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Appendix F: CO2 as feedstock for carbonate mineralisation
Carbon mineralisation is the conversion of CO2 to solid inorganic carbonates using chemical reactions. In this process, alkaline and alkaline-earth oxides such as magnesium oxide (MgO) and calcium oxide (CaO), which are present in naturally occurring silicate rocks such as serpentine and olivine are chemically reacted with CO2 to produce compounds such as magnesium carbonate (MgCO3) and calcium carbonate (CaCO3, commonly known as limestone). Mineral carbonation occurs naturally and is a very slow process. In order for carbonate mineralisation to be a viable method to capture and reuse CO2 from anthropogenic sources such as coal-fired power plants, this process must be accelerated considerably.
Current research and development activities in carbonate mineralisation are focused on achieving energy efficient reactions and reaction rates viable for storage of significant volumes of CO2 from industrial processes by using either:
- natural rock silicates; or
- industrial waste (flyash and waste water/brine).
The carbonates that are produced are stable over long time scales and therefore can be used for construction, mine reclamation or disposed of without the need for monitoring or the concern of potential CO2 leaks that could pose safety or environmental risks.
A flow diagram of the mineralisation process is presented below;
Process flow diagram for mineral carbonation11
The use of natural rock silicates in mineral carbonation is still in the research phase. This technology involves utilising the abundance of magnesium silicates such as serpentine and olivine containing high concentration of MgO for the carbonation reaction. For this to be viable for commercial scale mineralisation requires:
- efficient extraction or activation of the reactive component MgO from silicate mineral; and
- acceleration of the carbonation chemistry kinetics.
While most research is concentrating on methods using aqueous solutions, research using a fluidised bed reactor for gas/solid dry carbonation is being conducted in Finland. Although this technology is showing promising results, The process is energy intensive requiring high temperatures and high pressures (600°C and 100 bar). The technology is also limited by the fraction of silicate reserves that can be technically exploited, and the additional intensive operations of mining, crushing, milling and transporting the mineral-bearing ores to the processing plant for mineralisation. For these reasons commercial silicate mineral carbonation technology does not yet exist.
Carbonate mineralisation using industrial wastes (rather than natural rock silicates) are further developed with pilot scale plants in operation. In this process CO2 emissions (from power plant flue gas or cement manufacturing process) are chemically combined with water/brine to form solid mineral carbonates and bicarbonates. In particular, Calera has successfully used flyash as the source of alkalinity and for the production of cementitious materials (SCM), aggregate and other building related materials. The Calera carbonate mineralisation by aqueous precipitation (CMAP) process is shown below:
Calera's CMAP process
Calera has produced an alternative means of producing alkalinity in case there is an insufficient source or the available source is unable to complete the conversion of CO2 to carbonate. The current process for the production of alkalinity (e.g. sodium hydroxide) uses the high energy demand chlor-alkali process. To overcome this problem Calera has developed a new low voltage technology, Alkalinity Based on Low Energy (ABLE) to generate sodium hydroxide. The process uses an electrochemistry process to split salt to form an alkaline solution and acid and reduces overall electrical demands by 60 per cent of the current generation technologies.
Skyonic’s SkyMine& technology also removes CO2 from industrial waste streams and produces solid carbonate and/or bicarbonate materials. There is limited public information on the status of the technology other then it has been field tested on operational coal-fired power plants.
Currently two organisations are involved in developing the carbonate mineralisation technology using industrial emissions.
- Calera is operating a continuous pilot scale facility in Moss Landing, California which produces on average 5t/day of supplementary cementitious material (SCM). A demonstration plant is under construction in which a 10MW slipstream from the 1.5GW Dynergy Moss Landing gas fired power plant will be used as the source.Calera is currently undertaking a pre-feasibility study for a demonstration plant at TRUenergy’s Yallourn power station in the Latrobe Valley. It is reported that the project is due to start construction in 2010. The project plans to initially capture more than 300,000 tonnes of CO2 per year and convert it into more than 1Million tonnes of building materials per year. The project will expand to capture 1.35M tonnes of CO2 per year for conversion into 2.8M tonnes of building materials per year.Calera’s business plan is for the construction of multiple demonstration plants to validate the commercial viability of its technology. Hence Calera Corporation and Bechtel Power Corporation have formed a strategic alliance to deploy Calera’s technology worldwide.Calera is also currently constructing a pilot scale unit of their patented technology, Alkalinity Based on Low Energy (ABLE) which is capable of producing one tonne of NaOH per day. A demonstration scale unit consisting of fully commercial cells will be ready for operation in early 2011. This caustic production unit provides an alternative means of producing alkalinity for their CMAP technology.
- Skyonic Corporation has developed SkyMine& technology, a carbon mineralisation process which removes CO2 from industrial waste streams through co-generation of carbonate and/or bicarbonate materials. Phase 1 of Capitol-SkyMine demonstration facility has been initiated at Capitol Aggregates, Ltd cement plant in San Antonio, Texas, USA. Phase 1 includes modelling, simulation, design, costing, and procurement activities. Construction of the facility is anticipated to commence by the third quarter of 2010.The Capitol-SkyMine plant is targeted to capture 75,000 metric tonnes of CO2 from flue gas and mineralize carbon emissions to produce 143,000 metric tonnes of baking soda. The mineralized carbon dioxide (baking soda) will be used in several industrial applications and will be tested as feed-stock for bio-algae fuels.
CO2 utilisation and resource quantities
Adsorption of one tonne of carbon dioxide using carbonate mineralisation based on natural rock silicates with high pressure and temperature CO2 in a fluidised bed requires around three tonnes of serpentinite or equivalent ultramafic rock (or 6–7 tonnes of such rocks are required to absorb the carbon dioxide from the combustion of every tonne of coal) (Hunwick, 2009).
To store one tonne of CO2 as carbonates using wet carbonate mineralisation (based on natural rock silicates and aqueous solutions) requires:
- 2.4 tonne of NaOH and 2-4 tonne of make-up acid (Sebastion Zevenhoven et al, 2007).
Adsorption of one tonne of carbon dioxide using the Calera process (use of industrial waste (flyash), and alkalinity source – natural or manufactured) requires almost one tonne of brine or manufactured alkalinity (sodium hydroxide), and part flyash. Each tonne of mineral carbonation and cement formed by the Calera mineralisation process contains one-half tonne of CO2.
Potential markets for products generated from mineralisation include:
- Mine reclamation.
- Construction materials – aggregate.
- Supplant portion of cement.
The main markets for the use of the carbonates produced via the CMAP process are the cement and aggregates markets as alternatives to traditionally produced Portland cement and building aggregates. Calera claim that CMAP products can be made and sold competitively in the current market with estimates that approximately 1.5 billion tonnes of Portland cement could be substituted with carbonate cement, and another 30 billion tonnes of aggregate used in concrete, asphalt, and road base could be substituted.
Size of market
Calera has estimated that the current global demand for building materials is 32 billion tonnes per year and is expected to see year on year growth. According to the International Energy Agency, cement production is projected to grow by 0.8–1.2 per cent per year until 2050.
Acceptance of products as replacement for existing aggregate and cement supply.
Level of investment required (to advance the technology)
Calera plans to build a facility, Calera Yallourn, in the Latrobe Valley, Australia, which following a demonstration phase will be the first commercial scaled facility capable of capturing 200MWe of CO2. The CO2 will be captured from the flue gas of a local coal power station. Calera have estimated that the costs associated with the facility are as follows:
- CAPEX requirement (including CO2 capture and building materials) of US$300-380m; and
- a cost of CO2 capture of US$45-60/tonne of CO2.
Details of further operating and maintenance costs are not available.
Potential for revenue generation
There is a potential for this technology to produce sustainable revenues through the sale of the carbonate products. However this is dependent on the market accepting the product and the successful penetration of the market. Calera claim that price competitive products can be produced through the use of the CMAP process. However, this expectation should be treated with caution since the technology is not yet commercial.
The price of the technology will be affected by changes in demand for building products. The construction industry is typically cyclical so prices could be expected to vary over time.
There are no significant commercial benefits of the technology since it is unlikely that the products produced via the process will be superior to existing products in the market. Therefore, The commercialisation of the technology will largely be driven by the environmental benefits. There is potential for the technology to have a higher commercial benefit in regions where exists carbon trading scheme exists.
In general, mineralisation as a CO2 reuse option has a number of benefits. The major benefit is the permanence of CO2 storage. After mineral carbonation, CO2 would not be released to the atmosphere and the silica and carbonates that are produced are stable over long time periods. As a consequence, There would be little need to monitor the disposal sites and the associated risks would be very low.
In particular, The Calera process which is one of the most advanced mineralisation technologies has a number of benefits. These include:
- The utilisation of waste streams such as flyash and waste water.
- The technology does not require CO2 separation or compression or CO2 feed quality requirements.
- One of the by-products is fresh water that could be used as potable water, irrigation water, or an industrial water supply, which may alleviate the water deficit in some regions.
- The process captures other emissions including sulphur dioxide, particulate matter, mercury and other metals.
- The core technology and equipment can be integrated with base power plants and cement manufacturing very effectively.
- Process is designed to utilise flue gas from a range of emission sources and can operate with a wide range of CO2 concentrations.
- CMAP process removes hardness and other components from the brine, it allows for production of fresh water with lower energy consumption than raw brine. The separation of alkalinity, calcium, magnesium, and/or sodium chloride during the Calera process results in clean water that can be used as potable water, irrigation water, or an industrial water supply.
The mass of natural silicate rocks (containing magnesium ore) to store CO2 generated by coal combustion is calculated to be over eight times the mass of coal. Despite the large difference in mass, The mining operation is claimed to be of similar magnitude to that of coal (Herzog, 2002). Mineral carbonation using natural silicate rocks would be limited by:
- the fraction of silicate reserves that can be technically exploited;
- environmental consequences of large mining operation;
- environmental issues associated with the disposal of the carbonate (the volume of material increases as a result of the mineral carbonation process);
- legal and societal constraints at the storage location; and
- the energy intensity required for mining the resource and the carbonation technology itself.
It is likely that the carbonation process would need to take place at the mine, adding geographical constraints to this technology, raising similar issues to geological storage.
The Calera technology has the potential to be rejected by the cement industry (as it produces a product that is already produced in the manufacture of cement) and would require a carbon price to provide an incentive to cement manufacturers.
The success of Calera’s CMAP technology for the development at the Yallourn site in Australia is highly dependent on the availability of suitable subsurface waters (brine) to provide the requisite hardness and alkalinity required and within abundant supply. Without such brines, alkalinity will need to be manufactured which raises concerns over the current status of the proposed ABLE technology which is still in early demonstration (pilot scale) phase.
11 Courtesy IPCC CCS technical Summary report