Carbon capture: the added values of mineral carbonation

Although the global warming potential of carbon dioxide (CO₂) emissions has always been acknowledged by the scientific community and governments worldwide, CO₂ has been historically undervalued in the market place, which did not encourage companies to reduce emissions. In the European Union (EU), prices dropped from $45/tonne of CO₂ in 2008 to $4-10/tonne of CO₂ in 2013, due to the global economic crisis.

However, the US Interagency Working Group raised its estimate of the social cost of CO₂ to $65/tonne of CO₂ in 2010 and the UK added a carbon price floor of about $30/tonne of emitted CO₂ in order to prop up carbon pricing.

As regulations on CO₂ emissions become increasingly strict, and market forces move CO₂ prices higher, it is expected that there will be a push for companies to become more efficient and develop technologies for carbon sequestration and storage.

Suitable minerals

CO₂ sequestration in minerals, also known as mineral carbonation (MC), is one of the options considered by research centres and industries for mitigating the impact of power plant and transport emissions on the climate.

MC is based on the reaction of CO₂ with metal oxide bearing minerals to form insoluble, chemically stable carbonates; calcium (Ca) and magnesium (Mg) being the most reactive metals.

This process occurs spontaneously in silicates but is a very slow process.

Suitable minerals for MC include Mg-rich silicate rocks and minerals, such as serpentine and olivine, Ca-rich minerals, such as wollastonite, and alkaline industrial residues, such as slag from the steel industry or fly ash.

The process can take place in-situ, by injecting CO₂ in silicate-rich geological formations, in alkaline aquifers or in an ex-situ chemical processing plant after mining and pre-treating the minerals.

MC offers two main advantages to other carbon sequestration options, such as geological or ocean storage, having the advantage of abundant metal oxide bearing materials, in particular natural silicates, and the permanent storage of CO₂, which rules out any risk of leakage and cuts monitoring costs.

The reaction of CO₂ with serpentine and olivine produces stable magnesium carbonate, silica and heat, while the reaction with wollastonite produces heat, calcium carbonate and silica.

According to the Intergovernmental Panel on Climate Change, (IPCC), rocks containing magnesium silicates have the highest estimated storage capacity of 0.55kg of CO₂/kg of rock, while calcium-rich basaltic rocks have a theoretical storage capacity of 0.08kg of CO₂/kg of rock.

Accelerated mineral carbonation (ACM)

One of the main characteristics being investigated by scientists and industries is the possibility of accelerating the MC rate, either by increasing the temperature (up to 650⁰ C for serpentine) or by ultrafine grinding of the mineral, which has been used for olivine and wollastonite.

“The reaction is speeded up at higher temperatures and pressures. This is exactly our research [at the University of Leuven]. Most companies involved in mineralisation are working in these kinds of process conditions,” Pol Knops, research scientist at KU Leuven spin-off company, Innovation Concepts BV, told IM.

“Locations with an acidic environment and erosion sensitivity also show much faster reaction rates, but if no water is present, the reaction stops,” he added.

Research has been focusing on speeding up metal extraction from the input materials by using additives and catalysts in water solution, such as acetic acid and citric acid.

“The dissolution rate is a function of surface area, thus small grains are preferred,” Jens Hartmann, professor from the Institute for Biogeochemistry at the University of Hamburg, Germany, told IM.

Hartmann explained that low pH (acidic conditions) and longer residence time - time during which a water is in contact with the mineral surface - also increase the MC rate.

“The highest chemical weathering rates from volcanic areas have been reported. This could reach 250 tonnes CO₂/km² per year in very warm and wet environments, with high soil water permeability,” Hartmann added.

Australian Mg and Ca-based fertilisers and building materials producer, Calix, bought a processing technology from UK-based Novacem in 2012, which uses accelerated carbonation of magnesium silicates at temperatures of 180⁰C and a pressure of 150 bar to produce a magnesium oxide and hydrated magnesium carbonate-based cement.

Other companies which are developing a pilot plant to study ACM include Australia-based Greenmag, and US-based Skyonic.

IPCC’s figures show that AMC, using ground olivine dissolved in an aqueous solution of sodium chloride and sodium bicarbonate, and in contact with high pressure CO_2, recovered a maximum of 81% CO₂ in one hour, at a temperature of 185^oC and a CO₂ partial pressure of 150 bar.

The IPCC said that 1.6 tonnes of olivine could fix one tonne of CO₂, producing 2.6 tonnes of solid material for disposal, assuming 90% carbonation conversions and 10% losses.

Costs and feasibility ACM implies operations at the mines, including crushing, grinding, milling, some mechanical separation, such as magnetic extraction of magnetite and transportation.

A study from the US Department of Energy (DoE) on energy and economic costs on high-temperature and high-pressure flow-loop reactors, shows that costs range from $55/tonne of CO₂ stored for olivine with standard pre-treatment, to $64/tonne of CO₂ stored for activated wollastonite, with a percentage of CO₂ conversion after one hour varying between 61% and 82%.

According to the DoE, the cost of the most realistic technologies falls in the range of $50-100/tonne of CO₂ stored. Energy requirements have been estimated to be about 305 kWh/tonne of CO₂.

According to the IPCC, energy costs for thermal and mechanical activation are about 300 kWh/tonne and 70-150 kWh/tonne respectively.

“We calculated the CO₂ efficiency for mineral sequestration in mafic and ultramafic rocks, considering mining, crushing, grinding, transportation and application for most agricultural areas, globally. If the closest locations of identified sources are used, the CO₂ balance is always positive,” Hartmann told IM.

He added that the estimates do not include rates calculated in the CO₂ budget, and did not include further beneficial processes from the use of the material to increase biomass and crop productions.

Carbon capture in power plants and tailings

An AMC process was tested at the Jim Bridger 2120 MW coal-fired power plant in Wyoming, US, which is owned by PacifiCorp.

The study, which was published this year by the University of Wyoming and the Brigham Young University, demonstrated the mineralisation of fly ash particles at 70⁰C with CO₂ emissions at 60⁰C, and at a pressure of about 21 kPa (0.58 bar) with 16% moisture in the fluidised bed reactor (FBR).

Carbonation of the fly ash particles produced calcium carbonate at the plant, which increased from 3.11% in weight after 10 minutes to 3.86% in weight after two hours.

Scientists are planning to study the process on other coal-fired power plants under a wide range of conditions and evaluate the economic feasibility of the AMC process. They are also studying the use of carbonated fly ashes as a replacement for Portland cement in construction.

Researchers from The University of British Columbia, Canada, and from The University of Queensland, Monash University and Bond University, Australia, have recently explored the possibility of sequester CO₂ in mine tailings through MC.

By studying the natural MC at the abandoned Clinton Creek asbestos mine, the active Diavik diamond mines in Canada, and the active Mount Keith nickel mine in Australia, scientists found that different conditions of temperature and humidity, hydrology and tailings management practices impact differently in carbon sequestration.

While passive carbonation rates at the dry tailings at the Clinton Creek asbestos mine were estimated to be 6.2kg/m² of CO₂ per year, tailings at the Diavik mine captured only about 400g/m² CO₂ per year, despite similar climatic conditions; the majority of tailings at Diavik are submerged in a pond.

MC rates from tailings at Mount Keith are about 2.4 tonnes CO₂/m² per year, which is a faster rate than at the Diavik mine due to a warm, dry climate, a saline water environment and a design of the tailing storage which favours CO₂ adsorption.

Scientists are investigating possible strategies for AMC in the tailings, such as bioleaching of the ultramafic rocks, using acidophilic bacteria and increasing the number of tailings deposition points at the mine to enhance exposure to atmospheric CO₂.

Scientists estimated that complete carbonation of 11m tonnes of tailings, generated annually at Mount Keith, would sequester about 3.6 tpa of CO₂, accounting for about 11% of the total greenhouse gas emissions from the mine.

According to the study, the enhanced carbonation process at Mount Keith would have a value of between $1m/year and $4.7m/year, depending on the carbon price floor, and would have the potential to offset up to 74% of annual CO₂ emissions at the mine.

Added values of mineral carbonation

Knops said that carbon sequestration in olivine could also provide combined benefits, such as the use of magnesium carbonate as a replacement for constructing materials, or to release alkalinity and magnesium.

“The construction industry is rather conservative, but more and more civil applications are being developed, such as replacement in roofing materials and flooring applications,” Knop said.

Netherland-based company Olivine Group BV is also producing an olivine-based greenSand product for pH stabilisers in soil, acting as a source of bicarbonates to counteract acidification and delivering magnesium, manganese and iron plant nutrients.

Jens Hartmann, professor from the University of Hamburg, Germany, told IM that not only olivine, but mafic to ultramafic volcanic rocks are more suitable for CO₂ removal at a global scale.

“Mafic volcanic rocks contain, in addition to basic cations, geogenic nutrients, such as phosphorous, calcium and magnesium, which are beneficial for biomass increase and crop production,” Hartmann said.

Harmann explained that increasing forestation in South America and Africa could be helped by soil carbon improvement and nutrients supply through deeply weathered rocks in laterite areas.

“If other rocks are used, clay minerals will form in the soil and can have a stabilising effect on soil carbon. Also, clay minerals can be useful as cation exchange pools in the soil,” Hartmann told IM.

Hartmann said that due to an ever increasing population to feed, limited agricultural areas and the exhaustion of large phosphate mines within the next two-four decades, a long-term strategy to guarantee crop production needs to be considered, and this might include the use of enhanced weathering of volcanic rocks.

“It would be necessary to identify the best rock source by its composition and the ground rock would be the basis for a new soil or would be ploughed into existing soils,” Hartmann said.

“If enhanced weathering is to be applied as a large scale method in CO₂ management and food production, a new global infrastructure needs to be built up,” Hartman told IM.

“We want to establish contacts with mining companies, not only with olivine producers, but also with suppliers of volcanic rocks, in order to establish a cooperation which will be useful to analyse the life cycle of the minerals,” he added.

“Mining companies are slowly realising that this could be a new, additional outlet for [industrial minerals such as] olivine,” Knops said.

“But, as there are no existing markets it is hard for them to anticipate this market,” he added.

Innovation Concepts BV has started contacting companies to establish partnerships with, for example, Sibelco, Pasek and Nuova Cives.

*Conversion made July 2014