Y. Yanagisawa, “A new CO2 disposal process via artificial weathering of calcium silicate accelerated by acetic acid.” Energy, 2001. 26(4): p. 341–354.
73 “Australian national greenhouse accounts — National Inventory Report 2009” Dept of Climate Change and Energy Efficiency Canberra.
74 Zaelke D., Young O., Andersen S.O., “Scientific Synthesis of Calera Carbon Sequestration and Carbonaceous By-Product Applications”, Consensus Findings of the Scientific Synthesis Team, Donald Brent School of Environmental Science and Management, University of California, Santa Barbara, January 2011. Also: http://www.calera.com
75 Constanz, “Methods for sequestering CO2”, US Patent 7887694, 2011.
76 Hunwick, R., “System, apparatus and method for carbon dioxide sequestration”, Australian Patent AU2008/000232. 2008.
77 Hunwick, R. (ICS) and Duffy G. (CSIRO Energy Technology), Non-confidential Personal Communication, August 2011, with permission.
78 Duffy G. (CSIRO Energy Technology), Non-confidential Personal Communication, August 2011, with permission.
79 St. John Dawe, M., “Mineral Carbonation (MC) — A potential large scale solution for carbon storage and utilisation.”, in National CCS Week Conference, 2011. Melbourne. (From notes taken by the Task Force Chair during the presentation at that conference and St. John Dawe M, personal communication 2012 (with permission).
80 Rayson, M., M. Magill, R. Sault, G. Ryan and M. Swanson (2008). “Mineral Sequestration of CO2 — Group 2, Phase 3”, Discipline of Chemical Engineering, The University of Newcastle, NSW 2308. Report may be obtained by contacting the Discipline Secretary at the University of Newcastle.
81 Brent G., “Integrated Chemical Process”, Australian Patent Application AU2010100761 B4. Brent G., “Improved Integrated Chemical Process”, Australian Patent Application, AU 2010100762 B4. Chizmeshya V.G., Brent G. F., “High temperature treatment of hydrous materials”, Australian Patent Application AU 2010101031 A4.
82 Allen D. J., Brent G.F., “Sequestering CO2 by Mineral Carbonation: Stability against Acid Rain Exposure”, Environ. Sci. Technol., 2010, 44 (7), pp. 2735.
83 “Accelerating the Uptake of CCS: Industrial Use of Captured Carbon Dioxide”, Global CCS Institute, March 2011.